The invention relates to the filed of transmitting digital data by optical means. It is more particularly concerned with transmission of high bit rates with effective management of bandwidth.
Such transmission uses an optical transmitter connected to an optical receiver by the fiber. The transmitter generally modulates the power of an optical carrier wave from a laser oscillator as a function of the information to be transmitted. NRZ modulation is very frequently used and entails varying the power of the carrier wave between two levels: a low level corresponding to extinction of the wave and a high level corresponding to a maximum optical power. The variations of level are triggered at times imposed by a clock rate and this defines successive time cells allocated to the binary data to be transmitted. By convention, the low and high levels respectively represent the binary values xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d.
The maximum transmission distance is generally limited by the ability of receivers to detect without error these two power levels after the modulated wave has propagated in the optical link. The usual way to increase this distance is to increase the ratio between the average optical power of the high levels and that of the low levels, this ratio defining the xe2x80x9cextinction ratioxe2x80x9d which is one of the characteristics of the modulation.
Various modulation schemes for optical communications systems are known in the art. Frequency or phase modulation are utilized in optical communications technology in addition to intensity or amplitude modulation.
The Non-Return-to-Zero (NRZ) signal format is transmitting data in form wherein d(t)=0 is valid for a binary xe2x80x9c0xe2x80x9d and d(t)=1 is valid for a transmitted binary symbol xe2x80x9c1xe2x80x9d during the entire duration of a bit T (see for example, R. S. Vodhanel, Electronics Letters, Vol. 24 No. 3, pp. 163-165 (1988).
As the demand for faster communications increases, there has been a natural evolution towards a better usage of channel bandwidth. Optical fiber communications offers such a large usable bandwidth that efficient channel usage has not been an issue until recently. One modulation scheme for attacking the challenge of bandwidth management is the duobinary modulation This modulation may be the next step in the evolution of spectrally more efficient formats in optical fibers. Duobinary format is a binary NRZ signal with spectral shaping due to correlation between adjacent bits. This modulation scheme has four attractive features: (1) narrower bandwidth than binary format and hence suffers less from dispersion, (2) greater spectrum efficiency than binary format and hence allows tighter packing of wavelength division multiplexed channels, (3) less stimulated Brillouin backscattering, the major limiting factor in repeaterless transmission, and (4) ease of implementation. Duobinary format is new to the optical communications community and hence there are many unresolved issues. It is a relatively complicated modulation scheme which dispersion advantages depends from modulation, phase variation.
Another approach to achieve an effective bandwidth management is the Phase Shaped Binary Transmission (PSBT) scheme as described in D. Penninckx, xe2x80x9cEnhanced Phase Shape binary Trnasmissionxe2x80x9d, Electronic Letters, March 2000, page 478-480. This paper describes a variant of the duobinary transmission which is more tolerant towards chromatic dispersion than a pure NRZ modulation. With this modulation scheme the tolerance of signal-to noise ratio degradation is reduced.
In a wavelength division multiplex transmission scheme with a ITU grid of wavelength for example a transmission data rate of 40 Gbit/s is achieved with 16 different wavelengths and a 50 GHz spacing between the different wavelengths.
The ITU recommendations allows a wavelength comb with spacings of 100 GHz or 50 GHz. For the bandwidth per wavelength channel depends of the data rate a data rate of 10 Gbit/s can be transmitted with 50 GHz spacing. But with increasing bitrates the bandwidth per channels also increases.
One get to a special point when the spacing of 50 GHz is not broad enough to use the pure NRZ modulation method as described in the prior art at high bit rates Therefore one solution would be to increase the spacing between the wavelengths up to 100 GHz.
The aim of the invention is to propose a modulation scheme that is based on a NRZ amplitude modulation scheme but decreases the bandwidth per wavelength channels.
This new modulation scheme fits with the ITU wavelength grid for WDM transmissions over fiber in a bit range of 40 Gbit/s and more. The new NRZ modulation decreases the bandwidth of the channels. The new NRZ modulation scheme has a better tolerance to signal-to noise ratio SNR degradation.