[Enomoto, Sugimoto]

4/26 work

(The figure numbers are 1, 2, 3 ... from the left end)

We measured the OLTF by inserting a prototype notch filter to cancel the mechanical resonance of the laser PZT in the Y-side phase lock loop.

The notch filter was incorporated in The previous stage of the laser PZT.

Fig. 1 shows the notch filter design, and Fig. 2 shows the measured transfer function.

It was found from the previous measurement (8434) , that there is a large PZT resonance around 280 KHz-290 kHz. The notch filter was designed to its resonance.

Fig. 3 shows the OLTF measured before and the OLTF measured by inserting the notch this time.

The results of measurement with high resolution to see finer peaks (the frequency ranges were measured separately for 20kHz-100kHz, 100kHz interval from 100kHz to 500kHz) are shown in Fig. 4 .

It can be seen from this figure, that we have succeeded in cancelling the resonance.

On the other hand, the phase in UGF is delayed, and the phase margin, which was about 45 deg before the insertion of the notch, is about 30 deg.

This is not enough to raise the gain more than now.

The final stage of the common mode servo has a pole 100kHz passive low pass filter.This is expected to cancel out the PZT resonance which is above 100 kHz.

The results of simulating the case where the low-pass filter is removed by considering the insertion of the pole 100 kHz anti-low-pass filter are figs 6 and 7.

It can be seen that the phase margin recovers to about the same value as before the notch filter was inserted.