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The invention an optical system and a method for automatically controlling the gain of a receiver in an optical system. The optical system includes an optical receiver, a pulse capture unit, and an automatic gain control. The pulse capture unit includes a capture unit capable of capturing an optical

The invention an optical system and a method for automatically controlling the gain of a receiver in an optical system. The optical system includes an optical receiver, a pulse capture unit, and an automatic gain control. The pulse capture unit includes a capture unit capable of capturing an optical signal received by the optical receiver; and, a process unit capable of processing the captured optical signal. The automatic gain control is capable of controlling the gain of the optical receiver responsive to the content of the processed optical signal. The method includes comparing the intensity of at least one returned pulse, and typically a plurality of returned pulses, to a predetermined value; and controlling the gain of an optical detector responsive to the comparison. In addition, the maximum gain is controlled by a noise limit in some implementations.

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What is claimed: 1. An apparatus, comprising: an optical detector; a threshold unit capable of converting an analog optical signal received by the optical detector to a digital representation thereof; a capture unit capable of capturing the digital representation of the received optical signal; a p

What is claimed: 1. An apparatus, comprising: an optical detector; a threshold unit capable of converting an analog optical signal received by the optical detector to a digital representation thereof; a capture unit capable of capturing the digital representation of the received optical signal; a process unit capable of processing the captured digital representation; and an automatic gain control capable of controlling the gain of the optical detector responsive to the content of the processed digital representation, wherein the automatic gain control drives the gain higher responsive to determining that the intensity of a return pulse in the optical signal is lower than a target value and drives the gain lower responsive to determining that the intensity of the return pulse in the optical signal is higher than the target value, the automatic gain control including: an intensity median computation circuit capable of comparing the intensity of the return pulse to a predetermined value and outputting a first signal indicating the result of the comparison; an up/down counter capable of receiving the first signal, incrementing and decrementing responsive to the first signal, and outputting a second signal proportional to the count therein; a digital to analog converter capable of converting the second signal to an analog signal; and an attenuator receiving the analog signal and attenuating a gain signal responsive to the analog signal. 2. The apparatus of claim 1, wherein the optical detector comprises an array of photodiodes. 3. The apparatus of claim 1, wherein the threshold unit includes a bank of voltage comparators. 4. The apparatus of claim 1, wherein the capture unit samples the digitized representation of the received optical signal and encodes the resultant samples proportionally to the peak of the digitized representation at the time it is sampled. 5. The apparatus of claim 1, wherein the capture unit includes: a time demultiplexer capable of demultiplexing the digitized representation; and a line encoder capable of encoding the demultiplexed digitized representation. 6. The apparatus of claim 1, further comprising a memory in which the capture unit is capable of buffering the captured digital representation and from which the process unit is capable of reading the buffered captured digital representation. 7. The apparatus of claim 1, wherein the process unit includes: a convolution circuit capable of generating a filtered signal; and a peak detect circuit capable of detecting the peak amplitude of the filtered signal. 8. The apparatus of claim 1, further comprising a constant false alarm rate circuit capable of detecting a noise event and clamping the content of the up/down counter. 9. An apparatus, comprising: an optical detector; a threshold unit capable of converting an analog optical signal received by the optical detector to a digital representation thereof; a capture unit capable of capturing the digital representation of the received optical signal; a process unit capable of processing the captured digital representation; and an automatic gain control capable of controlling the gain of the optical detector responsive to the content of the processed digital representation, wherein: the automatic gain control drives the gain higher responsive to determining that the intensity of a return pulse in the optical signal is lower than a target value and drives the gain lower responsive to determining that the intensity of the return pulse in the optical signal is higher than the target value; and the target value is the median intensity value of the processed return pulses. 10. The apparatus of claim 9, wherein the optical detector comprises an array of photodiodes. 11. The apparatus of claim 9, wherein the threshold unit includes a bank of voltage comparators. 12. The apparatus of claim 9, wherein the capture unit samples the digitized representation of the received optical signal and encodes the resultant samples proportionally to the peak of the digitized representation at the time it is sampled. 13. The apparatus of claim 9, wherein the capture unit includes: a time demultiplexer capable of demultiplexing the digitized representation; and a line encoder capable of encoding the demultiplexed digitized representation. 14. The apparatus of claim 9, further comprising a memory in which the capture unit is capable of buffering the captured digital representation and from which the process unit is capable of reading the buffered captured digital representation. 15. The apparatus of claim 9, wherein the process unit includes: a convolution circuit capable of generating a filtered signal; and a peak detect circuit capable of detecting the peak amplitude of the filtered signal. 16. The apparatus of claim 9, further comprising a constant false alarm rate circuit capable of detecting a noise event and clamping the content of the up/down counter. 17. A method for automatically controlling the gain of a receiver in an optical system, comprising comparing the intensity of a returned pulse to a target value, including comparing the intensity to a median value of a plurality of returned pulses; and controlling the gain of an optical detector responsive to the comparison. 18. The method of claim 17, wherein controlling the gain of the optical detector includes: incrementing and decrementing an up/down counter; converting an output of the counter proportional to the content thereof to an analog signal; and attenuating a power supply signal proportionally to the amplitude of the analog signal. 19. The method of claim 17, further comprising gating the comparison. 20. The method of claim 19, wherein gating the comparison includes: enabling the comparison in a time period in which a true return pulse is expected; and disabling the comparison otherwise. 21. The method of claim 17, further comprising clamping the upper bound of the controlled gain. 22. The method of claim 21, wherein clamping the upper bound includes: detecting a noise event; and decreasing the gain. 23. The method of claim 22, wherein decreasing the gain includes: decrementing an up/down counter; converting an output of the counter proportional to the content thereof to an analog signal; and attenuating a power supply signal proportionally to the amplitude of the analog signal. 24. A method for automatically controlling the gain of a receiver in an optical system, comprising: comparing the intensity of a returned pulse to a target value; controlling the gain of an optical detector responsive to the comparison; clamping the upper bound of the controlled gain, including: detecting a noise event; and decreasing the gain; and gating the detection. 25. The method of claim 24, wherein gating the comparison includes: enabling the detection in a time period in which a false returned pulse is expected; and disabling the detection otherwise. 26. The method of claim 24, wherein comparing the intensity of the returned pulse to the target value includes comparing the intensity to a median value of a plurality of returned pulses. 27. The method of claim 24, wherein controlling the gain of the optical detector includes: incrementing and decrementing an up/down counter; converting an output of the counter proportional to the content thereof to an analog signal; and attenuating a power supply signal proportionally to the amplitude of the analog signal. 28. The method of claim 24, further comprising gating the comparison. 29. The method of claim 28, wherein gating the comparison includes: enabling the comparison in a time period in which a true return pulse is expected; and disabling the comparison otherwise. 30. The method of claim 29, further comprising clamping the upper bound of the controlled gain. 31. The method of claim 30, wherein clamping the upper bound includes: detecting a noise event; and decreasing the gain. 32. The method of claim 31, wherein decreasing the gain includes: decrementing an up/down counter; converting an output of the counter proportional to the content thereof to an analog signal; and attenuating a power supply signal proportionally to the amplitude of the analog signal. 33. A method comprising: receiving a plurality of return pulses; generating an optical signal including the received return pulses; capturing the return pulses in the optical signal; processing the captured return pulses; detecting a noise event in the capture of the return pulses; and automatically controlling the gain in generating the optical signal responsive to the intensity of the processed return pulses and responsive to the detected noise event, including comparing the intensity of the returned pulses to a target value, wherein comparing the intensity of the returned pulses to the target value includes comparing the intensity to a median value of a plurality of returned pulses. 34. The method of claim 33, wherein automatically controlling the gain in generating the optical signal includes: comparing the intensity of each true return pulse to a target value; and adjusting the gain of an optical detector responsive to the comparison. 35. The method of claim 33, wherein automatically controlling the gain includes: incrementing and decrementing an up/down counter responsive to the comparison; converting an output of the counter proportional to the content thereof to an analog signal; and attenuating a power supply signal proportionally to the amplitude of the analog signal. 36. The method of claim 34, further comprising gating the comparison. 37. The method of claim 33, further comprising clamping the upper bound of the controlled gain. 38. The method of claim 33, further comprising: generating a train of optical pulses; scanning a field of view while transmitting the train of optical pulses such that, upon encountering an object in the field of view, they are reflected as the return pulses. 39. The method of claim 33, wherein processing the captured return pulses generates a three-dimensional data set. 40. The method of claim 39, further comprising identifying a target from the three-dimensional data set. 41. The method of claim 33, wherein generating the optical signal includes: detecting the received return pulses; and digitizing the detected return pulses. 42. The method of claim 33, wherein capturing the return pulses includes: sampling the optical signal; and encoding the samples of the optical signal. 43. The method of claim 42, wherein capturing the return pulses includes buffering the encoded samples. 44. The method of claim 33, wherein processing the captured return pulses includes filtering the optical signal through a finite impulse response filter. 45. The method of claim 44, further comprising linearizing the captured optical signal.