DaveE said:
For 532nm it's the doubler xtal that's both the expensive and hard part.
The doubler crystal (typically KDP) requires a laser source to produce frequency-doubled green laser light. A diode pumped solid-state (DPSS) laser diode provides this at microscopic scale in a standard transistor-style package producing about a watt or so our optical power. It is not as nice, nor as powerful, as the rig used by the APOLLO group for Lunar Laser-ranging: it produces multiple output beams in an elliptical pattern, with multiple transverse modes of emission, rather than just one clean TEMoo green output beam. Some optical manipulation required before sending diode laser light toward the Moon.
I mentioned the possibility of repurposing a used tattoo removal laser if a DPSS laser diode should prove to be impractical. But you are correct regarding a DIY frequency-doubling crystal, such as KDP. No way a ham wants to get involved with such esoteric details unrelated to amateur radio. Some would call that "jumping the shark" but I call it a desperate last resort. I would probably just go do something else and give up on optical-eme if that was my only alternative. It's fine if someone else wants to give it a go... maybe a fellow ham in VK-land will do it. It's a big continent, and optical-eme could turn out to be a useful tool "down under" even if comms with the antipodes is impractical because of geometry.
I have no intention, nor the need, to duplicate what was done for lunar laser-ranging. It will be difficult enough to add digital modulation, using microsecond-wide PCM data bits to a short, millisecond, transmit window at a low duty cycle. I don't anticipate needing to leave the laser beam on for extended periods of time.
I also do not expect to receive enough signal reflected from the Moon to perform analog ranging operations using coherent multiplication of swept (chirped) RF modulation of the transmitted and received signals. The Moon is not a static target and the atmosphere is always somewhat in a state of turmoil and changing in density. These two things alone preclude a long-duration RF chirp being useful. This is a communications problem, not a ranging problem
per se.
We do need to know an approximate range (expressed as a round-trip signal delay) to align delayed receiver bits with the previously transmitted bits. At one megabit per second bit rates, knowing the delay to within a few hundred nanoseconds should be sufficient. The bit-rate can be lowered, if necessary to accommodate errors in delay measurement, but I think one megabit per second is a reasonable goal.
Approximately one millisecond of transmit window, followed by an identical period of non-transmitting, will be all that is needed for a short, repeated, message to be recovered from the noise after a sufficient number of repetitions. The time in between these two intervals is irrelevant if a bi-static rig with separate transmitting and receiving telescopes is used, but this "dead" time interval does determine the transmitter duty cycle and affects the average power transmitted.
For reasons stated elsewhere, the size of the transmitting telescope is not as important as the size of the receiving telescope. What IS important is the amount of transmitted power (affects the number of photons per unit time that are sent to the Moon) and the receiver aperture area (affects the number of photons per unit time that are detected back on Earth).
Baluncore said:
Ionospheric reflection is measured by ionosondes, that sweep from about 1MHz to 15MHz, at a rate of about 100kHz per second.
I am familiar with using swept-RF (a chirp) to create, in effect, a narrow-width ranging pulse. I wasn't aware of its application to measuring ionospheric height, which is presumably the purpose of the ionosondes you mention. Fine business that, but I doubt it is relevant to measuring the round-trip delay of signals bounced off the surface of the Moon.
There is only slightly (veeery slightly) more than zero signal returning to Earth from laser radiation sent to the Moon. This is true for any laser power currently available to the public. The APOLLO group, whose intent was the very precise measurement of the round-trip delay time of their pulsed laser beam, did not use chirp RADAR techniques because it is a non-starter. Only statistical photon counting techniques are viable.
Why? Because of the vast distance involved; the poor reflective properties of the Moon's regolith; the severe loss of signal upon retro-reflection using arrays left on the Moon for that purpose; and its propagation as an expanding beam back toward Earth is why.
My proposed communications solution involving PWM pulses, encoding a message that is repeated millions or more times, is one possible, previously demonstrated, way to solve the communications problem.
The measurement of the delay between transmitted pulses and received pulses is required to allow transmitted bits to be aligned with received bits, after accounting for the delay. It does not require picosecond resolution or accuracy.
Baluncore said:
For optical ranging of the Moon
I am not interested, except peripherally, in optical ranging of the Moon. That is the job for professional astronomers with big telescopes and powerful lasers. I am interested in using the Moon as a passive reflector of communications signals. Folks like the APOLLO group demonstrated "proof of concept" by sending a laser beam to the Moon and receiving reflected information (the round-trip time delay) back here on Earth.
Amateur radio operators have been doing so-called "Moonbounce" comms for over fifty years at VHF, UHF, and microwave wavelengths using Yagi-Uda antenna arrays and microwave dish antennas. I read about one ham who does this with a hand-held antenna and JT8 wsjt-x digital modulation. The only thing "new" about it, or the modulation techniques used to perform it, is the operating wavelength.