[elihu]

I would expect L1-to-earth communication to be problematic because you'd have to distinguish the signal from the background radiation of the sun.

It'd be interesting to know what the technical limits are in terms of output power and aim/focus. Generally, doubling distance means the signal power drops to 1/4th, and maximum data capacity of a communication link is proportional to the signal/noise ratio. So that would mean a 100 Gbps link might drop to 25 Gbps. You might be able to bring the signal/noise ratio back up by using a better detector or a more powerful laser, or aiming better. Or maybe the 100 Gbps data rate is limited by the transceiver, and there's actually plenty of S/N ratio margin that can be traded for range without affecting data rate at all.

[wmlavender]

If L1 is a problem, then L2 would be a problem as well. After all, at L2 the spacecraft is receiving its commands from the direction of the sun.

However, the problem is not quite as bad as it seems. Spacecraft at L1 and L2 Lagrange points actually are in a halo orbit that "orbits" around the Lagrange point. Attempting to stay at exactly the L1 or L2 point is unstable, since gravitational forces tend to knock you away from that point. The halo orbits are much more stable. And for a spacecraft in a halo orbit, you never have to point your antenna directly at the sun.

The problem is solvable for radio communication at least. There are currently 4 spacecraft orbiting the Earth-Sun L1 point (ACE, DSCOVR, SOHO, and WIND) as well as 3 spacecraft at the L2 point (Gaia, James Webb, and Spektr-RG).

[sansseriff]

Optical to Orion (O2O) is a plan to do a lasercomm demo on one of the future Orion moon missions.

When the Psyche spacecraft launches and heads to the asteroid belt (was supposed to launch in august) it will do the farthest (by far) lasercomm demo. I work in the group that made the SNSPD ground receiver. As my boss says, with a distance 1000x farther than previous space laser comm demos, closing the link is 1 million times harder...

Fun fact: when the Phyche comm laser is pointed at earth, the size of the spot will be roughly as large as California. Even with the largest optical telescopes, the loss in this link will be insane. That's why you need single photon detectors.

As you get to farther and father distances, one thing you can do is shift from on/off keying to large-M Pulse Position Modulation. This way you can save up the power on your satellite to send fewer but higher power laser pulses, each of which carries more bits of data. I believe the DSOC mission will go up to M=256. Meaning each pulse of photons received on earth will carry 8 bits of information based on when it arrives within an alphabet of 256 time bins.

[cycomanic]

The issue is not so much background radiation (you'd have similar issues with RF), and your SNR is going to be reduced because of diffraction (as you rightfully point out goes at r^2). However the SNR would still be much better for optics because diffraction scales as 1/lambda.

The reason why JWST did get an optical link is that people developing these things are rightfully conservative and optical links in space are really still under heavy development.