Browsing by Author "Klinger, Sandra"

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Germanium pin photodiodes on silicon and photonic integrated circuits : components for high-speed optical data communications
(2011) Klinger, Sandra; Berroth, Manfred (Prof. Dr.-Ing.)
Data communications based on optical fibres is already well established in our modern long distance communication networks. Due to the high data rates that can be achieved with optical data communications, light is nowadays omnipresent, even at smallest dimensions: high-speed optical communications is not only relied on regarding the communication between chips, but also between components on the chip. Most of the signal processing, however, and especially data storage is still realised in the electrical domain. This means that fast conversion from electrical to optical signals – and vice versa – is necessary at the transmitter and receiver. Hence, this work concentrates on optical links, with emphasis on receiver circuits. To achieve the desired high data rates of 100 Gbit/S and more with the already installed fibre networks, high order modulation and multiplexing schemes are applied. They require optical signal processing on the receiver side, which is provided by photonic integrated circuits (PICs). Such PICs are designed at the Institute of Electrical and Optical Communications Engineering, and they are externally fabricated in the material system SOI (Silicon on Insulator). Because of the high refractive index contrast of Silicon and Silicon Dioxide and the transparency at the telecommunication wavelengths 1310 nm and 1550 nm, SOI is well suited for the aspired compact signal guiding. Furthermore, it is compatible to the Silicon based Complementary Metal Oxide Semiconductor (CMOS) technology, in which fast mixed-signal integrated circuits can be realised. Among the PICs that are needed for fast integrated optical receivers are coupling elements and simple waveguide structures. The coupling of light from fibre to the chip that contains the PICs must be low-loss and simple. In this work, one-dimensional binary gratings are designed for coupling, and their transmission characteristic is measured. The grating couplers show a measured maximum coupling efficiency of about 37%. The grating couplers are optimised for transverse-electrical polarisation and a wavelength of 1550 nm and 1310 nm. Polarisation and wavelength dependence play a significant role considering coupling elements. With regard to waveguide structures, also single-mode operation as well as stray and bending losses must be considered. Different types of waveguides, like strip and rib waveguides, can be compared with each other. In this work, these aspects are studied theoretically. After the processing by the PICs, the optical signals must be converted. With Silicon as basis, Germanium is a suitable detector material: its absorption coefficient at the telecommunication wavelengths is sufficiently high; additionally, Germanium can be integrated into Silicon. However, the lattice mismatch between both semiconductor materials must be taken into account. In a common project with the Institute for Semiconductor Engineering (IHT), photodetectors with a 3 dB bandwidth of 49 GHz are demonstrated. The according Germanium pin photodiode is realised as a vertical two mesa structure. It is grown on Silicon at the IHT, with an IHT-process using a virtual substrate. Main focus of this work considering the project cooperation lies on simulation and measurement based characterisation as well as on layout-related optimisation. This optimisation mainly refers to the low responsivity of the photodiodes, which is due to the small dimensions of the structure in favour of a high bandwidth. The application of mirror layers and diffraction gratings is theoretically investigated. With such structures, responsivity can theoretically be tripled. Mirrors and gratings are, however, very resonant structures. This work also deals with the development of simulation models. They are needed to simulate the photodiodes together with adjacent electrical circuits. DC and small signal analysis are primarily examined. For further characterisation of the photodiodes, measurements in the time domain are carried out. They show bit rates of at least 25 Gbit/s. The signal that is available after the opto-electrical conversion must be pre-processed, e.g. amplified, before the actual signal processing. Therefore, a simple differential limiting amplifier in a Silicon Germanium bipolar process technology is designed and characterised in this work. Due to the high transit frequency of the process in use, a data rate of 50 Gbit/s is achieved.