Australian R&D group designing wavelet-based, dispersion-tolerant 40G links

JUNE 30, 2008 -- A private R&D Fibre-Optic Transmission (FOT) group in Canberra, Australia, is developing fiber-optic transmission systems by combining mixed time-frequency signal representations with photonic properties of light. The transmission system includes a standard 40-Gbit/sec intensity modulator and a standard direct detection receiver. The novelty consists of introducing a simple spectrum converter in the electronic driver of the electro-optic modulator, says the group.
 
The conventional approach of shaping signal waveforms, e.g. pulses, by means of controlling and tailoring the corresponding Fourier spectrum of the signal runs into the limitations associated with the Fourier analysis; the separation between the time domain and the frequency domain leads to the pulse duration being inversely proportional to the Fourier spectral bandwidth needed to shape the pulse.
 
In contrast, by mixing the time and frequency domains, the spectrum becomes a function of time, which provides an additional and significant element in shaping signals. In this context, wavelets--defined as short-duration sinusoidal waves--can break away from the Fourier constraints. The modulation format used will be the on-off keying (OOK) or binary ASK with return-to-zero (RZ) pulse structure to mitigate the effect of polarization-mode dispersion. The spectral efficiency will be higher than that of the dual-polarization QPSK.
  
The development is intended for multi-haul networks operating 10-Gbit and the emerging 40-Gbit fiber-optic transmission systems and will use commercially available components and devices. According to the group, three elements stand out: 
 
• The very narrow physical distribution of the optical spectrum that enables the elimination of dispersion compensation modules comprising specialized fiber and optical amplifiers;
 
• Reduction in the crosstalk between adjacent optical channels leading to enhanced optical signal to noise ratio (OSNR) and the elimination of forward error correction codes; and
 
• Simplification of the electronic driver, which will be designed in terms of digital combinational logic circuits rather than analog circuits requiring complex spectral shaping configurations.
  
In the quest for an optimal modulation format for an optical transmission channel bit rate of 40 Gbits/sec, the suppression of the optical carrier--in order to reduce the degradations induced by nonlinear effects in optical fibers--singled out the DPSK as the preferred technique. However, the need for phase-decoding and dispersion compensations, particularly for RZ pulses, increases the cost of deployment and operation.
 
The wavelet approach to high-speed optical pulse modulation has the capability of delivering high-quality transmission with simple setup configurations and reduced operational costs, claims the R&D group.
 
Details about the wavelet approach to high-speed optical transmission and its design considerations can be obtained from the group's R&D coordinator Andre Vatarescu.