Abstract: This application describes methods and apparatus for monitoring of rail networks using fibre optic distributed acoustic sensing (DAS), especially for condition monitoring. One method involves taking (902) a first data set corresponding to measurement signals from a plurality of channels of at least one fibre optic distributed acoustic sensor (100) having a sensing fibre (101) deployed to monitor at least part of the path of the rail network (201). The first data set corresponds to measurement signals acquired as a train (202) passed along a first monitored section of the rail network. The method involves identifying (903) a speed of the train through the first monitored section and dividing (904) the first data set into a plurality of time windows. Each time window contains a different subset of the first data set, with the measurement signal for each successive channel in a time window being delayed with respect to the previous channel by a time related to the identified train speed.

Abstract: The present disclosure describes semiconductor external cavity laser with wide bandwidth frequency modulation capabilities. The laser is preferably packaged in a standard form-factor package, such as a 14-pin butterfly package. The front end of the cavity comprises an integrated planar circuit (e.g., silica-on-silicon planar lightwave circuit with Bragg gratings), and the “back facet” of the laser is implemented as a high-reflection (HR) coated LiNbO3 phase tuning section in the double pass configuration. AC-voltage signal applied to the electrodes of phase tuning section modulates a refractive index of the propagating TE-polarization mode of external cavity and produces frequency modulation. Such frequency modulation is not associated with any thermal behavior of the gain element included in the external cavity laser, and has a negligible phase delay over a wide bandwidth.

Abstract: The present invention relates to distributed acoustic sensing using fiber-optic system. More particularly, the present invention describes use of frequency pulse labeling techniques and wavelength pulse labeling techniques for providing high bandwidth acoustic sensing in applications such as infrastructure monitoring. In one embodiment, a segmented sensing fiber is used with corresponding circulators in an architecture that controls the interrogation of each segment of the fiber. In another embodiment, a single continuous length of sensing fiber is used, but a plurality of pulse sequences with different wavelengths are used to interrogate. In both configurations, heterodyne beat frequency components are rejected by a processing scheme, resulting in a simple direct measurement of baseband phase information.