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I am reading the book: Kaushal, H., Jain, V.K. and Kar, S., 2017, Free space optical communication, New Delhi: Springer India.

The operating wavelengths for beacon and data transmission were discussed. Beacon wavelength window is 780 to 1064 nm, for data transmission it is 1520 to 1560 (1600) nm.

The following explanations are given for the data transmission wavelength window:

The 1550 nm wavelength is commonly used as data operating wavelength due to following reasons:

(i) Reduced background noise and Rayleigh scattering: The absorption coefficient of the Rayleigh scattering has functional dependence with the wavelength as 4: Consequently, there is almost negligible attenuation at higher operating wavelengths as compared to those at the visible range.

(ii) High transmitter power: At 1550 nm a much higher power level (almost 50 times) than at lower wavelengths is available to overcome various losses due to attenuation.

(iii) Eye-safe wavelength: The maximum permissible exposure (MPE) for eye is much higher at 1550 nm wavelength than at 850 nm. This difference can b explained by the fact that at 850 nm, approximately 50 % of the signal can reach the retina whereas at 1550 nm, the signal is almost completely absorbed by cornea itself. And therefore the signal received at the retina is negligibly small.

Why 780 to 1064 nm is chosen for beacon transmission wasn't explained.

Could someone explain why beacon signal operates in lower range?

What are reasons the difference of operating wavelengths?

Beacon signal sends information as well: "Where are you?", "hello I am here" and we send data via data link. What are the atmosphere effects on the beacon signal?

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  • $\begingroup$ What you quoted is on page 91 (not 60). If you read further the authors give all the technical reasons for wavelength selection. $\endgroup$
    – Ng Ph
    Nov 10, 2021 at 18:36

2 Answers 2

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Beacon wavelength window is 780 to 1064 nm, for data transmission is 1520– 1560 (1600) nm.

Could someone explain why beacon signal operates in lower range?

I don't have the book handy, but I can leave a partial answer until someone can offer a more complete one.

780 to 1064 nm for CW to medium speed but potentially very high power

These are common laser wavelength areas and have been used in space before.

780 nm (say 750 to 860 nm roughly) would be a near infrared semiconductor AlGaAs laser (and LED) wavelength range, used everywhere for laser scanners (especially self-driving cars, where they can burn out people's cameras!) Remote controls and security camera "invisible" LED lighting that shows up purple in photographs is in this area as well.

There is so much infrastructure making these lasers robust and indestructible as well as direct-modulation-able, they're a great choice for a reliable space laser.

1064 nm comes from an industrial, robust solid state Nd:YAG laser. There are lots of these in space as well.

That you'd want the highest power for a beacon should be self-evident; if you want to have a high photon flux over a high area to maximize the speed of target acquisition, raw power is one important consideration.

This system might not even share the same optics as the high speed data channel, needing a faster but less accurate rastering system.

1520–1560 (1600) nm for ultra high speed but not as high power

This is (part of) the wavelength range for the internet; when we write and read Stack Exchange posts we're doing it with laser light around this range.

Why? Germanium-doped core single mode optical fiber has a zero-crossing in dispersion ($dn/d \lambda$) in this area, so our pulses traveling down long-haul fiber receives a minimum of shape distortion. The whole high speed optical communications infrastructure and technology base is built up around reliable components in hostile environments (like the bottom of the ocean) working in this wavelength range.

This technology includes high speed amplitude and phase modulators to encode data at extremely high rates, and erbium doped waveguide/fiber amplifiers (also optical amplifier) to boost the power of the transmitted (and potentially the received) laser signals.

While erbium doped waveguide amplifiers are effective, they can't provide the same power that the shorter wavelength lasers can as a beacon. A system that's optimized for high speed and optimized modulation will likely always be of modest power.

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  • $\begingroup$ You wrote "...That you'd want the highest power for a beacon should be self-evident;...". If we use low range of wavelength, the effects of atmosphere will increase and it will be badly for our eyes (Eye-safe wavelength was discussed in the book)... Beacon signal sends data as well, right? It detects the position of a terminal and sends a response. Why is power more important than data rate? Sorry...i dont understand this concept how the beacon wavelenght is choosen... $\endgroup$ Nov 11, 2021 at 7:42
  • $\begingroup$ @NoelMiller Beacons send either little data slowly or none at all. They're mostly just a tone or "beep" saying "Hey! I'm here!" You'd like it to be seen even if not pointing in the right direction, so higher power, broader beam is important. Once the beacon's picked up and target acquired and locked by optical tracking, then the data can be transmitted via a much narrower beam at much higher data rate. By the way, if you have the passage you are asking about available, you should quote it. It's not good to ask about something written in a book without quoting the passage in question Thanks! $\endgroup$
    – uhoh
    Nov 11, 2021 at 9:09
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As mentioned, in the other answer, cost and availability for 780 and 1064 are important. At the systems level, You also may want to use a cheaper silicon imagining detector to detect your beacon. This can help find the signal and let you lock into it with a finer pointing and tracking system. Imaging detectors in the 1550 nm range are more expensive.

For data at 1550, eye safety and existing telecom detectors and other well developed technologies can be leveraged. Fiber amplifiers can be very high power these days.

As for atmospheric effects part of the question, at the shorter wavelengths you can get more scattering, also u your wavefronts will be distorted more, and more scintillation. For a beacon that can be less important, than for data.

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  • $\begingroup$ "and more scintillation" not substantially different until far into UV where the other mechanisms will still dominate. The index of refraction of air at 300 nm is only 0.002% higher than it is at 1 μm. (I think that's for 10% humidity average over the column, could be somewhat bigger when humidity is higher) Fig. 5 of researchgate.net/publication/… $\endgroup$
    – uhoh
    Jun 4 at 1:21
  • $\begingroup$ Agree that the refractive index difference is small, but over kms of optical path length the phase differences and small angle changes add up. $\endgroup$
    – UVphoton
    Jun 4 at 3:02
  • $\begingroup$ I wrote something about this in this answer in Astronomy SE, and now I'm getting really interested in this; how can blue and red and orange and green and yellow all be the furthest-bent wavelength at different moments in time? Hmm... I think I'll post a new question about that there; if so I'll ping you here. $\endgroup$
    – uhoh
    Jun 4 at 3:53

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