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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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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Using https://lightmachinery.com/optical-design-center/more-optical-design-tools/prism-designer/
| apex angle | incidence angle | 300 nm angle | 1100 nm angle | difference |
|---|---|---|---|---|
| 1 | 1 | 0.487 | 0.449 | 2.2 arcmin |
| 1 | 10 | 0.4981 | 0.4589 | 2.35 arcmin |
| 10 | 5 | 4.904 | 4.523 | 22.86 arcmin |
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This wedge pair needs to be tilted relative to incident beam. Five degree wedge angle seems good, as a pair. That gives us variation from zero to around 30 arcmin. For any sensible focal plane array of 4K x 4K
Jan 31 2024 CWS
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light source conceptual design
If we use a laser-driven Xenon light source, from Energetiq (we have one in lab, in a box) we get this output spectrum: (https://www.spectralproductsenergetiq.com/CM112/2232
RACBP Spring 2024 Parts List:
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We note the reflective designs in this paper which document a 4-element system that adequately* preserves the initial polarization / phase of light.
Galvez, E. (2001). Achromatic polarization-preserving beam displacer. Optics Letters, 26(13), 971–973. https://doi.org/10.1364/OL.26.000971eq99xfc-fiber-coupled-broadband-light-source)
If we operate with spectral bandwidth of around 1 nm, with a 200 micron diameter fiber we get around 10 uW/nm of spectral density. That would generate around 5 uA of photocurrent in photodiode, that's a lot! Even with factor of one hundred loss in transmission to photodiode we should be fine.
Possible double monochromator: https://www.spectralproducts.com/CM112/2232
they claim 1 nm bandwidth with 150 micron slit. Needs external order-blocking filter though, for lambda > 550 nm. Monochromator is f/3.9, laser driven light source is unknown.
notional system diagram:
References
Large optical wedge vendor, example: https://www.precisionoptical.com/precision-optics/optical-flats/unmounted-reference-flat/
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polarization-preserving configuration with 4 reflections ol-26-13-971.pdf; (Galvez, E. (2001). Achromatic polarization-preserving beam displacer. Optics Letters, 26(13), 971–973. https://doi.org/10.1364/OL.26.000971)
We note the reflective designs in this paper which document a 4-element system that adequately* preserves the initial polarization / phase of light.
| title | link | |
|---|---|---|
| Theory of the optical wedge beamsplitter | The_Theory_of_the_Optical_Wedge_Beam_Spl.pdf | |
| Achromatic polarization-preserving beam displacer | https://doi.org/10.1364/OL.26.000971 | |