Abstract
The ideal behavior of communication system requires a single frequency carrier. In optical communication system, light is used as a carrier. Practical laser source has a finite linewidth due to variations in the frequency of operation, hence, resulting in undesired phase perturbations in the signal whereas the ideal requirement is the delta function spectral shape at the carrier frequency. The spectral shape gets broadened due to phase noise and is modeled as lorentzian shape. Linewidth is a measure of stability of laser phase noise with time. Coherent Optical Orthogonal frequency division multiplexing (CO-OFDM) along with the spectrally efficient Quadrature Amplitude Modulation (QAM) formats is emerging as one of the best solutions for future high speed fiber transmission systems. Though the coherent, receivers have advantages in terms of sensitivity and selectivity, laser phase noise is the main limitation of such systems as the laser phase noise further causes common phase rotation of all the subcarriers per symbol and also results in inter carrier interference. QAM formats are also susceptible to laser phase noise. Phase noise in coherent systems is governed by laser linewidth. Hence, it is very important to investigate the impact of laser linewidth in CO-OFDM systems. This paper investigates the tolerable laser linewidths for different QAM formats in a 40 Gbps COOFDM system.
Highlights
With the tremendous increase in internet usage along with the requirement of high bandwidth applications, high speed and long haul optical communication systems are in great demand
This paper investigates the tolerable laser linewidths for different Quadrature Amplitude Modulation (QAM) formats in a 40 Gbps COOFDM system
In this paper laser linewidth requirements are investigated for the mentioned modulation formats and SER floors for different laser linewidth and different modulation formats are investigated
Summary
With the tremendous increase in internet usage along with the requirement of high bandwidth applications, high speed and long haul optical communication systems are in great demand. High performance of CO-OFDM system requires narrow linewidth transmitter and local oscillator lasers along with the DSP algorithms for phase and frequency estimation at the receiver. The software defined capability in COOFDM system presents numerous advantages in terms of different modulation formats for different subcarriers, mitigation of various imperfections such as laser drift, channel dispersion, optimization of various OFDM system parameters by monitoring of transmission channel characteristics. Due to these reasons, coherent detection is the most suitable choice for high-speed, high-spectral efficiency, dynamically reconfigurable optical networks suitable for long-distance transmission. The values of tolerable linewidths for various modulation formats at different OSNRs are obtained for a target SER of 1e−3
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