Wavelength division multiplexed system having reduced cross-phase modulation

In a wavelength division multiplexed (WDM) system including a transmitter for sending WDM signal channels over a plurality of serially connected amplified communication spans to a receiver, each span including an optical amplifier for receiving the WDM signal channels and transmitting them over a length of fiber to an adjacent span or the receiver, a method of reducing the cross-phase modulation (CPM) between at least one pair of adjacent WDM signal channels, comprising the steps of:

• at the receiver, measuring bit error rate (BER) at one or more channels of said at least one pair of adjacent WDM signal channels and
• at one or more spans, adjusting the clock phase difference between channels of said at least one pair of adjacent WDM signal channels to minimize the measured BER at said one or more channels.

In a wavelength division multiplexed (WDM) system including a transmitter for sending WDM signal channels over a plurality of serially connected amplified communication spans to a receiver, each span including an optical amplifier for receiving the WDM signal channels and transmitting them over a length of fiber to an adjacent span or the receiver, a method of reducing the cross-phase modulation (CPM) between at least one pair of adjacent WDM signal channels, comprising the steps of:

• at at least one span, determining the clock phase difference between said at least one pair of adjacent WDM signal channels at that span and
• at a different span, determining and adjusting the clock phase difference between said at least one pair of adjacent WDM signal channels to be different from the clock phase difference of said at least one pair of adjacent WDM signal channels at each of said at least one span.

In a wavelength division multiplexed (WDM) system including a transmitter for sending WDM signal channels over a plurality of serially connected amplified communication spans to a receiver, each span including an optical amplifier for receiving the WDM signal channels and transmitting them over a length of fiber to an adjacent span or the receiver, a method of reducing the cross-phase modulation (CPM) between at least one pair of adjacent WDM signal channels, comprising the steps of:

• determining the variation in clock phase differences at a fiber input of one or more spans and
• at the transmitter, adjusting the clock phase of a channel modulating data signal used to modulate one channel of said at least one channel pair to reduce cross-phase modulation.

A wavelength division multiplexed (WDM) system including a transmitter for sending WDM signal channels over at least one amplified communication span to a receiver, said at least one span comprising an optical amplifier for receiving the WDM signal channels and transmitting them over a length of fiber to an adjacent span or the receiver, said at least one span further comprising

• means for setting the difference in propagation delay, Δτ, for at least one channel pair of said WDM signal channels in any span so that (n+0.25)T<Δτ<(n+0.75)T, where T is the bit period of a channel modulating data signal, and n is a non-negative integer.

In a wavelength division multiplexed (WDM) system including a transmitter for sending WDM signal channels over at least one amplified communication span to a receiver, said at least one span including an optical amplifier for receiving the WDM signal channels and transmitting them over a length of fiber to an adjacent span or the receiver, a method of reducing the cross-phase modulation (CPM) at at least one pair of adjacent WDM signal channels, comprising the steps of:

• determining the differential time delay in one or more spans and
• adjusting the difference in propagation delay, Δτ, for at least one channel pair of said WDM signal channels in any span so that (n+0.25)T<Δτ<(n+0.75)T, where T is the bit period of a channel modulating data signal, and n is a non-negative integer.