Broadband optical coupling into fibre picocellular modules using pHEMTs

A low-cost, compact downlink for future wireless communications has been constructed using a single FET in an optoelectronic self-oscillating mixer configuration. The circuit oscillates at 27.7GHz at the second harmonic (55.4GHz) and was used for transmission of data across a short radio path. The circuit exhibits conversion gain, which was found to be a strong function of bias conditions and intermediate frequency. Injection locking was necessary in order to stabilize the local oscillator and reduce phase noise to an acceptable level. The circuit suffers a 25dB loss in optically induced power due to the mismatch in the shape of the fibre compared to the exposed active regions of the chip HEMT. A 7dB improvement in optically induced power was found when using ungated HEMTs. Ungated HEMTs require no additional MMIC processing stages and could function as an optical input port at the expense of slightly increased transmitter complexity