Opto-mechanical interactions in guided wave media are drawing great interest in fundamental research and applications. Forward stimulated Brillouin scattering, in particular, is widely investigated in optical fibres and photonic integrated circuits. In this work, we report a comprehensive study of forward stimulated Brillouin scattering over standard, panda-type polarization maintaining fibres. We distinguish between intra-polarization scattering, in which two pump tones are co-polarized along one principal axis, and inter-polarization processes driven by orthogonally polarized pump waves. Both processes are quantified in analysis, calculations and experiment. Inter-modal scattering, in particular, introduces cross-polarization switching of probe waves that is non-reciprocal. Switching takes place in multiple wavelength windows. The results provide a first demonstration of opto-mechanical non-reciprocity of forward scatter in standard fibre. The inter-polarization process is applicable to distributed sensors of media outside the cladding and coating boundaries, where light cannot reach. The process may be scaled towards forward Brillouin lasers, optical isolators and circulators and narrowband microwave-photonic filters over longer sections of off-the-shelf polarization maintaining fibres.

Fibre lasers based on backward stimulated Brillouin scattering provide narrow linewidths and serve in signal processing and sensing applications. Stimulated Brillouin scattering in fibres takes place in the forward direction as well, with amplification bandwidths that are narrower by two orders of magnitude. However, forward Brillouin lasers have yet to be realized in any fibre platform. In this work, we report a first forward Brillouin fibre laser, using a bare off-the-shelf, panda-type polarisation maintaining fibre. Pump light in one principal axis provides Brillouin amplification for a co-propagating lasing signal of the orthogonal polarisation. Feedback is provided by Bragg gratings at both ends of the fibre cavity. Single-mode, few-modes and multi-mode regimes of operation are observed. The lasing threshold exhibits a unique environmental sensitivity: it is elevated when the fibre is partially immersed in water due to the broadening of forward Brillouin scattering spectra. The results establish a new type of fibre laser, with potential for ultra-high coherence and precision sensing of media outside the cladding.

The monitoring of ionizing radiation is critical for the safe operation of nuclear and other high-power plants. Fiberoptic sensing of radiation has been pursued for over 45 years. Most protocols rely on radiation effects on the optical properties of the fiber. Here we propose a new concept, in which the opto-mechanics of standard fibers coated by thin layers of fluoroacrylate polymer are observed instead. The time-of-flight of radial acoustic waves through the coating is evaluated by forward stimulated Brillouin scattering measurements. The time-of-flight is seen to decrease monotonically with the overall dosage of gamma radiation from a cobalt source. Variations reach 15% of the initial value for 180 Mrad dose and remain stable for at least several weeks following exposure. The faster times-of-flight are consistent with a radiation-induced increase in the coating stiffness, observed in offline analysis. The effects on the coating are independent of possible changes in the optical parameters of the fiber. The combination of opto-mechanical analysis together with established fiber sensing protocols may help disambiguate the evaluation of multiple radiation metrics and reduce environmental crosssensitivities. The technique is suitable for online monitoring and may be extended to spatially distributed measurements.

Forward Brillouin scattering interactions support the sensing and analysis of media outside the cladding boundaries of standard fibers, where light cannot reach. Quantitative point-sensing based on this principle has yet to be reported. In this work, we report a forward Brillouin scattering point-sensor in a commercially available, off-the-shelf multi-core fiber. Pump light at the inner, on-axis core of the fiber is used to stimulate a guided acoustic mode of the entire fiber cross-section. The acoustic wave, in turn, induces photoelastic perturbations to the reflectivity of a Bragg grating inscribed in an outer, off-axis core of the same fiber. The measurements successfully analyze refractive index perturbations on the tenth decimal point and distinguish between ethanol and water outside the centimeter-long grating. The measured forward Brillouin scattering linewidths agree with predictions. The acquired spectra are unaffected by forward Brillouin scattering outside the grating region. The results add point-analysis to the portfolio of forward Brillouin scattering optical fiber sensors.

Scattering is among the most common and widely employed optical phenomena. The spatially resolved analysis of scattering contributions supports distributed sensing of quantities of interest. While optical backscatter events are readily mapped using time-of-flight considerations, the distributed analysis of forward scattering represents a fundamental and long-standing challenge. Interest in distributed analysis of forward scattering has reawakened in recent years, toward optical fiber sensors based on forward-stimulated Brillouin scattering. Existing protocols for distributed analysis of forward Brillouin scattering rely on secondary backscattering mechanisms and mandate the noise-prone differentiation of collected data with respect to position. Here we report on the direct, distributed analysis of forward scattering. The combined contributions of forward-stimulated Brillouin scattering and Kerr effect four-wave mixing are resolved with respect to position along polarization-maintaining fibers. The concept is based on the characteristics of intermodal scattering in such fibers: Forward scattering is initiated by a pair of orthogonally polarized and copropagating pump waves and observed through the nonlinear polarization switching of a counterpropagating probe. Measurements distinguish between dissimilar fibers connected in series, and between air and water outside a polyimide-coated fiber section in a specific location. The measurement range was 1.1 km. The spatial resolution currently achieved is estimated as 60 m, limited by the lifetimes of forward Brillouin scattering. The results provide preliminary proof of concept for distributed forward Brillouin fiber sensors that do not require the differentiation of data.