Mach-Zehnder Interferometers Based on Long Period Fiber Grating Coated With Titanium Dioxide for Refractive Index Sensing

• Published in: Journal of lightwave technology 2019-09, Vol.37 (18), p.4584-4589
• Main Authors: Vasconcelos, Helena Catarina Araujo Soares Guedes, Almeida, Jose Manuel Marques Martins de, Saraiva, Cristina Maria Teixeira, Jorge, Pedro Alberto da Silva, Coelho, Luis Carlos Costa
• Format: Article
• Language: English
• Subjects: Coating; Fourier transforms; Gratings (spectra); Long period fiber grating (LPFG); Mach-Zehnder interferometers; Optical device fabrication; optical fiber sensors; Optical refraction; Optical sensors; Refractivity; Sensitivity; Spectra; Spectral resolution; Spectral sensitivity; Temperature sensors; Titanium; Titanium dioxide; titanium dioxide (TiO<named-content xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="LaTeX"> _{2}</tex-math> </inline-formula> </named-content>)
• ISSN: 0733-8724; 1558-2213
• DOI: 10.1109/JLT.2019.2912829

The wavelength sensitivity and spectral resolution of Mach-Zehnder fiber interferometers obtained through a combination of two identical uncoated and titanium dioxide (TiO 2 ) coated long period fiber gratings (LPFGs) is presented and compared with single LPFGs-based refractometric sensors. A set of LPFGs were fabricated in single mode fiber with the resonance band having an amplitude of 3 dB in order to split in half the optical power between the core and the specific cladding modes. The separation between the pair of LPFG written in the fiber was varied between 1 and 3 cm and the thickness of the TiO 2 coating around the fiber ranged from 20 to 40 nm. A wavelength shift sensitivity of 216 nm/refractive index units (RIU) was achieved for the device with 3 cm and a 30-nm thick TiO 2 coating, which presented a spectral resolution of 1.1 × 10 −4  RIU. Despite the lower wavelength shift sensitivity of 142 nm/RIU, attained for a 2-cm long device and 30-nm thick TiO 2 coating, a spectral resolution of 1.8 × 10 −5  RIU was measured, which is one order of magnitude lower than a single LPFG.