Optical-EME Communications at 532nm

[Hop-AC8NS]

How did you find PF?

Member Delta Prime suggested it.

Not sure I really belong here. I started out to be physicist but eventually became an electrical engineer.

After participating in the summer of 1961 at a Physics Colloquium, held at Western Kentucky State College and sponsored by the National Science Foundation, I was looking forward to my senior year of high school in Smyrna TN.

My chemistry teacher had asked me to apply to attend that NSF summer session. It was competitive, but I suppose I did well enough on the SAT test I had to take to qualify. Did pretty well, acing the course and was invited to come back in the fall as an undergraduate student, skipping my senior year of high school.

Didn't work out: girls, girls, girls instead of study, study, study. But after graduating high school in Dayton OH I signed up for a four-year hitch in the Air Force. Then I landed an electronics technician job with the University of Dayton Research Institute (UDRI) in June 1967, one month after my active duty discharge. I still had a two-year inactive reserve obligation but was no longer in danger of getting drafted and sent to Vietnam.

After a year on the job, I was allowed to enroll as an undergraduate in the University of Dayton School of Engineering. Got all my tuition paid by UDRI but it took another ten years to graduate in 1978. Didn't have to use my G.I. Bill benefits, but I did anyway during my final year of study. I stayed with UDRI for another year, accepting a promotion from T4 tech to P1 staff. I had "maxed out" on my T4 salary, so the hundred dollar increase in salary was much appreciated.

I left the "ivory tower" of UD to work for a Department of Defense private contractor. I didn't really want to give up my cushy office and electronics lab, but I saw other techs who had stayed the course and graduated not being treated as "real" engineers because in a field chock full of BS, MS, and PhD holders it was hard to make the transition from technician to engineer and earn the respect of your new peers. I left first chance I got and never looked back. Um, I did attend a class reunion a few years later, but the only person I knew was the doctor who set my broken arm after my motorcycle accident.

I am now 80+ years old and retired from my previous career as a war monger... er, electrical engineer working for DoD contractors. It's been an exciting ride so far, but now I have a "bucket list" project that I could use some help with: optical communication using a visible-light laser, a small telescope, and the Moon as a passive reflector. The "plan" is a proof-of-concept demonstration that extends radio-frequency EME (Earth-Moon-Earth) communications (so-called moon-bounce) into the frequency spectrum above 275 GHz. If successful, this should make ham DX contesting a cinch: the Moon is visible from somewhere on Earth at all times.

 

[Greg Bernhardt]

Welcome to PF!

 

[berkeman]

Welcome to PF, and 73 from a fellow HAM.

I could use some help with: optical communication using a visible-light laser, a small telescope, and the Moon as a passive reflector.

Are you avoiding using the lunar retroreflector mirrors on purpose? What is the loss in optical reflective power for using the retroreflector mirrors vs. just the moon surface dust?

https://en.wikipedia.org/wiki/List_of_retroreflectors_on_the_Moon

 

[Hop-AC8NS]

Are you avoiding using the lunar retroreflector mirrors on purpose? Yes.

What is the loss in optical reflective power for using the retroreflector mirrors vs. just the moon surface dust? It is the fractional ratio of an area of 0.5m² versus (π)1000²m².

I got my "inspiration" for Optical-EME communications from those corner-cube arrays and went on a search... Don Lancaster had a term for that type of quest, but it currently escapes me. Early-onset dementia is making it difficult.

Anyhoo, A few years ago I found a paper published by the APOLLO (Apache Point Observatory Lunar Laser-ranging Operation) folks that describes how they used a modest-sized 3.5m reflecting telescope to make time-of-flight ranging measurements (optical RADAR) to any one of the three retroreflector arrays placed by our Apollo astronauts on the Moon, lo, those many years ago. Somewhat showing their age here in the 21st century, they were nevertheless an inspiration.

Unfortunately they are totally inadequate for amateur radio EME comms. The simple reason is the effective area of any of the arrays is tiny compared to the illumination area. Photon counting and statistical correlation is required to "see" photons reflected from the arrays for range measurements, but I don't care about measuring how far away the Moon happens to be during a QSO. I want to "tag" my transmitted photons by modulating the laser diode output with wave forms that are essentially rectangular pulses whose edges occur at precisely timed intervals, known a priora, but delayed with respect to their transmission by the approximately two and half seconds round-trip time.

The "message" is not important to a demonstration of proof-of-concept, but harking back to my Novice year (1966-1967), Morse Code transmitted at 10 wpm seemed like a suitable initial message signal. The message might look something like this: "CQ CQ MOON DE AC8NS K" Later, after a QSO has been established, the "message" can be anything that can be expressed in binary on/off modulation. ASCII bytes for example, but the message length and content must be constant during repeated transmissions.

The message would be transmitted, taking "t" seconds do so, followed by a "silent" period also "t" seconds long. Then it is repeated indefinitely. On Earth, a photon detector creates photon detection events or PDEs. Each PDE is counted in a series of consecutive bins with each bin corresponding to a specific time, determined by a GPS-disciplined oscillator controlling a clock displaying UTC. The same UTC clock is used to determine when each block of photons is transmitted and, after a delay, which photons are counted. Thus synchronous modulation followed (after a delay) by synchronous detection is "the method" used for optical-EME.

The fifty-percent "duty cycle" of the transmissions allows a huge narrowing of the "receiver" bandwidth because bins are counted up when photons are expected and counted down during the expected silent interval. That will allow statistical averaging such that only PDEs that occur during the transmission period are accumulated. All other PDEs represent background noise and will average to zero after a sufficient number of repetitions. Meanwhile, the PDEs representing the message accumulate in the bins until the signal rises above the noise floor sufficiently to be discriminated as binary on or off.

After this happens, I plan to use the recovered binary "data" representing "dits" and "dahs" to modulate a "side tone" audio oscillator so the EME operator can hear the Morse Code. Of course that is totally unnecessary: the data has been recovered and the Morse (or ASCII characters) could be decoded and displayed as characters on a computer screen. Later, for longer messages, error correcting coding could be added.

Um, it should go without saying that some serious software and a laptop computer is required to implement optical-EME comms. And some serious optics, too. From the point-of-view of the Moon, every light emission source of any practical size appears as a point-source because of the "vast" distances involved. The Earth's atmosphere is a gigantic hollow spherical concave-convex lens with a variable index of refraction. Nothing we can do about that, but it does cause a collimated beam to diverge through about one arc-second as it passes through the atmosphere on its way to the Moon. If you do the math, it means a collimated beam (of any practical diameter) arrives at the Moon with an illumination "footprint" about two kilometers in diameter. Compare the area of that with the effective aperture area of the A15 (largest) array (less the 0.5 square meters) and you will get some idea of why those arrays are useless for EME comms.

So, back on terra firma, we need to collect as many "tagged" photons as possible from that two kilometer diameter patch of illumination. Those photons are not being retroreflected back toward the transmitter... well, a few of them are if a retroreflector array is illuminated, but not enough to matter. Instead the Moon's regolith reflects and scatters the photons as if it is a "pebbled gray-body" with a rough surface and an average albedo of about 0.12. But that's the average over the entire surface facing the Earth. I assume the radiation is reflected in a Labertian (cosine) distribution with respect to the incident angle of the illumination, so a goodly percentage is directed toward Earth. The inverse square now comes into play: the radiation is expanding into a hemisphere of ever increasing area until it reaches (and zips right on by) the Earth. It is an exercise, left to whoever cares, to compute what part of that two kilometer sized patch actually illuminates the Earth. Well, actually it all does (we can see the Moon from anywhere on Earth!) but the power density is much reduced. For my size telescope (102mm aperture) and an assumed one watt laser power, only a few photons per second will make it into the aperture, and only about 20% of those will be detected by my 1P41 PMT, biased in single-photon detection mode. An APD would be better, with its higher quantuim efficiency, but those are expensive.

My telescope, a "Celestron Nextstar 102GT Computerized Telescope" has about one arc-second of optical resolution, so I should be able to point that illumination patch anywhere I desire on the Moon. I choose to point it at one of the "Highland" areas and avoid the dark mares. I don't NEED the telescope to collect photons. I plan to use an array of plastic Fresnel lenses, focused on a common spot, the cathode of the PMT, for my "receiver". This collection system does not require imaging the patch (Fresnel lens are not high-quality imagers) only the "funneling" of photons coming from the direction of the Moon onto a photocathode. Thanks to an Australian ham, Rex VK7MO, for this approach. He is doing line-of-sight and "cloud bounce" optical comms with a large Fresnel-lensed LED array and an APD viewing the received optical signal through another Fresnel lens. He said his APD cost him about $US 500!

I could go on to describe the other "problems" I have to solve, but if you have read this far, surely you have visited https://optical-eme.groups.io and now have some idea of what I and a few fellow hams are trying to accmplish. Thank you for your interest!

 

[berkeman]

Interesting stuff indeed. This part of the forums is generally just for brief introductions and not longer discussions, so if you'd like some help on this project, I'd suggest starting a new thread in the Astronomy or Electrical Engineering forum with the info above and any specific questions that you have. We have lots of EEs and Astronomy buffs here at PF, so you should get good help with your project.

 

[Hop-AC8NS]

Still learning how to navigate...

 

[DaveE]

At the risk of making the guy with the paint roller mad (congratulations! BTW)...

I worked at a DoD contractor that built a laser for ranging the reflectors on the moon. I was just starting as it was being retired. It was a VERY big laser. First I would look into power requirements and regulatory issues. You can't just build a big laser and shoot it into the sky. The paper work may be a non-starter.
I also worked on airborne laser communications with submarines. Again a VERY big laser. A lot of the effort was in filters and signal processing at the receiver using exotic materials. Huge laser, tiny signal to be recovered.

Start a new thread if you want to continue this. But I've probably exhausted my knowledge/memory on this subject.

 

[Hop-AC8NS]

Yep. I will coordinate with the FCC, FAA, and Space Force before aiming my 202mm collimated laser beam at the Moon. Space Force in particular would get involved if high power that could damage or disable certain assets in low-earth orbit were involved. I am familiar with the work done at Apache Point Observatory. They used a Nd:YAG infrared laser, frequency doubled into the green, and with picosecond pulse widths to obtain distance resolutions on the order of a few centimeters. Average power was less than a couple hundred watts, IIRC, but the pulsed power was huge. They also stationed an observer outside who could order the laser shuttered if an airplane was about to fly into the beam. None of this is applicable to radio amateurs doing optical-EME. The A15 retroreflector array is useless for communications, having an effective aperture of less than 0.5 m². The illumination area of a collimated beam (any practical size) sent from Earth to the Moon is a circular area about two kilometers (2000 m) in diameter. Do the math. It would also help if first responders would bother to visit the optical-eme discussion group before nay-saying this "bucket list" project.

My entire career, starting with my Air Force enlistment and continuing until my retirement in December 2014 was spent as a contractor working for the DoD. I helped spin up the Laser Window Evaluation Lab for the Airborne Laser Lab (ALL) aircraft at the Air Force Weapons Laboratory in Albuquerque NM in the 1970s. I worked with highly classified projects (TS/SCI) on digital image processing in the 1980s. In the 1990s, until retirement in 1994, I operated and maintained an ancient research particle accelerator (1.7MV Tandetron) and used it to make a GaAs PCSS invented at Sandia National Laboratories in the 1990s actually work. SNL abandoned it after using an array of cylindrical lens to guide switch conduction after optical triggering: they thought their "solution" was impractical. It was, but there is a simple fix which we discovered after following a suggestion by the Defense Threat Reduction Agency (DTRA), the military arm of the Department of Homeland Security.

I could be wrong, but I think my education and experience more than qualifies me for this "bucket list" quest. Grok 3 [beta] also agrees, for whatever that is worth. BTW, brute force often prevails in street fights and war, but finesse will sneak up and bite you in the ass. Thanks for responding!

73,
Hop AC8NS