Abstract

The wide developement of analog radio frequency (RF) optical fiber links for optically fed wireless communication systems and phased array antennas is partly conditioned by the availability of high speed, high efficiency photodetectors which can also deliver high RF power. Lumped or travelling-wave waveguide PIN photodetectors (WGPDs) [1,2] can achieve both high quantum efficiency and high bandwidth without any limitations in bandwidth efficiency product as in conventional PD. However in this WGPDs, carriers are generated in the first ten microns. As a consequence, under high illumination, the high photocarriers density in this small surface area, by space charge effects, modifies the internal electric field which in turn governs the carriers transport. This field screening induces non linearities of the electrical response of the PD for high optical power, and limits the maximum RF power which can be delivered. For higher power capabilities, diluted travelling-wave detectors (TWDs) have been proposed [3,4]. In order to further improve the power handling of the detectors and reduce the space charge effects, we propose and analyze a new microwave WGPD which distributes absorption over the maximum junction area available. The structure is based on a diluted singlemode WG with coupling and quantum efficiencies > 90% and can handle both high optical and electrical power. The photocarriers density is maintained constant along the whole propagation length of the diode (instead of being exponential) by controling and tailoring the modal absorption coefficient αΓ(z) along the axis of propagation (Fig.l). This design applies to lumped WGPDs as well as TWDs. For the distributed absorption WGPD presented here, the depleted region consists of a thin InGaAsP active layer sandwiched between transparent WG and contact layers and αΓ(z) can be varied by a factor of 10 by changing the WG ridge width from 2 to 4 µm. For a same junction area, the maximum RF output power can be 4 to 9 times higher for distributed absorption WGPDs as compared to diluted WGPDs [3,4] (lumped or TWDs). The structures are compared by using a drift-diffusion model which takes into account the heterointerfaces, device depolarization due to current flow in the load circuit and calculates the electric field in the depleted region in the linear regime (50 < E< 250kV/cm). At a given cut off frequency the figure of merit F is defined as the ratio of the maximum RF power to the incident optical power. F is optimized by choosing the maximum depleted region thickness (transit time limitation), the maximum junction area (RC time constant limitation for lumped detectors and 5 times more for TWDs) and by varying the reverse voltage and the optical power. Details of the design and performances of distributed absorption WGPD with coplanar transmission line will be presented in the conference.

© 1996 IEEE