Image Processing Reference
In-Depth Information
2
Digital Backward Propagation:
A Technique to Compensate Fiber Dispersion
and Non-Linear Impairments
Rameez Asif, Chien-Yu Lin and Bernhard Schmauss
Chair of Microwave Engineering and High Frequency Technology (LHFT),
Erlangen Graduate School in Advanced Optical Technologies (SAOT),
Friedrich-Alexander University of Erlangen-Nuremberg (FAU),
Cauerstr. 9, (91058) Erlangen
Germany
1. Introduction
Recent numerical and experimental studies have shown that coherent optical QPSK
(CO-QPSK) is the promising candidate for next-generation 100Gbit/s Ethernet (100 GbE)
(Fludger et al., 2008). Coherent detection is considered efficient along with digital signal
processing (DSP) to compensate many linear effects in fiber propagation i.e. chromatic
dispersion (CD) and polarization-mode dispersion (PMD) and also offers low required optical
signal-to-noise ratio (OSNR). Despite of fiber dispersion and non-linearities which are the
major limiting factors, as illustrated in Fig. 1, optical transmission systems are employing
higher order modulation formats in order to increase the spectral efficiency and thus fulfil
the ever increasing demand of capacity requirements (Mitra et al., 2001). As a result of
which compensation of dispersion and non-linearities (NL), i.e. self-phase modulation (SPM),
cross-phase modulation (XPM) and four-wave mixing (FWM), is a point of high interest these
days.
Various methods of compensating fiber transmission impairments have been proposed in
recent era by implementing all-optical signal processing. It is demonstrated that the fiber
dispersion can be compensated by using the mid-link spectral inversion method (MLSI)
(Feiste et al., 1998; Jansen et al., 2005). MLSI method is based on the principle of optical phase
conjugation (OPC). In a system based on MLSI, no in-line dispersion compensation is needed.
Instead in the middle of the link, an optical phase conjugator inverts the frequency spectrum
and phase of the distorted signals caused by chromatic dispersion. As the signals propagate to
the end of the link, the accumulated spectral phase distortions are reverted back to the value
at the beginning of the link if perfect symmetry of the link is assured. In (Marazzi et al., 2009),
this technique is demonstrated for real-time implementation in 100Gbit/s POLMUX-DQPSK
transmission.
Another all-optical method to compensate fiber transmission impairments is proposed in
(Cvecek et al., 2008; Sponsel et al., 2008) by using the non-linear amplifying loop mirror
(NALM). In this technique the incoming signal is split asymmetrically at the fiber coupler
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