Yuhang and Michael

The vertical alignment of the beam reflection from the pase shifter was improved. We can see that the incident and reflected beams stay at the same height for a long distance (figure 1).

Afterwards, the pitch efficiency of the IRMC was checked. In 2424, the phase shifter was given a DC voltage in order to cause mode mismatch in the IRMC. Much more light went into TEM01 than TEM10 at high offset - at 70V offset on the high voltage driver, the ratio of TEM01 to TEM00 was approximately 23%, as seen in figure 2 of 2424. This time, we did a quick measurement of the reflection spectrum at 70V offset and obtained 544 mV in TEM00, 132 mV in TEM01 and 32 in TEM10, corresponding to 18.6% power into TEM01. Which is improved but still not optimal.

From finesse simulation and experimental results, we theorize that the pitch misalignment is coming from bending of the piezo element controlling the phase shifter. In this case, there may be a static bending in the pitch direction causing misalignment when the piezo is excited. Ideally we only want longitudinal motion of the piezo. Yuhang also simulated the effect of phase shifter misalignment versus the phase shifter proximity to the beam waist, and found that the noise is reduced as the beam waist is brought closer to the phase shifter. In the previous arrangement, the beam waist after the 250 mm lens was located approximately two breadboard holes from the phase shifter (see 2393 for a screenshot of the relevant optical table layout). Now, the beam waist is located approximately one breadboard hole after reflection from the phase shifter, and we see a corresponding reduction in pitch noise from the measurements in 2407 compared to 1904.

Following this, we will move the phase shifter to the beam waist position. However, there isn't enough space on the optical table to move the phase shifter one hole backwards, so the IRMC mode matching lens setup may need to be reworked.