All-optical demultiplexing of closely spaced multimedia radio signals using sub-picometer fiber Bragg grating

Subcarrier multiplexed (SCM) transmission of multimedia radio signals such as CATV (5-860 MHz), cellular wireless (900 MHz) and wireless LAN (2.4 GHz) over fiber is frequently used to deliver broadband services cost effectively. These multi-channel radio-over-fiber (ROF) links have interesting applications and can connect enhanced wireless hotspots that will support high speed wireless LAN services or low speed cellular services to different customers from the same antenna. The SCM signals need to be demultiplexed, preferably in the optical domain for many reasons. Prefiltering of SCM signals with fiber-based optical filters warrants the use of inexpensive photodetectors and increases network flexibility. However, realizing optical demultiplexing as sub-GHz level is challenging and thus necessitates optical filters with high selectivity and low insertion loss and distortion. We developed a novel sub-picometer all-optical bandpass filter by creating a resonance cavity using two closely matched fiber Bragg gratings (FBGs). 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 showed that the filter is capable of separating two radio frequency (RF) signals spaced as close as 50 MHz without significant distortion. When this demultiplexer was employed to optically separate 2.4 GHz and 900 MHz radio signals, it was found to be linear from -38 dBm to +6 dBm with ~ 25.5 dB isolation. There was no significant increment in the BER of the underlying multimedia data. Results verified that the fabricated narrow bandpass filter can be a potential candidate in demultiplexing of RF signals in networks based on subcarrier multiplexed schemes.