Optical millimeter-wave generation techniques: An overview

With evolving bandwidth demands, there is a pressing need for transfer from lower to higher frequency spectrum especially in millimeter-wave range. Due to restricted frequency response of electronic devices, high frequency signals cannot be generated economically and efficiently. Therefore, generation of millimeter-waves in optical domain is proposed as a viable solution, as distribution of these high frequency signals can also be done using optical fiber in efficient manner. The fundamental concept behind microwave/millimeter-wave generation is optical heterodyne,wherein, light signals from two independent lasers operating at different wavelengths are combined using optical combiner and detected using photodiode. The photodiode provides electrical beat signal whose frequency is equal to frequency difference between two laser sources. As the two lasers are running independently, there is no phase correlation between the two output light signals. Therefore, resultant electrical beat signal is quite noisy. Different approaches have been proposed to enhance quality of resultant electrical beat signal, with good stability and frequency tunability. We generally refer these proposed approaches as optical millimeter-wave techniques. In this paper, an attempt has been made to explain working principle of different optical millimeter-wave generation techniques. Special emphasis has been provided to optical millimeter-wave generation using Mach-Zehnder modulators. Further, dependence of extinction ratio on optical band generation at different biasing points has been discussed. The millimeter-wave based radio-over-fiber systems have been classified in simplex, duplex and multichannel transmission networks. Also, modulator based systems with tunable frequency multiplication factor have been discussed.