Discretely tunable optical packet delays using channelized slow light

We describe a procedure for increasing the fractional delay (or delay-bandwidth product) of a slow-light system. A broadband input signal is sliced into several frequency bands. The light in each band propagates through a separate channel which possesses a highly reduced group velocity over a narrow frequency band. The output of each channel is then combined to form a single output field. For certain discretely distributed values of the group velocity of each channel, the output can replicate the input waveform without the need to adjust the output phase of each channel. Because this scheme makes use of many parallel channels, it can overcome a fundamental limit [D. A. B. Miller, Phys. Rev. Lett. 99, 203903 (2007)] to the delay-bandwidth product for single-channel devices. A practical design is proposed using spectral slicers and stimulated Brillouin scattering as the narrow-band slow-light process. Numerical simulation shows that such a channelized slow-light element can have nearly uniform gain and group index over very large bandwidths.

Figure 1

(Color online) Refractive index as a function of the detuning from some center frequency ${\nu }_0$ for an ideal slow-light medium and for a channelized slow-light medium. Here $\delta n=n_g^{\prime }{\Delta }_c∕{\nu }_0$ is the maximum refractive index variation for both types of medium within the frequency interval ${\Delta }_c$.

Figure 2

(Color online) False-color representation of the amplitude of the output signal propagating through an ideal channelized delay device as a function of time and of the value of $n_g^{\prime }$. Here the input signal is a Gaussian pulse with a half width to the $1/e$ intensity value of $T_0$, and the channel spacing ${\Delta }_c$ is set equal to $1∕\left(8T_0\right)$. The time axis is set so that $t=0$ indicates no delay as in the case in which $n_g^{\prime }=0$.

Figure 3

(Color online) Schematic diagrams of (a) a $1\times 8$ spectral slicer using cascaded half-band filters and (b) a channelized SBS slow-light delay element.

Figure 4

(Color online) (a) Transmission of each of the seven physical signal channels of a channelized SBS delay element, (b) corresponding pump field spectrum for one particular signal channel (channel No. 4), and the SBS-induced (c) gain and (d) refractive index variation as functions of the detuning for channel No. 4 (solid line) and for a broadened gain profile (dashed line), respectively.

Figure 5

(Color online) Outputs of a $40\mathrm{Gbps}$, RZ data train “1010” after propagating through a channelized SBS delay element with ${\Delta }_c$ of $10\mathrm{GHz}$ (solid line), a single channel narrow-band SBS delay element (dash-dotted line), and a single channel broadband SBS delay element (dashed line), respectively. The dotted line is the reference output.

Figure 6

(Color online) Eye openings of three channelized devices as functions of the standard deviation phase noise level $\delta \varphi$ among different spectral channels. The insets show typical eye diagrams for (a) an ideal channelized slow-light device, (b) a channelized device with realistic spectral slicers and ideal slow-light media, and (c) a realistic channelized SBS slow-light device. The corresponding eye openings are given on top of each eye diagram.