METHOD OF PRODUCING A GAUSSIAN LASER BEAM FILTER

Abstract: A method for constructing an optical filter for use in a holographic scanner, includes splitting a beam (22) from a laser (20), focusing the beams so obtained on a photosensitive film (34) positioned at a predetermined distance from the laser (20), and developing the film (34). A silver-halide gelatin film or a dichromate sensitized gelatin film is used to record in one or another regions of the spectrum. The developed film acts as a holographic phase filter when reinserted at said predetermined distance, causing the beam of light emerging from the filter to have a uniform irradiance. The phase filter according to the invention is highly resistant to thermal laser damage and is comparatively cheap to produce.

Description: METHOD OF PRODUCING A GAUSSIAN LASER BEAM FILTER Technical Field The present invention is directed to a method of producing a Gaussian laser beam filter and more particularly to the production of a holographic phase filter capable of converting a high-powered Gaussian laser input into a uniform irradiance output. Background Art The science of holography deals with a method used to produce three dimensional images by laser light and to record on a photosensitive emulsion the interference patterns from which an image can be reconstructed. A hologram is the name give to a thin film having an interference pattern recorded thereon. The interference pattern is usually produced by a source of coherent light which is normally split into two beams, one of which is directed onto the sheet of photosensitive film and the other of which is directed onto a subject. The beam directed onto the subject is reflected onto the sheet of film with waves of varied orientation and wave front curvature. The reflected light waves and the undisturbed beam are recombined at the film where they interfere as patterns of light and dark areas which are optically recorded. Another way of defining a hologram is a recording of the three dimensional intensity distribution of the interference pattern produced by the interaction of two or more mutually coherent monochromatic waves. Holograms have been utilized in scanning an object such as a universal product code (UPC) label to read the data contained therein. An example of such a scanning arrangement may be found in United States Patent No. 4,224,509. 6/04155 - 2 - In manufacturing the holographic system used in the above-mentioned scanning apparatus, a laser is used as a source of coherent light along with a plurality of holograms. Where it is required that coherent light be of- uniform illumination, objective lenses are used to expand the beam and then limit the expansion with a collimating lens since the laser projects a beam whose cross-sectional profile is approximately Gaussian. In order to overcome this deficiency in a laser beam, optical filters have been proposed. However, the manufacture of such optical filters is rather costly. Disclosure of Invention It is an object of the present invention to provide a low loss, high efficiency, holographic phase filter which is highly resistant to thermal laser damage and which is comparatively cheap to produce. Thus according to the invention, there is provided a method of producing an optical filter, characterized by the steps of providing a source of coherent light having an intensity which varies from point to point over its cross-sectional area in accordance with a predetermined Gaussian function; positioning a photosensitive film in the beam of light at a predetermined distance from said source of light; exposing the photosensitive film to said beam of light; and developing the exposed photosensitive film to produce a holographic phase-type filter which, when reinserted at said predetermined distance from said source of light, will cause the beam of light emerging from the filter to have a uniform irradiance, wherein a silver-halide-sensitized gelatin film is used for recording in the red or near infrared region of the spectrum, and a dichromate-sensitized gelatin film is used for recording in the blue-green region of the spectrum. An advantage of the filter obtained by the method according to the invention is that it uses a single diffraction order to smooth out the beam profile. Brief Description of the Drawing One embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a schematic representation of a system for constructing the Gaussian beam filter of the present invention. Fig. 2 is a diagram of the theoretical power transmission of a series of phase Gaussian filters; and Fig. 3 is a diagram of the measured power transmission of the filters shown in Fig. 2. Best Mode for Carrying out the Invention Referring now to Fig. 1, there is shown a schematic representation of a system for constructing a holographic disk which is used in a holographic scanning system and illustrates the fabrication geometry for a holographic Gaussian filter. The system includes a 50 mW helium-neon laser 20 whose light output 22 is directed into a beam splitter 24 splitting the beam into two light beam segments 26 and 28. The beam segment 26 is directed toward a mirror 30 which deflects the beam segment as an object beam 32 toward a Gaussian type filter 34. The Gaussian filter 34 in turn directs the light beam as a segment 36 toward a microscope lens 38 and then through a pinhole 40. In order to reduce the cost of manufacturing the holographic filter 34, it was found that by exposing a photosensitive film to the output of a laser and after developing the film, reinserting the exposed film at its original position, the output beam of the filter was of a uniform illumination. The phase filter can be recorded in the red or near infrared region of the spectrum using a high resolution photographic plate followed by siiver- halide gelatin (SHG) processing or in the blue-green region of the spectrum using a dichromate-sensitized gelatin (DG) film followed by water-isopropanol processing. It is noted that either process will produce a low loss, high efficiency, volume phase-type filter. In fabricating such a filter, a photographic plate was inserted in front of the helium-neon laser 20 at a specified distance, thereby exposing the plate to the light output of the laser. The exposed plate was then developed for 5 minutes in developer D-19, treating the plate for 1 minute in the stop bath SB-1, washing in water for 5 minutes, bleaching in R-ΪO tanning bleach for 2 minutes, water washing for 5 minutes, fixing the plate for 10 minutes in a rapid fixer solution, washing the plate for 5 minutes in a 50/50 solution of water and isopropanol, washing the plate for 5 minutes in a 100% solution of isopropanol, and air drying the plate. After the plate has been dried, a cover plate is cemented over the plate for environmental protection. After developing, the developed plate was reinserted at its original position where it was found that the plate diffracted most strongly the central portion of the laser output beam, thus eliminating the original Gaussian irra¬ diance profile of the laser beam. Where that portion of the laser beam has the highest light irradiance, the corresponding portion of the developed plate has the highest diffraction efficiency (plus 1 order), thereby limiting the amount of the light passing through the plate. For those portions of the laser beam having the lowest light irradiance, the corresponding portion of the plate will be the lowest diffraction efficiency. In measuring the light output of the filter 34, the filtered output is directed through the microscopic object lens 38 and spatially filtered by the pinhole filter 40 which produces a noise-free, constant irradiance, coherent laser beam projected on an output plane 42 where the amount of light is detected and measured by an optical detector 44. Referring now to Fig. 2, there is shown a diagram of the theoretical output irradiance for various exposure times of filters constructed in accordance with the present invention and reflects theoretical Gaussian filter transmittance T vs.. radial distance r (cm). Fig. 2 shows the theoretical filter transmission while Fig. 3 shows the measured transmission for the same filter. Curve 50 of Fig. 2 discloses the irradiance profile of the power transmission of the laser without a filter showing the pronounced Gaussian effect. Curve 52 shows the output of a filter exposed to the output of a laser for 2.5 milliseconds, curve 54 is exposed for 3.5 milli¬ seconds, curve 56 is exposed for 4.5 milliseconds, and curve 58 is exposed for 5.5 milliseconds. It will be seen from Fig. 2 that as the time of exposure of the filter to the laser output increases, a flattening of the sharply peaked Gaussian input beam transmitted through the phase filter is effected. In a similar manner. Fig. 3 shows the measured irradiance profile of a number of filters recorded on photographic plates. Curve 60 illustrates the measured Gaussian filter transmittance T vs. radial distance r (arbitrary units) of a glass plate, curve 62 shows a hologram exposed at 2.5 milliseconds, curve 64 another hologram at 3.5 milliseconds, curve 66 a further hologram at 4.5 milliseconds and curve 68 a final hologram exposed at 5.5 milliseconds. It will — o — be seen in comparing the curves of Fig. 3 with those of Fig. 2 that both are very similar in that both show • a flattening of the Gaussian input beam. At higher exposures over-modulation is seen to produce a "hole" in the beam center. It is seen that the holographic thin film phase type photographic filter constructed in accordance with the present invention is very inexpensive compared to the manufactured metallic thin film filters. The performance of these filters is superior to the use of Gaussian lens correctors, and it has been found also that holographic filters work well if the input is assymetrical or not a true Gaussian. Further, the disclosed filter operates on the unexpanded laser beam with scattered light being eliminated by the spatial filter so that anti- reflection coatings are not necessary. Contrasted with absorption type filters, the phase-type filters are not susceptible to thermal laser damage at high beam power. The holographic phase filters produced by processing dichromated-gelatin or siiver-halide gelatin-sensitized films operate by diffraction and are thus highly resistant to such laser damage.

Claims: Claims 1. A method of producing an optical filter, characterized by the steps of providing a source of coherent light (20) having an intensity which varies from point to point over its cross-sectional area in accordance with a predetermined Gaussian function; positioning a photosenstive film (34) in the beam of light (22) at a predetermined distance from said source of light; exposing the photosenstive film (34) to said beam of light; and developing the exposed photosenstive film (34) to produce a holographic phase-type filter which, when reinserted at said predetermined distance from said source of light (20), will cause the beam of light (36) emerging from the filter, to have a uniform irradiance, wherein a silver-halide-sensitized gelatin film (24) is used for recording in the red or near infrared region of the spectrum, and a dichromate-sensitized gelatin film (34) is' used for recording in the blue-green region of the spectrum. 2. Method according to claim 1, characterized by the additional step of splitting the beam of light (22) from said source of coherent light (20) into a reference beam (28) and an object beam (26, 32), and focusing the beams onto said photosensitive film (34) positioned at said predetermined distance.

Inventors: FERRANTE RONALD ALFRED, FERRANTE, RONALD, ALFRED
Applicants: NCR CO, NCR CORPORATION
Application year: 1985
Main classification: G02B5
IPC codes: G02B5, G02B27, G03H1