What should be the conditions to launch such light into MMF with the help of 40x lens? How much distance should we maintain between the lens and fiber?
there is an equation relating waist of your beam, supposed to be collimated before the lenses, to the waist of the beam after the lenses and is w0=lambda*f/(pi*w01) where lambda is the wavelenght of the ligth you are using, f is the focal lenght of your microscope objective and w0 and w01 respectively the waist of your beam before and after the lens supposing always in the paraxial approssimation, pi is 3.14.
Regards
Ideally, you want your distance between the objective and the fiber to be the same as the working distance of the objective. This of course is considering that your beam is collimated before entering the objective. Anyhow, even if your beam is diverging of converging slightly, the position of the focus should not change much (maybe just a few microns) for a 40X objective.
Regards
Ale
Launching light into a MM fiber is not that much difficult. You should first check out the core diameter of the MM fiber being used. Theoretically, maximum launching is achieved when the beam waist and the core diameter coincide (overlap). Best way to get optimum distance is that you may mount the objective on a moving rail in front of the input end of fiber. By changing the distance you can observe maximum launch into fiber by detecting the output power of the fiber simultaneously. In this way by replacing your objective at a time, you can choose an optimum objective with suitable focusing power.
With 40X objective your spot size and confocal parameter is very small, so you will have very low tolerance if your alignment is not very precise(it is nearly impossible if you are not using some translation stages etc.). Also depending on your pulse energy you can burn your fiber input. Use 10X or 20X objective at maximum, We are achieving nearly %80 coupling efficiency to a 20-micron-core fiber using 10X objective at 1030 nm wavelength...
Dear Alessio Tierno, we dont know the focal length of the lens, and your equation will tell us about beam waist after that lens. but we want to calculate optimum distance between MMF and 40X microscope objective lens for maximum coupling of light.
Alexandro- how many micron for 40x lens and how we can calculate this distance??
Syedi Yavas - How much distance you took in your case? we are using 630 nm wavelength source. if i use 10x lens, theoretically how much distance we should maintain between MMF and Lens?
If you want to calculate it, you need to know the focal length of your lens. If you cant do this then set the system up and adjust the lens to fibre distance to maximise the output from the other end of the fibre.
We are using Thorlabs 10X objectives for this purpose, and it has 1 cm of working distance. We are putting fiber 1 cm away from the objective and using same company's fiber adjustment stages we are getting pretty good results. Working distance should not depend very much on wavelength but be careful about the AR-coating of the objective. If you use some objective with AR-coating with different wavelength than yours, you may have losses, or for worst case, you may make damage on objective at high average powers since you have pulsed laser.
The distance varies, depending on the objective you are using, it could vary from a few hundred microns up to a centimeter or so for long working distance objectives. Check the characteristics of your objective to find what is the working distance of it.
What is NA of your fiber?
NA of standard MM fibers is 0.2, so the optimal microscope objectives are 10x or 20x. NA of 40x objective is ~0.5, so part of the optical power will be lost into cladding modes.
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