[Tanaka, Hirose, Saito]

We constructed the PLL control system in which a feedback signal was generated using a mixer, a local oscillator, a SR560, and a Moku:Lab, and fed back to the PZT of the sub-laser. Unfortunately, the control system did not function successfully.
 

• Since the previous observation of the beat signal (klog:36835), the sub-laser had remained on with its temperature kept at 31.57 ℃. Under this condition, a beat signal was observed; however, its amplitude changed significantly, and its frequency drifted by about 5 MHz over approximately one minute. Because the frequency was close to 0 Hz and the range over which it could be further reduced was limited, we increased the sub-laser temperature to 31.60 ℃ to raise the beat signal frequency to about 112.1 MHz (Photo 1). Next, this beat signal and a local oscillator signal (112.1 MHz, 10 dBm) were input to the mixer to generate an error signal. The mixer output was then fed into the SR560, where it was amplified and passed through a first-order low-pass filter with a cutoff frequency of 1 MHz. The output of the SR560 was subsequently input to the Moku:Lab, where an integrator was applied using a PID controller (Photo 2). The output of the Moku:Lab was connected to the PZT of the sub-laser, and the overall gain was varied. However, the error signal did not decrease, and no change was observed in the feedback signal. The error signal amplitude was 1.090 V; however, when the local oscillator frequency was set to 90 MHz, it became 409.5 mV, and at 80 MHz, it became −154.0 mV, indicating that it varied with the local oscillator frequency (Photo 3). On the other hand, changing the temperature of the sub-laser did not result in any noticeable change in the error signal amplitude. This suggests that the signal being treated as the error signal may in fact be a different signal. It is also possible that the Moku:Lab was operated incorrectly.