William D. Gunter

Optical devices to increase photocathode quantum efficiency

Maximum quantum efficiencies attainable with commercially available semitransparent photocathodes are near 30% (for blue light). Work in the Instrumentation Division at Ames Research Center, NASA, has achieved quantum efficiencies as high as 58% with these same photocathodes through the use of optical enhancement. Improvement ratios in the red and near ir are even larger, but of course, the original sensitivities are smaller at these wavelengths. In addition to simply improving sensitivity, effort is also directed toward extending the applicability of the technique. A new class of devices recently conceived at Ames Research Center will allow application of optical enhancement to TV camera tubes, image intensifiers, and other imaging detectors. This paper describes the optical enhancement work in detail with major emphasis on results in the areas mentioned above.

Enhancement of photomultiplier sensitivity by total internal reflection

Photomultiplier tube sensitivity enhancement with very thin semitransparent photocathode and total internal reflection

Dual Enhancement of Photocathode Sensitivity in the Near Infrared

Near IR sensitivity enhancement by external field assisted photoemission from semitransparent photocathode

New properties of afocal lens pairs with Gaussian beams

Some unexpected properties of Gaussian beams when they are used with afocal lens pairs are presented. It is shown that the output confocal parameter is simply equal to the input confocal parameter multiplied by the square of the magnification of the afocal lens pair. It is not a function of the location of the input beam waist. Applications of these analytical results are discussed in a general way, and one particular application is discussed in detail.

Increased Ultraviolet Sensitivity in the Sodium Salicylate–Photomultiplier Combination

For many years, the sodium salicylate-coated photomultiplier has been used as a detector of uv radiation.When used to detect weak signals, the multipliers are often cooled to reduce dark current and the coating thickness optimized to increase sensitivity. The purpose of this letter is to suggest that , a t least for cases of favorable geometry, an additional increase in sensitivity can be gained with little effort. Sodium salicylate emits approximately as much light away from the photomultiplier as it does toward it. If part of the light normally lost is directed back toward the photocathode, an increase in sensitivity results. In order to demonstrate this effect, light from the exit slit of a monochromator was allowed to pass through an aperture cut in a hemispherical reflector. The reflector was fastened to a sodium salicylate-coated photomultiplier as shown in Fig. 1. The thickness of sodium salicylate used was 1.0 mg/cm , shown to be a reasonable choice for normal use. This was sprayed on from a methanol solution and thickness was determined by weighing. The photomultiplier tube was an E M I 9514 B (5 cm diam) connected as a diode, with 400 V between the cathode and the rest of the tube elements. The reflector was a hemisphere (2.5 cm i.d.) formed by standard glassblowing techniques. A 0.64 cm × 1.5 cm aperture was cut to admit the light. Then the inside surface of the hemisphere was aluminized. Measurements of dark current, photocurrent with reflector, and photocurrent without reflector were then made at a number of wavelengths. Dark currents were in the range of 10 1 0 A to 1 0 u A, and photocurrents were in the range of 10 8 A to 10 1 0 A. Estimated error for the data is 3 % . Values shown in Table I represent the increase of photocurrent with reflector over photocurrent without reflector expressed as percent. The increase in output was about 3 5 % for the wavelengths at which tests were conducted. An upper limit of 100% improvement, corresponding to capturing and using all the light emitted in the reverse direction, would be degraded by losses through the aperture, losses in

Spectral dependence of deep-space communications capability

A simple model of communications capability projected for the mid-1970s is formulated. The spectral dependence of the future state of the art is examined in terms of antennas, noise, diffraction effects, etc. The model suggests that the frequency spectrum in the vicinity of 1010 Hz would allow maximum information transfer and that radio-frequency techniques may be superior to the laser.

Fiber optics transmission of LV signal.

The first use of a long optical fiber for transmitting megahertz frequencies in a laser velocimeter (LV) receiver system is reported. The fiber comprises a 600-micron diameter fused silica core, a silicon polymer cladding and a plastic jacket. The fiber numerical aperture is 0.22, corresponding to a maximum entrance half-angle of 0.22 rad. The 10-m length used results in a 5.6% attenuation loss. The fiber is found to transmit an 80-MHz signal with excellent resolution. It is established that an LV receiver using fiber optics sends a clean signal in electronically noisy and high-pressure environments and allows velocity measurements in places too small for a photomultiplier tube.

Optical enhancement of photomultipliers at ultraviolet wavelengths.

Optical enhancement of photomultipliers extended to UV wavelengths, using suprasil and spectrosil fused silica with high flat transmission curves as optical materials

Laser Doppler Velocimeter Development

A new two-channel laser Doppler velocimeter developed for the Ames High Reynolds Channel No. II is described. Design features required for the satisfactory operation of the optical system in the channel environment are discussed. Fiber optics are used to transmit the megahertz Doppler signal to the photodetectors located outside the channel pressure vessel, and provision is made to isolate the optical system from pressure and thermal strain effects. Computer-controlled scanning mirrors are used to psoition the laser beams in the channel flow. Techniques used to seed the flow with 0.5-¿-diam polystyrene spheres avoiding deposition on the test-section windows and porous boundary-layer removal panels are described. Preliminary results are presented with a discussion of several of the factors affecting accuracy.