Yan Han

Coherent optical communication using polarization multiple-input-multiple-output

Polarization-division multiplexed (PDM) optical signals can potentially be demultiplexed by coherent detection and digital signal processing without using optical dynamic polarization control at the receiver. In this paper, we show that optical communications using PDM is analogous to wireless communications using multiple-input-multiple-output (MIMO) antennae and thus algorithms for channel estimation in wireless MIMO can be ready applied to optical polarization MIMO (PMIMO). Combined with frequency offset and phase estimation algorithms, simulations show that PDM quadrature phase-shift keying signals can be coherently detected by the proposed scheme using commercial semiconductor lasers while no optical phase locking and polarization control are required. This analogy further suggests the potential application of space-time coding in wireless communications to optical polarization MIMO systems and relates the problem of polarization-mode dispersion in fiber transmission to the multi-path propagation in wireless communications.

Phase-and-amplitude regeneration of differential phase-shift keyed signals using a phase-sensitive amplifier

DPSK phase-and-amplitude regeneration with a NOLM-based phase-sensitive amplifier is demonstrated experimentally. For a highly degraded input signal, maximum differential phase errors were reduced from 82 degrees to 41 degrees , while the SNR was improved by more than 5-dB. Differential phase Q-factor improvement was better than 6-dB. The PSA was operated free of excess noise due to stimulated Brillouin scattering by using a binary phase modulated pulse train as the pump. The impact of pump fluctuations on regeneration performance is clarified. The regenerated signal was characterized by measurement of the constellation diagram by linear optical sampling, giving the first directly measured evidence of DPSK phase regeneration.

Demonstration of phase-regeneration of DPSK signals based on phase-sensitive amplification

Amplification and simultaneous phase regeneration of DPSK signals is demonstrated using a phase-sensitive amplifier. Phase-sensitive gain is achieved in a Sagnac fiber interferometer comprised of nonpolarization maintaining, highly nonlinear fiber operating in the un-depleted pump regime. Both the pump and signal are RZ-DPSK pulse trains. The amplifier is capable of producing greater than 13 dB of phase-sensitive gain for an average pumping power of 100 mW, and easily reduces the BER of the regenerated DPSK signal by two orders of magnitude compared to the un-regenerated signal, corresponding to a negative power penalty of 2 dB. Careful optimization of the regenerator reveals much stronger BER Improvement.

Direct-detection optical differential 8-level phase-shift keying (OD8PSK) for spectrally efficient transmission

An implementation of optical differential 8-level phase-shift keying (OD8PSK) is proposed for spectrally efficient high capacity long-haul optical fiber transmission systems. Interferometric demodulation and direct detection at the receiver yield three output binary sequences identical to the three input binary sequences. This is accomplished by proper design of electrical encoding and optical encoding at the transmitter. Three optical encoding schemes are proposed with corresponding differential electrical encoding schemes. Numerical simulations are performed for a single channel transmission to evaluate the transmission performances of OD8PSK systems.

Direct detection differential polarization-phase- shift keying based on Jones vector

In this paper, differential polarization-phase-shift keying (DPolPSK) is proposed for digital lightwave communication, in which information is encoded in both polarization and phase. The DPolPSK symbols are demodulated by optical delayed interference and electrical multilevel detection, without using any polarization control or polarization selection at the receiver. Simulation results show that single-channel error-free transmission at 20 Gb/s (10 Gsymbol/s) through 25 spans of 100 km standard single-mode fiber is possible. The constellation diagram of DPolPSK can be represented by a set of Jones vectors. Hence DPolPSK can also be called differential Jones-vector shift keying (DJSK).

WDM Transmission over 320 km EDFA-Amplified SSMF Using 30 Gb/s Return-to-Zero Optical Differential 8-Level Phase-Shift Keying (OD8PSK)

Fiber transmission of optical differential 8-level phase-shift keying (OD8PSK) signals is demonstrated for the first time. Co-polarized 8 WDM channels of 10 Giga-symbol/s or 30 Gb/s return-to-zero (RZ) OD8PSK signals with a channel spacing of 50 GHz were transmitted over 320 km of standard single mode fiber (SSMF) with an EDFA spacing of 80 km. The BER of the worst WDM channel after transmission of 320 km was 2.3x10-5.

All-optical phase and amplitude regeneration of DPSK signals based on phase-sensitive amplification

All-optical regeneration of differential phase-shift keyed signals by an interferometric phase-sensitive amplifier is described theoretically and demonstrated experimentally. Measurements reveal simultaneous reduction of both phase and amplitude noise when the device functions as a limiting amplifier

Noise statistics of and BER estimation using demodulated signals for direct detection differential polarization-phase-shift keying

The noise statistics of optically demodulated signal of the recently proposed modulation format, namely differential polarization-phase-shift keying, are analyzed. It is found that, in the linear regime, the Gaussian approximation is reasonably accurate in estimating the BER. In the nonlinear regime, the noise of the demodulated signal is dominated by the effect of nonlinear phase noise and obeys chi-squared statistics with a degree of freedom of one. Gaussian approximation underestimates the BER in the nonlinear regime. A two-step BER estimation method accounting for both linear beat noise and nonlinear phase noise is proposed. The effectiveness of this two-step BER estimation method at all power levels has been established by comparison with direct error counting.

Experimental demonstration of coherent optical polarization multiple-input-multiple-output

Electrical polarization-demultiplexing of 10 Gb/s BPSK signals is experimentally demonstrated without using optical polarization control. The signal processing is based on the coherent polarization multiple-input-multiple-output (PMIMO) architecture, which can manage various polarization-related issues in optical communications.

Nonlinear phase noise in polarization-division-multiplexed DPSK systems

Nonlinear phase noise (NPN) in polarization-division-multiplexed DPSK systems is analyzed. It is found that the absolute NPN is inversely proportional to the square-root of channel spacing, while the differential NPN depends on the launch condition.