Title of Invention

"A LASER FREQUENCY STABILISATION SYSTEM FOR GAS LASERS"

Abstract

This invention relates to a frequency stabilization system for gas lasers, in combination with a laser resonator. The system comprises a drive unit for providing drive waveform to a piezo transducer, a feedback chain for providing signal to the said drive unit. The piezo transducer is coupled with 100% reflecting mirror of the gas laser resonator. The feed back chain has a feed back signal processing unit coupled with a photo detector which is further coupled with output mirror of the laser resonator for sensing the output from the gas laser resonator. The drive unit comprises a cascaded arrangement of an updown counter, a digital-to-analog computer and high-voltage amplifier

Full Text

FIELD OF INVENTION: This invention relates to a frequency stabilisation system for gas lasers. PRIOR ART: Ideally, a gas laser should have a single frequency of operation but it is never so and it always suffers from a frequency broadening. The frequency broadening is due to different kinds of broadening mechanism associated with the operation of gas laser. In general, these various broadening mechanisms, presents in a typical gas laser, are natural broadening, collision broadening and Doppler broadening. The natural broadening, present in a gas laser, is due to finite radiative lifetime of the metastable state of the lasing material. Similarly, collision broadening is due to the perturbations in the state of the excited atoms or molecules caused by presence of inter-atomic forces. The Doppler broadening is present due to random thermal motion of excited atoms or molecules, which makes them move randomly with different velocities. A given atom or molecule, thus, emits radiation only at that frequency whose Doppler shifted frequency component is in resonance with the natural emission of the atom or molecule in question. As the molecular motion is a random one, the gain curve also gets broadened. In most of the gas lasers, contributions from natural broadening and collision broadening mechanisms are insignificant as compared to that from Doppler broadening. For example, a Carbon Dioxide laser emitting at 10.6 micron has a Doppler broadened line width of 60 MHz, while a Helium-Neon laser emitting at 0.6328 micron has a Doppler broadened gain curve as wide as 1400 MHz. The width of the gain versus frequency curve is an indicator of the uncertainty in the frequency of the emitted laser radiation. In variety of applications involving gas lasers, it is desirable to have a gas laser with stabilised laser frequency at any point on the Doppler broadened gain curve, preferably, on the center of the Doppler broadened curve where the gain is maximum. A variety of laser frequency stabilisation systems, known in prior art, are employed in order to stabilize the frequency of a gas laser One of the laser frequency stabilisation system, known in the prior art, is Dither frequency stabilisation. In a dither frequency stabilisation system, one of the mirrors of the laser resonator is mounted on a piezoelectric (PZT) element that Is driven by a sinusoidal or square signal at a frequency typically somewhere between 100 and 1000 Hz. The movement of the PZT element resulting in the movement of the mirror, along the laser resonator axis, produces modulation in the laser cavity length. This cavity length modulation, known as dithering, leads to the modulation in the laser output frequency. This frequency modulation manifests itself as amplitude modulation of the output laser beam power as per the gain versus frequency curve of the laser. The amplitude of the output signal depends upon the proximity of the instantaneous emitted laser frequency to the center of the gain versus frequency curve. Farther the emitted frequency is from the line canter, larger is the amplitude. The phase of the amplitude-modulated signal with reference to the phase of the dithering signal depends upon the side of the curve center the emission is located. If the instantaneous laser emission frequency is towards the left of the line center, which has a positive slope, it leads to an in-phase-modulated signal. On the other hand, if the instantaneous laser emission frequency is located towards the right of the curve, which has a negative slope, it produces a 180° out-of-phase modulated signal. The signal is phase sensitively detected with respect to the reference signal used for dithering the cavity and the discriminant, produced as a result, after proper filtering, is fed back to the PZT driver circuit for necessary correction. However, this Dither frequency stabilisation system, known in the prior art, suffers from many disadvantages. Main disadvantage of his stabilisation system, known in the prior art, is that this system involves the dithering /modulation of the cavity which may not be desirable. Another disadvantage of this stabilisation system, known in the prior art, is that this system is generally an analog system, thus has size, weight and portability constraints. Moreover, the system is less flexible due to its analog nature. Yet another disadvantage of this stabilisation system, known in the prior art, is that there is always a sinusoidal or square signal present to dither/modulate the laser cavity length. The amplitude of this modulating signal is decided by the minimum detectable modulated output obtained at the feedback port hence the stabilisation accuracy is limited by the laser gain profile itself. Still further disadvantage of this stabilisation system, known in the prior art, is that incidental frequency modulation is present in frequency stabilised laser output due to the fact that the cavity is always being dithered /modulated in order to produce the desired feedback signal. Opto-galvanic stabilisation system is another laser frequency stabilisation system, which is known, in the prior art. In this stabilisation system, the gain medium of the laser is driven by a constant current source. The movement of the PZT-mounted mirror, as a result of application of dithering signal, produces plasma impedance changes. This plasma impedance changes leads to a changing voltage representing the location of the laser's operating frequency with respect to the line center of the gain versus frequency curve. This changing voltage signal is processed to get a frequency stabilised output. However, this opto-galvanic system, known in the prior art, also suffers from several disadvantages. Main disadvantages of this system, known in the prior art, is that it requires a change in the gain of the laser medium and, as such, it needs a high efficiency medium to be able to produce a workable modulated signal. Another disadvantage of this system, known in the prior art, is that the stabilised output suffers from incidental frequency modulation present in the stabilised output. Yet another disadvantage of this system, known in the prior art, is that this system is purely analog in nature thereby suffering from the disadvantage of the analog systems mentioned above. Stark-Cell stabilisation system is another laser frequency stabilisation system, which is known, in the prior art. In this laser frequency stabilisation system, it is not the laser cavity that is dithered but it is a cell filled with the medium, having an absorption transition very close to the laser transition of the interest, that is dithered. The absorption transition is then stark shifted to make it coincide with the laser transition, which is to be stabilised. For example, de-utrated ammonia has an absorption transition that can be stark shifted to coincide with the 10.6-rnicron transition of the Carbon Dioxide laser. The absorption cell, called Stark-Cell, is dithered and the laser beam passing through the cell gets amplitude modulated. The cell may be placed intra-cavity or external to the laser cavity. The modulated signal is phase sensitively detected and fed to the PZT element to achieve frequency stabilisation. However, this system also suffers from the disadvantages. The main disadvantage of this system, known in the art, is that it needs an intra-cavity or extra-cavity gas cell whose one of the absorption lines is stark shifted to coincide with the wavelength of the laser to be stabilised. Another disadvantage of this system, known in the art, is that it is highly cumbersome to implement in hardware. Yet another disadvantage of this system, known in the prior art, is that this system can be used only for those lasers for which another species, having a matching absorption transition, is available. Still further disadvantage of this system, known in the prior art, is that this system is also essentially purely analog in nature thereby suffering from the disadvantages associated with the analog systems mentioned above. Saturation absorption laser frequency stabilisation system is another system for the frequency stabilisation of a gas laser, which is known, in the prior art. The saturation absorption laser frequency stabilisation system comprises a cell that is filled with a gas having an absorption line coinciding with the center of the spectral line to be stabilised. This cell may be placed intra-cavity or placed external to the laser cavity. The laser frequency is stabilised around the center of the saturation absorption dip also called inverted lamb dip. Lamb dip is a phenomenon that leads to flattened peak and under favorable conditions even a clearly defined dip in the gain versus frequency curve of the Doppler broadened gas lasers. The saturation absorption dip, also called inverted lamb dip, occurs due to saturation of the absorption coefficient at the center of the absorption line. This system is similar to the stark-cell stabilisation system. However, in this case, the gas laser is stabilised to the center of inverted lamb dip This frequency stabilisation system also suffers from the disadvantages as mentioned in the case of stark-cell stabilisation system. Mam disadvantage of this system, known in the prior art, is that it needs an absorption cell. Another disadvantage of this system, known in the prior art, is that it requires the gas laser, to be stabilised, to cause a saturated absorption. Yet another disadvantage of this system is that it is hard to implement in practical terms. In fact, so far, this system has been exploited as a laboratory set-up for carrying out spectroscopic studies. This system does not adapt well for building portable frequency stabilised laser sources for outdoor applications. As such, there is need for a frequency stabilisation system that overcomes the disadvantages of the prior art related to the frequency stabilisation systems enumerated above. OBJECTS OF THE INVENTION: Primary object of the invention is to provide a frequency stabilisation system for gas lasers, which can be used to stabilize the frequency of a gas laser around the center of the gain versus frequency characteristics of the laser. Another object of the invention is to provide a frequency stabilisation system for gas lasers, which does not suffer from the presence of incidental frequency modulation in the frequency stabilised output. Yet another object of the invention is to provide a frequency stabilisation system for gas lasers, which is capable of providing both short-term and long-term frequency stability for a gas laser. Yet further object of the invention is to provide a frequency stabilisation system for gas lasers, which is piezo driven type frequency stabilisation system with the drive waveform being a digital ramp waveform and not a symmetrical sinusoidal or square waveform. Still further object of the invention is to provide a frequency stabilisation system for gas lasers in which the size and time duration of the step of the digital ramp waveform is just the one required optimally to obtain the minimum changes in the laser output in a minimum amount of time. Yet further object of the invention is to provide a frequency stabilisation system for gas lasers which utlises a predominantly digital technique for the stabilisation purposes thereby significantly reducing the hardware requirement. Still further object of the present invention is to provide a frequency stabilisation system for gas lasers, which is capable of providing significantly higher stabilisation accuracy by using large number of bits in the various digital building blocks which generates a digital ramp drive waveform. DESCRIPTION OF INVENTION According to this invention there is provided a frequency stabilisation sysyem for gas lasers, in combination with a gas laser resonator (3), comprising of: a drive unit for providing a drive unit for providing a drive waveform to a piezo transducer (2), a feedback chain for providing a signal to the said drive unit, the said piezo transducer (2) coupled with the 100% reflecting mirror (1) of a said gas laser resonator (3), the feedback chain having a said feedback signal processing unit (9) coupled with a photodetector (5), which photodetector (5) is coupled with the output mirror C4) of the said laser resonator (3) for sensing the output from the said gas laser resonator (3). The laser frequency stabilisation system of the present invention utilises improved hardware components, which include a high voltage amplifier specifically designed to efficiently drive the capactive load in the piezo transducer. The system comprises a cascaded arrangement of an updown counter, a digital -to—analog converter and a high voltage amplifier, which together form a drive unit for a drive waveform to the piezo transducer mounted on 100% reflecting mirror of the gas laser. Further, it comprises a cascaded arrangement of a feedback signal processing unit, an analog-to—digital converter, a digital magnitude comparator and a timing control unit which together form a feedback chain for providing a correcting signal to piezo transducer depending upon the output of the gas laser. The high voltage amplifier, used in the present system, is built around a high performance operationl, amplifier, Mhich takes into account the desired drive requirement for the capacitive load of the piezo transducer. The drive waveform is a digital ramp whose step size and duration is chosen so as to keep the laser output frequency around the center of the spectral line width within the desired accuracy. DESCRIPTION OF THE DRAWINGS: Any further characteristics, advantages and applications of the invention will become evident from the detailed description of the preferrend embobiment, which has been described and illustrated, with the help of following drawings wherein: Fig (1) is a typical representation of the effect of broadening in the gain v frequency curve of the gas laser Fig (2) is a block diagram showing the functioning of the frequency stabilisation system in combination with a typical gas laser resonator Fig (3) shows the schematic layout of the frequency stabilisation system of the present invention, in combination with a gas laser resonator Fig (4 a) shows the laser output of a typical He-Ne gas laser without frequency stabilisation Fig (4 b) shows a typical Helium-Neon gas laser output when frequency stabilised with the frequency stabilisation system of the present invention. DESCRIPTION OF INVENTION WITH REFERENCE TO DRAWINGS: The frequency stabilisation system of the present invention essentially comprises a drive unit for providing drive waveform to piezo transducer assembly and a feedback chain for monitoring the status of the gas laser-output and providing feedback accordingly. The drive waveform is a digital ramp whose step size and duration is chosen so as keep the laser output frequency around the center of the spectral line width within the desired accuracy. The heart of the drive waveform generator in the present system is an up-down counter whose output is fed to a digital-to-analog converter. The counter and digital to analog converter combination generates the digital-ramp waveform. A feed back processing unit monitors the status of the laser output power in real time. At any time, irrespective of the magnitude of the peak laser power output, the center point of the gain versus frequency characteristics always corresponds to a changeover from a positive slope region to a negative slope region or vice versa. The digital ramp waveform starts from a zero output voltage and then increments in small steps with the size of the step being primarily governed by the number of bits in the counter and gain of the high voltage amplifier. With each incremental input, the laser output power changes, which is being sensed by the feedback signal processing unit and represented in a digital format at the output of the analog to digital converter. The laser output increases or decreases from the immediately previous value depending upon whether the instantaneous laser frequency is towards the left or right of the line center. The digital magnitude comparator compares the magnitudes of two adjacent samples. The control logic built into the circuit ensures that whenever [A -B] changes from a positive output to a negative output, it indicates crossing of the line center. Here [A] is the ADC (analog to digital converter) output for the nthsample and [B] is the ADC output for the (n-1)th sample. Whenever such a situation occurs, the counter's up/down control input changes from up-mode to down-mode or down-mode to up-mode depending upon whether we are approaching the line center from left or right of the gain versus frequency characteristics. Figure (1), represents a theoretical (ideally desired) gain versus frequency curve and a practical gain versus frequency curve of a gas laser affected by broadening mechanisms. Referring to figure (2), the frequency stabilisation system of the present invention has been shown in conjunction with the gas laser, which is to be frequency stabilised. Essentially, the frequency stabilisation system generates a digital ramp waveform to drive the piezo transducer and thereby producing a change in the cavity length limiting the frequency uncertainty of the broadened gas laser. The movement of the piezo transducer and, thereby, the movement of the mirror attached to it, introduces change in the laser cavity length. This change in the cavity length is based on the real time status of the broadened laser. The real time status of the broadened laser is obtained from the laser feedback. The feedback chain compares the broadened laser position from its line center position and generates the error signal accordingly and corrects the position of the piezo transducer so as to always bring the laser to its line center position. In this way the laser is stabilised in frequency on the Doppler broadened curve. Referring to figure (3), the laser resonator (3), to be frequency stabilised in combination with the frequency stabilisation system of the present invention, comprises a 100% reflecting mirror (1) and an output mirror (4). A piezo transducer (2) is coupled with the 100% reflecting mirror (1) and an application of voltage to the piezo transducer (2) changes its position and hence the position of the mirror (1). This axial movement in the position of the mirror (1) results in the change in the cavity length of the laser resonator (3). A portion of the laser output obtained at the output mirror (4) is fed back to the feedback signal processing unit (9) through a photo-detector (5) connected with the output mirror (4). A cascaded arrangement of an up-down counter (6), a digital -to -analog converter (DAC) (7) and a high voltage amplifier (8) all together form the drive unit of the present system. The drive unit provides the drive waveform to the piezo transducer (2), which is coupled with the 100% reflecting mirror (1) of the gas laser (3). The high voltage amplifier (8) is either a dedicated monolithic amplifier or built around a high voltage bipolar transistor whose base terminal is driven by the operational amplifier (OPAMP) output with the collector terminal of the transistor connected to the inverting input of the OPAMP. The up-down counter (6) output serves as digital inputs to the digital-to-analog converter (7). The clock frequency of the up-down counter (6) is chosen according to the speed of response of the piezo transducer (2). The counter up/down control input is obtained from the feedback chain comprising of the feedback signal processing unit (9), analog to digital converter (ADC) (10), digital magnitude comparator (11) and a precise timing control unit (12). The number of input bits of digital-to-analog converter (7), which is nothing but the bit-size of the up-down counter (6), decides the step size in the digital ramp signal. The digital-to-analog converter (7) output serves as analog input for the high voltage amplifier (8). The high voltage amplifier (8) provides the required drive current and drive voltage depending on the type and the specifications of high voltage amplifier device chosen for the application and doesn't alter the shape of the ramp waveform present at the digital-to-analog converter (7) output. The gain of the high voltage amplifier (8) and hence its output voltage is chosen so that for the given application, the transducer provides an effective length changes of the order of one (λ) [λ = wavelength of the gas laser under consideration] over the full signal range of the digital ramp input. A cascaded arrangement of a feedback signal processing unit (9), an analog-to-digital converter (10), a digital magnitude comparator (11) and a timing control unit (12), together form a feedback chain for providing a signal to the piezo transducer (2) depending upon the output of the gas laser (3). The feedback signal processing unit (9) does the necessary optoelectronic conversion of the sample of laser power output and generates an equivalent DC voltage. This DC voltage is then fed to the analog-to-digital converter (10). The analog-to-digital converter (10) transforms this DC voltage representing a particular sample of laser power output to a digital format. In fact, analog-to-digital converter (10) output updates itself with every successive sample generated by the application of the digital ramp signal to the piezo drive assembly. Analog-to-digital converter (10) is used in continuous operation mode so that it monitors the status of the laser output in real time. The output of analog-to-digital converter (10) feeds the inputs of the digital magnitude comparator (11) at different intervals of time controlled precisely by timing control, so that the digital magnitude comparator (11) compares the past and present values of the sampled laser power output. The minimum value to which the digital magnitude comparator resolves to depend on the number of output bits of analog-to-digital converter (10). The up/down control of the counter (6) is decided on the basis of comparison between the present sample value and the immediately preceding sample value of the laser output power. When the present sample value (A) of the laser power output is greater than the immediately preceding sample value (B), the digital magnitude comparator (11) generates a signal A>B. This signal makes up-down counter (6) to count up and the piezo transducer (2) is fed with an increasing ramp voltage. Similarly, when the present sample value (A) of the laser power output is smaller than the immediately preceding sample value (B), the digital magnitude comparator (11) generates a signal A constant provided that (he changes in the laser power output due to minor changes in the laser frequency around the line center is less than the voltage step equivalent of one least significant bit (LSB) of analog-to-digital converter (10) Around the frequency stabilised condition as the frequency tries to drift in either direction, the counter counts either one count up or one count down depending upon direction of drift and the generation of control signal by digital magnitude comparator (11) and brings the laser frequency back to the line center. When the system is initially switched on. irrespective of which side of the line center the laser is. the piezo transducer (2) voltage starts increasing. The feedback chain does the necessary comparison between the past and the present sample values of the laser power output. Based on this comparison, an up/down control signal is generated. This signal makes the counter to count up or down thereby presenting an increasing or decreasing ramp voltage to the piezo transducer (2) and allowing it to take the laser output towards the line center of the broadened gain curve. Once the line center frequency point is achieved, the laser output is maintained around this point with the help of the closed loop. Referring to fig (4 a) and fig (4 b), these figures together show efficacy for stabilising the frequency of a Helium-Neon gas laser around the line center of its Doppler broadened gain curve. Figure (4 a) shows the inherent frequency drift when the feedback loop of the frequency stabilisation system is kept open. This large drift, shown in the figure, indicates the amount of change in the laser frequency on the broadened gain curve. Figure (4 b) shows the results when the feedback loop is closed. A long-term drift (100 minutes) of ± 2.5 MHz and a short-term drift (6 minutes) of ± 0.5 MHz was observed. This reduction in frequency uncertainty is brought about by the laser frequency stabilisation system of the present invention. Similarly, the present frequency stabilisation system will stabilise the frequency of any gas laser around the center of its broadened gain curve. The proposed system optimally provides the long term and short term frequency stability of the gas laser by ensuring that the laser always operates at the line center of the broadened gain curve of the gas laser. The present embodiment of the invention, which has been set forth above, was for the purpose of illustration and is not. intended to limit the scope of the invention. It is to be understood that various changes, adaptations and modifications can be made in the invention described above by those skilled in the art without departing from the scope of the invention which has been defined by following claims. I CLAIM 1. A frequency stabilisation sysyem for gas lasers, in combination with a gas laser resonator (3), comprising of: a drive unit for providing a drive unit for providing a drive waveform to a piezo transducer (2), a feedback chain for providing a signal to the said drive unit, the said piezo transducer (2) coupled with the 100X reflecting mirror (i) of a said gas laser resonator (3), the feedback chain having a said feedback signal processing unit (9) coupled with a photodetector (5), which photodetector (5) is coupled with the output mirror (4) of the said laser resonator (3) for sensing the output from the said gas laser resonator (3) . 2. A frequency stabilisation system for gas lasers, as claimed in claim 1, wherein said drive unit comprises a cascaded arrangement of updown counter (6) connected to a digital to analog converter (7), said converter connected to a high voltage amplifier (8), said transducer (2) coupled to amplifier (8). 3. A frequency stabilisation system for gas lasers, as claimed in claim 1, wherein said feedback chain comprises a cascaded arrangement of a feedback signal processing unit (9) connected to an analog to digital converter (10), said converter connected to a digital magnitude comparator (11) the comparator being connected to a timing control unit (12). 4. A frequency stabilisation system for gas lasers, as claimed in claim 3, wherein the high voltage amplifier (8) is coupled to said transducer. 5. A frequency stabilisation system for gas lasers, as claimed in claim 1, wherein the output frequency of the said laser resonator (3) is continuously monitored by the said feedback signal processing unit (9). 6. A frequency stabilisation system for gas lasers, as claimed in claim 1, wherein said piezo transducer (2) changes the cavity length of the said laser resonator (3) to modify the frequency of the said laser resonator (3). 7. A frequency stabilisation system far gas lasers, as claimed in claim 1, wherein the said piezo transducer (2) is driven by a digital ramp waveform. 8. A frequency stabilisation system for gas lasers, as claimed in claim 1, wherein the said up—down counter (6) counts up or down depending upon the control signal present at its up/down control input. 9. A frequency stabilisation system for gas lasers, as claimed in claim 1, wherein the feedback signal is processed and the amplitudes of two successive the samples are compared in a digital magnitude comparator (11) to generate control signal for the said up—down counter (b) with the timing control unit (12) ensuring coordination of the function of various digital building blocks. 10. A frequency stabilisation system, for gas lasers substantially as described and illustrated herein.

Documents:

1184-del-2002-abstract.pdf

1184-del-2002-claims.pdf

1184-del-2002-correspondence-others.pdf

1184-del-2002-correspondence-po.pdf

1184-del-2002-description (complete).pdf

1184-del-2002-drawings.pdf

1184-del-2002-form-1.pdf

1184-del-2002-form-18.pdf

1184-del-2002-form-2.pdf

1184-del-2002-form-3.pdf

1184-del-2002-gpa.pdf