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    $\begingroup$ If the goal is to increase the SNR, that can be achieved either by increasing the numerator or reducing the denominator. Can you demonstrate or cite any work that shows that the silica glass used by LIGO is less absorbent at IR wavelengths than say red wavelengths or that coatings that reflect almost all light could not be made at red wavelengths? $\endgroup$ Commented Apr 28 at 10:43
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    $\begingroup$ I can't see the point of you getting annoyed about me asking for clarification and reliable sources for the information you present. The numerator and denominator I refer to are the signal/noise ratio. Decreasing the laser wavelength increases the signal. If it increases the noise by more, then that would possibly answer my question. $\endgroup$ Commented 2 days ago
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    $\begingroup$ @ProfRob I'm not sure that the answer is that 1064 nm + fused silica is strictly optimal for absorption. I think it's more a multi-objective engineering problem (laser stabilization, coatings with low absorption AND thermal noise, photodiode efficiency, ...) and the technology for all the various components exists and is mature at around 1064 nm. I'm not sure if any one reference explores all the tradeoffs. Interestingly, for low frequency ET the plan is to use 1550 nm and silicon mirrors (einsteintelescope-emr.eu/wp-content/uploads/2024/05/…) $\endgroup$ Commented 2 days ago
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    $\begingroup$ One of the chief limitations of LIGO is the noise introduced by the glass in the mirrors. See this from the Berkeley physics department - Hellman Lab Races to Find Perfect Glass for LIGO For information on coatings, see the RP Photonics Encyclopedia article on Bragg Mirrors. $\endgroup$ Commented 2 days ago
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    $\begingroup$ In fact, 1064 nm is not optimal for attenutation, they don't use it for fiber communications for that reason. 1550 nm is universal for long-haul fiber; it's the lowest glass absorption and you can get fiber amplifiers. 1310 nm is a bit cheaper for transceivers and is almost as good in attenuation, but no fiber amplification (semiconductor optical amplifiers are starting to appear). 1064 nm's benefits are raw power: you can get very high power laser diodes, solid-state lasers, fiber amplifiers, and very good Si detectors work. Common applications for 1064 are welding, marking, and surgery. $\endgroup$ Commented yesterday