Baluncore said:
Thank you. I hope I won't bore anyone here.
Baluncore said:
I would suggest that the laser diode be on-off modulated with an RF chirp from a DDS that covers a wideband. The received data can be multiplied by the transmitted data, which is both synchronous detection and down conversion. You will need a stable clock, such as a GPS disciplined frequency reference, so the transmitted and received data can maintain low phase noise.
Yes, the proposed modulation scheme is a form of amplitude modulation that uses digital correlation with noise cancellation to improve a horrendous signal-to-noise ratio. RF chirp is out of the question because I don't have an affordable method of doing that. My Elecraft KX3 amateur radio transceiver uses DDS to cover all the HF amateur radio bands, and it also transmits coherent electromagnetic radiation. I don't know how to do this with laser diodes.
Also, I cannot mix the transmitted laser signal with a local oscillator optical signal to perform down conversion: the laser emission line is too wide and not frequency stable. The stuff we do with radio frequencies is just not practical with lasers that I can afford.
Baluncore said:
Take the FFT of the TX*RX product, so frequency will give you range. If the FFT has one million samples, you will get a noise reduction, due to transform gain of; √n = 1000 times.
I do plan on taking lots of "samples" of the same "message" and "stacking" them to improve the S/N ratio proportional to the square root of the number of identical "message frames" transmitted.
I don't know what performing an FFT of the TX*RX product would do, or if it is even possible to do that. The TX part is well known, since I generate that here on Earth, but the RX part is in the form of individual photons, some of which are signal photons transmitted from Earth, and some of which are noise photons reflected from the Moon by Earthshine, starshine, sunshine from crescent Moon limbs. What distinguishes the two is
a priori knowledge of when the transmitted photons occur, determined by synchronous bit transitions synchronized to an atomic clock with a GPS-disciplined oscillator and also UTC clock derived from same. The received photons are also synchronized, after a very slowly varying delay, to the same oscillator. Once this delay is determined, the received photons will be counted up during the transmission interval and counted down during the following silent interval. The noise photons will average toward zero counts because noise photons are counted up and then down while transmitted photons are only counted up. Therefore transmitted photons accumulate in bit counters while noise photons average toward zero in the same counting bins.
All of this is quite similar to using a light chopper to add amplitude modulation information to a transmitted beam and then using a lock-in amplifier, synchronized to the chopper frequency and phase, to recover the signal. I performed this experiment in the 1970s using a Harshaw Chemical pyroelectric detector for the receiver, a fifty percent duty-cycle mechanical chopper, and a Princeton Applied Research lock-in amplifier.
I placed the pyroelectric detector, without any optics, pointing at a doorway on the other side of the lab, a distance of about fifty feet. The chopper was interposed in the path between the detector and the doorway. With the chopper running, and the PAR lock-in set for about a 100 second integration time, I could detect the presence (or not) or a human being standing in the doorway. Too bad I didn't have two of those early device to play with because I might have discovered passive infrared (PIR) detectors, so ubiquitous today for the important purpose of turning lights on when motion is detected, or opening doors when a person approaches. PIR detectors were the death-kneel for optical interrupters and pressure-sensitive foot pads previously used to open and close electrically operated doors.
Baluncore said:
A low pass filter will eliminate atmospheric backscatter, while things at the distance of the Moon will fall in a particular frequency band proportional to range.
Atmospheric backscatter during frame transmission is a huge problem. It cannot be eliminated by filtering because it is the same wavelength as the laser transmitter. What I can do is mute the receiver while transmitting. This is the normal way that hams perform a QSO: using a single-pole, double-throw relay, the transmitter is connected to the antenna and the receiver is disconnected, and also possibly electronically muted.
A very narrow-band optical filter MUST be used at the entrance to the quantum photon detector to prevent it from becoming saturated with ambient light on the Moon. I found a 532nm inteference filter that is affordable, but my laser emission occurs over a fairly wide bandwidth of +/- 10nm. I cannot afford a laser with a one nanometer wide spectral emission line, much less an interference filter that small a bandpass. And then there is the problem of laser line emission wavelength stability. Gotta keep the laser emissions within the filter band width... maybe with temperature control? But even if that works, it still leaves the problem of getting the laser to emit in the center of the filter's bandwidth response.
When I was a Novice in the late 1970s, I built my transmitter with a NE-2 type of neon lamp that had a third "trigger" terminal. When I keyed the transmitter I also turned on the neon lamp, which was wired in series with a small-valued resistor, across the SB-300 antenna input terminals. The RF output from the transmitter kept the neon lamp lit when transmitting, and that was enough to keep damaging RF out of the receiver. Actually, it was a little more compicted than that because the contact closure of the telegraph key had to be "conditioned" with an RC filter to avoid transmitting "clicks" caused by too-rapid rise and fall times of the circuit. Fun stuff, figuring all that out. As soon as I got re-licensed as an Extra-class (AC8NS) operator I bought an Elecraft KX3 and an Elecraft 100 watt linear amplifier to go with it. But by then I was making a decent income and was about to retire, so DIY rigs were mainly limited to antenna construction.
This type of "listen while transmitting" operation is called full break-in keying or QSK. It was not common during that era, and I never did a QSO with a ham who had similar capability. But I was able to receive signals in between the dits and dahs of my telegraph key while transmitting. Can't do this very well with optical-eme because of the two and half second round-trip time delay, even with a bi-static rig that would easily allow it, maybe even require it because of the backscatter problem.
Baluncore said:
Lasers, aimed at retroreflectors on the Moon, transmit PRBS "ranging codes" that enable precise transit times to be measured. https://en.wikipedia.org/wiki/Gold_code
Yeah, but I am not trying to measure precise transit times. I am only incidentally measuring the distance to the Moon, to determine the time delay required for alignment of received bits with transmitted bits. No need for extreme precision either. With one micosecond wide bits in the message, delay measurement to within a few hundred nanoseconds (NOT picoseconds) will be more than adequate.
The Gold codes might be useful for initial signal acquisition, but I haven't looked further into that. Some help will be appreciated once the transmitter and receiver are bult. The plan is to use an online ephemeris to obtain an initial delay and then "seek" with a software servomechanism the actual delay. This may be as simple as decreasing the "message" bit density by combining adjacent bits, and perhaps manually "tweaking" the delay to maximize the received signal before transmitting the "real" message frames. I am going to try to do this "tweaking" in software after building the transmitter and bi-static receiver. In it's simplist form, the transmitter will remain on during the entire transmit interval, repeating this transmission perhaps at a faster rate so more photons are collected. Lots of experimenting needed to see what works, but again the KISS principle applies.
Baluncore said:
Optical DX on Earth between mountains, uses LED, not laser sources because the laser light rapidly looses collimation in the atmosphere, so 150 watts of LED with a lens is actually more productive than laser.
LEDs have a large-area emitter compared to a diode laser. They do not collimate well because their light output is incoherent. Their main advantage for communications experiments is they do not currently require any sort of license.
As for laser light rapidly losing collimation as it propagates through the atmosphere... this will be true for ANY light source. Nothing special about lasers except optical coherence, and apparently that IS important for collimation as well as efficient power transmission. Anything emitting light through the Earth's atmosphere experiences a minimum of one arc-second of divergence because of the atmosphere's negative focal-length lensing effect.
The Earth is surrounded by a thin layer of gas that is concave at the surface and convex as you move further out into space. We live inside a "weak" spherical lens and nothing can be done about it unless you want to operate in the vacuum of outer space, like Hubble or JWST. Our atmosphere also has a variable index of refraction, affected by turbulance and aerosols, but mainly decreasing with altitude because the atmospheric pressure decreases with elevation above sea level.
I am "pretty sure" those are all good suggestions, but I would like to do optical-eme as simple as possible, so hams worldwide all have a chance to participate. Marconi started with sparks for his transmitters, the Wright brothers with bicycles before building airplanes, and I stand firmly on the shoulders of Claude Shannon and others (such a Joe Taylor K1JT who created wsjt-x weak-signal communication protocols) trying now to send a few zillion photons to the Moon and receiving some of them back for statistical quantum detection.
What could possibly go wrong with that? Some asshat could decide to write a regulation that prohibits optical moonbounce without jumping through a string of buerocratic hoops. I would then have to somehow move into international waters for optical-eme if that were occur, maybe set up on an oil platform? Or on a position-stabilized ship. I wonder how much those gigantic cruise ships rock-and-roll in the ocean waves? Does anyone do serious astronomy while at sea? I bet the salt-water laden air ruins the "seeing". It sure doesn't help me, living within a mile of the Gulf.
Thanks for responding y'all! I apologize for the late reply.