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Mynaric is one of the main suppliers of laser terminal for free space communication. In one of its latest publications I have read, they will increase the data rate of the last lasercom terminal up to 100 Gbps. How it will be done was not explained.

In the lasercom terminal, LD is the main part if we speak about data rate. It creates a narrow beam to transmit data to a ground station.

If data rate is increased up to 100 Gbps, does it mean they will improve the characteristics of LD? What do we use to improve data rate in optical free space communication?

LD- laser diode

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  • $\begingroup$ What does LD stand for? $\endgroup$
    – Ng Ph
    Nov 4, 2021 at 11:13
  • $\begingroup$ @NgPh laser diode $\endgroup$
    – Anna Koroleva
    Nov 4, 2021 at 12:55
  • $\begingroup$ Thanks! In your other question, you linked to a MIT Phd thesis. Have you read the discussions on design trade-offs for High-Power Laser Diodes in this thesis to see whether it has answers to your question here? $\endgroup$
    – Ng Ph
    Nov 4, 2021 at 13:47
  • $\begingroup$ @NgPh Thank you for this hint. I will read the thesis one more time. One question, ATP (acquisition, tracking, and pointing) and PAT(pointing, acquisition and tracking) systems are different concepts? it seems different systems [link] (aidic.it/cet/15/46/170.pdf) $\endgroup$
    – Anna Koroleva
    Nov 4, 2021 at 15:07
  • $\begingroup$ IMO, the paper by the Chinese you linked to uses two acronyms for the same functional concept. I don't know why. Using a lot of acronyms just confuses the readers. At least for me, it doesn't impress, on the contrary. $\endgroup$
    – Ng Ph
    Nov 4, 2021 at 16:08

1 Answer 1

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You need to look carefully at how it's measured. One of the major contributors to data rate is availability: with low Earth orbit satellites they might only have a few minutes of downlink opportunity each 90 minutes. With relays, constellations and/or networks of ground stations you can increase that availability of downlink up to 100%.

Technically there is a limit given by the modulation technology: whilst the infrared carrier frequency is around 200-300 THz, the switching of the beam is usually less than 40 GHz, though 100+ GHz switching does exist. You can 'switch' amplitude, switch polarisation, switch frequency, or simply turn the light on or off, or do something more fancy with multiple overlaid modes.

It matters how data is encoded. In theory you can get more than 1 bit of information from just one photon, e.g. by using pulse position modululation (and encoding information in the timing).

Another factor is how much error correction code you need (like Reed-Solomon code). You send redundant data, for error checking and correction. The noisier the channel (i.e. more atmosphere or the less power used) the more error correction you need. That reduces the data rate. Or on the contrary, boosting the power and the link margin means you use less error correction and get a higher data rate.

Another factor is how many channels are being sent. You could in theory send multiple channels along the same path, or via having a satellite or ground station with multiple lasercom terminals.

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    $\begingroup$ "a few minutes of downlink opportunity each 90 minutes" is pretty optimistic, since the Earth is rotating underneath? You'd need a very special orbit and a lot of ground stations to do that, no? $\endgroup$
    – uhoh
    Nov 3, 2021 at 22:41
  • $\begingroup$ @uhoh, the implicit assumption is that the satellite maintains its beam to point to a particular Earth point (=tracking). This is what Stalink satellites do anyway. The "few minutes" is the time for the satellite to remain above a certain elevation angle of the Earth station. $\endgroup$
    – Ng Ph
    Nov 4, 2021 at 11:13
  • $\begingroup$ @NgPh Starllinks don't have footprints of 20 meters. You can't compare diffraction-limited radio to diffraction limited optical. $\endgroup$
    – uhoh
    Nov 4, 2021 at 11:15
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    $\begingroup$ Re. "limit given by the modulation technology" In fiber communication systems, one way around that problem is wave division multiplexing. (I.e., send multiple, distinct bit streams down the same physical on different wavelengths of light. IDK if it's ever been done in a free-space optical system. $\endgroup$
    – Solomon Slow
    Nov 5, 2021 at 21:00
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    $\begingroup$ @uhoh For low Earth orbit, the orbital periods are usually around 90 - 100 minutes. Ng Ph gave a good answer too. The best you could hope for without relays would be a few minutes each orbit, which would occur for example if your ground station were within sight of an equatorial orbit, or towards the poles in case of a polar orbit. These cases are in fact common choices, and those downlinks are achievable with just one ground station. Then using intersatellite optical links as relays significantly improves downlink availability. $\endgroup$
    – Polar_Bear
    Nov 7, 2021 at 20:05

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