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CRC Press eBooks

Reconfigurable Signal Processing Using Slow-Light-Based Tunable Optical Delay Lines

Jacob B. Khurgin, R.S. Tucker

Published Oct 8, 2018• 0 citations
Publisher

Abstract

As a potential enabling technology, slow light has captured much research interest over the past few years for achieving a continuously tunable optical delay line [1]. In principle, slow light is generated by tailoring an enhanced group-index resonance within a given nonlinear medium. This effective refractive index change is experienced by the data stream passing through and thus induces a controllable group delay onto the signal. Themain feature of a slow-light-based optical delay line is the capability for fine-grain temporal manipulation of optical pulses. This fine tunable delay element is envisioned to be useful for various high-bandwidth signal processing functions and applications.

DOI
10.1201/9781315219059-26

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As a potential enabling technology, slow light has captured much research interest over the past few years for achieving a continuously tunable optical delay line [1]. In principle, slow light is generated by tailoring an enhanced group-index resonance within a given nonlinear medium. This effective refractive index change is experienced by the data stream passing through and thus induces a controllable group delay onto the signal. Themain feature of a slow-light-based optical delay line is the capability for fine-grain temporal manipulation of optical pulses. This fine tunable delay element is envisioned to be useful for various high-bandwidth signal processing functions and applications.
Optical tunable delay lines have many applications for high-performance optical switching and signal processing. Slowlight has emerged as an enabling technology for achieving continuously tunable optical delays. The reconfigurable delay opens up a whole new field of nonlinear signal processing using slow light. In this paper, we review recent advances in slow-light-based optical signal processing, with a focus on the data fidelity after traversing the slow light elements. The concept of slow-light-induced data-pattern dependence is introduced and is shown to be the main signal degrading effect. We then propose and experimentally demonstrate phase-preserving slow light by delaying 10-Gb/s differential-phaseshift- keying (DPSK) signals with reduced DPSK data pattern dependence. Spectrally-efficient slow light using advanced multi-level phase-modulated format is further described. With this technique, doubled or even tripled bit-rate signals can be transmitted through a bandwidth limited slow light element. We finally show several novel slow-light-based signal processing modules. Unique features such as multi-channel operation and variable bit-rate capability are highlighted.
Tunable optical delay lines have many applications for high-performance optical switching and signal processing. Slow light has emerged as an enabling technology for achieving continuously tunable optical delays. Delay reconfigurability opens up a whole new field of nonlinear signal processing using slow light. In this paper, the authors review recent advances in slow-light-based optical signal processing, with a focus on the data fidelity after traversing the slow light elements. The concept of slow-light-induced data pattern dependence is introduced and is shown to be the main signal degrading effect. We then propose and experimentally demonstrate phase-preserving slow light by delaying 10 Gb/s differential phase-shift keying (DPSK) signals with reduced DPSK pattern dependence. Spectrally efficient slow light using advanced multilevel phase-modulated formats is further described. With this technique, doubled bit-rate signals can be transmitted through a bandwidth-limited slow light element. We finally show several novel slow-light-based signal processing modules. Unique features such as multichannel operation, variable bit-rate capability, and simultaneous multiple functions are highlighted.
An active involvement of optics in information processing depends on the existence of dynamically controllable all-optical buffers. There has been a steady interest in the development of such devices and in the investigation of the fundamental principles that could lead to their development. Unlike usual optical delay lines that rely upon the phase delay of the signal light, the new generation of the controllable optical buffers is based on group delays. Phase delay τph = Ln0/c depends on the refractive index of thematerial n0, length of the delay line L, and speed of light in the vacuum c. Group delay τg = d(ωLn0(ω)/c)/dω is built upon the dispersion of the material, i.e., upon the frequency (ω) dependence of the refractive index n0(ω).
Light propagation at slow speed, delaying light, has a tremendous role in a wide range of advanced technology from peta-bit optical networks to even laser fusion technology [1,2]. Recently, it was realized that delaying of an optical signal is useful for a number of applications such as optical buffering, signal processing, optical sensing and enhancement of optical nonlinearity in materials [3]. Also, as a consequence of important role of fiber lasers in laser fusion technology, coherent beam combining of several fiber lasers for achieving high power output is a crucial problem in future fusion technology. This task can be overcome by using proper engineered photonic components for achieving specific group delay time [2].
All resonant systems throughout science and engineering, independent of their physical implementation have a bandwidth that is inversely related to the decay time. A similar relation exists in all slow-light systems, where the group index (and therefore the delay for a given footprint) is inversely related to the bandwidth. Therefore, resonant or slow-light systems can either store a broad signal for a short time, or a narrow signal for a long time, but cannot achieve large delay for broad bandwidth signals.Here we discuss our recent work on non-reciprocal optical systems that are not constrained by the delay-bandwidth limit. We show that large, broadband optical delay is not a pipe dream and is achievable with current optical technology. We discuss the underlying physics of delay and bandwidth in non-reciprocal optical systems and present an experimental implementation, based on a figure-9 cavity. We demonstrate a delay-bandwidth product 30 times above the seemingly fundamental time-bandwidth limit of traditional systems. Furthermore, we show that the optical pulse can be released after an arbitrary number of round trips, providing the control and tunability lacking from conventional spiral waveguide or fibre loop delay lines.
The possibility of dynamically controlling the speed of light can be extremely useful in all the optical applications where a variable delay is needed, such as bit and frame synchronization and optical storage. To this aim, coupled resonator slow-wave structures (SWSs) represent a promising approach to slow down the propagation of optical signals with as large bandwidths as required by optical communication systems. We show that by suitably setting the rings' resonances, SWSs can act efficiently as tunable delay lines, with the overall time delay being continuously adjustable from zero to a maximum value, typically limited by the effects of loss and dispersion. Every ring of the structure can be used as an optical gate and optical pulses can also be trapped and released by the SWS, so as to realize an optical memory or buffer. A time delay of 300 ps and a slowing ratio of 7.5 over a 3.3 GHz bandwidth are experimentally demonstrated in SWSs realized in silicon oxynitride technology