The use of diffuse reflectance materials in laser pump reflector design can lead to significant improvements in laser performance over reflectors employing more traditional, specular (or mirror-like) reflectors. Diffuse reflectors provide a more predictable and uniform beam profile, and reduced susceptibility to parasitic oscillations. Since laser pumping involves multiple reflections within the pump chamber, the efficiency of a laser pump chamber can be significantly affected by relatively small changes in reflectance. For example, a chamber with a reflectance factor of approximately 99% over the 400 to 1000 nm range, can provide a 15% gain in performance over a comparable 98% reflective chamber, even though the reflectance factor is only 1-2% lower. Much larger gains are possible over typical ceramic reflectors. This paper will examine high performance PTFE as a reflector in laser pump chambers compared to other materials. Gains in performance through reflectance and diffuseness are shown through mathematical models, experimental results and real world case studies.
The velocity of ordinary ("first") sound in liquid helium 11 has been measured at a frequency of 1 Mc/sec between approximately 0.1 °K and 1.7°K under the saturated vapor pressure, using a highresolution technique previously described.^ A signal from a continuously running, crystal-controlled oscillator, fed through a gated amplifier, provides a transmitter pulse which excites a quartz crystal immersed in the liquid. The resulting ultrasonic pulse is received by a second crystal, amplified, and fed into a 6BN6 phase detector, where its phase is compared with that of the cw reference s^nal from the oscillator. To make results independent of the detailed characteristics of the phase-detector circuit, a Helidel^ delay line, continuously variable up to 1 iisec, is used to adjust the phase of the reference signal so that the output of the phase detector is balanced at a null. The total time required for the ultrasonic pulse to traverse the 5.051-cm path used in the experiments was about 210 jLtsec. Changes in this transit time could be observed with a resolution of ±1 nanosecond, corresponding to changes in velocity of ±5 parts per million. In addition to this random error, there may be systematic error amounting to as much as ±2% of the observed velocity difference between two given temperatures; below 0.8°K, where the velocity varies so little, this error is negligible compared with the scatter of the data.Temperatures below 1°K were reached by adiabatic demagnetization, using a cryostat of the type invented by Ashmead.^ Warmup times from the lowest temperature to 0.9°K were typically half an hour. Temperatures were determined by measureing the susceptibility of the iron-ammonium-alum cooling salt with a conventional ac mutual-inductance bridge, and are estimated to be accurate to about 1 %.Values of the velocity derived from phase measurements made with different signal amplitudes, and on different days, are shown in Fig. 1. Only changes in velocity could be observed directly; to convert our data to the results plotted, we have adjusted the values of phase obtained in different demagnetizations so that they agree with one another at the lowest temperatures. The absolute value of the velocity has then been chosen so that at 1.6°K it agrees with the results of earlier measurements.^'^ With the smallest signal the signal-to-noise ratio was poor, and the scatter of the results was larger than for the other runs. The results obtained with the largest signal show a slight systematic deviation from the rest, perhaps as a result of finite-amplitude effects. The observed change in velocity between the lowest temperatures and 1.6°K is 3.66 ± 0.1 m/sec, and the resulting value for the velocity extrapolated to absolute zero is ^^(0) = 238.27 ± 0.1 m/sec. It should be noted that the estimated experimental error in u^{0) does not include any 243
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