This paper presents the fundamental principles and recent advances in the field of linearity enhancement of Mach–Zehnder modulators in microwave photonic systems using all-optical signal processing.A review of the fundamentals and applications that implement the linearity improvement is also provided.
Using differential detection,we perform polarization-multiplexing 160-Gb/s optical non-return-to-zero(NRZ) differential quadrature phase shift keying(DQPSK) signal transmission over 100-km standard single mode fiber at a bit error rate(BER) of less than 10-9.The enabling technology includes clock recovery,fine dispersion compensation,and polarization tracking for de-multiplexing.Furthermore,a hybrid clock recovery scheme is proposed.The scheme is realized with ordinary devices using an optoelectrical modulator to down-convert the clock frequency and a phase-locked loop for filtering,which can provide an indication signal that simultaneously monitors residual dispersion and tracking polarization.
A novel architecture of converged radio-over-fiber(RoF) and wavelength division multiplexed passive optical network(WDM-PON) system,namely RoF-WDM-PON,is demonstrated.20-GHz 1-Gb/s radio frequency(RF) signals and 1-Gb/s baseband(BB) signals are simultaneously generated and transmitted using optical carrier suppression(OCS) modulation techniques.The proposed scheme is compatible with the conventional RoF and PON system.25-km single-mode fiber(SMF) transmission is successfully achieved.
The principle of error vector magnitude (EVM) against modulator nonlinearity for vector modulation signal (VMS) transmission in radio-over-fiber (RoF) systems is theoretically and experimentally investigated. A highly linear modulation scheme is proposed and demonstrated using a single-drive dual parallel Mach-Zehnder modulator (MZM). This method improves EVM performance and enlarges the linear input dynamic range of the VMS transmission. An index of maximum allowable input power difference (MAIPD) that reflects the difference of upper input power limits between these two schemes is measured. An EVM limitation of 5% MAIPD has 5 dB. Both 16and 64-QAM results indicate that the proposed scheme supports VMS transmission better than the MZM one.
The relation between the phase shift and the mean optical power(MOP) output from a delay-line interferometer (DLI) port applied for phase-shift keying(PSK) signal demodulation is proven of a cosine law irrelevant to signal modulation condition.The variation amplitude of the MOP is proportional to the transition duration of the modulation pulses.This phenomenon is interpreted as the result of the statistical and waveform characteristics of the PSK.The conclusions verified by simulation and experiment are generalized to other modulation formats and then applied to phase detuning monitoring,delay time judgment of DLI,and independence of modulation data assessment.
