I bought a ST-ST Simplex 125 um SM UPC-polish optical fiber patch cord in order to transmit visible light (400-700 nm) from a collimating cannon (that I designed myself) into a spectrometer. Trouble is the fiber does not transmit light in sufficient intensity. I can see it does transmit light if I look at an end and shine light into the other, and that works for a UV laser, green light, red light and white sources (LED, incandescent, fluorescent, sun). I'm no pro in optical fibers, so I don't know what would be best: get a larger core (250, 500 um?) or something else?
Best would be to have a fiber corresponding to the light source image size and numeric aperture in your 'colimating cannon'. I usually use 1 mm core fibers with NA 0.22, it's easier to come close to these parameters, than to 125 um.
For a simple inexpensive lamp to fiber coupling system, 10% efficiency can be considered quite satisfactory.
Fiber size and alignment are probably your key issues. If your fiber is not well aligned with the output of your collimator, or undersized relative to the collimated beam, then the throughput of the fiber is likely not the issue but the amount of light launched into the fiber may be the problem. Launching a laser beam into a .22 NA fiber, which is probably what you are using [numerical aperture, NA = n sin(theta)] with a 100-500mm focal length plano convex lens should give you acceptable results with a 100+ micron diameter fiber (larger is better, obviously).
Well. My lens is 25 mm wide in diameter and its f = 4.85 cm. That gives a semi-angle theta = arctg (2.5 / (2 * 4.85)) = 14.5º. That is my lens cone has an aperture of 29º. If my fiber has indeed NA = 0.22, then its semi-angle is theta = arcsin (NA / n) = arcsin (0.22 / 1) (fiber in air) = 12.6, e.g., a cone of ~25º. The lens cone is wider than the acceptance light cone in the fiber. Supposing everything else is aligned (which I doubt), maximum light transmission will not be over 86%. Sad...
The second part of losses is a mismatch of the fiber core diameter and the image size. E.g. if the hot spot of your lamp is 2 mm, the above described coupler produces an image of it of same 2 mm size, and 75% of light is outside of the core of a 1 mm fiber (and 99% outside of the core of a 200 um fiber)...
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