In-phase/quadrature modulation using directly reflectivity-modulated laser

Abstract

In the last decade, coherent optical transmission technology has dominated the provision of high-capacity communications in metro and long-haul networks and is expected to expand to short-reach networks such as data center and passive optical networks. Capacities of more than 1.5 Tb/s have been demonstrated for a single wavelength. In-phase/quadrature (IQ) modulation allows information to be encoded in both the phase and amplitude of light (or in-phase and quadrature components) and is typically achieved by nested multiple Mach–Zehnder modulators (MZMs). MZMs have advantages including low chirp, broadband operation, and easy to achieve high-order quadrature amplitude modulation (QAM), but they are typically large and have high RF power consumption and excess losses. A small IQ transmitter with low RF power consumption is therefore in demand for low-cost and highly integrated coherent modules. In this paper, we experimentally demonstrate a directly reflectivity-modulated laser that can potentially achieve this goal. We demonstrate the device on a hybrid silicon/III-V platform, where silicon photonics offers compact components for filters, modulators, and reflectors, and the III-V material provides gain. In principle, the device could inherit the benefits of conventional directly modulated lasers while overcoming their speed limits. We demonstrate 32/50 Gbaud quadrature phase-shifted keying with low bit error ratios and experimentally prove the feasibility of 30 Gbaud 16-QAM.