Patent GB-1367922-A (GB)
Title: Adaptive power redistribution systems
IPC: G01S17/02
Filed: 1971-11-24
Abstract: 1367922 Lasers HUGHES AIRCRAFT CO 24 Nov 1971 [24 Feb 1971] 54548/71 Heading H1C [Also in Divisions H4 and G1] In a laser transmission system in which energy at optical frequencies is transmitted along a number of paths 14a-14i each path having a distinctive modulation applied thereto by modulators 22a-22c, the system power transmission efficiency is measured by receiving at 40 (e.g. via reflectors 16, 34, 36) a portion of each beam or path and utilizing modulation components in the received energy to provide a separate signal for each path indicative of the transmission efficiency of each path. Radiation for the three paths 14a-14c is derived from a laser 18. Each path contains a phase shifter 22a-22c (e.g. a movable mirror or an electro-optical device) driven at respective frequencies # 1-3  by oscillators 52a-52c, and a variable gain laser power amplifier 24a-24c, e.g. of the electronically pumped CO 2  type. The beams directed from apertures 26a-26c, which may include telescopes, are received at photodiode mixer 40 where they are heterodyned with the signal from a laser 42. After amplification the envelope of the amplified signal is applied to passband filters 48a-48c. Considering just channel 12a, its filter is centred at 2# 1 , # 1  being the phase modulation frequency of the transmitted beam 14a. It is shown in the Specification that if the phase of the beam is such that the beam intensity attains one of its cyclically occurring maxima at the target 16, Fig. 4 (not shown), then the amplitude-modulated component of the reflected beam at a frequency 2 # 1  is proportional to the transmission efficiency of the path. Accordingly the output from filter 48a and hence from PSD 50a is indicative of the transmission efficiency of the path 14a. The Specification also shows that the maximum power transmitting efficiency of the entire system is achieved when the gain of each amplifier 24a-24c is proportional to the transmission efficiency of the corresponding path. Accordingly the output from PSD 50a is supplied to a power control unit 28a which also receives, via normalization unit 72, the outputs from the PSD's 50b, 50c of the other channels. In this way each control unit 28 adjusts the gain of its corresponding amplifier 24 to maintain the system efficiency at a maximum. A control unit 28 is shown in detail in Fig. 2. The computation unit 74 receives the output A m  from the corresponding PSD 50 and also the output B from the normalization unit 72. The output W m  from 74 feeds a sample and hold circuit 76. During "control" periods the output from 76 is fed via switch 78 to excitation control unit 80, which controls the gain of the associated power amplifier 24. During "calibration" periods unit 80 is responsive to the output from a reference source 82. The reference source signal is the same in all channels, so that all power amplifiers 24 have the same gain during "calibration" periods, which allows a meaning- ful comparison of their relative-contributions to the total received energy. The "calibration" and "control" periods alternate, and during "calibration" periods unit 74 computes the current value of W m  which is stored at 76 until the next "control" period. The relative duration of these periods is governed by monostable multivibrator 86, which controls circuit 76 via differentiator 88, and by monostable multivibrator 84. Both multivibrators are controlled by clock pulses from a synchronization unit 70. In order to maintain the correct in-phase condition at the target each channel may be provided with a phase control feed-back loop, Fig. 5 (not shown). An additional filter (90a) centred at # 1  receives the output from the envelope detector 46. The filter output is supplied to a further PSD (92a) where it is phase detected against a signal at # 1  from oscillator 52a. The signal from the PSD (92a) is then summed with the reference signal # 1  in a transformer device (94a) whose output is applied to the phase shifter 22a. In this way it can be shown that the correct phase condition is maintained.