Summary
For the initial phase of BHD testing, we recently discussed whether the mode-matching telescopes could be built with 100% stock optics. This would allow the optical system to be assembled more quickly and cheaply at a stage when having ultra-low loss and scattering is less important. I've looked into this possibility and conclude that, yes, we do have a good stock optics option. It in fact achieves comprable performance to our optimized custom-curvature design [ELOG 15357]. I think it is certainly sufficient for the initial phase of BHD testing.
Vendor
It turns out our usual suppliers (e.g., CVI, Edmunds) do not have enough stock options to meet our requirements. This is for two reasons:
- For sufficient LO1-LO2 (AS1-AS4) Gouy phase separation, we require a very particular ROC range for LO1 (AS1) of 5-6 m (2-3 m).
- We also require a 2" diameter for the suspended optics, which is a larger size than most vendors stock for curved reflectors (for example, CVI has no stock 2" options).
However I found that Lambda Research Optics carries 1" and 2" super-polished mirror blanks in an impressive variety of stock curvatures. Even more, they're polished to comprable tolerances as I had specificied for the custom low-scatter optics [DCC E2000296]: irregularity < λ/10 PV, 10-5 scratch-dig, ROC tolerance ±0.5%. They can be coated in-house for 1064 nm to our specifications.
From modeling Lambda's stock curvature options, I find it still possible to achieve mode-matching of 99.9% for the AS beam and 98.6% for the LO beam, if the optics are allowed to move ±1" from their current positions. The sensitivity to the optic positions is slightly increased compared to the custom-curvature design (but by < 1.5x). I have not run the stock designs through Hang's full MC corner-plot analysis which also perturbs the ROCs [ELOG 15339]. However for the early BHD testing, the sensitivity is secondary to the goal of having a quick, cheap implementation.
Stock-Part Telescope Designs
The following tables show the best telescope designs using stock curvature options. It assumes the optics are free to move ±1" from their current positions. For comparison, the values from the custom-curvature design are also given in parentheses.
AS Path
The AS relay path is shown in Attachment 1:
- AS1-AS4 Gouy phase separation: 71°
- Mode-matching to OMC: 99.9%
| Optic |
ROC (m) |
Distance from SRM AR (m) |
| AS1 |
2.00 (2.80) |
0.727 (0.719) |
| AS2 |
Flat (Flat) |
1.260 (1.260) |
| AS3 |
0.20 (-2.00) |
1.864 (1.866) |
| AS4 |
0.75 (0.60) |
2.578 (2.582) |
LO Path
The LO relay path is shown in Attachment 2:
- LO1-LO2 Gouy phase separation: 67°
- Mode-matching to OMC: 98.6%
| Optic |
ROC (m) |
Distance from PR2 AR (m) |
| LO1 |
5.00 (6.00) |
0.423 (0.403) |
| LO2 |
1000 (1000) |
2.984 (2.984) |
| LO3 |
0.50 (0.75) |
4.546 (4.596) |
| LO4 |
0.15 (-0.45) |
4.912 (4.888) |
Ordering Information
I've created a new tab in the BHD procurement spreadsheet ("Stock MM Optics Option") listing the part numbers for the above telescope designs, as well as their fabrication tolerances. The total cost is $2.8k + the cost of the coatings (I'm awaiting a quote from Lambda for the coatings). The good news is that all the curved substrates will receive the same HR/AR coatings, so I believe they can all be done in a single coating run. |