Modulation Bandwidth Enhancement for Dual DFB Lasers Coupled with a High-Q Microring Resonator

A form of mutually injection-locked lasers by butt coupling two distributed feedback (DFB) lasers with a high-Q microring resonator (MRR) is proposed and investigated. With the help of the MRR, two DFB lasers can achieve stable locking and carrier suppression output as the optical carriers and modulation sidebands are separated. The carriers of the lasers can inject to each other with a frequency near the resonate frequency of the MRR efficiently, while the modulation sidebands are passed through the MRR without interference. We establish a theoretical model based on coupled-mode rate equations to investigate the modulation characteristics. At appropriate injection strength, the 3-dB modulation bandwidth of the laser module can reach up to 24.4 GHz, which exhibits a more than two-fold enhancement compared with 9.1 GHz of the laser under the free-running condition. In addition, the influence of laser parameters such as frequency detuning and bias current on the modulation bandwidth is also studied.

I. Introduction

Direct modulation semiconductor lasers, with their unique superiority in size, weight, and power efficiency (SWaP), have been widely used in radio-over-fiber (RoF) [1], short-haul data communication [2], and many other fields. Currently, two main kinds of technical schemes are categorized to improve the modulation bandwidth of semiconductor lasers. One is to increase the carrier-photon resonance (CPR) frequency by shortening cavity length or redesigning active layer structure, but these methods undoubtedly increase the manufacturing difficulty and cost [3]. The other approach is to introduce the photon-photon resonance (PPR) effect [4], [5]. The PPR effect can be activated by unidirectional external optical injection locking, self-injection locking, and mutual-injection locking. For unidirectional external optical injection, master-slave structures are generally adopted. However, the discrete devices such as variable optical attenuators and isolators not only have a large volume, but also make the whole system very sensitive to environmental disturbances. For self-injection locking, due to the use of passive external cavities to realize optical feedback, the cost can be significantly reduced. In our previous work, a butt-coupled microring reflector was used as the optical feedback cavity. The hybrid laser could achieve nearly two folds of direct modulation bandwidth enhancement and two orders of magnitude of linewidth reduction [6].