Wireless technology is a cost-effective means to bring broadband communications to both mobile users and home consumers; however, deploying next generation, multi-GHz wireless systems is currently too expensive. For these systems, photonic technologies can bring cost reduction as well as an increase in performance, mainly due to the ultra low-loss property of optical fibers. One approach to signal distribution is to capitalise on the vast fibre-optic distribution networks deployed within and between cities. A microwave carrier can be optically deployed from central offices to remote antenna sites using these optical links. This paper will discuss the generation of such a microwave carrier using a dual-wavelength, external-cavity laser (ECL).
Two different dual-wavelength ECL's, constructed with fiber-Bragg-gratings (FBG's), have been investigated. One uses a semiconductor gain chip with a dual-FBG acting as an external reflector. The other uses two similar dual-FBG reflectors on each side of a semiconductor optical amplifier (SOA). In both cases the wavelength separation between the gratings is 0.25 nm.
We will demonstrate that a dual-wavelength emission can be temporarily stabilized in the gain-chip ECL if a specific phase relation, between the external feedback from the FBG's and the residual feedback from the gain chip, is satisfied at both lasing wavelengths. The power of the RF beat signal generated by the dual-wavelength optical signal was typically 25 dB above the noise floor. The 3-dB linewidth of the RF signal was approximately 2 MHz and it can be tuned over a frequency range of 200 MHz. The physical mechanisms underlying the observed laser instability will be briefly discussed.