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More about wiggler problems
There are 4 ways that a wiggler can fail to deliver good power into the optical trap:
(1) If the polarization of the light emanating from the fiber end is wrong, then much of the power splits in the wrong direction at the first beamsplitter cube. To test for this problem, remove the beam block from underneath the prism box and use an IR indicator card to compare the wasted power coming out the bottom of the box between the A and B prism boxes. To fix such a problem, reposition and re-tape the fiber between the laser and the top of the wiggler while watching the SUM reading change on the computer monitor.
(2) While disconnecting / re-connecting the wiggler fiber from the laser, some dirt may have become trapped on the surface of the FC/PC connectors. Use a fiber inspection scope such as Thorlabs FS201-FC to inspect the connector on the wiggler. If dirt is observed, it may be removed by wiping the surface with lens tissue moistened with a drop of alcohol. Note that big dirt on the wiggler connector indicates matching dirt inside the laser connector which must be cleaned by opening the laser case. Be careful of the fragile fiber inside and avoid static electricity.
(3) Perhaps dirt has become stuck to the emission end of the wiggler fiber. To test for this problem, obtain or build a fiber-coupled visible laser, such as Thorlabs LPS-635-FC + EK1101, and couple it to the wiggler using a mating sleeve, e.g., Thorlabs ADAFCB1. Use a piece of paper to observe the visible light cone emanating from the wiggler tip. The spot should be circular and Gaussian with no extra lobes. To fix dirt on the fiber tip, try touching it with adhesive tape and peeling off the dirt. Next, try alcohol or acetone to loosen such dirt. Finally, it may be necessary to replace the whole fiber.
(4) During step 3 with visible light, if a bright spot of light is seen coming out somewhere along the yellow furcation tubing leading into the wiggler, then the fiber is broken inside the tube and the whole fiber must be replaced. In the future, try to avoid making sharp bends in the wiggler optical fiber.
P.S. Some of the visual-light tests can also be performed with a cheap cell phone camera which “sees” out past 800 nm. However, such a camera is not as sensitive as the human eye viewing 630 nm red light.
Contributed by: Steve Smith
Last update: 25/04/2023 20:15