A novel scheme to simultaneously generate an on–off keying (OOK) impulse radio ultra-wideband (IR-UWB) signal, a 60-GHz millimeter-wave (mmW) signal, and a baseband signal in the optical domain using a Sagnac loop is proposed and demonstrated. In the proposed system, a polarization beam splitter (PBS), a fiber Bragg grating (FBG), and two back-to-back connected polarization modulators (PolMs) are incorporated in the Sagnac loop. An OOK Gaussian pulse signal is modulated on a clockwise transmitted optical carrier by the first PolM and then converted to an OOK UWB impulse signal at the FBG serving as an edge filter, and the counterclockwise transmitted optical carrier is simultaneously modulated by a baseband signal and a 30-GHz mmW signal at the second PolM. By introducing a phase shift between the clockwise and counterclockwise optical carriers, the optical carrier of the 30-GHz signal is suppressed when applied to a polarizer. As a result, a frequency-doubled mmW signal at 60 GHz is generated by beating the two first order sidebands at a photodetector. Due to the velocity mismatch between the counterclockwise light wave and the clockwise microwave carrier, the OOK signal and the baseband signal can travel through the other PolM with negligible modulation; thus no interference from another signal would be introduced. Error-free transmission of a UWB signal at 2.5 Gbps and a wired baseband signal at 2.5 and 5 Gbps over a 25-km single-mode fiber is achieved. A frequency-doubled mmW signal at 60 GHz is also obtained.
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