Eye-diagram mask for optical pulses
In an optical data transmission system, high frequency pulsed optical signals are transmitted along an optical fiber. These pulses are attenuated and distorted by transmission over long optical fibers making detection of data difficult at the receiving end. The allowable pulse profile is determined by an eye diagram which is non-symmetrical with respect to logic 1 and 0 levels. By positioning the eye diagram closer to the logic 0 level, detection in the presence of noise introduced during transmission of the optical pulses, can be significantly improved.
1. An optical transmission system, comprising:
a) an optical transmitter for generating and transmitting optical pulses having logic one and logic zero levels along an optical fiber, a shape of the optical pulses which are transmitted being defined by an eye diagram, the pulse shapes for the logic one and logic zero levels being asymmetrical with respect to an amplitude midpoint such that a duty cycle of the pulses is reduced; and
b) a receiver for receiving the optical pulses at a far end of the optical fiber, and means for utilizing an eye mask to facilitate detection of the logic one and logic zero levels in a presence of noise introduced during transmission of the optical pulses, wherein the eye mask has a mean amplitude which is closer to the logic zero level than to the logic one level.
2. The optical transmission system as claimed in claim 1 , wherein the optical transmitter generates and transmits pulses at a rate of 3 Gb/s or less.
3. The optical transmission system as claimed in claim 1 , wherein the eye mask is of rectangular shape and has an upper limit of approximately 0.65 of the logic one level, and a lower limit of approximately 0.15 of the logic zero level.
4. The optical system as claimed in claim 1 , wherein the eye mask is a rectangle having a time position centered on a peak amplitude position of one of the optical pulses.
5. The optical system as claimed in claim 1 , wherein the eye mask has a time duration which is 0.2 of a bit interval of the eye diagram.
6. The optical system as claimed in claim 1 , and a converter for converting the pulses received at the far end of the optical fiber to an electrical signal, and threshold decision means for detecting the pulses, the threshold decision means being AC coupled to the converter.