Numerical Simulations for the Analysis of the Dynamical Behaviour of an Ultra-Fast InP/InGaAsP Optoelectronic Modulator F. M. De Paola a# , V. d’Alessandro a , A. Irace a , J. H. den Besten b and M. K. Smit b a Department of Electronics and Telecommunication Engineering University of Naples “Federico II”, Naples, Italy b Opto-Electronic Devices Group, Faculty of Electrical Engineering Eindhoven University of Technology, NL-5600 MB Eindhoven, the Netherlands ABSTRACT In this paper we present a simulation strategy for the accurate prediction of the functionality of an InP based optoelectronic modulator. The device is composed of an InP/InGaAsP p-i-n diode embedded in a rib waveguide and a Mach-Zehnder interferometer. Finite Element Analysis for both semiconductor and optical equations solution is exploited. The presented numerical results, indicating a reverse bias voltage of 5.5 V for a 180° phase shift in a 2 mm- long device, are confirmed by measured data. Transient simulation performed report that this structure is suitable for 40 Gbit/s operation. Keywords: electro-optic effect, finite element analysis, InP/InGaAsP, Mach-Zehnder modulator. 1. INTRODUCTION InP-based optoelectronic integrated circuits [1-2] have been a very attractive playfield for researchers in recent years for many obvious reasons. First of all, InP technology offers the opportunity of monolithically integrating optical functionalities with electronic circuitry. Further, the will to compete with the inherently faster lithium niobate based devices with a cheaper technology has driven the efforts towards the design and fabrication of different kinds of optical modulators. Among the others, modulating structures based on interferential effects (such as Mach-Zehnder, Fabry- Perot or DBR), which exploit phase shift conversion into optical amplitude modulation, have proved to be the fastest and less power consuming. In this contribution, to extend the results obtained in a previously published paper [3], we will present the results of numerical simulations performed on an InP/InGaAsP optical modulator by means of a novel strategy involving the use of Finite Elements Method (FEM) for both electrical and optical operations. We will demonstrate how the usage of a state of the art electrical simulator together with an in-house developed code for semiconductor optical waveguide analysis can lead to highly reliable and fast-to-obtain results. 2. ELECTRICAL AND OPTICAL OPERATION With reference to Fig. 1, the electrical device consists of an InP/InGaAsP p-i-n diode. Starting from an InP substrate, two high-doped buffer layers are grown for optical confinement and ohmic contact purpose. An InGaAsP bulk-layer makes the guiding filmlayer while a p-doped cladding ensures the other contact. The difference in refractive index between the film layer and the substrate/cladding guarantees that the optical radiation is all confined where the depletion of the p-i-n diode under reverse bias condition takes place. The principle of operation of this device is very simple: by depleting the guiding layer it is possible to vary its refractive index due to plasma dispersion effect and band-filling effect; moreover, refractive index variation takes also place due to the Pockels and Kerr mechanism inside the material. The relationship between injected (depleted) free carriers, electric field and refractive index variation are recalled in the next Section. # email: fdepaola@unina.it ; phone: 0039-081-7683142 ; web: http://www.die.unina.it