Bi-directional optical transmission system

DETAILED DESCRIPTION The present invention provides the desired bi-directional transmission system.
The present invention achieves the bi-directional transmission capability with reduction or avoidance of optical beat interference by providing a unique combination of transmitters and receivers at the access points of the network.
In accordance with an embodiment of the present invention, the transmitters at two different locations along the single optical fiber produce transmission signals in accordance with two different optical spectra.
The optical spectral characteristics for the two transmitters are selected so as to assure that the wavelength of the optical carriers are different during the operation of the system.
This avoids the production of optical beat interference.
The selection is made so as to assure that even as the wavelength of the transmitters may vary based on certain stimuli such as temperature (such variation also being referred to as drift), the wavelengths will have a very low probability of overlapping thereby insuring a reduction or avoidance of optical beat interference.
In one of the embodiments the optical carriers for the two transmitters are specifically selected to have optical frequencies which differ by more than the maximum frequency employed as a sub-carrier (fmaxsc).
The optical carrier's wavelength is equal to the speed of light "c" divided by the optical frequency f (.
lambda.
=c/f).
Therefore, this sets a specification on how the wavelengths of the optical carriers must differ (.
vertline.
.
DELTA.
.
lambda.
.
vertline.
=.
DELTA.
f/C .
lambda.
.
sup.
2).
This requirement must be increased to account for the laser linewidth (variations in the optical frequency due to noise), chirp (variations in the optical frequency due to modulation of the optical carrier), and drift.
Temperature controllers could be provided with these transmitters so as to stabilize the lasers to avoid drift.
However, if the wavelength difference is sufficient, then even without temperature control the effects of optical beat interference should be minimized