Application Cases

Experimental Name: System Analysis of Phase-Locked Loop for Repetition Rate Stabilization

Experimental Content: To address repetition frequency drift, two phase-locked loop (PLL) systems were introduced to provide feedback control, locking the repetition rates of two lasers to the same stable clock source. This chapter primarily elaborates on the principles of classical PLLs, the theory and key components of PLLs for repetition rate stabilization, and the analysis of results.

Test Equipment: High-voltage amplifier, photodetector, low-pass filter, proportional-integral controller, PZT, etc.

Figure 1: Structure Diagram of the Phase-Locked Loop System for Repetition Rate Stabilization

Experimental Process:

The system structure is shown in Figure 1. A portion of the light from the NPR mode-locked fiber laser is coupled into a photodetector with a bandwidth of 5 GHz, which converts the optical signal into an electrical signal and simultaneously extracts the repetition frequency. This electrical signal passes through a DC block and then enters a mixer (phase detector) along with an RF signal from a stable clock source. The repetition frequency and the clock source's RF signal are mixed, producing sum and difference frequency signals. The difference frequency signal, which is the required error signal, passes through a low-pass filter (LPF) to remove the sum frequency component. The difference frequency signal then enters a Proportional-Integral (PI) controller. Here, the LPF and PI controller together constitute the loop filter of the PLL. The output error voltage from the PI controller ranges from –10V to +10V, which is insufficient to drive the Piezoelectric Ceramic Transducer (PZT). It first enters a High-Voltage Amplifier (HVAMP) where it is amplified tenfold before being applied to the PZT. Different voltage values cause varying degrees of expansion/contraction in the PZT, exhibiting a roughly linear relationship. As the PZT cannot directly change the cavity length within the fiber laser cavity, it drives a translation stage. The fiber cavity is wound around this translation stage, thus allowing the PZT's expansion/contraction to induce changes in the cavity length and adjust the laser repetition frequency. This forms a closed-loop circuit where the repetition frequency tracks the clock source's RF signal. When the laser's repetition frequency matches the clock source's reference frequency, the error voltage becomes a DC level, the PZT state stabilizes, and the laser's repetition frequency locks onto the reference frequency. If external environmental factors disturb the laser cavity, breaking the stable state, the error voltage signal fluctuates, and the feedback mechanism reactivates, adjusting the cavity length to quickly restore repetition frequency stability. After establishing one PLL system to stabilize the first mode-locked laser's repetition rate, the entire PLL process is repeated to lock the repetition frequency of the second mode-locked laser, using a reference frequency generated by the same clock source, ensuring relative stability between the repetition rates of the two lasers.

Experimental Results:

Figure 2: Time-domain waveforms at various points in the PLL intermediate chain.

After successfully building the PLL, various points in its intermediate chain were tested. Figure 2(a) shows the error voltage signal after mixing the repetition frequency with the clock source's reference frequency. Oscillatory ripple within the envelope is observable, indicating the presence of both the difference frequency and residual sum frequency signals. Figure 2(b) shows the waveform after the error voltage signal passes through the low-pass filter, extracting the difference frequency. Figure 2(c) shows the error voltage signal after processing by the PI controller. Figure 2(d) shows the error voltage signal becoming a DC level once the stable state is achieved.

High-Voltage Amplifier Recommendation: ATA-2048 High-Voltage Amplifier

Figure: ATA-2048 High-Voltage Amplifier Specifications and Parameters

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