Contents lists available at ScienceDirect Optik journal homepage: www.elsevier.com/locate/ijleo Original research article Strain resolution enhancement in Rayleigh-OTDR based DSS system using LWT-MPSO scheme Ramji Tangudu ⁎ , Prasant Kumar Sahu School of Electrical Sciences, Indian Institute of Technology Bhubaneswar, Argul, Odisha, 752050, India ARTICLE INFO Keywords: Strain resolution Optical time domain reflectometry (OTDR) Lifting wavelet transform (LWT) Modified particle swarm optimization (MPSO) Fourier wavelet regularized deconvolution (ForWaRD) ABSTRACT This paper presents a scheme for performance improvement of a Rayleigh-optical time domain reflectometry (OTDR) based distributed strain sensing (DSS) system using the combined effect of lifting wavelet transform (LWT) and modified particle swarm optimization (MPSO) method. In our proposed scheme, MPSO evolutionary algorithm produces an optimum threshold and helps in minimizing the noisy lifting wavelet coefficients effectively. This allows an order-of-magnitude increase in strain resolution of the proposed system. The proposed sensing system is capable of measurement up to 50 km using a 10 mW of input laser source power. We have applied 30 με strain at 30 km position of the fiber under test. The proposed signal processing scheme offers better performance over a conventional system. The system achieved a strain resolution of ∼ 1.8 με . Relevant numerical simulations are presented using MATLAB 15.0. 1. Introduction Now a days, structural health monitoring (SHM) has been attracting both research and development as it aims to give on a real time/pseudo real time manner, a diagnosis of the “state” of the constituent materials, and of the different parts of the structure under test. Distributed fiber optic sensing (DFOS) system is highly promising for sensing defects at an early stage in civil SHM applications [1]. This system utilizes the fiber as transmission medium and sensing medium. DFOS system has major benefits such as, flexibility, reliability, ease of integration, electromagnetic interference (EMI) immunity, less complexity, resistance to corrosion, small in size and long operating life time under harsh environmental situations. Optical time domain reflectometry (OTDR) technique is the most prominent technique in DFOS system for SHM operation [1,2]. An OTDR directs a laser signal into the fiber under test (FUT), and the Rayleigh backscattered signal’s intensity can be obtained for the full length of the FUT. From this intensity, we can estimate the fiber losses due to attenuation, connectors, splices, bending and breaks along the fiber and also we can find the amount of strain that is applied on the FUT [3]. The backscattered signal is extracted from the same end of the fiber through which light signal has been launched. The received backscattered signal is directed to a highly sensitive photodetector [4]. X. Bao et al., [5] proposed a distributed sensor system using polarization maintained fiber (PMF) and photonic crystal fiber (PCF) for simultaneous temperature and strain measurement. Though these fibers have very high strain and temperature sensitivity (strain accuracy of 10–30 με and temperature accuracy of 1–2 ℃), but offers meter range sensing only. In [6], A.W. Brown et al., presented a distributed strain sensing (DSS) system using dark pulse based Brillouin optical time domain analysis (DP-BOTDA) technique. Using this technique, the authors demonstrated a spatial resolution of 50 mm with a strain measurement accuracy of 6 με on a 100 m sensing fiber. This technique is based on double ended access concept. The presented system suffers from nonlinear effects and design https://doi.org/10.1016/j.ijleo.2018.09.060 Received 11 June 2018; Received in revised form 6 September 2018; Accepted 14 September 2018 ⁎ Corresponding author. E-mail address: rt10@iitbbs.ac.in (R. Tangudu). Optik - International Journal for Light and Electron Optics 176 (2019) 102–113 0030-4026/ © 2018 Elsevier GmbH. All rights reserved. T