The optical fiber deployed in the metropolitan infrastructure carrying high-speed WDM traffic data is used also for network surveillance, identifying and localizing the onset of vibrations and dynamic stress events, potentially dangerous for the integrity of the fiber telecom asset. A coherent approach is adopted in a counter-propagating interferometer to develop a sensing system exploiting the ring layout of the metro net- work. With respect to usual dual Mach-Zehnder interferometric sensors, where typically the reference arm is kept isolated, in our arrangement we use as reference arm a fiber inside the same deployed cable, minimizing the impact of the city strong environmental noise. Preliminary assessments of the proposed sensing solution have been experimented in a metro ring of 32 km standard single-mode fiber deployed in the city of Turin, Italy, devoted to telecom applications. The sensor is demonstrated to operate preserving the coexistence with the high-speed WDM telecom traffic. With respect to complex and expensive sensing solutions based on pulsed phase-OTDR techniques, in the present case continuous-wave counter-propagating optical signals are exploited and the spatial resolution in dynamic event localization depends only on the receiver sampling rate: in our implementation, by adopting off-the shelf low-cost commercial 20MS/s sampling boards, 10-meter spatial resolution is experimentally achieved. The proposed sensing solution provides significant added value to the deployed metro fiber infrastructure and the same interferometric approach based on the counter-propagating layout can be easily exploited in metro networks even in case of presence of optical nodes equipped with reconfigurable optical add-drop multiplexers useful for WDM traffic routing.

Birefringence affects all the optical circuits both in free-space and guided-wave optics. It perturbs the state of polarization of the propagating light, causing unwanted detrimental effects in many practical situations. A retracing circuit offers the potentiality of compensating the birefringence, but it is not universal because the birefringence is of different kinds, that is, either linear or circular, as well as either reciprocal or nonreciprocal, and because the common mirror does not hold all the requested symmetries. This paper reviews the compensation techniques suitable for each kind of birefringence, taking into account the introduction of two further generalized mirrors, the mirrored Faraday rotator and the mirrored quarter-wave plate. The main retracing schemes are analyzed and presented with the help of the Poincaré sphere and the Jones matrices. Examples of full compensation of all the main cases of birefringence that occurs in practical optical circuits are given. The different compensation properties of the three mirrors can be interpreted by means of a unified vision in the abstract space related to the Poincaré sphere representation.

The results of an experimental investigation of the impact of the bending-induced linear birefringence on a fiber-optic current sensor are presented. A polarimetric configuration is used for measuring a 50-Hz AC current, exploiting the Faraday effect in a coil of standard optical fiber. Different values of diameter and number of turns of the coil are considered, evaluating the loss of sensitivity due to the birefringence as reduction of an effective value of the Verdet constant. It will be experimentally demonstrated that for a coil with a diameter sufficiently large, i.e. equal or larger than 14 cm, the effect of the linear birefringence plays only a marginal role and it can be neglected.

A novel fiber optic sensing technology for high frequency dynamics detection is proposed in this paper, specifically tailored for structural health monitoring applications based on strain wave analysis, for both passive impact identification and active Lamb wave monitoring. The sensing solution relies on a fiber optic-based interferometric architecture associated to an innovative coherent detection scheme, which retrieves in a completely passive way the high-frequency phase information of the received optical signal. The sensing fiber can be arranged into different layouts, depending on the requirement of the specific application, in order to enhance the sensor sensitivity while still ensuring a limited gauge length if punctual measures are required. For active Lamb wave monitoring, this results in a sensing fiber arranged in multiple loops glued on an aluminum thin panel in order to increase the phase signal only in correspondence to the sensing points of interest. Instead, for passive impact identification, the required sensitivity is guaranteed by simply exploiting a longer gauge length glued to the structure. The fiber optic coherent (FOC) sensor is exploited to detect the strain waves emitted by a piezoelectric transducer placed on the aluminum panel or generated by an impulse hammer, respectively. The FOC sensor measurements have been compared with both a numerical model based on Finite Elements and traditional piezoelectric sensors, confirming a good agreement between experimental and simulated results for both active and passive impact monitoring scenarios.