Contents I. Introduction
Variable delay is required for the synchronization of optical signals and for signal buffering in future all-optical communication systems. (a) Schematic diagram illustrating the variable optical delay lines device working principle. (b) Device layout (2×6 cm<sup>2</sup>), and the 2×2 switch layout (drawing not to scale). For any of these applications, it is advantageous to develop a programmable optical delay system, where true time delay can be varied continuously over a large range of delay [1]. Prior approaches use a combination of 2×2 switches and fixed delay paths varying in length by a factor of two. The minimum path length defines the delay resolution of such a device. Large delays can be also achieved in optical fibers; however, they are bulky and must ultimately be combined with switches. The approach proposed in this letter is potentially realized by integrating coarse delay steps with a device that can be used to tune continuously between steps. One approach to implement variable delays in waveguides is using all-pass filters (APFs) [2], [3]. Ring resonator optical APFs have been proposed for tunable delay lines [5]. By cascading a number of thermally tuned stages, a flat group delay can be achieved. Since in this approach there is an inherent group delay ripple, a tradeoff between bandwidth and the group delay ripple has to be made. This approach allows for high-resolution control of the delay, limited only by the group delay ripple. In most optical transmission platforms, a resolution of less than 1 bit can be easily implemented. Integrating these devices in high index contrast waveguides has the advantage of attaining large delay times in a single compact chip in a low cost platform.
