Yojiro Mori

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.

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.

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.

Multi-impairment monitoring from adaptive finite-impulse-response filters in a digital coherent receiver.

We propose a novel and unified algorithm that estimates linear impairments in optical transmission systems from tap coefficients of an adaptive finite-impulse response (FIR) filter in a coherent optical receiver. Measurable impairments include chromatic dispersion (CD), differential group delay (DGD) between two principal states of polarization, second-order polarization-mode dispersion (second-order PMD), and polarization-dependent loss (PDL). We validate our multi-impairment monitoring algorithm by dual-polarization quadrature phase-shift keying (QPSK) transmission experiments.

Novel configuration of finite-impulse-response filters tolerant to carrier-phase fluctuations in digital coherent optical receivers for higher-order quadrature amplitude modulation signals

We propose a novel configuration of the finite-impulse-response (FIR) filter adapted by the phase-dependent decision-directed least-mean-square (DD-LMS) algorithm in digital coherent optical receivers. Since fast carrier-phase fluctuations are removed from the error signal which updates tap coefficients of the FIR filter, we can achieve stable adaptation of filter-tap coefficients for higher-order quadrature-amplitude modulation (QAM) signals. Computer simulations show that our proposed scheme is much more tolerant to the phase noise and the frequency offset than the conventional DD-LMS scheme. Such theoretical predictions are also validated experimentally by using a 10-Gsymbol/s dual-polarization 16-QAM signal.

Multi-impairments monitoring from the equalizer in a digital coherent optical receiver

We propose a simple and precise algorithm to monitor multiple impairments such as CD, PMD, and PDL, which are jointly mitigated in the adaptive equalizer of a digital coherent optical receiver. We validate our algorithm by dual-polarization QPSK transmission experiments.

Phase-noise tolerance of optical 16-QAM signals demodulated with decision-directed carrier-phase estimation

Phase-noise tolerance of 10-Gsymbol/s 16-QAM signals is investigated both theoretically and experimentally. Optimizing the decision-directed phase-locked loop for carrier-phase estimation, we can obtain acceptable BER performance when the laser linewidth is in the 100-kHz range.

Bit-error rate performance of Nyquist wavelength-division multiplexed quadrature phase-shift keying optical signals

We measure bit-error rates of polarization-multiplexed Nyquist-WDM QPSK signals, which have rectangular spectral shapes and channel spacing equal to the symbol rate. The spectral efficiency of 4 bit/s/Hz is achieved with negligible power penalty.

Demodulation of 480-Gbit/s 8PSK OTDM signal with digital coherent receiver

We demodulate a 480-Gbit/s 8PSK signal with a digital coherent receiver having time-division demultiplexing function. The receiver sensitivity is -20 dBm at BER of 10-5.

Ultrafast digital coherent receiver based on parallel processing of decomposed frequency subbands

We propose a novel digital coherent receiver, where frequency subbands of an ultrafast optical signal are detected in parallel and synthesized with DSP. A proof-of-concept experiment performing two-subband decomposition and reconstruction for PSK signals proves effectiveness of the proposed scheme.