For a diffuse source like the filament (I'm thinking of monochromatic diffuse light), what are the coupling efficiencies we could achieve? Couldn't I grab light from a bigger solid angle and focus it on the correct MFD to couple more light in? I understand MFD and gaussian profile are the needed conditions to couple, but not the NA of my focusing lens
This is a tricky question. The answer, as I understand it, is that the best coupling efficiency you can achieve would be through 1:1 imaging of the hottest part of the filament onto the fiber’s core. That’s what the equations I show in the webinar are based on. If you change the imaging setup to something like 2:1, then the spatial mode will be focused onto a larger area of the core and therefore all of the light won’t be coupled in. Even though the focusing lens is focusing more light from the source, you’ll actually end up with less light coupled into the fiber, compared with the 1:1 setup. Here are some references if you want more info: Book: Fiber Optic Test and Measurement, by Dennis Derickson. ISBN 0-13-534330-5. Section 9.8.1, Pages 370-372 www.rp-photonics.com/spotlight_2008_02_22.html -Dave
@@thorlabs Thank you Dave for the answer and references! I just need to understand why you cannot focus monochromatic light to a smaller spot than where you caught it from.
Excellent question! This is due to the concept of “conservation of etendue” in optics. The idea is that you can focus light into a smaller spot, but by doing so you’re “catching” less light from the source due to the magnification you need in the lens system, etc. For coupling into a SM fiber there’s no way to cheat here. For more information on the conservation of etendue, you can read about it here: en.wikipedia.org/wiki/Etendue#Conservation_of_etendue
Thank you all for making these videos! The information is very helpful and well presented - the field has needed this!
Thank you! We are glad you found it helpful!
For a diffuse source like the filament (I'm thinking of monochromatic diffuse light), what are the coupling efficiencies we could achieve? Couldn't I grab light from a bigger solid angle and focus it on the correct MFD to couple more light in? I understand MFD and gaussian profile are the needed conditions to couple, but not the NA of my focusing lens
This is a tricky question. The answer, as I understand it, is that the best coupling efficiency you can achieve would be through 1:1 imaging of the hottest part of the filament onto the fiber’s core. That’s what the equations I show in the webinar are based on. If you change the imaging setup to something like 2:1, then the spatial mode will be focused onto a larger area of the core and therefore all of the light won’t be coupled in. Even though the focusing lens is focusing more light from the source, you’ll actually end up with less light coupled into the fiber, compared with the 1:1 setup.
Here are some references if you want more info:
Book: Fiber Optic Test and Measurement, by Dennis Derickson. ISBN 0-13-534330-5. Section 9.8.1, Pages 370-372
www.rp-photonics.com/spotlight_2008_02_22.html
-Dave
@@thorlabs Thank you Dave for the answer and references! I just need to understand why you cannot focus monochromatic light to a smaller spot than where you caught it from.
Excellent question! This is due to the concept of “conservation of etendue” in optics. The idea is that you can focus light into a smaller spot, but by doing so you’re “catching” less light from the source due to the magnification you need in the lens system, etc. For coupling into a SM fiber there’s no way to cheat here.
For more information on the conservation of etendue, you can read about it here: en.wikipedia.org/wiki/Etendue#Conservation_of_etendue
@@thorlabs Yes, this is the concept that was failing. Thanks
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