David Notebook: Difference between revisions

'''5/23/14'''

The last few days were spent trying to improve the 633 signal and better isolate it. We used a [[Thorlabs PDA36A Si Switchable Gain Detector|Thorlabs PDA36A]] photodiode and pushed it right up again the fiber, but could only get a very weak signal. However it did seem to correspond with locking the laser. The laser locked well enough to try walking the fiber connection and the signal was improved, but eventually it was discovered that the 780 beam was blocked at the source. This means that some other light was getting to the fiber, even past all of the filters we had set up. 1064 seems a likely candidate. We added in an old cavity window, which reflects almost all 1064, realigned the 780 beam and found a signal we believe comes at least mostly from 633. We also switched to the much more sensitive [[UDT Sensors PIN 5DP Photodiode|PIN 5DP Photodiode]] and [[Stanford Research Systems SR570 Low Noise Current Preamplifier|SR570 Preamplifier]] combination. While the previous signal was responsive mostly to adjusting the fiber coupling and little response to the 780 beam alignment, this signal was responsive to both. We were able to improve the signal by adjusting both the fiber coupling and 780 alignment. We also tried using a power meter, and while the results were not entirely clear, we believe we might have seen a signal of a few tenths of nW when locking.

 

We wanted to be more sure that the signal we were looking at was entirely 633, which can't be done with our current method even with numerous filters in place. We instead sent the cavity beams through a prism and aligned an approximately 2 foot long piece of PVC pipe and a small iris until we could only see the 633 beam through it by eye. Further adjustments greatly reduced the amount of scattered ASE that was previously visible. We then put in a 30mm -B lens at the end of the pipe and lined it up to the power meter. Measuring with the power meter proved too difficult even when covering the setup and blocking most external light to the room, but using the preamp and photodiode, we could detect a signal. There was some background noise, likely from light leakage, but when locking the laser we saw spikes between 100-300 mV from the photodiode. At 633 nm, the photodiode has a sensitivity of about <math> 0.4 </math> <math> Amps/Watt </math>. The preamp was set at <math> 10^9 </math> <math> V/Amp </math>. [[Converting Photodiode Signal to Laser Power|Calculating the beam power]], we found the 300 mV signal to be 0.75 nW, which was consistent with our previous observations, although we expected this to be somewhat higher now that this was not coupled through a fiber.

 

Next time maybe try removing filters to reduce 633 losses, since we will spatially separate it from the other beams with the prism.