Yanzhen Tan

Hollow-core fiber Fabry–Perot photothermal gas sensor

A highly sensitive, compact, and low-cost trace gas sensor based on photothermal effect in a hollow-core fiber Fabry-Perot interferometer (FPI) is described. The Fabry-Perot sensor is fabricated by splicing a piece of hollow-core photonic bandgap fiber (HC-PBF) to single-mode fiber pigtails at both ends. The absorption of a pump beam in the hollow core results in phase modulation of probe beam, which is detected by the FPI. Experiments with a 2 cm long HC-PBF with femtosecond laser drilled side-holes demonstrated a response time of less than 19 s and noise equivalent concentration (NEC) of 440 parts-per-billion (ppb) using a 1 s lock-in time constant, and the NEC goes down to 117 ppb (2.7×10-7 in absorbance) by using 77 s averaging time.

Hollow-Core Microstructured Optical Fiber Gas Sensors

Recent progress in gas detection with hollow-core microstructured optical fibers (HC-MOFs) and direct absorption/photothermal interferometry spectroscopy are reported. For direct-absorption sensors, the issue of mode interference noise is addressed and techniques to minimize such a noise are experimentally demonstrated. Large-scale drilling of hundreds of low-loss micro-channels along a single HC-MOF is performed, and reduction of diffusion-limited response time from hours to ∼40 s is demonstrated with a 2.3-m-long HC-MOF. For photothermal inteferometry sensors, novel detection configurations based on respectively a Sagnac interferometer and an in-fiber modal interferometer are experimentally demonstrated. The Sagnac configuration avoids the need for complex servo-control for interferometer stabilization while the in-fiber configuration simplifies the detection, reducing the size and cost of the sensor system. Sub ppm gas detection can be achieved easily with photothermal interferometry HC-MOF sensors but is difficult to achieve for direct-absorption sensors with the current commercial HC-MOFs.

Optical Fiber Photoacoustic Gas Sensor With Graphene Nano-Mechanical Resonator as the Acoustic Detector

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 report an all-optical fiber photoacoustic gas sensor with a graphene nano-mechanical resonator as the acoustic detector. The acoustic detector is a Fabry-Perot interferometer formed by attaching a 100-nm-thick, 2.5-mm-diameter multilayer graphene diaphragm to a hollow cavity at the end of a single-mode optical fiber. 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 2.11 μPa/Hz1/2 at 10.1 kHz is demonstrated. Detection of acetylene gas is demonstrated with a distributed feedback semiconductor laser tuned to the P(9) absorption line of acetylene and a lower detection limit of 119.8 parts-per-billion (ppb) is achieved with 123.9-mW pump power. Theoretical analysis shows that by increasing the Q-factor of the resonator, which may be achieved by operating at low gas pressures, ppb level gas detection is possible. The all-fiber photoacoustic gas sensor is immune to electromagnetic interference, safe in flammable and explosive environment, and would be ideally suited for remote, space-limited applications and for multipoint detection in a multiplexed fiber optic sensor network.

Distributed gas sensing with optical fibre photothermal interferometry

We report the first distributed optical fibre trace-gas detection system based on photothermal interferometry (PTI) in a hollow-core photonic bandgap fibre (HC-PBF). Absorption of a modulated pump propagating in the gas-filled HC-PBF generates distributed phase modulation along the fibre, which is detected by a dual-pulse heterodyne phase-sensitive optical time-domain reflectometry (OTDR) system. Quasi-distributed sensing experiment with two 28-meter-long HC-PBF sensing sections connected by single-mode transmission fibres demonstrated a limit of detection (LOD) of ∼10 ppb acetylene with a pump power level of 55 mW and an effective noise bandwidth (ENBW) of 0.01 Hz, corresponding to a normalized detection limit of 5.5ppb⋅W/Hz. Distributed sensing experiment over a 200-meter-long sensing cable made of serially connected HC-PBFs demonstrated a LOD of ∼ 5 ppm with 62.5 mW peak pump power and 11.8 Hz ENBW, or a normalized detection limit of 312ppb⋅W/Hz. The spatial resolution of the current distributed detection system is limited to ∼ 30 m, but it is possible to reduce down to 1 meter or smaller by optimizing the phase detection system.

High finesse hollow-core fiber resonating cavity for high sensitivity gas sensing application

We present all-fiber resonating Fabry-Perot gas cells made with a piece of hollow-core photonic bandgap fiber (HC-PBF) 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.

Highly sensitive optical fibre gas sensors

Fibre-based photothermal and photoacoustic sensors have demonstrated ppb - ppm level detection limit. The use of optical fibres and near infrared semiconductor lasers would allow compact and cost-effective sensors with remote detection capability.