Effect of High Frequency Gain on the Performance of Optical Costas Loop in Face of Loop Delay

Abstract

Objectives: Optical costas loops (OCLs) are widely used in optical communication as homodyne receivers. Due to use of different electronic counterparts and fibre optic cable inherent loop delay always presents in the system. The steady state behaviours of optical costas loop are highly affected by the presence of loop delay. Different nonlinear behaviours may be observed due to presence of delay. There are two main objectives of this article. Firstly, how OCL can be operated as stable receiver up to some large value of loop delay by using a proportional plus integrating type loop filter (LF). Secondly, how a controlled chaotic optical signal can be generated from OCL by choosing the system parameters in correct manner. Methods: To investigate system behaviours of OCL both analytical and numerical methods have been used. Stability analysis of OCL has been done by Routh-Hurwitz method. From stability analysis, it is possible to predict the stable and unstable behaviour of the OCL in presence of delay and how system stability can be improved by high frequency gain value of LF. Numerical methods have also been used to solve the nonlinear equation of OCL to observe real time behaviours. Findings: Analytical findings show that loop stability can be improved by increasing the value of high frequency gain of LF. For large value of loop delay, chaotic oscillation of phase error may be observed in OCL. The chaotic oscillation can also be controlled by high frequency gain with certain extent value of loop delay. All the numerical findings have been properly verified with numerical results. Novelty: This article describes how effects of loop delay can be controlled to run OCL in steady as well as in unsteady state. From designer's point of view this study would be helpful to choose correct values of system parameters to improve the performance of OCL in optical communication. It also gives the idea for generation of chaotic optical signal, which is used in secured communications.