Entwicklung und Realisierung von Wanderwellen-Photodetektoren für Hochfrequenz-Übertragungssysteme
This work describes the development and the realisation of travelling-wave photodetectors for the millimeterwave regime. The used travelling-wave concept for the realised optoelectronic devices omits RC-time limitation, which is valid for traditional high-speed photodetectors. To overcome this limitation, the travelling-wave photodetector uses wave propagation effects for the optical input, the optoelectronic conversion, and for the electrical output. The travelling-wave photodetector is fabricated in the InGaAlAs material system lattice-matched to InP using a molecular beam epitaxy. This material system is suited for operation at 1.3 µm to 1.55 µm optical wavelength. The realised devices consist of a coplanar Schottky-contact electrical transmission line. The optical waveguide is located underneath the centre conductor inside the mesa. The interaction between optical waves and electrical waves is realised using an optical absorbing layer. A reverse-biased Schottky-diode between centre conductor and outer conductor separates electrons and holes which are generated in the optical absorbing layer generating a current on the electrical transmission line. This work describes several tools, which are developed for the simulation of travelling-wave photodetectors. The simulation tools allow the calculation of the local optical intensity inside the optical waveguide with BPM and analytical methods. The optical intensity is used to calculate the distributed photo current, which is generated in the distributed photo current source due to carrier generation. The current of this distributed current source is finally used to calculate the currents and voltages of the distributed equivalent circuit of the coplanar electrical waveguide. The realised simulation tools allow a complete description of the optoelectronic conversion of the travelling-wave photodetector including optical input, electrical output, and optoelectronic conversion including wave propagation effects. The simulation tools are used to optimise the travelling -wave photodetector. The simulations are validated using results from different characterisation methods. Two optoelectronic measurement setups for generation of optical heterodyne signals with beating frequencies of up to 60 GHz are realized and used for this work. Several new material systems are developed for the fabrication of travelling-wave photodetectors. The realised devices are the first InP travelling-wave photodetectors with an absorption length of approx. 900 µm for operation at 60 GHz with 1.3 µm to 1.55 µm optical wavelength. Saturation effects are not visible at 60 GHz for optical input powers of up to 12.5 dBm due to the distributed absorbing layer. The realised travelling-wave photodetectors have a efficiency of 0.23 A/W at 1.55 µm optical wavelength and operation at 60 GHz. A comparison with other high-speed photodetectors shows, that the travelling-wave photodetector belongs to the most powerful optoelectronic converters in the millimeterwave regime.
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