All optical fiber photothermal spectroscopic sensors for high sensitivity gas detection

Abstract

Detection of trace gases sensitively and selectively is of importance in many fields such as environmental monitoring, safety and industrial process control as well as national security applications. Laser absorption spectroscopy (LAS) is a powerful technique for trace gas detection with characteristics of high selectivity and sensitivity. Optical fiber-based technologies have been employed for trace gas detection due to the advantages of compact size, remote interrogation and sensor networking. However, the performance of direct absorption based optical fiber gas sensors, including those made of micro-optic lenses with fiber pigtails and solid or hollow core microstructured optical fibers, is limited to 1 to 10 parts-per-million (ppm). The detection sensitivity of LAS sensors can be improved by using longer absorption cell. Using of a longer length of hollow-core photonic bandgap fiber (HC-PBF) as absorption cell could improve the detection sensitivity. However, it is very difficult to achieve parts-per-billion (ppb) level detection because it would require a very long length of HC-PBF (e.g., 100 meters). This will also make the sensor response extremely slow because of the very long time needed to fill gas sample into the hollow-core. So the current direct absorption gas sensors are difficult to achieve high sensitivity and fast response simultaneously. In this thesis, we aim to investigate all optical fiber gas sensors based on photothermal (PT) spectroscopy which can achieve high sensitivity and fast response. PT spectroscopy is an alternative indirect method which can provide high sensitivity for gas detection. Trace gas detection based on photothermal effect has been investigated by many instrumental methods such as photoacoustic spectroscopy (PAS) and photothermal interferometry (PTI). These methods have demonstrated sub-ppb level detection sensitivity. The basic process of the PAS involves the conversion of light absorption of the target gas into the acoustic pressure wave via the photoacoustic effect. The generated acoustic pressure wave can be detected by the acoustic detector which can achieve fast response. Here we propose a fiber Fabry-Perot (F-P) interferometer with graphene diaphragm as the acoustic detector. It is made by attaching a multilayer graphene diaphragm to a hollow cavity at the end of a single mode optical fiber. As an acoustic detector, its pressure and frequency responses are firstly investigated. By operating at one of the mechanical resonances of the diaphragm, the sensitivity for acoustic detection is enhanced and a noise equivalent minimum detectable pressure of ~2 μPa/Hz¹/² at ~10 kHz is demonstrated. To our knowledge, this is a very sensitive result since it is better than the commercial B&K4189 microphone (~8 μPa/Hz¹/²). Then the acetylene detection by optical fiber photoacoustic gas sensor is demonstrated by using the proposed acoustic detector. A lower detection limit of ~120 ppb can be achieved when the modulation frequency is operating at one of the mechanical resonances of the diaphragm. Theoretical analysis shows that the detection sensitivity can be further improved by increasing the Q-factor of the resonator at low gas pressures. In order to improve the detection sensitivity and achieve faster response, we also propose a novel resonating HC-PBF photonic microcell for high performance gas detection. This absorption cell is formed by jointing a piece of HC-PBF with two single mode fibers (SMFs) with mirrored ends, and light travels back and forth through the absorbing gas in the absorption cell and the effective absorption path length can be enhanced by a factor proportional to the cavity finesse. The finesse of a resonating absorption cell made with 6.75-cm-long HC-1550-06 fiber is measured to be 128, corresponding to an effective optical path length of ~5.5 m. So such a resonating gas cell enables us to use a shorter HC-PBF (e.g., a few to tens of centimeters) to achieve high sensitivity, overcoming the problem of slow response associated with the use of long HC-PBF. Gas detection experiments based on direct absorption with a resonating HC-PBF microcell are conducted. With a cavity length of 9.4 cm and a finesse of 68, a detection limit of 7 ppm can be achieved. Compared with a single-path non-resonating HC-PBF, the use of a resonating HC-PBF microcell can improve the detection sensitivity. We also conduct cavity enhanced PT gas detection experiments with the resonating HC-PBF microcell. Experiments with a 6.2-cm-long resonating microcell with a finesse of 45 for pump beam and 41 for probe beam demonstrated a detection limit of 126 ppb acetylene, 1-2 orders of magnitude better than the same gas cell based on direct absorption. The cavity enhancement of such a resonating HC-PBF microcell can simultaneously amplify the intracavity build-up intensity for the pump beam inside the HC-PBF and improve the slope of the operating point for the probe beam by a factor proportional to the cavity finesse.