OCDM reaches terabit capacity

Oct. 1, 2001
TOKYO—Researchers at the Communications Research Laboratory, along with Prof. Kenichi Kitayama of Osaka University (Osaka), have succeeded in terabit-capacity multiplexing transmission using optical code-division multiplexing (OCDM).

Incorporating news from O plus E magazine, Tokyo

TOKYO—Researchers at the Communications Research Laboratory, along with Prof. Kenichi Kitayama of Osaka University (Osaka), have succeeded in terabit-capacity multiplexing transmission using optical code-division multiplexing (OCDM). The Japanese government has endorsed an initiative called "e-Japan"—a focused plan to create a society with high-level information transmission capability—and has stated that ultrahigh-speed, high-capacity optical networks on the petabit (1015-bit) level will be necessary by the year 2010. The OCDM method is a new type of multiplexing that differs from standard optical time-division multiplexing and wavelength-division multiplexing (WDM).

In OCDM, information is transmitted by encoding the signal on the sender side using different wave forms and decoding the signal on the recipient side using the same code as a key. Using this principle, multiple channels with the same wavelength can be multiplexed at the same time. In addition, routing and other complex manipulations can be performed in the optical regime, as opposed to at higher levels. Therefore, higher performance can be expected. Also, higher security can be anticipated because the information is encoded in the optical regime.

In this experiment, the wavelengths of a 20-Gbit/s signal were converted to wideband by the emission of supercontinuum light. The signal was then multiplexed in four channels using OCDM and separated into 19 wavelengths. The optical signal was transmitted over 80 km at 1.52 Tbit/s. The recipient decomposed this signal using an arrayed-waveguide grating with 200-Ghz spacing and, after decoding, accepted the 20-Gbit/s signal using a nonlinear-loop-mirror optical switch.

The optical transversal filter used to encode and decode the signal consists of a tunable optical coupler, an optical delay line, a thermo-optic phase shifter, and a wave mixer. The input transmission pulse is separated into five branches using a coupler and each segment temporally delayed by 5 ps (200 GHz). The carrier phase of each segment is shifted by either 0 or p using a phase shifter and the signals recombined.

The recipient uses the same optical transversal filter. The filter takes the nine pulses separated by the coupler and performs a threshold operation on autocorrelation waveforms that show a sharp peak only when the codes match. Only the matching codes are recognized.

This method, called binary phase-shift keying, is able to encode and decode purely in the optical regime. In addition, in the decoding step, a time gate is included, enabling higher-speed encoding and decoding. Also, the signa-to-noise ratio is superior to previous methods. This encoding method enables an optical pulse with a wide wavelength band to be filtered through the optical transversal filter only once to encode many wavelengths. As a result, improvements in OCDM technology can also be applied to the development of WDM.

This experiment involved many optical processes that enabled the transmission and processing of optical signals without electrical processing. In the sending regime, supercontinuum light emission was used for wavelength multiplexing. In the transmission regime, a dispersion-flat transmission path that prevents the degradation of wideband optical signals due to wavelength dispersion and nonlinear effects was used. This path included a single-mode fiber with zero dispersion in the 1300-nm band, along with reverse-dispersion fiber. In addition to overseeing wavelength dispersion, frequency chirping of the optical signal itself was also used. On the receiving end, the time gate for the OCDM requires time-gate qualities of a 200-Gbit/s OTDM. In order to take this into account, a nonlinear-loop-mirror optical switch with higher nonlinear properties than conventional optical fibers was used.

Courtesy O plus E magazine, Tokyo

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