Photothermal interferometry is an ultra-sensitive spectroscopic means for trace chemical detection in gas- and liquid-phase materials. Previous photothermal interferometry systems used free-space optics and have limitations in efficiency of light–matter interaction, size and optical alignment, and integration into photonic circuits. Here we exploit photothermal-induced phase change in a gas-filled hollow-core photonic bandgap fibre, and demonstrate an all-fibre acetylene gas sensor with a noise equivalent concentration of 2 p.p.b. (2.3 × 10−9 cm−1 in absorption coefficient) and an unprecedented dynamic range of nearly six orders of magnitude. The realization of photothermal interferometry with low-cost near infrared semiconductor lasers and fibre-based technology allows a class of optical sensors with compact size, ultra sensitivity and selectivity, applicability to harsh environment, and capability for remote and multiplexed multi-point detection and distributed sensing.

Silicon nitride (Si3N4) has emerged as a promising material for integrated nonlinear photonics and has been used for broadband soliton microcombs and low-pulse-energy supercontinuum generation. Therefore understanding all nonlinear optical properties of Si3N4 is important. So far, only stimulated Brillouin scattering (SBS) has not been reported. Here we observe, for the first time, backward SBS in fully cladded Si3N4 waveguides. The Brillouin gain spectrum exhibits an unusual multi-peak structure resulting from hybridization with high-overtone bulk acoustic resonances (HBARs) of the silica cladding. The reported intrinsic Si3N4 Brillouin gain at 25 GHz is estimated as 7×10 −13 m/W. Moreover, the magnitude of the Si3N4 photoelastic constant is estimated as |p12| = 0.047 ± 0.004. Since SBS imposes an optical power limitation for waveguides, our results explain the capability of Si3N4 to handle high optical power, central for integrated nonlinear photonics. arXiv:1908.09815v1 [physics.optics]

Directly accessing the middle infrared, the molecular functional group spectral region, via supercontinuum generation processes based on turn-key fiber lasers offers the undeniable advantage of simplicity and robustness. Recently, the assessment of the coherence of the mid-IR dispersive wave in silicon nitride (Si 3 N 4 ) waveguides, pumped at telecom wavelength, established an important first step towards mid-IR frequency comb generation based on such compact systems. Yet, the spectral reach and efficiency still fall short for practical implementation. Here, we experimentally demonstrate that large cross-section Si 3 N 4 waveguides pumped with 2 μm fs-fiber laser can reach the important spectroscopic spectral region in the 3–4 μm range, with up to 35% power conversion and milliwatt-level output powers. As a proof of principle, we use this source for detection of C 2 H 2 by absorption spectroscopy. Such result makes these sources suitable candidate for compact, chip-integrated spectroscopic and sensing applications.

This paper investigates the effect of modal interference on the performance of hollow-core photonic bandgap fiber (HC-PBF) gas sensors. By optimizing mode launch, using proper length of sensing HC-PBF, and applying proper wavelength modulation in combination with lock-in detection, as well as appropriate digital signal processing, an estimated lower detection limit of less than 1 part-per-million by volume (ppmv) acetylene is achieved.

Among all the nonlinear effects stimulated Brillouin scattering offers the highest gain in solid materials and has demonstrated advanced photonics functionalities in waveguides. The large compressibility of gases suggests that stimulated Brillouin scattering may gain in efficiency with respect to condensed materials. Here, by using a gas-filled hollow-core fibre at high pressure, we achieve a strong Brillouin amplification per unit length, exceeding by six times the gain observed in fibres with a solid silica core. This large amplification benefits from a higher molecular density and a lower acoustic attenuation at higher pressure, combined with a tight light confinement. Using this approach, we demonstrate the capability to perform large optical amplifications in hollow-core waveguides. The implementations of a low-threshold gas Brillouin fibre laser and a high-performance distributed temperature sensor, intrinsically free of strain cross-sensitivity, illustrate the potential for hollow-core fibres, paving the way to their integration into lasing, sensing and signal processing.

High resolution and fast detection of molecular vibrational absorption is important for organic synthesis, pharmaceutical processes, and environmental monitoring, and is enabled by mid-infrared (mid-IR) laser frequency combs via dual-comb spectroscopy. Here, we demonstrate a novel and highly simplified approach to broadband mid-IR dual-comb spectroscopy via supercontinuum generation, achieved using unprecedented nanophotonic dispersion engineering that allows for ultra-broadband and flat-envelope mid-IR frequency combs. Our mid-IR dual-comb spectrometer has an instantaneous bandwidth covering the functional group region from 2800-3600 cm −1 , comprising more than 100,000 comb lines, enabling parallel gas-phase detection with a high sensitivity, sub-Doppler spectral resolution, and a high speed. In addition to the traditional functional groups, their isotopologues are also resolved in this supercontinuum-based dual-comb spectroscopy. Our approach combines well established fiber laser combs, digital coherent data averaging, and integrated nonlinear photonics, each in itself a state-of-the-art technology, signaling the emergence of mid-IR dual-comb spectroscopy for use outside of the protected laboratory environment.

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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.