Reservoir computing using integrated semiconductor lasers with delayed feedback

The concept of delay-based reservoir computing, using only a single nonlinear node with delayed feedback instead of a very big random network, was introduced some years ago as a means of limiting hardware complexity in photonic systems. In essence, the idea of delay line reservoir computing constitutes an exchange between space and time: what is normally done spatially with many nonlinear nodes, is now done in a single node that is multiplexed in time. A virtual network configuration is created through the inertia of the system. After the first mixed analog/digital implementations in electronics, high processing speeds have been demonstrated based on the transient response to optical data injection in nonlinear optical systems such as semiconductor lasers. While previous efforts have focused on signal bandwidths limited by the semiconductor laser’s relaxation oscillation frequency, we have demonstrated numerically that the much faster optical phase response makes significantly higher processing speeds attainable. Using complex nonlinear interactions between optical field and material, we demonstrate that it is possible to reduce the delay time to within lengths that can be integrated on chip. Relying on these unique nonlinear optics properties, we show that multiple optical modes can be used to either process in parallel several independent computational tasks or use them in conjunction to work on the same problem. We will illustrate our approach on a single-longitudinal mode semiconductor ring laser with optical feedback and on a semiconductor laser with multiple longitudinal modes. Finally, we have further developed the structure of delay-based reservoir computing to accommodate a cascade of several reservoir computers. In electronics, we have already demonstrated that such a cascade can perform several complex audio-processing tasks without the need for any audio-related pre-processing. With integrated delay-based reservoir computers, such a scheme could be achieved in optics.