[Ushiba, Takano, Saito]

The VCO efficiency was set to match the efficiency of the sub-laser PZT, which is 1.871 MHz/V. Under this condition, a filter was designed so that the UGF of the open-loop transfer function became 10 kHz. Since the PFD requires an input signal larger than 300 mVpp, while the actual beat signal is only about 0.2 mVpp, it may be difficult to use the PFD in practice. PLL operation was also successfully achieved using a mixer instead of the PFD. Furthermore, it was confirmed that if the beat signal fluctuation is smaller than approximately 936 kHz, lock acquisition is possible by turning on the integrator at the appropriate timing.
 

• To match the efficiency of the VCO in Moku:Lab to that of the sub-laser PZT, the VCO efficiency was set to 1.871 MHz/V. Then, as in the previous experiment, the open-loop transfer function was measured, and the filter was adjusted so that the UGF became 10 kHz. The filter used is shown in Photo 1. According to klog:35917, resonance of the main laser PZT is observed above 80 kHz, and the resonance frequency of the sub-laser PZT is expected to be similar. Therefore, setting the UGF to 10 kHz is considered sufficiently safe. In addition, the PFD requires an input signal larger than 300 mVpp, whereas the current PLL beat signal (klog:36919) is only about 0.2 mVpp. Since amplification by more than a factor of 1000 would be required, the PFD is not suitable for use in the PLL system.

• When a mixer was used instead of the PFD, the filter was designed in the same manner as for the PFD case, and PLL operation with a UGF of 10 kHz was successfully achieved. Furthermore, because the frequency of the actual beat signal fluctuates, an additional signal with an amplitude of 500 mVpp and a frequency of 100 mHz was applied to verify whether PLL operation could still be maintained under frequency fluctuations. This corresponds to a fluctuation of approximately 936 kHz. If the same filter used for the non-fluctuating case was applied directly, oscillation occurred. Therefore, one of the two integrators was turned off, and lock acquisition was achieved by turning the integrator on when the error signal frequency became sufficiently low.