Modulated TEA Laser Receiver

This invention is in the field of receivers for information modulated onto light waves. In particular, it concerns information carried by the "tail" of a TEA laser. Light-wave communications are well known in the art, including those systems employing lasers as the channel. Such systems, when using lasers, are usually point-to-point using low-power, continuous-wave (CW) laser outputs modulated by some electro-optical device. Pulsed laser outputs are not normally used because of the difficulties of modulating extremely short (and often high peak power) output pulses. The pulsed TEA laser, however, has an output envelope which may be used to advantage. U.S. Pat. No. 4,393,517, by the present inventors and one other, shows a TEA laser pulse-code transmitter. This envelope has a fast-rise-time and high-intensity pulse which rapidly decays into a relatively long low-intensity tail. A receiver for receiving CW lasers might be damaged by such a high-intensity pulse, or at least temporarily blinded. The ideal requirements for a receiver of information on a TEA laser output are: low sensitivity during the high-intensity portion of the output, and high sensitivity during the tail thereof. The instant invention meets these requirements.

The invention is a receiver for information modulated onto the tail portion of a pulsed TEA laser. The receiver includes a photodetector which can be biased to either a high-sensitivity or to a low-sensitivity rate. In the absence of a laser pulse, and during the high-intensity portion of the pulse, the photodetector is biased to low sensitivity. When the photodetector detects the high-intensity portion of the laser pulse, a timing circuit operates (after a delay) to bias the photodetector to high sensitivity, and to connect a demodulator to the photodetector output.

The single drawing FIGURE is a schematic diagram of the invention.

The invention may be best understood if this description is taken in conjunction with the drawing. In the drawing, a photodetector 10 is exposed to radiation 11 from a pulsed TEA laser. Although not shown, it should be understood that a telescope or lens may be used to focus 11 onto 10. Radiation 11 will have an envelope (as shown) with a relatively short and high-amplitude tail. The amplitude of the pulse portion is such that it might damage a sensitive photodetector. A voltage -V biases photodetector 10 through choke L and load resistor R.sub.L such that 10 is in a low-sensitivity state during the absence of radiation 11 and during the high-amplitude portion of 11. Photodetector 10 is a photoresistor or photoconductor which varies in resistance (or conductivity) in response to radiation 11, such that the voltage at junction 12 likewise varies. These variations are coupled by d-c blocking capacitor C.sub.1 to signal inputs of gates 13 and 14. Gate 13 is normally closed, and 14 is normally open, such that a signal coupled by C.sub.1 passes through 14, is amplified by amplifier 15, and passes to comparator 16. This comparator compares the output of 15 to a reference voltage V.sub.1 and produces an output when the output of 15 equals or exceeds V.sub.1. The output of 16 feeds both a disable input to gate 14 (to close the gate) and delay 17. The output of 17 operates monostable multivibrator 18, which multivibrator produces an output pulse. This pulse feeds inverter 19, and the enable input of gate 13. The output of 19 feeds through coupling capacitor C.sub.2 and current-limiting resistor R to junction 12, and more highly biases photodetector 10 than it is biased by -V. This higher bias makes 10 more sensitive. The delay in the circuit including 14-19, C.sub.2 and R is such that 10's sensitivity only increases after the high-amplitude pulse portion of radiation 11. The photodetector is thus biased to high sensitivity during the tail portion of radiation 11 while, the information of interest being modulated onto said tail portion. Gate 13, which was enabled by the output of 18, passes the signals from 10 to amplifier 20. The output of 20 is a signal input to comparator 21; V.sub.2 is a reference input thereto. When the signal equals or exceeds V.sub.2, 21 has an output to shift register 22, which stores the signal on the tail of radiation 11. This signal, which may be cryptographically encoded, is decoded by message decoder 23. The decoder is well known in the art, and is not, per se, necessary to the invention. An uncoded signal may be taken directly as the output of 22.

A receiver has thus been described which has a photodetector with a first (low) sensitivity during (and before) the high-amplitude pulse portion of a pulsed TEA laser output, and has a second (high) sensitivity during the tail of the laser output. Each of blocks 13-23 is well known in the art, but, taken together, provide a unique combination. Comparators 16 and 21, as examples, are Schmitt triggers or equivalent.