Typical cavity ring-down signals. The fitting residuals are shown in the lower panel.
The qualitative and quantitative analysis to trace gas in exhaled human breath has become a promising technique in biomedical applications such as disease diagnosis and health status monitoring. This paper describes an application of a high spectral resolution optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) for ammonia detection...
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... laser is instantaneously switched off and the resonance light decays exponentially. The fast ring-down decay is recorded by a high sampling rate digitizer (CS320A, 100 MHz, Cleverscope, Auckland, New Zealand), and ring-down time was calculated by an exponential fit, as shown in Figure 2. Sensors 2019, 19, x FOR PEER REVIEW 6 of 11 ...
... laser is instantaneously switched off and the resonance light decays exponentially. The fast ring-down decay is recorded by a high sampling rate digitizer (CS320A, 100 MHz, Cleverscope, Auckland, New Zealand), and ring-down time was calculated by an exponential fit, as shown in Figure 2. At 1531.6 nm the ring-down time was measured as τ0 = 7.75 µs with dry N2 flowing through the V-cavity, corresponding to a cavity finesse of 14,610 (effective absorption path length of 2.325 km). ...
... It also prevents the direct reflection light at the incident mirror from being fed back to the laser, causing oscillations in the laser output pattern. The folded-cavity is a classical cavity structure in the application of optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) . However, such a structure seems to make the cavity selective for the polarization of the incident laser. ...
... A minimum detectable absorption coefficient of α min = 7.6 × 10 −9 cm −1 can be obtained in a single laser scan of 10 s. This trace gas detection sensor has a compact structure and potential to provide stable performance in breath applications according to our previous work . Potential applications of this phenomenon are also proposed in our paper. ...
A special phenomenon of resonance mode separation is observed during the study of a high sensitivity folded-cavity enhanced absorption spectroscopy for the measurement of trace gases. The phenomenon affects the measurement of gas absorption spectrum in the cavity. This resonant mode separation phenomenon of the resonant cavity is different from the resonant modes previously observed in linear-cavity enhanced absorption spectroscopy systems. To explore the mechanism of this phenomenon, a series of hypotheses are proposed. The most likely reason among these hypotheses is based on the different reflectance properties of the plane mirror at the fold of the cavity for S-polarized light and P-polarized light. Based on the matrix calculation method, the different reflectance and phase shift of the plane mirror for S-polarized light and P-polarized light are analyzed theoretically, and the results are in better agreement with the phenomena observed in the experiment. Finally, in order to eliminate the resonant mode separation phenomenon, line polarizers were added. By improving the system, the cavity enhanced absorption spectrum of residual water vapor in the cavity was successfully measured, and a minimum detectable absorption coefficient of αmin = 7.6 × 10-9 cm-1 can be obtained in a single laser scan of 10 s.
Selectivity is one of the most crucial figures of merit in trace gas sensing, and thus a comprehensive assessment is necessary to have a clear picture of sensitivity, selectivity, and their interrelations in terms of quantitative and qualitative views. Recent reviews on gas sensors/techniques are limited to specific sensors, sensors with unconventional materials, various technological exploitation, or specific applications. However, the selectivity is either unexplored in most cases or explained concerning the materials/techniques involved in a demonstration. Therefore, there is a pressing need to identify the possible ways to improve the selectivity of a gas sensor/technique with low or zero cross-sensitivity to other compounds/gases present in the working environment. Analytical techniques involving spectroscopic and mass-spectrometry-based methods are excellent in selectivity but have limited applicability for field deployment compared to the miniatured solid state sensors. Solid state sensors are the mainly studied gas sensors due to their flexibility, portability, and cost-effectiveness, and being technologically favorable but suffer from low selectivity in harsh and humid environments. This review will evaluate the limitations and possible solutions to selectivity issues in a wide variety of gas sensors. Here, we have discussed the gas-sensor technologies and underlying sensing mechanisms in two main groups - spectroscopic and non-spectroscopic. Recent state-of-the-art techniques and fundamental challenges are discussed to improve the selectivity and other gas sensor indicators and future perspectives.
A novel technique for performing fiber pigtailed DFB laser and linear Fabry-Pérot cavity based optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) is proposed. A fiber-coupled electro-optic modulator (f-EOM) with x-cut y-propagation LiNbO3 waveguide is employed, instead of PZT used in traditional OF-CEAS, to correct the feedback phase, which improves the compactness and applicability of OF-CEAS. Through the efficient and real-time control of the feedback phase by actively changing the input voltage of the f-EOM, a good long-term stability of the signal has been achieved. Consequently, a detection sensitivity down to 7.8×10-10 cm-1, better than the previous by PZT based OF-CEAS, has been achieved over the integration time of 200 s, even by use of a cavity with moderate finesse of 2850.
Spectroscopic gas sensing technologies based on absorption and Raman scattering are fast, nondestructive, long-term stable and highly selective. By making use of an optically resonant cavity, the sensitivity can be enhanced hundreds to thousands of times. However, frequency-locking technology is necessary to maintain a stable resonant condition. In this review, various applications of Pound-Drever-Hall (PDH) and optical feedback (OF) frequency-locking cavity-enhanced spectroscopy (cavity ring-down spectroscopy, cavity-enhanced absorption spectroscopy and cavity-enhanced Raman spectroscopy) for highly sensitive gas sensing are presented. The effect of frequency-locking cavity-enhanced spectroscopy for improving the sensitivity of gas sensing is demonstrated. In addition, the key parameters and advantages as well as limitations of different frequency-locking cavity-enhanced spectroscopy are summarized and discussed.
Investigating the use of exhaled breath analysis to diagnose and monitor different diseases has attracted much interest in recent years. This review introduces conventionally used methods and some emerging technologies aimed at breath analysis and their relevance to lung disease, airway inflammation, gastrointestinal disorders, metabolic disorders and kidney diseases. One section correlates breath components and specific diseases, whereas the other discusses some unique ideas, strategies, and devices to analyze exhaled breath for the diagnosis of some common diseases. This review aims to briefly introduce the potential application of exhaled breath analysis for the diagnosis and screening of various diseases, thereby providing a new avenue for the detection of non-invasive diseases.
Laser absorption spectroscopy combined with spectral analysis has been extensively investigated in detection and measurements of gas samples because of their broad applicability for measuring more than one hundred species, such as O2, CO2, H2O, HCl, NH3, NOx, and hydrocarbons. Recently, real-time detection and measurements of gas samples based on laser absorption spectroscopy have attracted considerable interest from various fields owing to their performance of robustness, fast response speed, high sensitivity, and good precision. Here, we review the commonly used infrared laser absorption spectroscopy techniques for real-time measurements and the significant optical equipment utilized in measuring systems. Then, we discuss practical applications based on infrared laser absorption spectroscopy which require real-time determination of gas component concentrations, including atmospheric environment monitoring, breath analysis, combustion diagnostics, and industrial applications. Finally, perspectives of real-time determination of gas component concentrations based on infrared laser absorption spectroscopy are presented.