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We report the observation and theoretical analysis of photothermal phase modulation in an optical nanofiber (NF) immersed in light-absorbing gas. The phase modulation arises from the contrasting photothermal processes experienced by the mode field within and outside the NF, which exhibits significant wavelength and pressure dependence in the nanoscale waveguides. By designing the diameter of the NF, the photothermal phase modulation can be readily controlled, and a nullification of phase modulation is achieved at a specific wavelength. Experiments on NFs with different diameters demonstrated pressure-controllable phase modulation up to 0.058π rad/mW, potentially allowing the development of nanoscale all-optical modulators and sensors with optimal performance. [ABSTRACT FROM AUTHOR]

On‐chip waveguide spectroscopic sensors have attracted considerable attention due to its potential for large‐scale integration. However, existing waveguide gas sensors based on direct absorption spectroscopy (DAS) suffer from limited sensitivity and measurement range. Here waveguide‐based on‐chip photothermal spectroscopy (PTS) is demonstrated for gas detection with high sensitivity and large dynamic range. On‐chip photothermal field due to non‐radiation relaxation of gas molecules and the resulted photothermal phase modulation are analyzed. By selecting chalcogenide glass (ChG) as the core‐layer material and fabricating thermally‐isolated ChG‐on‐SU8 waveguide for thermal field accumulation, a twofold increase in photothermal phase modulation is achieved as compared to ChG‐on‐SiO2 waveguides. Different from the major concern of multi‐path etalon noise in DAS, piezoelectric transducer noise in the interferometer is identified as the main source in this PTS. For a fair comparison, acetylene (C2H2) detection experiments are conducted using PTS and DAS with a 2 cm‐long ChG‐on‐SU8 waveguide. A remarkable sensitivity of 4 parts‐per‐million (ppm) is achieved, which is 16 times better than that of DAS. The dynamic range extends over five orders of magnitude for PTS, ≈3 orders of magnitude larger than that of DAS. Such high performance opens the possibility of fully‐integrated chip‐level sensors for low‐power, light‐weight applications. [ABSTRACT FROM AUTHOR]

We demonstrate a compact all-fiber oxygen sensor using photothermal interferometry with a short length (4.3 cm) of hollow-core negative curvature fibers. The hollow-core fiber has double transmission windows covering both visible and near-infrared wavelength regions. Absorption of a pump laser beam at 760 nm produces photothermal phase modulation and a probe Fabry-Perot interferometer operating at 1550 nm is used to detect the phase modulation. With wavelength modulation and first harmonic detection, a limit of detection down to 54 parts per million (ppm) with a 600-s averaging time is achieved, corresponding to a normalized equivalent absorption of 7.7 × 10−8 cm−1. The oxygen sensor has great potential for in situ detection applications. [ABSTRACT FROM AUTHOR]

Photothermal interferometry (PTI) spectroscopy is a background-free and ultrasensitive gas or chemical sensing method. To date, most works have focused on applying different techniques to improve the sensitivity of PTI gas sensors via enhancing the photothermal (PT) phase modulation. Herein, we develop an all-fiber dispersion turning point (DTP)-enhanced PTI gas sensor with a taper-based mode interferometer in which the PT phase detection sensitivity of the interferometer is enhanced in a millimeter-long single fiber PTI configuration, which has not been studied yet. The design is demonstrated by conducting the measurement of acetylene with a 3-mm-long, 2.29-μm-diamter microfiber interferometer. A lower detection limit of 965 parts per billion (ppb) acetylene is achieved when the probe wavelength is operating near DTP, showing 16 times enhancement. Besides, theoretical analysis shows that the sensitivity of phase detection would be enormously enhanced when the dispersion factor approaching zero. By optimal design of the taper-based mode interferometer, together with thinner fiber diameter and longer fiber length, gas detection may be further improved by double enhancement of both PT phase generation and phase detection. With advantages of high sensitivity, compact size and low cost, the proposed all-fiber DTP-enhanced PTI gas sensor is potentially promising for ultra-sensitive trace chemical or biochemical sensing. • A dispersion turning point (DTP)-enhanced photothermal interferometry gas sensor is proposed. • The sensor is a single microfiber interferometer and acts as absorption region simultaneously. • The sensor focuses on improving the phase detection sensitivity of the interferometer. • The proposed sensor demonstrated a lower detection limit of 965 ppb acetylene. • Results showed that the signal was enhanced by a factor of 16 with probe beam near DTP. [ABSTRACT FROM AUTHOR]