Complex Field Modulation in Direct Detection Systems

Abstract

Even though fiber optics communication provides a high bandwidth channel to achieve high-speed data transmission, there is still demand for higher spectral efficiency, faster data processing speeds with reduced resource requirements due to ever increasing data and media traffic. Also, lately the demand for online streaming because of remote working has increased significantly. Various multilevel modulation and demodulation techniques are used to improve spectral efficiency. Although spectral efficiency is improved, there are other challenges that arise. Such as requirements for high speed electronics, receiver sensitivity degradation, chromatic dispersion, operational flexibility, effects of nonlinearity impairments etc. Here, we investigate complex bandwidth efficient field modulation and coding techniques to improve spectral efficiency while reducing the digital signal processing (DSP) resources required for implementations using FPGAs or ASICs and compensation for linear and nonlinear impairments that appear in fiber optic communication systems. In this dissertation we investigated and developed solutions for various limitations and impairments in a direct-detection transmission system with complex field modulated optical signal. The solutions that we developed to compensate the fiber optical impairments can be implemented using DSP either at transmitter side or the receiver. By employing DSP based approach to mitigate the optical impairments and limitations we can achieve more flexibility in the optical transceivers while achieving higher spectral efficiency. We proposed and demonstrated digital-analog hybrid subcarrier multiplexing (SCM) technique which can reduce the speed requirement of high-speed digital electronics such as ADC and DAC, while providing wideband capability, high spectral efficiency, operational flexibility and controllable data-rate granularity. Hybrid SCM is a modular approach in which multiple digitally generated subcarriers are aggregated through RF oscillators and IQ mixers for frequency up- and down-conversions. Next, to achieve maximum spectral efficiency with conventional Quadrature Phase Shift Keying (QPSK) we need highly spectral efficient Nyquist filters which require large amount of FPGA resources for digital signal processing (DSP). Hence, we investigated Quadrature Duobinary (QDB) modulation as a solution to reduce the FPGA resources required for DSP while achieving spectral efficiency of 2bits/s/Hz. We compared QDB with QPSK in a digital-analog hybrid subcarrier multiplexing system and we show that with minor changes in transmitter design we can achieve 2bits/s/Hz spectral efficiency, which is same as the Nyquist QPSK with relaxed resource requirements for DSP. We investigated and developed a solution to digitally compensate the nonlinearities introduced by semiconductor optical amplifiers (SOA). In a field modulated direct-detection system, due to square-law detection of the photodiode, leads to an interference called signal-signal beat interference (SSBI). To eliminate SSBI we can use Kramers-Kronig (KK) receiver as we can retrieve the phase information from the direct detected optical signal for the class of signals called as minimum phase signals. However, it is under the assumption that the entire transfer function of our optical transmission system is linear except for photodiode. However, when the system transfer function is non-linear due to SOA nonlinearities when operated in gain saturation region. By using electrical forward propagation method for pre-compensation of nonlinearities caused by SOA we show that we can simultaneously restore the efficiency of KK receiver and as well achieve electronic dispersion post-compensation.