>> Standard CCD and CMOS cameras can "see" 1570 nm sources -- reason is unclear, since silicon cuts off at around 1200 nm and the absorption depth at 1570 nm is something like many centimeters, way thicker than the 20 um silicon die thickness. One possible explanation is that the SiO[sub]2[/sub] is known for 2nd harmonic generation and that's what we're seeing.
>> The Sensors Unlimited 320M InGaAs imager, with a known cuton around 900nm can "see" 650 nm signals, sort of. Turns out that the InP substrate on which the InGaAs detector is built is responsive to 650 nm, so the generated carriers are able to move into the InGaAs epi layer and behave like normally generated photoelectrons. Since the electrons diffuse quite a ways, it's no good for serious beamwidth measurements, but can be used for beam alignment tasks with 1570 sources.
