Katsumi Emura

Characteristics of optical duobinary signals in terabit/s capacity, high-spectral efficiency WDM systems

Optical duobinary signals have been applied to dense wavelength division multiplexing (WDM) systems with high-spectral efficiency to fully utilize a limited gain bandwidth of about 35 nm (4.4 THz) for an erbium-doped fiber amplifier (EDFA). These signals are one type of partial response signals and have narrower bandwidth than conventional intensity modulation (IM) signals. Thus, the use of these signals should make possible to attain ultradense WDM systems. In this paper, characteristics of optical duobinary and IM signals in ultradense WDM systems are compared through experimental evaluations at 20 Gb/s. High-spectral efficiency of 0.6 b/s/Hz was reached in this demonstration.

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. >

Feasibility study on QPSK optical-heterodyne detection system

The feasibility of a QPSK (quadrature-phase-shift-keying) optical heterodyne detection system for a high-speed and long-span coherent transmission is investigated. The key factors, which are the modulation method and the spectral linewidth requirement for optical light sources, are discussed. Through theoretical analysis and the simulation experiment, it is confirmed that the required beat spectral linewidth for QPSK coherent detection is less than 7.5*10/sup -5/*bit rate. It is also shown that the required spectral linewidth for QPSK differentially coherent detection is less than 2.5*10/sup -5/*bit rate. These results show that the gigabits-per-second range QPSK optical heterodyne detection can be realized with conventional external cavity laser diodes. >

Photonic wavelength-division and time-division hybrid switching system utilizing coherent optical detection

The application of coherent optical detection to a photonic wavelength-division (WD) and time-division (TD) hybrid switching system for a large-capacity switching is studied. Wavelength switching time is 1.8 ns, and the wavelength is stable after wavelength switching as wide as 3-4 AA. The results suggest the feasibility of switching over 20 channels (TD 4 channels, WD approximately 6 channels) for 100-Mb/s signals. This channel number can be increased to 256 channels (WD 64 channels) in view of the maximum wavelength sweep range (40 AA) of wavelength tunable laser diodes obtained so far.<<ETX>>

System design and long-span transmission experiments on an optical FSK heterodyne single filter detection system

The influence of LD phase noise on a heterodyne noncoherent detection system was evaluated. Based on the evaluation, an optical FSK heterodyne single filter detection system with large frequency deviation and wide-band IF filter has been developed to allow use of stand-alone DFB LDs. In the system, a phase tunable DFB LD was used as an FSK transmitter light source to improve the FSK modulation characteristics. An IF filter with appropriate bandwidth evaded the influence of LD phase noise. With these configurations, long-span (243 km at 140 Mbit/s and 204 km at 280 Mhit/s) transmission experiments have been successfully carried out on this single filter detection system. To the contrary, influence of LD phase noise appeared in a limited IF bandwidth case, which agrees well with the theoretical evaluation.

Optimum system design for CPFSK heterodyne delay demodulation system with DFB LDs

An optimum system configuration for an optical continuous-phase frequency-shift-keying (CPFSK) heterodyne delay demodulation system with distributed feedback laser diodes (DFB LDs) is discussed. The optimum modulation index was determined by evaluating the LD phase noise effect and the IF noise effect. The IF noise effect was investigated in detail, considering the noise conversion effect through delay demodulation. In the case of 10-MHz IF beat spectral width, the modulation index m=1.5 is optimum for a 1.2-Gb/s system. With this optimum modulation index, a 204-km long-span transmission experiment, with -41.5 dBm receiver sensitivity, has been successfully performed. The feasibility of using stand-alone DFB LDs for a high-sensitivity CPFSK delay demodulation system has been confirmed through this experiment. >

Feasibility studies on an optical fiber feeder system for microcellular mobile communication systems

The optical fiber feeder system is a wideband and low-loss radio signal feeder system for use between a base station (BS) and micro-base stations (micro-BSs). This system can realize very compact micro-BSs and easy radio channel assignment control in the BS. Calculations and experiments confirmed that the optical feeder can transmit a 1.5 GHz band digital mobile telephone signal, composed of five carriers (pan-European GSM-type eight-channel TDM), through 20 km of optical fiber with more than 75 dB CNR and less than -80 dBc intermodulation distortion.<<ETX>>

A coherent optical FDM CATV distribution system

A coherent optical frequency-division-multiplexing (FDM) experimental system for an optical CATV distribution service has been developed. This system employs a channel frequency spacing locked optical FDM transmitter and a random access optical heterodyne receiver. In the transmitter, ten 1.54- mu m wavelength tunable distributed-Bragg-reflector laser-diode (DBR LD) modules were FSK modulated with a 400-Mb/s PN pattern. A reference pulse method is used for channel space control. Individual channel spacings for ten LDs are stabilized to 8 GHz. The random access optical heterodyne receiver is realized with a wavelength tunable local DBR LD, polarization diversity reception technique, and random access automatic frequency controller. A current address method realizes the random access function. The results of a ten-channel FDM transmission experiment carried out to evaluate these techniques are presented. It is estimated that over 80 channel high-definition TV signals can be distributed to 2000 subscribers with 500-GHz frequency tunable DBR LD. The feasibility of expanding the subscriber number to over 10000 was confirmed by an experiment with a traveling-wave optical amplifier. >

Feasibility study on over 1 bit/s/Hz high spectral efficiency WDM with optical duobinary coding and polarization interleave multiplexing

In this paper, we report the feasibility of high spectral efficiency WDM transmission through the use of the polarization interleave multiplexing, in which each channel is orthogonally polarized to the neighboring channels. The expected deterioration factors are the imperfection of the demultiplex operation and the destruction of the polarization orthogonality between the neighboring channels due to the polarization.