# Injecting a laser beam into an optical fibre

### Introduction.

Injecting  a laser beam into an optical fibre is a very common task in optical  laboratories. For example it is extremely useful for the alignment and collimation of optical components in instruments like fibre-fed spectrographs.  Our youtube video Injecting a laser beam into an optical fiber describes this process.

For thin fibres the direct injection of the laser beam does not provide the best coupling efficiency. In addition the pattern of the output projected beam  strongly depends on the injection angle to the fibre axis. The video shows:

• the effect on the output beam with the injection angle of the laser beam,
• the variation of the output pattern with bends on the fibre (the well known focal ratio degradation effect in coupling spectrographs to telescope with fibres),
• a very easy way to measure the numerical aperture of the fibre and
• an simple way to use a microscope objective to substantially increase the flux injected into the fibre

#### A word about the measurement of the numerical aperture

The numerical aperture (NA) of the fibre has been described in one of our previous posts (look here). The NA is related to the beam aperture into the fibre by

$NA = \sin (\tan ^{ - 1} ({1 \over {2F/\# }}))$

where F/#  is the beam aperture of the injected beam. In the video, F/# = d/Φ is the ratio of the distance between the fibre to the screen to the biggest ring on the screen. For small injection angles, the equation can be simplified to

$NA \approx {1 \over {2F/\# }}$

### Used material

• 400 μm core step index fibre, 10 m length
• Green diode laser
• Screen with concentric circles
• 10x microscope objective
• Mechanical supports, x,y,z translation stage, etc

Injecting a laser beam into an optical fibre by CAOS group is an article licensed under a Creative Commons Attribution-Non-Commercial-No Derivative Works 3.0 Germany License.
Based on a work at spectroscopy.wordpress.com