Pulsed photothermal interferometry (PTI) gas sensor with hollow-core photonic bandgap fibre (HC-PBF) is demonstrated with a Sagnac interferometer-based phase detection system. Under the condition of constant peak pump power, the optimal pulse duration is found to be > 1:2 <i>μs</i> for detecting low-concentration of trace gases in nitrogen, limited by thermal conduction of gases within the hollow-core. Preliminary experiments with a 0.62-mlong HC-PBF gas cell, low peak power ( ~ 20:2<i>mW</i>) and a boxcar averager with 10k average times demonstrated a detection limit of 3:3 <i>p:p:m</i> acetylene. Detection limit down to ppb or lower is expected with high peak power pump pulses.
We report a highly sensitive all-fiber hydrogen sensor based on continuous-wave stimulated Raman gain spectroscopy with a hollow-core photonic crystal fiber operating around 1550 nm. A pump-probe configuration is used, in which the frequency difference between the pump and the probe lasers is tuned to the S<sub>0</sub>(0) transition of para-hydrogen with a Raman shift of 354 cm<sup>-1</sup>. Preliminary experiments demonstrate a detection limit down to 17 ppm with a 250 s averaging time, and further improvement is possible. The all-fiber configuration operating in the telecommunication wavelength band would enable cost-effective and compact sensors for high sensitivity and high-resolution trace analysis.
We present all-fiber resonating Fabry-Perot gas cells made with a piece of hollow-core photonic bandgap fiber (HCPBF) sandwiched by two single mode fibers with mirrored ends. A HC-PBF cavity made of 6.75-cm-long HC-1550-06 fiber achieved a cavity finesse of 128, corresponding to an effective optical path length of 5.5 m. Such HC-PBF cavities can be used as absorption cells for high sensitivity gas detection with fast response. Preliminary experiment with a 9.4-cm-long resonating gas cell with a finesse of 68 demonstrated a detection limit better than 7.5 p.p.m. acetylene.
We demonstrate an all-optical-fiber photoacoustic (PA) spectrometric gas sensor with a graphene nano-mechanical resonator as the acoustic detector. The acoustic detection is performed by a miniature ferrule-top nano-mechanical resonator with a ∼100-nm-thick, 2.5-mm-diameter multilayer graphene diaphragm. Experimental investigation showed that the performance of the PA gas sensor can be significantly enhanced by operating at the resonance of the grapheme diaphragm where a lower detection limit of 153 parts-per-billion (ppb) acetylene is achieved. The all-fiber PA sensor which is immune to electromagnetic interference and safe in explosive environments is ideally suited for real-world remote, space-limited applications and for multipoint detection in a multiplexed fiber optic sensor network.
We exploit photothermal effect in gas-filled hollow-core photonic bandgap fibers, and demonstrate remarkably sensitive all-fiber (acetylene) gas sensors with noise equivalent concentration of 1-3 parts-per-billion and an unprecedented dynamic range of nearly six orders of magnitude. These results are two to three orders of magnitude better than previous direct absorption-based optical fiber gas sensors. The realization of photothermal spectroscopy in fiber-optic format will allow a new class of sensors with ultra-sensitivity and selectivity, compact size, remote and multiplexed multi-point detection capability.
We demonstrate a gas sensor based on mode interference in a hollow-core photonic bandgap fiber. Gas absorption of a pump laser beam induces phase modulation of a propagating probe beam, which is detected by use of an in-fiber modal interferometer. An estimated detection limit of ~2 ppm acetylene (~7x10<sup>-5</sup> in terms of noise equivalent absorbance or NEA) is achieved with 30-cm-long HC-PBF operating at the near infrared wavelength. This NEA is ~22 times better than state-of-the-art HC-PBF gas sensors based on direct absorption spectroscopy.
Phase sensitivity of the fundamental mode of hollow-core photonic bandgap fiber to gas pressure applied internally to its core is investigated. The measured phase sensitivity for a 95-cm-long fiber is 9.92 rad/kPa, over two orders of magnitude higher than that to external pressure. The large phase sensitivity is attributed mainly to the pressure-induced refractive index change of air inside the fiber core. Such an effect may be exploited for high sensitivity pressure sensing and biochemical and environmental process analysis involving pressure variations.
The effects of modal interference (MI) on the performance of hollow-core photonic bandgap fiber (HC-PBF) gas sensors are investigated. By optimizing mode launch, applying wavelength modulation with proper modulation parameters as well as appropriate digital signal processing, an estimated lower detection limit of <1 ppmv acetylene is achieved with 13-m long HC-PBF. The impacts of drilling side-hole on the MI and response time are also studied. With a 62-cm long sensing HC-PBF drilled with multiple side-holes, an acetylene sensor with a lower detection limit of 11 ppmv and a recovery time of 2 minute is demonstrated.
The acoustic pressure sensitivities of hollow-core photonic bandgap fibers (HC-PBFs) with different thicknesses of silica outer-cladding and polymer jacket were experimentally investigated. Experiment with a HC-PBF with 7 μm-thick silica outer cladding and 100 μm-thick Parylene C jacket demonstrated a pressure sensitivity 10 dB higher than the commercial HC-1550-02 fiber and 25 dB higher than a standard single mode fiber. The significant enhancement in sensitivity would simplify the design of fiber hydrophones and increase the number of sensors that could be multiplexed in a single fiber.