Application Cases

Experiment Name: Measurement of NH₃ Concentration Based on NICE-OHMS Technology Using Fiber Lasers

Research Direction:

NICE-OHMS (Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy) technology was initially developed to achieve highly stable frequency references by combining FMS (Frequency Modulation Spectroscopy) and CEAS (Cavity-Enhanced Absorption Spectroscopy). In 1996, Ye Jun and colleagues from the JILA group in the United States proposed cavity-enhanced frequency modulation spectroscopy. Using a highly stable Nd:YAG laser and a high-finesse cavity with a finesse of 10⁵, they locked the laser to the sub-Doppler signal of C₂HD at 1064 nm, achieving a detection sensitivity of 1×10⁻¹⁴ cm⁻¹ and a frequency stability of 1×10⁻¹¹ (1 s integration time). Subsequently, researchers recognized the potential of this technology for gas detection, leading to experiments measuring the absorption of different gas molecules using various lasers. To obtain complete absorption line profiles, numerous tunable lasers were employed to measure gas absorption signals under Doppler broadening. In NICE-OHMS, to combine FMS and CEAS, while locking the laser carrier frequency to the TEM₀₀ mode of the high-finesse cavity using PDH (Pound-Drever-Hall) technique, the modulation frequency in FMS must be locked to the free spectral range (FSR) of the high-finesse cavity using DVB (Dichroic Atomic Vapor Laser Lock) technique. Since both the carrier and sidebands in FMS are affected by frequency-noise interference from the PDH lock, their beat signal cancels out this interference, making NICE-OHMS immune to such noise, which is a major factor limiting the detection sensitivity of CEAS. These attributes make NICE-OHMS one of the most sensitive trace gas detection technologies in the world, as well as an efficient laser frequency stabilization technique.

Test Equipment: High-voltage amplifier, Function generator, Fiber laser, Fiber-coupled acousto-optic modulator, Low-pass filter, etc.

Experimental Process:

Figure 1: Schematic diagram of the NICE-OHMS experimental setup

FL, Fiber laser; f-AOM, Fiber-coupled acousto-optic modulator; f-EOM, Fiber-coupled electro-optic modulator; OI, Optical isolator; MML, Mode-matching lens; λ/2, Half-wave plate; λ/4, Quarter-wave plate; PBS, Polarizing beam splitter; Len, Focusing lens; PD, Photodetector; PID, Proportional-Integral-Derivative controller; HVA, High-voltage amplifier; LP, Low-pass filter; DBM, Double-balanced mixer; FG, Function generator; φ, Phase shifter; PID, Proportional-Integral-Derivative controller

The experimental setup is shown in Figure 1. Optical path: The laser generated by the fiber laser (FL) passes through a fiber-coupled acousto-optic modulator (f-AOM), where the +1 order diffraction shifts the laser frequency by 110 MHz. The frequency-shifted laser then passes through a fiber-coupled electro-optic modulator (f-EOM), which is a waveguide-type EOM that only allows laser light polarized along the e-axis to pass, avoiding the influence of residual amplitude modulation. The laser exiting the f-EOM is collimated by a fiber collimator and output into free space. The emitted spatial light first passes through an optical isolator (OI) to prevent reflected light from optical components from entering the fiber and damaging the fiber devices. It is then transformed by a mode-matching lens (MML) to adjust the beam waist radius and curvature radius before passing through a half-wave plate (λ/2). The λ/2 adjusts the polarization direction of the laser to align with the transmission polarization direction of the polarizing beam splitter (PBS). The light transmitted through the PBS passes through a quarter-wave plate (λ/4) and is coupled into the high-finesse cavity. The reflected light from the cavity passes through the λ/4 again. Passing through the λ/4 twice results in the polarization direction of the laser being perpendicular to the polarization direction of the light initially transmitted through the PBS. Thus, it is reflected by the PBS, focused by lens 1 (len1), and enters detector 1 (PD1) for PDH and DVB locking. The transmitted light from the cavity is converged by lens 2 (len2) and detected by detector 2 (PD2) to obtain the NICE-OHMS signal. Optical surfaces of components are tilted where possible and positioned according to the Etalon-immune distance (EID) to avoid generating Etalon noise that could affect the final signal.

Circuit part: The radio frequency ν_fsr = 380 MHz, used to demodulate the transmitted light to obtain the NICE-OHMS signal, is directly generated by function generator 1 (FG1). Function generator 2 (FG2) generates ν_dvb = 355 MHz. The beat signal of the two is passed through a 35 MHz low-pass filter (LP1) to obtain ν_PDH = 25 MHz. Both ν_fsr and ν_PDH are applied to the f-EOM to modulate the laser, with modulation indices of 0.8 and 0.19, respectively. The AC signal detected by PD1 is split into two paths, used for PDH locking and DVB locking, respectively. The signal mixed with ν_PDH is passed through low-pass filter 2 (LP2) and sent to proportional-integral-derivative controllers 1 and 2 (PID1 and PID2, self-made). The output signal from PID1 is amplified by a high-voltage amplifier (HVA) and applied to the laser, while the output signal from PID2 is directly fed to the f-AOM. Together, they achieve a PDH lock with a bandwidth of 100 kHz. The signal mixed with ν_dvb is passed through low-pass filter 3 (LP3) and fed by proportional-integral-derivative controller 3 (PID3) back to the frequency control port of FG1, achieving a DVB lock with a bandwidth of 100 kHz. After locking, the laser frequency matches the cavity mode frequency. The cavity mode frequency is scanned by changing the length of the piezoelectric transducer on the F-P cavity, thereby scanning the laser frequency. When the laser frequency scans across the molecular transition line of the target absorber, the laser transmitted through the cavity is received by detector 2. The AC signal output from detector 2 is mixed with ν_fsr and then passed through low-pass filter 4 (LP4) to obtain the NICE-OHMS signal.

Experimental Results:

Figure 2: Gas absorption signals measured based on the NICE-OHMS system; (a) CEAS signal; (b) NICE-OHMS signal at the absorption phase; (c) NICE-OHMS signal at the dispersion phase

Figure 2 shows the system signals obtained when the intracavity pressure was maintained at 70 mTorr. Figure 2(a) is the measured CEAS signal. Affected by frequency-amplitude noise, the signal-to-noise ratio, i.e., the ratio of signal amplitude to linewidth, is only 4.3. As analyzed earlier, the broadening lineshape of the gas at 70 mTorr pressure is primarily Gaussian. The red line in Figure 2(a) is the result of Gaussian lineshape fitting, showing a high degree of fit between the measured signal and the fitted lineshape. Figures 2(b) and 2(c) show the NICE-OHMS signals obtained by demodulating the transmitted cavity mode using ν_fsr at the absorption phase and dispersion phase, respectively. The signals are symmetric and smooth, indicating good locking performance between the laser and the cavity during the laser frequency scan. The amplitude of the dispersion signal in the figure is larger than that of the absorption signal. Measurement of the dispersion signal shows a signal amplitude-to-linewidth ratio of 188, which is a 43-fold improvement compared to the cavity-enhanced signal. The wavenumber of the laser used in this measurement corresponds to 1530.58 nm, targeting the absorption line of NH₃ at 6533.4515 cm⁻¹. According to the HITRAN database, the transition line strength is 4.436×10⁻²² (cm⁻¹/molecule/cm⁻²), corresponding to a detection sensitivity of 3.7×10⁻¹⁰ cm⁻¹.

High-Voltage Amplifier Recommendation: ATA-2022B

Figure: ATA-2022B High-Voltage Amplifier Specifications

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