Silicon still prime candidate for cryo @ 20K and 120 K.
Technically doable (45 cm size, purity), but how to obtain it.
Silicon TPA ok for high power operation @ 1550 nm.

• We can get 45 cm silicon substrates with magnetically assisted Czochalski method in sufficient purity.
• Large Sapphire substrates can be made, but we do not know the purity. Small Sapphire substrates with promising absoption (xxxx ppm/cm) have been demonstrated.
• Nothing new concerning Fused Silica (stacked thin 3001 can be fused to thick 3001, but with reduced 3D homogeneity. 3002 = 2D homogeneous material is avalable at desired size and weight)

To our knowledge 45cm MA CZ Silicon has been demonstrated but cannot be 'bought'. Can we 'convince' industry to grow large boules? Do we have to develop our own X-tal growing facilities?

Info in presentation of Jerome: et-0024a-12.pdf (seems to contain a different number 0.8 cm/GW) than the literature. (close enough)

Lin et al., Dispersion of silicon non-linearities in the near infrared region: paper_2007_05.pdf
Lin et al., Nonlinear optical phenomena in silicon waveguides: Modeling and applications: oe_lin_painter_review_nonlinear_optics_silicon_2007.pdf

From Angus, about non-linear absorption in silicon at 1550nm: The nonlinear absorption coefficient is about 0.5 cm/GW. So 1MW/cm^2 would give 500ppm/cm, 2 kW/cm^2 would give 1ppm/cm I have not had a chance to check the power in the mirror substrates in the Voyager design yet.

Harald: back of the envelope calculation: with 2 MW in the arm and a generic Finesse in the 500 range we would have order of mag. 50kW in the ITM. With a beam radius of ca. 5 cm we get 600W / cm^2 (http://www.ophiropt.com/laser--measurement/power-density-calculator), meaning less than 1 ppm absorption

Ken: The Voyager Blue numbers I found are: power = 3MWx2 arms, finesse 450, so power build-up ~300, i.e. roughly 10kW/ITM, and the simple average over r=2.95cm ITM beam-size is of order 400W/cm^2, the proper Gaussian peak absorption and correctly weighted average may be more than that. [Edited by Ken]

Two photon absorption creaes electron-hole pairs which influence the refractive index. The fluctuations of those carriers leads to refractive index noise in transmitted optics. et2014heinert.pdf

et2014heinert.pdf

Info in literature
ionbeametchingsilicon_ii.pdf
ionbeampolishingsilicon.pdf
Ion beam figuring of Silicon does not increase µ-roughness above 0.15 nm, but (after a quick search) no info @ 0.05nm level coul dbe found.

• find out how to obtain substrates
• polishing issues with silicon? Surface absorption is an important issue here. A method for polishing without surface absorption has been identified (http://dx.doi.org/10.1088/1361-6382/aa8aac) but more work is in progress on trying to polish to the required specification without surface absorption.
• inhomogeneities in refractive index
• charge carrier density noise limits
• …

in collaboration with GWIC 3G R&D and Voyager team.