Nishino

This is a continous work of 3734.

I measured the actuator efficiency (A) of the piezo-atattched mirror with a Michelson interferometry(img0959.jpg, layout.png). The result is: there still exists resonance around 660 Hz, which prevents to increase UGF.

1) Setup

Optical setup is a simple Michelson. One mirror is the PCM and the other is a super mirror with the same curvature. The piezo is connected to a voltage amplifier (*10 in amplitude), which is connected to the output port of the filter F (Moku:Go). F is a first-order filter shown in filter.png. The interferometer is locked in mid-fringe. The output (Vout) is fedback to the piezo (diagram.png).

2) Estimation of the optical gain (H)

Scanning the Vin over a fringe, one can estimate the optical gain (H) of the system. One fringe takes 1.33*10 V to the piezo (mokuoscilloscopedata20240918145453screenshot.png), which means:

H = 532 nm/(13.3 V) ~ 4e-8 [m/V] = 40 [nm/V]

In spec, PA44LEW has an efficiency of 2.6 um/150 V = 17 [nm/V], which is lower than the measured value. 

3) Open loop transfer function (OLTF)

OLTF is shown in OLTFandAH10.png. There still exsists peaks around 660 Hz. This prevents to expand the UGF.

4) Estimation of the actuator efficiency (A)

A.png is the actuator efficiency. It is flat at low frequencies but has a structure around 600-700 Hz. This is an independent result that shows an oscillating mechanism in the piezo-bonded PCM.

5) Discussion

According to Akutsu-san and Takano-san, steering mirrors with springs for adjustment have resonance around 500 Hz. Their suggestion is to mount the PCM to a solid mount without steering and prepare another mirror only for alignment. This hypothesis can be checked by replacing the current mount to a different one and see if the resonant frequency shifts.