Application Cases

Experiment Name: Design and Construction of a Gas Raman Spectroscopy Detection Device

Test Purpose: To conduct research on gas Raman spectroscopy detection technology and design a gas Raman spectroscopy detection device based on optical feedback cavity enhancement technology. The device uses a visible diode laser as the base light source and a high-finesse V-type three-mirror cavity as the gas cell. The Raman spectroscopy signal of the gas is introduced into the spectrometer through the slit, and the computer software parameters are set to control the spectrometer and CCD camera for real-time observation and collection of the signal. This chapter introduces the overall structure of the optical feedback cavity-enhanced gas Raman spectroscopy detection device based on the V-type three-mirror cavity and the experimental environment requirements. The device can be divided into three key parts: the laser control module, the gas cell, and the external optical path. The device combines wavelength modulation to achieve laser frequency locking, ensuring sufficient laser power within the gas cell and collecting and detecting the gas Raman spectroscopy signal on the side of the gas cell.

Testing Equipment: High-voltage amplifier, function generator, laser controller, detector, etc.

Experiment Process:

Laser Control Module: The role of this part is to control the temperature and current of the diode laser through the laser controller, enabling the laser to operate under suitable conditions to produce a single-mode stable laser with higher power and lower frequency noise. It also involves outputting a low-frequency scanning signal from the function generator and scanning the output current of the laser. The high-frequency modulation signal from the phase-locked amplifier is combined with the low-frequency scanning signal from the function generator using an adder, and then input into the laser controller to modulate the final output of the laser.

Gas Cell: This part serves as the gas sample cell, controlling the internal gas state through the inlet and outlet. The gas cell is composed of a V-type three-mirror cavity, including an Invar cavity body, two concave cavity mirrors, one flat cavity mirror, two broadband high-transmission windows, an inlet, an outlet, and a pressure gauge interface.

External Optical Path: This part is responsible for collimating and shaping the output light from the diode laser, measuring the beam quality using the knife-edge method, and matching the mode to couple the output light into the gas cell composed of the V-type three-mirror cavity. When the cavity's transmitted light is fed back into the laser, the feedback rate and phase in the optical feedback process are controlled.

Figure 1: Structure of the Frequency Locking Module

Optical Feedback Injection Locking: This paper selects wavelength modulation for laser frequency locking, with the structural design shown in Figure 1, including a phase-locked amplifier, PID, high-voltage amplifier, and piezoelectric ceramic PZT. The experiment sets the phase-locked amplifier to output a high-frequency sine modulation signal with a frequency of 20kHz, amplitude of 0.026V, and phase of 180°. This signal is sent to the adder to be combined with the low-frequency scanning signal from the function generator, used to modulate the laser output, and then mixed with the V-type three-mirror cavity mode signal to demodulate and generate an error signal. Figure 2 shows the cavity mode signal and error signal before laser frequency locking. It can be seen that the error signal has a zero-crossing point with a negative slope, and the zero point corresponds to the maximum value of the cavity mode signal. The error signal is sent to the PID module, and the control signal generated is connected to the high-voltage amplifier, whose output is connected to the piezoelectric ceramic PZT attached to the back of the high-reflection mirror. In the experiment, the high-voltage amplifier is fine-tuned to control the extension and contraction of the piezoelectric ceramic PZT to achieve the best feedback phase. The external optical path is precisely maintained at three times the length of the cavity arm, and the cavity mode signal appears as a symmetrical arch shape. Before locking, the scanning signal is turned off, and the laser in the cavity oscillates irregularly. The PID settings are P=8, I=10^3, and D=0. At this point, the slope of the error signal is eliminated, and the feedback phase is locked to the optimal point.

Figure 2: Cavity Mode Signal and Error Signal Before Laser Frequency Locking

Raman Light Lateral Collection Path: A lateral collection path for Raman light is designed and constructed, including a concave mirror, focusing lens, notch filter, spectrometer, CCD camera, and computer.

Figure 3: Structure of the Raman Light Lateral Collection Path

Experimental Results:

This chapter focuses on the gas Raman spectroscopy detection technology based on optical feedback cavity enhancement and the construction of the V-type three-mirror cavity experimental device. The device must be maintained in a constant temperature, vibration isolation, and dark experimental environment. It includes three key parts. The laser control module controls the laser temperature and scans and modulates the laser current to produce a mode-stable laser. The gas cell allows the laser entering the cavity to resonate and interfere, enhancing the interaction with the gas medium inside the cavity, producing gas Raman scattering light. The external optical path collimates, shapes, and mode-matches the output light from the diode laser before coupling it into the gas cell. At the same time, the transmitted light from the gas cell is fed back into the laser along the original path, improving the laser's output performance. The wavelength modulation method locks the cavity laser to the cavity mode peak, ensuring that the cavity laser is always at its maximum value, enhancing the intensity of the Raman spectroscopy signal inside the cavity. The lateral collection path for Raman light is used to collect and analyze the Raman spectroscopy signal generated inside the gas cell.

Voltage Amplifier Recommendation: ATA-7020

Figure: Specification Parameters of the ATA-7020 High-Voltage Amplifier

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