Some Aspects of Scanned Reference Beam Holography J, C. Palais and I. C. Vella

College of Engineering Sciences, Arizona State University, Tempe, Arizona 85281. Received 6 October 1971.

In this letter we are concerned with limited hologram exposure times due to stability of the components or due to object motion. Some new aspects and experiments using scanned beam holo­graphy1 are presented.

The hologram construction is arranged in the conventional manner with losses minimized so that all available light is used. The beam ratio is set at the optimum value, the light intensities are measured, and the exposure time is computed. If the re­sulting exposure time is too large, the reference beam is condensed to illuminate a smaller region of the photographic plate. This in­creases the reference beam intensity and decreases the exposure time. The reference beam is scanned over the photographic plate and successive exposures made to fill the desired area. If the problem is one of vibration with displacements on the order of tens of microns or less, the various exposures can be made with long times between them. If the object is moving, the total elapsed period for making all the exposures must be short enough such that the largest distance traveled is less than that resolvable by the viewer. Consider that an object moves at velocity v. Choosing the criteria that the object must not move more than a tenth of a wavelength during any individual exposure, at wave­length λ the maximum exposure time is te =λ /10v. If the desired resolution is r meters, the total time for the complete exposure of the entire hologram plate is te = r/v. This means the hologram can be divided into te/te = 10r/λ segments at most. For example, if λ = 0.63 μm and r = 100 μm, the total number of segments can be as much as 1590. Numbers approaching this would be pos­sible if the time between exposures is negligible compared to the individual exposure times. A stepper motor Can be used to rotate or translate a mirror in steps to accomplish this result with a cw laser as the source. This arrangement minimizes the problem of unwanted reference beam phase shifts due to scanning. The Geneva starwheel used to advance motion picture film in steps could also be used to move the mirror. If a repetitively pulsed laser, with pulses as short as the minimum exposure time, is used, the mirror can be continuously moved.

The resolution problem can be eased by vertical scanning of horizontal lines so that both eyes of the viewer "wall see the same

Fig. 1. (a) Line scanned hologram and (b) its reconstruction.

February 1972 / Vol. 11, No. 2 / APPLIED OPTICS 481

exposure or group of several exposures. Using this technique, horizontal motion of the object parallel to the photographic plate would be evidenced by tilting of any vertical lines in the object rather than the more objectionable double or blurred image. Although a double image of 100 μm might be detectable by the eye, it is unlikely that a tilt of 100 μm along a 1-cm vertical length of object would be significant to a viewer. (Possibly this effect could be useful in measuring the motion of an object.)

A line scanned hologram is illustrated in Fig. 1(a). It is made up of ten 1-mm high segments. The segment length is 8 cm. The reconstructed image, shown in Fig. 1(b), exhibits all the three-dimensional properties of a conventional hologram. The image is clearly viewable with two eyes. The object beam il­luminated the entire hologram during each exposure. The object was not in motion. The Agfa-Gevaert 10E75 plate was hyper-sensitized by bathing in Photo Flo for 5 min at 20°C and drying overnight. For point sources we measured that this procedure

reduced the optimum exposure by a factor of about 3 while negUgibly affecting the noise or contrast of the image. The ex-posure used was 1.75 μJ/cm2. With a conventional construction the exposure time was found to be 6.2 sec. By condensing the reference beam the scanned exposure time was 0.62 sec. The re­duction in exposure time by a factor of 10 results in a lowering of the efficiency by this same factor. The combination of hyper-sensitization by bathing and scanning gave a total reduction of 30 for the individual exposure times as compared to a conven­tional hologram.

This work was supported by U.S. Air Force Office of Scientific Research under Project Themis.

References 1. J. C. Palais, Appl. Opt. 9, 709 (1970).

482 APPLIED OPTICS / Vol. 11, No. 2 / February 1972

These photographs of Frank Cooke were taken by Tom Vogl of Westinghouse Research Labs at the Panel on Optical Fabrication and Testing, at which session of the OSA Chateau Laurier meeting last October Cooke demonstrated

his optical flat lapping machine.