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Nick Brewer: Difference between revisions
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'''<big>2/8/17</big>'''
Some RF power from the VCO seemed to be getting through even when it was not being triggered by the delay box. Also, when the VCO was connected to the 15 V supply it seemed to add noise to the SHG error signal. So I took it out and now I'm just using the RF generators to look at one frequency at a time.
I think the backburned peak I'm trying to see EIT on is too wide. A rough calculation is showing that the EIT peak should be a couple kHz wide, and the backburned peak is closer to 4 MHz. I'm trying to narrow it by adjusting the time and power that go into creating it.
It's difficult to get more than ~1.4 W of green light from the SHG cavity and have it be stable for a while.
The best bet might be focusing the beams tighter to get more intensity in the EIT beam.
'''<big>1/30/17</big>'''
Been trying to see EIT. No luck yet.
There is around 1.3 W of 527 nm light coming out of the SHG cavity. The beam goes through one beam splitter and as much light as possible is reflected, the remaining transmitted light is enough for the repump beam. The next beam splitter again reflects as much light as possible, this reflected beam goes to the control beam AOM. The remaining transmitted light through the second beam splitter is enough for the probe and goes to an AOM in a double pass configuration (so if I sweep out a spectral hole 20 MHz wide, the RF generator only sweeps over a 10 MHz range).
I try to keep the maximum RF power going into any AOM less than 5 W (37 dBm).
The control and repump beam are coupled into the same fiber using a polarizing beam cube and a polarization maintaining fiber. The output of the fiber is sent through a glan taylor polarizer and a 1/2 wave plate. The probe is also sent through a fiber with a glan taylor and 1/2 wave plate at the output. There is about a microwatt of repump and probe power. I have seen control beam powers before the crystal up to 350 mW. If the RF is on for any length of time the fiber seems to become misaligned (presumably from the AOM heating up) so the power needs to be checked quickly. The beam can be walked through the fiber at low power, and optimized by using the delay box to send short (~10 microsecond) pulses through at a time that can be seen on a photodiode.
The two beams are directed parallel to each other, but horizontally displaced by ~3/4 inches, and sent through a 200 mm lens so that they focus and overlap each other in the crystal. This geometry is used so that the probe can be separated from the control/repump beams. There is another 200 mm lens on the other side of the crystal to recollimate the beams.
I keep the cryostat between 4.5 K and 5 K mostly by adjusting the pressure in the dewar. I try to keep the pressure around 3 psi, and at this pressure the cryostat is around 5 K. Over the course of a few hours the pressure gets closer to 6 psi at which point the cryostat is usually close to 4.5 K. At that point I vent it back to 3 or 4 psi.
I have a Matlab program that controls the delay box and the RF generators to the probe, control, and repump beams.
RF1 is the probe
RF2 is the repump
RF3 is the control
RF2 is on all the time except when the EIT sequence is executed. I start by sweeping out a 20 MHz wide spectral hole for 5 seconds with the probe beam at the max RF power (+5 dBm for the current RF generator and amplifier). Then for one second I turn on the control beam at -30 dBm. At this point a spectral hole should be swept out and a backburned population with width of ~2 MHz should be in the middle of the trough.
An RF switch changes the input to the probe to a VCO instead of the RF generator. An integrator integrates a 10 microsecond pulse from the delay box to create a triangle wave, which is used as the tuning voltage of the VCO. The circuit is built so that the offset and amplitude of the triangle wave can be adjusted by potentiometers, therefore changing the range that the VCO scans over.
'''<big>1/4/16</big>'''
Today the cavity is working great. There were a few small peaks between the 0,0 peaks but they were small and I was able to get rid of them by adjusting the mirrors and mode matching lenses. Yesterday's problems might have been a laser issue due to the weather. This morning the humidity was 30%, I'm not sure what it was yesterday but I should keep an eye on how that affects how well the cavity is behaving.
I must have done the <math>I_r/I_{in}</math> measurement wrong on 5/21/14. Today I measured the same finesse that I got two days ago (~65.5
I redid the mode matching calculation with g=0.953, <math>R_{in}=0.
According to my Matlab program, we should expect 15.9% conversion efficiency. I was getting 10 mW of green for 65.5 mW of IR, which is 15.3%.
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