Variable Attenuator
This is a tough one, with no immediately obvious path to successful completion. We need the capability to vary the attenuation of the receiver from virtually clear to perhaps O.D. 2.0 in a matter of 20 ms or so. This is because we would be lucky indeed if the corner-cube return from within the telescope enclosure happened to be the exact right signal strength (a few photons per pulse) given that the total attenuation will be something like 10 orders of magnitude. It is more likely that we will have too little attenuation than too much. Thus a variable attenuator within the receiver both cuts down the corner-cube return and helps to eliminate scattered laser light from hitting the detector. See below for a likely position.
I saw such a device at the McDonald Laser Ranging Station, pittured below. Perhaps we should ask Jerry Wiant where he got his. Though the picture is fuzzy, you can make out that the aluminum holding block is about 3/8 of an inch thick, making the PLZT something like 0.25-inch thick and maybe 2-inches square (this holds with my memory as well).
The PLZT takes something like 300 volts—I think to make it clear. According to Jerry, they "like" to be modulated, though he applied D.C. to his. The way I imagine this working for us is: we would send a TTL logic signal to indicate when the PLZT should be clear vs. opaque, plus an analog voltage (0-10 V) to indicate the amplitude of modulation. This would then be scaled to the appropriate high-voltage amplitude and appled to the PLZT.
Of course all this is moot unless we can get our hands on a PLZT (or steal Jerry's). We would also do well to look at alternatives that would accomplish the same thing. Fast turn-on is advantageous but not absolutely necessary. If the thing were fast, we could base the logic on the laser fire request, which comes some 200-300 microseconds before the light.