The question How does SpaceX plan to make money with StarLink Broadband? has got me thinking about how the satellites will link to each other.

Wikipedia says it's something greater than 10,000 GHz and cites Wiltshire, William M. (April 20, 2017). "Re: Space Exploration Holdings, LLC, IBFS File No. SAT-LOA-20161115-00118". FCC Space Station Application. Retrieved 15 February 2018. which I quote below and refer to as a "wall of text".

To me this wall of "neither you nor the DoD can tell us what we can or can not do here" ending it a single sentence of "but we'll play nice with other operators if we have to".

But my question is about the frequency greater than 10,000 GHz. While the wall of text refers to lasers, >10,000 GHz means only that the wavelength is shorter than 30 microns, and 10,000 GHz is just 10 THz, and Wikipedia's Terahertz radiation says:

Terahertz radiation occupies a middle ground between microwaves and infrared light waves known as the “terahertz gap”, where technology for its generation and manipulation is in its infancy.

Is there any reasonable chance that they will be using perhaps quantum cascade lasers or even more conventional technology for THz beams to form the high bandwidth data links between satellites? Or is it most likely they will stick to what we'd call "optical lasers" based on direct bandgap III-V lasers, similar to what's been used in free space optical links in space and on Earth already, and the "10,000 GHz" is more of staking a claim (or drawing a line in the sand) to establish precedent for far future use of THz waves? Or perhaps something else entirely?

  1. Please indicate whether optical inter-satellite links will be coordinated with other systems proposed in FCC applications and with the DoD's laser clearing house, and, if such coordination has commenced, please address the status of coordination.

SpaceX plans to operate its optical ISLs at a frequency greater than 10,000 GHz. The Commission has consistently held that these optical transmissions fall outside its jurisdiction over radio communications. This conclusion is consistent with international norms as well, where the ITU Convention defines the term “radio waves” as “electromagnetic waves of frequencies arbitrarily lower than 3000 GHz, propagated in space without artificial guide.” Indeed, the one time that the ITU looked into the possibility of adopting procedures for free-space optical links, the U.S. took the position that “interference between inter-satellite links would also be rare due to directed and narrow beamwidths, and the vast geometry of space,” and that therefore “there is no evidence to suggest procedures for free space optical links are needed.” The ITU agreed, and the underlying resolution to look into this issue was deleted. Regulation would be especially unnecessary for the optical ISLs SpaceX plans to use, which operate at a very low power level – low enough to qualify as Class 1 laser, which cannot emit laser radiation at levels that are known to cause eye or skin injury during normal operation and thus are not generally subject to regulation

These same considerations effectively obviate the need for coordination of such ISLs between satellite operators. And, unlike lower frequency emissions within the Commission’s regulatory purview, beams at or near optical frequencies exhibit virtually no emissions outside of that narrow beam. In addition, because optical receivers are also highly directional, interference on an optical link would only be likely if another satellite (1) itself used ISLs, (2) passed through the narrow optical beam from SpaceX, and (3) did so with its ISL receiver aligned parallel to the SpaceX beam. This combination of events is extremely unlikely and makes any formal coordination unnecessary. Moreover, even if such an unlikely event were to occur, orbital dynamics ensure that the conjunction would only last fractions of second and could only be repeated through deliberate action.

Likewise, SpaceX is aware of no general requirement for users of optical ISLs to coordinate with the Department of Defense (“DoD”) Laser Clearinghouse. This clearinghouse is intended for DoD users and others using lasers intentionally directed at DoD assets. SpaceX’s proposed system does not meet either of these criteria. More importantly, as noted above, the very low power level and extremely narrow beam effectively mitigates any risk of harm. Notably, the DoD Laser Clearinghouse is operated by JSpOC. As discussed in its application, SpaceX works closely with JSpOC on a range of issues, including planning for its proposed constellation. SpaceX would, of course, coordinate with other operators to the extent any such requirement is adopted or imposed.

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    $\begingroup$ "SpaceX plans to operate its optical ISLs at a frequency greater than 10,000 GHz. " Greater than 10 THz also includes the range 430 - 750 THz, the range of visible light. I guess they want to use lasers in the IR to UV range. Much higher than 10 THz, but less than 1 PHz (petahertz). Just to use an unregulated frequency range. $\endgroup$ – Uwe Oct 28 '18 at 10:25

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