Today Tara and I worked on getting a noise measurement for the bare laser diode using a Michelson interferometer with different arm lengths. The setup is attached. However, at a differential arm length of 20 cm we were unable to see interference because it was too difficult to focus the beams. Tara suggested I use a symmetric Michelson interferometer to see if I can get interference, since the noise levels might be too high for such a large arm length. I then tried much smaller differential arm lengths and I was able to get interference at 1 cm and 5 cm.
I took background measurements of the noise from the SR785 (about 20 nV/rtHz) and from the blocked photodiode (electrical noise, about 50 nV/rtHz). Since these were both small, we can be confident that the measurements we took are mostly the noise from the frequency of the laser diode.
The results from the 1 cm and 5 cm measurements are attached. We seem to have noise levels close to what we predicted (1 MHz/rtHz), which seems odd since there will be extra noise from mechanical components, temperature fluctuations, and a worse current driver than we planned to use. In addition, this doesn't explain why we weren't able to get interference at a differential arm length of 20 cm. The 5 cm measurements have even lower noise levels for some reason. I'm not sure if I'm doing something wrong with factoring in the gain, so I'm going to check my math. Gain still confuses me a little since there's a different gain on each machine I used. Overall, the measurements seem suspiciously low noise.
I'm going to check these calculations again this weekend to make sure I didn't mess up. I will also revise my presentation so that I will be ready to present on the LLO SURF field trip.