However, the implementation penalty of such subsystems varies from 2dB to 4dB at bit-error rate of 10 −3, which is non-negligible for optical networks, especially when multiple wavelength conversion nodes are included in the networks. It is imperative to suppress phase noise in optical signal processing subsystems to allow compatibility phase-noise sensitive high-order QAM formats.Īs one of the basic optical signal processing techniques, several all-optical wavelength conversion (AOWC) schemes have been demonstrated to realize AOWC functions of OFDM, 8ary phase-shift keying (8PSK), 16QAM, and 64QAM by using the second-order nonlinear effect in periodically-poled Lithium Niobate (PPLN) waveguide, four-wave mixing (FWM) in highly-nonlinear fibers (HNLF), semiconductor optical amplifier (SOA), or silicon waveguide. However, for these high-order QAM signals, the increasing number of states in the constellation makes the signal more sensitive to the intensity and phase noise.
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It is desirable to exploit optical signal processing schemes suitable for these advanced optical modulation formats. On the other hand, recently, lots of advanced modulation formats like single-carrier high-order QAM like 64QAM or multi-carrier optical orthogonal frequency-division multiplexing (OFDM) have been introduced and realized in optical communications for enabling spectrally-efficient and ultra-fast optical transmissions. As basic optical network functionalities, all-optical wavelength conversion, wavelength data exchange, and wavelength multicasting play important roles in the all-optical networks to enhance the re-configurability and non-blocking capacity, and facilitate the wavelength management in future transparent optical networks. However, optical signal processing is still highly desirable and appreciable in order to overcome the electronics bottlenecks, support the transparency and ultra-fast processing in future optical networks. It dramatically simplifies the reception of multi-level and multi-dimensional modulation formats such as high-order quadrature amplitude modulation (QAM), thus making high-order QAM become a promising and practical approach for achieving higher bit rate and higher spectral efficiency.
![optical ofdm with quadrature modulator optisystem optical ofdm with quadrature modulator optisystem](https://i.ytimg.com/vi/XFgDXwtCsMQ/maxresdefault.jpg)
Recently, digital signal processing (DSP) is playing an increasingly important role in coherent detection for reconstructing the complex field of signal and compensating for the transmission impairments. All of the discussions are based on experimental investigation. We will first investigate the effective monitoring approach to optimize the performance of wavelength conversion for avoiding undesired nonlinear phase noise and distortions, and then propose coherent pumping scheme to eliminate the linear phase noise from local pumps in order to realize pump-phase-noise-free wavelength conversion, wavelength exchange and multicasting for high-order QAM signals. To design high-performance optical signal processing subsystems, both linear and nonlinear phase noise and distortions are the main concerns in the system design. Due to the susceptibility of high-order QAM signals against phase noise, it is imperative to avoid the phase noise in the optical signal processing subsystems.
![optical ofdm with quadrature modulator optisystem optical ofdm with quadrature modulator optisystem](https://www.frontiersin.org/files/Articles/749497/fphy-09-749497-HTML-r1/image_m/fphy-09-749497-g009.jpg)
In this book chapter, optical signal processing technology, including optical wavelength conversion, wavelength exchange and wavelength multicasting, for phase-noise-sensitive high-order quadrature-amplitude modulation (QAM) signals will be discussed.