Investigation of Linear and Nonlinear Pre-Equalization of VCSEL

Conference: Breitbandversorgung in Deutschland - 11. ITG-Fachkonferenz
03/29/2017 - 03/30/2017 at Berlin, Deutschland

Proceedings: Breitbandversorgung in Deutschland

Pages: 5Language: englishTyp: PDF

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Shivashankar, Veena; Kottke, Christoph; Jungnickel, Volker; Freund, Ronald (Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institute, Einsteinufer 37, 10587 Berlin, Germany)

Current trends for short-reach data interconnects are determined by the necessity of higher bitrates as well as the need of reducing the power consumption. Attention is focused on optical solutions to face these challenges. In particular, the use of directly modulated 850 nm vertical cavity surface emitting lasers (VCSEL) as an inexpensive reliable light source with wide bandwidth and low-energy consumption, together with the use of advanced modulation techniques, like pulse amplitude modulation (PAM) or discrete multi-tone (DMT) to increase the spectral efficiency are considered as keys to achieve low-cost fast optical interconnects. However, these advanced modulation techniques require additional signal processing to compensate the inherent linear and nonlinear VCSEL properties. In this paper, a simulative and experimental investigation of linear and nonlinear compensation of the VCSEL characteristic is presented. We show that nonlinear pre-equalization of VCSEL-based transmitters can be used to improve the sensitivity of IM/DD based links, however with limitations when high-bandwidth signals are considered. Investigations of the performance of equalizers designed using Volterra series for compensating linear and nonlinear distortions that arise in VCSELs are carried out. The simulation environment is built using the simulation tools Matlab for the signal processing and VPItransmissionMaker for the optical link. The designed system is using 8-PAM modulation with multiple symbol rates. A detailed VCSEL model, based on semi-empirical rate equation, is utilized in the simulations. The model takes into account effects like relaxation oscillations, thermal impedance or thermal rollover. To verify the simulative results, the experimental realization of a pre-compensated high-speed optical transmission, based on PAM is also shown.