Experimental Study on FPGA Based Nonlinearity Reduction in a Laser Diode
Tony Jose1, Vijayakumar Narayanan2

1Tony Jose, Fiber Optics & Photonics Laboratory, Department of Electronics & Communication Engineering, College of Engineering Trivandrum, Thiruvananthapuram (Kerala), India.
2Vijayakumar Narayanan, Department of Electronics & Communication Engineering, Government Engineering College, Barton Hill, Thiruvananthapuram (Kerala), India.  

Manuscript received on 13 June 2017 | Revised Manuscript received on 20 June 2017 | Manuscript Published on 30 June 2017 | PP: 145-148 | Volume-6 Issue-5, June 2017 | Retrieval Number: E5022066517/17©BEIESP
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© The Authors. Blue Eyes Intelligence Engineering and Sciences Publication (BEIESP). This is an open access article under the CC-BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Abstract: This work aims to reduce the Total Harmonic Distortion (THD) in a laser diode by predistorting the input signal. A predistorted signal is generated using FPGA and this signal is fed to the inherently nonlinear laser diode. The combined effect of predistorted signal and the nonlinear diode characteristics render an overall linear characteristics for the optical transmitter. The design of the FPGA predistorter is the crux of the present work. The optical power versus injection current graph is plotted for the laser diode and based on the characteristics, certain mathematical manipulation is performed to obtain an analytical expression to faithfully reproduce the entire curve. In order to design the predistorter, the inverse function of the nonlinear expression is computed. Based on these data, a lookup table based VHDL code is downloaded onto the Field Programmable Gate Array (FPGA) chip. The FPGA operates entirely in the digital regime, thus suitable interfacing circuitry were rigged up. The harmonic distortion is studied using spectrum analyzer.
Keywords: FPGA, Harmonic Distortion, Laser Diode, Nonlinearity, Predistortion.

Scope of the Article: Software Engineering Case Study and Experience Reports