Method and apparatus for high speed, sustained recording of infrared laser beam patterns

A method of photographically recording at high speed the far field pattern of a high energy infrared laser beam over a selected period of time for providing rapid repetitive samples of information on beam parameters such as far field beam intensity, jitter, and absolute beam size and power. A sample from a high energy infrared laser beam is directed into an enclosure that is light tight to visible radiation and contains an operating lensless movie camera loaded with suitable movie film. The infrared beam is focused on the film plane of the camera so that consecutive frames of the film are irradiated by the infrared beam each time the camera shutter is open. The period of irradiation is of sufficient duration to sensitize the film to visible radiation in the areas where the infrared radiation impinges on the film. The position of the camera shutter is detected and an electrical trigger signal generated just before the shutter is closed for each frame. The trigger signal is applied to appropriate circuitry which actuates a light source to generate a pulse of visible light that irradiates the film plane of the camera near the end of each shutter movement to expose each frame of the film. The exposed film strip is developed to provide a continuous record of the infrared laser beam characteristics over the selected period of time.

BACKGROUND OF THE INVENTION 
This invention is in the general field of infrared (IR) radiation 
photography. More particularly it pertains to the high speed photographic 
recording of far field IR laser beam patterns during the entire period 
when a high energy laser is operating so that the characteristics of the 
laser beam over the entire cycle of laser operation can be determined. 
There is extensive research being conducted on IR laser systems at the 
present time and one of the areas under continuous investigation is how to 
improve the evaluation of the various characteristics of the laser beam. 
This evaluation is often referred to as beam diagnosis. Some of the beam 
characteristics to be diagnosed or determined are far field beam 
intensity, near field phase, jitter and absolute power of the beam. The 
usual approach to monitoring IR beams for these parameters is to use 
electronics intensive electro-optic techniques requiring very expensive 
photodetectors, recorders and signal processing equipment. Since these 
techniques are expensive and time consuming to use, other simpler 
approaches are needed. 
It has been suggested in the literature by Frazier et al., Applied 
Optics/Vol. 15, No. 6/June 1976, that IR photography might be attractive 
for photography of far-field high energy laser mode patterns. Frazier 
discloses in U.S. Pat. No. 4,018,608 a process for photographing IR laser 
beam patterns on silver halide film by first impinging an IR laser beam on 
the film and then flashing the film with visible light. Both of the 
Frazier publications identified above contain a discussion of IR 
presensitization photography. IR sensitization theory is also discussed by 
Naor et al. in Applied Optics/ Vol 20, No. 14/15 July 1981. Naor et al. 
theorizes that the sensitization of the photographic film results from the 
heating produced in the photographic emulsion by the IR radiation. Naor et 
al. also explains that the film may be sensitized or desensitized 
depending upon the duration of the visible exposure that follows the IR 
exposure. Naor et al. further reports that sensitivity effects were 
improved by delays shorter than 0.5 msec between the IR and visible 
exposure. The basic approach used in all of the investigations discussed 
above, and in the invention disclosed in this application, utilizes direct 
photographic recording of IR radiation on silver halide film. Two 
sequential exposures are made, an IR exposure to sensitize a portion of 
the film, followed immediately by a uniform visible exposure of all the 
film in the frame. After development those areas of the film that have 
been exposed to IR and visible radiation are darker than the areas that 
have only been exposed to visible radiation. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention to provide a method and 
apparatus that can be used to determine the characteristics of a 
continuous wave high energy laser beam over a cycle of operation that 
extends for several seconds. 
It is another object of the invention to provide a laser beam monitoring 
method and apparatus that is economical to utilize and capable of being 
quickly set up and used at any target site. 
These and other objects of the invention are accomplished as follows. A 
high speed movie camera and several optical components are mounted on a 
portable optical table and then enclosed in an enclosure light tight to 
visible radiation. The visible light tight enclosure is provided with an 
infrared window so that an IR laser beam sample, which is a faithful 
rendition of the relative intensity and absolute phase of the laser beam 
being sampled, can be reflected into the enclosure for recording by the 
camera equipment mounted therein. The IR laser beam sample is reflected 
into the enclosure by a parabolic mirror. Once inside the enclosure the IR 
beam is focused on the film plane of the movie camera. The camera moves 
the film in steps at a desired rate so a series of film frames are 
sequentially exposed to the IR radiation through a shutter mechanism in 
the camera. The film advancing and shutter operating mechanisms are 
synchronized so that each film frame is stationary while being exposed. A 
timing mechanism is connected to the film advancing and shutter operating 
mechanism and synchronized therewith for generating and electrical trigger 
signal just before the shutter mechanism closes. Shutter closing blocks 
radiation from the film in the camera. The trigger signal from the timing 
mechanism is connected into appropriate circuitry which in turn actuates a 
strobe. The strobe is one of the optical components mounted in the light 
tight enclosure and it is oriented so that it irradiates the film plane of 
the camera with a uniform field of visible light. 
There results a method of photographing an IR laser beam by focusing the 
beam on the film plane of a movie camera. The IR beam is a small diameter 
beam of coherent radiation so it only irradiates a small area of the film 
relative to the available area of each film frame. The IR sensitizes the 
film in the areas it irradiates. Just prior to each shutter closure the 
strobe is actuated to flood the film frame with a very short uniform flash 
of visible light. The IR beam continues to impinge on the film during and 
after the time when the film is flashed with visible light. However, any 
IR energy striking the film after the visible flash is essentially 
impotent and does not affect the resulting photographic record.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 of the drawing is a schematic illustration of an actual IR sensor 
and recording system that was used in a downrange ground test of a gas 
dynamic laser. The IR output beam of the gas dynamic laser was sampled in 
a known manner employing diffraction techniques to provide a relatively 
low power IR beam that is a faithful rendition of the relative intensity 
and absolute phase of the sampled laser beam. Beam 10 shown in FIG. 1 is 
the sample beam and it is directed at a large collection mirror 12 which 
reflects beam 12 to the sensing and recording system 14. System 14 
consists of a portable optical table 16 surrounded by a visible light 
tight enclosure made up of four vertical walls 18, 20, 22 and 24. The 
enclosure is shown with its top cover (not shown) removed so as to 
disclose the position of other system components mounted on optical table 
16. The floor of the enclosure is provided by optical table 16 which rests 
on a flat surface that seals off all mounting holes in table 16. IR 
radiation enters the enclosure through a germanium window 25 mounted in 
wall 20. The IR radiation is focused on the film plane by a ZnSe lens 27 
mounted on the optical table by means of a conventional lens holder (not 
shown). 
Camera 26 is a high speed (144 frames per second) 35 mm step and index type 
movie camera, Model GC, manufactured by Mitchell Camera Corporation. The 
camera has a Redlake Model 15-0001 speed control designated by numeral 28. 
A detachable film container 30 is loaded with several hundred feet of 35 
mm, KODAK 5369, silver halide film. A shutter mechanism (not shown) 
located behind side 32 of the camera has an opening therein through which 
radiation may pass to reach the film plane (not shown) located inside the 
camera. Since the camera is well known its operation and construction will 
not be described in complete detail except for the timing circuit 
modifications shown in FIGS. 2 and 3 that was added thereto to make the 
camera capable of providing a timing signal just before each shutter 
closure. 
A timing mechanism 34 is mounted on the side of the camera in the manner 
shown in FIG. 2. A chopper wheel 36 having a suitable number of holes 38 
formed at spaced intervals around the periphery thereof is mounted on 
shaft 40 by bolt 39. Shaft 40 was available on the camera and it rotates 
with the camera shutter operating mechanism when the camera is operating. 
Wheel 36 thus rotates with shaft 40 when the camera is operating. The 
position of wheel 36, and holes 38, relative to the shutter mechanism of 
camera 26 can be adjusted by loosening bolt 39 and rotating wheel 36 
relative to shaft 40. This makes it possible to adjust the timing of the 
triggering signals from timing mechanism 34 as desired. In some 
applications it may be desirable to replace the circular openings in the 
chopper wheel with rectangular slots to obtain more precise chopping of 
the radiation passing through the openings in the wheel. This function 
will be described more fully hereafter in the description of how the 
invention operates. A support block 42, suitably mounted to the side of 
camera 26 adjacent shaft 40, includes two support members 44 and 46 that 
extend to a position adjacent the periphery of chopper wheel 36. A light 
emitting diode 48 is mounted on the end of support member 44 and a photo 
diode 50 is mounted on support member 46. 
Referring again to FIG. 1, the timing mechanism is connected to a 5 volt 
Hewlett Packard Model 6220B power supply 52 and to a strobe light 54, 
which is a General Radio Company Strobotac type 1531-2B. Strobe 54 is 
mounted to the optical table 16 and oriented so that it floods the film 
plane of camera 26 with uniform field of visible light when triggered by 
timing mechanism 34. The power supply 52 and strobe 54 are connected to a 
bus bar 56 mounted on the optical table. Speed control 28 of the camera is 
connected to a source of 110V power 58 which is also connected to bus bar 
56. 
The timing mechanism and strobe operates as follows when camera 26 is 
running and power is applied to the system. As shown in FIG. 4, light 
emitting diode 40 is connected to the 5 volt power supply through resistor 
60 and to ground so that it emits radiation in the direction shown by the 
arrows. This radiation passes through holes 38 in chopper plate 36 and 
strikes photo diode 50 causing it to conduct and apply a signal to the 
base of transistor 62. This turns transistor 62 on and connects the strobe 
to ground. Grounding the strobe causes it to flash. Thus it is possible to 
trigger the strobe at desired intervals by selecting an appropriate 
diameter and hole spacing in the chopper wheel and then adjusting the 
chopper wheel relative to shaft 40. 
The overall system operates in the following manner assuming synchronized 
start up of the laser being tested and system 14. IR laser beam 10 enters 
the light tight enclosure via window 25. The beam passes through focusing 
lens 27 and on through the opening in the shutter mechanism (not shown) to 
strike the film plane of the camera. Normally side 32 of the camera has 
several lenses mounted thereon that may be rotated over the opening into 
the shutter mechanism for focussing purposes, but they are removed for 
this application. The ZnSe lens 27 images the IR radiation reflected by 
large diameter parabolic mirror 13 onto the film plane of the camera. 
Mirror 12 provides a large area in which the beam may jitter and still be 
directed into the clear aperture of lens 27 and imaged onto the camera 
film plane. The shutter speed and film indexing rate of the camera are 
adjustable so as to control the frames per second exposed and the length 
of time each frame is exposed. In the actual downrange laser test to 
demonstrate the operability of the invention a film indexing rate of 100 
frames per second, and a shutter opening time of 1 millisecond was 
employed. This exposure time was fast enough to freeze out motion induced 
blur. The IR radiation strikes the film frame during the entire period the 
shutter is open and sensitizes those areas where the IR radiation 
impinges. Just before shutter closure, and after almost 1 millisecond of 
IR exposure, the strobe is triggered. The radiation therefrom enters the 
camera and exposes the film frame to a 2 microsecond flash of visible 
light. It was determined that the IR energy coming in after the visible 
flash was essentially impotent and did not further sensitize the film. In 
preliminary investigations a 2 millisecond flash from a flash lamp was 
used to expose film to visible light. It was found that transitioning to a 
shorter light pulse (the 2 microsecond strobe) resulted in a dramatic 
increase in modulation of the film. The photographic optical density of 
the developed beam image was increased by the shorter visible exposure. 
This result is illustrated in FIG. 5. This means that the area of each 
film frame exposed to IR radiation is darker when exposed to a short pulse 
of visible light rather than a longer pulse. 
This completes the detailed description of the invention. The particular 
embodiment disclosed herein was designed for use on a particular laser 
system; however, since the invention's operation depends on thermal 
effects, it is intrinsically scalable to permit use with laser systems 
whose outputs are shorter wavelength IR beams.