Optoelectronic mixing in heterojunction bipolar transistors

This Thesis is concerned with optoelectronic mixing in heterojunction bipolar transistors (HBTs). Optoelectronic mixing is classified into the electrically pumped and optically pumped categories. In electrically pumped mixing, the local oscillator (LO) source is applied electrically to an optoelectronic mixer which photodetects and converts a radio frequency (RF) modulated optical signal to an electrical signal at an intermediate frequency (IF). In optically pumped mixing, the LO is applied optically to an optoelectronic mixer which converts an electrical input signal at RF to another electrical output signal at IF. Optoelectronic mixing is required in many microwave over fibre and fibre-radio systems, and the conventional approach is to employ a separate photodiode for optical-to-electrical conversion and a microwave mixer for frequency conversion. In this work, optoelectronic mixing has been carried out using a two-terminal edge-coupled InP/InGaAs HBT for the first time and a three-terminal normal-incidence InP/InGaAs HBT. This single HBT approach to optoelectronic mixing offers a simpler alternative to the conventional method. The two HBT optoelectronic mixers have been experimentally characterised in terms of the mixing conversion gain, frequency response for the three-terminal HBT, signal-to-noise ratio and spurious-free dynamic range, obtained from two-tone third-order intermodulation distortion measurement using a two-laser approach. The distortion characteristics for HBT optoelectronic mixers are reported for the first time. Harmonic-balance techniques have been applied for the first time to model the conversion gain characteristics of the two electrically pumped HBT optoelectronic mixers. A large-signal equivalent circuit in the T-topology has been used for the three-terminal HBT. However a quasi-static model has been used for the two-terminal HBT because of the absence of an electrical base terminal which would otherwise allow the internal transistor parameters to be determined through DC and s-parameter measurements and the construction of a more physical HBT model. A number of fibre-radio system architectures employing HBT optoelectronic mixers are discussed. (author)