SuperK Fiber Tip Quality Check

If the etalon light intensity is low or if the light is highly chromatic, the fiber alignment should be checked. One quick check before alignment is to see if the fiber tip needs cleaning (or is damaged). The easiest way to do that is to look at the mode field output by the SuperK fiber at the input to the Etalon.

Disconnect the yellow fiber from the SuperK at the edge of the Cal Bench (at the input to the black etalon fiber) and shine the output on to a card. The mode field should look very “tidy” as in the photo below. The light is a continuum, but is very bright so take proper safety precautions when handling the fiber.

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A “good” mode field from the SuperK fiber.

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This is an example of a “bad” mode field.

If the mode field is bad, clean the fiber tip. If cleaning does not improve it, the fiber tip may be damaged. Use a fiber microscope to inspect it and replace the fiber with a spare or re-polish the end if needed.

SuperK Fiber Alignment

There is a fiber alignment coupling between the NKT SuperK light source and the etalon which may need to be aligned if disturbed.

The alignment jig is at the bottom of the etalon rack. It is the “SuperK Connect” box attached to the “SuperK Split” box. The fiber coming out of the alignment jig is visibly glowing as can be seen in the photo below.

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Photo of the bottom of the Etalon rack showing the “SuperK Connect” alignment jig.

To align the fiber, disconnect the fiber from the mounting at the corner of the cal bench (the other end of the glowing fiber seen in the photo above). And put the output in to a laser power meter. Monitor the power level while adjusting the four gold colored knobs on the “SuperK Connect” box. These are very sensitive and it only takes a very small turn to make a large difference.

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Example of the power level measured at the output of the SuperK connect Box in April 2025.

Peaking up this power level after adjusting the four knobs on the SuperK Connect achieves the alignment.

Other Checks

It can be valuable to check the power level at other locations in the optical chain. The output of the etalon itself can be checked by putting the power meter on the output fiber (black).

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Output of the etalon measured in April 2025. This is (very roughly) 3 orders of magnitude less than the input to the Etalon as measured above, which is broadly consistent with the amount of filtering expected.

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This is the output of the orange fiber which is the input to the cal bench. There is one additional alignment jig on the cal bench (outside the enclosed part of the cal bench) which can be used to adjust this. This power level should be broadly similar to that measured at the output of the etalon (minus some modest losses in the fiber couplings).