Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of communicating the signals over long distances with very low loss. An optical network may be configured to combine modulated signals at various wavelengths or optical frequencies (also known as “channels”) into a single optical fiber. Each disparate channel may include optically encoded information to be communicated throughout the optical network. Such combining of various channels into a single fiber is known as wavelength-division multiplexing (WDM). Each optical wavelength (or carrier) may carry multiple sub-carriers using frequency-division multiplexing (FDM).
Orthogonal frequency-division multiplexing (OFDM) is a FDM scheme in which a plurality of closely-spaced orthogonal sub-carriers is used to carry data. The data is divided into several parallel data channels, one for each sub-carrier. OFDM modulation may be implemented using inverse discrete Fourier transformation (IDFT) and an optical modulator instead of using multiple modulators and oscillators for subcarriers as is the case in traditional FDM. The demodulation is also achieved using discrete Fourier transformation (DFT) instead of using multiple filters and oscillators for subcarriers. The separation of subcarrier channels is the integer multiple of the inverse observation period for a symbol to assure orthogonality.
An optical signal comprised of disparate subcarriers may experience optical dispersion (and in particular, chromatic dispersion). Because of dispersion, sub-carriers of differing frequencies may propagate with different speeds in an optical fiber. Optical subcarriers have phase noise due to finite linewidth of the laser providing a source of electromagnetic energy for a fiber. Such laser phase noise combined with dispersion, if not compensated, may lead to non-orthogonality between sub-channels, thus degrading the OFDM signal. Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform. Signal degradation due to laser phase noise combined with chromatic dispersion may be significant, especially when optical OFDM signals are created using a distributed feedback (DFB) laser.