Application Cases

Experiment Name: Automatic Re-locking of Optical Resonators

Testing Equipment: Power amplifier, signal generator, STEMlab board, piezoelectric ceramics, etc.

Figure: STEMlab Development Board

Experiment Process:

The basic parameters of the mode cleaner used to test the automatic re-locking function are shown in Table 1. The frequency of the triangular wave signal output by the arbitrary signal generator is set to 42Hz, and the high-voltage amplifier bias is set to 178/180. Adjust the mode-matching lens in the optical path to maximize the transmission peak. At this point, higher-order modes are maximally suppressed, and only the fundamental mode resonates within the cavity.

Table 1: Basic Parameters of the Mode Cleaner

Figure 2: Error Signal and Transmission Signal in Scan Mode

As shown in Figure 2, the error signal obtained in the experiment has a large slope on either side of the center point, indicating high sensitivity, making it a qualified PDH error signal suitable for feedback control. The peak of the transmission signal corresponds to the point where the slope of the error signal is the greatest. At this point, the error signal fluctuates around zero, and the ideal resonance state between the optical resonator and the reference laser frequency is achieved at this point. After finding the optimal mode of the transmission signal, the arbitrary signal generator can be turned off, and the error signal is sent to the fuzzy PID controller. The output signal of the fuzzy PID controller is converted by the DAC and amplified by the high-voltage amplifier to control the cavity length change of the optical resonator. At this point, the servo feedback control loop has been formed, with the control signal and cavity length mutually constraining each other, and the system enters the locking mode. Figure 3 shows the direct current transmission signal of the optical resonator and the error signal at this time, collected by the oscilloscope integrated inside the FPGA in the locking mode.

Figure 3: Error Signal and Transmission Signal in Locking Mode

The re-locking mechanism of the optical resonator is achieved by a program on the computer terminal that automatically switches the PDH locking system between scan mode and locking mode according to the triggering conditions. The complete locking system model is shown in Figure 4.

Figure 4: Automatic Re-locking System Block Diagram

Experimental Results:

The STEMlab board, optical resonator, and photodetector components were connected, and the optical resonator was locked to the reference frequency using PDH technology, entering the locking mode. At this point, the re-locking program was started, which cyclically judged the transmission signal collected by the photodetector and the value of the FPGA integrator during operation. To verify the automatic re-locking function, the incident light beam was manually blocked in the locking mode to unlock the optical resonator. The direct current monitoring transmission signal collected by the photodetector dropped from the original locked position to the ground line. Figure 5 shows the process of the system from locking to unlocking and then re-locking. The optical resonator unlocked at 0.9s and relocked at 3s.

Figure 5: Direct Current Transmission Signal During Re-locking Process

It can be seen that in the actual test of blocking the injected signal light field for 2 seconds, the optical resonator can successfully automatically return from the unlocked state to the locking mode, and the locking position is almost the same as before unlocking, without manual operation. After multiple experimental tests, the time required for the system to complete this process is usually no more than 2 seconds, meeting the basic requirements for general experiments.

In the experiment, the use of FPGA instead of traditional analog circuit components effectively improved the degree of automation of each link in the servo system, saving some repetitive manual adjustment work for experimenters. Moreover, it has significant advantages in terms of cost and space occupation. However, due to the relatively large electronic noise of digital circuits, output limiting, and limited resolution, their control accuracy for locking optical resonators is sometimes still not as good as analog circuits. When locking most resonators with lower finesse, FPGA can be used. For high-finesse resonators, analog servo circuits are more suitable.

Power Amplifier Recommendation: ATA-7030

Figure: Specification Parameters of the ATA-7030 High-Voltage Amplifier

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