Application Cases

Experiment Name: Laser Noise Filtering by a Mode Cleaner

Testing Equipment: High-voltage amplifier, electro-optic modulator, power splitter, phase shifter, low-pass filter, proportional-integral controller, etc.

Experimental Process:

Figure 1: Locking System of the Mode Cleaner, (a) Electronic Circuit for Cavity Locking, (b) Frequency Discrimination Curve

In the experiment, the cavity of the mode cleaner is made of Invar, a material with an extremely low thermal expansion coefficient. The finesse of both the 671nm and 1.34μm mode cleaners is 400, with a linewidth of 750kHz each. We use the Pound-Drever-Hall (PDH) frequency stabilization technique to lock the cavity length of the mode cleaner to the resonance frequency of the incident laser. The transmission rates of the 671nm and 1.34μm mode cleaners are 50% and 60%, respectively.

Figure 1(a) shows the experimental setup for locking the cavity length of the mode cleaner using PDH technology. The process of locking the mode cleaner cavity consists of two steps: frequency discrimination signal adjustment and locking. The frequency discrimination signal adjustment process is as follows: First, a signal source outputs a 30Hz triangular wave, which, after being amplified by the high-voltage amplifier, is used to drive the piezoelectric ceramic (PZT) that fixes the concave mirror in the mode cleaner to periodically change the cavity length of the mode cleaner. Meanwhile, a high-frequency signal source outputs a 49MHz sine signal. A power splitter is used to divide this modulation signal into two equal-power signals, S1 and S2. S1 is amplified by a power amplifier and then used to drive the electro-optic phase modulator to phase-modulate the incident laser. S2, serving as the local signal, is first phase-adjusted by a phase shifter and then fed into a mixer to mix with the AC signal output by the detector. After the mixing signal passes through a low-pass filter to remove high-frequency signals, the error signal shown in Figure 1(b) is obtained. By optimizing the parameters of the phase shifter and the amplitude of the modulation signal, the best frequency discrimination curve is obtained. Then, the triangular wave signal for periodic cavity length scanning is removed. The error signal is amplified by a proportional-integral-differential controller (PID) and a high-voltage amplifier (HV) and fed back to the PZT that fixes the concave mirror in the mode cleaner to control its cavity length, thereby achieving cavity length locking.

Experimental Results:

Figure 2: Noise Characteristics of 671nm/1.34μm Dual-Wavelength Laser Output; (a) Noise Characteristics of 1.34μm Laser, (b) Noise Characteristics of 671nm Laser

After locking the cavity length of the mode cleaner to the resonance frequency of the incident laser using the locking loop shown in Figure 1(a), the noise characteristics of the 671nm and 1.34μm lasers filtered by their respective mode cleaners were measured under the same spectrometer parameters. The results are shown in Figure 2. In the figure, curve (i) represents the shot noise limit (SNL) benchmark, (ii) and (iii) are the intensity and phase noise of the 671nm and 1.34μm lasers, respectively, and (iv) and (v) are the intensity and phase noise of the 671nm laser. It can be seen from the figure that the intensity noise of both the 671nm and 1.34μm lasers reaches the SNL at 1MHz, and the phase noise reaches the SNL at 1.3MHz. Comparing with the results without mode cleaner filtering (shown in Figure 3), it can be found that the intensity and phase noise of the 671nm/1.34μm laser have been significantly improved after filtering by the mode cleaner.

Figure 3: Noise Characteristics of 671nm/1.34μm Dual-Wavelength Laser Output; (a) Noise Characteristics of 1.34μm Laser, (b) Noise Characteristics of 671nm Laser

Recommended High-Voltage Amplifier: ATA-7020

Figure: ATA-7020 High-Voltage Amplifier Specifications

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