Waveguide-integrated Photodetector For 60 GHz Microwave Transmission at 1.55 /spl mu/gm

Abstract

A waveguide-integrated photodetector with 0.3 N W responsivity and 70 GHz bandwidth is presented. Millimeter-wave transmission experiments were successfully performed at 64 GHz, demonstrating linear operation up to +10 dBm optical power level. Introduction In view of future cellular broadband communications networks operating in the 60 GHz band and therefore allowing extensive frequency reuse it will be advantageous to distribute the modulated microwave signal via optical fibres to the numerous pico-cells. This system concept allows the remote generation of high quality microwave carriers in the control stations and leads to reduced costs of the base stations in the pico-cellslV2. These base stations will have to be capable of receiving this optical signal and to convert it into a microwave signal, which simply has to be further amplified and fed into an antenna3. For noise considerations an optical power level as high as possible is desired. Therefore, ultrafast photodetectors are required as optic/microwave converters, which provide linear operation up to high input power levels. In the literature several approaches have been reported to realise ultrafast photodetectors. However, in surface illuminated conventional p-i-n photodiodes or metal-semiconductormetal photodetectors4 a high bandwidth entails a limited responsivity. On the other hand, structures with illumination perpendicular to the electric field vector, such as waveguide detectors536 or waveguide integrated photodiodes7 provide high responsivities at ultrahigh frequencies. Furthermore, the uniform distribution of the light absorption over an extended region in these detectors leads to the capability of handling high optical input powers without suffering from carrier induced field screening effects. In this paper the suitability of waveguide integrated p-i-n photodiodes for highly effective conversion of microwave signals in the 60 GHz band at high power levels is demonstrated with respect to application as optic/microwave converters in the base stations of future picocellular communications systems. Detector structure and fabrication The photodetector is formed by a p-i-n diode evanescently coupled to a feeding strip loaded waveguide. The layer stack is grown in a single run by MOVPE on a semi-insulating 1nP:Fe substrate and consists of a 1000 nm thick and a 200 nm thick Ga1nAsP:Fe (Ag= 1.06 pm) waveguide slab and rib layer, respectively, separated by a thin 1nP:Fe etch stop layer. The detector layers start with an n-doped Ga1nAsP:Si (Ag= 1.3 pm) contact layer, followed by an undoped GaInAs absorber layer with a thickness of 400 nm. Selective Zndiffusion in an RTP-furnace is used to form the pn-junction at a depth of about 100 nm. This layer structure was optimised to give a large effective absorption of the evanescently coupled