OPTICAL COMMUNICATIONS

OPTICAL COMMUNICATIONS

Advanced multiplexing sends high-speed data

Paul Mortensen

Researchers at Nippon Telegraph and Telephone (NTT; Yokosuka, Japan) have successfully completed a large-capacity optical-transmission experiment at 400 Gbit/s over 100 km by combining time division multiplexing (TDM) and wavelength division multiplexing (WDM) technologies for the first time. The high data rate was achieved by carrying information at 100 Gbit/s on each of 4-ps-pulse streams of different wavelengths extracted from a single laser light source.

The company`s researchers see TDM technology, in which multiple pulse trains from a single source are arranged so they do not overla¥in time, as the best route to substantial increases in transmission capacity. Without TDM, information is transmitted with multiple semiconductor lasers whose oscillating wavelengths are precisely selected and controlled. This can lead to problems with the reliability, stability, and cost of the light sources.

The NTT researchers addressed these problems by developing what they call a supercontinuum pulse generator that has an ultrawide bandwidth and short pulse width. According to NTT, a supercontinuum is the light emitted when powerful short light pulses are introduced into a nonlinear optical medium. The resulting continuous optical spectrum retains the short-pulse nature. The supercontinuum pulse generator consists of an actively modelocked erbium-doped fiber ring laser that outputs 6.3-GHz, 1561.5-nm, 4.8-ps pum¥pulses; an erbium-doped fiber amplifier (EDFA) to amplify the pum¥pulses to a peak power of 1.5 W; and a 3-km-long, single-mode, dispersion-shifted fiber for supercontinuum generation (see Fig. 1 on p. 20).

Multiple-wavelength source

The new light source has an emission spectrum of 200 nm with a corresponding bandwidth of 200 nm (25 THz) and can generate multiple streams of coherent, low-noise short optical pulses of arbitrary center wavelengths within the spectrum. Because it can generate multiple-wavelength short optical pulses of less than one picosecond at an arbitrary wavelength and its wavelength stability is ten times that of a conventional semiconductor laser, NTT says that the light source will play a major role in realizing low-cost, reliable large-capacity transmission based on optical TDM and WDM. By selecting multiple-wavelength components from the single supercontinuum light source and carrying information on each wavelength, one can transmit wavelength-division-multiplexed optical signals in an optical fiber using a single light source. The researchers at NTT say the potential transmission capacity of the super source will exceed 5 Tbit/s.

The optical transmitter selects four pulse streams of different wavelengths from a single supercontinuum light source by using a WDM multiplexer. It then multiplexes the 6.3-Gbit/s pulse signals by 16 through optical TDM to generate 100-Gbit/s signals at each wavelength. The collective 400-Gbit/s (that is, 100 Gbit/s at each of the four wavelengths) signals are then amplified by a broadband erbium-doped fiber amplifier and transmitted through a 100-km optical fiber. An optical receiver then demultiplexes the signals (see Fig. 2).

Terabit barrier broken

Meanwhile, researchers at Fujitsu Laboratories Ltd. (Atsugi, Japan) have achieved a transmission rate of 1.1 Tbit/s over optical fiber using WDM technology. Crossing the 1-Tbit/s threshold for WDM has been an active goal of research laboratories worldwide.

In their experiment, Fujitsu re searchers used 55 diode lasers in the 1.55-µm wavelength range, where fiber has the lowest transmission loss, and set the channel spacing to 0.6 nm. They used in-line repeaters and a common preamplifier for simultaneously amplifying the 55 signals and broadband EDFAs as postamplifiers. All signals were transmitted through 150 km of 1.3-µm zero-dispersion single-mode fiber with an amplifier spacing of 50 km. Lithium niobate Mach-Zehnder-type external modulators produced a rate of 20 Gbit/s for each signal, for a total capacity of 1.1 Tbit/s.

A key factor in the success of this experiment was the use of dispersion-compensating fiber (DCF) with a large negative dispersion. Without DCF, a 20-Gbit/s signal can be transmitted only 10 to 20 km.

By overcoming this problem, the experiment confirmed a transmission capacity about 400 times that of current 2.5-Gbit/s one-wavelength systems and established the potential of 1.3-µm zero-dispersion single-mode fiber as a terabit-per-second transmission medium using WDM technology.