Coherent 16-QAM bidirectional fiber-wireless hybrid access network with optimal use of the optical local oscillator signal

Abstract

This work reports on the optimal use of an optical local oscillator signal from a gain-switched vertical cavity surface emitting laser (VCSEL) optical frequency comb (OFC) in wavelength division multiplexed (WDM) fixed-wireless access networks. The system was tested for its ability to transmit full-duplex error-free data signals to and from a fixed/wireless user. The idea of wavelength reuse was used for the first time to our knowledge, for simultaneous upstream, downstream, and photonic up-conversion to high-frequency RF carrier. Two optical carrier signals were filtered from the generated OFC. One of the two optical carriers was modulated with 16-QAM 28-Gbps data signal. The second optical carrier was amplitude-modulated with an 8.5-Gbps on-off keying (OOK) data signal. These two modulated optical carriers were multiplexed and transmitted over 24 km of standard single mode fiber (SSMF). At the remote site, the second optical carrier carrying the 8.5-Gbps data was selected, demodulated, and recovered. The recovered optical wavelength was used both for photonic up-conversion and for upstream data transmission. Error-free performance was obtained for both downstream, upstream, and wireless data transmission. The wireless carrier obtained after photonic up-conversion using the recovered wavelength was transmitted over a 4-m wireless distance. An EVM of 7% was obtained for the 16-QAM signal. These results are attractive as they demonstrate and motivate the possibility of using gain-switched VCSELs as OFC sources in future fixed-wireless WDM access networks. Wavelength reuse improves the spectral efficiency in the optical and RF domain. The use of VCSELs results in power-efficient access networks.