Passive broadband Faraday isolator for hybrid integration to photonic circuits without lens and external magnet

Abstract

Optical isolation based on a non-reciprocal effect is crucial for proper operation of several high-performance photonic devices such as in telecommunications, light detection and ranging, and even quantum platforms. The magneto-optical Faraday rotation is the most commonly used non-reciprocal effect as it offers unique advantages, including broadband operation, wide input optical power range, low insertion losses and high optical isolation, but it is currently not conducive to miniaturization. Two major impediments hinder the direct integration of Faraday isolators into photonic chips: the need for bulky external magnets and the challenging fabrication of low-loss waveguides that would eliminate the need for free-space coupling optics. Here we have addressed both challenges using a new femtosecond laser writing technique to create waveguides within the bulk of latched bismuth-doped iron garnet slabs without altering its magneto-optic functionality. As a result, we have achieved a Faraday rotator waveguide exhibiting <0.15 dB insertion loss with a figure of merit of 346° dB⁻¹. By interposing this Faraday rotator between two 30-μm-thick polarizers, we further demonstrate a miniaturized optical isolator waveguide with >25 dB isolation ratio and <1.5 dB insertion loss over the entire optical telecom C-band for hybrid integration to photonic circuits without lenses and external magnet.