March 12, 2022
While successful, implementation of collimated beam projector to date has two issues: 1) dynamic range mismatch between monitor diode (wants nA of current) and astronomical instrument (100,000 electrons per pixel max), and 2) not directly monitoring beam at exit pupil, due to size mismatch.
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Fused Silica index is < 1.5 for lambda > 275 nm. So concave-convex FS lens has 1/f=(0.47)(1/r) and f=2.12 R.
Desired attenuation
Imagine we want nA of photocurrent in monitor diode, and (spread over 100 pixels) 100*100,000 in 10 sec = 1e6 emitted photons/sec.
For unity QE that is photon rate on diode of 1e-9 Coul/sec * 1 e/1.6E-19 Coul = 6e9 photons/sec. That's an attenuation of 6000.
For 4% reflection here is attenuation vs number of bounces:
n=1 n=2 n=3 n=4
25.0000e+000 625.0000e+000 15.6250e+003 390.6250e+003
So n=3 looks favorable.
A more elegant implementation:
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At 808 nm:
| angle (degrees) | Rp | Rs |
|---|---|---|
| 0 | 0.041365 | 0.041365 |
| 5 | 0.040948 | 0.041784 |
| 10 | 0.039693 | 0.043069 |
| 15 | 0.037593 | 0.045301 |
| 20 | 0.034640 | 0.048630 |
If angle of incidence is 10 degrees, reflected beam rotates by 20 degrees. If beam of diameter D rotates by theta, distance it must travel to not have incident and reflected overlap obeys tan(theta)=D/L so L/D=1/tan(theta)
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Bought Omegon 203mm aperture, 2436mm collimator. A 100 micron diameter fiber subtends an angle of 100E-6/2.436 radians = 8.5 arcsec.
Beam is f/12. Back focal length from flange is 23 cm, from focuser is 14 cm. Allowable aperture for 2 arcsec diffraction limit (so it is subdominant relative to geometrical optics)
is 10E-6=400 nm/D do D > 400e-9/10e-6 => D> 40mm so 2 inch (50mm) optics should be fine.
f*lambda is 12*0.5 microns = 6 microns. Launcher could be 10 micron pinhole in front of fiber. Or a 10 micron multimode fiber?
Feb 2023 Design
References
Large optical wedge vendor, example: https://www.precisionoptical.com/precision-optics/optical-flats/unmounted-reference-flat/
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