A Novel Optical Vector Spectral Analysis Technique Employing a Limited-Bandwidth Detector
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A Novel Optical Vector Spectral Analysis Technique Employing a Limited-Bandwidth Detector C. K. Madsen Department of Electrical Engineering, Texas A&M University, College Station, TX 77843, USA Email: [email protected] Received 23 April 2004; Revised 6 December 2004 A new technique for obtaining frequency-dependent magnitude and phase information across an optical channel is presented using tunable allpass optical filters and a detector with a small bandwidth relative to the full channel width. This technique has application to optical monitoring of intersymbol interference distortions, including chromatic and polarization-mode dispersion effects, and thus can provide vector information for input to the control of adaptive optical filters. A method for generating a test signal spanning the spectrum of a modulated data signal without introducing intersymbol interference is discussed. This technique can also be used to characterize an optical pulse source and does not scale in complexity or cost as the bandwidth of the source increases. Keywords and phrases: intersymbol interference, chromatic and polarization-mode dispersion, optical channel monitoring, optical channel estimation.
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INTRODUCTION
A novel optical vector spectral analysis (OVSA) technique is proposed that measures the phase and amplitude of a signal using a detector having a limited bandwidth with respect to the full signal bandwidth. When a known signal with spectral components spanning the channel width is launched into a wavelength-division-multiplexed (WDM) optical communication system, it allows the frequency response of the system across the channel bandwidth to be determined. Given the importance of chromatic dispersion and polarizationmode dispersion (PMD) in high-bitrate optical communication systems, the OVSA can be implemented to characterize both the frequency-dependent relative phase and the signal’s polarization-dependence. The goal of characterizing the channel, or system response across a limited spectral width, is to provide accurate information on signal distortion. This information may then be used as a signal quality monitor or to update an adaptive filter that compensates for distortions [1, 2] and subsequently allows a higher bitrate times distance product to be achieved for the system. The proposed technique also allows a periodically modulated optical source to be fully characterized. A previous OVSA technique suggested the use of narrowband optical filters to extract sidebands equally spaced around a return-to-zero (RZ) modulated carrier [3], from which relative delay and dispersion information could be obtained without introducing any special test signals at the
transmitter. One narrowband filter (NBF) was required per tone, so a minimum of three filters were required, one at the carrier and one for each sidetone. The detector bandwidth was necessarily on the order of the signal spectral width. It is desired to keep the detector bandwidth as small as possible for low cost and to minimize the complexity of the optical circu
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