Dear Mijanur Rahim,

The obtained information is, potentially, useful: the focused beam intensity distribution could be generated by modes distribution. The need to couple more energy on the APD remains.

Thank you,

Alexandru Rusu.

Dear Alexandru Rusu

If you want to have a gaussian beam comming out of your fiber you have to use a single mode fiber. The core of a single mode fiber has a diameter of about 5um. In order to the small core only the fundamental mode can propagate through the fiber.

There are some disadvantages comming along with a single mode fiber. It is much harder to couple more intensinty into the fiber than for a multi mode fiber. Especially when the incomming beam differs much from a gaussian beam. Also the wavelength is important. Each single mode fiber is just for a special wavelength range single mode.

I hope this information will help you to understand better the influences of the fiber on your beam. If you have a gaussian beam it is easier to simulate the propagation through an optical system.

Best regards

Julian Robertz

Dear Julian Robertz,

Thank you for the answer.

My fiber is multimode one, so is the light source. For the moment, I want to keep on these characteristics.

Is it necessary to consider the exiting beam from the optic fiber as a real gaussian beam?

Dear Alexandru Rusu

It isn't necessary. But it is a possible explanation why your focus is larger than expected.

A gaussian beam is well defined and therefore it is easy to simulate its propagation through an optical system. It is also possible to do this simulations for beams with several modes but it is more complex. You can find an overview about transverse modes here: https://en.wikipedia.org/wiki/Transverse_mode#Laser_modes. There is also one paragraph about modes in optical fibers.

Maybe the beam quality factor (https://en.wikipedia.org/wiki/M_squared) helps you to do some calculations and to understand, why a gaussian beam has the smallest focus.

Thank you,

Alexandru Rusu.