Contents I. Introduction
The explosive increase in network traffic has led to a need for high-speed signal processing. Recently, a photonic network based on optical signal processing has attracted much attention. The key components of optical signal processing are optical buffers and compact, high-speed memories with low power consumption. A bistable laser is an attractive candidate for an optical memory because of its compactness, low switching power, and stable switching without holding power. In previous studies, various types of bistable lasers have been demonstrated. A bistable laser using two polarized lights from vertical-cavity surface emitting lasers (VCSEL) was applied to all-optical 4-bit buffer memories with a shift register function [1], [2]. To reduce the number of optical components, planar semiconductor optical devices would be advantageous for large-scale integrated optical memories. Waveguide-based bistable lasers with saturable absorbers using a multimode-interferometer (MMI) [3], and a master-slave type coupled micro-cavity laser [4] have been demonstrated. In this work, we propose the optical flip-flop operation of a distributed Bragg reflector (DBR) laser using sidemode injection locking [5]. Laser injection locking has been demonstrated in a Fabry–Pérot laser and a distributed feedback (DFB) laser [6], [7]. However, flip-flop operation has not been demonstrated since the lasing spectrum returns to its initial state once the input light is removed. We focused on the mode suppression induced by spectral hole burning between two longitudinal modes to realize stable bistable operation. Since the optical memory can be controlled by the operating wavelength, access to each memory in a photonic integrated circuit becomes easy by combining this approach with the wavelength routing techniques developed for wavelength division multiplexing (WDM) systems. Dynamic flip-flop operation is demonstrated both theoretically and experimentally.
