Application Cases

Experiment Name: Principle of Terahertz Laser Output Stabilization Control

Test Equipment:
Voltage amplifier, THz laser detector, data acquisition card, etc.

Experimental Process:

Figure 1: Structure of the THz Laser Stabilization Control System

The principle of the THz laser stabilization control system is shown in Figure 1. It mainly includes a THz laser detector, a data acquisition card, and a voltage amplifier. The THz laser power meter detects the THz laser and outputs a DC voltage signal. The data acquisition card, controlled by LabVIEW graphical programming on a computer, collects the THz laser power signal, analyzes it, and generates an error voltage signal. This signal controls the D/A converter on the data acquisition card to output an analog voltage signal, which is amplified by the voltage amplifier and fed back to the piezoelectric ceramic in the pump laser resonator. If the voltage increases and the power increases, the voltage change direction remains unchanged, and the voltage continues to increase. If the power decreases, the voltage change direction reverses. This ensures that the THz laser power is always maintained at the maximum value. This method is referred to as the "maximum value method," and its flowchart is shown in Figure 2.

Figure 2: Flowchart of the THz Laser Stabilization Control System

Test Results:

Figure 3: Active Stabilization Control Setup for Optically Pumped THz Laser

The experimental setup is shown in Figure 3. The THz laser uses a mechanically chopped Q-switched CO₂ laser with DC excitation as the pump source. By adjusting the grating angle to output the 9P(36) line and setting the repetition frequency to 20 kHz, the maximum average power of the pump laser is 6.7 W. Methanol working gas is filled into the THz oscillator at a pressure of 15 Pa, achieving a maximum average power of 7.2 mW for the pulsed THz laser. The THz laser is split by a high-resistivity silicon mirror. One beam is reflected to a P4 pyroelectric detector, and the detected pulse signal is sent to the control system. After amplification and rectification, the voltage signal is collected by the data acquisition card controlled by the computer. The error voltage signal generated after analysis is amplified by the voltage amplifier and fed back to the piezoelectric ceramic of the pump laser resonator to adjust the pump laser frequency, stabilizing the THz laser at the maximum value. The other beam passes through the silicon mirror and enters the THz laser power meter, where the power changes of the THz laser are monitored and recorded in real time by the computer.

Figure 4: Drift of Freely Running THz Laser Power

When the optically pumped THz laser system operates freely, the THz laser power changes are measured and recorded over 15 minutes. The experimental results, as shown in Figure 4, indicate that the THz laser power varies between 1.5 mW and 6 mW, with a certain degree of randomness. The maximum power change rate is 0.088 mW/s. The control system response time is less than 50 ms, and the minimum control voltage step is 500 V/4096 = 0.12 V, meeting the precision requirements. The calculated laser power stability is 0.75. The maximum expansion of the piezoelectric ceramic is 10 µm. When the piezoelectric ceramic voltage changes from 0 to 500 V, the pump laser frequency variation period for the F-P resonator and the pump laser wavelength of 9.69 µm is greater than 2 s, meeting the tuning requirements. The pump laser resonator length is 1.5 m, with a free spectral range of approximately 100 MHz. The resonant absorption frequency of methanol gas is about +24 MHz from the center of the pump laser line, falling within the tunable range of the pump laser. Roughly estimated, the pump laser power at the optimal pump frequency lies between the maximum and minimum power levels.

Recommended Voltage Amplifier: ATA-214

Figure: ATA-214 High-Voltage Amplifier Specifications

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