To allow optical fibers to transmit both telecom (1550 nm) wavelength-division-multiplexed (WDM) signals and millimeter-wave radio signals to serve broadband wireless communication systems, the WDM signals can be upconverted to the millimeter-wave region for simultaneous transmission with radio-over-fiber (ROF) signals. This hybrid network is possible using external intensity, phase, or cross-absorption modulation techniques; however, all of these methods suffer high conversion loss and polarization sensitivity. But by exploiting four-wave mixing (FWM) in a nonlinear fiber medium, researchers at NEC Laboratories America (Princeton, NJ) and the Georgia Institute of Technology (Atlanta, GA) have succeeded in demonstrating a low-loss, all-optical, polarization-insensitive hybrid telecom/ROF network.
The process begins with two pump beams generated by a single laser source and locked in phase with each other. Through FWM in a nonlinear fiber medium between the WDM signal beam and the two pump beams, two new peaks (locked and with the same polarization) are generated. Each new peak is a copy of the original signal, which is realized by polarization-insensitive wavelength conversion, and there are two identical new peaks for each WDM channel. After removing the original signals with an optical filter (an optical interleaver is optimal), only the converted new peaks exist. When the new converted peaks are detected by a high-speed photodiode, they are beat together and generate upconverted 60 GHz millimeter electrical signals. Contact Jianjun Yu at [email protected].