Optical communication system employing coherent detection and method

DETAILED DESCRIPTION In view of the above, the present invention provides for an optical communication system and method which allows high density wavelength division multiplexed communications of modulated carriers on an optical bus without the use of a local oscillator at the terminal devices and yet provides optical energy generated by each terminal device that can be used for the development of a modulated carrier for transmission to other terminal devices on the optical bus or for coherent detection of received modulated carriers including detection by both homodyne and heterodyne techniques.
In one form of the invention, the communications system is defined by an optical pathway, such as an optical fiber bus, in which terminal devices are coupled to the bus for communications with one another.
A system wide reference carrier source is coupled to the bus for introducing at least one highly stable system wide carrier for propagation throughout the system.
Terminal devices within the system can communicate by removing a portion of the system wide reference carrier, effecting a frequency shift to a communications frequency, and modulating the frequency shifted carrier for introduction onto the system bus as an information signal for propagation to other terminal devices in the system.
The terminal devices within the system, in order to receive a information signal from another terminal device, remove a portion of the information signal from the bus as well as a portion of the reference carrier transmitted by the system wide reference carrier source.
To effect heterodyne detection, the removed carrier energy is frequency shifted in the receiving terminal device to a frequency offset from that of the received information signal to generate a heterodyned intermediate frequency substantially equal to the difference in frequency between the received information signal and the locally frequency shifted reference carrier.
Alternatively, the removed carrier energy is not shifted in frequency so that heterodyning with the received information signal generates an "intermediate frequency" which is equal to the displacement in frequency of the information signal from the reference carrier