Haruhito Shimizu

Polarization control method for suppressing polarization mode dispersion influence in optical transmission systems

Recent progress in long-span optical repeater systems indicates that the polarization-mode dispersion (PMD) influence has become one of the main degradation factors in high speed and wideband systems. This paper discusses polarization-control methods for suppressing the PMD influence for both the coherent FDM system and the IM-DD optical repeater system. A principal-state transmission method, which can avoid PMD influence in coherent FDM common polarization control, has been proposed. In this scheme, FDM light is launched at one of the principal states in the fiber using two polarization controllers, located at both ends of the fiber. The feasibility of this scheme was confirmed through 2.5-Gb/s CPFSK, three-channel FDM transmission experiments. For a 150-km long fiber, a 700-GHz optical bandwidth can be used with the principal-state transmission method. This bandwidth is about three times wider than that for conventional common polarization control in a 150-km long fiber. The principal-state transmission method has been modified to apply to a long-span optical-repeater transmission system which includes optical isolators. In this method, small frequency modulation was added to the signal light to search for the principal state for the total transmission line. The modified method can avoid inter-symbol interference (ISI) degradation due to accumulated PMD in long-span optical fibers and optical components. Power-penalty-free operations were successfully demonstrated with up to one bit time PMD value in 5-Gb/s IM-DD, 1000-km transmission experiments. This method is expected to apply to a long-span undersea optical-repeater transmission system. >

Highly practical fiber squeezer polarization controller

The authors developed a fiber squeezer polarization controller satisfying these requirements: (1) stable endless operation, (2) high response speed in the endless operation, and (3) protection from fiber break, a particular problem in a fiber squeezer polarization controller. The developed controller realized a stable endless operation with less than 0.1-dB power penalty under 40 degrees C fiber squeezer temperature change. In addition, a practically sufficient response speed of 15 pi rad/s was obtained in an endless operation. A metal-coated fiber used in the fiber squeezers was confirmed to be effective in preventing fiber break by a repetitive load application test (5-10 kg, one million times) to the metal-coated fiber. These techniques could realize a practical and high performance polarization controller for a coherent transmission system. >

2.5 Gb/s CPFSK coherent multichannel transmission experiment for high capacity trunk line system

A 2.5-Gb/s CPFSK four-channel FDM transmission experiment has been conducted with a 150-km-long nondispersion-shifted fiber. This system employed a wideband channel space controller, a common polarization control, and a channel identification function. These were indispensable for realizing a high-capacity, coherent FDM trunk line system. The results of the experiment confirm the feasibility of trunk line systems with over 160-Gb/s capacity.<<ETX>>

A bidirectional common polarization control method for coherent optical FDM transmission system

A bidirectional common polarization control method with a single polarization controller for a coherent optical FDM bidirectional transmission system is discussed. This method is promising for realizing a reliable and low-cost terminal in a coherent multichannel distribution system because the polarization controller can be placed in common only at a central office. Theoretical and experimental results show that the frequency separation delta f has to be 6*bit rate< delta f<120/ Delta tau (GHz) for FSK bidirectional transmission. The method is effective not only for reducing the complexity of the subscriber terminal but also for improving the reliability in a coherent CATV system.<<ETX>>

Key technologies for a coherent optical trunk line system

A high capacity continuous phase frequency shift keying (CPFSK) heterodyne transmission system is proposed from the components point of view. In the coherent trunk line system, reliability, compactness and high sensitivity are important characteristics to achieve a practical system. To realize such characteristics, the authors developed several key components, such as a compact frequency distributed-feedback laser diode package, a reliable filter squeezer polarization controller, a novel dual-PIN high electron mobility transistor receiver, an efficient chromatic dispersion equalizer, and an intelligent frequency controller. By applying these components to a 2.5 Gb/s CPFSK transmission system, -47 dBm receiver sensitivity and stable operation in 764 km normal dispersion fiber transmission were achieved. This high sensitivity value and a long span transmission capability will be effective for both transmission capacity upgrades with frequency division multiplexing (FDM) and also for transmission span expansion with optical amplifier repeaters.<<ETX>>

Cold start 2.5-Gbit/s CPFSK optical heterodyne receiver

A high speed, high sensitivity coherent optical FDM transmission system is expected to realize a large capacity and long span trunk line for broadband-ISDN. In the practical multichannel trunk line system, individual channels should be operated at definite optical frequencies, and the receiver should start quickly and automatically.

Important technologies for highly stable coherent system operation

Essential technologies for stabilizing the operation of coherent optical-fiber communication systems, such as packaging, several automatic control technologies, and an automatic start-up procedure are described. In particular, polarization fluctuation compensation is shown to be indispensable for coherent system operation. Both a polarization control system and a polarization diversity receiver were developed, and stable operation was achieved in each system, which greatly improved the practicability of the coherent systems. Using several technologies, 10-h error-free operation was achieved in a 400-Mb/s-162-km FSK (frequency shift keying) heterodyne single filter detection system. this 10-h error-free operation, which corresponds to less than a 7*10/sup -14/ error rate, greatly advanced the feasibility of achieving an actually operating coherent system.<<ETX>>