Application Cases

Experiment Title: Frequency Stabilization Technology Research

Testing Purpose:Laser frequency stabilization technology selects a stable reference frequency standard. When the frequency of the laser to be locked deviates from a specific frequency standard, the deviation is identified and an error signal that reflects this deviation is generated. The error signal is then fed back to the laser system to be locked through a servo system. Common reference frequency standards can be broadly divided into two categories: one is the center frequency of atomic or molecular transition spectral lines as the reference standard; the other is the resonance frequency of optical resonators (including Fabry-Perot cavities (F-P cavities), mode cleaners, frequency-doubling cavities, optical parametric oscillators, etc.) as the reference standard. There are various methods to achieve laser frequency stabilization, such as saturation absorption spectroscopy stabilization based on atomic or molecular transition spectral lines, modulation transfer spectroscopy stabilization, and dual-color spectroscopy stabilization; and PDH (Pound-Drever-Hall) stabilization, lock-in phase stabilization, and tilt-locking stabilization based on the resonance frequency of optical resonators. Since the spectral range of atomic or molecular transition lines is limited, frequency stabilization can only be performed for lasers of certain specific wavelengths. In contrast, frequency stabilization methods based on the resonance frequency of optical resonators are not limited by wavelength.

Testing Equipment:High Voltage Amplifier, Signal Generator, Oscilloscope, PZT, Laser Detector, etc.

Experiment Process:

Figure 1: Schematic diagram of the structure of automatic frequency stabilization based on an analog circuit. SG: Signal Generator; Comp.: Comparator; Laser: Laser; OI: Optical Isolator; EOM: Electro-Optic Phase Modulator; PBS: Polarizing Beam Splitter; Optical Cavity: Optical Cavity; PD: Photodetector; LO: Local Oscillator; PS: Phase Shifter; Mixer: Mixer; LF: Low Pass Filter; HVAmp.: High Voltage Amplifier

An infrared laser with a wavelength of 1064nm and a power of 1.3W is used. The infrared light passes through an optical isolator to prevent reflected light from entering the laser and affecting its normal operation. It then passes through an electro-optic modulator to modulate the phase of the light and enters the optical cavity (using a mode cleaner). The reflected light from the optical cavity is detected by a photodetector to obtain an electrical signal, which is mixed with the modulated signal in a mixer. After passing through a low-pass filter, an error signal is obtained. The error signal is processed by an automatic frequency stabilization device, which includes a PI circuit module and a comparison module. If the input electrical signal is compared with the standard signal and the input signal is greater than the standard signal, the device automatically switches to the cavity locking mode. The cavity length of the laser is controlled through a high voltage amplifier to control the frequency of the laser. If the input signal is less than the standard signal, the automatic scanning device scans the optical cavity. The electrical signal output by the photodetector is used to determine whether the laser frequency is locked or scanned. The schematic diagram of the automatic frequency stabilization device is shown in Figure 1.

Experimental Results:

Figure 2: Automatic Frequency Stabilization Results

Based on PDH frequency stabilization, an automatic frequency stabilization device is added to the feedback control loop system. Another output signal from the photodetector (VD) is connected to the automatic frequency stabilization device and compared with the reference signal (VR) in the device. The reference signal can be provided by the photodetector of another beam of light (set VR≥1/2VD). If VD>VR, the PID is automatically activated, connecting the PID and high voltage amplifier I, and disconnecting the signal source and high voltage amplifier II, then PDH frequency stabilization is performed; if VD<VR, the signal source and high voltage amplifier II are connected, i.e., scanning is performed. The automatic frequency stabilization device can automatically detect whether the system is in a locked state. If the lock is lost, the device can automatically start the scanning function until the resonance frequency of the optical resonator is found again, and then laser frequency stabilization is resumed.

High Voltage Amplifier Recommendation: ATA-7020

Figure: Specifications of the ATA-7020 High Voltage Amplifier

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