Application Cases

Experiment Title: Study on the PDH Locking System of Fiber Ring Resonators

Testing Equipment: High Voltage Amplifier, Signal Generator, Oscilloscope, Electro-Optic Phase Modulator, Analog PID Controller, Piezoelectric Ceramic, etc.

Experiment Process:

Figure 1: (a) Experimental setup of the fiber ring resonator. (b) Fiber stretching holder: PZT: Piezoelectric Ceramic. The piezoelectric ceramic is placed at the center of the slit, and the fiber is fixed at the ends of the U-shaped holder arms using UV glue.

Based on the stable fiber ring resonator mentioned above, we combined the PDH (Pound-Drever-Hall) feedback locking technique to lock its resonance frequency. The resonance frequency of the fiber ring resonator is controlled by adjusting the fiber length. The cavity length control device has been introduced in Figure 1(b). The schematic diagram of the fiber ring resonator locking device is shown in Figure 2. The electro-optic modulator phase-modulates the laser in free space before coupling it into the fiber ring resonator. The output laser from the fiber ring resonator is split by a fiber beam splitter and then output to the AC detector 1 and DC detector 2 for generating the error signal required by the PDH frequency stabilization system and monitoring its reflection spectrum. The signal generator produces the EOM modulation signal, which is split into two paths by a power divider and input into the EOM and phase shifter for phase modulation and delay of the laser. The signal after phase delay is mixed with the cavity reflection signal detected by the AC detector 1 in the mixer, and then successively passes through a low-pass filter, PID controller, and high voltage amplifier before being fed back to the piezoelectric ceramic used for cavity length control as shown in Figure 1(b). This real-time adjustment and control of the fiber ring resonator cavity length are used to lock the resonance frequency of the FRR. We use the oscilloscope to simultaneously monitor the driving voltage of the PZT, the error signal, and the reflection spectrum of the FRR.

Figure 2: Schematic diagram of the PDH locking experimental setup. EOM: Electro-Optic Phase Modulator, HVAmplifiers: High Voltage Amplifiers, PZT: Piezoelectric Ceramic, PID: Analog PID Controller. Solid lines represent the optical path, and dashed lines represent the electrical circuit.

Experimental Results:

Figure 3: (a) Locking results with phase modulation power at 12dBm: The cyan solid line is the direct output reflection spectrum of the fiber ring resonator, the black curve is the reflection spectrum after passing through the low-pass filter, the red curve is the frequency-discriminated signal, and the blue curve is the reflection signal after locking; (b) Locking results with phase modulation power at -9dBm: The black curve is the direct output reflection spectrum of the fiber ring resonator, the red curve is the frequency-discriminated signal, and the blue curve is the reflection signal after locking.

The reflection spectrum of the fiber ring resonator is obtained by scanning the length of the piezoelectric ceramic. Figures 3(a) and (b) show the reflection spectra when the EOM modulation signal power is 12dBm and -9dBm, respectively. In Figure 3(a), the cyan and black curves are the direct output and filtered reflection spectra of the fiber ring resonator, respectively. The electro-optic modulator phase-modulates the laser beam in the optical path, producing two sidebands. Due to the sensitivity of the fiber resonator to phase, the interference between the main frequency of the resonator and the sidebands causes intensity modulation of its reflection signal. The red curve is the frequency-discriminated signal of the locking system, and the blue curve is the reflection signal of the fiber ring resonator after locking. In Figure 3(b), the black curve is the filtered reflection spectrum of the fiber ring resonator output signal, the red curve is the frequency-discriminated signal of the locking system, and the blue curve is the reflection signal of the fiber ring resonator after locking.

From Figure 3(a), we can see that the direct output reflection signal of the fiber ring resonator carries intensity modulation at the same frequency as the phase modulation signal from the electro-optic modulator, which is extremely unfavorable for its subsequent applications. Therefore, we performed frequency analysis on the reflection signal of the fiber ring resonator after locking, and the measurement results are shown in Figure 3(a). There is an intensity modulation signal at the frequency equal to the cavity reflection signal and the phase modulation signal frequency. The power of the phase modulation signal should be reduced in the experiment to minimize the impact of this modulation signal on the intensity modulation of the cavity reflection signal. In Figure 3(a), the red, blue, black, and purple curves are the spectra of the cavity reflection signal when the phase modulation signal power is -9dBm and 24dBm, the background noise of the spectrum analyzer, and the electronic noise of the detector, respectively.

High Voltage Amplifier Recommendation: ATA-7020

Figure: Specifications of the ATA-7020 High Voltage Amplifier

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