Title of Invention	AN APPARATUS FOR GENERATION OF COLLIMATED HOLLOW LASER BEAM
Abstract	The present invention provides for an apparatus and process for generation of a collimated hollow laser beam using polarizing beam splitters, quater wave plates, and a pair of axicon mirrors, wherein the axicon mirrors are convex axicon mirror and concave axicon mirror. Both the axicon mirrors are of same base angle. The present invention also provides for a process for generation of collimated hollow laser beam of variable diameter.

Title of Invention

AN APPARATUS FOR GENERATION OF COLLIMATED HOLLOW LASER BEAM

Abstract

The present invention provides for an apparatus and process for generation of a collimated hollow laser beam using polarizing beam splitters, quater wave plates, and a pair of axicon mirrors, wherein the axicon mirrors are convex axicon mirror and concave axicon mirror. Both the axicon mirrors are of same base angle. The present invention also provides for a process for generation of collimated hollow laser beam of variable diameter.

Full Text	FORM-2 THE PATENTS ACT, 1970 (39 OF 1970) & The Patents Rules, 2003 COMPLETE SPECIFICATION [See section 10 and rule 13] 'AN APPARATUS AND PROCESS FOR GENERATION OF COLLIMATED HOLLOW LASER BEAM- DEPARTMENT OF ATOMIC ENERGY, a Department of the Government of India, of Anushakti Bhawan, Chatrapati Shivaji Maharaj Marg, Mumbai- 400 001, Maharashtra, India The following specification particularly describes the invention and the manner in which it is to be performed. FIELD OF THE INVENTION The present invention relates to an apparatus and method for generation of hollow laser beams. More specifically the invention relates to generation of collimated hollow laser beam with variable diameter. BACKGROUND OF THE INVENTION Various methods are being used for generating hollow beams for e.g. By transverse mode selection in laser resonators, hollow beams with dark center, namely, doughnut beams were generated. Using specially designed mirrors in resonators, hollow beams were produced in the laser output. Computer generated hologram(s) have been used to convert a laser beam into a dark centered beam (hollow beam). Use of intra-cavity hologram generated a bottle beam (hollow beam) output from a laser Hollow optical beams have also been generated by illuminating a hollow fiber and a photonic crystal fiber with a laser beam. Use of refractive axicon lens(es) to convert a laser beam into a hollow beam having minimum intensity at the beam axis has been made in different geometries and combinations (US Patent No. -4,887,592). Using a liquid crystal (LC) a hollow beam whose diameter depends upon input laser power was obtained; and using a specially designed LC ceil which modulates the phase of the incident beam, hoflow laser beam was obtained from a Gaussian laser beam. Using spatial light modulators, hollow beams have been obtained from the laser beams, and illuminating a polished cylindrical glass rod with a laser beam resulted in a hollow conic beam. Illuminating a metal axicon mirror with a laser beam resulted in a hollow conic beam. However all the above methods for generating hollow laser beams have some drawbacks. For e.g. Transverse mode selection in laser resonators to generate hollow beams or using specially designed mirrors in resonators is based on suppression of lasing of lowest mode. This results in significant loss of laser power due to unused gain medium around the resonator axis. Computer generated hologram also inserts significant loss to input beam power, therefore it decreases overall efficiency of conversion from Gaussian beam to hollow beam. Both hollow optical fiber and photonic crystal fiber provide diverging hollow beam. The power conversion efficiency from input beam to the hollow beam is also low. The efficiency is limited mainly due to coupling of input laser beam power to the fiber. Reported value of conversion efficiency with photonic crystal fibre was ~ 15 %. Generation of hollow beam using refractive axicon lens(es) is efficient, but these are difficult to fabricate and expensive. 2 Though the efficiency to generate hollow beam using a liquid crystal (LC) was reasonable (-76 %), the diameter of hollow beam was dependent on input laser power, which is undesirable. Similarly, using a specially designed LC cell, a high efficiency (~ 100 %) was claimed, but the design of this cell is complicated. Using spatial light modulators, the efficiency for hollow beam generation was limited to diffraction efficiency of SLM, which is usually low (40-60 %). Illuminating a polished cylindrical glass rod with a laser beam resulted in a conic hollow beam, which was diverging and asymmetric in intensity distribution. A single metal axicon mirror also provided a diverging hollow beam. U S Patent 4,887,592 and European Patent EP 0402250 describe the use of concave axicon lens for generation of ring shaped laser beam. The resulting hollow beam was diverging which was focused using a spherical lens to appropriate size for cutting/surgery of cornea. Since intensity of beam varies significantly with propagation distance, the position of target is a critical parameter. Also, in both these devices, the separation between the axicon lens and spherical lens governs the ring diameter, width and intensity at the target surface. U S Patent 3,547,526 discloses the use of combination of various optical elements to form a device that alters the energy distribution of an incident light beam across the beam area. Using this refractive optical device a solid cross-section beam of light can be converted into a beam of hollow tubular shape. The device can be formed using refractive elements like cones, prisms, pyramids or rhombs cemented together at their base, or can be a single dished cone, prism or pyramid. A drawback of this proposal is that the parameters of the generated hollow tubular shaped beam are not variable and are fixed according to the dimensions of the components forming device. Apart from complexity to fabricate these components, refractive elements are susceptible to generate aberrations. WIPO document WO/2004/002668, U S Patent 4,179,708 and Chinese patent CN1560666 also relate to generation of the hollow laser beams. All these documents disclose the use of convex axicon lens(es). Said axicon lens(es) being a refractive element, may be susceptible to cause aberrations in the beam. U S Patent 5,946,282 relates to use of hollow laser beams to improve the optical resolution of an optical recording/reproducing apparatus. Hollow laser beam was generated via blocking 3 the central part of an incident Gaussian laser beam using a light interceptor. As central part of the input beam which contains the maximum energy gets blocked to generate a hollow light beam, this method of producing hollow beam is less efficient. U S Patent 4,514,850 and U S Patent 4,516,244 relate to the design of laser resonators for generation of annular/hollow laser beams. The annular beam was generated either by selecting a transverse mode in laser resonator or by using specially designed conical mirrors for the laser cavity. Both these methods are based on suppression of lasing in the lowest mode. This results in significant loss of laser power due to unused gain medium around the resonator axis and increased diffraction losses. Hence these methods provide less efficiency for power/energy. The hollow laser beams may also be generated by transverse mode selection in laser resonators using a mechanical aperture. This approach has drawback of being less efficient due to use of an aperture to prevent lasing action in the central region of the gain medium around the resonator axis. A significant laser power is lost and overall process to generate hollow beam is less efficient. The generation of hollow laser beams using specially designed mirrors in resonators is also less efficient because of un-utilized gain medium in the central region surrounding the beam axis. Otherwise, the geometry calls for re-shaping the active medium to fit the lasing beam profile, which is difficult in many cases. The generation of hollow laser beams using the computer holograms also inserts significant loss to input beam power, therefore it decreases overall efficiency to convert a Gaussian beam into a hollow beam. The generation of the hollow laser beams using optical fibre has the drawback of the poor power conversion efficiency from a laser beam to the dark centred hollow beam. The efficiency in this case is mainly limited due to coupling of input beam to the fiber. The use of refractive axicon lens(es) to convert a laser beam into a hollow beam though result in the generation of hollow beam comparable in efficiency to the present invention setup, use of these axicon lenses has a drawback that these axicon lenses differ significantly in material and fabrication method from the axicon mirrors used in the present invention. The axicon mirrors used in the present invention are simple to fabricate and inexpensive. Moreover, being a refractive element, axicon lens(es) are susceptible to cause aberrations in the beam. The use of Liquid Crystal (LC) for generation of hollow laser beams has a drawback that the diameter of the generated beam was dependent on input laser power. The generation of a 4 hollow beam from a Gaussian laser beam using a specially designed LC cell has a drawback that the design of this specially designed cell is complicated. Therefore, the device is expected to be expensive and difficult to fabricate. The hollow beam may be generated by using spatial light modulators (SLMs). The main drawback of SLMs is their low diffraction efficiency (40-60 %), which limits the efficiency for ho/fow beam generation, (n addition, SLMs need active control to generate the shape of the beam. The generation of hollow beam by illuminating a polished cylindrical glass rod with a laser beam has a drawback that the generated hollow beam would be diverging and asymmetric in intensity distribution. The use of metal axicon mirror in generation of hollow beam by illuminating a metal axicon mirror with a laser beam has been disclosed in an article (S. R. Mishra, S. K. Tiwari, S. P. Ram and S. C. Mehendale, Opt. Eng. 46, 084002, 2007). However, the published article no where discloses or teaches or suggests the use of a second axicon mirror in generation of hollow laser beam. Further use of a single axicon mirror results in a diverging hollow beam. OBJECTIVE OF THE INVENTION The drawbacks of use of axicon lenses and other methods of generating hollow laser beam can be overcome in the present invention. The apparatus of present invention generates a collimated hollow laser beam using two axicon mirrors. The main objective of the present invention is to propose an apparatus and method to generate a high efficiency collimated hollow laser beam of variable diameter. Another objective of the present invention is to generate a collimated hollow laser beam without any substantive loss of power during conversion. It is another objective of the present invention to provide for an apparatus using two axicon mirrors of same base angle for generation of a collimated hollow laser beam whose diameter can be varied and depends upon distance between the said two axicon mirrors. It is yet another objective of the invention to provide for an apparatus and method which is relatively simple, low cost and low weight for generation of a collimated hollow laser beam. It is another objective of the present invention to provide a technique for the efficient generation of the collimated hollow laser beam from a linearly polarized laser beam from any laser such as He-Ne laser, diode laser, C02 laser, Nd-laser, Ti-sapphire laser, dye laser, etc. Other objectives and advantages of the present invention will be readily appreciated as the same become better understood by reference to the ensuing non-limiting detailed description when considered with reference to the accompanying drawings. 5 SUMMARY OF THE INVENTION Accordingly, the present invention provides for generating a collimated hollow laser beam by an apparatus comprises of a laser generating source, a pair of axicon mirrors having same base angle, wherein one of the axicon mirror is convex axicon mirror and another is concave axicon mirror. A cubical polarizing beam splitter made of two right angled prisms joined together from the hypotenuse sides and a pair of quarter wave-plates for in-coupling and out-coupling of the laser beam. The basic idea for the present invention is that the input laser beam upon reflection from a convex axicon mirror becomes a diverging hollow conical beam, when this diverging hollow conical beam falls on a concave axicon mirror of same base angle a collimated hollow laser beam is generated. The diameter of the. generated collimated hollow laser beam can be varied depending upon the distance between the two axicon mirrors. The use of combination of a pair of axicon mirrors having same base angle provides an easy, less expensive and efficient way for generation of collimated hollow laser beam. STATEMENT OF INVENTION In accordance with the present invention there is disclosed, an apparatus for converting a linear polarized laser beam into a collimated hollow laser beam, the apparatus comprises of a polarizing beam splitter for reflecting the linear polarized laser beam in orthogonal direction; a first quarter wave-plate for converting the beam received from the polarizing beam splitter into a circularly polarized laser beam; a convex axicon mirror for reflecting the said circularly polarized laser beam received from the first quarter wave-plate into a hollow conical laser beam for again passing through the first quarter wave-plate which again convert it into a linearly polarized hollow conical laser beam for passing through the polarizing beam splitter; a second quarter wave-plate for receiving the said linearly polarized hollow conical laser beam from the polarizing beam splitter and converting it into a circularly polarized hollow conical laser beam; a concave axicon mirror having a base angle identical to that of the convex axicon mirror for converting the incident circularly polarized hollow laser beam into a circularly polarized collimated hollow laser beam; the circularly polarized collimated hollow laser beam being converted in to a linearly polarized collimated hollow laser beam after passing through the second quarter wave-plate and reflected orthogonally through polarizing beam splitter 6 thereby producing the desired collimated hollow laser beam. In accordance with the present invention there is also disclosed a process for generation of collimated hollow laser beam of variable diameter comprising the steps of directing a linearly polarized laser beam on to the polarizing beam splitter; reflecting the said incidental linearly polarized beam for passing through the quarter wave-plate for conversion into a circularly polarized laser beam; reflecting said circularly polarized laser beam from the convex axicon mirror thereby converting it into a circularly polarized hollow conical laser beam for again passing through the first quarter wave-plate thereby converting it into a linearly polarized hollow conical laser beam for passing through the polarizing beam splitter; directing the said linearly polarized conical hollow laser beam through the second quarter wave-plate thereby converting it into a circularly polarized hollow laser beam incident onto the concave axicon mirror; reflecting and converting said circularly polarized conical hollow laser beam incident onto concave axicon mirror into a circularly polarized collimated hollow laser beam for passing again through the second quarter wave-plate which again converts said circularly polarized collimated hollow laser beam into a linearly polarized collimated hollow laser beam after passing through it; reflecting said linearly polarized collimated hollow laser beam via the polarizing beam splitter outside the apparatus. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1, shows a schematic view illustrating the working principle of the device; Figure 2, shows a schematic view of the actual device including beam-coupling optics; Figure 3, shows the images and corresponding intensity profiles of the generated hollow laser beams at different distances from PBS for a fixed separation of 70 mm between axicon mirrors; Figure 4, shows the CCD images and corresponding intensity profiles of the generated collimated hollow laser beams measured at a fixed distance of 410 mm from PBS for different values of separation between axicon mirrors; and Figure 5, shows measured variation of hollow beam dark diameter with separation (d) between axicon mirrors. The inset in this figure presents the definition of dark diameter estimated from the measured intensity profile of the hollow beam. DETAILED DESCRIPTION OF THE INVENTION 7 Reference is now be made in detail to the embodiments of the present invention as illustrated in the accompanying drawings and description. The invention relates to a novel apparatus for generation of the collimated hollow laser beam. In accordance with the present invention there is provided an apparatus comprising of a pair of axicon mirrors of same base angle, one convex and the other concave axicon mirror, a polarizing beam splitter and two quarter wave-plates. Each of such elements has been mounted in the apparatus by suitable mounting means. The axicon mirrors as used in the present invention may be easily fabricated. The mirrors may preferably be made from a disc of copper metal. The disc of diameter 25 mm and thickness 5 mm is found suitable for the purposes of the invention. Its one surface is flat and other is conical with convex meniscus. The conical surface (having base angle of 1°) was finished with diamond turning machine (DTM) and then gold-coated to obtain high reflectivity of - 93 %. Similarly the other axicon mirror is prepared which is conical with concave meniscus. The polarizing beam splitter used is a commercially available optical element of cubical shape. In the invented apparatus for example a cube of dimension 25x25x25 mm3 was found suitable. The cube is made of two identical right angle prisms (of optical material BK-7) cemented together, hypotenuse-face-to-hypotenuse-face, with a multilayer dielectric coating in between. This PBS is a variable ratio beam splitter whose output depends upon the polarization of the input beam. The quarter wave-plate used in the invented apparatus is a commercially available optical element. It is made of a uniaxial optical crystal. It changes the polarization of the incident beam. Along with PBS, the quarter wave-plate was used to in-couple the incident laser beam to axicon mirrors and out-couple the generated collimated hollow beam from the setup. As shown schematically in Figure 1, when a laser beam is incident on a convex axicon mirror (AXi) along its axis, the reflected beam is a hollow conical beam. If it is made to fall on the concave axicon mirror (AX2) of same base angle, the reflected beam is a collimated hollow beam whose diameter depends on the separation between the two axicon mirrors and, 8 hence, can be varied by varying the separation. This is the principle on which the invention is based. In accordance with the present invention a linearly polarized laser beam is made incidental onto the polarizing beam splitter (PBS). The polarizing beam splitter cube splits an arbitrarily polarized beam into two orthogonal linearly polarized beams, when the input beam passes through PBS and is incident on the multilayer dielectric film. The PBS transmits the p-polarized light component (i.e. polarization parallel to the plane of incidence at the multi layer film) in the incident beam direction and reflects the s-polarized light component (i.e. polarization perpendicular to the plane of incidence at the multi layer film) at right angle from the incident beam direction. Thus a linearly polarized input light beam (having both s- and p-polarization components) gets split into two beams whose power values depend on the ratio of s- and p-polarization components. Hence, if incident beam is purely p- or purely s-polarized, it gets, respectively, fully transmitted or fully reflected. It is preferred that the input beam was kept to be s-polarized so that it was fully reflected by PBS. This way, the input beam was directed towards the convex axicon mirror (AXi) by the PBS. The PBS used in the present invented apparatus is a commercially available optical element. It provides either complete reflection or complete transmission depending upon whether Incident beam is s-polarized or p-polarized respectively. This polarization selective reflection or transmission of the laser beam by PBS has been used for in-coupling of the incident laser beam to the axicon mirrors and out-coupling of the generated hollow beam from the setup, with suitably oriented quarter wave-plates. A person skilled in the art would know that when a light beam passes through a quarter wave-plate, it results a 90° phase retardation between two orthogonal polarization components (represented as ordinary ray and extra-ordinary ray inside a uniaxial crystal). Thus if quarter wave-plate is oriented suitably, a circularly polarized light is obtained at the output for a linearly polarized input beam. As indicated in (Fig.2), input light passes twice through the quarter wave-plate 5(i) (between PBS and convex axicon AXi). First, the input beam reflected by PBS, and then, the conic beam from the convex axicon AXi. Due to two passes of light through quarter wave-plate, polarization of hollow conic beam (from AXi) gets rotated by 90° with respect to the input beam. Thus hollow conic beam is seen as p-polarized at PBS and hence is fully transmitted through PBS. Similarly, quarter wave-plate 5(ii) also rotates the 9 polarization of incident light beam by 90° in two passes - first after exit from PBS and second after reflection from concave axicon (AX2). Thus the coJIimated hollow beam from concave axicon mirror (AX2) is seen as s-polarized at the PBS and is reflected by PBS. This way the generated collimated hollow beam is out-coupled from the setup. Using simple setup, a collimated hollow laser beam has been generated whose diameter can be varied by varying separation between axicon mirrors. Since focusing of hollow beam using a lens results in nondiffracting Bessel-like beam, it should be possible to generate a nondiffracting beam of variable central spot size and propagation range using this setup. Figure 1, shows a schematic diagram to illustrate the working principle of the device. When a laser beam (3) is incident on a convex axicon mirror AXi (1) having a base angle, along its axis, the reflected beam is a hollow conical beam. If this reflected beam is made to fall on another concave axicon mirror AX2 (2) of same base angle, the reflected beam is a collimated hollow beam (4) whose diameter depends on the separation between the axicon mirrors and, hence, can be varied by varying the separation. Neglecting diffraction, the generated hollow beam (4) has a dark central region of diameter [phi] given by § = 2d.tan(2y) [phi = 2d.tan(2 gamma)], and a bright outer shell of thickness (a) for input laser beam (3) of size (2a). Using, for the first time, a combination of metal axicon mirrors, the generation of a collimated hollow beam from a Gaussian laser beam with high conversion efficiency (i.e. percentage ratio of the generated hollow beam power to input beam power) of ~ 76 % has been achieved. The present invention also demonstrates that the diameter of the central dark region of generated hollow beam can be varied by changing the separation between the axicon mirrors. The apparatus overcomes drawbacks of known devices described above. The invented apparatus is expected to operate efficiently for a range of laser wavelengths from visible to infrared, covering various lasers such as dye lasers, semiconductor lasers, He-Ne laser, Nd-lasers, Ti-sapphire laser, CO2 laser, etc. The construction of the actual device including beam coupling optics is shown in Figure 2. Both the axicon mirrors (1) and (2) were made from copper discs of 25 mm diameter. Their reflecting conical surfaces were generated on a computer numerical control (CNC) lathe and these were further finished on a diamond turning machine (DTM). The finished surfaces were subsequently gold-coated for corrosion protection and reflectivity enhancement. Both the 10 axicon mirrors had base angle of 1 degree (i.e. half vertex cone angle of 89 degree). The quarter wave-plates (5(i) and 5(ii), and polarizing beam splitter (PBS) (6) were used for in-and out-coupling of beams. A linearly polarized laser beam (3) having a nearly Gaussian spatial profile, having full width at half-maximum (FWHM) = 0.774 mm and power ~ 16 mW, from a 780 nm diode laser was made incident (as shown in Figure 2) on the convex axicon mirror (1) via a PBS (6) and a quarter wave-plate (5(i)). In the setup, upon reflection from axicon mirror (1), a conic shaped hollow beam gets generated, which propagates back through the optics and falls on the concave axicon mirror (2). Upon reflection from this axicon mirror (2) (which is a concave mirror with same base angle), the beam becomes a coliimated hollow beam. The appropriate orientation of another quarter wave-plate (5(ii)) allows this beam to out-couple through PBS (6), as is indicated in Figure 2. The measured power conversion efficiency in our setup was ~ 76 %. The diameter and intensity profiles of generated hollow beam (4) were measured at different distances (z) from PBS (6) and for different values of separation (d) between axicons ((1) and (2)). This was done by taking images of a calibrated screen (7), on which hollow beam (4) was incident, using a digital CCD camera and computer controlled frame grabber system. Figure 3 and Figure 4 show the CCD images and intensity profiles of the generated hollow beam using our setup. The quality of the generated hollow beam depends on the spatial profile of the input beam and can be further improved in the present setup by using a better quality input beam. The diameter of hollow beam can also be changed by choosing a different base angle of the axicon mirrors. Accordingly, The diameter of the generated hollow beam can be varied by two different ways i.e. either by varying separation between axicon mirrors or by using the axicon mirrors of another base angle. In our experimental geometry, the diameter of the coliimated hollow beam is equal to the diameter of diverging conic beam incident at the concave axicon mirror (AX2) which depends on the separation between two mirrors as well as on the value of base angle of the axicon mirrors. Thus in our setup diameter of coliimated hollow beam can be varied by varying the position of concave axicon mirror (AX2) with respect to convex axicon mirror (AXi). In a different situation where separation between concave axicon mirror (AX2) and convex axicon mirror (AXi) is to be kept fixed, the option to change the diameter of generated coliimated hollow beam can be exercised by using both the axicon mirrors of 11 changed base angle. Among the above two methods, it is preferred that the separation between the axicon mirrors is implemented to change the diameter of collimated hollow laser beam while keeping the base angle fixed. This is an easy way to change the hollow beam diameter as people who wish to use the invented apparatus do not have to replace the axicon mirrors. Example 1: Figure 3 shows images and corresponding intensity profiles ((i) to (iv)) of the generated hollow beam at various distances (z) in the beam path ((i) z = 150 mm, (ii) z = 410 mm, (iii) z = 780 mm and (iv) z = 1150 mm) for d = 70 mm separation between the axicon mirrors (1) and (2). The results show that the diameter (peak to peak intensity) of the generated hollow beam (4) is nearly constant for various propagation distances (z). A person skilled in the art would readily appreciate the good quality of collimation in this way. Example 2: The setup allows change in diameter of hollow beam by changing the separation (d) between the axicon mirrors (1) and (2). Figure 4 shows the images and corresponding intensity profiles (from (v) to (x)) of the hollow beam (4) at a fixed position in the beam path (at z = 410 mm) for different values of separation (d). The values of d for images from (v) to (x) in Figure 4 were 70 mm, 90 mm, 110 mm, 130 mm, 150 mm and 170 mm, respectively. Using this data, the hollow beam dark diameter for different separation between mirrors (d) has been obtained (see Figure 5). Here we define the dark diameter of hollow beam as separation between half-maximum intensity points (inner side), as shown in the inset in Figure 5. Graph in Figure 5 (filled circles) shows the results of our measurements of hollow beam dark diameter. The plot shows a linear variation of dark diameter with separation (d). These measurements should serve as calibration of the hollow beam diameter for different values of separation (d). The straight line in Figure 5 shows the variation of diameter § with separation (d). The main advantages of the invention are that the invented apparatus here utilizes the metal axicon mirrors which are easy to fabricate and relatively inexpensive. The invention produces collimated hollow beam whose diameter can be continuously varied by changing separation between mirrors therefore no external interference. The apparatus demonstrated here works in reflection geometry. Hence, it should to be free from aberrations commonly appearing in 12 refractive elements such as lenses. Further, the axicon mirrors used in the present apparatus are less expensive and aberration free as compared to other element such as axicon lenses. The use of combination of axicon mirrors of this type provides an easy, inexpensive and efficient way to generate a collimated hollow beam with variable diameter. Hollow laser beams with dark center are used in a variety of R&D and industrial applications such as trapping and guiding of atoms and particles, laser machining, generation of nondiffracting beams (beams with large depth of focus), light detection and ranging (LIDAR), etc. The nondiffracting beams (generated using hollow beams) find applications in corneal surgery, compact disc (CD) read/write devices, optical traps/tweezers for atoms and particles, laser machining, etc. The invention described here is for efficient generation of collimated, variable size hollow optical beam from a laser beam. A range of laser wavelengths from visible to infrared, covering various lasers such as dye lasers, semiconductor lasers, He-Ne laser, Nd-lasers, Ti-sapphire laser, C02 laser, etc. can serve as input for the invented device. While the best mode for carrying out the invention has herein been described in detail, those skilled in the art to which this invention relates will recognize various alternative constructions and embodiments for practicing the present invention as defined by the following claims. The invention as described above should not be limited to the disclosed embodiments and features. Therefore any modifications, substitutions and improvements to the present invention are possible without departing from the spirit and scope of the invention. 13 We claim: 1. An apparatus for converting a linear polarized laser beam into a collimated hollow laser beam, the apparatus comprises of: - a polarizing beam splitter (6) for reflecting the linear polarized laser beam in orthogonal direction; - a first quarter wave-plate (4) for converting the beam received from the polarizing beam splitter (6) into a circularly polarized laser beam; - a convex axicon mirror (1) for reflecting the said circularly polarized laser beam received from the first quarter wave-plate (5(i)) into a diverging hollow conical laser beam for again passing through the first quarter wave-plate (5(i)) which converts it into a linearly polarized hollow conical laser beam for passing through the polarizing beam splitter; - a second quarter wave-plate (5(ii)) for receiving the said linearly polarized hollow conical laser beam from the polarizing beam splitter and converting it into a circularly polarized hollow laser beam; - a concave axicon mirror (2) having a base angle identical to that of the convex axicon mirror (1) for converting the circularly polarized diverging hollow beam from the second quarter wave-plate (5(ii)) into a circularly polarized collimated hollow beam; the circularly polarized collimated hollow laser beam being converted in to a linearly polarized collimated hollow laser beam after passing through the second quarter wave-plate (5(ii)) and reflected orthogonally through polarizing beam splitter thereby producing the desired collimated hollow laser beam. 2. An apparatus for generation of collimated hollow laser beam as claimed in claim 1, wherein both axicon mirrors are metal axicon mirrors. 3. An apparatus for generation of collimated hollow laser beam as claimed in claim 1, wherein polarizing beam splitter is made of two right angle prisms. 4. An apparatus for generation of collimated hollow laser beam as claimed in claim 3, wherein the pair of the prisms js cemented together along their hypotenuse faces with 14 a multilayer dielectric coating in between. 5. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the source laser beam is of Gaussian intensity profile or of uniform intensity profile. 6. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the input laser beam is linearly polarized and is of nearly Gaussian intensity profile. 7. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the diameter of the generated coliimated hollow laser beam is variable depending upon the distance between the two axicon mirrors. 8. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the diameter of the coliimated hollow laser beam is dependent upon the base angle of the two axicon mirrors. 9. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the axicon mirrors are made of copper discs. 10. A process for generation of coJJimafed hollow Jaser beam of variable diameter comprising the steps of: - directing a linearly polarized laser beam on to the polarizing beam splitter; - reflecting the said incident linearly polarized beam for passing through the quarter-wave-plate for conversion into a circularly polarized laser beam; - reflecting said circularly polarized laser beam from the convex axicon mirror thereby converting it into a diverging hollow conical laser beam for again passing through the first quarter wave-plate thereby converting it into a linearly polarized hollow conical laser beam for passing through the polarizing beam splitter; 15 - directing the said linearly polarized conical hollow laser beam through the second quarter wave-plate thereby converting it into a circularly polarized hollow laser beam incident onto the concave axicon mirror; - reflecting and converting said circuferfy pofarized conical hollow faser beam into a circularly polarized collimated hollow laser beam for passing again through the second quarter wave-plate which again convert said circularly polarized collimated hollow laser beam into a linearly polarized collimated hollow laser beam after passing through it; - reflecting said linearly polarized collimated hollow laser beam via the polarizing beam splitter outside the apparatus. 11. A process for generation of collimated hollow laser beam as claimed in claim 10, wherein the conversion efficiency of the generated collimated hollow laser beam is upto 76 % of the input laser beam. 12. A process for generation of collimated hollow laser beam as claimed in claim 10, wherein the collimation of the generated hollow laser beam remains constant for various propagation distances. 13. A device for generation of the collimated hollow laser beam substantially as herein described with reference to the accompanying description and drawings. 14. A process for generation of the collimated hollow laser beam substantially as herein described with reference to the accompanying description and drawings. Dated this 31st day of October, 2008. (Anuradha Salhotra) Of LALL LAHIRI & SALHOTRA AGENT FOR THE APPLICANTS 16

Full Text

FORM-2
THE PATENTS ACT, 1970
(39 OF 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
[See section 10 and rule 13]

'AN APPARATUS AND PROCESS FOR GENERATION OF COLLIMATED
HOLLOW LASER BEAM-

DEPARTMENT OF ATOMIC ENERGY, a Department of the Government of India, of Anushakti Bhawan, Chatrapati Shivaji Maharaj Marg, Mumbai- 400 001, Maharashtra, India
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention relates to an apparatus and method for generation of hollow laser beams. More specifically the invention relates to generation of collimated hollow laser beam with variable diameter.
BACKGROUND OF THE INVENTION
Various methods are being used for generating hollow beams for e.g. By transverse mode selection in laser resonators, hollow beams with dark center, namely, doughnut beams were generated. Using specially designed mirrors in resonators, hollow beams were produced in the laser output. Computer generated hologram(s) have been used to convert a laser beam into a dark centered beam (hollow beam). Use of intra-cavity hologram generated a bottle beam (hollow beam) output from a laser Hollow optical beams have also been generated by illuminating a hollow fiber and a photonic crystal fiber with a laser beam. Use of refractive axicon lens(es) to convert a laser beam into a hollow beam having minimum intensity at the beam axis has been made in different geometries and combinations (US Patent No. -4,887,592). Using a liquid crystal (LC) a hollow beam whose diameter depends upon input laser power was obtained; and using a specially designed LC ceil which modulates the phase of the incident beam, hoflow laser beam was obtained from a Gaussian laser beam. Using spatial light modulators, hollow beams have been obtained from the laser beams, and illuminating a polished cylindrical glass rod with a laser beam resulted in a hollow conic beam. Illuminating a metal axicon mirror with a laser beam resulted in a hollow conic beam.
However all the above methods for generating hollow laser beams have some drawbacks. For e.g. Transverse mode selection in laser resonators to generate hollow beams or using specially designed mirrors in resonators is based on suppression of lasing of lowest mode. This results in significant loss of laser power due to unused gain medium around the resonator axis. Computer generated hologram also inserts significant loss to input beam power, therefore it decreases overall efficiency of conversion from Gaussian beam to hollow beam. Both hollow optical fiber and photonic crystal fiber provide diverging hollow beam. The power conversion efficiency from input beam to the hollow beam is also low. The efficiency is limited mainly due to coupling of input laser beam power to the fiber. Reported value of conversion efficiency with photonic crystal fibre was ~ 15 %. Generation of hollow beam using refractive axicon lens(es) is efficient, but these are difficult to fabricate and expensive.
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Though the efficiency to generate hollow beam using a liquid crystal (LC) was reasonable (-76 %), the diameter of hollow beam was dependent on input laser power, which is undesirable. Similarly, using a specially designed LC cell, a high efficiency (~ 100 %) was claimed, but the design of this cell is complicated. Using spatial light modulators, the efficiency for hollow beam generation was limited to diffraction efficiency of SLM, which is usually low (40-60 %). Illuminating a polished cylindrical glass rod with a laser beam resulted in a conic hollow beam, which was diverging and asymmetric in intensity distribution. A single metal axicon mirror also provided a diverging hollow beam.
U S Patent 4,887,592 and European Patent EP 0402250 describe the use of concave axicon lens for generation of ring shaped laser beam. The resulting hollow beam was diverging which was focused using a spherical lens to appropriate size for cutting/surgery of cornea. Since intensity of beam varies significantly with propagation distance, the position of target is a critical parameter. Also, in both these devices, the separation between the axicon lens and spherical lens governs the ring diameter, width and intensity at the target surface.
U S Patent 3,547,526 discloses the use of combination of various optical elements to form a device that alters the energy distribution of an incident light beam across the beam area. Using this refractive optical device a solid cross-section beam of light can be converted into a beam of hollow tubular shape. The device can be formed using refractive elements like cones, prisms, pyramids or rhombs cemented together at their base, or can be a single dished cone, prism or pyramid. A drawback of this proposal is that the parameters of the generated hollow tubular shaped beam are not variable and are fixed according to the dimensions of the components forming device. Apart from complexity to fabricate these components, refractive elements are susceptible to generate aberrations.
WIPO document WO/2004/002668, U S Patent 4,179,708 and Chinese patent CN1560666 also relate to generation of the hollow laser beams. All these documents disclose the use of convex axicon lens(es). Said axicon lens(es) being a refractive element, may be susceptible to cause aberrations in the beam.
U S Patent 5,946,282 relates to use of hollow laser beams to improve the optical resolution of an optical recording/reproducing apparatus. Hollow laser beam was generated via blocking
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the central part of an incident Gaussian laser beam using a light interceptor. As central part of the input beam which contains the maximum energy gets blocked to generate a hollow light beam, this method of producing hollow beam is less efficient.
U S Patent 4,514,850 and U S Patent 4,516,244 relate to the design of laser resonators for generation of annular/hollow laser beams. The annular beam was generated either by selecting a transverse mode in laser resonator or by using specially designed conical mirrors for the laser cavity. Both these methods are based on suppression of lasing in the lowest mode. This results in significant loss of laser power due to unused gain medium around the resonator axis and increased diffraction losses. Hence these methods provide less efficiency for power/energy.
The hollow laser beams may also be generated by transverse mode selection in laser resonators using a mechanical aperture. This approach has drawback of being less efficient due to use of an aperture to prevent lasing action in the central region of the gain medium around the resonator axis. A significant laser power is lost and overall process to generate hollow beam is less efficient. The generation of hollow laser beams using specially designed mirrors in resonators is also less efficient because of un-utilized gain medium in the central region surrounding the beam axis. Otherwise, the geometry calls for re-shaping the active medium to fit the lasing beam profile, which is difficult in many cases. The generation of hollow laser beams using the computer holograms also inserts significant loss to input beam power, therefore it decreases overall efficiency to convert a Gaussian beam into a hollow beam. The generation of the hollow laser beams using optical fibre has the drawback of the poor power conversion efficiency from a laser beam to the dark centred hollow beam. The efficiency in this case is mainly limited due to coupling of input beam to the fiber. The use of refractive axicon lens(es) to convert a laser beam into a hollow beam though result in the generation of hollow beam comparable in efficiency to the present invention setup, use of these axicon lenses has a drawback that these axicon lenses differ significantly in material and fabrication method from the axicon mirrors used in the present invention. The axicon mirrors used in the present invention are simple to fabricate and inexpensive. Moreover, being a refractive element, axicon lens(es) are susceptible to cause aberrations in the beam. The use of Liquid Crystal (LC) for generation of hollow laser beams has a drawback that the diameter of the generated beam was dependent on input laser power. The generation of a
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hollow beam from a Gaussian laser beam using a specially designed LC cell has a drawback that the design of this specially designed cell is complicated. Therefore, the device is expected to be expensive and difficult to fabricate. The hollow beam may be generated by using spatial light modulators (SLMs). The main drawback of SLMs is their low diffraction efficiency (40-60 %), which limits the efficiency for ho/fow beam generation, (n addition, SLMs need active control to generate the shape of the beam. The generation of hollow beam by illuminating a polished cylindrical glass rod with a laser beam has a drawback that the generated hollow beam would be diverging and asymmetric in intensity distribution.
The use of metal axicon mirror in generation of hollow beam by illuminating a metal axicon mirror with a laser beam has been disclosed in an article (S. R. Mishra, S. K. Tiwari, S. P. Ram and S. C. Mehendale, Opt. Eng. 46, 084002, 2007). However, the published article no where discloses or teaches or suggests the use of a second axicon mirror in generation of hollow laser beam. Further use of a single axicon mirror results in a diverging hollow beam.
OBJECTIVE OF THE INVENTION
The drawbacks of use of axicon lenses and other methods of generating hollow laser beam can be overcome in the present invention. The apparatus of present invention generates a collimated hollow laser beam using two axicon mirrors. The main objective of the present invention is to propose an apparatus and method to generate a high efficiency collimated hollow laser beam of variable diameter. Another objective of the present invention is to generate a collimated hollow laser beam without any substantive loss of power during conversion. It is another objective of the present invention to provide for an apparatus using two axicon mirrors of same base angle for generation of a collimated hollow laser beam whose diameter can be varied and depends upon distance between the said two axicon mirrors. It is yet another objective of the invention to provide for an apparatus and method which is relatively simple, low cost and low weight for generation of a collimated hollow laser beam. It is another objective of the present invention to provide a technique for the efficient generation of the collimated hollow laser beam from a linearly polarized laser beam from any laser such as He-Ne laser, diode laser, C02 laser, Nd-laser, Ti-sapphire laser, dye laser, etc. Other objectives and advantages of the present invention will be readily appreciated as the same become better understood by reference to the ensuing non-limiting detailed description when considered with reference to the accompanying drawings.
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SUMMARY OF THE INVENTION
Accordingly, the present invention provides for generating a collimated hollow laser beam by an apparatus comprises of a laser generating source, a pair of axicon mirrors having same base angle, wherein one of the axicon mirror is convex axicon mirror and another is concave axicon mirror. A cubical polarizing beam splitter made of two right angled prisms joined together from the hypotenuse sides and a pair of quarter wave-plates for in-coupling and out-coupling of the laser beam. The basic idea for the present invention is that the input laser beam upon reflection from a convex axicon mirror becomes a diverging hollow conical beam, when this diverging hollow conical beam falls on a concave axicon mirror of same base angle a collimated hollow laser beam is generated. The diameter of the. generated collimated hollow laser beam can be varied depending upon the distance between the two axicon mirrors. The use of combination of a pair of axicon mirrors having same base angle provides an easy, less expensive and efficient way for generation of collimated hollow laser beam.
STATEMENT OF INVENTION
In accordance with the present invention there is disclosed, an apparatus for converting a linear polarized laser beam into a collimated hollow laser beam, the apparatus comprises of a polarizing beam splitter for reflecting the linear polarized laser beam in orthogonal direction; a first quarter wave-plate for converting the beam received from the polarizing beam splitter into a circularly polarized laser beam; a convex axicon mirror for reflecting the said circularly polarized laser beam received from the first quarter wave-plate into a hollow conical laser beam for again passing through the first quarter wave-plate which again convert it into a linearly polarized hollow conical laser beam for passing through the polarizing beam splitter; a second quarter wave-plate for receiving the said linearly polarized hollow conical laser beam from the polarizing beam splitter and converting it into a circularly polarized hollow conical laser beam; a concave axicon mirror having a base angle identical to that of the convex axicon mirror for converting the incident circularly polarized hollow laser beam into a circularly polarized collimated hollow laser beam; the circularly polarized collimated hollow laser beam being converted in to a linearly polarized collimated hollow laser beam after passing through the second quarter wave-plate and reflected orthogonally through polarizing beam splitter
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thereby producing the desired collimated hollow laser beam.
In accordance with the present invention there is also disclosed a process for generation of collimated hollow laser beam of variable diameter comprising the steps of directing a linearly polarized laser beam on to the polarizing beam splitter; reflecting the said incidental linearly polarized beam for passing through the quarter wave-plate for conversion into a circularly polarized laser beam; reflecting said circularly polarized laser beam from the convex axicon mirror thereby converting it into a circularly polarized hollow conical laser beam for again passing through the first quarter wave-plate thereby converting it into a linearly polarized hollow conical laser beam for passing through the polarizing beam splitter; directing the said linearly polarized conical hollow laser beam through the second quarter wave-plate thereby converting it into a circularly polarized hollow laser beam incident onto the concave axicon mirror; reflecting and converting said circularly polarized conical hollow laser beam incident onto concave axicon mirror into a circularly polarized collimated hollow laser beam for passing again through the second quarter wave-plate which again converts said circularly polarized collimated hollow laser beam into a linearly polarized collimated hollow laser beam after passing through it; reflecting said linearly polarized collimated hollow laser beam via the polarizing beam splitter outside the apparatus.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1, shows a schematic view illustrating the working principle of the device;
Figure 2, shows a schematic view of the actual device including beam-coupling optics;
Figure 3, shows the images and corresponding intensity profiles of the generated hollow laser
beams at different distances from PBS for a fixed separation of 70 mm between axicon
mirrors;
Figure 4, shows the CCD images and corresponding intensity profiles of the generated
collimated hollow laser beams measured at a fixed distance of 410 mm from PBS for different
values of separation between axicon mirrors; and
Figure 5, shows measured variation of hollow beam dark diameter with separation (d)
between axicon mirrors. The inset in this figure presents the definition of dark diameter
estimated from the measured intensity profile of the hollow beam.
DETAILED DESCRIPTION OF THE INVENTION
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Reference is now be made in detail to the embodiments of the present invention as illustrated in the accompanying drawings and description. The invention relates to a novel apparatus for generation of the collimated hollow laser beam.
In accordance with the present invention there is provided an apparatus comprising of a pair of axicon mirrors of same base angle, one convex and the other concave axicon mirror, a polarizing beam splitter and two quarter wave-plates. Each of such elements has been mounted in the apparatus by suitable mounting means.
The axicon mirrors as used in the present invention may be easily fabricated. The mirrors may preferably be made from a disc of copper metal. The disc of diameter 25 mm and thickness 5 mm is found suitable for the purposes of the invention. Its one surface is flat and other is conical with convex meniscus. The conical surface (having base angle of 1°) was finished with diamond turning machine (DTM) and then gold-coated to obtain high reflectivity of - 93 %. Similarly the other axicon mirror is prepared which is conical with concave meniscus.
The polarizing beam splitter used is a commercially available optical element of cubical shape. In the invented apparatus for example a cube of dimension 25x25x25 mm3 was found suitable. The cube is made of two identical right angle prisms (of optical material BK-7) cemented together, hypotenuse-face-to-hypotenuse-face, with a multilayer dielectric coating in between. This PBS is a variable ratio beam splitter whose output depends upon the polarization of the input beam.
The quarter wave-plate used in the invented apparatus is a commercially available optical element. It is made of a uniaxial optical crystal. It changes the polarization of the incident beam. Along with PBS, the quarter wave-plate was used to in-couple the incident laser beam to axicon mirrors and out-couple the generated collimated hollow beam from the setup.
As shown schematically in Figure 1, when a laser beam is incident on a convex axicon mirror (AXi) along its axis, the reflected beam is a hollow conical beam. If it is made to fall on the concave axicon mirror (AX2) of same base angle, the reflected beam is a collimated hollow beam whose diameter depends on the separation between the two axicon mirrors and,
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hence, can be varied by varying the separation. This is the principle on which the invention is based.
In accordance with the present invention a linearly polarized laser beam is made incidental onto the polarizing beam splitter (PBS). The polarizing beam splitter cube splits an arbitrarily polarized beam into two orthogonal linearly polarized beams, when the input beam passes through PBS and is incident on the multilayer dielectric film. The PBS transmits the p-polarized light component (i.e. polarization parallel to the plane of incidence at the multi layer film) in the incident beam direction and reflects the s-polarized light component (i.e. polarization perpendicular to the plane of incidence at the multi layer film) at right angle from the incident beam direction. Thus a linearly polarized input light beam (having both s- and p-polarization components) gets split into two beams whose power values depend on the ratio of s- and p-polarization components. Hence, if incident beam is purely p- or purely s-polarized, it gets, respectively, fully transmitted or fully reflected. It is preferred that the input beam was kept to be s-polarized so that it was fully reflected by PBS. This way, the input beam was directed towards the convex axicon mirror (AXi) by the PBS. The PBS used in the present invented apparatus is a commercially available optical element. It provides either complete reflection or complete transmission depending upon whether Incident beam is s-polarized or p-polarized respectively. This polarization selective reflection or transmission of the laser beam by PBS has been used for in-coupling of the incident laser beam to the axicon mirrors and out-coupling of the generated hollow beam from the setup, with suitably oriented quarter wave-plates.
A person skilled in the art would know that when a light beam passes through a quarter wave-plate, it results a 90° phase retardation between two orthogonal polarization components (represented as ordinary ray and extra-ordinary ray inside a uniaxial crystal). Thus if quarter wave-plate is oriented suitably, a circularly polarized light is obtained at the output for a linearly polarized input beam. As indicated in (Fig.2), input light passes twice through the quarter wave-plate 5(i) (between PBS and convex axicon AXi). First, the input beam reflected by PBS, and then, the conic beam from the convex axicon AXi. Due to two passes of light through quarter wave-plate, polarization of hollow conic beam (from AXi) gets rotated by 90° with respect to the input beam. Thus hollow conic beam is seen as p-polarized at PBS and hence is fully transmitted through PBS. Similarly, quarter wave-plate 5(ii) also rotates the
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polarization of incident light beam by 90° in two passes - first after exit from PBS and second after reflection from concave axicon (AX2). Thus the coJIimated hollow beam from concave axicon mirror (AX2) is seen as s-polarized at the PBS and is reflected by PBS. This way the generated collimated hollow beam is out-coupled from the setup.
Using simple setup, a collimated hollow laser beam has been generated whose diameter can be varied by varying separation between axicon mirrors. Since focusing of hollow beam using a lens results in nondiffracting Bessel-like beam, it should be possible to generate a nondiffracting beam of variable central spot size and propagation range using this setup. Figure 1, shows a schematic diagram to illustrate the working principle of the device. When a laser beam (3) is incident on a convex axicon mirror AXi (1) having a base angle, along its axis, the reflected beam is a hollow conical beam. If this reflected beam is made to fall on another concave axicon mirror AX2 (2) of same base angle, the reflected beam is a collimated hollow beam (4) whose diameter depends on the separation between the axicon mirrors and, hence, can be varied by varying the separation. Neglecting diffraction, the generated hollow beam (4) has a dark central region of diameter [phi] given by § = 2d.tan(2y) [phi = 2d.tan(2 gamma)], and a bright outer shell of thickness (a) for input laser beam (3) of size (2a).
Using, for the first time, a combination of metal axicon mirrors, the generation of a collimated hollow beam from a Gaussian laser beam with high conversion efficiency (i.e. percentage ratio of the generated hollow beam power to input beam power) of ~ 76 % has been achieved. The present invention also demonstrates that the diameter of the central dark region of generated hollow beam can be varied by changing the separation between the axicon mirrors. The apparatus overcomes drawbacks of known devices described above. The invented apparatus is expected to operate efficiently for a range of laser wavelengths from visible to infrared, covering various lasers such as dye lasers, semiconductor lasers, He-Ne laser, Nd-lasers, Ti-sapphire laser, CO2 laser, etc.
The construction of the actual device including beam coupling optics is shown in Figure 2. Both the axicon mirrors (1) and (2) were made from copper discs of 25 mm diameter. Their reflecting conical surfaces were generated on a computer numerical control (CNC) lathe and these were further finished on a diamond turning machine (DTM). The finished surfaces were subsequently gold-coated for corrosion protection and reflectivity enhancement. Both the
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axicon mirrors had base angle of 1 degree (i.e. half vertex cone angle of 89 degree). The quarter wave-plates (5(i) and 5(ii), and polarizing beam splitter (PBS) (6) were used for in-and out-coupling of beams.
A linearly polarized laser beam (3) having a nearly Gaussian spatial profile, having full width at half-maximum (FWHM) = 0.774 mm and power ~ 16 mW, from a 780 nm diode laser was made incident (as shown in Figure 2) on the convex axicon mirror (1) via a PBS (6) and a quarter wave-plate (5(i)). In the setup, upon reflection from axicon mirror (1), a conic shaped hollow beam gets generated, which propagates back through the optics and falls on the concave axicon mirror (2). Upon reflection from this axicon mirror (2) (which is a concave mirror with same base angle), the beam becomes a coliimated hollow beam. The appropriate orientation of another quarter wave-plate (5(ii)) allows this beam to out-couple through PBS (6), as is indicated in Figure 2. The measured power conversion efficiency in our setup was ~ 76 %. The diameter and intensity profiles of generated hollow beam (4) were measured at different distances (z) from PBS (6) and for different values of separation (d) between axicons ((1) and (2)). This was done by taking images of a calibrated screen (7), on which hollow beam (4) was incident, using a digital CCD camera and computer controlled frame grabber system. Figure 3 and Figure 4 show the CCD images and intensity profiles of the generated hollow beam using our setup. The quality of the generated hollow beam depends on the spatial profile of the input beam and can be further improved in the present setup by using a better quality input beam.
The diameter of hollow beam can also be changed by choosing a different base angle of the axicon mirrors. Accordingly, The diameter of the generated hollow beam can be varied by two different ways i.e. either by varying separation between axicon mirrors or by using the axicon mirrors of another base angle. In our experimental geometry, the diameter of the coliimated hollow beam is equal to the diameter of diverging conic beam incident at the concave axicon mirror (AX2) which depends on the separation between two mirrors as well as on the value of base angle of the axicon mirrors. Thus in our setup diameter of coliimated hollow beam can be varied by varying the position of concave axicon mirror (AX2) with respect to convex axicon mirror (AXi). In a different situation where separation between concave axicon mirror (AX2) and convex axicon mirror (AXi) is to be kept fixed, the option to change the diameter of generated coliimated hollow beam can be exercised by using both the axicon mirrors of
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changed base angle. Among the above two methods, it is preferred that the separation between the axicon mirrors is implemented to change the diameter of collimated hollow laser beam while keeping the base angle fixed. This is an easy way to change the hollow beam diameter as people who wish to use the invented apparatus do not have to replace the axicon mirrors.
Example 1:
Figure 3 shows images and corresponding intensity profiles ((i) to (iv)) of the generated hollow beam at various distances (z) in the beam path ((i) z = 150 mm, (ii) z = 410 mm, (iii) z = 780 mm and (iv) z = 1150 mm) for d = 70 mm separation between the axicon mirrors (1) and (2). The results show that the diameter (peak to peak intensity) of the generated hollow beam (4) is nearly constant for various propagation distances (z). A person skilled in the art would readily appreciate the good quality of collimation in this way.
Example 2:
The setup allows change in diameter of hollow beam by changing the separation (d) between the axicon mirrors (1) and (2). Figure 4 shows the images and corresponding intensity profiles (from (v) to (x)) of the hollow beam (4) at a fixed position in the beam path (at z = 410 mm) for different values of separation (d). The values of d for images from (v) to (x) in Figure 4 were 70 mm, 90 mm, 110 mm, 130 mm, 150 mm and 170 mm, respectively. Using this data, the hollow beam dark diameter for different separation between mirrors (d) has been obtained (see Figure 5). Here we define the dark diameter of hollow beam as separation between half-maximum intensity points (inner side), as shown in the inset in Figure 5. Graph in Figure 5 (filled circles) shows the results of our measurements of hollow beam dark diameter. The plot shows a linear variation of dark diameter with separation (d). These measurements should serve as calibration of the hollow beam diameter for different values of separation (d). The straight line in Figure 5 shows the variation of diameter § with separation (d).
The main advantages of the invention are that the invented apparatus here utilizes the metal axicon mirrors which are easy to fabricate and relatively inexpensive. The invention produces collimated hollow beam whose diameter can be continuously varied by changing separation between mirrors therefore no external interference. The apparatus demonstrated here works in reflection geometry. Hence, it should to be free from aberrations commonly appearing in
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refractive elements such as lenses. Further, the axicon mirrors used in the present apparatus are less expensive and aberration free as compared to other element such as axicon lenses. The use of combination of axicon mirrors of this type provides an easy, inexpensive and efficient way to generate a collimated hollow beam with variable diameter.
Hollow laser beams with dark center are used in a variety of R&D and industrial applications such as trapping and guiding of atoms and particles, laser machining, generation of nondiffracting beams (beams with large depth of focus), light detection and ranging (LIDAR), etc. The nondiffracting beams (generated using hollow beams) find applications in corneal surgery, compact disc (CD) read/write devices, optical traps/tweezers for atoms and particles, laser machining, etc. The invention described here is for efficient generation of collimated, variable size hollow optical beam from a laser beam. A range of laser wavelengths from visible to infrared, covering various lasers such as dye lasers, semiconductor lasers, He-Ne laser, Nd-lasers, Ti-sapphire laser, C02 laser, etc. can serve as input for the invented device.
While the best mode for carrying out the invention has herein been described in detail, those skilled in the art to which this invention relates will recognize various alternative constructions and embodiments for practicing the present invention as defined by the following claims. The invention as described above should not be limited to the disclosed embodiments and features. Therefore any modifications, substitutions and improvements to the present invention are possible without departing from the spirit and scope of the invention.
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We claim:
1. An apparatus for converting a linear polarized laser beam into a collimated hollow laser
beam, the apparatus comprises of:
- a polarizing beam splitter (6) for reflecting the linear polarized laser beam in orthogonal direction;
- a first quarter wave-plate (4) for converting the beam received from the polarizing beam splitter (6) into a circularly polarized laser beam;
- a convex axicon mirror (1) for reflecting the said circularly polarized laser beam received from the first quarter wave-plate (5(i)) into a diverging hollow conical laser beam for again passing through the first quarter wave-plate (5(i)) which converts it into a linearly polarized hollow conical laser beam for passing through the polarizing beam splitter;
- a second quarter wave-plate (5(ii)) for receiving the said linearly polarized hollow conical laser beam from the polarizing beam splitter and converting it into a circularly polarized hollow laser beam;
- a concave axicon mirror (2) having a base angle identical to that of the convex axicon mirror (1) for converting the circularly polarized diverging hollow beam from the second quarter wave-plate (5(ii)) into a circularly polarized collimated hollow beam; the circularly polarized collimated hollow laser beam being converted in to a linearly polarized collimated hollow laser beam after passing through the second quarter wave-plate (5(ii)) and reflected orthogonally through polarizing beam splitter thereby producing the desired collimated hollow laser beam.

2. An apparatus for generation of collimated hollow laser beam as claimed in claim 1, wherein both axicon mirrors are metal axicon mirrors.
3. An apparatus for generation of collimated hollow laser beam as claimed in claim 1, wherein polarizing beam splitter is made of two right angle prisms.
4. An apparatus for generation of collimated hollow laser beam as claimed in claim 3, wherein the pair of the prisms js cemented together along their hypotenuse faces with
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a multilayer dielectric coating in between.
5. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the source laser beam is of Gaussian intensity profile or of uniform intensity profile.
6. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the input laser beam is linearly polarized and is of nearly Gaussian intensity profile.
7. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the diameter of the generated coliimated hollow laser beam is variable depending upon the distance between the two axicon mirrors.
8. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the diameter of the coliimated hollow laser beam is dependent upon the base angle of the two axicon mirrors.
9. An apparatus for generation of coliimated hollow laser beam as claimed in claim 1, wherein the axicon mirrors are made of copper discs.
10. A process for generation of coJJimafed hollow Jaser beam of variable diameter comprising the steps of:
- directing a linearly polarized laser beam on to the polarizing beam splitter;
- reflecting the said incident linearly polarized beam for passing through the quarter-wave-plate for conversion into a circularly polarized laser beam;
- reflecting said circularly polarized laser beam from the convex axicon mirror thereby converting it into a diverging hollow conical laser beam for again passing through the first quarter wave-plate thereby converting it into a linearly polarized hollow conical laser beam for passing through the polarizing beam splitter;
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- directing the said linearly polarized conical hollow laser beam through the second
quarter wave-plate thereby converting it into a circularly polarized hollow laser beam
incident onto the concave axicon mirror;
- reflecting and converting said circuferfy pofarized conical hollow faser beam into a
circularly polarized collimated hollow laser beam for passing again through the second
quarter wave-plate which again convert said circularly polarized collimated hollow laser
beam into a linearly polarized collimated hollow laser beam after passing through it;
- reflecting said linearly polarized collimated hollow laser beam via the polarizing beam
splitter outside the apparatus.
11. A process for generation of collimated hollow laser beam as claimed in claim 10, wherein the conversion efficiency of the generated collimated hollow laser beam is upto 76 % of the input laser beam.
12. A process for generation of collimated hollow laser beam as claimed in claim 10, wherein the collimation of the generated hollow laser beam remains constant for various propagation distances.
13. A device for generation of the collimated hollow laser beam substantially as herein described with reference to the accompanying description and drawings.
14. A process for generation of the collimated hollow laser beam substantially as herein described with reference to the accompanying description and drawings.
Dated this 31st day of October, 2008.

(Anuradha Salhotra)
Of LALL LAHIRI & SALHOTRA
AGENT FOR THE APPLICANTS
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Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=bUCkxEFKvH/G/s0NjYpsTw==&loc=vsnutRQWHdTHa1EUofPtPQ==

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Patent Number

269505

Indian Patent Application Number

2333/MUM/2008

PG Journal Number

44/2015

Publication Date

30-Oct-2015

Grant Date

26-Oct-2015

Date of Filing

03-Nov-2008

Name of Patentee

DEPARTMENT OF ATOMIC ENERGY

Applicant Address

ANUSHAKTI BHAWAN, CHATRAPATI SHIVAJI MAHARAJ MARG, MUMBAI

Inventors:

#	Inventor's Name	Inventor's Address
1	TIWARI, SANJIV, KUMAR	LASER PHYSICS APPLICATIONS DIVISION, RAJA RAMANNA CENTRE FOR ADVANCED TECHNOLOGY, INDORE-452013,
2	MISHRA, SATYA, RAM	LASER PHYSICS APPLICATIONS DIVISION, RAJA RAMANNA CENTRE FOR ADVANCED TECHNOLOGY, INDORE-452013,
3	RAM, SURJYA, PRAKASH	LASER PHYSICS APPLICATIONS DIVISION, RAJA RAMANNA CENTRE FOR ADVANCED TECHNOLOGY, INDORE-452013,

PCT International Classification Number

H01S3/08

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA