Actually, it’s worse the R[sup]2[/sup] losses. It’s actually R[sup]4[/sup], because you have the inverse-square loss both coming and going. So a target at twice the range has only one-sixteenth the return.
All active remote sensors face this loss (radar, sonar, lidar, etc). You make up for it in two primary ways: 1) a brighter source through more power and better directionality, and 2) a more sensitive receiver.
There’s often a third way – modulation of the signal. This allows you to discriminate it from noise and from other signals of the same wavelength, and to use phase-lock methods to aid detection.
Yup, though I don’t know how much help it would be in a situation where you’re essentially doing single photon counting. How much can you modulate that?
(And here my complete lack of background in signals comes into play. For all I know, you can encode enough information onto a single photon that if you detect one, you’ll know exactly what it hit, when it hit, how it hit it, and what it had for breakfast.)
Yeah, to keep things simple I didn’t mention processing gain, but that is an important method to improve signal to noise. You can do some truly amazing things with the right signal processing.
The phase information that most signal processing makes use of is the Heisenberg conjugate of particle number. So, like position and momentum, there’s a fundamental limit to how precisely we can simultaneously measure both the number of particles and the phase of the waveform. This means that traditional signal processing is useless in low-photon situations.
Actually, w.r.t. making a laser visible using dust / mist / whatever, wouldn’t this set off the device?
After all, the detector would be receiving significantly less light.
Or are they set up to ‘fire’ only if the beam is completely blocked?
Oh, and, I’ve thought of another: could I in theory walk into a room with laser sensors with a very bright light source of the same wavelength as the lasers, and cross the beams (i.e. with my EM source keeping the detectors from triggering)?