Abstract
Previously, we discussed an optical delay device consisting of a directional coupler of two different photonic crystal (PC) waveguides. It generates wideband and low dispersion slow light. However, it is easily degraded by a large reflection loss for a small imperfection of the coupling condition. In this paper, we propose and theoretically discuss a PC coupled waveguide, which allows more robust slow light with lower loss. For this device, unique photonic bands with a zero or negative group velocity at the inflection point can be designed by the structural tuning. Finite difference time domain simulation demonstrates the stopping and/or back and forth motion of an ultrashort optical pulse in the device combined with the chirped structure. For a signal bandwidth of 40 GHz, the average group index of the slow light will be 450, which gives a 1 ns delay for a device length of 670 microm. The theoretical total insertion loss at the device and input/output structures is as low as 0.11 dB.
Highlights
First-in-first-out (FIFO) optical buffer, which compress, stores and extracts optical signals on demand, is desired for sophisticate optical signal processing such as photonic routing
The phenomenon first studied for slow light was the electromagnetic induced transparency (EIT) [1]
We theoretically showed that such ideal band and light propagation are equivalently realized in a directional coupler consisting of two different chirped photonic crystal (PC) waveguides with opposite group velocity dispersion (GVD) [21]
Summary
First-in-first-out (FIFO) optical buffer, which compress, stores and extracts optical signals on demand, is desired for sophisticate optical signal processing such as photonic routing. We proposed the chirped PC waveguide, in which some structural parameters are gradually changed so that photonic bands are smoothly shifted [20] Due to the imperfect periodicity of the chirped structure, the light does not completely stop but soon moves forward with the positive GVD In this process, the GVD is perfectly compensated, if the photonic band is symmetric against the inflection point. The GVD is perfectly compensated, if the photonic band is symmetric against the inflection point Such light propagation is obtained in a desired bandwidth by designing the chirped structure. Schematic of ideal band shifted by chirping against the frequency of incident light (dashed line)
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