Improvement of linearity in EAM by composite QWs

We propose a novel structure that improves the linear characteristic of a multiple quantum well (MQW) electro-absorption modulator (EAM). The transfer functions of EAMs composed of 8 nm, 9 nm and 10 nm quantum well (QW) were obtained and the combined transfer functions were investigated We found that the IMD3 of composite-type QW in which 12 nm : 8 nm = 1 : 3 was suppressed about 20 dBc compared with single-type QW.

1. Introduction

RF-photonics links require a large spurious-free dynamic range (SFDR) which depends on the nonlinear characteristics of optical transmitter. As an E/O converter, an external modulator is advantageous for a photonics link because one could avoid large nonlinear distortion by the frequency chirping that is common in a direct modulating laser diode. An electro-absorption modulator (EAM) is well known to be a good candidate for a key component in RF-photonics link^[1] due to its small size, low driving voltage, large bandwidth and potential for monolithic integration with other devices like a photodiode^[2] or a laser diode. It has an exponentially decaying transfer function behavior and has more complicated nonlinearity than a LiNbO3 modulator. Many methods to linearize an EAM were presented; an optical feedforward technique^[3], electrical predistortion method^[4] and dual parallel modulation scheme^[5], Although these were demonstrated improved SFDR and low distortion, they needed predistortion electronic circuit which had response limit and another optical source or a modulator.