Fibre Misalignment and Light Coupling

[irfanskp]

Hi,

I have got a single mode fibre (SMF) with core diameter 8.4 microns. Fibre is aligned in front of horizental reflective surface at a distance of 500 microns. Light from SMF make a spot size of 24 microns and is reflected and coupled back into SMF. However, if I tilt the reflective surface (mirror) by 1 degree how much off-set or light is lost that is coupled back into SMF. Please find attached the diagram.

Answer to the question would be highly appreciated.

Thanks

Irfan

 

[Andy Resnick]

It should be simple to estimate this using geometry- given the NA of light exiting the fiber face, the distance to and and the tilt angle of the reflective surface, you can calculate what fraction of light is incident on the fiber face. I'd be surprised if it was more than a few percent.

 

[irfanskp]

Thanks Andy.
When I do simple geometery calculations I get about 18% of loss with 2 degree. However, When I use fibre efficiency formula for tilt misalignment its surprisingly huge. So, I ve got confused which one is correct. Please find attached diagram that gives few more numbers. If you have a look on that then we can discuss in further to solve the problem.

I appreciate you answer!

Irfan

 

[Andy Resnick]

Going by your diagram, I calculate only 0.8% of the light will re-enter the fiber (about 21 dB loss). And that doesn't take into account any reflection off the air-glass interface.