Sub-carrier multiplexing in broadband optical networks

DETAILED DESCRIPTION In accordance with one embodiment of our invention, a WDM fiber-optic network includes a unique laser transmitter which enables signals to be routed at three hierarchical levels: at one level discrimination among signal paths is based on N WDM wavelength channels, at another level discrimination is based on m AM subcarrier frequency channels, and at a third level discrimination is based on n FM subcarrier frequency channels.
Thus, a total of Nmn distinguishable optical channels can be accommodated.
In this embodiment each combination of the Nmn parameters constitutes a separate address for routing signals to Nmn users.
The laser transmitter comprises a broadband, tunable semiconductor laser which includes an intracavity, integrated composite reflector to which a tuning voltage and an FM dither signal are applied, an intracavity gain section to which drive current is applied, and an extracavity, integrated electroabsorption modulator to which an information (e.
g.
, data, voice, video) signal and an AM dither signal are applied.
The tuning voltage provides coarse wavelength tuning among N WDM channels, whereas the FM dither signal produces an additional m channels via FM-SCM.
The AM dither likewise produces an additional n channels via AM-SCM.
In a preferred embodiment, the tunable laser provides an optical output signal in any one of N different longitudinal modes at wavelengths corresponding, for example, to the N channels of the WDM system.
The laser comprises an MQW active region, a DFB region for selecting the nominal wavelength of stimulated emission (i.
e.
, laser light) generated by the active region, and a first waveguide optically coupled to the active region to permit egress of the laser output signal.
The tunable laser is characterized in that a composite second reflector is coupled to one end of the first waveguide so as to form a cavity resonator with the DFB region.
The second reflector includes an MQW second region optically coupled to the MQW active region, a second waveguide having one end optically coupled to the first waveguide, and a high reflectivity dielectric layer disposed at the other end of the second waveguide