Orbital angular momentum beam excitation through direct phase-matched coupling is experimentally demonstrated using an all-fiber weakly fused mode selective coupler consisting of a single-mode fiber and a ring-core fiber. Experimental results showing the excited OAM mode purity of up to 75% measured through the standard ring technique not only demonstrate the proof of concept but also provide a baseline for further improvement.

The fact that light carries spin angular momentum (SAM) of $\sigma \hslash$ ($\sigma =\pm 1$) was known as early as 1909, when Poynting demonstrated that a circularly polarized beam of light could cause a piece of birefringent material suspended on a thin wire to rotate about its center.¹ However, it was not until 1992 that Allen et al.² proved that an optical beam with azimuthal varying phase $\exp (il\varphi )$ carries orbital angular momentum (OAM) of $l\hslash$ per photon, with $l$ being the topological charge of the beam. The past three decades have seen tremendous growth of such optical beams from being a mere scientific curiosity to being a key enabler in a wide variety of cutting-edge applications, such as super-resolution imaging, optical trapping, and classical and quantum optical communications. In all these applications, the first step is the generation of an OAM mode. Experimentally, there are numerous methods to generate the desired OAM modes.^{3 – 5} One of the first reports on the generation of OAM mode was simply using two cylindrical lenses.⁶ Since then, there has been tremendous growth in this field, and the current state of the art includes several different design techniques such as binary holographic optical designs, spiral phase plate (SPP) designs, and planar metasurface designs. In general, OAM mode generation is achieved by modulation of one or more of the incident light beam’s characteristics through an optical element fabricated lithographically using electron beam, ion beam, and/or photolithography, or by using active devices, such as liquid crystal on silicon (LCoS)-based spatial light modulators (SLMs) and digital micromirror devices (DMDs).

The performance of post-processing techniques carried out on the Brillouin gain spectrum to estimate the Brillouin frequency shift (BFS) in standard Brillouin distributed sensors is evaluated. Curve fitting methods with standard functions such as polynomial and Lorentzian, as well as correlation techniques such as Lorentzian Cross-correlation and Cross Reference Plot Analysis (CRPA), are considered for the analysis. The fitting procedures and key parameters for each technique are optimized, and the performance in terms of BFS uncertainty, BFS offset error and processing time is compared by numerical simulations and through controlled experiments. Such a quantitative comparison is performed in varying conditions including signal-to-noise ratio (SNR), frequency measurement step, and BGS truncation. It is demonstrated that the Lorentzian cross-correlation technique results in the largest BFS offset error due to truncation, while exhibiting the smallest BFS uncertainty and the shortest processing time. A novel approach is proposed to compensate such a BFS offset error, which enables the Lorentzian cross-correlation technique to completely outperform other fitting methods.

We report the experimental demonstration of a reference-free Distributed Anti-Stokes Raman Thermometry (DART) scheme for real-time power line monitoring in overhead power transmission (OPGW/OPC) cables. Our work is based on the loop configuration in which only the anti-Stokes intensity is captured and processed to determine the temperature, thereby providing a self-calibrated solution that is tailor-made for a rugged field measurement. Temperature experienced by the optical fiber embedded in the power cable is estimated through a simple heat transfer model and is experimentally validated using a homebuilt DART system with a root mean square (RMS) temperature error of 0.33 °C.

In this paper, we propose the use of a Cross Recurrence (CR) Plot Analysis based technique applied on signals captured using fiber Bragg grating sensors to improve the accuracy of detection and localization of partial discharges. With the help of detailed simulations, the performance of the proposed technique in estimating the Time Difference of Arrival (TDOA) is evaluated. A novel technique of extracting the frequency from CR plots is also proposed and frequencies of single as well as multiple-tones signals are extracted successfully. Controlled experiments are conducted to verify the technique and the estimates of location are found to be within an error of less than a centimeter.