Mode-expanded 1.55-μm InP-InGaAsP Fabry-Perot lasers using ARROW waveguides for efficient fiber coupling

We report on a new concept for InGaAsP-InP 1.55-μm lasers with integrated spot-size converters based on antiresonant reflecting optical waveguides (ARROWs). The mode expanders consist of a tapered active region on top of a fiber-matched passive vertical ARROW waveguide. The large fundamental leaky mode with its low propagation loss makes ARROW waveguides useful for fiber coupling functions and avoids typical growth-related problems as encountered with traditional designs. The tapers exhibit a low transformation loss and narrowed far-field emission patterns (10.4°×22°) and reduce the coupling loss to standard single-mode fibers from 8 to 2.6 dB. We also present the design and the results obtained with a relaxed ARROW design with thinner ARROW layers to reduce the overall layer stack thickness considerably, without affecting the fiber-coupling performance. The antiresonant effect has also been used for the lateral confinement of the fiber-matched mode. This feature makes the presented spot-size transformer as simple to fabricate as a standard waveguide, only requiring a planar growth step and a single conventional etch process. The fabricated tapers exhibit a low transformation loss and minimum far-field divergence angles of 13.8°×30.8°, reducing the coupling loss to a standard single-mode fiber from 8 to 4 dB. We also analyze by simulation two variants of the concept proposed in this work, including a taper structure for a buried waveguide, which are expected to show better performance. Simulation results show fiber-coupling efficiencies as low as 2.4 and 1.1 dB and reduced far-field divergence angles as low as 7.2°×14° and 7.2°×9° for both variants.