Katsushi Iwashita

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>>

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>>

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. >

Damping factor influence on linewidth requirements for optical PSK coherent detection systems

Spectral linewidth requirements for optical phase-shift-keying (PSK) coherent detection systems are found to depend on the phase-locked loop (PLL) parameters. Until now, the damping factor of the PLL has been assumed to be 1/ square root 2 when deriving the required spectral linewidth of a light source, because it is at this value that an electrical PLL offers near optimum performance in many cases. By increasing the PLL damping factor above 1/ square root 2, it is shown that there exists a maximum value of the required linewidth that achieves a received optical power penalty of 1 dB at a bit error rate of 10/sup -10/. The required beat linewidths so obtained are 50% larger than previously reported results (which assume a damping factor of 1/ square root 2). As for PLL frequency acquisition performance, it is shown that raising the camping factor above 1/ square root 2 does not seriously affect the hold-in limit or the pull-in limit. It is also shown that the normalized loop gain that optimizes PLL performance is roughly one half the normalized loop gain at which the PLL oscillation commences. >

Optical PSK synchronous heterodyne detection transmission experiment using fiber chromatic dispersion equalization

This letter demonstrates an 8-Gb/s optical PSK (phase shift keying) synchronous detection transmission experiment using external cavity laser diodes. A 188-km 1.3- mu m zero-dispersion fiber is used as the transmission medium at the wavelength of 1.55 mu m. Fiber chromatic dispersion is successfully compensated with a microstrip-line delay equalizer.<<ETX>>

Cross-phase modulation influence on a two-channel optical PSK homodyne transmission system

The residual amplitude modulation of adjacent channels causes cross-phase modulation which is responsible for degradation in the error rate performance of a phase detection system. This influence on a two-channel 10 Gb/s optical phase-shift-keying (PSK) homodyne transmission system was experimentally evaluated. A penalty was observed for more than 6 dBm fiber input power with 28% residual amplitude modulation passing through a 100 km dispersion shifted-fiber. Since cross-phase modulation does not require phase matching, which is required for four-wave mixing, the optical frequency range in which the influence occurs is broadband. These results show that residual amplitude modulation in synchronous optical frequency division multiplexing transmission systems severely limits the fiber input power.<<ETX>>

Phase Noise Cancelled OFDR With cm-Level Spatial Resolution Using Phase Diversity

Optical frequency domain reflectometry (OFDR) has high spatial resolution and high sensitivity compared with optical time domain reflectrometry, provided the phase noise of optical source is cancelled. A new method was proposed to cancel phase noise in the OFDR. The proposed method detects short-distance and long-distance breakpoints with high spatial resolution. Double side band with a suppress carrier modulation was used with phase diversity detection. Experiments confirmed that the proposed method was achievable. Breakpoints at approximate distance 120 m were detected as short-distance breakpoints and ~20 as long distance. Performance of the proposed method has been further demonstrated by measuring expansion in fiber length due to change in temperature.

Linewidth influence on 10-Gb/s optical PSK homodyne detection

Optical phase-shift-keying (PSK) homodyne detection offers the best sensitivity of any binary signaling technique and requires only the same electrical bandwidth as the bit rate. Therefore, it is suitable for multigigabit coherent transmission systems.1,2 However, it requires lasers with extremely narrow linewidths. Laser linewidth requirements depend on the configuration of the phase-locked loop (PLL). A decision- driven PLL can tolerate a larger laser linewidth (e.g., 3.1 × 10−4 times the bit rate 3) than can a pilot-carrier optical PLL, which requires a linewidth of 6 × 10−6 times the bit rate.4 Moreover, the requirement is restricted by the PLL propagation time.5 PLL propagation time is important for high-speed homodyne detection systems using broad-linewidth laser diodes. This paper reports the linewidth requirements for 10-Gb/s PSK homodyne detection in a decision-driven PLL, taking into consideration the loop propagation delay.

A novel wavelength converter based on optical single-sideband modulator and arrayed waveguide grating

A novel wavelength converter composed of an optical SSB (OSSB) modulator and AWGs is proposed. A 100-GHz wavelength conversion using 25GHz channel spacing AWG with four conversions at OSSB is performed at 5Gbps NRZ signal.