Title of Invention	PUMPED LASER SYSTEM USING FEEDBACK TO PUMP MEANS
Abstract	A laser system according to the invention comprises pump generating means (xO2, xO3) for generating at least a first and a second, preferably focused, pump beam, and lasing means (xO6, xO7) for emitting radiation by being appropriately pumped. The lasing means (xO6, xO7) is disposed in a first resonator so as to receive the first pump beam in order to generate a first beam (x21) having a first frequency, and the lasing means (xO6, xO7) is disposed in a second resonator so as to receive the second pump beam in order to generate a second beam (x22) having a second frequency. At least one Q-switch (xO8; xl7, xl8) is disposed in the first and the second resonator, so that the first beam and the second beam both pass a Q-switch (xO8; xl7, xl8). The laser system (x01) has an output (x13) generated from said first beam (x21) and said second beam (x22), and at least a part of said output (x13) is fed back to a regulation system (xl4), said regulation system (xl4) controlling said pump generating means (xO2, xO3).

Title of Invention

PUMPED LASER SYSTEM USING FEEDBACK TO PUMP MEANS

Abstract

A laser system according to the invention comprises pump generating means (xO2, xO3) for generating at least a first and a second, preferably focused, pump beam, and lasing means (xO6, xO7) for emitting radiation by being appropriately pumped. The lasing means (xO6, xO7) is disposed in a first resonator so as to receive the first pump beam in order to generate a first beam (x21) having a first frequency, and the lasing means (xO6, xO7) is disposed in a second resonator so as to receive the second pump beam in order to generate a second beam (x22) having a second frequency. At least one Q-switch (xO8; xl7, xl8) is disposed in the first and the second resonator, so that the first beam and the second beam both pass a Q-switch (xO8; xl7, xl8). The laser system (x01) has an output (x13) generated from said first beam (x21) and said second beam (x22), and at least a part of said output (x13) is fed back to a regulation system (xl4), said regulation system (xl4) controlling said pump generating means (xO2, xO3).

Full Text	Pumped laser system using feedback to pump means Technical Field The following invention relates to a laser system comprising pump generating means for generating at least a first and a second, preferably focused, pump beam, lasing means for emitting radiation by being appropriately pumped, said lasing means being disposed in a first resonator so as to receive the first pump beam in order to generate a first beam having a first frequency, and said lasing means being disposed in a second resonator so as to receive the second pump beam in order to generate a second beam having a second frequency, and at least one Q-switch being disposed in the first and the second resonator, so that the first beam and the second beam both pass a Q- switch, and said laser system having an output generated from said first beam and said second beam. Laser systems, which comprise two or more lasers or gain elements or alternatively a gain element generating two or more laser lines (i.e. two or more wavelengths), and in which a so-called sum frequency generation or another form of nonlinear conversion is involved, are a relatively new area of research. In order to achieve a more efficient con- version, the lasers can be pulsed. The individual lasers can be pulsed by pulsing a pump source (not that efficient) or they can be pulsed by periodically changing the so- called Q-factor of the cavity of the laser by modulating the losses in the cavity. This can be achieved by using a so-called Q-switch. The Q-switch can either be active, such as an electro-optical or acousto-optical Q-switch, or passive using a saturable absorber. One of the main problems of such systems is to synchronise the individual lasers since they usually have different pulse build-up times. Thereby, the pulses that propagate in the cavity will not temporally overlap. This is a problem, since the nonlinear conversion only can occur if the pulses overlap. If the overlap between the individual pulses is not in any way controlled or regulated, the overlap between the pulses will be random and therefore not optimised. Consequently, the nonlinear conversion will not be optimised either. One method for optimising the overlap is to regulate the focus of the emitted light from the pump source in the laser element. This is of course only possible in systems, where the light from the pump source is focused into the laser element. Thus, if the pump source is a gas discharge lamp or a blitz lamp, it is not possible to use this type of regulation. In any case, it is difficult to change the focus of the pump source beam in commercial laser systems, since the functionality of the laser is highly dependent on an accurate focus. Furthermore, mechanical systems for changing the focal point have in- herent inaccuracies and will additionally considerably increase the overall cost of the laser system. Another method for regulating the pulse overlap relates to changing or translating one or more mirrors of the cavity in order to change the eigen-focus in the laser element. This changes the overlap between the cavity mode (i.e. the spot size in the laser ele- ment) and the pump source, whereby the gain of the laser element is changed. In addi- tion, the change in cavity mode itself can change the gain. Once again, this method is mechanically based and therefore involves a considerable increase to the overall cost of the system as well as introducing a source for misalignment. A third possibility is to change the position of the laser element with respect to the focal point of the pump source. However, this is only possible if the laser element is shorter than the cavity arm, in which the laser element is disposed. Additionally, it must be pos- sible to translate the laser element, which again means that mechanical translation means are necessary. Thus, this method has the same disadvantages as the afore- mentioned methods. A fourth possibility is to employ a method, where the cavities have static or variable, in- dividual losses so that the difference in intracavity losses outbalance the difference be- tween the respective gains of the laser elements. A fifth possibility is to utilise a plurality of Q-switches and a variable electronic delay be- tween them. This is a somewhat more complex solution, which demand for separate electrical supplies for the individual Q-switches as well as an electronic variable delay line. If the system operates in the nanosecond range, which is often the case for such systems, such electronics are relatively expensive and sensitive. Also, if every laser line of the system uses a separate Q-switch, this increases the overall cost of the sys- tem, since Q-switches generally are relatively expensive. US patent no. 5,121,402 discloses a multiple gain-element ring laser, which has the capability of being scaled to higher pulsed output powers. The ring laser comprises a plurality of pump sources and corresponding gain elements. The resulting laser beams from the individual gain elements all pass the same Q-switch in the ring cavity. Disclosure of Invention The purpose of the present invention is to provide a new and improved laser system, in which the pulse overlap between two or more laser lines of the system is optimisable. This is according to the invention achieved by at least a part of said output being fed back to a regulation system, which regulates the build-up time of individual laser pulses of the first and second beam by controlling the intensity output of said pump generating means. The regulation system is thereby adapted to control the intensity output of the pumping means, which in turn regulates the gain of the laser means, thereby controlling the build-up time of the individual laser pulses of said first and second beam. The overlap between the individual pulses can thereby be optimised in a simple manner. In a preferred embodiment of the invention, said at least one Q-switch is an active Q- switch, such as an electro-optical or an acousto-optical Q-switch. Active Q-switches are favourable as compared to passive Q-switches, since the latter only have a limited working interval with respect to the repetition frequency and the pulse effect. Therefore, typically an invariable working point has to be chosen. According to a preferred embodiment of the invention, said at least one Q-switch is disposed in both the first resonator and the second resonator, so that the first beam and the second beam both pass the same Q-switch. According to a preferred embodiment, the pump means are a single pump source, such as a laser diode. This single pump source is thus used to generate at least said first and said second pump beam. In this case, it is possible to regulate the individual pump beam effect by use of methods known per se, such as by use of a phase re- tarder, a polarising beam splitter or a birefringent crystal. Alternatively, the pump means comprise at least a first pump source, such as a first la- ser diode, for generating the first pump beam, and a second pump source, such as a second laser diode, for generating the second pump beam. Thereby, a particular sim- ple embodiment of the pump means is achieved. According to a preferred embodiment according to the invention, the lasing means is a single gain element or laser element. This single laser element is thus used to generate at least two pulsed laser beams having different frequencies or wavelengths. Alternatively, the lasing means comprise at least a first gain element or lasing element for generating said first beam, and a second gain element or laser element for generat- ing said second beam. Thereby, a particular simple embodiment of the lasing means is achieved, in which separate laser elements are used for generating the individual laser lines (or wavelengths) of the system. The laser system can of course comprise three, four or even more pump sources as well as three, four or more gain elements, the resulting laser system thus generating beams with three, four or more frequencies. The gain element(s) can for instance be a Nd:YAG or a Nd:YV04 crystal. According to a preferred embodiment of the invention, the first beam follows a first path in said first resonator, and the second beam follows a second path in said second resonator, and wherein at least a part of the first path and the second path are coincid- ing. In other words, said first resonator and said second resonator have a coinciding in- tracavity optical propagation axis. The first laser element and the second laser element can thus be positioned in the parts of the first cavity and second cavity, respectively, which are not coinciding. In an embodiment according to the invention, said first resonator and/or said second resonator are linear resonator cavities. In another embodiment according to the inven- tion, said first resonator and/or said second resonator are ring resonator cavities. Com- mon for all these embodiments is that the cavities usually comprise a plurality of mir- rors, one or more of these mirrors typically being common for all the resonator cavities. The cavity mirrors are preferably reflection coated to yield a high reflection coefficient for at least said first and said second frequency. According to a preferred embodiment, the first resonator and the second resonator have a common output mirror. Said output mirror is typically antireflection coated to output the desired wavelength or wavelengths. According to a particularly preferred embodiment of the invention, the laser system ad- ditionally comprises a nonlinear conversion element. The nonlinear conversion element can thus be used for inter alia sum frequency mixing or frequency difference mixing us- ing both the fundamental modes and the harmonics of the first and second beam, re- spectively. The conversion element can for instance be a periodically poled KTP crystal or another nonlinear optical element. According to an embodiment of the invention, the nonlinear conversion element is posi- tioned in the coinciding intracavity optical propagation axis. The conversion of the first and second beam can thus be achieved before being output through the common out- put mirror. According to an alternative embodiment, the nonlinear conversion element is posi- tioned after the common output mirror. In this case both the first beam and the second beam are output from the common output mirror and thus converted outside of their re- spective cavities. However, the nonlinear conversion element can also be positioned in the first cavity but outside the second cavity. In a particularly preferred embodiment of the invention, at least a part of the output from the nonlinear conversion element is fed back to the regulation system. This yields a particular simple way to regulate the pump sources using the regulation system, since the output from the conversion element directly indicates the efficiency of the conversion and thereby also the degree of overlap between the pulses. The laser system can comprise two or more Q-switches, which for instance are posi- tioned in the parts of the cavities, which are not coinciding. This embodiment is particu- larly useful in systems, in which three or more laser beams are generated, or in sys- tems where the individual laser lines have a large wavelength difference. According to a preferred embodiment of the invention, the time difference between ac- tivation of said Q-switches is fixed. Alternatively, the time difference between activation of said Q-switches is variable. In a preferred embodiment according to the invention, the output from the laser system is a nonlinear converted beam. The frequency of the converted beam is thus for in- stance the sum or the difference of the frequencies of the individual beams of the reso- nator cavities or their harmonics. It is for instance possible to use individual resonator laser beams lasing at 1064 nm and 1342 nm, respectively, in order to generate a yel- low-orange laser beam at 593 nm. According to another preferred embodiment of the invention, the output from the sys- tem has multiple laser lines or frequencies. These individual laser lines can for instance be the individual resonator laser beams, i.e. said first and said second beam. It is also possible to achieve a multi laserline output using the nonlinear conversion element. In the case using individual resonator laser beams lasing at 1064 nm and 1342 nm and further using the harmonics of these beams, it is possible to achieve individual output lines of 532 nm, 671 nm and 593 nm, i.e. green, red and yellow output. The system is usable for a wide range of applications. The laser system can for in- stance be used in particle image velocimetry (PIV) systems, which often utilise laser beams of different wavelengths. Similarly, the laser system can also be used for laser Doppler anemometry systems using two or more wavelengths. The laser system can also be used for inter alia RGB-displays, since the system can generate the individual laser lines needed for such displays. The laser system can also be used for generating solid state lasers with a yellow out- put. Yellow laser systems are today only found commercially on a very limited scale. One of the most promising areas of use for the system is in opto-medical applications, such as dermatology, photodynamic therapy and ophthalmology. Blood has a high ab- sorption coefficient for yellow light, and this can for instance be utilised for treatment of vascular related damages, such as port-wine stains, in acne treatment and skin rejuve- nation. Furthermore, the laser system can be used for photodynamic therapy, which is a promising method for treating skin cancer. The opto-medical applications mentioned are today often based on dye lasers. The laser system according to the invention can for instance generate yellow light with a pulse length in the nanosecond range with a repetition frequency in the kilohertz range and with an average effect of several watts. The laser system is also applicable for laser isotope separation. Brief Description of the Drawings The invention is explained in greater detail below with reference to the accompanying drawings, in which Fig. 1 shows a first embodiment of a laser system according to the invention, Fig. 2 a second embodiment of a laser system according to the invention, Fig. 3 a third embodiment of a laser system according to the invention, Fig. 4 a fourth embodiment of a laser system according to the invention, Fig. 5 a fifth embodiment of a laser system according to the invention, Fig. 6 a schematic diagram of a feedback regulation system according to the invention, Fig. 7 the conversion efficiency as a function of the pump power of the first and the second pump sources, Fig. 8 the conversion efficiency as a function of the pump power of the second pump source for a given fixed pump power for the first pump source, and Fig. 9 a series of measurement demonstrating the dependency between the pump in- tensity and the pulse overlap. Best Modes for Carrying out the Invention Fig. 1 shows a first embodiment of the laser system according to the invention. The la- ser system 101 comprises a first pump source 102 and a second pump source 103, preferably in form of a first laser diode and a second laser diode. The light emitted from the first pump source 102 is focused by a first condensing lens 104 and into a first gain element or laser element 106. When this laser element 106 is pumped, a laser pulse starts building up and is emitted from the laser element 106. A resulting emitted first la- ser beam 121 having a first frequency is emitted in both directions in a first linear reso- nator cavity, which comprises three mirrors M11, M12 and M15. The three mirrors M11, M12 and M15 are preferably reflection coated in order to have high reflectance at the first frequency. Similarly, the light emitted from the second pump source 103 is focused by a second condensing lens 105 and into a second gain element or laser element 107. When the second laser element 107 is pumped, a laser pulse starts building up and is emitted from the laser element 107. A resulting emitted laser beam 122 is emitted in both direc- tions in a second linear resonator cavity, which comprises three mirrors M13, M14 and M15. The first resonator and the second resonator path have a coinciding beam path part between mirror M14 and M15. A Q-switch 108 and a nonlinear conversion element 109 are positioned in this coinciding intracavity propagation path. The Q-switch 108 is pref- erably an active Q-switch, such as an electro-optical Q-switch or an acousto-optical Q- switch. The nonlinear conversion element 109 converts the first beam 121 and the second beam 122 by generating a converted beam having a frequency, which is the sum or the difference between the first frequency and the second frequency or the harmonics of the first and second frequencies. The output from the nonlinear conver- sion element 109 can also contain several laser lines, i.e. several beams of different wavelengths/frequencies. The converted light beam, and if desired also the first 121 and the second beam 122, is output through the common output mirror M15, which preferably is anti-reflection coated to output the desired wavelength of the converted beam. A feedback part 112 of the output 113 is fed back via a beam splitter 110 to for instance a photo diode or a power meter 111. The detected signal is processed by a regulation system or micro- processor 114, which controls the first pump source 102 and/or the second pump source 103. The pump sources 102, 103 in turn regulate the pulse build-up times for the laser elements 106 and 107, since the pulse build-up time is dependent on the in- tensity of the pump beam. Since the converted signal is a direct measure for the effi- ciency of the pulse overlap between the pulses of the first beam 121 and the second beam 122, the signal detected by the photo diode 111 can be used for regulating the intensity output of the first pump source 102 and/or the second pump source 103, and thereby the corresponding build-up times of the individual laser pulses. It is thereby possible to optimise the pulse overlap between the pulses of the first beam 121 and the second beam 122 by varying the pump intensity and finding the maximum output of the feedback signal 112. The nonlinear conversion element 109 can also be positioned after the common output mirror M15. If the laser system for instance is a multiline laser system emitting laser beams of the first frequency and the second frequency only, the nonlinear conversion element 109 can be positioned in the path of the feedback beam 112, thereby only us- ing the nonlinear conversion element 109 for the feedback regulation. The first embodiment can also be modified so that it comprises two Q-switches, where a first Q-switch is positioned in the first resonator in the path between mirror M11 and M14, and a second Q-switch is positioned in the second resonator in the path between mirror M13 and M14. In this case, the Q-switches can be temporally synchronised, for instance if the difference in wavelengths between the first and second beam is rela- tively large. Fig. 2 shows a second embodiment of the laser system according to the invention, in which like reference numerals refer to like parts of the first embodiment. The laser sys- tem 201 comprises a first pump source 202 and a second pump source 203, preferably in form of a first laser diode and a second laser diode. The light emitted from the first pump source 202 is focused by a first condensing lens 204 and into a first gain element or laser element 206. When this laser element 206 is pumped, a laser pulse starts building up and is emitted from the laser element 206. A resulting emitted first laser beam 221 having a first frequency is emitted in both directions in a first ring resonator cavity, which comprises four mirrors M21, M22, M25 and M26. The four mirrors M21, M22, M25 and M26 are preferably reflection coated in order to have high reflectance at the first frequency. Similarly, the light emitted from the second pump source 203 is focused by a second condensing lens 205 and into a second gain element or laser element 207. When the second laser element 207 is pumped, a laser pulse starts building up and is emitted from the laser element 207. A resulting emitted laser beam 222 is emitted in both direc- tions in a second ring resonator cavity, which comprises four mirrors M23, M24, M25 and M26. The first resonator and the second resonator path have a coinciding beam path part from mirror M24 to M25 and then from M25 to M26 and on to M23. A Q-switch 208 and a nonlinear conversion element 209 are positioned in this coinciding intracavity propa- gation path. The Q-switch 208 is preferably an active Q-switch, such as an electro- optical Q-switch or an acousto-optical Q-switch. The nonlinear conversion element 209 converts the first beam 221 and the second beam 222 by generating a converted beam having a frequency, which is the sum or the difference between the first frequency and the second frequency or the harmonics of the first and second frequencies. The output from the nonlinear conversion element 209 can also contain several laser lines, i.e. several beams of different wavelengths/frequencies. The converted light beam, and if desired also the first 221 and the second beam 222, is output through the common output mirrors M25 and M26, which are preferably anti- reflection coated to output the desired wavelength/frequencies. A feedback part 212 of a first output 213 is fed back via a beam splitter 210 to for instance a photo diode or a power meter 211. The detected signal is processed by a regulation system or micro- processor 214, which controls the first pump source 202 and/or the second pump source 203. The pump sources 202, 203 in turn regulate the pulse build-up times for the laser elements 206 and 207, since the pulse build-up time is dependent on the in- tensity of the pump light. Since the converted signal is a direct measure for the effi- ciency of the pulse overlap between the pulses of the first beam 221 and the second beam 222, the signal detected by the photo diode 211 can be used for regulating the intensity output of the first pump source 202 and/or the second pump source 203, and thereby the corresponding build-up times of the individual laser pulses. It is thereby possible to optimise the pulse overlap between the pulses of the first beam 221 and the second beam 222 by varying the pump intensity and finding the maximum output of the feedback signal 212. The nonlinear conversion element 209 can also be positioned behind the common out- put mirror M25. if the laser system for instance is a multiline laser system emitting laser beams of the first frequency and the second frequency only, the nonlinear conversion element 209 can be positioned in the path of the feedback beam 212, thereby only us- ing the nonlinear conversion element 209 for the feedback regulation. The embodiment can also be modified so that it comprises two Q-switches, where a first Q-switch is positioned in the first resonator in the path between mirror M21 and M22, and a second Q-switch is positioned in the second resonator in the path between mirror M23 and M24. in this case, the Q-switches can be temporally synchronised, for instance if the difference in wavelengths between the first and second beams is rela- tively high. In the shown embodiment, the first pump source 202 is positioned behind mirror M21 and the second pump source behind mirror 24. However, the first pump source 202 may be positioned behind mirror M22 instead and/or the second pump source 203 can be positioned behind mirror M23. Also, the ring resonator cavities can comprise an op- tical diode so that the first beam 221 and second beam 222 only propagate in one di- rection in the ring resonator cavities. Alternatively, the laser system 201 can comprise a second output 215 through for instance mirror M26. Fig. 3 shows a third embodiment of the laser system according to the invention, in which like reference numerals refer to like parts of the first and second embodiments. The laser system 301 comprises a first pump source 302 and a second pump source 303, preferably in form of a first laser diode and a second laser diode. The light emitted from the first pump source 302 is focused by a first condensing lens 304 and into a first gain element or laser element 306. When this laser element 306 is pumped, a laser pulse starts building up and is emitted from the laser element 306. A resulting emitted first laser beam 321 having a first frequency is emitted in both directions in a first linear resonator cavity, which comprises four mirrors M31, M32, M35 and M36. The four mir- rors M31, M32, M35 and M36 are preferably reflection coated in order to have high re- flectance at the first frequency. Similarly, the light emitted from the second pump source 303 is focused by a second condensing lens 305 and into a second gain element or laser element 307. When the second laser element 307 is pumped, a laser pulse starts building up and is emitted from the laser element 307. A resulting emitted laser beam 322 is emitted in both direc- tions in a second ring resonator cavity, which comprises four mirrors M33, M34, M35 and M36. The first resonator and the second resonator path have a coinciding beam path part from mirror M34 to M35 and then from M35 to M36 and on to M33. A Q-switch 308 and a nonlinear conversion element 309 are positioned in this coinciding intracavity propa- gation path. The Q-switch 308 is preferably an active Q-switch, such as an electro- optical Q-switch or an acousto-optical Q-switch. The nonlinear conversion element 309 converts the first beam 321 and the second beam 322 by generating a converted beam having a frequency, which is the sum or the difference between the first frequency and the second frequency or the harmonics of the first and second frequencies. The output from the nonlinear conversion element 309 can also- contain several laser lines, i.e. several beams of different wavelengths/frequencies. The converted light beam, and if desired also the first 321 and the second beam 322, is output through the common output mirror M35, which preferably is anti-reflection coated to output the desired wavelength/frequencies. A feedback part 312 of the output 313 is fed back via a beam splitter 310 to for instance a photo diode or a power meter 311. The detected signal is processed by a regulation system or microprocessor 314, which controls the first pump source 302 and/or the second pump source 303. The pump sources 302, 303 in turn regulate the pulse build-up times for the laser elements 306 and 307, since the pulse build-up time is dependent on the intensity of the pump light. Since the converted signal is a direct measure for the efficiency of the pulse over- lap between the pulses of the first beam 321 and the second beam 322, the signal de- tected by the photo diode 311 can be used for regulating the intensity output of the first pump source 302 and/or the second pump source 303, and thereby the corresponding build-up times of the individual laser pulses. It is thereby possible to optimise the pulse overlap between the pulses of the first beam 321 and the second beam 322 by varying the pump intensity and finding the maximum output of the feedback signal 312. The nonlinear conversion element 309 can also be positioned after the common output mirror 325. If the laser system for instance is a multiline laser system emitting laser beams of the first frequency and the second frequency only, the nonlinear conversion element 309 can be positioned in the path of the feedback beam 312, thereby only us- ing the nonlinear conversion element 309 for use in the feedback regulation. Fig. 4 shows a fourth embodiment of the laser system according to the invention. The laser system 401 comprises a first pump source 402 and a second pump source 403, preferably in form of a first laser diode and a second laser diode. The light emitted from the first pump source 402 is focused by a first condensing lens 404 and into a first gain element or laser element 406. When this laser element 406 is pumped, a laser pulse starts building up and is emitted from the laser element 406. A resulting emitted first la- ser beam 421 having a first frequency is emitted in both directions in a first linear reso- nator cavity, which comprises two mirrors M41 and M43, and beam combining means 416. The two mirrors M41 and M43 are preferably reflection coated in order to have high reflectance at the first frequency. Similarly, the light emitted from the second pump source 403 is focused by a second condensing lens 405 and into a second gain element or laser element 407. When the second laser element 407 is pumped, a laser pulse starts building up and is emitted from the laser element 407. A resulting emitted laser beam 422 is emitted in both direc- tions in a second linear resonator cavity, which comprises two mirrors M42 and M43 and the beam combining means 416. The beam combining means 416 can for instance be a birefringent crystal and ensures that the two beams 421, 422 having different wavelengths follow two different paths. The first resonator and the second resonator path have a coinciding beam path part between mirror M43 and the beam combining means 416. A nonlinear conversion ele- ment 409 is positioned in this coinciding intracavity propagation path. A common Q- switch is disposed next to the two laser elements 406, 407 so that it overlaps both the first beam 421 and the second beam path 422. The Q-switch 408 is preferably an ac- tive Q-switch, such as an electro-optical Q-switch or an acousto-optical Q-switch. The nonlinear conversion element 409 converts the first beam 421 and the second beam 422 by generating a converted beam having a frequency, which is the sum or the dif- ference between the first frequency and the second frequency or the harmonics of the first and second frequencies. The output from the nonlinear conversion element can also contain several laser lines, i.e. several beams of different wave- lengths/frequencies. The converted light beam, and if desired also the first and the second beam, is output through the common output mirror M43, which preferably is anti-reflection coated to output the desired wavelength/frequencies. A feedback part 412 of the output 413 is fed back via a beam splitter 410 to for instance a photo diode or a power meter 411. The detected signal is processed by a regulation system or microprocessor 414, which controls the first pump source 402 and/or the second pump source 403. The pump sources 402, 403 in turn regulate the pulse build-up times for the laser elements 406 and 407, since the pulse build-up time is dependent on the intensity of the pump light. Since the converted signal is a direct measure for the efficiency of the pulse overlap between the pulses of the first beam 421 and the second beam 422, the signal de- tected by the photo diode 411 can be used for regulating the intensity output of the first pump source 402 and/or the second pump source 403, and thereby the corresponding buiid-up times of the individual laser pulses. Thereby, it is possible to optimise the pulse overlap between the pulses of the first beam 421 and the second beam 422 by varying the pump intensity and finding the maximum output of the feedback signal 412. The nonlinear conversion element 409 can also be positioned after the common output mirror M43. If the laser system for instance is a multiline laser system emitting laser beams of the first frequency and the second frequency only, the nonlinear conversion element 409 can be positioned in the path of the feedback beam 412, thereby only us- ing the nonlinear conversion element for use in the feedback regulation. Fig. 5 shows a fifth embodiment of the laser system according to the invention, in which like reference numerals refer to like parts of the fourth embodiment. Therefore, only the difference between the two embodiments is described. The fifth embodiment only dif- fers from the fourth embodiment in that it comprises two separate Q-switches. A first Q- switch 517 is disposed in the first beam path 521 next to the first laser element 506. A second Q-switch 518 is disposed in the first beam path 522 next to the first laser ele- ment 507. Fig. 6 shows a schematic illustration of the functionality of the feedback regulation sys- tem. As described in the five previous embodiments, a first pump source 602 is used for generating a first beam having a first frequency f1f and a second pump source 603 is used for generating a second beam having a second frequency f2. A nonlinear con- version element 609 is used for generating a converted beam having a third frequency f3. At least a part of this beam is detected by a photo diode or a power meter 611. The feedback regulation system 614 functions by letting the first pump source 602 regulate the output effect of the resulting converted beam, and the second pump source 603 has to perform an optimisation routine in order to maximise the output ef- fect of the converted beam for the chosen effect of the first pump source 602. The regulating of the system functions by comparing 634 the detected signal with a refer- ence signal 633. A resulting error signal 635, which is the difference between the de- tected signal and the reference signal 633 is fed to a control system 631, which based on the error signal 635 generates a control signal 636, which in turn regulates the out- put power of the first pump source 602. Based on the chosen set point for the first pump source 602, the feedback system uses the detected signal to perform an optimi- sation using an optimisation system 632 in order to generate a control signal 637, which in turn regulates the output power of the second pump source 603. In order for the regulation system 614 not to become unstable due to the optimisation routing, the optimisation routine has to be performed more often than the regulation routine. Hereby, it is also possible to average out the contribution from the optimisation. Fig. 7 shows an example on how the conversion efficiency  can manifest itself from the pulse overlap from the two laser beams as a function of the pump power P1 of the first pump source 602 and the pump power P2 of the second pump source 603. As in- dicated in Fig. 7 the regulation of the output power of the converted beam will be a lin- ear function over a large interval for the pump power of the pump sources 602, 603. Fig. 8 shows the conversion efficiency  as a function of the pump power P2 of the second pump source 603 for a given set point, i.e. a fixed pump power P1 of the first pump source 602 set via the regulation routine. The graph shows that the conversion efficiency has a Gaussian-like or bell-shaped dependence on the pump power P2 of the second pump source. In order to optimise the system, the optimisation routine is run using the optimisation system 632 in order to find the vertex of the graph or in other words to maximise the conversion efficiency  by regulating the pump power P2 of the second pump source 603.. Figs. 9a-f show a number of measurements, which demonstrate the relationship be- tween the pump intensity and the pulse overlap. During the measurements the first pump source (a first laser diode for a 1319 nm laser) was driven with a constant cur- rent, while the current of the second pump source (a second laser diode for a 1064 nm laser) was varied. Fig. 9a shows the sum frequency output power in arbitrary units as a function of the second pump source current. The pump power is linearly proportional to the current. As seen from the measurements and the shown curve fit, the sum fre- quency output power shows a nearly Gaussian relationship with the pump current (or power). Fig. 9b-f show oscillator traces from the measurements with the frequency de- picted on the x-axis (in arbitrary units) and the power output from the 1064 nm laser (marked with reference numeral 50) and the power output from the 1319 nm (marked with reference numeral 60) on the y-axis, respectively. The corresponding sum fre- quency output power is shown in the top right corner of the individual figures. The measurements clearly show that the sum frequency output power is highly affected by the current of the pump source with a non-linear relationship. Fig. 9c additionally shows spill-over effects from the 1319 nm pulses to the 1064 nm pulses. The spill-over effects are detection artefacts, which can be seen do not affect the sum frequency output power. The invention has been described with reference to a preferred embodiment. However, the scope of the invention is not limited to the illustrated embodiment, and alterations and modifications can be carried out without deviating from said scope of the invention. For instance do the individual laser resonators need not have a coinciding intracavity propagation axis, and the individual beams may instead be converted outside of the resonator or at a point, where the individual resonators cross each other. List of reference numerals In the numerals, X indicates a number, and x refers to a particular embodiment. Thus, for instance 403 refers to the second pump source of the fourth embodiment. x01 laser system x02 first pump source x03 second pump source x04 first focusing lens x05 second focusing lens x06 first gain element / laser crystal x07 second gain element / laser crystal x08 Q-switch x09 nonlinear conversion element / nonlinear crystal x10 partially reflecting mirror x11 photo detector / power meter x12 feedback x13 output x14 feedback regulation circuit / microprocessor x15 output x16 beam combining means x17 first Q-switch x18 second Q-switch x21 first beam x22 second beam x31 control system x32 optimsation system x33 reference signal x34 comparing means x35 error signal x36 control signal x37 control signal MX mirror FX frequency Claims 1. A laser system (x01) comprising: - pump generating means (x02, x03) for generating at least a first and a second, preferably focused, pump beam, - lasing means (x06, x07) for emitting radiation by being appropriately pumped, said lasing means (x06, x07) being disposed in a first resonator so as to receive the first pump beam in order to generate a first beam (x21) having a first fre- quency, and said lasing means (x06, x07) being disposed in a second resonator so as to receive the second pump beam in order to generate a second beam (x22) hav- ing a second frequency, - at least one Q-switch (x08; x17, x18) being disposed in the first and the sec- ond resonator, so that the first beam and the second beam both pass a Q-switch (x08; x17, x18), and - said laser system (x01) having an output (x13) generated from said first beam (x21) and said second beam (x22), wherein at least a part of said output (x13) is fed back to a regulation system (x14), which regu- lates the build-up time of individual laser pulses of the first beam (x21) and second beam (x22) by controlling the intensity output of said pump generating means (x02, x03). 2. A laser system according to claim 1, wherein said at least one Q-switch (x08; x17, x18) is an active Q-switch, such as an electro-optical or an acousto-optical Q-switch. 3. A laser system according to claim 1 or 2, wherein said at least one Q-switch (x08; x17, x18) is disposed in both the first resonator and the second resonator, so that the first beam (x21) and the second beam (x22) both pass the same Q-switch. 4. A laser system according to any of claims 1-3, wherein the pump means (x02, x03) is a single pump source, such as a laser diode. 5. A laser system according to any of claims 1-3, wherein the pump means (x02, x03) comprises at least a first pump source (x02), such as a first laser diode, for gener- ating the first pump beam, and a second pump source (x03), such as a second laser diode, for generating the second pump beam. 6. A laser system according to any of claims 1-5, wherein the lasing means (x06, x07) is a single gain element or laser element. 7. A laser system according to any of claims 1-5, wherein the lasing means (x06, x07) comprise at least a first gain element or lasing element (x06) for generating said first beam (x21), and a second gain element or laser element (x07) for generating said second beam (x22). 8. A laser system according to any of the preceding claims, wherein said first reso- nator and said second resonator have a coinciding intracavity optical propagation axis. 9. A laser system according to any of the preceding claims, wherein said first reso- nator and said second resonator are linear resonator cavities and/or ring resonator cavities. 10. A laser system according to any of the preceding claims, wherein both said first resonator and said second resonator are ring resonator cavities. 11. A laser system according to any of the preceding claims, wherein the first resona- tor and the second resonator have a common output mirror. 12. A laser system according to any of the preceding claims, wherein the system ad- ditionally comprises a nonlinear conversion element (x09). 13. A laser system according to claims 8 and 12 or claim 8, 12 and any of claims 9- 11, wherein the nonlinear conversion element (x09) is positioned in the coinciding in- tracavity optical propagation axis. 14. A laser system according to claims 11 and 12, wherein the nonlinear conversion element (x09) is positioned after the common output mirror. 15. A laser system according to claim 12, wherein the nonlinear conversion element (x09) is positioned in the first cavity but outside the second cavity. 16. A laser system according to any of claims 12-15, wherein at least a part of the output from the nonlinear conversion element (x09) is fed back to the regulation system (x14). 17. A laser system according to any of the preceding claims, wherein the laser system comprises two or more Q-switches (x17, x18). 18. A laser system according to any of the preceding claims, wherein the output (x13) from the laser system is a nonlinear converted beam. 19. A laser system according to any of claims 1-18, wherein the output (x13) from the system has multiple laser lines or frequencies. A laser system according to the invention comprises pump generating means (xO2, xO3) for generating at least a first and a second, preferably focused, pump beam, and lasing means (xO6, xO7) for emitting radiation by being appropriately pumped. The lasing means (xO6, xO7) is disposed in a first resonator so as to receive the first pump beam in order to generate a first beam (x21) having a first frequency, and the lasing means (xO6, xO7) is disposed in a second resonator so as to receive the second pump beam in order to generate a second beam (x22) having a second frequency. At least one Q-switch (xO8; xl7, xl8) is disposed in the first and the second resonator, so that the first beam and the second beam both pass a Q-switch (xO8; xl7, xl8). The laser system (x01) has an output (x13) generated from said first beam (x21) and said second beam (x22), and at least a part of said output (x13) is fed back to a regulation system (xl4), said regulation system (xl4) controlling said pump generating means (xO2, xO3).

Full Text

Pumped laser system using feedback to pump means
Technical Field
The following invention relates to a laser system comprising pump generating means
for generating at least a first and a second, preferably focused, pump beam, lasing
means for emitting radiation by being appropriately pumped, said lasing means being
disposed in a first resonator so as to receive the first pump beam in order to generate a
first beam having a first frequency, and said lasing means being disposed in a second
resonator so as to receive the second pump beam in order to generate a second beam
having a second frequency, and at least one Q-switch being disposed in the first and
the second resonator, so that the first beam and the second beam both pass a Q-
switch, and said laser system having an output generated from said first beam and said
second beam.
Laser systems, which comprise two or more lasers or gain elements or alternatively a
gain element generating two or more laser lines (i.e. two or more wavelengths), and in
which a so-called sum frequency generation or another form of nonlinear conversion is
involved, are a relatively new area of research. In order to achieve a more efficient con-
version, the lasers can be pulsed. The individual lasers can be pulsed by pulsing a
pump source (not that efficient) or they can be pulsed by periodically changing the so-
called Q-factor of the cavity of the laser by modulating the losses in the cavity. This can
be achieved by using a so-called Q-switch. The Q-switch can either be active, such as
an electro-optical or acousto-optical Q-switch, or passive using a saturable absorber.
One of the main problems of such systems is to synchronise the individual lasers since
they usually have different pulse build-up times. Thereby, the pulses that propagate in
the cavity will not temporally overlap. This is a problem, since the nonlinear conversion
only can occur if the pulses overlap. If the overlap between the individual pulses is not
in any way controlled or regulated, the overlap between the pulses will be random and
therefore not optimised. Consequently, the nonlinear conversion will not be optimised
either.
One method for optimising the overlap is to regulate the focus of the emitted light from
the pump source in the laser element. This is of course only possible in systems, where
the light from the pump source is focused into the laser element. Thus, if the pump

source is a gas discharge lamp or a blitz lamp, it is not possible to use this type of
regulation. In any case, it is difficult to change the focus of the pump source beam in
commercial laser systems, since the functionality of the laser is highly dependent on an
accurate focus. Furthermore, mechanical systems for changing the focal point have in-
herent inaccuracies and will additionally considerably increase the overall cost of the
laser system.
Another method for regulating the pulse overlap relates to changing or translating one
or more mirrors of the cavity in order to change the eigen-focus in the laser element.
This changes the overlap between the cavity mode (i.e. the spot size in the laser ele-
ment) and the pump source, whereby the gain of the laser element is changed. In addi-
tion, the change in cavity mode itself can change the gain. Once again, this method is
mechanically based and therefore involves a considerable increase to the overall cost
of the system as well as introducing a source for misalignment.
A third possibility is to change the position of the laser element with respect to the focal
point of the pump source. However, this is only possible if the laser element is shorter
than the cavity arm, in which the laser element is disposed. Additionally, it must be pos-
sible to translate the laser element, which again means that mechanical translation
means are necessary. Thus, this method has the same disadvantages as the afore-
mentioned methods.
A fourth possibility is to employ a method, where the cavities have static or variable, in-
dividual losses so that the difference in intracavity losses outbalance the difference be-
tween the respective gains of the laser elements.
A fifth possibility is to utilise a plurality of Q-switches and a variable electronic delay be-
tween them. This is a somewhat more complex solution, which demand for separate
electrical supplies for the individual Q-switches as well as an electronic variable delay
line. If the system operates in the nanosecond range, which is often the case for such
systems, such electronics are relatively expensive and sensitive. Also, if every laser
line of the system uses a separate Q-switch, this increases the overall cost of the sys-
tem, since Q-switches generally are relatively expensive.
US patent no. 5,121,402 discloses a multiple gain-element ring laser, which has the
capability of being scaled to higher pulsed output powers. The ring laser comprises a

plurality of pump sources and corresponding gain elements. The resulting laser beams
from the individual gain elements all pass the same Q-switch in the ring cavity.
Disclosure of Invention
The purpose of the present invention is to provide a new and improved laser system, in
which the pulse overlap between two or more laser lines of the system is optimisable.
This is according to the invention achieved by at least a part of said output being fed
back to a regulation system, which regulates the build-up time of individual laser pulses
of the first and second beam by controlling the intensity output of said pump generating
means.
The regulation system is thereby adapted to control the intensity output of the pumping
means, which in turn regulates the gain of the laser means, thereby controlling the
build-up time of the individual laser pulses of said first and second beam. The overlap
between the individual pulses can thereby be optimised in a simple manner.
In a preferred embodiment of the invention, said at least one Q-switch is an active Q-
switch, such as an electro-optical or an acousto-optical Q-switch. Active Q-switches are
favourable as compared to passive Q-switches, since the latter only have a limited
working interval with respect to the repetition frequency and the pulse effect. Therefore,
typically an invariable working point has to be chosen.
According to a preferred embodiment of the invention, said at least one Q-switch is
disposed in both the first resonator and the second resonator, so that the first beam
and the second beam both pass the same Q-switch.
According to a preferred embodiment, the pump means are a single pump source,
such as a laser diode. This single pump source is thus used to generate at least said
first and said second pump beam. In this case, it is possible to regulate the individual
pump beam effect by use of methods known per se, such as by use of a phase re-
tarder, a polarising beam splitter or a birefringent crystal.
Alternatively, the pump means comprise at least a first pump source, such as a first la-
ser diode, for generating the first pump beam, and a second pump source, such as a

second laser diode, for generating the second pump beam. Thereby, a particular sim-
ple embodiment of the pump means is achieved.
According to a preferred embodiment according to the invention, the lasing means is a
single gain element or laser element. This single laser element is thus used to generate
at least two pulsed laser beams having different frequencies or wavelengths.
Alternatively, the lasing means comprise at least a first gain element or lasing element
for generating said first beam, and a second gain element or laser element for generat-
ing said second beam. Thereby, a particular simple embodiment of the lasing means is
achieved, in which separate laser elements are used for generating the individual laser
lines (or wavelengths) of the system.
The laser system can of course comprise three, four or even more pump sources as
well as three, four or more gain elements, the resulting laser system thus generating
beams with three, four or more frequencies. The gain element(s) can for instance be a
Nd:YAG or a Nd:YV04 crystal.
According to a preferred embodiment of the invention, the first beam follows a first path
in said first resonator, and the second beam follows a second path in said second
resonator, and wherein at least a part of the first path and the second path are coincid-
ing. In other words, said first resonator and said second resonator have a coinciding in-
tracavity optical propagation axis. The first laser element and the second laser element
can thus be positioned in the parts of the first cavity and second cavity, respectively,
which are not coinciding.
In an embodiment according to the invention, said first resonator and/or said second
resonator are linear resonator cavities. In another embodiment according to the inven-
tion, said first resonator and/or said second resonator are ring resonator cavities. Com-
mon for all these embodiments is that the cavities usually comprise a plurality of mir-
rors, one or more of these mirrors typically being common for all the resonator cavities.
The cavity mirrors are preferably reflection coated to yield a high reflection coefficient
for at least said first and said second frequency.

According to a preferred embodiment, the first resonator and the second resonator
have a common output mirror. Said output mirror is typically antireflection coated to
output the desired wavelength or wavelengths.
According to a particularly preferred embodiment of the invention, the laser system ad-
ditionally comprises a nonlinear conversion element. The nonlinear conversion element
can thus be used for inter alia sum frequency mixing or frequency difference mixing us-
ing both the fundamental modes and the harmonics of the first and second beam, re-
spectively. The conversion element can for instance be a periodically poled KTP crystal
or another nonlinear optical element.
According to an embodiment of the invention, the nonlinear conversion element is posi-
tioned in the coinciding intracavity optical propagation axis. The conversion of the first
and second beam can thus be achieved before being output through the common out-
put mirror.
According to an alternative embodiment, the nonlinear conversion element is posi-
tioned after the common output mirror. In this case both the first beam and the second
beam are output from the common output mirror and thus converted outside of their re-
spective cavities. However, the nonlinear conversion element can also be positioned in
the first cavity but outside the second cavity.
In a particularly preferred embodiment of the invention, at least a part of the output
from the nonlinear conversion element is fed back to the regulation system. This yields
a particular simple way to regulate the pump sources using the regulation system,
since the output from the conversion element directly indicates the efficiency of the
conversion and thereby also the degree of overlap between the pulses.
The laser system can comprise two or more Q-switches, which for instance are posi-
tioned in the parts of the cavities, which are not coinciding. This embodiment is particu-
larly useful in systems, in which three or more laser beams are generated, or in sys-
tems where the individual laser lines have a large wavelength difference.
According to a preferred embodiment of the invention, the time difference between ac-
tivation of said Q-switches is fixed. Alternatively, the time difference between activation
of said Q-switches is variable.

In a preferred embodiment according to the invention, the output from the laser system
is a nonlinear converted beam. The frequency of the converted beam is thus for in-
stance the sum or the difference of the frequencies of the individual beams of the reso-
nator cavities or their harmonics. It is for instance possible to use individual resonator
laser beams lasing at 1064 nm and 1342 nm, respectively, in order to generate a yel-
low-orange laser beam at 593 nm.
According to another preferred embodiment of the invention, the output from the sys-
tem has multiple laser lines or frequencies. These individual laser lines can for instance
be the individual resonator laser beams, i.e. said first and said second beam. It is also
possible to achieve a multi laserline output using the nonlinear conversion element. In
the case using individual resonator laser beams lasing at 1064 nm and 1342 nm and
further using the harmonics of these beams, it is possible to achieve individual output
lines of 532 nm, 671 nm and 593 nm, i.e. green, red and yellow output.
The system is usable for a wide range of applications. The laser system can for in-
stance be used in particle image velocimetry (PIV) systems, which often utilise laser
beams of different wavelengths. Similarly, the laser system can also be used for laser
Doppler anemometry systems using two or more wavelengths.
The laser system can also be used for inter alia RGB-displays, since the system can
generate the individual laser lines needed for such displays.
The laser system can also be used for generating solid state lasers with a yellow out-
put. Yellow laser systems are today only found commercially on a very limited scale.
One of the most promising areas of use for the system is in opto-medical applications,
such as dermatology, photodynamic therapy and ophthalmology. Blood has a high ab-
sorption coefficient for yellow light, and this can for instance be utilised for treatment of
vascular related damages, such as port-wine stains, in acne treatment and skin rejuve-
nation. Furthermore, the laser system can be used for photodynamic therapy, which is
a promising method for treating skin cancer. The opto-medical applications mentioned
are today often based on dye lasers. The laser system according to the invention can
for instance generate yellow light with a pulse length in the nanosecond range with a
repetition frequency in the kilohertz range and with an average effect of several watts.

The laser system is also applicable for laser isotope separation.
Brief Description of the Drawings
The invention is explained in greater detail below with reference to the accompanying
drawings, in which
Fig. 1 shows a first embodiment of a laser system according to the invention,
Fig. 2 a second embodiment of a laser system according to the invention,
Fig. 3 a third embodiment of a laser system according to the invention,
Fig. 4 a fourth embodiment of a laser system according to the invention,
Fig. 5 a fifth embodiment of a laser system according to the invention,
Fig. 6 a schematic diagram of a feedback regulation system according to the invention,
Fig. 7 the conversion efficiency as a function of the pump power of the first and the
second pump sources,
Fig. 8 the conversion efficiency as a function of the pump power of the second pump
source for a given fixed pump power for the first pump source, and
Fig. 9 a series of measurement demonstrating the dependency between the pump in-
tensity and the pulse overlap.
Best Modes for Carrying out the Invention
Fig. 1 shows a first embodiment of the laser system according to the invention. The la-
ser system 101 comprises a first pump source 102 and a second pump source 103,
preferably in form of a first laser diode and a second laser diode. The light emitted from
the first pump source 102 is focused by a first condensing lens 104 and into a first gain
element or laser element 106. When this laser element 106 is pumped, a laser pulse
starts building up and is emitted from the laser element 106. A resulting emitted first la-

ser beam 121 having a first frequency is emitted in both directions in a first linear reso-
nator cavity, which comprises three mirrors M11, M12 and M15. The three mirrors M11,
M12 and M15 are preferably reflection coated in order to have high reflectance at the
first frequency.
Similarly, the light emitted from the second pump source 103 is focused by a second
condensing lens 105 and into a second gain element or laser element 107. When the
second laser element 107 is pumped, a laser pulse starts building up and is emitted
from the laser element 107. A resulting emitted laser beam 122 is emitted in both direc-
tions in a second linear resonator cavity, which comprises three mirrors M13, M14 and
M15.
The first resonator and the second resonator path have a coinciding beam path part
between mirror M14 and M15. A Q-switch 108 and a nonlinear conversion element 109
are positioned in this coinciding intracavity propagation path. The Q-switch 108 is pref-
erably an active Q-switch, such as an electro-optical Q-switch or an acousto-optical Q-
switch. The nonlinear conversion element 109 converts the first beam 121 and the
second beam 122 by generating a converted beam having a frequency, which is the
sum or the difference between the first frequency and the second frequency or the
harmonics of the first and second frequencies. The output from the nonlinear conver-
sion element 109 can also contain several laser lines, i.e. several beams of different
wavelengths/frequencies.
The converted light beam, and if desired also the first 121 and the second beam 122, is
output through the common output mirror M15, which preferably is anti-reflection
coated to output the desired wavelength of the converted beam. A feedback part 112 of
the output 113 is fed back via a beam splitter 110 to for instance a photo diode or a
power meter 111. The detected signal is processed by a regulation system or micro-
processor 114, which controls the first pump source 102 and/or the second pump
source 103. The pump sources 102, 103 in turn regulate the pulse build-up times for
the laser elements 106 and 107, since the pulse build-up time is dependent on the in-
tensity of the pump beam. Since the converted signal is a direct measure for the effi-
ciency of the pulse overlap between the pulses of the first beam 121 and the second
beam 122, the signal detected by the photo diode 111 can be used for regulating the
intensity output of the first pump source 102 and/or the second pump source 103, and
thereby the corresponding build-up times of the individual laser pulses. It is thereby

possible to optimise the pulse overlap between the pulses of the first beam 121 and the
second beam 122 by varying the pump intensity and finding the maximum output of the
feedback signal 112.
The nonlinear conversion element 109 can also be positioned after the common output
mirror M15. If the laser system for instance is a multiline laser system emitting laser
beams of the first frequency and the second frequency only, the nonlinear conversion
element 109 can be positioned in the path of the feedback beam 112, thereby only us-
ing the nonlinear conversion element 109 for the feedback regulation.
The first embodiment can also be modified so that it comprises two Q-switches, where
a first Q-switch is positioned in the first resonator in the path between mirror M11 and
M14, and a second Q-switch is positioned in the second resonator in the path between
mirror M13 and M14. In this case, the Q-switches can be temporally synchronised, for
instance if the difference in wavelengths between the first and second beam is rela-
tively large.
Fig. 2 shows a second embodiment of the laser system according to the invention, in
which like reference numerals refer to like parts of the first embodiment. The laser sys-
tem 201 comprises a first pump source 202 and a second pump source 203, preferably
in form of a first laser diode and a second laser diode. The light emitted from the first
pump source 202 is focused by a first condensing lens 204 and into a first gain element
or laser element 206. When this laser element 206 is pumped, a laser pulse starts
building up and is emitted from the laser element 206. A resulting emitted first laser
beam 221 having a first frequency is emitted in both directions in a first ring resonator
cavity, which comprises four mirrors M21, M22, M25 and M26. The four mirrors M21,
M22, M25 and M26 are preferably reflection coated in order to have high reflectance at
the first frequency.
Similarly, the light emitted from the second pump source 203 is focused by a second
condensing lens 205 and into a second gain element or laser element 207. When the
second laser element 207 is pumped, a laser pulse starts building up and is emitted
from the laser element 207. A resulting emitted laser beam 222 is emitted in both direc-
tions in a second ring resonator cavity, which comprises four mirrors M23, M24, M25
and M26.

The first resonator and the second resonator path have a coinciding beam path part
from mirror M24 to M25 and then from M25 to M26 and on to M23. A Q-switch 208 and
a nonlinear conversion element 209 are positioned in this coinciding intracavity propa-
gation path. The Q-switch 208 is preferably an active Q-switch, such as an electro-
optical Q-switch or an acousto-optical Q-switch. The nonlinear conversion element 209
converts the first beam 221 and the second beam 222 by generating a converted beam
having a frequency, which is the sum or the difference between the first frequency and
the second frequency or the harmonics of the first and second frequencies. The output
from the nonlinear conversion element 209 can also contain several laser lines, i.e.
several beams of different wavelengths/frequencies.
The converted light beam, and if desired also the first 221 and the second beam 222, is
output through the common output mirrors M25 and M26, which are preferably anti-
reflection coated to output the desired wavelength/frequencies. A feedback part 212 of
a first output 213 is fed back via a beam splitter 210 to for instance a photo diode or a
power meter 211. The detected signal is processed by a regulation system or micro-
processor 214, which controls the first pump source 202 and/or the second pump
source 203. The pump sources 202, 203 in turn regulate the pulse build-up times for
the laser elements 206 and 207, since the pulse build-up time is dependent on the in-
tensity of the pump light. Since the converted signal is a direct measure for the effi-
ciency of the pulse overlap between the pulses of the first beam 221 and the second
beam 222, the signal detected by the photo diode 211 can be used for regulating the
intensity output of the first pump source 202 and/or the second pump source 203, and
thereby the corresponding build-up times of the individual laser pulses. It is thereby
possible to optimise the pulse overlap between the pulses of the first beam 221 and the
second beam 222 by varying the pump intensity and finding the maximum output of the
feedback signal 212.
The nonlinear conversion element 209 can also be positioned behind the common out-
put mirror M25. if the laser system for instance is a multiline laser system emitting laser
beams of the first frequency and the second frequency only, the nonlinear conversion
element 209 can be positioned in the path of the feedback beam 212, thereby only us-
ing the nonlinear conversion element 209 for the feedback regulation.
The embodiment can also be modified so that it comprises two Q-switches, where a
first Q-switch is positioned in the first resonator in the path between mirror M21 and

M22, and a second Q-switch is positioned in the second resonator in the path between
mirror M23 and M24. in this case, the Q-switches can be temporally synchronised, for
instance if the difference in wavelengths between the first and second beams is rela-
tively high.
In the shown embodiment, the first pump source 202 is positioned behind mirror M21
and the second pump source behind mirror 24. However, the first pump source 202
may be positioned behind mirror M22 instead and/or the second pump source 203 can
be positioned behind mirror M23. Also, the ring resonator cavities can comprise an op-
tical diode so that the first beam 221 and second beam 222 only propagate in one di-
rection in the ring resonator cavities. Alternatively, the laser system 201 can comprise a
second output 215 through for instance mirror M26.
Fig. 3 shows a third embodiment of the laser system according to the invention, in
which like reference numerals refer to like parts of the first and second embodiments.
The laser system 301 comprises a first pump source 302 and a second pump source
303, preferably in form of a first laser diode and a second laser diode. The light emitted
from the first pump source 302 is focused by a first condensing lens 304 and into a first
gain element or laser element 306. When this laser element 306 is pumped, a laser
pulse starts building up and is emitted from the laser element 306. A resulting emitted
first laser beam 321 having a first frequency is emitted in both directions in a first linear
resonator cavity, which comprises four mirrors M31, M32, M35 and M36. The four mir-
rors M31, M32, M35 and M36 are preferably reflection coated in order to have high re-
flectance at the first frequency.
Similarly, the light emitted from the second pump source 303 is focused by a second
condensing lens 305 and into a second gain element or laser element 307. When the
second laser element 307 is pumped, a laser pulse starts building up and is emitted
from the laser element 307. A resulting emitted laser beam 322 is emitted in both direc-
tions in a second ring resonator cavity, which comprises four mirrors M33, M34, M35
and M36.
The first resonator and the second resonator path have a coinciding beam path part
from mirror M34 to M35 and then from M35 to M36 and on to M33. A Q-switch 308 and
a nonlinear conversion element 309 are positioned in this coinciding intracavity propa-
gation path. The Q-switch 308 is preferably an active Q-switch, such as an electro-

optical Q-switch or an acousto-optical Q-switch. The nonlinear conversion element 309
converts the first beam 321 and the second beam 322 by generating a converted beam
having a frequency, which is the sum or the difference between the first frequency and
the second frequency or the harmonics of the first and second frequencies. The output
from the nonlinear conversion element 309 can also- contain several laser lines, i.e.
several beams of different wavelengths/frequencies.
The converted light beam, and if desired also the first 321 and the second beam 322, is
output through the common output mirror M35, which preferably is anti-reflection
coated to output the desired wavelength/frequencies. A feedback part 312 of the output
313 is fed back via a beam splitter 310 to for instance a photo diode or a power meter
311. The detected signal is processed by a regulation system or microprocessor 314,
which controls the first pump source 302 and/or the second pump source 303. The
pump sources 302, 303 in turn regulate the pulse build-up times for the laser elements
306 and 307, since the pulse build-up time is dependent on the intensity of the pump
light. Since the converted signal is a direct measure for the efficiency of the pulse over-
lap between the pulses of the first beam 321 and the second beam 322, the signal de-
tected by the photo diode 311 can be used for regulating the intensity output of the first
pump source 302 and/or the second pump source 303, and thereby the corresponding
build-up times of the individual laser pulses. It is thereby possible to optimise the pulse
overlap between the pulses of the first beam 321 and the second beam 322 by varying
the pump intensity and finding the maximum output of the feedback signal 312.
The nonlinear conversion element 309 can also be positioned after the common output
mirror 325. If the laser system for instance is a multiline laser system emitting laser
beams of the first frequency and the second frequency only, the nonlinear conversion
element 309 can be positioned in the path of the feedback beam 312, thereby only us-
ing the nonlinear conversion element 309 for use in the feedback regulation.
Fig. 4 shows a fourth embodiment of the laser system according to the invention. The
laser system 401 comprises a first pump source 402 and a second pump source 403,
preferably in form of a first laser diode and a second laser diode. The light emitted from
the first pump source 402 is focused by a first condensing lens 404 and into a first gain
element or laser element 406. When this laser element 406 is pumped, a laser pulse
starts building up and is emitted from the laser element 406. A resulting emitted first la-
ser beam 421 having a first frequency is emitted in both directions in a first linear reso-

nator cavity, which comprises two mirrors M41 and M43, and beam combining means
416. The two mirrors M41 and M43 are preferably reflection coated in order to have
high reflectance at the first frequency.
Similarly, the light emitted from the second pump source 403 is focused by a second
condensing lens 405 and into a second gain element or laser element 407. When the
second laser element 407 is pumped, a laser pulse starts building up and is emitted
from the laser element 407. A resulting emitted laser beam 422 is emitted in both direc-
tions in a second linear resonator cavity, which comprises two mirrors M42 and M43
and the beam combining means 416. The beam combining means 416 can for instance
be a birefringent crystal and ensures that the two beams 421, 422 having different
wavelengths follow two different paths.
The first resonator and the second resonator path have a coinciding beam path part
between mirror M43 and the beam combining means 416. A nonlinear conversion ele-
ment 409 is positioned in this coinciding intracavity propagation path. A common Q-
switch is disposed next to the two laser elements 406, 407 so that it overlaps both the
first beam 421 and the second beam path 422. The Q-switch 408 is preferably an ac-
tive Q-switch, such as an electro-optical Q-switch or an acousto-optical Q-switch. The
nonlinear conversion element 409 converts the first beam 421 and the second beam
422 by generating a converted beam having a frequency, which is the sum or the dif-
ference between the first frequency and the second frequency or the harmonics of the
first and second frequencies. The output from the nonlinear conversion element can
also contain several laser lines, i.e. several beams of different wave-
lengths/frequencies.
The converted light beam, and if desired also the first and the second beam, is output
through the common output mirror M43, which preferably is anti-reflection coated to
output the desired wavelength/frequencies. A feedback part 412 of the output 413 is
fed back via a beam splitter 410 to for instance a photo diode or a power meter 411.
The detected signal is processed by a regulation system or microprocessor 414, which
controls the first pump source 402 and/or the second pump source 403. The pump
sources 402, 403 in turn regulate the pulse build-up times for the laser elements 406
and 407, since the pulse build-up time is dependent on the intensity of the pump light.
Since the converted signal is a direct measure for the efficiency of the pulse overlap
between the pulses of the first beam 421 and the second beam 422, the signal de-

tected by the photo diode 411 can be used for regulating the intensity output of the first
pump source 402 and/or the second pump source 403, and thereby the corresponding
buiid-up times of the individual laser pulses. Thereby, it is possible to optimise the
pulse overlap between the pulses of the first beam 421 and the second beam 422 by
varying the pump intensity and finding the maximum output of the feedback signal 412.
The nonlinear conversion element 409 can also be positioned after the common output
mirror M43. If the laser system for instance is a multiline laser system emitting laser
beams of the first frequency and the second frequency only, the nonlinear conversion
element 409 can be positioned in the path of the feedback beam 412, thereby only us-
ing the nonlinear conversion element for use in the feedback regulation.
Fig. 5 shows a fifth embodiment of the laser system according to the invention, in which
like reference numerals refer to like parts of the fourth embodiment. Therefore, only the
difference between the two embodiments is described. The fifth embodiment only dif-
fers from the fourth embodiment in that it comprises two separate Q-switches. A first Q-
switch 517 is disposed in the first beam path 521 next to the first laser element 506. A
second Q-switch 518 is disposed in the first beam path 522 next to the first laser ele-
ment 507.
Fig. 6 shows a schematic illustration of the functionality of the feedback regulation sys-
tem. As described in the five previous embodiments, a first pump source 602 is used
for generating a first beam having a first frequency f1f and a second pump source 603
is used for generating a second beam having a second frequency f2. A nonlinear con-
version element 609 is used for generating a converted beam having a third frequency
f3. At least a part of this beam is detected by a photo diode or a power meter 611.
The feedback regulation system 614 functions by letting the first pump source 602
regulate the output effect of the resulting converted beam, and the second pump
source 603 has to perform an optimisation routine in order to maximise the output ef-
fect of the converted beam for the chosen effect of the first pump source 602. The
regulating of the system functions by comparing 634 the detected signal with a refer-
ence signal 633. A resulting error signal 635, which is the difference between the de-
tected signal and the reference signal 633 is fed to a control system 631, which based
on the error signal 635 generates a control signal 636, which in turn regulates the out-
put power of the first pump source 602. Based on the chosen set point for the first

pump source 602, the feedback system uses the detected signal to perform an optimi-
sation using an optimisation system 632 in order to generate a control signal 637,
which in turn regulates the output power of the second pump source 603.
In order for the regulation system 614 not to become unstable due to the optimisation
routing, the optimisation routine has to be performed more often than the regulation
routine. Hereby, it is also possible to average out the contribution from the optimisation.
Fig. 7 shows an example on how the conversion efficiency  can manifest itself from
the pulse overlap from the two laser beams as a function of the pump power P1 of the
first pump source 602 and the pump power P2 of the second pump source 603. As in-
dicated in Fig. 7 the regulation of the output power of the converted beam will be a lin-
ear function over a large interval for the pump power of the pump sources 602, 603.
Fig. 8 shows the conversion efficiency  as a function of the pump power P2 of the
second pump source 603 for a given set point, i.e. a fixed pump power P1 of the first
pump source 602 set via the regulation routine. The graph shows that the conversion
efficiency has a Gaussian-like or bell-shaped dependence on the pump power P2 of the
second pump source. In order to optimise the system, the optimisation routine is run
using the optimisation system 632 in order to find the vertex of the graph or in other
words to maximise the conversion efficiency  by regulating the pump power P2 of the
second pump source 603..
Figs. 9a-f show a number of measurements, which demonstrate the relationship be-
tween the pump intensity and the pulse overlap. During the measurements the first
pump source (a first laser diode for a 1319 nm laser) was driven with a constant cur-
rent, while the current of the second pump source (a second laser diode for a 1064 nm
laser) was varied. Fig. 9a shows the sum frequency output power in arbitrary units as a
function of the second pump source current. The pump power is linearly proportional to
the current. As seen from the measurements and the shown curve fit, the sum fre-
quency output power shows a nearly Gaussian relationship with the pump current (or
power). Fig. 9b-f show oscillator traces from the measurements with the frequency de-
picted on the x-axis (in arbitrary units) and the power output from the 1064 nm laser
(marked with reference numeral 50) and the power output from the 1319 nm (marked
with reference numeral 60) on the y-axis, respectively. The corresponding sum fre-
quency output power is shown in the top right corner of the individual figures. The

measurements clearly show that the sum frequency output power is highly affected by
the current of the pump source with a non-linear relationship. Fig. 9c additionally shows
spill-over effects from the 1319 nm pulses to the 1064 nm pulses. The spill-over effects
are detection artefacts, which can be seen do not affect the sum frequency output
power.
The invention has been described with reference to a preferred embodiment. However,
the scope of the invention is not limited to the illustrated embodiment, and alterations
and modifications can be carried out without deviating from said scope of the invention.
For instance do the individual laser resonators need not have a coinciding intracavity
propagation axis, and the individual beams may instead be converted outside of the
resonator or at a point, where the individual resonators cross each other.

List of reference numerals
In the numerals, X indicates a number, and x refers to a particular embodiment. Thus,
for instance 403 refers to the second pump source of the fourth embodiment.
x01 laser system
x02 first pump source
x03 second pump source
x04 first focusing lens
x05 second focusing lens
x06 first gain element / laser crystal
x07 second gain element / laser crystal
x08 Q-switch
x09 nonlinear conversion element / nonlinear crystal
x10 partially reflecting mirror
x11 photo detector / power meter
x12 feedback
x13 output
x14 feedback regulation circuit / microprocessor
x15 output
x16 beam combining means
x17 first Q-switch
x18 second Q-switch
x21 first beam
x22 second beam
x31 control system
x32 optimsation system
x33 reference signal
x34 comparing means
x35 error signal
x36 control signal
x37 control signal
MX mirror
FX frequency

Claims
1. A laser system (x01) comprising:
- pump generating means (x02, x03) for generating at least a first and a second,
preferably focused, pump beam,
- lasing means (x06, x07) for emitting radiation by being appropriately pumped,
said lasing means (x06, x07) being disposed in a first resonator so as
to receive the first pump beam in order to generate a first beam (x21) having a first fre-
quency, and
said lasing means (x06, x07) being disposed in a second resonator
so as to receive the second pump beam in order to generate a second beam (x22) hav-
ing a second frequency,
- at least one Q-switch (x08; x17, x18) being disposed in the first and the sec-
ond resonator, so that the first beam and the second beam both pass a Q-switch (x08;
x17, x18), and
- said laser system (x01) having an output (x13) generated from said first beam
(x21) and said second beam (x22), wherein
at least a part of said output (x13) is fed back to a regulation system (x14), which regu-
lates the build-up time of individual laser pulses of the first beam (x21) and second
beam (x22) by controlling the intensity output of said pump generating means (x02,
x03).
2. A laser system according to claim 1, wherein said at least one Q-switch (x08; x17,
x18) is an active Q-switch, such as an electro-optical or an acousto-optical Q-switch.
3. A laser system according to claim 1 or 2, wherein said at least one Q-switch (x08;
x17, x18) is disposed in both the first resonator and the second resonator, so that the
first beam (x21) and the second beam (x22) both pass the same Q-switch.
4. A laser system according to any of claims 1-3, wherein the pump means (x02,
x03) is a single pump source, such as a laser diode.
5. A laser system according to any of claims 1-3, wherein the pump means (x02,
x03) comprises at least a first pump source (x02), such as a first laser diode, for gener-

ating the first pump beam, and a second pump source (x03), such as a second laser
diode, for generating the second pump beam.
6. A laser system according to any of claims 1-5, wherein the lasing means (x06,
x07) is a single gain element or laser element.
7. A laser system according to any of claims 1-5, wherein the lasing means (x06,
x07) comprise at least a first gain element or lasing element (x06) for generating said
first beam (x21), and a second gain element or laser element (x07) for generating said
second beam (x22).
8. A laser system according to any of the preceding claims, wherein said first reso-
nator and said second resonator have a coinciding intracavity optical propagation axis.
9. A laser system according to any of the preceding claims, wherein said first reso-
nator and said second resonator are linear resonator cavities and/or ring resonator
cavities.
10. A laser system according to any of the preceding claims, wherein both said first
resonator and said second resonator are ring resonator cavities.
11. A laser system according to any of the preceding claims, wherein the first resona-
tor and the second resonator have a common output mirror.
12. A laser system according to any of the preceding claims, wherein the system ad-
ditionally comprises a nonlinear conversion element (x09).
13. A laser system according to claims 8 and 12 or claim 8, 12 and any of claims 9-
11, wherein the nonlinear conversion element (x09) is positioned in the coinciding in-
tracavity optical propagation axis.
14. A laser system according to claims 11 and 12, wherein the nonlinear conversion
element (x09) is positioned after the common output mirror.
15. A laser system according to claim 12, wherein the nonlinear conversion element
(x09) is positioned in the first cavity but outside the second cavity.

16. A laser system according to any of claims 12-15, wherein at least a part of the
output from the nonlinear conversion element (x09) is fed back to the regulation system
(x14).
17. A laser system according to any of the preceding claims, wherein the laser system
comprises two or more Q-switches (x17, x18).
18. A laser system according to any of the preceding claims, wherein the output (x13)
from the laser system is a nonlinear converted beam.
19. A laser system according to any of claims 1-18, wherein the output (x13) from the
system has multiple laser lines or frequencies.

A laser system according to the invention comprises pump generating means (xO2, xO3) for generating at least
a first and a second, preferably focused, pump beam, and lasing means (xO6, xO7) for emitting radiation by being appropriately
pumped. The lasing means (xO6, xO7) is disposed in a first resonator so as to receive the first pump beam in order to generate a first
beam (x21) having a first frequency, and the lasing means (xO6, xO7) is disposed in a second resonator so as to receive the second
pump beam in order to generate a second beam (x22) having a second frequency. At least one Q-switch (xO8; xl7, xl8) is disposed
in the first and the second resonator, so that the first beam and the second beam both pass a Q-switch (xO8; xl7, xl8). The laser
system (x01) has an output (x13) generated from said first beam (x21) and said second beam (x22), and at least a part of said output
(x13) is fed back to a regulation system (xl4), said regulation system (xl4) controlling said pump generating means (xO2, xO3).

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Patent Number

280020

Indian Patent Application Number

154/KOLNP/2009

PG Journal Number

06/2017

Publication Date

10-Feb-2017

Grant Date

07-Feb-2017

Date of Filing

13-Jan-2009

Name of Patentee

ADVALIGHT

Applicant Address

ADVALIGHT,A Danish Entityof Diplomvej 373, 1DK-2800 Kongens Lyngby Denmark

Inventors:

#	Inventor's Name	Inventor's Address
1	TIDEMAND-LICHTENBERG, PETER	KLØVERVANG 11, DK-2970 HØRSHOLM
2	MORTENSEN, JESPER LILTORP	SNOGEGÅRDSVEJ 138 ST., TH., DK-2860 SØBORG
3	THORHAUGE, MORTEN	BAGSVAERD HOVEDGADE 99, 8B, DK-2880 BAGSVAERD

PCT International Classification Number

H01S 3/102,H01S 3/23

PCT International Application Number

PCT/DK2007/000350

PCT International Filing date

2007-07-10

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	60/830,718	2006-07-12	EUROPEAN UNION
2	06388051.2	2006-07-12	EUROPEAN UNION