All-optical devices enable 160Gbps transmission

To satisfy the demand for more transmission capacity, vendors are boosting fiber capacity by increasing both the number of wavelength channels and the time-division multiplexed bit rate (TDM) per wavelength channel. A channel bit rate of 40Gbps will soon be installed and the next-generation TDM bit rate of 160Gbps is under active study in several research laboratories. Since electrical signal processing is not available at 160Gbps today, the TDM bit rate of 160Gbps requires optical time-division multiplexing (OTDM) techniques.

In a typical OTDM system, an optical pulse source on the transmitter side generates a pulse train with a repetition rate corresponding to the base rate (electronic data rate) used. Therefore, these systems use a base rate of 40Gbps, which is the highest electronic data rate available at present. The 40GHz pulse train is coupled into four optical branches, in which modulators driven by electrical signals at the base data rate generate 40Gbps optical return-to-zero data signals. These four optical data signals are bit-interleaved by a delay-line multiplexer to generate the multiplexed 160Gbps optical data signal. This signal is launched into a single-mode fiber-transmission line. While traveling down the transmission line, the signal is degraded by the attenuation, nonlinearity, chromatic dispersion and polarization-mode dispersion (PMD) of the fiber.

Pulse sources used for high-bit-rate transmission experiments include mode-locked or gain-switched laser diodes, mode-locked fiber-ring lasers, continuous-wave lasers externally modulated by a Mach-Zehnder modulator or an electro-absorption modulator and supercontinuum pulse generation. Gain-switched laser diodes and externally modulated continuous-wave lasers have an insufficient pulse width and an insufficient extinction ratio. They require some sort of subsequent pulse compression and optical regeneration. The fiber-ring laser needs harmonic mode locking and stability is a critical issue. For 160Gbps transmission, a semiconductor mode-locked laser diode fulfills all requirements. It can be realized as a very compact and stable device.

Different fiber types have been investigated for high-speed transmission. It became clear that fibers with high local dispersion and low nonlinearity perform better. For 160Gbps, it is necessary to compensate not only for chromatic dispersion but also for dispersion slope. To date, the most mature technique of dispersion compensation is based on slope-compensated dispersion compensating fiber (SC-DCF).

For a demultiplexer in ultrahigh-speed OTDM transmission systems, clock recovery is an essential operation. A phase-locked loop (PLL), which is often used in conventional electrical time-division multiplexed (ETDM) systems, is also a promising technique in ultra-high-speed OTDM systems. Various switching devices were used for optical demultiplexers up to 640Gbps. In the most advanced experiments, optical switching for optical demultiplexing was based on fiber Kerr nonlinearity (nonlinear optical loop mirror and four-wave mixing in fibers). Polarization-independent, all-optical demultiplexing of a 200Gbps data signal was demonstrated using FWM in a semiconductor optical amplifier (SOA). Optical demultiplexing based on an electrically controlled electro-absorption modulator was demonstrated in a 160Gbps data transmission experiment.

Today, the most promising approach for an all-optical demultiplexer is to use gating devices based on cross-phase modulation in an SOA in conjunction with interferometric structures, like Sagnac (Slalom, or Toad-Terahertz optical asymmetric demultiplexer), ultrafast nonlinear interferometer (UNI), Mach-Zehnder and Michelson configurations.

The advantage of the semiconductor optical amplifier-based switching devices is the high resonant optical nonlinearity of the SOA resulting in low optical power and, therefore, low power for all-optical switching. As an example, we report here a 160Gbps return-to-zero transmission experiment over field-installed fiber G.652. In the transmitter, a mode-locked semiconductor laser was operated at 1,550nm with a repetition rate of 10GHz. The 10GHz transform-limited, 1.2ps pulse train was intensity modulated with a pseudo-random bit-sequence using an external Mach-Zehnder modulator.

The 10Gbps data signal was then multiplexed by a fiber delay-line multiplexer (four stages) to a 160Gbps, single-polarization return-to-zero data signal. The optical demultiplexer used was a UNI with a semiconductor optical amplifier operating in the "gain-transparent" mode. We achieved error-free performance for all transmission experiments. However, a manual adjustment of the state of polarization at the fiber input was required to minimize degradation due to PMD.

Despite daytime temperature changes during the experiments (environment: 10:C to 25:C, lab: 20:C to 30:C), no readjustment of the dispersion compensation was needed.

Heinrich-Hertz

Institut fur Nachrichtentechnik