Diode mixers with optical local oscillator injection
The results of an investigation into the use of diodes as mixers with optical local oscillator injection are presented. Such 'optically pumped mixers' might be employed in complex microwave systems where the optical distribution of the local oscillator is attractive. The photocurrent generation, through which the optical local oscillator is coupled into the device, the frequency conversion and noise mechanisms in optically pumped mixers are all investigated. Computer models of optically pumped mixing have been developed, and are shown to give very good agreement with experimental measurements. A novel optically pumped mixer structure using the tunnelling nonlinearity in a metal contact to heavily doped gallium arsenide has been investigated theoretically and experimentally. The prototype device has been found to be limited to relatively low frequencies (\simeq 100MHz) as an optically pumped mixer, although for a device of smaller area low conversion loss may be achieved at frequencies up to 1GHz. The structure is limited by the large capacitance per unit area, the generally poor responsivity, and the dependence of the responsivity on relatively slow, minority-carrier diffusion, current transport mechanisms. The results from the above investigation have enabled an improved, Mott diode structure to be proposed for optically pumped mixers. Predictions from the accurate computer model indicate efficient operation should be achievable at X-band frequencies and beyond using gallium arsenide lasers of moderate output power (\geqslant 3mW average). Similar performance at lower power levels should be achievable with shorter wavelength illumination. Due to the simplicity of using a single device, it is shown that optically pumped mixers may be more attractive than photodetector-mixer combinations in many complex microwave systems.