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David Notebook: Difference between revisions
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The power of the 780 reaching the cavity must first be maximized. Use the magnetic mirror on the 780 side of the cavity to send the beam towards the middle of the table and measure the power. This is the only place where the power meter can be easily put in.
Adjust the alignment of the seed laser to the TA to maximize the power measured at the cavity--usually just one mirror is sufficient, and the horizontal adjustment tends to have a much larger affect than the vertical. Then re-walk to the isolator, and then finally to the fiber. These adjustments have a small enough effect on the overall position of the beam that it is not necessary to adjust the placement of the power meter. Repeating these alignments once more usually gives an additional 5% or so in power since the aligning to the TA and isolator will slightly affect the coupling to the fiber.
Assuming that these adjustments have been small and that the 780 is still mostly aligned to the cavity, center two irises on the 1064 side of the cavity on the 780 beam with the irises placed a foot or two apart. Then maximize the HeNe power through these irises (close them all the way and walk it to get maximum power after the second iris) to ensure that it is overlapped with the 780 beam. This is usually a small enough adjustment that the HeNe is still centered on the photodiode, but occasionally re-centering it or even re-aligning it to the PVC pipe is necessary. Then align the 1064 beam as usual (cool the cavity, perform threshold tests, walk beam to get good free-spectral ranges). 633 should be visible in the flip mirror past the grating now when ramping the cavity. A signal may be detectable with the lock-in/photodiode at this point when locking the 1064, but note that it's possible that the 633 can be visible in the mirror and be aligned to the photodiode and still produce no signal on the Thorlabs PDA10A detector. This photodiode is better for getting an absolute power measurement of the 633 since it has a faster response time and produces good square waves, but the UDT Sensors PIN 5DP is less noisy and can be used at higher gain settings. The power of the 633 can easily vary by a factor of 10 with little change in perceived brightness in the mirror, so rather than try to re-align to the grating/PVC pipe/photodiode at this point, it's better to first switch out the photodiode, quickly recenter it using the HeNe (adjusting the photodiode position, not the HeNe alignment), and then maximize whatever signal is found by walking the 780. The alignment change by walking the 780 is small enough that it should still be overlapped well with the HeNe, but if the 780 alignment was very off it might be necessary to use the irises to overlap the HeNe again and check that it is still centered on the photodiode.
I measured conversion efficiency vs pump power--it is fairly linear as expected. See [https://wiki.physics.wisc.edu/yavuz/images/8/85/633conversionefficiency.PNG graph]
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