A high-sensitivity quartz enhanced photoacoustic spectroscopy (QEPAS) system based on acousto-optic Q-switched fiber laser for C ₂ H ₂ detection was proposed and experimentally demonstrated. The gas sensor system coupled an erbium-doped fiber laser with a custom-made transmission quartz tuning fork structure. An acousto-optic modulator was used to perform amplitude modulation at a modulation frequency of 32.76 kHz and the system achieved a maximum intra-cavity peak pulse power of 6 W. The system response relationship between the peak pulse power and QEPAS signals confirms that saturation does not occur with the laser power less than 6 W. Preliminary measurements of the sensor response to C ₂ H ₂ concentrations have been made with pure N ₂ as a carrier gas, and the experimental results showed the relationship between QEPAS signal amplitude and the C ₂ H ₂ concentrations have a good linearity (R-square = 0.99918). Finally, trace gas measurements have been assessed and a minimum detection limit of 101 pptv within 20 ms integration time for C ₂ H ₂ has been achieved. This paper provides a valuable gas sensor for practical project application.

The temperature fluctuations inside commercially distributed feedback laser diode were experimentally measured and analyzed in detail in this study. In tunable diode laser absorption spectroscopy (TDLAS) based gas sensing systems, the temperature of the laser source is usually fixed and a current signal is used to drive the laser for wavelength scanning or modulation. Even if the laser is controlled by a robust temperature controller, the internal temperature still fluctuates due to the delayed response of the temperature controller when rapidly changing the drive current. These temperature fluctuations result in distortions in the measured absorption signals which can be effectively suppressed by limiting the drive current modulation to the resonance frequency of the temperature control proportional-integral-derivative loop. This study demonstrates a reduction in the temperature dependent absorption distortions and improves the detection uncertainty using this technique.

A new defined parameter, tuning efficiency (TE), is proposed to evaluate the tuning performance of distributed feedback laser diode (DFB-LD). The TE is defined as the ratio of output wavelength response to power response of DFB-LDs when working in a current driving mode. Simulations and experiments are performed to study its influence on harmonic signal, signal-to-noise ratio (SNR), and residual amplitude modulation (RAM) effect in wavelength modulation spectroscopy (WMS). It is the first time to be found that the parameter TE would change as driving frequency of DFB-LD getting changed; in addition, different DFB-LDs result in changes in TE. A WMS-based water vapor sensing system is constructed to discuss the TE's influence on gas detection applications, and the experimental results show highly consistent with the simulations. In conclusion, the TE is a robust parameter guiding us to choose favorable DFB-LDs for gas sensing applications.

Fixed wavelength modulation spectroscopy (FWMS) featured as that wavelength is modulated around a constant value instead of a dynamic scanning ramp in the standard wavelength modulation spectroscopy (WMS). To eliminate the influence of center wavelength drift on absorption measurement in the FWMS system, a software-based proportional integral derivative (PID) controller is developed to lock the laser wavelength at methane line 1653.72 nm for CH ₄ detection in this study. Moreover, two crucial parameters of PID, gain and integral time are optimized to improve the laser wavelength locking performance. As a result, the laser wavelength is locked to the target CH ₄ line with a 1σ fluctuation less than 406 kHz (better than 3.7×10 ^-3 pm) with a robust 100% locking success rate, which is the best result reported so far to our knowledge. Based on the wavelength locked FWMS CH ₄ sensor, linearity is tested to be R ² = 0.998 in low concentration range of 0-1000 ppm. Detection sensitivity is 52 ppb at 0.1 s and could be improved to 36 ppb if the integration time is extended to 1.4 s. Compared with standard WMS, CH ₄ detection stability is improved by 5 times.