Why don't optical phased arrays use something like an oil in the delay line instead of air?

[ELS]

Wouldn't it cut down the required length of the delay line by about 25%?
Is it too hard to get good enough optical performance out of a fluid? I presume it would for glass, but you could use a hydrocarbon with a high refractive index too, no?
 

[robin_astro]

I would suggest the modest decrease in length would be offset by the increased cost and complexity of a fluid filled system. CHARA for example used vacuum filled tubes (presumably for stability reasons) to  transport the beam from the telescopes and for the main path length adjustment which is done with a series of fixed length tubes , with the remaining continuously variable path length adjustment done  in air. eg see this article

https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=867e2665f9df6f9adfdee8bb99cfbfdadfb75034

EDIT, actually vacuum is probably necessary for the majority of the path length to avoid problems with loss of coherence due to dispersion of the beam (where different wavelengths travel though a medium  at different velocities) This would rule out the use of a high refractive index material

Cheers

Robin

Edited August 27 by robin_astro

[ELS]

Well, would it really change dispersion? It would be the same as in air that was 25% longer. So the dispersion isn't different for the required length of each and cancels out.
I guess it's just cheaper and easier to build a longer line than buy a bunch of very clear fuel. Which would probably also come with higher annual costs because it would be a fire hazard.

[Gfamily]

22 minutes ago, ELS said:

Well, would it really change dispersion? It would be the same as in air that was 25% longer. So the dispersion isn't different for the required length of each and cancels out.

Dispersion is not just proportional to refractive index as it comes as a result of the refractive index varying by wavelength of the light, so you'd need a low dispersion oil as well as a highly transparent one. 

[robin_astro]

7 hours ago, Gfamily said:

so you'd need a low dispersion oil as well as a highly transparent one. 

That's why CHARA used vacuum for most of the journey with only the last adjustable bit in air. Even so, they still had to compensate for the dispersion 

"7.2. Longitudinal Dispersion Correction: LDC

Since our continuous delay lines are not under vacuum, at the shorter wavelengths we can suffer from a reduction in correlation due to differential dispersion between the light arriving from different telescopes.This can be corrected by introducing different amounts of glass to equalize the dispersion in each arm of the interferometer.The LDCs consist of two opposed wedges that can be moved past one another to place an accurately controlled amount of glass into each arm"

Edited August 28 by robin_astro
typo

[robin_astro]

7 hours ago, ELS said:

Well, would it really change dispersion? It would be the same as in air that was 25% longer. So the dispersion isn't different for the required length of each and cancels out.
 

It would only cancel at a particular wavelength and a particular direction the telescopes are pointing in. The two paths from the telescopes are very different in length and vary depending on the direction they are pointing 

https://www.chara.gsu.edu/public/basics-of-interferometry