Efficiencies of Coupling Light into a Fiber

Question

I am in AMO Physics and work a lot with optics. I just wanted to get an idea of what coupling efficiencies one "should" get in a "reasonable time"* by coupling light into a fiber using different couplers, like collimators, mounting plates, mounted lens systems, etc. I understand that this dependents on a lot of factors, so I will narrow it down for my specific case but would appreciate if ppl report some numbers with a short info about their setups.

We use a cage system system with 5 mm lens and APC fiber plate to couple 633 nm laser light into an APC single mode fiber. Pretty much everything of the optical equipment is from Thorlabs.com.

The reason for my question: My PI told me I should get efficiencies up 80% percent with our coupling scheme but I can only get 30%. I hope to get a better feeling for coupling efficiencies with this answer.

Thanks for your input,

n3rd

*With "should" and "reasonable time" I refer to coupling efficiencies one can achieve at time scales on the order of tens of minutes rather than hours, days, weeks.

Answer 1

Some more practical tips:

1. After having finished fiber coupling, i.e. positioning the tip of the fiber as precisely as possible in the beam path, beam walking (usually performed with two mirrors that lead the beam to the fiber-coupling stage) can also help in elevating the efficiency and can be performed in a matter of minutes.

2. This one is time-consuming but if the project is very efficiency-hungry, then it's worth a try. Assuming you have chosen the correct lens and incoming spot-size of the beam (matched to the NA of the fiber i.e.) but a high efficiency still eludes you, then:

   • mount a 1:1 telescope on an optical rail before the fiber-coupling stage,
   • change this ratio in tiny amounts and optimize the coupling every time,
   • Perform the optimization mentioned in point 1 above.

Perform them iteratively and, with luck, you may get higher values.

Answer 2

How much light couples into the fiber depends on the NA (numerical aperture) of the fiber, the diameter of the beam of light entering the collimating lens, and the focal length of the lens. Typical fibers accept light only from a limited angle about the axis of the fiber. This is referred to as the NA of the fiber. If the NA is large (e.g. 0.7) the fiber can accept light at up to about 44 degrees from the axis. For an optical fiber this would be a remarkable NA. Most fibers are in the 0.2 range and can accept light only within about 10-12 degrees from the axis.

Assuming that you have collimated light to start with, the diameter of the beam and the focal length of the lens then determine the effective NA of the light converging on the fiber. The NA is the ratio of the radius of the beam to the focal length of the lens (this is a little approximate since NA is defined as the sin of the half angle, but for normal NA's it is very close). If the converging beam of light has too great an angle, the outer parts of the beam will fail to couple into the core of the fiber.

Finally, if the beam is not collimated, i.e. it doesn't focus to a small spot on the core of the fiber, then the part of the beam that misses the core will not couple into the fiber.