Manufacturing and Characterization of Femtosecond Laser-Inscribed Bragg Grating in Polymer Waveguide Operation in an IR-A Wavelength Range

Jan Meyer,Gion Joel Strobel,Leonhard Ganzer,Christian Kelb,Antonio Nedjalkov,Wolfgang Schade

doi:10.3390/s20010249

Abstract

Optical sensors, such as fiber Bragg gratings, offer advantages compared to other sensors in many technological fields due to their outstanding characteristics. This sensor technology is currently transferred to polymer waveguides that provide the potential for cost-effective, easy, and flexible manufacturing of planar structures. While sensor production itself, in the majority of cases, is performed by means of phase mask technique, which is limited in terms of its degrees of freedom, other inscription techniques enable the manufacture of more adaptable sensor elements for a wider range of applications. In this article, we demonstrate the point-by-point femtosecond laser direct inscription method for the processing of polymer Bragg gratings into waveguides of the epoxy-based negative photoresist material EpoCore for a wavelength range around 850 nm. By characterizing the obtained grating back-reflection of the produced sensing element, we determined the sensitivity for the state variables temperature, humidity, and strain to be 45 pm/K, 19 pm/%, and 0.26 pm/µε, respectively. Individual and more complex grating structures can be developed from this information, thus opening new fields of utilization.

Highlights

Since the first discovery of fiber photosensitivity in the late seventies, fiber Bragg gratings (FBG)have been extensively studied and their outstanding performance for many different purposes are of high interest to this day [1]
We present, to the best of our knowledge, the first Bragg grating at an operating wavelength of around 850 nm, inscribed in a planar EpoCore polymer waveguide with near infrared point-by-point Ti/sapphire femtosecond laser technique
Since automatized visual structure recognition is applied for the inscription of the Bragg grating, an exposed waveguide is formed in the initial processing step

Summary

Introduction

Since the first discovery of fiber photosensitivity in the late seventies, fiber Bragg gratings (FBG)have been extensively studied and their outstanding performance for many different purposes are of high interest to this day [1]. These periodic refractive index modulations within a waveguide core are used as narrowband reflectors (e.g., for fiber lasers or laser wavelength stabilization), highly reflective mirrors (e.g., pump reflector in fiber amplifiers), or gratings (e.g., dispersion compensation for long-haul transmission) [1,2] Besides these implementations, FBG are regarded as excellent sensor elements because their measurand information is wavelength-encoded, and intrinsically immune to disturbances, such as power loss, and enable the measurement of the physical variables temperature and strain [1,3,4]. FBG offer many advantages, as they are lightweight, flexible, and show high temperature tolerance and immunity to electromagnetic disturbance.

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