Integrated optical circuits in fiber cladding by tightly focused femtosecond laser writing

Abstract

Femtosecond laser direct writing in glass materials represents a simple single-step approach to generate threedimensional (3D) optical circuits that cannot be constructed with traditional fabrication techniques. In this paper, we present an attractive extension of such femtosecond laser processing to the writing of optical circuits directly inside the cladding of single-mode optical fiber. To enable the formation of strongly guided and undistorted waveguide modes within the small cylindrical fused silica volume (125 μm diameter), frequency-doubled (λ = 522 nm) Ytterbium fiber-amplified femtosecond laser light at high repetition rate (500 kHz) was tightly focused with a high 1.25 numerical aperture (NA) oil immersion lens. In this way, low-loss waveguides could be arbitrarily located in various cladding positions without generating ablation damage. Basic components such as directional couplers were demonstrated that present a new means for dense integration of optical elements that couple with the nearby fiber core. Such 3D all-fiber optical circuits represent practical tools to bypass tedious assembly and packaging steps such as fiber pigtailing with planar lightwave components. This formation of optical circuits directly within the cladding of optical fiber opens new prospects for manufacturing compact and functional optical and optofluidic microsystems for Telecom, sensing and lab-on-a fiber applications.