Novel hollow-core optical fiber gas and acoustic sensors

Abstract

Hollow-core photonic bandgap fibers (HC-PBFs) have drawn increasing attentions over the past fifteen years. The hollow-core and the air-silica cladding with high air-filling ratio in HC-PBF enable some important properties, which could be exploited to create novel photonic sensors. This thesis studies the use of HC-PBFs for higher performance gas and (acoustic) pressure sensors. A HC-PBF can confine a fluidic sample (e.g., gas) and light modes simultaneously within the hollow-core, and the light-sample overlap approaches 100%. Light focus in a HC-PBF is much tighter and the interaction length much longer and hence the overall efficiency of light-sample interaction could be made significantly higher than in a free-space system. These properties would allow highly sensitive absorption-based gas detectors. However, two issues or obstacles were identified, and need to be overcome in order to develop high performance HC-PBF gas sensor for practical applications. One is the noise associated with mode interference (MI) since current HC-PBFs are not true single mode fibers; the other is the slow response limited by gas diffusion into the hole-columns if a longer HC-PBF is going to be used for the purpose of enhanced detection sensitivity. We have conducted detailed investigations on both of the issues and achieved good results. By optimizing mode launch,using proper length of sensing HC-PBF, applying proper wavelength modulation in combination with lock-in detection, and using appropriate digital signal processing after signal-detection, we successfully demonstrated an acetylene sensor with a detection limit of less than 1 part-per-million (ppm),with a 13-meter-long HC-PBF. To our knowledge, this is the best result reported so far for HC-PBF gas sensors based on direct absorption. We demonstrated significant reduction of MI in HC-PBFs by fusion splicing HC-PBF to standard single mode fiber (SMF) with proper fusion parameters to collapse the cladding holes and by splicing an intermediate LMA-5 fiber between the HC-PBF and the SMF.Such techniques minimize the excitation of the higher order cladding modes and enables significant reduction of MI even for a short length (e.g., <1m) of HC-PBF. We also studied the effect of drilling micro-channels with a femtosecond infrared laser on the insertion loss, MI noise and response time of the HC-PBF sensors. It was found that high quality low-loss micro-channels introduce very small loss of less than <0.02 dB per channel and do not have significant negative effect on MI. Hence many micro-channels could be made along a longer length of HC-PBF to achieve higher sensitivity without compromising the response time. By use of a 0.62-m-long HC-1550-02 fiber with 15 micro-channels and spliced to an input SMF with optimal fusion parameters, we demonstrate detection of acetylene with a detection limit of 11 ppm and 2 minutes recovery time. Despite our effort,it seems hard to further significantly enhance sensitivity of direct-absorption-based HC-PBF sensors. We then studied a new technique for gas detection. Instead of detecting the absorption-induced change of light intensity, we studied the absorption-induced phase change via photothermal (PT) effect. With this technique, we demonstrated an all-fiber gas sensor with a detection limit of parts-per-billion (ppb) level and an unprecedented dynamic range of six orders of magnitude (120 dB) with low cost near infrared semiconductor laser sources. The novel detection technique will allow a new class of optical fiber sensors with compact size, ultra sensitivity and selectivity, applicability to harsh or difficult environment,and capability for remote and multiplexed multi point detection and distributed sensing. The hollow-core and the large air filling ratio of the air-silica cladding region mean that the elasto-optic property of the HC-PBF can be very different from the conventional solid fibers. These HC-PBFs can be optimized to produce more sensitive pressure and acoustic sensors. We carried out experimental and theoretical investigations on the phase sensitivity of the fundamental mode to pressure waves applied externally to the fiber,and demonstrated experimentally that, by post-processed (i.e.,etching the cladding away and re-coating with polymer) the commercial HC-1550-02 fiber, the sensitivity can be increased by ~10 dB, which is ~25 dB higher than the standard SMF. The phase sensitivity of HC-PBF to gas pressure applied internally to the hollow-core was also theoretically and experimentally investigated. The accumulated phase of the fundamental mode to internal pressure was measured to be 1.044×10 -2 rad/(Pa·m), which is over two orders of magnitude higher than that to external pressure. These properties of the HC-PBF would allow high sensitivity optical fiber acoustic/pressure sensors.