Photonic generation and delivery of millimeter-wave reference signals

Abstract

Methods for the photonic generation of stable millimetre-wave reference signals are examined and compared. In particular, the generation of optical comb lines with microwave frequency separation in an amplified fibre ring and by sideband generation with an optical phase modulator are reported. Two comb lines at a millimetre-wave difference frequency can be selected using optical filters and heterodyned. The fibre ring can produce comb lines over a broad range of up to about 1 THz, whereas the sideband generation scheme is limited to frequencies of about 160 GHz. Both methods produce stable, low-phase-noise millimetre-wave signals useful as phase/frequency references. The transmission of such reference signals through optical fibre links of up to 9 km is also investigated. Differential dispersion effects can cause a power penalty in the received millimetre-wave signal, through the interaction of chromatic dispersion and SPM/XPM effects in the fibre, and through differential polarisation changes in the fibre causing non-alignment of the two optical fields at the photodiode. For the transmission of phase reference signals, the effects of differential dispersion, both chromatic and PMD, will cause phase variations in the received millimetre-wave signal, with the PMD effect being more serious due to its stochastic behaviour.