The optical waveguide entry facet of high-speed, 40 GHz waveguide photodetectors is usually obtained by manual cleaving, which has limited accuracy (± 10 μm) and reduces fabrication yields. In our new fabrication process, the waveguide facet is obtained with Chemically Assisted Ion Beam Etching (CAIBE). Length is therefore precisely controlled by photolithography. The antireflection coating is also deposited collectively on the whole wafer, which further reduces costs. The bandwidth of the photodiodes is 50 GHz, and their optical responsivity is 0.6 A/W at 1.55 μm wavelength. Other techniques, such as Inductively Coupled Plasma Etching (ICP), were also investigated for reducing leakage current.
This paper presents recent developments in the packaging of 40GHz receivers for radio over fiber applications. First, we present a hybrid 40GHz photoreceiver module. It integrates 40GHz waveguide photodiode technology and narrow band MMIC amplifier. We give key elements for the design and the realisation of such photoreceiver. With an input optical power of 0dBm, an RF ouptut power of -3dBm is obtained in the 38-44GHz frequency range with a flatness of ±0.5dBm. The maximal input optical power is +6dBm. Secondly, a small footprint 40GHz photoreceiver scheme is presented. It includes high optical coupling tolerances 40GHz tapered waveguide photodiodes and the same type of MMIC amplifier than used in the hybrid package. The interconnect of the several elements (MMIC, capacitors, photodiode biasing network) uses Microwave High Density Integration (MHDI) technology on a Si substrate. The photodiode is flip-chip mounted on the same substrate. This technology is very promising for packaging of optoelectronic devices, since it combines compactness and reproducibility thanks to wire bonding suppression.