Femtosecond Laser Inscription of Photonic and Optofluidic Devices in Fiber Cladding

Abstract

Femtosecond laser internal microstructuring has emerged as a powerful tool for inscribing devices inside transparent materials. This chapter addresses the challenge of laser processing inside cylindrically shaped optical fibers to provide technical solutions for embedding highly functional photonic devices that efficiently interconnect with the fiber core waveguide. Chemical etching of laser modification tracks is further introduced to open microfluidic and other structures that, together with photonic devices, define a promising all-fiber platform of photonic, optofluidic, and microelectromechanical systems of broad interest to telecommunication, sensing, and biomedical applications. Aberration-free focusing of the femtosecond laser light with high numerical aperture oil-immersion lenses was developed for distortion-free writing of three-dimensional optical and optofluidic devices in arbitrary positions anywhere within the core and cladding of single-mode and coreless optical fibers. Various approaches for efficient coupling of light from core to cladding-formed waveguides are presented, building into demonstrations of more functional photonic devices including Mach-Zehnder interferometers and shape-temperature sensors based on distributed Bragg-grating waveguide circuits. Waveguide birefringence is exploited to define in-fiber polarization splitters and polarization-selective taps while laser trimming of waveguides with femtosecond laser-stressing tracks is shown to offer strong birefringence tuning up to 2 × 10−3 on which submillimeter length wave retarders were embedded in fiber. Femtosecond laser irradiation with chemical etching was further harnessed to form three-dimensional microfluidic networks, reservoirs, and micro-optical resonators with optically smooth sidewall roughness that were combined with cladding waveguides to demonstrate an in-fiber fluorescence detector and optofluidic Fabry-Perot refractive index sensor. The techniques presented in this chapter enable new directions for fabricating highly functional photonic microsystems and lab-in-fiber devices for complex laboratory-level diagnostics in a compact and flexible optical fiber platform.