Stimulated raman dispersion spectroscopy with hollow-core fiber : from fundamental principles to applications

Abstract

Identification and measurement of chemical substances with high sensitivity and good selectivity is of great importance for many practical applications, including pollutant analysis, industrial safety monitoring and health examination. Optical fiber-based laser absorption spectroscopy and its derivatives have been regarded as powerful techniques for sensitive detection. Measurements of absorptive gases (e.g., methane, acetylene and ammonia) have been extensively demonstrated using laser absorption spectroscopy and limit-of-detection from parts-per-million (ppm) to ppb parts-per-billion (ppb) levels has been achieved. However, these methods are not applicable to gases that have no absorption in the low-loss transmission window of optical fibers, which is in the near-infrared. Raman spectroscopy is a versatile technology which can detect Raman-active gases that do not necessarily have absorption in the near-IR. The recent employment of the hollow-core fiber for Raman spectroscopy has greatly enhances both the spontaneous and stimulated Raman signals compared with the free-space system. Sensitive gas detection (e.g., hydrogen, nitrogen and methane) has been achieved with detection limit down to a few hundreds of ppm using lock-in amplifier with 1s time constant at ambient condition. Such system uses pump source in visible band and need sophisticated process to couple light beams in to hollow-core fibers. Recently, our group has demonstrated all-fiber hydrogen gas sensors based on stimulated Raman gain spectroscopy with 15-m-long hollow-core photonic crystal fiber. The detection system operates in the telecom-wavelength-band (i.e., from 1530 to 1625 nm), where cost-effective fiber-optic components are available. The all-fiber systems avoid complex optical alignment, have the capability for remote detection, and are flexible for practical applications. Detection limit of ~140 ppm with 1 second lock-in time constant is realized. However, further performance improvement is limited by the intensity fluctuation of the received signal at the photodetector, a problem common to the intensity-detection-based techniques. In this thesis, we develop a new branch of Raman spectroscopy to sensitively detect hydrogen based on stimulated Raman dispersion spectroscopy with hollow-core photonic crystal fibers. Stimulated Raman dispersion spectroscopy monitors the refractive index change during the Raman scattering process by sensitive optical fiber interferometers in the vicinity of a stimulated Raman transition. It essentially has better immunity to the intensity noise and large dynamic range. The physical process of the stimulated Raman scattering induced dispersion involves two incident laser beams (i.e., a pump beam and a probe beam). The phase change of the probe beam can be modulated when the frequency difference between the pump and probe matches a Raman transition. For the first measurement of the Raman-induced dispersion change, a fiber Mach Zehnder interferometer with 7m-long hollow-core fiber is used to detect the accumulated phase change of the probe beam. The interferometer is stably operated at its quadrature point. By wavelength modulating the pump beam, measurement of the dispersion change with high signal-to-noise ratio is achieved. Applying the optimal modulation depth, this technique has been demonstrated for hydrogen detection with a normalized noise-equivalent concentration of 17.4 ppm/(m·W) at 3.5 bar and a dynamic range over 4 orders of magnitude. To our best knowledge, this is the first demonstration of the measurement of stimulated Raman scattering induced dispersion and its application for sensitive gas detection. To develop a compact hydrogen sensor with compact size and fast response time, a Fabry Perot interferometer based on hollow-core fiber is implemented. Different from the Mach Zehnder interferometer, the typical length of the hollow core fiber is several centimeters, which makes it much faster for gas molecules to diffuse into the hollo core. The Fabry Perot interferometer-based hydrogen sensors show excellent long-term stability since the time delay of the two reflected probe beams are extremely short (i.e., <0.2nm). Moreover, the single mode fibers connected to the hollow-core fiber are not parts of the interferometer, which ensures the environment perturbations imposed on the single mode fiber would not interfere the interferometer performance. With a 3.5 cm-long how-core Fabry Perot interferometer, hydrogen detection with detection limit of 0.42% using 1s integration time constant is demonstrated. Lower detection limit of 264 ppm may be realized by conducting the Allan deviation analysis and applying the optimal integration time constant of ~1300s. The linewidth of hydrogen Raman transitions are quite narrow (i.e., typically a few hundred MHz). Though the Raman-induced refractive index change is small, the rapid variation of the refractive index change near Raman resonances can make considerable group refractive index change of the gas medium, which can be used to actively control the group velocity of optical pulses by adjusting the pump power level and gas pressure. Optical pulse delay as much as 1.42 ns with Raman gain of 10 dB in an 80-m-long hollow-core fiber filled with 2.5-bar hydrogen is achieved.