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.

Optical Signal Processing Using Tunable Delay Elements Based on Slow Light

We investigate nonlinear carrier dynamics in a multiquantum-well semiconductor optical amplifier (SOA) in the context of ultrafast all-optical logic. A rate-equation model is presented that accounts for two-photon absorption, free-carrier absorption, self- and cross phase modulation, carrier heating, spectral, spatial hole burning, and self- and cross polarization modulation. The nonlinear refractive index dynamics is investigated theoretically and experimentally. We find nonlinear phase changes larger than /spl pi/ radians, which recovers on a timescale in the order of 1 ps. We also investigate a nonlinear AND gate that consists of an SOA that is placed in an asymmetric Mach-Zehnder interferometer. We show that the gate can be operated using 800-fJ optical pulses with duration of 200 fs while having a contrast ratio larger than 11 dB.

/pdf/all-optical-logic-based-on-ultrafast-gain-and-index-dynamics-3gajec9shw.pdf

All-optical logic based on ultrafast gain and index dynamics in a semiconductor optical amplifier

We demonstrate the operation of a novel all-optical flip-flop. The flip-flop consists of a slave Fabry-Perot laser diode (FP-LD) and a specially designed master FP-LD which has a built-in external cavity and operates in single longitudinal mode oscillation. The set and reset pulses were generated by external modulators at 1 Gbit/s. The rising and falling times of the output signal in on-off operation of the flip-flop were about 50 ps. The required powers of both set and reset pulses were less than -9 dBm.

All-optical flip-flop based on the bistability of injection locked Fabry-Perot laser diode.

Independent delay control and bit-level synchronization of multiple data channels are demonstrated through a single stimulated Brillouin scattering (SBS)-based slow-light element. Multiple, independently tunable SBS gain resonances are generated in a single piece of highly nonlinear fiber from multiple SBS pumps. Each pump is broadened by noise modulating the laser current so that the bandwidth is compatible with high-speed gigabit-per-second data signals. Bit-level synchronization on three 2.5-Gb/s nonreturn-to-zero on-off keying channels shows independent and continuous delay control of up to 112ps. Bit-error-rate measurement on the synchronized channels shows 3.5-dB power penalty. Effect of one channel on the other channel's delay is also investigated and the reason is attributed to the nonlinear crosstalk.

A Single Slow-Light Element for Independent Delay Control and Synchronization on Multiple Gb/s Data Channels

We investigate ultrafast refractive index dynamics in a multiquantum-well InGaAsP-InGaAs semiconductor optical amplifier that is operated in the gain regime by using a pump-and-probe approach. The pump-and-probe pulses are cross-linearly polarized. We observe a phase shift of 200/spl deg/ if the amplifier is pumped with 120 mA of current, but find that the phase shift vanishes if the injection current is increased to 160 mA. Our results indicate a contribution of two-photon absorption to the nonlinear phase shift that opposes the phase shift introduced by the gain. Finally, we observe that the phase shift comes up and disappears within a picosecond.

/pdf/ultrafast-refractive-index-dynamics-in-a-multiquantum-well-4jkr09rsnv.pdf

Ultrafast refractive-index dynamics in a multiquantum-well semiconductor optical amplifier

Error-free all-optical packet switching is demonstrated by use of a multiwavelength mutual injection-locked Fabry-Perot laser diode. A 10-Gbit/s data signal is switched on and off with an extinction ratio of 16.9 dB when an optical control signal is turned off and on with a power difference of only 3 dB.

/pdf/demonstration-of-an-all-optical-switch-by-use-of-a-1nvsofdfo2.pdf

Demonstration of an all-optical switch by use of a multiwavelength mutual injection-locked laser diode.

Simultaneous all-optical header processing and control- packet switching are demonstrated by using multiwavelength injection- locking in a single Fabry-Perot laser diode. A special two-level control packet is used to process the header of the data packet at 5 Gb/s. The entire control packet is all optically switched ON or OFF. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 342-345, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20629

https://eie.polyu.edu.hk/~wai/journal/motl_05.pdf

All‐optical header processing using an injection‐locked Fabry‐Pérot laser diode

We have demonstrated all-optical packet add–drop for all-optical packet-switched networks. Intelligent all-optical add–drop of packets is performed, based on all-optical processing of packet headers. The header and payload rates are 5 and 10? Gbits?s, respectively.

/pdf/all-optical-add-drop-node-for-optical-packet-switched-55zqpggrbs.pdf

All-optical add-drop node for optical packet-switched networks

https://eie.polyu.edu.hk/~wai/journal/oc_04a.pdf

All-optical header processing using control signals generated by direct modulation of a DFB laser

We demonstrated a 1xN all-optical packet switch with all-optical header processing and packet switching at 10 Gb/s using multi-wavelength mutual injection-locking in a single Fabry-Perot laser diode. The packet header is proceed at 10 Gb/s.

L. Y. Chan

Papers

Demonstration of an all-optical switch by use of a multiwavelength mutual injection-locked laser diode.

All‐optical header processing using an injection‐locked Fabry‐Pérot laser diode

All-optical add-drop node for optical packet-switched networks

All-optical header processing using control signals generated by direct modulation of a DFB laser

1×N All-optical packet switch at 10 Gb/s