Distributed optical vibration sensors (DOVS) have attracted much attention recently since it can be used to monitor mechanical vibrations or acoustic waves with long reach and high sensitivity. Phase-sensitive optical time domain reflectometry (Ф-OTDR) is one of the most commonly used DOVS schemes. For Ф-OTDR, the whole length of fiber under test (FUT) works as the sensing instrument and continuously generates sensing data during measurement. Researchers have made great efforts to try to extract external intrusions from the redundant data. High signal-to-noise ratio (SNR) is necessary in order to accurately locate and identify external intrusions in Ф-OTDR systems. Improvement in SNR is normally limited by the properties of light source, photodetector and FUT. But this limitation can also be overcome by post-processing of the received optical signals. In this context, detailed methodologies of SNR enhancement post-processing algorithms in Ф-OTDR systems have been described in this paper. Furthermore, after successfully locating the external vibrations, it is also important to identify the types of source of the vibrations. Pattern classification is a powerful tool in recognizing the intrusion types from the vibration signals in practical applications. Recent reports of Ф-OTDR systems employed with pattern classification algorithms are subsequently reviewed and discussed. This thorough review will provide a design pathway for improving the performance of Ф-OTDR while maintaining the cost of the system as no additional hardware is required.

Long period grating (LPG) at turn-around-point (TAP) has been studied with a view to enumerate the dependence of sensitivity of a particular resonant mode at the TAP to surrounding refractive index on the initial coupling strength. It has been shown theoretically and also validated experimentally that sensitivity can be enhanced significantly by tailoring the coupling strength of the cladding mode at the resonant wavelength near the TAP. Sensitivity characteristics have been studied for surrounding refractive index in the range 1.335-1.360, which is of interest in the field of biosensors, where the sensitivity of conventional LPGs is relatively small. We could attain a sensitivity of ~1850 nm/RIU using a TAP-LPG with ~3-dB attenuation at resonance.

In this paper, a detailed investigation on the modeling of long-period fiber grating (LPFG) sensors is discussed with the aim of providing a more realistic solution for their use in biosensing. Add-layer sensitivity, i.e., sensitivity of the sensor to an additional layer adhered onto the fiber surface, is quantified and a clear and complete analysis about the influence of the average thickness of the deposited biological sensing layers, as well as the change in refractive index of these layers, on the resonant wavelength of the cladding modes of an LPFG is provided. Add-layer sensitivity of LPFG sensors close to mode transition (MT) and also at turn-around point (TAP) are taken into account. Adsorbed layer thicknesses, as estimated from measured wavelength shifts of the LPFG, are found to have a good match with the values obtained through other measurement techniques.

A new temperature fiber ring laser (FRL) sensor based on a cascaded Sagnac loops fiber structure is proposed and experimentally demonstrated. The optical FRL sensor consists of cascaded Sagnac loops by inserting two polarization-maintaining optical fibers (PMF) with slightly different lengths. PMF with length of 56 cm and 75 cm are used in Sagnac loops as a filter and sensing unit in laser cavity. The working principle of the sensor is based on the phase shift (θ) caused by birefringence between two principal polarization modes and enhance the sensitivity by constructing a Vernier-scale. In an appropriate temperature range (25 °C - 31 °C), the detection sensitivity of FRL sensor based on a cascaded Sagnac structure is significantly higher than other FRL sensors. Thanks to the laser sensing system, the sensitivity can be modulated by changing the free spectral range (FSR). The experimental results show that the temperature sensitivity of the cascade Sagnac structure sensor is - 4.031 nm / °C, which is five times higher than that of FRL sensor base on the single Sagnac structure.

A highly sensitive optical fiber sensor for the measurement of axial force and refractive index (RI) is presented. The optical sensor consists of distributed Bragg reflector (DBR) based fiber laser, the laser cavity of DBR is polished to D-shape by the wheel fiber polishing system (WFPS). Birefringence has been introduced with side polishing of the fiber and then it is utilized for sensing applications. The working principle of the device is based on the principle of the change in beat frequency of the two orthogonal polarization modes of DBR fiber laser with external perturbations. In this work, it has been shown that the dual polarization beat frequency (DPBF) can be used for refractive index-based measurement applications along with measurement of axial force. The sensitivity of the DBR fiber laser sensor with change in axial force enhanced significantly with side polishing method. Such a device is robust, simple in fabrication and most importantly low-cost interrogation techniques can be used for the detection purpose. A resolution of 5.6014×10-8 N has been achieved in case of recognition of axial force and 1.9259×10-5 RIU is the observable detection limit for the RI based measurement.

We present our theoretical study on the design of LPFG sensor where its Add-Layer sensitivity is enhanced. Add-Layer sensitivity quantifies the sensitivity of the sensor to the changes taking place within few tens of nanometers around the receptor molecules. Two different methodologies: the use of dual overlay layer and tailoring of the intermodal separation between two cladding modes, have been used to enhance the Add-layer sensitivity. Using coupled mode analysis we compute several examples to carry out a detailed comparative analysis between the results obtained, focusing on the cladding mode near mode transition.