Laser light detection system

#This invention relates to the field of pulse detection.

#In accordance with a known technique, short laser impulses illuminating an aircraft during radar surveillance are detected by providing a photodetector which produces electrical impulses which are amplified and fed to the input circuit of an integrator. These impulses consist of signal impulses, one being produced for each burst of laser light detected by the photodetector, together with noise impulses due to ambient light such as sunlight and sunlight reflection from, for example, helicopter rotor blades. Should an integrator integrate noise and signal impulses during a time interval of for example, 0.1 seconds, a ramp voltage is produced having a final amplitude at the end of the interval which is proportional to the integral of the noise and signal impulses of one polarity. Upon the commencement of the receipt of laser signal impulses, the final integrator ramp voltage will increase above a given threshold voltage to provide for the detection of the laser illumination. By applying such ramp voltage to the input circuit of comparator, a comparator output signal may be produced indicating the fact that signal (laser) impulses are present, since the final ramp voltage will exceed a reference threshold voltage.

#However, the photodetector output may change, typically 4-1, between shade and full sunlight, which may cause the integrator to saturate, which in turn causes the comparator output to be unreliable as a high noise level may be mistaken for signal impulses due to laser illumination.

#The sole FIGURE illustrates a circuit diagram of an embodiment of the invention.

#In the sole FIGURE, video input circuit 1 is coupled to input lead 2 of automatic gain control (AGC) means 3 which could employ a field effect transistor. Photodetector 5 converts laser light impulses 15 to electrical signals fed to video input 1. The signal impulses applied to AGC circuit 3 could typically have a duration of 50 nanoseconds and a pulse repetition frequency of 2-kHz, where the present invention is employed to detect certain types of laser illumination impulses. The output signal of AGC 3 is illustrated at 4, and the signal impulses 6 and 7 may be seen riding above the noise impulses 8. The signal impulses, in contrast with the noise impulses, extend positively, but are not negative going owing to the biasing of AGC circuit 3. A first uni-directional impedence circuit 9 and video amplifier 11 are coupled in tandem between the input circuit 12 of first integrator 13 and the output circuit of AGC 3 as illustrated. Such uni-directional impedence means would typically include a diode having a polarity which causes both the positive going signal impulses 6 and 7 and the positive going noise signals 8 to be integrated by integrator 13, but not the negative noise signals. In the contemplated environment of this invention, integrator 13 would integrate during a 0.1 second interval and become reset during a 1 millisecond interval, pursuant to the operation of clock 14 which controls switch means 16; the integrator being reset upon the brief closure of switch means 16. Upon the opening of switch means 16, the aforesaid ramp voltage is produced having a terminal amplitude which is proportional to the amplitudes of signal impulses 6 and 7, for a given pulse repetition frequency. Comparator 18, having reference threshold device 19 such as a potentiometer, produces an output signal should the final amplitude of the ramp voltage represented by pulse diagram 21 exceed a pre-determined threshold as previously explained.

#As set forth above, the photodetector noise output amplitude may change 4-1 between shade and full sunlight, thereby to cause first integration means 13 to saturate to produce a false alarm indication. This problem is overcome by providing second noise integration means 22 which, in contrast with integration means 13, only integrates the noise level rather than the signals.

#Pursuant to this goal, a second uni-lateral impedence device 24 is coupled between the output circuit of AGC 3 via video amplifier 11. Since the polarity of device 24 is opposite the polarity of device 9 with respect to ground, the negative going impulses, which do not include signals 6 and 7, are integrated by the second integrator 22, the output of which is amplified by amplifier 26 and fed back to control lead 27 of AGC device 3. This feedback circuit causes a reduction in the gain of AGC 3 upon an increase in the noise level, thereby to maintain the noise level applied to the first integrator 13 substantially constant. Hence, the afore mentioned adverse effect caused by considerable variations in the level of noise upon integrator 13 is eliminated, and improved sensitivity and reliability is provided.