System and method of optical transmission
View Patent ↗Minimum shift keying (MSK) is used as the coding scheme in a high bit rate optical transmission system, and the signal format is either RZ (return-to-zero) or NRZ (non-return-to-zero). The system can combine multiple individual channels with different wavelengths in a WDM or dense wavelength division multiplexed (DWDM) arrangement. Dispersion management can be provided using several techniques, such as quasi-linear transmissions or conventional RZ transmissions. At the transmitter, an optical MSK transmitter is used to modulate the phase of a stream of high bit rate (e.g., 40 Gbit/s) optical signals. Many such data streams can be combined in a wavelength division multiplexer and transmitted to a remote receiver, where the signal is wavelength division demultiplexed. The encoded data in each wavelength channel is then recovered by an MSK receiver, which may consist of a delay demodulator and a balanced detector.
1. An apparatus for encoding a sequence of optical pulses in accordance with a sequence of input data, comprising:
means for generating a series of optical pulses, and
means for modulating the phase of said optical pulses such that a digital “1” in the input data is represented by a π/2 phase shift of a current optical pulse with respect to the previous pulse, and a digital “0” in the input data is represented by a-π/2 phase shift of a current optical pulse with respect to the previous pulse;
said modulating means including:
a continuous wave (CW) laser source for generating optical pulses at a desired clock rate;
an optical pulse generator for generating, responsive to said optical pulses at said desired clock rate, a 90-degree-alternate-phase (90°-AP) output in which successive pulses have a 90 degree phase difference,
means for generating a differential MSK encoded signal in accordance with the input data such that the differential MSK encoded signal represents the result of exclusive OR'ing (a) a first data signal representing said input data and (b) a clock signal having a rate that is one half of the rate of said desired clock rate, and
a chirp-free DPSK data modulator for encoding the phase of said 90-degree-alternate-phase (90°-AP) output in accordance with said differential MSK encoded signal.
2. The invention defined in claim 1 wherein said chirp-free DPSK data modulator is a Mach-Zehnder modulator biased at it's null point.
3. An optical transmission system to transmit an input data encoded optical pulses comprising:
a continuous wave (CW) laser source;
an optical pulse generator for generating optical carrier pulses at a desired clock rate having a 90-degree-alternate-phase (90°-AP) output such that successive pulses in the carrier pulse stream have a 90 degrees phase difference;
a differential MSK encoder for encoding the optical carrier pulses in accordance with a stream of digital data from a data source;
a chirp-free DPSK data modulator for encoding the phase of said 90-degree-alternate-phase (90°-AP) output in accordance with said differential MSK encoded signal to generate a modulated optical output signal;
a transmitter to transmit the said modulated optical signal to a remote receiver over an optical transmission medium; and
a receiver for recovering said digital data from the said encoded signal at the end of transmission;
such that a digital “1” in the input data is represented by a π/2 phase shift of a current optical pulse with respect to the previous pulse, and a digital “0” in the input data is represented by a −π/2 phase shift of a current optical pulse with respect to the previous pulse.
4. The invention defined in claim 3 wherein said differential MSK encoder generates a signal code as a result of ‘Exclusive OR’ operation of a first data signal representing an input data from a data source and a clock signal having a rate that is one half of the rate of said desired clock rate.
5. The invention in claim 4 wherein a “1” in said input data is represented by a change in the value of said first data signal and “0” in said input data is represented by the absence of a change in the value of said first data signal.
6. The invention in claim 4 where the optical pulses may be in RZ or NRZ format.
7. The invention in claim 3 wherein the transmission medium is dispersion managed.
8. The invention in claim 3 wherein the transmission medium is quasi-linear.
9. The invention in claim 3 wherein said receiver includes a one-bit-delay interferometer and a balanced receiver.
10. An optical communication method for transmitting data encoded optical pulses the method comprising the steps of:
generating optical carrier pulses from a cw light source at a desired clock rate having a 90-degree-alternate-phase (90°-AP) such that successive pulses in the carrier pulse stream have a 90 degrees phase difference;
generating a differential MSK code for encoding the optical carrier pulses in accordance with a stream of digital data from a data source;
modulating said 90-degree-alternate-phase (90°-AP) optical pulses output in accordance with said differential MSK code using a chirp-free DPSK data modulator thereby encoding the phase of signal to generate a modulated optical output signal;
transmitting the modulated optical signal to a remote receiver over an optical transmission medium; and
recovering said digital data from the said encoded signal at the end of transmission;
such that a digital “1” in the input data is represented by a π/2 phase shift of a current optical pulse with respect to the previous pulse, and a digital “0” in the input data is represented by a −π/2 phase shift of a current optical pulse with respect to the previous pulse.