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In this paper, we demonstrate automatic vehicle detection and counting by processing data acquired using a phase-sensitive optical time-domain reflectometer ( ϕ -OTDR) distributed optical fiber sensor. The acquired data are processed using the Hough transform, which detects the lines in the images formed by representing the acquired data in the space-time domain. A rough classification of the vehicles (heavy versus light vehicles) is also proposed, based on the amplitude of the vibration data along the detected lines. The method has been experimentally tested by performing ϕ -OTDR measurements along a telecommunication fiber cable running in a buried conduit along the state road SS18 (province of Salerno, Italy), opened to normal traffic. Comparison with ground-truth data, manually generated by inspecting video recordings, allowed us to estimate a vehicle detection success rate up to 73%, while heavy vehicles were fully detected.

In this paper, we analyze the performance of a distributed acoustic sensor at two different interrogation wavelengths. We show theoretically that, in a coherent optical time-domain reflectometry (OTDR) operating at 850 nm, the dynamic signal-to-noise ratio (SNR) is enhanced, compared to an identical configuration operating at 1550 nm. Such enhancement is maximum at the interrogating pulse input section, while decreasing along the fiber in virtue of the higher loss. Experimental tests, carried out using two heterodyne C-OTDR detection schemes operating at the analyzed wavelengths, confirm the SNR improvement.

The application of a Brillouin distributed sensor for the monitoring of railway traffic is presented in this work. The field test is performed on the Italian regional line San Severo-Peschici, operated by Ferrovie del Gargano. A single-mode optical fiber sensor was attached along a rail length of about 60 m. The strain associated with train passage was acquired along the monitored rail length at 31 Hz acquisition rate and 1 m spatial resolution. The data acquired by the sensor demonstrates its capability of retrieving useful information in railway traffic monitoring, such as train identification, axle counting, speed detection, and dynamic load calculation.

In this work, we analyze the effects of Brillouin gain and Brillouin frequency drifts on the accuracy of the differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA). In particular, we demonstrate numerically that the differential gain is highly sensitive to variations in the Brillouin gain and/or Brillouin shift occurring during the acquisition process, especially when operating with a small pulse pair duration difference. We also propose and demonstrate experimentally a method to compensate for these drifts and consequently improve measurement accuracy.

An integrated optical waveguide refractometer, believed to be novel, is presented. The sensor is based on an antiresonant reflecting optical waveguide and uses the strong attenuation dependence on the refractive index of antiresonant cladding layers as the sensing principle. The theory and the operation of the sensor are discussed in terms of one- and two-dimensional geometry. The theoretical predictions and numerical analysis show that a versatile sensor can be realized. The design trade-offs are discussed, and the sensitivity and measurement range are presented.

We describe a novel apparatus for the real-time characterization of transverse-mode-mixing effects by monitoring the fluctuations of the M(2) factor defined by Siegman [Proc. Soc. Photo-Opt. Instrum. Eng. 1224, 2 (1990)]. A comparison between the results provided by our approach and those obtained by the use of a standard measurement apparatus has shown a satisfactory agreement within the experimental uncertainty.

A quantitative measurement of laser-beam quality can be performed by determination of the presence of multiple transverse modes of the laser oscillator and by calculation of their power content. Along this line of argument, we discuss a new approach that, starting from near-field and far-field intensity measurements, can evaluate the complex excitation coefficients of the transverse modes in a laser beam. The exploitation of near-field measurements sharply improves the performances of the technique in those cases in which only far-field measurements are used. The validity of the method is confirmed by several accurate numerical simulations and by some experimental results relative to a multimode Q-switched Nd:YAG laser.

We have developed a numerical code that, starting from far-field intensity measurements, is able to evaluate the excitation coefficients of the transverse modes in a laser system.Both the coherent and incoherent mode cases are addressed, and, while the incoherent case is shown to be equivalent to a linear problem, the coherent case is discussed through its equivalence to the phase-retrieval problem. Problems arising from both ill posedness and the nonlinearity are discussed in detail.The validity of our approach is confirmed by several numerical simulations and some experimental results on the characterization of a Q-switched Nd:YAG laser.