10.1117/2.1200712.0987 Silicon-based nonlinear devices for high-speed optical communications Haisheng Rong, Simon Ayotte, Shengbo Xu, Walid Mathlouthi, and Mario Paniccia High optical nonlinearity and strong light confinement in silicon waveguides allow chip-scale spectral inverters to compensate for chro- matic dispersion in optical fibers. In optical fiber communications, dispersion compensation is essential for transmitting data at high speeds over long dis- tances. Pulses representing digital data spread as they prop- agate through optical fibers because of chromatic dispersion: that is, different spectral components (colors) travel at differ- ent speeds. As the distance increases, the pulse width broadens, and eventually the individual pulses, or data bits, cannot be dis- tinguished. This effect becomes more severe as the transmission speed increases. For example, a 10Gb/s signal can be sent over 80km in standard optical fibers without dispersion compensa- tion, but at a data rate of 40Gb/s, this distance decreases to less than 5km. Various techniques to counter this problem have been developed. One of the most successful and widely deployed uses dispersion compensating fibers (DCFs) and is appropriate for standard optical fibers. DCFs typically have much higher loss than the standard medium, and therefore require additional am- plifiers. Depending on the transmission distance, multiple dis- persion compensation modules (DCMs) consisting of DCFs and amplifiers may be used in fiber links. The overall cost associated with this approach scales with the link distance. Another promising technique that could offer a cost-effective alternative to DCFs for long-distance optical communication is mid-span spectral inversion (MSSI). 1 Instead of employing dispersion-compensating materials, the method manipulates the signal itself. When an optical signal’s spectrum is inverted in the middle of the transmission link, the spectral components that travel slower in the first half-span will become faster in the sec- ond half-span and vice versa. The net result is that the pulse Figure 1. Experimental setup for multichannel dispersion compensation. MZM: Mach–Zehnder modulator. BSF: Band-selective filter. A/D: Add/drop filter. SI: Spectral inverter. SSMF: Standard single-mode fiber. BPF: Band-pass filter. Rx: Receiver. spread is reversed and the dispersion effect is canceled out. The MSSI technique is independent of the transmission fiber’s dis- persion properties as long as the same type of fiber is used for both halves of the link. The beauty of this approach is that only one spectral inverter device is required at the midpoint of the link, independent of its distance. The inverter is a nonlinear optical device that mixes the in- put signal at carrier frequency ν 1 and a pump at ν 0 to generate a new or converted signal at ν 2 , where energy conservation re- quires ν 2 = 2 ν 0 - ν 1 . The intensity of the converted signal is proportional to the input signal, so the same optical data is car- ried, but the data spectrum is inverted (note the opposite signs of ν 1 and ν 2 ). MSSI has been demonstrated effectively using four-wave mix- ing (FWM) in semiconductor optical amplifiers (SOAs) 2 and difference-frequency generation in periodically poled lithium niobate (PPLN) waveguides. 1 The advantage of SOAs is that they require low pump power to generate efficient optical Continued on next page