All-Optical Demultiplexing of WLAN and Cellular CDMA Radio Signals

Subcarrier multiplexed transmission of multimedia radio signals over fiber is often done to deliver broadband services cost effectively. These signals need to be demultiplexed, preferably in the optical domain, to avoid loss and noise due to optical-to-electrical conversion. However, it is challenging to optically isolate signals at subgigahertz range due to the need for very narrow optical bandpass filters with high selectivity and low insertion loss and distortion. We developed such a novel subpicometer all-optical bandpass filter by creating a resonance cavity using two closely matched fiber Bragg gratings. This filter has a bandwidth of 120 MHz at -3 dB, 360 MHz at -10 dB, and 1.5 GHz at -20 dB. Experimental results show that this filter optically separates two RF signals spaced as close as 50 MHz without significant distortion. This paper analytically and experimentally investigates the scenario when this filter was used with 2.4-GHz (wireless local area network) and 900-MHz (cellular wireless) radio signals. The bit-error rate of the underlying baseband data is related to the linearity and isolation of the filter.