OCDM-based fibre access


Oki Electric says that its new technology — reported at the 2002 Institute of Electronics, Information and Communication Engineers Society Conference in September — was developed in collaboration with Osaka University. It allows service providers to guarantee transmission capacity of up to 10Gbit/s in bandwidth for bi-directional communications over an access network. Service providers will also be able to offer flexible services for a variety of user needs, such as multiple services on one network.

"Oki believes that this technology will become indispensable in providing valuable new services that flexibly respond to customer requirements on bandwidth," said Harushige Sugimoto, general manager of Oki's Corporate Research and Development Center. "Customers will be able to enjoy multiple services such as motion pictures, voices, and data simultaneously under the real broadband environment. Our new method will expand the optical communications market."

With current broadband services, it is difficult to provide customers with both large data-transmission capacity and QoS (quality of service) at the same time, since these are best-effort type services involving shared bandwidths among a number of users. With the new OCDM technology, the bandwidth of a wavelength in a fibre is further multiplexed and de-multiplexed by optical codes.

This technology also allows the allocation of services and users using these optical codes, which enables service providers to control many bandwidths flexibly according to user requests. Oki has proved that the new technology will allow service providers to guarantee bandwidth up to 10Gbit/s, 100–1000 times the capacity of current services.

Oki has succeeded in enhancing the data bit rate of a fibre Bragg grating (FBG)-type optical encoder/decoder, enabling encoding of signals at various speeds (1–10Gbit/s) with one encoder/decoder. Since the equipment does not require an electrical power supply, a much smaller optical transmission system becomes possible at lower cost.

Using this newly developed technology, Oki intends to develop commercial optical encoder/decoder products and an optical encoder/decoder and optical code transceiver/receiver within a year.

OCDM is a method used to multiplex signals by diffusing them via various optical codes on an optical fibre; this method enables much more effective use of optical signal frequency than possible with WDM technology. Receivers acquire the coincident signals by selecting codes. Passive optical elements such as FBG can be used for an encoder/decoder, which results in a faster signal rate, due to the lack of electrical limitation.

In post-deadline paper PD4.2 by B Zhu et al at September's European Conference on Optical Communications in Copenhagen, OFS reported a record-setting capacity–distance demonstration at 40Gbit/s. This was done by transmitting 3.2Tbit/s (80 x 100GHz-spaced WDM channels of a 42.7Gbit/s pseudo-random bit stream data pattern) over 52 x 100km spans of UltraWave Terrestrial dispersion-managed fibre, i.e. a total capacity–distance of 16.6 petabits-km/s.

The demonstration used return-to-zero differential-phase-shift-keyed modulation (RZ-DPSK), balanced detection and all-Raman amplification. It was conducted at the OFS Laboratories in New Jersey with collaborators from Lucent Technologies/Bell Labs. The result surpasses the previous record of 10 petabit-km/s (2.56Tbit/s over 4000km), which used RZ-DPSK with balanced detection, giving a 3dB advantage in receiver sensitivity over on-off-keyed (OOK) modulation formats (A H Gnauck et al, OFC2002 paper FC2).

Previous 40Gbit/s experiments using 100km single-fibre-type spans and OOK have demonstrated 10.2Tbit/s over 300 km (Y Frignac et al, OFC2002 paper FC5) and 3.2Tbit/s over 2000km (B Zhu et al, OFC2002 paper FC8).

For the latest experiment, figure 1 shows the received optical spectrum. The launched power per channel was –6dBm. Figure 2 shows an uncorrected bit-error-rate (BER) of 9x10-5, giving BER <10-13 for all channels with FEC.

"Fewer regenerators are necessary, as dispersion managed fibre spans offer the potential to further increase transmission distance," says Janice Haber, VP for Market Strategy and Systems Engineering Research.

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