[agoodthrowaway]

Unfortunately FMCW lidar is sweeping the lasers across the same band. You don’t need the exact frequency just a beat frequency that’s within your detection Bw.

Also balanced detectors have something called common mode rejection. This is not infinite. In high volume applications it’s difficult for this to be >25dB but you can buy some devices >35dB.

Given that Lidar dynamic range is ~100dB you will definitely see the DC. I’ve not thought about this too much but it seems like an issue for the AGC as your demodulator won’t be bothered by it.

[kyzyl]

It's true that the laser frequency is sweeping, but it very well may not be over the same band. The sweep bandwidth in a typical lidar is likely in the 1-10GHz range. The carrier frequency of the laser that this modulation is riding on is probably in the neighborhood of 200THz. Let's say you're using a telecom laser at 1550nm. The actual wavelength of that laser will centered on some channel in the 1530-1580nm band, with each channel spaced by say 100GHz. So already each laser might intentionally be in a different channel, depending on chance and how many cars are there. But even if they are in the same channel, the chirp bandwidth is small compared to the channel bandwidth, so there will likely be at most only partial overlap, depending on where the respective center frequencies actually are. Unless your lidar is using a very expensive, very fiddly laser system, this center frequency will be drifting around within the channel all the time. It varies with temperature, mechanical stress, output power and a bunch of other stuff, depending on the type of laser. However, even if the lasers are magically in the same channel, and perfectly locked to the same center frequency, you still need the light be coherent to produce an interfering RF signal. They will not be coherent.

Certainly the balanced detectors will have finite CMRR. In general you definitely have to make a good detector but it doesn't need to reject to 100dBc. A photodiode might have 100dB of dynamic range, but most likely your RF front end does not, and more importantly for most applications you will be dominated by photon shot noise, so you don't need to push common mode signals all the way to your electronic noise floor. 35dB of rejection works wonders.