Specialty fibers have introduced new functionalities and opportunities in distributed fiber sensing applications. Particularly, Brillouin optical time domain analysis (BOTDA) systems have leveraged the unique features of specialty fibers to achieve performance enhancement in various sensing applications. This paper provides an overview of recent developments of the specialty fibers based BOTDA technologies and their sensing applications. The specialty fibers based BOTDA systems are categorized and reviewed based on the new features or performance enhancements. The prospects of using specialty fibers for BOTDA systems are discussed.

Tide monitoring is essential for coastal management and protection. In this study, a commercial submarine cable was used to measure the vibration signals induced by the tide-edge on the coast. An intensity-based distributed acoustic sensing (DAS) interrogator was used to acquire the vibration datasets along the submarine cable up to the first repeater. The preliminary results demonstrated that the DAS system and the existing submarine cable can achieve sea level and shoreline wave induced vibration strength monitoring in the manner of on-line and distributed measurement. The vibration strength experienced by the submarine cable in the land section and the sea section induced by the tide edge are different, forming a clear boundary that can be analyzed further for coastal structure monitoring. Experimental results indicate the potential of DAS for effective tide level monitoring, revealing the standard deviations of 0.221 m and a mean difference of 0.004m in tide level variation and the standard deviation of 0.120 m/h and a mean speed difference of 0.003 m/h in the rate of tide level fluctuations. This study foresees the potential for developing a submarine sensor network using DAS technology, coupled with existing or customized fiber optic submarine cables to monitor coastal structures and enhance coastal protection capabilities in coastal cities.

For the measurement of Mueller matrix in an optical system with birefringence and small polarization-dependent loss or gain (PDL/G), we theoretically derive the statistical relationship between the Mueller matrix measurement error and three input states of polarization (SOP). Based on this theoretical relation and simulation results, it can be concluded that the three input SOPs, that are coplanar with an angle of 120° between any two of them in Stokes space, can be considered as a substitute for the best input SOPs which can statistically lead to the minimum measurement error. This conclusion is valid when the PDL/G of the optical system under test is less than 0.35dB.

The properties of the state of polarization (SOP) and the degree of polarization (DOP) of Rayleigh backscattered light (RBL) in single-mode fibers (SMF) are investigated theoretically and experimentally when the incident probe is a perfectly coherent continuous-wave (CW) light. It is concluded that the instantaneous DOP of the coherently superposed RBL is always 100%, and the instantaneous SOP is determined by the distributions of the birefringence and the optical phase along the SMF. Therefore, the instantaneous SOP of the coherently superposed RBL does not have a constant relationship with the SOP of the incident CW probe. Furthermore, the instantaneous SOP varies randomly with time because the optical phase is very sensitive to ambient temperature and vibration even in the lab environment. Further theoretical derivation and experimental verification demonstrate, for the first time, that the temporally averaged SOP of the coherently superposed RBL has a simple constant relationship with the SOP of the incident CW probe, and the temporally averaged DOP is 1/3 in an SMF with low and randomly distributed birefringence. The derived formulas and obtained findings can be used to enhance the modelling and improve the performances of phase-sensitive optical time-domain reflectometry and other Rayleigh backscattering based fiber-optic sensors.