Kazuhiro Katoh

Coherent demodulation of 40-Gbit/s polarization-multiplexed QPSK signals with 16-GHz spacing after 200-km transmission

40-Gbit/s polarization-multiplexed QPSK signals with 16-GHz spacing are demodulated after 200-km transmission by using a homodyne phase-diversity receiver. The highlights of our scheme are electrical post-filtering and digital signal processing that enhance the BER performance.

Unrepeated 200-km transmission of 40-Gbit/s 16-QAM signals using digital coherent receiver

We demonstrate unrepeated 200-km transmission of 40-Gbit/s 16-QAM signals using a digital coherent receiver, where the decision-directed carrier-phase estimation is employed. The phase fluctuation is effectively eliminated in the 16-QAM system with such a phase-estimation method, when the linewidth of semiconductor lasers for the transmitter and the local oscillator is 150 kHz. Finite-impulse-response (FIR) filters at the receiver compensate for 4,000-ps/nm group-velocity dispersion (GVD) of the 200-km-long single-mode fiber and a part of self-phase modulation (SPM) in the digital domain. In spite of the launched power limitation due to SPM, the acceptable bit-error rate performance is obtained owing to high sensitivity of the digital coherent receiver.

Coherent demodulation of optical multilevel phase-shift-keying signals using homodyne detection and digital signal processing

We demonstrate coherent demodulation of optical multilevel (M-ary) phase-shift-keying (PSK) signals. Since the carrier phase is estimated accurately through digital signal processing after phase-diversity homodyne detection, the system performance is highly tolerant to the carrier phase noise. By off-line bit-error-rate measurements using distributed feedback semiconductor lasers with linewidths of 150 kHz as a transmitter and a local oscillator, it is shown that binary PSK (M=2), quadrature PSK (M=4), and eight-PSK (M=8) signals are successfully demodulated at the symbol rate of 10 Gsymbol/s

Unrepeated transmission of 20-Gb/s optical quadrature phase-shift-keying signal over 200-km standard single-mode fiber based on digital processing of homodyne-detected signal for Group-velocity dispersion compensation

We demonstrate unrepeated optical transmission of 20-Gb/s quadrature phase-shift-keying (QPSK) signals over a 200-km-long standard single-mode fiber (SMF) without using any optical dispersion compensator. By employing optical homodyne detection, which can restore the entire information of the complex amplitude of the transmitted signal, group-velocity dispersion (GVD) of the SMF can be compensated electrically by a linear equalizer at the receiver. From off-line bit-error-rate measurements, we find that a simple transversal filter implemented in digital signal processing circuits after homodyne detection can effectively cancel the fiber GVD of up to 4000 ps/nm, enabling successful 20-Gb/s QPSK transmission.

Unrepeated 210-km transmission with coherent detection and digital signal processing of 20-Gb/s QPSK signal

We demonstrate an unrepeated transmission of 20-Gb/s QPSK. signals over 210 km using coherent detection and digital signal processing. Phase estimation of the homodyne-detected QPSK signal improves the bit-error rate performance compared to differential demodulation.

Unrepeated 200-km transmission of 40-Gbit/s 16-QAM signals using digital coherent optical receiver

We demonstrate unrepeated 200-km transmission of 40-Gbit/s 16-QAM signals. In spite of the launched power limitation due to SPM, the acceptable BER performance is obtained owing to high sensitivity of a digital coherent optical receiver.

Vegetation restoration by seasonal exclosure in the Kerqin Sandy Land, Inner Mongolia

Grazing control has been reported to be effective for the control of desertification in semi-arid regions. However, economic reasons often make complete inhibition of grazing (complete exclosure) difficult to carry out. Grazing control has been applied to the Kerqin Sandy Lands, Inner Mongolia, China, by means of seasonal exclosure, whereby grazing is allowed from November to April. The harvesting of hay is also allowed once during September - October. The aim of the reported study was to evaluate the effectiveness of this seasonal exclosure on vegetation restoration. Species compositional data were obtained from 356 quadrats and ordinated by Detrended Correspondence Analysis (DCA). Ordination indicated that landform was the most important factor influencing the species composition of the vegetation. Regardless of landform and type of grazing control, however, vegetation coverage, vegetation height and species richness were higher at sites where grazing had been controlled, than at sites lacking any control. Perennial species were dominant at the former while annual species were dominant at the latter. Both shrub and tree species were quite rare at the sites where seasonal exclosure had been carried out. It is concluded that seasonal exclosure is sufficient to restore and maintain grassland vegetation in and around the study area. When shrubby or tree vegetation is needed for reasons such as fixing sands or preventing sand dune remobilization, complete exclosure is recommended.

coherent demodulation of optical 8-phase shift-keying signals using homodyne detection and digital signal processing

We demonstrate coherent demodulation of optical 8-phase shift-keying signals for the first time. Since the carrier phase is estimated accurately through digital signal processing, the system performance is highly tolerant toward the carrier phase noise.

Ultrafast Operation of Digital Coherent Receivers Using Their Time-Division Demultiplexing Function

The digital coherent receiver, which is a combination of a phase-diversity optical homodyne receiver and digital signal processing (DSP), can demodulate any multilevel coded optical signals without relying upon an optical phase-locked loop. However, the maximum symbol rate processed by such a receiver is limited by the speed of electric analog-to-digital converters and digital signal processors. Although real-time operation at 10 Gsymbol/s using an application-specific integrated circuit has recently been demonstrated, it is still difficult to increase the symbol rate beyond 40 Gsymbol/s. In order to cope with this difficulty, we propose a novel scheme, which employs a local oscillator (LO) pulsed at the subharmonic frequency of the symbol rate, enabling time-division demultiplexing of the signal at the digital coherent receiver. We demonstrate that the new type of digital coherent receiver operating at 10 Gsymbol/s can demodulate the aggregate symbol rate of 160 Gsymbol/s. From these results, we can expect ultrafast coherent optical fiber communication in the future.

Adjustable dispersion-compensation devices with wavelength tunability based on enhanced thermal chirping of fiber Bragg gratings

We demonstrate a dispersion compensation device based on thermal chirping of a glued fiber Bragg grating. The device can change the group velocity dispersion (GVD) while maintaining a center wavelength or change the center wavelength while preserving a GVD value. The GVD can be tuned from -122.5 to -57 ps/nm with a center wavelength at 1552.9 nm. On the other hand, the center wavelength can be shifted by 2.2 nm with a GVD value around -105 ps/nm.