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value of S. If the exit end of the detector is cut for the laser beam incidence at the critical angle (14.5"), then S= (1-F') =0.64. The term H arises from the phonon absorption in germanium [4], which depends on the hole density P and the hole density P' ( = 7 X 1018 cm-r) a t which the absorption equals the phonon absorption (K' = 0.1 cm-1).The choice of resistivity is rather an importint design parameter for increasing the detectivity. The resistivity p is inversely proportional to P and p, and also the absorption is proportional to the doping level P. Thus the value of K needs to be optimized by (aQ/aK) =0, which on substitution givesThe value of L is chosen to be 2 cm, which provides a good experimental convenience. Equation (2) yields the optimum value of K=O.28 cm-1. At this value of absorption, P=4XlO14 cm-8 and p = 8 Q.cm (obtainable from [SI and [6]). FABRICATION AND PERFORMANCEDetector bars of suitable size are cut from single crystals of germanium, and the bar ends are polished to about 1-pm finish. Ohmic contacts need to be put as close to the bar ends as possible. Reliable ohmic contacts have been made using a flux of 3:l mixture of saturated zinc chloride solution and concentrated hydrochloric acid, and directly soldering onto the germanium with the usual lead-tin solder. Finally, the detector is rigidly mounted in a brass holder and the signal is obtainable via a BNC connector.For detecting the Q-switched laser pulses, the response time of the detector (governed by its RC time constant) need not be better than 40 ns. Detector bars of 6-8 Q.cm resistivity and about +by 3by 20-mm size are used, and the exit end of the detector is cut for critical angle incidence of the laser beam. Since the resistance of the detector is around 5-7 kQ and the capacitance is around 10 pF, the RC time constant of the detector is -40 ns. The responsivity of the detectors [as calculated from ( l ) ] is 60-80 mV/kW. The Q-switched laser pulses of about 500-W peak power have been directly measured using a Tektronix 545A oscilloscope. Rise time of the laser pulses is -120 ns and the detector faithfully displays these pulses. Actual responsivities of the detectors are found to be 40-60 mV/kW, which are in reasonable agreement with the calculated values. The photon-drag EMF generated in the aforementioned detectors can be suitably amplified by about 60 dB and then the overall responsivity becomes -40 mV/W. A chopped CW laser signal of about 0.5 W is thus measurable. The noise in the amplifiers, however, limits the detectivity. Investigations are in progress along these lines.Laser pulses of subnanosecond duration are expected from the mode-locked TEA COZ lasers [3]. The output of such lasers is in the megawatt range and hence the responsivity optimization becomes unimportant. To obtain fast response, the detector resistance must be low, and preferably 50 17 to match the connecting coaxial cables. Detectors suitable for this purpose are fabricated with bars of dimensions 2 by 2 by 20 mm of l.0-Q.crn p-type germanium. The openc...

The intermodulation properties of a memoryless non-linear amplifier with two carriers having large level difference are in the limit determined by the properties of the amplitude characteristic of the amplifier at a single point, i.e. at the operating point determined by the stronger carrier. The paper derives the mathematical expression for the level of the dominating third-order intermodulation product, and also shows how this level can be determined graphically from a given amplitude characteristic.

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