Seiji Norimatsu

An 8 Gb/s QPSK optical homodyne detection experiment using external-cavity laser diodes

A quadrature-phase-shift-keying optical homodyne detection experiment was conducted at a bit rate of 8 Gb/s. External-cavity laser diodes and a single-body QPSK phase modulator were used. The optical carrier was recovered by a decision-directed phase-locked loop. The first reported measurement of bit-error-rate performance is given. A receiver sensitivity of -27.8 dBm was achieved.<<ETX>>

Accurate Q-factor estimation of optically amplified systems in the presence of waveform distortions

We present an accurate method for the Q-factor estimation even in the presence of waveform distortions in optically amplified systems. The method stands on the assumption that the dominant noises over the bit error rate (BER) are those superimposed on the nearest rails to the decision level. It is clarified that the assumption is usually satisfied. The results also show that, in numerical system evaluation, we should evaluate not only the eye opening but also the probability of occurrence of the rail level.

PLL propagation delay-time influence on linewidth requirements of optical PSK homodyne detection

Phase-locked loop (PLL) propagation delay-time influence on optical homodyne detection was investigated both theoretically and experimentally. Applying the Pade approximation, which is often used in the control system, to the calculation of the phase-error variance with the nonzero loop delay time, a high-accuracy analytic expression phase-error variance is obtained. The linewidth requirement with the nonnegligible loop delay time for phase-shift-keying (PSK) homodyne detection is obtained as delta nu =2.04*10/sup -3// tau where delta nu (hertz) is beat linewidth and tau (seconds) is the loop delay time. The linewidth requirement with small delay time approaches delta nu =6.2*10/sup -4/ R/sub b/ where R/sub b/ (bits-per-second) is the system bit rate. Results were confirmed by a 10-GB/s optical PSK homodyne detection experiment using external cavity laser diodes. Receiver sensitivity degradations due to loop delay time and beat linewidth are in good agreement with theoretical results. >

PSK optical homodyne detection using external cavity laser diodes in Costas loop

5-Gb/s optical PSK (phase-shift keying) homodyne detection experiments are discussed. In these experiments, the optical carrier is recovered by a Costas optical phase-locked loop using a multielectrode local oscillator (DFB) laser diode at 1.55 mu m with a flat FM response. Although the beat linewidth of 80 kHz is broad compared to the loops in other phase-locked loop (PLL) experiments, phase locking with Costas loop is confirmed at 5 Gb/s by increasing the loop natural frequency. The receiver sensitivity is -42.2 dBm or 93 photon/bit for a 2/sup 7/-1 pseudorandom bit sequence (PRBS) in front of a 90 degrees hydride.<<ETX>>

Statistical analysis on stimulated Raman crosstalk in dispersion-managed fiber links

Power impairments due to stimulated Raman scattering (SRS) in dispersion-managed (DM) fiber links are evaluated theoretically. We extend previous work on the statistical analysis of SRS crosstalk to the case of multiple fiber segments. Closed-form formulas are derived, and the applicable range is presented by comparison with simulation results. The SRS crosstalk in DM fiber links is evaluated using derived formulas, and the preferable configuration of DM fiber links for suppressing SRS crosstalk is discussed. We also evaluate the exact power penalty induced by the SRS crosstalk in consideration of the log-normal waveform distribution due to SRS and non-Gaussian noise and clarify the system bounds in some typical DM fiber links. The developed approach provides a design rule for DM fiber links from the viewpoint of SRS crosstalk suppression.

A 1580-nm band WDM transmission technology employing optical duobinary coding

This paper reports 1580-nm band wavelength division multiplexed (WDM) transmission employing optical duobinary coding over dispersion-shifted fibers. By using the 1580 nm band, the generation of four-wave mixing (FWM) over dispersion-shifted fibers (DSFs) can he suppressed. Optical duobinary coding is dispersion-tolerant because of its narrow bandwidth, and enables the use of the conventional binary intensity modulated direct detection (IM-DD) receiver. First, comparisons are made for WDM transmission performance in the 1580-nm band between conventional binary nonreturn-to-zero (NRZ) coding with and without postdispersion compensation, and optical duobinary coding by computer simulation is described. From the numerical simulations, it is found that the optical duobinary coding has superior transmission performance to the conventional binary coding without any dispersion compensation, and that the difference in the transmission performance between two coding methods is very small even if postdispersion compensation at the optical receiver is applied to the NRZ coding method. Second, transmission performance between the conventional binary NRZ and the optical duobinary signals without any dispersion compensation is compared with the straight-line experiment over 500-km dispersion-shifted fiber. The experimental results reveal that the transmission distance with optical duobinary coding is doubled in comparison with that of the conventional binary NRZ signals. Finally, 16-channel, 10-Gb/s optical duobinary WDM signals in the 1580-nm band are successfully transmitted over 640 km (80 km/spl times/8) of DSF without any dispersion compensation or management.

Dispersion compensation for homodyne detection systems using a 10-Gb/s optical PSK-VSB signal

This letter demonstrates fiber chromatic dispersion compensation for optical homodyne detection systems, using a VSB filtered optical PSK signal. An optical PSK signal suitable for SSB and VSB filtering is newly employed. A 10-Gb/s optical PSK-VSB signal is transmitted with an optical carrier through 126 km conventional single-mode fiber. Penalty-free transmission is achieved by fiber chromatic dispersion compensation in the baseband with a 10-cm microstrip line.<<ETX>>

Waveform distortion due to stimulated Raman scattering in wide-band WDM transmission systems

The influence of stimulated Raman scattering (SRS) taking into account the random modulation and the walk-off effect on wide-band wavelength division multiplexing (WDM) transmission systems are studied theoretically. First, it is shown that power depletion due to SRS can be separated into average power loss and waveform distortion. The waveform distortion is evaluated for various types of pulse shapes and fibers, and simple equations for evaluating the waveform distortion due to SRS are derived. These equations can be easily applied for designing wide-band WDM transmission systems from the viewpoint of the SRS waveform distortion. We also compare results obtained from our method with those from the split-step Fourier method, and confirm validity of our method. Our method should be useful in the design of wide-band WDM transmission systems, in which SRS is expected to be a serious limitation.

An optical 90/spl deg/-hybrid balanced receiver module using a planar lightwave circuit

An optical 90/spl deg/-hybrid coherent receiver module is constructed that uses the planar lightwave circuit technique. The module consists of an optical 90/spl deg/-hybrid silica planar waveguide and two 14 GHz balanced O/E converters; the solder-bump technique is employed. For both sets of paths between the optical coupler and the twin-p-i-n photodetector pairs, the effective propagation time differences were reduced to within 4 ps. A 10 Gb/s BPSK homodyne detection experiment confirms the feasibility of the receiver.<<ETX>>

The influence of cross-phase modulation on optical FDM PSK homodyne transmission systems

A combination of optical frequency division multiplexing (FDM) and phase-shift-keying (PSK) homodyne detection can increase transmission capacity. However, phase sensitive transmission systems, especially repeatered ones, suffer from data-dependent optical amplitude fluctuation that is converted to phase fluctuation by fiber nonlinearity. The authors discuss how this data-dependent amplitude fluctuation affects the error rate performance of optical FDM PSK homodyne detection systems. If only the optical amplitude fluctuation induced by phase modulators is taken into account, the allowable power fluctuation to keep the power penalty at 0.5 dB at a bit error rate (BER) of 10/sup -10/ is below 0.17 mW for BPSK homodyne detection and 0.09 mW for QPSK homodyne detection. However, if only the amplitude fluctuation induced by the fiber chromatic dispersion is taken into account, the allowable number of repeaters to keep a 0.5-dB power penalty due to XPM at a BER of 10/sup -10/ is 1 for BPSK homodyne detection and below 5 for QPSK homodyne detection. >