RESEARCH ARTICLE Determination of physical location of fiber Bragg gratings using traversal heat-induced phase shift Noor Shafikah Md Rodzi | Mohd Ridzuan Mokhtar | Zulfadzli Yusoff Research Institute for Digital Connectivity, Multimedia University, Cyberjaya, Selangor, Malaysia Correspondence Mohd Ridzuan Mokhtar, Research Institute for Digital Connectivity, Multimedia University, Cyberjaya, Selangor 63100, Malaysia. Email: ridz@mmu.edu.my Funding information Ministry of Higher Education, Malaysia, Grant/Award Number: [FRGS/2/2014/TK03/MMU/01/1] Abstract In certain application, the exact location of the grating structure of fiber Bragg grating (FBG) is crucial for prop- erly packaging, altering or exploiting the device. This article proposes a simple way to determine the location of FBG by applying heat induced phase shift. A travers- ing thin heating element is continuously scanned across the FBG while the local optical spectral profile is ana- lyzed. The experiments are performed on an apodised uniform FBG and a chirped FBG. The results show that the symmetry of the local spectral profiles and promi- nence of the phase-induced spectral dip can be used to determine the structure center and exact physical location of the apodised FBG. Meanwhile, the location of the lin- early chirped FBG can be determined by simply observ- ing the wavelength of the spectral dip. KEYWORDS apodised grating, chirped grating, fiber Bragg grating, phase shifts 1 | INTRODUCTION Fiber Bragg grating (FBG) is well known nowadays because of their numerous applications in fiber optic communications and sensing. This is attributed to the unique features offered by the FBG such as low insertion loss, low cost, compact size, and fiber-based. The most unique feature of the FBG is the flexibility for achieving desired spectral features, wherein various physical parameters in its structure can be manipulated, which comprises refractive index modulation, length of grating, and grating period. 1 The increasing number of fiber devices due to technol- ogy demand in market place has driven the modern grating writing technique, 2 which allows any spectral and phase pro- file to be designed for fiber optic communication and sens- ing applications. In some applications, the location of the physical structure of the FBG is required to be precisely known. This requirement arisen especially for the purpose of proper positioning in a specifically designed packaging, 3,4 ultra-high spatial resolution sensing, 5 or modification of structure. 6 Refractive index variation that constitutes the FBG structure is not physically visible, thus making it diffi- cult to determine its exact location. Typically, researcher and engineers merely rely on the marks crudely sketched on the optical fiber surface by the fabricators. Astutely, Maier et al. used side scattering technique based on cladding mode cou- pling to verify the location of grating. 7 However, it involves a tedious procedure and a longer time is required to process the necessary wide window of the expected spectrum to avoid spectral aliasing. Alternatively, a soldering iron was used by Raussel et al. 8 to produce heat for inducing local Bragg wavelength shift that is used to identify the index modulation structure of FBGs. In order to get fine spatial resolution, the position of hot finger is required to be adjusted close to the grating structure. However, it is diffi- cult to achieve good spatial resolution because of the rela- tively large heated region and poor heat confinement due to air thermal convection. This article demonstrates a simple and low-cost tech- nique to locate FBG by using traversal heat-induced phase shift. Analysis of the evolution of the local spectra yields correlation to the excursion of the phase shift. Consequently, mere observation of spectra can be directly translated into the physical location of the FBG structure. Received: 18 October 2018 DOI: 10.1002/mop.31719 Microw Opt Technol Lett. 2019;1–6. wileyonlinelibrary.com/journal/mop © 2019 Wiley Periodicals, Inc. 1