Nick Brewer: Difference between revisions

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 controlscontrol 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.

Right 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.

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 looked at the reflected signal before it went through the beam cube and got a similar measurement for <math>I_r/I_{in}</math> (5.4/7.02 = 0.769). I think the reflectivity needs to be measured once the temperature is optimized for green generation. When I did this I got <math>I_r/I_{in} = 0.239</math> (accounting for a 106 mV noise the photodiode was seeing).

I redid the mode matching calculation with g=0.953, <math>R_{in}=0.05239476</math>, and <math>I_r/I_{in} = 0.239</math>. With this numbers I get <math>m=0.759</math>. This is good because it means the cavity is mode matched pretty well, and also that the cavity must be pretty well impedance matched. If the impedance matching is perfect, I think the mode matching coefficient, m, is just <math>1-I_r/I_{in}</math>. In this case, <math>1-I_r/I_{in}=0.761</math>.