Adaptive distortion compensation with integrated optical finite impulse response filters in high bitrate optical communication systems

In high-bit-rate optical transmission systems, distortions due to dynamic chromatic dispersion, polarization mode dispersion, and power changes are larger than the distortion tolerances of the system. To meet the tolerances and the desired quality of service, an adaptive equalizer is necessary. We demonstrate the capabilities of planar lightwave circuit integrated optical finite impulse response filters for mitigating distortions of the transmission channel, and we investigate two adaptive equalization approaches. The first approach uses an adaptive feedback generated from electrical spectrum monitoring; the second one uses intersymbol interference minimization with a least mean square error algorithm. We successfully demonstrated adaptive equalization of chromatic dispersion, self-phase modulation, and polarization mode dispersion, as well as combinations of these distortions.

I. Introduction

IN current and next-generation high-speed optical transmission systems, solutions for an adaptive distortion compensation are of high interest. With increasing bit rates and an increasing complexity of the optical layer, the allowed distortion tolerances of the system are decreasing while the sources for signal distortions are increasing. The temporal and frequency-varying changes of the transmission channel due to chromatic dispersion (CD), polarization mode dispersion (PMD), and nonlinear distortions, such as self-phase modulation (SPM), easily exceed the tolerable amount for an error-free operation of the transmission system. Reasons for the changing transmission characteristics are, e.g., temperature variations. The dispersion coefficient of the optical transmission fiber is temperature dependent (e.g., zero dispersion wavelength shift: 0.03 nm/°C for Tera Light fiber, [1]). The birefringent characteristics change with external induced stress and temperature, which leads to PMD variations. Not every path throughout the network has equal characteristics; therefore rerouting changes the transmission characteristics of a transmission link. Power variations influence the nonlinear characteristics of the transmission channel. An increased optical power leads to increased signal distortions due to nonlinear fiber effects. However, to compensate for all these time- and frequency-varying distortions and to meet the system tolerances, a static compensation approach is not sufficient anymore [1]. An adaptive equalizer is necessary.