Depending what you mean by "slimmed down". Do you mean power reduction? and spectral narrowing?

Dear prof Sze

Thank you for your reply

It is not, I mean the signal in RF spectrum with wavelenght after transmitting it is being narrow and thin.

Regards

@Sze Yun Set it is spectral narrowing

Dear @Vincent Lecoeuche

Thank you for your reply

The transmitter is LED source over 4 km multi-mode fiber,

The received optical signal has a spectral narrowing than the transmitted one,

Regards

It would be nice to see the spectral response that you are getting: both the input and the output spectrum. I am aware that, depending how you inject light into a fiber, the spectral response tends to shift towards longer wavelengths: it is a small shift but it is perceptible. I don't call that a slim down but just a spectral shift.

Let me know what you have.

Dear Egalon,

Thank you for your reply

I have attached a picture of transmitting and receiving optical signal in MMF optical fiber.

There is no spectral shift

Best regards

As Vincent mentioned, definitely this is a laser.

Dear Vincent and claudio

I will try to see, Thank you for your replies.

Best regards

Cela est dû tout simplement à la dispersion intermodale dans la fibre. Multimode.

Dear Martin,

Thank you for your reply

Best regards