March 12, 2022
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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. How much flux? the unattenuated beam is 6e9 photons per sec. At 1E-19 joules per photon we want 6e-10 Watts so nW of power.
That should be OK....
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?
Also bought wedges on eBay, > 2 inches in size.
Feb 2023 Design
HgAr source, 12V 3W so around 500mA@12V:
Beam source
either tunable laser or else modulated photodiodes. If we want a 10 micron spot at f/12, need to inject the input beam at slow f/# as well.
a 50 micron fiber needs to be 12*50 = 600 microns away from a pinhole to generate an f/12 beam.
Disperser
Say we want adjustable dispersion with Risley prism arrangement. At n1sin(theta1) = n2sin(theta2). For small angles we get 1.5*theta1=theta2. deflection is theta1-theta2 = theta1-1.5 theta1 = theta1 *(1-n(lambda)).
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 |
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
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.energetiq.com/eq99xfc-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/
plano-concave high quality mirror blanks, up to 50mm diahttps://www.lambda.cc/product/plano-concave-mirror-blanks-pccm/
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 | |