Title of Invention

"A PROCESS FOR FABRICATION OF DIFFRACTIVE OPTICAL ELEMENTS"

Abstract

This invention relates to a process for the fabrication of diffractive optical elements (DOEs) for use in thick waveguide based optical instruments such as night vision goggles, image guide and telescope. According to the process a laser beam is splitted into a plurality of split beams. The splitted beams are interfered through a roof prism having a photo-sensitive film at the base. The exposed photo-sensitive film is developed and then mounted on a thick optical glass.

Full Text

FIELD OP INVENTION This invention relates to a process for fabrication of diffractive optical elements which can be used in the thick waveguide based optical instruments such as image guide, night vision goggles or telescope . PRIOR ART Conventional glass elements like lenses, mirror, etc are basically refractive elements. When these optical elements are used in optical instruments, the wave front propagates in the free space. Due to this free space propagation, considerable spaces are required between different optical elements. The optical instruments based on these conventional glass elements have therefore several drawbacks. The first drawback of the optical instruments based on conventional glass elements is that these optical instruments are bulky. Another drawback of the optical instruments based on conventional glass elements are that these are prone to misalignment which degrades their performance considerably. Further drawback of the optical instruments based on conventional glass elements is that these instruments have considerable weight as the components are made of glass etc. Still further disadvantage of these instruments is that these instruments involve complexity of tubes, fixtures, spacers, baffles, etc which occasionally have the tendency of misalignment and hence need frequent service. Yet further disdavantage of the above type of optical instruments is that these instruments cannot be reduced in size or weight. Even if the size is reduced by beam folding, additional mirror, etc have to be added which results in more weight. Still further disadvantage of the above type of optical instruments is the loss of light at different surfaces and in the glass. In the multiple element instrument, the loss could be as high as 90%. Another type of instruments known in the art are based on optical waveguide. Optical waveguide is a thin film of thickness comparable to its operating wavelength ( ). These instruments are very small sized and light weight. However, the limitation of these instruments is that these instruments can carry very small size (0.2um) wavefronts and are not suited to carry wavelengths of size of several mm, as required in imaging devices. Another disadvantage of the above type of optical waveguide based instruments is that these instruments are unable to accept incoherent wavefronts. Holographic optical elements(HOEs) are diftractive optical and are currently being used in optical instrumentation replacing the conventional glass optics. Using HOEs, the number of optical elements can be reduced and since the HOE is light weight, the instrument can be made smaller in size and less weight. However the Holographic optical elements(HOEs) used in above-mentioned type of optical instruments, also have the drawback that these elements allow free space propagation of light waves. These instruments also have severe misalignment problems. OBJECTS OF THE PRESENT INVENTION The primary object of the present invention is to propose a process for fabrication of diffractive optical elememts(DOES) for use in thick waveguide based optical instruments. Another object of the present invention is to propose a process for fabrication of DOEs where in multiplexed holographic elements lead to reduction in the number of elements thereby leading to reduction in the total weight and size of the optical instrument. Still another object of the present invention is to propose a process for fabrication of DOEs wherein the reduction in the number of optical elements due to multiplexing of holographic elements, eliminates: the misalignment problems associated with the optical instruments known in the art. Yet another object of the present invention is to propose a process to fabricate DOEs which ensure higher diffraction efficiency. Further object of the present invention is to propose a process for fabrication of DOEs which can carry large size of image wavefront of the order of several mm. Still further object of the present invention is to propose a process which leads to diffractive optical elements(DOEs) which can act as eyepiece for incoherent image wavefront. Yet further object of the present invention is to propose a process which leads to diffractive optical elements which can be used as beam-splitter in equal ratio (1:1) at ±45° to the surface normal. Still further object of the present invention is to propose a process which leads to diffractive optical elements which can act as coupler for coupling the splitted images in the waveguide. Yet further object of the present invention is to propose a process which leads to DOEs which can act as decoupler for decoupling the beam for normal viewing. STATEMENT OF INVENTION According to the invention, there is provided a process for the fabrication of diffractive optical elements (DOEs) for use in thick waveguide based optical instruments such as night vision goggles, image guide, telescope which comprises in the steps of: a. splitting a laser beam into a plurality of split beams; b. interfering the splitted beams through a roof prism having a photo-sensitive film at the base; c. developing the exposed photo-sensitive film preferably photopolymer film as herein described. d. mounting the developed film on a thick optical glass. DESCRIPTION OF FIGURES Further advantages and objects of the present invention would be apparent from the following description which is to be read in conjunction with the accompanying drawings wherein FIGURE 1: illustrates the fabrication process for coupler DOE using three beam holographic interferometry technique; FIGURE 2: illustrates the fabrication process for the decoupler DOE using three beam holographic interferometry technique; FIGURE 3: illustrates the construction of an optical instrument using DOEs; FIGURE 4: illustrates an example of the construction of a telescope using DOEs. BRIEF DECSRIPTION OF INVENTION In accordance with the present invention, the fabrication of diffractive optical elements is carried out by a novel technique involving splitting of a laser light beam into three beams and interfacing these beams through a roof prism and then developing an exposed film placed at the base of the prism. In the present invention, DOEs have been designed to act as granting coupler, objective lense, eyepiece, beam splitter etc.. The free space propagation is confined to the optical waveguide. and the image is split in two wavefronts for bi-ocular vision. The image can be viewed through another DOE, two in number, one for each eye. This DOE acts as decoupler and eyepiece. Thus, the light wave is guided through a glass plate similar to that in an optical fibre. Such a glass plate of thickness 10mm mounted with such DOEs work as telescope. Thus, achieving enormous reduction in size and weight of the instrument. The DOEs can be fabricated to work in transmission or reflection mode. DOEs can be made multifunctional also. Such DOEs are expected to revolutionise the scenario in the optical and opto-electronic instrumentation. The DOES are designed to receive the incoming wavefront at right angles to the DOEs. The DOE acting as a objective splits the incoming beam from the object in two beams and couples these beams at ± 45° to the normal, into the glass plate acting as waveguide. The DOEs acting as eyepiece, decouples these beams directing these at normal angles for viewing by the eye. DESCRIPTION OF FABRICATION PROCESS The figure-l explains the process for the fabrication of coupler diftractive optical element(DOE). The fabrication of this type of DOE is carried out by a three beam holographic interferometry technique. According to this technique, the laser light beam from an Ar-ion laser at 488 mm is split into three beams(1,2 & 3) of intensity ratio of 1:1:0.707, the normal beam(l) being of less intensity. The two other beams (2 & 3) are at ±45° to the normal beam(1). These three beams(1,2 & 3) are made to interfere through a roof prism(4) with surfaces coated with anti-reflection coating at 489mm. The photo-sensitive film(5) silver halide or photopolymer or photoresist or dichromated gelatin , preferably photopolymer, is placed at the base of the prism(4). The exposed film is developed to achieve high efficiency. To facilitate the exposure of the interference pattern on the photosensitive film/plate, an index matching liquid is appl'ied on the base of the roof prism. The recording arrangement is further secured by placing a 3-4mm glass plate(6). The nature of interfering beams determines the type of DOE recorded. This process can record DOEs upto 100mm diameter and f/no. 2 . Fig-2 explains the fabrication process for the decoupler diffractive optical element(DOE) . For this type of DOE, the recording technique is based on two beam interference method. The normal beam(7) and another beam(8) at +45° are interfered in the roof prism(9) such that the interference pattern is located on the base. Photo-sensitive film (10) is placed at the base of the- roof prism(9) . A securing glass plate(11) of thickness 3-4mm is placed under the film(lO). The construction of optical instruments based on use of DOEs is explained in fig-3. For such type of optical instrument, an arrangement of coupler and decoupler in conjuction with a thick glass plate is used. The glass plate acts as a thick optical waveguide and is about dimension 170 mmx30mmxl0mm. A coupler DOE(13) is mounted on the top face of the thick waveguide(14). The two numbers of decoupler DOEs (15 & 16) are mounted on the bottom face of the thick waveguide(14). The coupler DOE(13) acts as coupler for incoming free space incoherent object wavefront(12) and simultaneously splits the wavefront into two wavefronts(17 & 18) at ± 45° to the normal. The size of the coupler DOE(13) is equal to the size of the exit aperture of the objective of the instrument for which it has to function. The optimum range of thickness of optical waveguide is from half the size of exit aperture of the objective to the maximum 40mm for inter-ocular aperture of 80mm. EXAMPLE Fig-4 gives an example of the construction of a telescope using DOEs. The beam(19) from a distant object enters the objective DOE which splits the beam(19) into two beams ( 22 & 23) at ±45° to. the normal and guides into the glass slab waveguide 21. The binocular view is made available by the eyepieces DOE (24 & 25). It is to be understood that the present invention is subject to changes, modifications, adaptations, by the persons skilled in the art. Such modifications are intended to be covered within the scope of the present invention which is set forth by the following claims:- WE CLAIM: 1. A process for the fabrication of diffractive optical elements (DOEs) for use in thick waveguide based optical instruments such as night vision goggles, image guide, telescope which comprises in the steps of: a. splitting a laser beam into a plurality of split beams; b. interfering the splitted beams through a roof prism having a photo-sensitive film at the base; c. developing the exposed photo-sensitive film preferably photopolymer film as herein described. d. mounting the developed film on a thick optical glass. 2. A process as claimed in claim 1 wherein the said laser beam used is Ar-ion laser. 3. A process as claimed in claims 1 & 2 wherein said laser beam is split into a normal beam and three inclined beams for decoupler DOE/coupler DOE. 4. A process as claimed in claim 1 wherein the said inclined beams are at an angle of ± 45° to the normal. 5. A process as claimed in claims 1 to 4 wherein the intensity ratio of said split three beams for coupler DOE is in the ratio of 1:1:0.707 where the normal beam is of less intensity. 6. A process as claimed in claims 1 to 5 wherein said splitting of the said laser beam for decoupler DOE, is into two beams of the intensities in the ratio if 1:1. 7. A process as claimed in claims 1 to 6 wherein the said photosensitive film used is photopolymer or silver halide or photoresist preferably photopolymer film. 8. A process for fabrication of diffractive optical elements substantially as herein described and illustrated.

Documents:

236-del-1997-abstract.pdf

236-del-1997-claims.pdf

236-del-1997-correspondence-others.pdf

236-del-1997-correspondence-po.pdf

236-del-1997-description (complete).pdf

236-del-1997-drawings.pdf

236-del-1997-form-1.pdf

236-del-1997-form-19.pdf

236-del-1997-form-2.pdf

236-del-1997-form-3.pdf

236-del-1997-gpa.pdf