We present a continuous-wave fiber cavity ringdown (FCRD) pressure-sensing method based on frequency-shifted interferometry (FSI). Compared with traditional CRD or FCRD techniques, this FSI-FCRD scheme deduces pressure by measuring the decay rate of continuous light exiting the fiber ringdown cavity (RDC) in the spatial domain (i.e., the CRD distance), without the requirement for optical pulsation and fast electronics. By using a section of fiber with the buffer layer stripped in the fiber RDC as the sensor head, pressures were measured within the range from 0 to 10.4 MPa. The sensitivity of 0.02356/(km∙MPa) was obtained with a measurement error of 0.1%, and the corresponding pressure resolution was 0.05 MPa. It was found that the measurement sensitivity can be improved by enlarging the interaction length of the sensor head. The results show the proposed sensor has the advantages of simple structure, low cost, high sensitivity, and high stability in pressure detection.

We present a multipoint fiber magnetic field sensing system based on magnetic fluid (MF) and frequency-shifted interferometry fiber loop ringdown (FSI-FLRD). This system contains a number of cascaded sensing channels, and shares a common continuous-wave light source and slow electronics instead of a pulsed light source and high-speed electron devices as required in traditional multichannel FLRD techniques. Each sensing channel is formed by incorporating an MF-coated etched fiber as the sensing head inside a fiber ringdown loop. Each sensing head is mounted on a temperature control device. We experimentally fabricated a dual-channel FSI-FLRD magnetic field sensing system. First the system's temperature response was investigated, which indicates the MF-based sensing heads had temperature cross sensitivity. Then after keeping the temperature at 20 °C for improving testing accuracy and applying different magnetic fields, the two sensing channels were separately shown to have sensitivities of - 9.38×10 ^-5 and - 2.28×10 ^-5 /(km ·Oe) with a linear response in the range of 221-721 Oe. The obtained sensing channel stabilities were respectively 0.02% and 0.04%, which enhance more than one order of magnitude compared with those of most conventional FLRD techniques.

A novel multipoint fiber loop ringdown (FLRD) strain sensing system using frequency-shifted interferometry (FSI) is proposed and experimentally validated. Compared to conventional multipoint FLRD techniques, this scheme measures the decay rate of the continuous wave (CW) light in the space domain and thus greatly reduces the cost without the requirement of expensive devices. A serial dual-point strain sensing system was experimentally constructed and a biconical tapered multimode fiber (MMF) as the sensor head was used for obtaining the large measuring range. By applying different strains on the sensor heads through translation stages, a linear response between strain and additional loss induced by strain sensor was obtained, and the static strain sensitivities of 0.13676 dB/mε and 0.19665 dB/mε were achieved, corresponding to the detection limit of 0.0123 dB and 0.0360 dB, respectively. Moreover, a large measuring range of approximately 6 mε was achieved for both strain sensors. The experimental results indicate that our proposed method offers a promising multipoint strain sensor which has the advantages of low cost, a simple sensing structure and a large measuring range.

In this letter, we demonstrate a low-cost fiber loop ring-down (FLRD) magnetic field sensor using frequency-shifted interferometry. A D-shaped fiber coated with magnetic fluid was used as the sensor head for enhancing the sensitivity and improving the stability. With this scheme, the magnetic field was sensed in the space domain by measuring the ring-down distance, and its measurement accuracy was improved by introducing a temperature controller. The experimental results showed that a sensitivity of 2.53 × 10 ^-3 (km ^-1 ċG ^-1 ) was achieved, and a stability of 0.146% was obtained, which is better than those of the current time-domain FLRD sensors based on evanescent field effect.

We present a multichannel continuous-wave (CW) fiber cavity ringdown (FCRD) gas sensing method based on frequency-shifted interferometry (FSI). This scheme detects gas concentration by measuring the intensity decay rates of continuous light from different ringdown cavities in the spatial domain, unlike conventional FCRD techniques, which measure the decay rates of pulse light in the time domain. This method shares one CW light source, one slow detector, and one slow data collector. In order to illustrate the theory, acetylene gas concentration measurement in a two-channel FSI-FCRD system was experimentally conducted in the range of 0%–1%. A linear relation was established between concentration and absorption loss, which is proportional to the intensity decay rate, and the measurement resolutions of 3.871%/dB and 3.658%/dB were achieved, respectively. The results reveal that the proposed system has the advantages of low cost, high sensitivity, high precision, and good stability in multichannel gas detection.

In industries such as coal mining, natural gas transportation and waste-to-energy, methane detection is an essential step. In order to reduce the risk and improve accuracy, laser sensors are used to detect methane. Aiming at the characteristics of the absorption peak of methane gas at 1653.7 nm, the 1653.7 nm distributed feedback laser was obtained from the multi quantum well materials design and ridge-wide pattern design to device packaging by using metal-organic chemical vapor epitaxial deposition, holographic exposure, and nanoimprint lithography. The laser performance achieves a side-mode suppression ratio of 54 dB, a slope efficiency of 0.372 W/A, a threshold current not greater than 12 mA, a saturated optical power greater than 20 mW, and stable optical and electrical properties. Based on this laser, for methane gas with a concentration of 0% to 3%, the loss is stable with the change of gas concentration, and the absorption sensitivity to methane is 0.20237 dB/%.