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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 is 1520– 1560 (1600) nm.

The followin 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 choosen for beacon transmission wasnt explained.

Coud someone explain why beacon signal operates in lower range?

What are reasons the difference of operating wavelengths?

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

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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 '21 at 18:36
  • $\begingroup$ I’m voting to close this question because the answer is in the book cited by the OP. $\endgroup$
    – Ng Ph
    Nov 10 '21 at 18:37
  • $\begingroup$ @NgPh but why keep the answer secret from everyone except those who have a copy of the book handy? Why is blocking any user from posting an answer and preventing any future reader from seeing it the best of all possible actions here? It seems that you just want to punish the question, but SE is all about answers. If you've got the answer right in front of you, please consider writing up a short answer for everyone's benefit! $\endgroup$
    – uhoh
    Nov 10 '21 at 23:56
  • $\begingroup$ @NgPh you are partially right. In the book, the reasons why the 1550 nm wavelength is commonly used as data operating wavelength are given. I have asked about beacon signal. This was not explained. $\endgroup$ Nov 11 '21 at 7:26
  • $\begingroup$ @uhoh (& Noel Miller too), exactly. The answer is not available to those not having access to the book, nor the background of the question. And that's what the OP has chosen to do: quoting just one sentence of a non-available resource (out of a chapter explaining how to select laser wavelength vs application). This is bad practice in general (not only for SE), IMO. That's what I wanted to flag. $\endgroup$
    – Ng Ph
    Nov 11 '21 at 9:11
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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 '21 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 '21 at 9:09

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