Investigation of a Bragg Grating-Based Fabry–Perot Structure Inscribed Using Femtosecond Laser Micromachining in an Adiabatic Fiber Taper

• Published in: Applied Sciences
• Main Authors: Aayush Madan, Stephanie Hui Kit Yap, Varghese Paulose, Wonkeun Chang, Perry Ping Shum, Jianzhong Hao
• Format: Article
• Language: English
• Published: MDPI AG 2020-02-01
• Subjects: fabry–perot; microfiber sensor structure; femtosecond laser micromachining; direct writing; point-by-point fabrication; harsh environment fiber sensor; sensor for structural health monitoring
• Online Access: https://www.mdpi.com/2076-3417/10/3/1069
• ISSN: 2076-3417

This paper presents the fabrication of a fiber Bragg grating (FBG)-based Fabry&#8722;Perot (FP) structure (7 mm total length) in an adiabatic fiber taper, investigates its strain and temperature characteristics, and compares the sensing characteristics with a standard polyimide coated FBG sensor. Firstly, a simulation of the said structure is presented, followed by the fabrication of an adiabatic fiber taper having the outer diameter reduced to 70 <inline-formula> <math display="inline"> <semantics> <mi mathvariant="sans-serif">&#956;</mi> </semantics> </math> </inline-formula>m (core diameter to 4.7 <inline-formula> <math display="inline"> <semantics> <mi mathvariant="sans-serif">&#956;</mi> </semantics> </math> </inline-formula>m). Next, the sensing structure, composed of two identical uniform FBG spaced apart by a small gap, is directly inscribed point-by-point using infrared femtosecond laser (fs-laser) micromachining. Lastly, the strain and temperature behavior for a range up to 3400 <inline-formula> <math display="inline"> <semantics> <mrow> <mi mathvariant="sans-serif">&#956;</mi> <mi mathvariant="sans-serif">&#949;</mi> </mrow> </semantics> </math> </inline-formula> and 225 <inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mo>&#176;</mo> </msup> </semantics> </math> </inline-formula>C, respectively, are investigated for the fabricated sensor and the FBG, and compared. The fabricated sensor attains a higher strain sensitivity (2.32 pm/<inline-formula> <math display="inline"> <semantics> <mrow> <mi mathvariant="sans-serif">&#956;</mi> <mi mathvariant="sans-serif">&#949;</mi> </mrow> </semantics> </math> </inline-formula>) than the FBG (0.73 pm/<inline-formula> <math display="inline"> <semantics> <mrow> <mi mathvariant="sans-serif">&#956;</mi> <mi mathvariant="sans-serif">&#949;</mi> </mrow> </semantics> </math> </inline-formula>), while both the sensors experience similar sensitivity to temperature (8.85 pm/<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mo>&#176;</mo> </msup> </semantics> </math> </inline-formula>C). The potential applications of such sensors include continuous health monitoring where precise strain detection is required.