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Numerical aperture problem (lens + optic fiber)

woor90

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Sep 27, 2018
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Hi all,

I'm facing an issue: I'm using a NA = 0.16 lens to focus light into an optic fiber with NA = 0.5 (I'm using that lens because I want it to be compatible even with a NA = 0.22 optic fiber I want to use in the future), but what is the aperture of the output of the optic fiber? I'd say it's still 0.16 because internal reflections shouldn't change it, but I'm not sure.

Thanks
 



Singlemode Laser

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Jun 25, 2017
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Hi,

your question makes no sense at all. The aperture of your multi mode fibre and its core diameter are two independent things and are listed with the fiber datasheet. How can we help you?

Singlemode

Ps: Please introduce yourself in the appropriate thread.
 

woor90

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Imagine this: you have a collimated source, to collect light into the optic fiber (NA= 0.5) you use a plano convex lens (NA = 0.16). The numerical aperture of the lens is small enough so that the light reach the optic fiber with an angle smaller than the acceptance angle of the fiber and is efficiently collected. Then what is not clear to me is what happens at the other end of the optic fiber: has the cone of the leaving light the numerical aperture of the optic fiber (0.5) or has still the NA of the lens (0.16)?
The reflections inside the optic fiber shouldn't change the angle of the incident light, right? so the light shouldn't exit at the other end with an angle greater than that permitted by the NA 0.16 lens (even if the optic fiber has a greater aperture).
 

Singlemode Laser

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Now your question is clear to me:

Imagine this: you have a collimated source, to collect light into the optic fiber (NA= 0.5) you use a plano convex lens (NA = 0.16). The numerical aperture of the lens is small enough so that the light reach the optic fiber with an angle smaller than the acceptance angle of the fiber and is efficiently collected. Then what is not clear to me is what happens at the other end of the optic fiber: has the cone of the leaving light the numerical aperture of the optic fiber (0.5) or has still the NA of the lens (0.16)?
The reflections inside the optic fiber shouldn't change the angle of the incident light, right?
Only when your fiber is perfectly straight. Any deviation from that will change the incident (and then the reflected angle) until the angle is redistributed over all possible angles and then leaves the fibre with an NA of 0.5 in your case.

so the light shouldn't exit at the other end with an angle greater than that permitted by the NA 0.16 lens (even if the optic fiber has a greater aperture).

Like described above, this is only valid for a straight and very short fiber (some mm's lenght).

If you need a specific beam diameter after the fiber use a fibre with a smaller NA so collimation is easier than with the NA = 0.5 one.


Singlemode
 




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