High-speed electro-absorption modulators for fiber optic communications

Abstract

Features such as large bandwidth, low drive voltage, compact size, and feasibility for monolithic laser integration make electro-absorption modulators (EAM) attractive candidates for ultra high-speed fiber-optical time division multiplexing (TDM). EAM with traveling-wave (TW) electrodes have successfully been demonstrated as a way to considerably increase the modulation bandwidth without sacrificing modulation efficiency. However, for reasonable modulation efficiency a low characteristic impedance (≈25Ω) has to be accepted. Termination with a matched load is necessary to benefit from the TW configuration. Thus, TWEAM with continuous impedance-matched transmission lines (TML) provide very high bandwidths, but suffer from high electrical return loss when using a 50Ω driver. A solution to this problem is to split up the modulator and insert passive TML segments between the active parts. The passive segments can be designed for a higher characteristic impedance than the active modulator parts with their inherently low impedance. In this way, the impedance seen at the electrical modulator input can be tailored for values that deliver optimum performance in combination with the available driving electronics (usually 50Ω). Only little bandwidth is sacrificed with the segmented design. Recently, we have demonstrated state-of-the-art performance of segmented TWEAM. These devices exhibit low electrical return loss (<-15dB) and a flat small signal modulation response in the characterized frequency range of 0.04 to 50GHz. 50Gb/s operation is demonstrated. An extinction of 10dB with 3Vp-p is achieved at 40Gb/s.