Nonlinear photoluminescence imaging applied to femtosecond laser manufacturing of fiber Bragg gratings.

Cyril Hnatovsky,Dan Grobnic,Stephen J Mihailov

doi:10.1364/oe.25.014247

Abstract

Nonlinear photoluminescence imaging is used to visualize the intensity distribution of femtosecond laser pulses inside the optical fiber during Bragg grating inscription based on side illumination through a phase mask. This technique, which results in direct imaging of the inscription laser field inside the optical fiber, facilitates i) the characterization of the laser focus in the vicinity of the fiber core and ii) the optimization of the fiber alignment with respect to the laser focus while using pulses with energies several times lower than those used during the actual inscription process. The applicability of this imaging technique is demonstrated for Bragg grating inscription in different optical fibers, including direct inscription through the fiber coating.

Highlights

Fiber Bragg gratings (FBGs) are typically made by inducing a periodic or aperiodic modulation of refractive index in the fiber core with intense laser radiation
Nonlinear photoluminescence imaging is used to visualize the intensity distribution of femtosecond laser pulses inside the optical fiber during Bragg grating inscription based on side illumination through a phase mask
This technique, which results in direct imaging of the inscription laser field inside the optical fiber, facilitates i) the characterization of the laser focus in the vicinity of the fiber core and ii) the optimization of the fiber alignment with respect to the laser focus while using pulses with energies several times lower than those used during the actual inscription process

Summary

Introduction

Fiber Bragg gratings (FBGs) are typically made by inducing a periodic or aperiodic modulation of refractive index in the fiber core with intense laser radiation. Nonresonant multiphoton absorption of laser light inside transparent media can be achieved with focused ultrashort pulses [3]. Ultrafast lasers can be used for direct FBG inscription in almost any optical fiber, as compared with UV lasers where some photosensitization of the fiber core is required to make the UV light absorption possible [5]. The use of a phase mask in order to produce an interference pattern in the fiber has its own advantages [2]. This approach is more robust and easier to introduce than the majority of interference schemes with long optical paths [8, 9]

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