Application Cases

Experiment Name: Reducing 1.5μm Laser Noise Using a Mode Cleaner

Testing Equipment: High-voltage amplifier, signal generator, fiber laser, optical isolator, photodetector, etc.

Experiment Process:

Figure 1: Experimental setup and mode cleaner cavity locking system. HWP: Half-wave plate; OI: Optical isolator; EOM: Electro-optic modulator; L: Lens; MC: Mode cleaner; PD: Photodetector; HV: High-voltage amplifier

In the experiment, the laser was initially used to adjust the closure of the mode cleaner cavity. Then, another signal generator produced a triangular wave signal with a frequency of around 30Hz, which was amplified by the high-voltage amplifier and connected to the piezoelectric ceramic for scanning the mode cleaner cavity length. The transmission peak was adjusted to the highest point using the transmission signal, and then the mode matching between the laser and the mode cleaner was adjusted using a matching lens. After the adjustment, the measured finesse for p-polarization was approximately 220, the bandwidth for s-polarization was 1.36MHz, and the finesse for s-polarization was approximately 900.

To proceed with the subsequent experiments, we must lock the mode cleaner to stabilize the laser transmission. In the experiment, we used the sideband locking method, i.e., the Pound-Drever-Hall (PDH) frequency stabilization method, to lock the mode cleaner cavity to the frequency of the incident laser. The cavity locking system is shown in Figure 1. A high-frequency signal generator produces a modulation signal with a frequency of about 60MHz, which is split into two equal paths by a power splitter. One path passes through a phase delay element and is input into the mixer, while the other path is amplified by a power amplifier and input into the electro-optic phase modulator, adding the modulation signal to the incident signal light. Thus, the signal light transmitted through the concave mirror of the mode cleaner cavity carries the cavity's detuning information. After being focused by a lens, it enters the photodetector. The detected AC signal is amplified and mixed with the local signal in the mixer to obtain the error signal. After passing through a low-pass filter and a proportional integral differential circuit, the signal is input into the high-voltage amplifier HV and then loaded onto the piezoelectric ceramic of the mode cleaner's concave mirror to change the cavity length of the mode cleaner, thereby locking the cavity to the frequency of the incident laser. After a simple adjustment of the frequency discriminator curve, the cavity can be locked. In our experimental system, due to the losses of various optical components, the maximum laser power reaching the front of the mode cleaner MCI is 1.73W. After the cavity is locked, the measured transmission power for p-polarization is 1.04W, with a transmission efficiency of 60%, and the power stability after cavity locking is better than 1%. The transmission efficiency for s-polarization is only 1%, and due to the low transmission power, it is not possible to use the s-polarization for subsequent experiments after filtering.

Experimental Results:

Figure 2 shows the intensity noise spectrum of the laser before and after the mode cleaner measured using the balanced homodyne detection method. The detector's electronic noise is far below the shot noise limit. In the figure, curve a represents the shot noise benchmark, curve c represents the laser intensity noise spectrum before the mode cleaner, and curve b represents the laser intensity noise spectrum after the mode cleaner.

Figure 2: Intensity noise of a 1.5μm laser measured at 1mW

From the figure, it can be seen that the output laser from the fiber laser has high intensity noise at low frequencies, and the laser's intensity noise is still far above the shot noise benchmark even at 30MHz. However, after passing through the mode cleaner, the laser's intensity noise reaches the shot noise benchmark at 15MHz.

High-Voltage Amplifier Recommendation: ATA-7030

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

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