OKI Develops Highly Precise Technology to Suppress Polarization-Mode Dispersion, the Main Cause of Interference for 100Gbps Ultra High-speed Optical Communication
Succeeding in operational test at 160Gbps and accelerating the realization of high-speed communication over existing fiber networks
TOKYO, April 17, 2009 -- OKI Electric Industry (TSE: 6703) and the National Institute of Information and Communications Technology (NICT) today announced that OKI has developed a technology to precisely suppress Polarization-Mode Dispersion (PMD)(*1), which is the main cause of interference when attempting high-speed optical communication greater than 100Gbps. Using a "PMD Suppressor," OKI and NICT succeeded in an operational test using a 160Gbps optical signal, which is faster and more susceptible to PMD than 100Gbps. Using this technology, ultra high-speed optical communication not limited to short distances due to PMD can be achieved over existing fiber networks. The study results are based on NICT's commissioning research.
The research that led to this achievement was conducted as part of the "Research and Development on λ (Lambda) Utility Technology," under the auspices of NICT, as part of Japan's Ministry of Internal Affairs and Communications (MIC)'s "Research and Development for Photonic Network Technology."
Background
Efforts to achieve 100 Gbps-class high-speed optical communications are accelerating as networks rapidly become broadband, and will intensify as the 40G/100G Ethernet standard is applied in the world in 2010. Although the 100Gbps optical communication system has 10 times the capacity of the current 10Gbps system, it is more easily influenced by PMD, which is the major cause of communication quality loss in such systems. In Japan, over 50% of current optical fiber networks (total length of backbone network: approx. three hundred thousand kilometers) are of a type influenced greatly by PMD(*2). Thus, in order to achieve a 100Gbps system, it is important to reduce the PMD effect.
Achieved results
PMD causes a distortion to the optical signal wave. When a PMD is generated, the received signal expands (becomes thicker) which makes it difficult to transmit data accurately. Because PMD randomly changes depending on the installed condition of the optical fibers and external environment, communication quality deteriorates. In particular, the faster the transmission-rates, the more likely it is to be affected of PMD, and thus, for a 100Gbps system, PMD is the major cause of degradation of communication quality.
OKI succeeded in developing a PMD suppressor, which is an optical circuit for restoring the distorted received signal wave to its original form. Using this suppressor, OKI succeeded in restoring a wave distorted by PMD to its original form by generating a pseudo PMD in an operational experiment using a 160Gbps signal. This was achieved by applying OKI's technology to generate an ultra-small optical path difference and technology to monitor the PMD. <Refer to Reference>
Even when using a relatively new optical fiber with low PMD characteristics at a transmission-rate of 100Gbps, signals can be transmitted only up to several hundred kilometers. Moreover, with optical fibers of the kind built in the beginning of the 1990s, signals can be transmitted only several tens of kilometers. However, with OKI's newly developed technology, the transmission distance is not limited by PMD, while achieving ultra high-speed transmission.
Future prospects
The newly developed PMD suppressing technology does not depend on the transmission-rate, making it advantageous for low power consumption. In the future, it can also be applied to other areas, such as for transmission-rates exceeding 100Gbps and for energy-saving-use. OKI will continue to ensure long-term stability and achieve smaller size of this PMD suppressor and to aim for wider commercialization.
[Reference] The structure of the PMD suppressor and the results of the movement experiments
Fig. 1 shows an image of the PMD suppression structure. The PMD suppressor consists of two polarization controllers, an ultra high time-resolution variable DGD generator, and a polarization-beam-splitter (PBS). For PMD suppressors, the monitoring method is very important for maintaining the optimum control states automatically. To find out the effective control state, we employed the carrier wavelength intensity as a PMD monitor.
(1) shows the image of the transmitter output signal. The signal is split into two components by PMD, and the speed of each component in the fiber is different, which results in DGD (Differential Group Delay). Moreover, as the speed increases, more complicated wave distortion occurs, as seen by the purple line in (2). The PMD suppressor is controlled to restore the distorted signal((2)) back to its original wave form ((1)). In (3), the fast component is given a delay by the variable DGD generator, to compensate for the difference in timing. And finally in (4), the complicated distortion factor is suppressed by the PBS.
In general, when changing the amount of DGD using conventional DGD generators, the output state-of-polarizations (SOP) are large and show sudden fluctuations. Therefore, polarization dependent devices, such as PBS, could not be used behind the DGD generators. On the other hand, the newly developed DGD generator is capable of generating ultra small DGD (0.05 fs). Thanks to this small-step feature, the amounts of change induced by DGD generations can be controlled as needed. With this technology, it is now possible to simultaneously employ variable DGD generators for highly precise DGD compensation and PBS for depolarization suppression.
One of the PBS output signals is used as a PMD monitor. The PMD suppressor is controlled to minimize the carrier wavelength intensity of the monitor signal. With this monitor and control method, PMD suppression that is both effective and automatic can be realized.
Fig. 2 and Fig. 3 show the results of the PMD suppression experiments using 160 Gbps signals. Fig. 2 (a) is the optical sampling waveform of a 160 Gbps transmitter output signal, (b) is the PMD emulated signal, and (c) is the PMD suppressed signal based on OKI's technologies.
Fig. 3 shows the optical signal spectrum. The solid-line (blue) is the PMD monitor input signal when the PMD suppressor is controlled at an optimum state. The dashed-line is the signal source for reference. As shown in these figures, effective suppression is recognized when the carrier wavelength intensity of the signal being inputted to the monitor is minimized.
The Q-factors which indicate transmission performance were 27dB at transmitter outputs, and the values were restored to 26dB after PMD suppression.
[Fig. 2] The experimental results for PMD suppression experiments using 160Gbps signal.(Optical sampling waveform)
Transmitter output signal
(Q-factor: 27dB)
The distortion signal by PMD
The PMD suppressed signal:
Q-factor: 26dB.
(3 ps/div.)
[Fig.3] The monitor signal for the states where the PMD is optimally controlled (blue-solid) and transmitter output signal (red-dashed for reference)
Glossary
- *1 :PMD (Polarization-Mode Dispersion)
Generally, optical signals have a wave characteristic and when a signal is transmitted, oscillating in constant direction, it is called polarization. The ideal cross-section of an optical fiber to transmit optical signals should be a perfect circle. However, realistically, because of the difference in manufacturing and the effects from the external environment, it is slightly ellipse-shaped. In an ellipse-shaped optical fiber, the signal is split into two beams and their propagation directions are 90-degree different to each other. This disruption is called polarization-mode, and these two optical signal components reach the receiver at different speeds. This time difference is called differential group delay (DGD), and provides a scale to represent the size of PMD. In addition, optical signal oscillation direction (polarization-mode direction) and the size of DGD change depending on the conditions of deployed fiber. Theses phenomena are known as polarization-mode dispersion (PMD).
When the DGDs grow large, it is difficult to differentiate between neighboring signals, and it becomes difficult to accurately identify the "0s" and "1s" of digital data when optically overlapped, thus, reliable communication cannot be established. The DGD generated in the optical fiber can be compensated for by creating an opposite DGD to the optical signal. However, because the PMD can change momentarily depending on the external environment, it is important to accurately monitor the changes and develop a technology to automatically track DGD compensation at all times.
The states of PMD generation also depend on frequency. Generally, as transmission-rates increase, the frequency-bands of optical-signal become wider. Thus, the faster the speed of transmission, the more serious effects there are on wave distortion due to PMD.
- *2 :The relationship between impact of PMD and year of optical fiber installation
The size of the PMD also depends on the optical fiber manufacturing technology. Therefore, the old fibers have the large PMD. The quantity of PMD is represented as PMD-coefficient (unit: ps/km0.5). The PMD-coefficient for optical fibers that are relatively new is under 0.1 ps/km0.5 but many of those that were installed in the 1980's and early 1990's have a PMD coefficient of 0.2 ps/km0.5 to 1 ps/km0.5. The PMD-coefficient is an indicator to show how large a DGD there is in the PMD on average. For example, an optical fiber path of 1 ps/km0.5 means an average of 10ps of DGD is generated for a distance of 100km.
About Oki Electric Industry Co., Ltd.
Founded in 1881, Oki Electric Industry Co., Ltd. is Japan's first telecommunications manufacturer, with its headquarters in Tokyo, Japan. OKI provides top-quality products, technologies and solutions to its customers through its info-telecom system business and printer business. All businesses function as a collective force to create exciting new products and technologies that satisfy a spectrum of customer needs in various markets. Visit OKI's global web site at http://www.oki.com/.
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