System of holographic optical elements for testing laser range finders

A focusing, collimating, and beamsplitting test system for testing laser rangefinders for their beam quality, ranging accuracy, and sight unit alignment. The system includes holographic optical elements in place of conventional mirrors, lenses and beamsplitters. The advantages of this system are that it not only reduces the size and weight of optical elements, but also reduces the number of required elements. Consequently, the resulting test equipment requires less labor to assemble and is more compact and much lighter than conventional test systems.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of testing equipment for 
electro-optical systems such as laser rangefinder/designators, and more 
particularly to an improved test system which employs holographic optical 
elements. 
A laser rangefinder is an equipment which can send out laser pulses and 
detect the returning pulses reflected from the target; the delay time of 
the pulses determines the target range. The equipment also contains a 
sight unit for the operator to aim at the selected target. 
A laser designator is an equipment similar to the laser rangefinder, except 
that its purpose is to illuminate the selected target. The scattered laser 
energy is used to guide "smart" bombs or artillery for converging into the 
target. 
The laser designator and the laser rangefinder are sometimes combined into 
one piece of equipment. 
For a laser rangefinder or designator to be useful, its laser output must 
meet certain energy level and beam divergence requirements. The alignment 
between the sight unit and the laser pointing must be properly maintained. 
And, the subsystem for measuring the delay of return signals must function 
correctly. It is thus necessary to test the equipment periodically for its 
output, alignment and ranging accuracy. 
Presently, the laser beam quality can be tested with, for example, an 
automatic laser test set (ALTS) marketed by Hughes Aircraft Company. The 
ALTS equipment uses diode matrix array to record the laser beam intensity 
profile. The analysis of this profile will give the information on beam 
energy, divergence and pointing. When coupled with a visual reticle by the 
use of a beamsplitter, the ALTS can also be used to test beam alignment. 
The ranging accuracy can be tested with, for example, a simulated optical 
range target (SORT), also developed by Hughes Aircraft Company. The SORT 
equipment includes focusing optics and an optical fiber delay line. The 
laser energy is focused into one end of the fiber and reflected back from 
the other end. The laser energy re-emits from the first end with proper 
delay time and attenuation to simulate the return signal. The return 
signal then travels back to the rangefinder for ranging test. 
Conventional lenses, beamsplitters and mirrors can be used to combine the 
ALTS and the SORT equipment with the visual reticle into a compact package 
to test the rangefinder for its beam quality, alignment and ranging 
accuracy. However, the disadvantages of this approach include the 
following: (1) traditional mirrors, lenses, and beamsplitters are bulky 
and heavy; and (2) mechanical design and assembly processes are 
complicated by the relatively large number of optical elements involved. 
SUMMARY OF THE INVENTION 
It would therefore be advantageous to provide a test set for testing 
electro-optical systems, which has optical elements of reduced size and 
weight in comparison to existing test set equipment. 
It would further represent an advance in the art to provide a test set for 
electro-optical systems which employs fewer optical elements than existing 
test set equipment. 
In accordance with the invention, a system for testing a laser rangefinder 
device is described. The system comprises a visual test reticle image, an 
optical delay line having a predetermined optical path length, and a 
detector array responsive to incident laser energy generated by the 
rangefinder. 
In accordance with the invention, the system comprises a first holographic 
optical element, comprising means for collimating reticle image light 
emitted from the visual reticle and directing the collimated light along a 
first optical path into the sight-unit of the rangefinder, and means for 
focusing laser light emitted along a second optical path from the laser 
transmitter onto the optical delay line and for directing light from the 
optical delay line back to the laser receiver of the rangefinder. 
A second holographic optical element is disposed in the optical path 
between the first holographic optical element and the delay line and 
comprises means for reflecting and refocusing a portion of the laser light 
energy emitted by the laser onto the detector array. 
The positions of the visual reticle and the detector array are selected so 
that when the sight unit of the rangefinder is aligned such that the 
visible reticle is visible therein, laser light emitted from the laser 
transmitter is directed to the center of the detector array when the sight 
unit and the laser transmitter are properly aligned. 
The system further provides the capability of range calibration of the 
rangefinder since the optical delay line provides a simulated optical 
target image at a predetermined, simulated range.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention exploits the multi-functional property of holographic optical 
elements (HOEs). Holographic optical elements are holograms that function 
as ordinary optical elements, such as lenses, mirrors or beamsplitters. 
Most holograms are made by recording the interference pattern of two or 
more waves of mutually coherent light. See, "Handbook of Optical 
Holography," H.J. Caulfield, Academic Press, 1979, at page 573. As will be 
described, a preferred embodiment of the invention employs two HOEs. Each 
of the two HOEs has both focusing and beamsplitting functions. Moreover, 
the off-axis configuration of this embodiment avoids the unwanted 
zerothorder interference, thus providing a high performance system. As is 
well known in the HOE art, the zeroth-order interference results, in the 
case of a transmissive mode HOE, from light which passes directly through 
the HOE undiffracted, or in the case of a reflective mode HOE, from light 
which is simply reflected from the HOE, as from an ordinary mirror. The 
off-axis configuration of the system of claim 1 results in the 
zeroth-order light being directed away from the directions of interest. 
FIG. 1 is a simplified block diagram of a portable rangefinder test set 20 
employing HOEs in accordance with the invention. A typical rangefinder 15 
under test comprises a sight unit 17 operating at visible wavelengths and 
a laser transmitter-receiver unit 19 operating at an IR wavelength (for 
example, 1.06 .mu.m). For collimated incident IR and visible energy, the 
first HOE (HOE 1) splits the visible wavelength portion of the incident 
energy from the IR wavelength portion of the incident energy. The visible 
wavelength portion is focused to a visual reticle 22 and the IR part is 
focused to the fiber optic delay line 24. A second HOE (HOE 2) is placed 
in the path of the converging IR energy beam portion such that a portion 
of the IR energy incident on the HOE 2 is reflected and focused to the 
photodiode detector array 26, with the rest of the incident IR energy 
passing through HOE 2 and continuing to converge to the optical fiber 
delay line 24. 
The optical fiber delay line 24 comprises an optical fiber of predetermined 
length. The light entering the fiber at one end 25 will be reflected back 
from the other end 18 re-emit with a certain delay time. An exemplary 
commercially available optical fiber delay line suitable for the purpose 
is the QFS-200W model, available from Quartz Products Corporation, 688 
Somerset St., Plainfield, N.J. 07061. 
The photodiode detector array 26 is a solid state imaging device such as is 
used in common CCD video cameras. An exemplary commercially available 
array is the MC520 model, available from EG&G Reticon, 345 Potrero Avenue, 
Sunnyvale, Calif. 94086. 
The visual reticle 22 generates a cross-hair reticle image which is 
directed onto HOE 1 and reflected to the sight-unit 17 of the rangefinder 
15. An exemplary visual reticle suitable for the purpose is commercially 
available as the model 04RET003 from Melles Griot, 1770 Kettering Street, 
Irvine, Calif. 92714. 
The rangefinder 15 is aligned so that the visual reticle image is seen in 
the eye-piece of the sight unit 17. Once the rangefinder has been so 
aligned, the IR laser transmitter 19 of the rangefinder 15 is fired to 
generate an IR laser beam. The position of the visual reticle 22 is 
related to that of the photodetector array 26 such that, if the sight unit 
17 of the rangefinder is properly aligned with the laser transmitter unit 
19, the IR laser energy will be incident at the center of the detector 
array 26. If the laser energy is not centered on the array 26, this is an 
indication of misalignment. 
A portion of the laser light from the rangefinder 15 is also incident of 
the optical fiber delay line 24, and is reflected at the line end 18, so 
that the reflected light is directed back at the rangefinder 15. The delay 
line 24 has a known optical length, which can be used to test the 
rangefinding accuracy of the device 15 under test. 
In operation, HOE 1 functions as a collimator for the visual reticle 22. 
That is, the light emitted from the reticle 22 is collimated through HOE 1 
and directed into the rangefinder's sight unit 17 for aiming. HOE 1 also 
functions as the focusing optics for focusing the IR energy at the input 
to the optical fiber delay line 24. At the same time, HOE 1 combines with 
HOE 2 to form a telephoto system. A telephoto system is a two-element 
optical system with one converging and one diverging optical element. When 
properly arranged, a telephoto system can provide a very long focal length 
with a relatively short system length. The advantage of a telephoto system 
in the apparatus of FIG. 1 is to provide large magnification in a compact 
system. 
HOE 1 can be implemented as a single hologram with dual exposures, or as 
two holograms glued together, one for IR wavelength selection and the 
other for visual wavelength selection. FIG. 2 shows an illustrative set-up 
for the construction of the IR hologram comprising HOE 1. The laser 52 
generates a laser beam at a selected wavelength, e.g., 0.528 micron. The 
laser 52 may comprise, for example, an Argon laser. The laser beam is 
split into two paths by beamsplitter 54. The first path is through lens 
56, which focuses the incident energy at an aperture 58 formed in plate 
60, thereby creating a point source. The location of the aperture 58 
corresponds to the location of the input 25 to the optical delay line 24 
of FIG. 1. The second path forms a collimated beam, the light transmitted 
through the beamsplitter 54 being reflected from mirrors 62 and 64 onto 
lens 66. The lens 66 focuses the light at aperture 68 in plate 70, forming 
a second point source whose light is directed through collimating lens 72 
to form a collimated light beam. The interference of the two wavefronts 
from the point source at aperture 58 and the collimated beam from lens 72 
will produce a reflective HOE with the desired focusing property. 
For the visual light selective hologram comprising HOE 1, a similar set-up 
to that shown in FIG. 2 can be used with the location of the point source 
being shifted to the place of the visual reticle 22 of FIG. 1. FIG. 3 
illustrates such a set-up, with the elements 52'-72' corresponding to the 
elements 52-72 of FIG. 2. 
HOE 2 can be constructed with two point sources as shown in FIG. 4. Here, 
the laser 100 light beam is incident on beamsplitter 102 which divides the 
beam into two portions, a first portion directed to lens 104 and the 
second portion directed to mirror 110. Lens 104 focuses the incident light 
at aperture 106 formed in plate 108, thereby forming a point source of 
light projected on HOE 2. The point source at aperture 106 is located at 
the position of the input 25 to the optical fiber delay line -24 of FIG. 
1. The light incident on mirror 110 is reflected through lens 112 and 
reflected again by mirror 114. Lens 112 focuses the light at aperture 116 
in plate 118, thereby forming a second point source located at the plane 
of the photodiode detector array 26 of FIG. 1. 
Because most holographic materials are not sensitive to IR wavelength light 
at 1.06 .mu.m, one of the following methods may be used to construct the 
IR hologram comprising HOE1 and HOE 2. The first method uses visible light 
with the recording angle different from the playback angle. The necessary 
angular shifting can be calculated from the anticipated wavelength 
shifting. For example, if the desired playback angle is .theta., the 
recording angle should be .theta.'=.mu..theta., where .mu. is the ratio of 
the playback light wavelength to the recording wavelength. See, "Optical 
Holography," R.J. Collier et al., Academic Press, 1971, page 76. 
The second method is the gelatin expansion technique, which can be used 
during dichromated gelatin processing. The curing time in the processing 
will determine the expansion factor. This method expands the small grating 
spacings that are recorded with a shorter (visible) wavelength into larger 
grating spacings that will respond to a longer (IR) wavelength. The 
expansion factor should be just the ratio of the playback wavelength to 
the recording wavelength. See, "Handbook of Optical Holography," H.J. 
Caulfield, Academic Press, 1979, at page 284. 
The third method uses computer-generated holograms; that is, uses a 
computer to calculate the required interference pattern and to drive a 
plotter to draw the pattern. See, e.g., "Optical Holography," R.J. Collier 
et al., Academic Press, 1971, at Chapter 19. 
The system of this invention, as shown in FIG. 1, contains only two light 
weight HOEs which can be simply mounted. Also, the use of a telephoto 
configuration gives a compact system with a long focal length. 
Conventional designs contain many traditional optical elements which are 
heavy and bulky, and require complicated mechanical design to mount these 
elements. The new design has advantages of being light, compact, and easy 
to assemble and maintain. 
It is understood that the above-described embodiment is merely illustrative 
of the possible specific embodiments which may represent principles of the 
present invention. Other arrangements may readily be devised in accordance 
with these principles by those skilled in the art without departing from 
the scope of the invention.