Yokogawa and Fujitsu Develop 40 Gbps DQPSK Optical Transmission Technology

Yokogawa Electric Corporation and Fujitsu Limited today announced the joint development of the world’s first practical 40 Gbps optical transmission technologies using differential quadrature phase shift keying (DQPSK)(1). The two companies are respectively planning to incorporate the technologies in various new products for 40 Gbps ultra high-speed optical transmission networks.

The new technologies were developed under the strategic partnership established between Yokogawa and Fujitsu in March of 2006 to jointly develop core system technologies and key components for ultra high-speed optical transmission systems, and with the cooperation of Fujitsu Laboratories Ltd. Sample products were on display at the Optical Fiber Communication Conference and Exposition (OFC) and the National Fiber Optic Engineers Conference (NFOEC) 2007, held in Anaheim, California from March 27 - 29, 2007.

Background of Development
The dissemination of optical access networks that directly connect households via optical fibers and the construction of next generation networks have led to a increasing demand for greater capacity in inter-city optical transmission networks. In order to meet this demand, increasing the maximum transmission speed from 10 Gbps used in current optical transmission systems to 40 Gbps is being considered.

Overview of Development Challenge
Typically, when transmission speed is increased, distortion caused by polarization mode dispersion (PMD)(2) becomes pronounced, limiting transmission reach. For example, in the case of transmission of a 40 Gbps signal using a standard binary modulation(3) in an optic fiber that meets the PMD specification (0.2 ps /√km) recommended by the ITU-T(4), transmission reach is limited to a maximum of 100 km. Thus, inter-city transmission, which requires long-distance transmission of more than several hundred kilometers, has not been possible.

Seeking to overcome this obstacle, advances are being made in investigating DQPSK-type transmission, which is tolerant to waveform distortion due to PMD, and its high performance has been confirmed in the laboratory experiments. However, the configuration of a DQPSK format is complex, and thus large size and high power consumption of the optical transceivers have proved to be challenges.


The Newly Developed Technologies
Yokogawa Electric and Fujitsu Limited, in cooperation with Fujitsu Laboratories Ltd, have successfully developed the world’s first practical 40 Gbps DQPSK optical transmission technologies:

1. DQPSK LN Optical Modulator:
The LN optical modulator(5) for DQPSK modulation, which was developed by Fujitsu and operates with the world’s lowest drive voltage, enables a compact optical transmission component design and lower power consumption.

2. Dedicated ICs and Devices that Enable DQPSK:
Compact, low power consumption dedicated ICs and other devices that enable DQPSK, including a driver device optimized for the DQPSK LN optical modulator mentioned above, optical / electrical conversion devices that operate stably despite PMD waveform distortion, and clock and data recovery devices, were developed with Yokogawa Electric’s InP HBT(6) technology.

3. Compact Optical Transmission Module:
Control technology was developed that allows the newly developed key devices to operate in a stable manner as well as mounting technology that enables a compact size, making possible a compact 110 mm x 320 mm x 40 mm package equipped with all functions necessary for 40 Gbps DQPSK in the transmission equipment and a low-power consumption of 35 W (with case temperature of 72°C).

Results
100 units of 40 Gbps DQPSK optical transceivers were manufactured and transmission performance and stable operation despite environmental changes, such as temperature fluctuations and variations in supply voltages, was confirmed. Furthermore, the transmission reach as limited by PMD was found to be approximately eight times better than that of standard binary modulation (figure 2).

The resulting technology is expected to significantly reduce the time it will take to implement major inter-city high-capacity optical networks.

Moving Forward
Utilizing the newly developed technology, Yokogawa Electric and Fujitsu are respectively planning to bring various products for 40 Gbps ultra high-speed optical transmission networks to the market.

Notes
1 Differential Quadrature Phase Shift Keying (DQPSK): A modulation method for digital signals whereby 2 bits of data are allocated to each of four modulated optical phases. The use of four types of optical phases enables a pulse width that is double that of the binary modulation scheme, reducing the effect of waveform distortion due to PMD.

2 Polarization Mode Dispersion (PMD): Phenomenon in which light travels at different speeds in the two polarization modes of an optical fiber causing signal distortion at the optical receiver.

3 Binary Modulation: A modulation method whereby the digital singles “0” and “1” are assigned to either “bright” or “dark” optical signals, or to “0°” or “180°” optical phases.

4 International Telecommunication Union - Telecommunication standardization sector (ITU-T): An international organization that produces standards in the telecommunications field.

5 LN Optical Modulator: A device that converts an electrical data signal into an optical data stream via the electro-optic effect of a LiNbO3 crystal. For more detailed information, please refer to the press release on March 27, 2007.

6 InP Hetero-Junction Bipolar Transistor (InP HBT): A compound semiconductor transistor with outstanding high-speed performance and high drive capability with low IC power consumption.

For more information:
Yokogawa Electric Corporation (http://www.yokogawa.com/)
Fujitsu Limited (http://www.fujitsu.com/global/)