We present a fiber-optic multibeam Mach-Zehnder interferometer (m-MZI) for simultaneous multi-parameter measurement. The m-MZI is comprised of a section of photonic crystal fiber integrated with two independent cores of distinct construction and birefringence properties characterized for torsion, strain and temperature sensing. Due to the presence of small core geometry and use of a short fiber length, the sensing device demonstrates inter-modal interference in the small core alongside the dominant inter-core interference between the cores for each of the orthogonal polarizations. The output spectrum of the device is characterized by the three-beam interference model and is polarization-dependent. The two types of interferometers present in the fiber m-MZI exhibit distinct sensitivities to torsion, strain and temperature for different polarizations, and matrix coefficients allowing simultaneous measurement of the three sensing parameters are proposed in experiment.

A new fiber sensor configuration based on the cascaded in-line fiber interferometers has been proposed and experimentally demonstrated for simultaneous measurement of strain and temperature. In order to make the cascaded in-line interferometers, we used two different types of two-core photonic crystal fibers (TCPCFs) having two cores in the symmetry and asymmetry structures. The TCPCFs have different transmission characteristics owing to the distinct geometric structures of the integrated cores, this causes the different strain and temperature sensitivities and thus the two parameters can be discriminated. The measurement resolutions are achieved to be 11.1 με and 1.10 °C for strain and temperature, respectively, in the 10 pm wavelength resolution.

A highly sensitive and simple all-fiber interferometric two-dimensional (2D) bending vector sensor is demonstrated. Fiber interferometer works in reflection mode and formed by manually splicing a small section of elliptic two-core photonic crystal fiber (ETC-PCF) at the end of a single-mode fiber (SMF). Due to birefringence nature of the two cores, fiber device exhibit orientation-dependent bend sensitivity which is different for different polarization states. A high sensitivity of 6.49 nm/mm is achieved for bending along x direction, and the rms deviations for bending in x and y directions are estimated to be 0.015 mm and 0.098 mm, respectively by using matrix method. The device is immune to bend-induced power fluctuation.

We propose a weighted singular value decomposition (WSVD) to reduce the random noise in the Rayleigh backscattering signal of the phase-sensitive optical time domain reflectometry (Φ-OTDR) to enhance the detection performance of the distributed vibration sensing. A 2D image is formed by assembling the raw Rayleigh backscattering traces into a matrix form, and slowly varying fluctuation and random noise can be removed using the WSVD. Consequently, the location information and the frequency of vibration induced by the external vibration event can be extracted. A vibration event with 9 m spatial resolution is detected along a 2.4 km single mode fiber. The signal-to-noise ratio (SNR) of location information for the 102 Hz physical vibration and the 525 Hz acoustic vibration was found to be 10.7 and 12.2 dB, respectively. The SNR of the vibration events demonstrate an increase of 6–7 dB compared to the conventional method, showing the excellent denoising capability of this new approach.

In this article, we present comparative study on the transmission and the sensing characteristics of an all-fiber Sagnac loop interferometer (SLI) made by different kinds of the highly-birefringent (HB) two-core photonic crystal fiber (TCPCF), respectively for strain, torsion, and temperature sensing. The fiber device is analyzed theoretically and experimentally. Transmission spectrum of the SLI using the HB TCPCF shows six spatial frequencies due to superposition of multiple two-beam interferences originating from the intra-core and the inter-core interactions between the fundamental polarization modes propagating in the two birefringent cores. Our results reveal that the transmission characteristics of the fiber device and the sensing features of its multiplexed interferences are strongly dependent on the optical and the structural properties of the two birefringent cores such as their intrinsic birefringences, the effective index contrast between them, and the relative orientation between the (slow) birefringence axes. Finally, the application of the SLI made by the specific HB TCPCF is discussed in the simultaneous measurement of three parameters.

We demonstrate a multiparameter distributed sensing system based on optical frequency-domain reflectometry (OFDR) with a bandwidth-division multiplexing (BDM) technique. With the BDM-assisted OFDR, the sensor system can help interrogate multiple parallel sensing optical fibers by scanning the fibers within distinct spectral ranges without any cross talk. A dual-parameter distributed sensing system was set up by multiplexing two types of sensing fibers for the simultaneous independent measurement of strain and temperature. The rms deviations between the applied and measured values of the two parameters are $19~ {\mu } {\varepsilon } $ and 1.4 °C, respectively, for a sensing spatial resolution of 10 cm. The proposed multiparameter distributed sensor system offers inexpensive potential solution to continuously monitor various parameters and is effective for the health monitoring of complex distributed structures.