optical fiber and detector

[harryd]

hi
i am doing an experiment to measure the beam width. my task right now is measure the light coming out from the fiber using a simple common collector ( phototransistor). now , the detector is working because when i apply the laser pointer( 633nm 5mw) on the detecotr it reads around 3 volts. i , also, polished the fiber and it transmits light ( a small red dot is seen at the end of the fiber).

However, when i apply the laser in the fiber and the fiber infront of the detector it doesnt read anything and i mean anything ,abloute zero.

so i concluced one of these 2 solutions:
1- change the laser pointer with a stronger one
2- change the phototransistor with something more sensitive ( suggest plz)

plz help asap

 

[singhania12]

how much is the fiber length

 

[harryd]

1 meter
are u suggesting that the power loss in the fiber makes the signal so weak by the time it reches the end ??

 

[IanP]

If you use one of standard optical fibres for tests the diameter of the usable core is well below 100um ..
In the normal circumstances there is always a little bit of optics involved, for example to focus the beam on the right spot ..
Also, alignment of the source of light at one end and the receiver device at the other end is of very high importance, taking into account the diameter of the core ..

You have to do it with absolute precision ..

Rgds,
IanP
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