Hi.
I have a Lumencor Sola solid state light engine which uses a 5mm liquid light guide (LLG). I would like to use this for a spinning disk confocal setup which uses an FC port for optic fibres. I would like to stay away from lasers for now.
With some primitive calculations based on my objective lenses, I decided to purchase a 400um multimode (MM) optic fibre for UV-VIS (FC-SMA905 plugs). The Sola outputs IR as well, I have decided to either cut out that component with an IR cut filter or disconnect the LED module physically from the board. I do not think heat is good for the fibre.
The problem I am facing now is "squeezing" the output of the Sola into my 400um fibre. Realistically, an efficiency of 20% would be decent.
For the optical scheme, I basically plagiarised Thorlabs' solution for their stabilised light sources, which coincidentally also uses a 400um fibre bundle.
https://www.thorlabs.com/images/TabImages/SLS201L_Simplified_Drawing_D1-780.gif
They appear to be using a 40mm best-form lens to collimate the output and an aspherical lens to focus it into the fibre.
I suppose the Lumencor Sola uses a similar method. I will have to open it and check, but I do recall a couple lenses being used, presumably to focus the light into the 5mm LLG. I do not wish to move those lenses and I also do a lot of widefield fluorescence imaging.
Therefore, I suppose I am attempting to collimate the output for a 5mm LLG and then focus it into my 400um MM fibre. I can design and 3D print a bracket for the Sola's output port which will enable a cage system for all the optics.
Another rather unusual method which I am unsure of would be focusing the output light with a microscope objective, straight from the Sola into the MM fibre.
Will my method(s) work? Is there a better method to achieve this with minimal alterations made to the Lumencor Sola?
Thank you for your help and any advice is appreciated!
Hello Daniel,
a quick word of advice, squeezing light from a big incoherent source into a small fiber is not only difficult, it is actually physically impossible. This is due to the rule of étendue, which is the optical manifestation of the second principle of thermodynamics.
While if you look up the rule of ètendue you get complex definitions talking about solid angles and apertures, the main, most important consequence of the conservation of étendue is that, given an incoherent source such as an LED or the output of a liquid wave guide, the light intensity per unit of surface can only decrease when passing through a passive optical system. This means that, with your 5mm diameter wave guide, and your 0.4mm diameter multimode fiber, you can at best couple in the fiber a fraction of the light equal to the ratio of the areas, so (0.4^2)/(5^2)=0.6%. You will not be able to couple in anything close to 20%, no matter how complex your optical system is.
Sorry for giving you the bad news, i would suggest looking into multimode lasers for your application, or finding a way to illuminate the disk in your spinning disk without passing through the FC port
Paolo Pozzi I was aware of ètendue from the perspective of a microscope objective, in that there is only a finite amount of light that can go through, regardless of how powerful the light source is. Thanks for illuminating the path with regard to coupling, which is something I am a novice at. The 0.6% figure looks scary. The manufacture of the lighting system offers a coupling device with a claim of 25% efficiency into a 1500um MM fibre. The input is still 5mm. It could be more complicated.
Edit for a minor correction: the LLG input diameter is 5mm but the core is 3mm, I believe. Efficiency is still very low.
So 1.5^2/3^2=25%, that checks out!
Now, 0.4^2/9=1.78%.
Perhaps replacing the FC port is indeed the way forward.
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