[Torrey, Jeff, Daniel]
Continuing work here, we maximized the power of the beam that's shifted twice by the AOM by changing the tip and tilt of the curved mirror. We are fairly confident that this is the correct beam because it doesn't move (very much) on the camera (~6" downstream of the AOM) when we change the input frequency of the AOM by 30 MHz. The power of this beam is 0.87 mW. I then aligned this beam into a fiber collimator and got a power of 0.73 mW. We then swept the AOM frequency to evaluate how much power still gets into the beam.
Below is a table with some data. The Assumed Output Power uses the AOM specs here. It assumes for reference a 0.87 mW power for 200 MHz and is proportional to the square of the efficiency.
| AOM Input Frequency (MHz) | Assumed Twice Shifted Output Power (mW) | Measured Twice Shifted Power Through Fiber (mW) | Fiber Efficiency |
|---|---|---|---|
| 200 | 0.87 | 0.73 | 84% |
| 170 | 0.60 | 0.6 | 100% |
| 185 | 0.82 | 0.69 | 84% |
| 215 | 068 | 0.61 | 90% |
| 230 | 0.60 | 0.48 | 80% |
I definitely don't trust the efficiency above ~85%, but I think this gives a very good indication that we can move the AOM frequency by ~60 MHz and still have enough light to lock the cavity. 0.48 mW would saturate the fast PD, so we need to use an ND filter anyway.
I think we should copy this scheme for future AOM setups and maybe go back and adjust the mirrors. I should note that I didn't do any fine adjustment of the location of the curved mirror whatsoever. Maybe we got lucky, but this hopefully means that the path is fairly easy to set up.
The next step is to verify that this is twice shifted light. I think the easiest way is to use it in a cavity.