Michael and Yuhang

We completed the setup as per 2393. The spectrum of aligment noise from excitation of the IRPS was made and compared to 1904.

The measurement was taken using a 500 Ohm PSD placed after the IRMC and homodyne flipping mirror. We measure the dark noise, followed by the jitter noise when transmitting the local oscillator, and then the jitter noise when the phase shifter PZT is excited by random noise from the spectrum analyser, applied via the high voltage driver. In the original measurement of 1904, the PZT was excited using 200 mVpk random noise, where the beam had a horizontal angle of incidence of ~ 45 degrees. Here, the beam has perpendicular incidence, so we only use 200/sqrt(2) ~ 140 mVpk excitement for the yaw measurement, but keep 200 mVpk for the pitch measurement.

For yaw, we seen that the noise floor seems to be the same or perhaps slightly better than 1904, it is hard to tell just from the picture. However, the new measurement reaches the noise floor at approximately 30 Hz, compared to ~ 10 Hz in the old measurement. Also, both measurements have similar values at either end of the measurement frequency window, but the new measurement looks to be a lot higher in the mid frequencies than that of the old measurement. The PD dark noise is consistent in both cases.

For pitch, the measurement is very strange. Unlike yaw in this case and both degrees in 1904, the LO jitter noise does not converge to the PSD dark noise at high frequency. Both the LO jitter and excitation jitter seem to be about 15-20 dB higher than their yaw counterparts.

Afterwards, we measured the transfer function of input excitation (from the spectrum analyser) to measured PSD noise. We see that the pitch transfer function has significantly more coherence from the excitation to the PSD. The yaw coupling at 140 mVpk excitation is also much lower than for pitch excitations.

A few other notes:

- The IRMC alignment seems to drift quite a lot. Last week it had to be relocked about 4 or 5 times during the afternoon. It also had dropped to about 1.6 mW transmitted power before starting the measurement described in this elog. I realigned it for the optimal transmission each time (1.8 mW).

- The phase shifter is not quite flat, it has some pitch tilt. As a result, the incoming alignment is not perfectly horizontal, but rather is adjusted so that the outgoing beam maintains a constant height of 76 mm.

- PSD output voltage was zeroed to within 0.5 mV for both X and Y