|Title:||High sensitivity spectroscopic gas sensing with hollow-core optical fibers|
|Advisors:||Jin, Wei (EE)|
|Department:||Department of Electrical Engineering|
|Pages:||xx, 146 pages : color illustrations|
|Abstract:||Trace gas sensing is of importance in many areas including environmental and air-pollution monitoring, industrial process control and safety, and medical breath analysis. Among various kinds of gas sensing methods, laser absorption spectroscopy (LAS) offers advantages in sensitivity and selectivity as well as non-contact real-time measurement. Traditional LAS, including tunable diode LAS, cavity-enhanced LAS and photoacoustic LAS, uses a bulk multi-pass gas cell or resonating optical cavity. The size of gas cell and the need for careful optical alignment and/or cavity locking limit the application of traditional LAS in a real-world environment.|
Hollow-core fiber (HCF) has been proved to be an efficient platform for light-gas interaction. With HCFs, compact gas cells can be made, which reduces gas consumption as well as benefits space-limited applications. HCF-based gas cells can be integrated with fiber-based components to provide advantages such as remote sensing and multi-point interrogation. However, since the first demonstration of HCF-based LAS in 2004, its sensitivity has long remained in tens of parts per million (ppm) level, which is insufficient for many precision applications. In 2015, the first HCF-based laser gas sensor with parts per billion (ppb) level sensitivity was demonstrated with photothermal spectroscopy (PTS). In this thesis, we explore several methods to further improve the sensitivity of HCF-based PTS gas sensors, and apply HCFs to stimulated Raman spectroscopy to detect gases with weak or no strong absorption in the operating wavelength range.
A straightforward way to improve the sensitivity of PTS is to increase the optical pump power level. For this, we developed an intracavity PTS system by placing a HCF gas cell inside a fiber ring laser cavity. The intracavity laser power was shown to be significantly higher than the laser output power. With a 0.62-m-long photonic bandgap HCF (PBG-HCF), we realize a sensitivity of 176 ppb acetylene in nitrogen. We build a model to study the PTS in the fiber ring laser. Theoretical study shows that the sensitivity of intracavity PTS can be further enhanced to ppb level by reducing the optical loss inside the laser cavity.
Apart from increasing the pump power level, the gas detection sensitivity may be improved by reducing the system noise level. As an interferometric gas sensing method, part of the noises of PTS comes from the optical interferometer. We discovered that an in-fiber modal interferometer with a dual-mode anti-resonant HCF (AR-HCF) gives a much better signal-to-noise ratio (SNR). A gas sensing method named mode-phase-difference PTS (MPD-PTS) is demonstrated. The photothermal effect is measured by the phase difference between the two optical modes in the AR-HCF. The common-mode noise is greatly reduced and hence the MPD-PTS shows a much higher SNR. A multi-physics model is built to investigate the robustness of the dual-mode interferometer. Simulation shows that the sensitivity of MPD-PTS to environmental temperature and pressure perturbations is reduced by two orders of magnitude when compared to the previous HCF-based PTS. By using a 4.67-m-long AR-HCF, the sensitivity of tracing acetylene reaches parts per trillion (ppt) level with 3630-s integration time.
HCFs are made of fused silica, which is not good at confining heat. The heat generated by optical absorption quickly dissipated into the environment via the silica claddings. Hence, we investigated another subsequent effect of optical absorption: photoacoustic effect. The large acoustic impedance mismatch between the silica and air makes HCF a good acoustic resonator. A gas sensing method named photoacoustic Brillouin spectroscopy (PABS) is then demonstrated. A theoretical model is built to understand the acoustic-optical interaction and the thermodynamics in the HCF. The shifting boundary of the silica capillary of AR-HCF introduces an additional change of refractive index of the optical mode. With a 0.3-m-long AR-HCF, a sensitivity of 8-ppb acetylene in nitrogen has been demonstrated. Isotope detection at low pressures and characterization of the fiber microstructure have also been realized by PABS. Theoretical simulation shows that the sensitivity of PABS can be further enhanced by several orders of magnitude by using specially designed optical fibers.
For gases with no strong optical absorption such as hydrogen, Raman spectroscopy provides a solution for tracing them. The small air-core of HCF provides a perfect platform for light-gas interaction with high optical intensity, which enables highly efficient stimulated Raman scattering (SRS). We demonstrated a point hydrogen sensor and a distribution hydrogen sensor based on stimulated Raman gain spectroscopy of the S0(0) transition of hydrogen. With a 15-m-long PBG-HCF, a sensitivity of 20 ppm hydrogen has been demonstrated. By using backward SRS with a pulsed laser, distributed hydrogen sensing is demonstrated with a 2.7-m spatial resolution and a sensitivity of 833 ppm hydrogen over a length of 100 meters. The response time of the distributed hydrogen sensor is less than 60 s. Distributed pressure sensing is also demonstrated by decoding the Raman linewidth.
SRS process also generates heat, which may be exploited for gas detection. The indirect measurement of SRS via the generated heat would provide a higher gas sensing sensitivity because we could allow more pump photons to transform into Stokes photons without saturating the PD. We demonstrated Raman spectroscopy in an AR-HCF by detecting the SRS-induced photothermal phase modulation by using a setup similar to the MPD-PTS, which is named Raman photothermal interferometry. A theoretical model is built to investigate the SRS-induced photothermal effect in HCFs. Theoretical simulation shows that part of the temperature change generated by SRS can be measured by the phase difference between two optical modes in the AR-HCF. The broad transmission band of AR-HCF enables the use of the Q1(1) transition of hydrogen, which greatly enhances the heat generated by SRS. By using a 3.9-m-long AR-HCF, a sensitivity of 3.2 ppm hydrogen has been demonstrated at a pressure of 6 bar.
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