Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2003-03-20
2004-08-31
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S015000, C385S027000
Reexamination Certificate
active
06785446
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to optical equalization arrangements and, more particularly, to a method and apparatus for implementing a multi-channel optical equalizer for intersymbol interference mitigation.
BACKGROUND OF THE INVENTION
Equalizers in the electrical domain to mitigate various impairments in digital optical communication systems are well known [see References 1-3 in Appendix]. An equalizer in the optical domain has been proposed but not demonstrated [4]. This proposed equalizer is a multistage lattice filter with many adjustable phase parameters, the parameters chosen by complicated optimization algorithms and compensates only one wavelength channel at a time. There have been demonstrations of single impairment optical compensators [4a], such as chromatic dispersion (CD) compensators [5-7] and polarization-mode dispersion (PMD) compensators [8], but these address only one source of impairment at a time.
What is desired is a simple multi-channel optical equalizer for intersymbol interference mitigation which can compensate many wavelength channels of a multiwavelength signal simultaneously and which requires few adjustable parameters.
SUMMARY OF THE INVENTION
In accordance with the present invention, we disclose a method and apparatus for implementing a less-complex multi-channel optical equalizer for intersymbol interference mitigation which can compensate many wavelength channels simultaneously and which requires fewer adjustable parameters. Our equalizer method and apparatus is extremely simple, having only two control signals (four in special cases), and yet can compensate many wavelength channels simultaneously. In one embodiment our equalizer includes a single Mach-Zehnder interferometer (MZI) with a free-spectral range of 50 GHz (an integer multiple of the channel spacing) and tunable couplers. In electrical equalizer terminology, our equalizer is a single-tap linear equalizer [3]. Our equalizer can dramatically improve the performance of non-return-to-zero (NRZ), carrier-suppressed return-to-zero (CSRZ), and possibly other transmission formats impaired by intersymbol interference. It can mitigate many impairments simultaneously, including those due to transmitter and/or receiver imperfections, filter narrowing, CD, and PMD. Our equalizer is especially dramatic in its improvement of the non-return-to-zero (NRZ) amplitude-shift keying (ASK) format, which is significant since this is the least expensive format to generate and detect.
More particularly, our optical signal equalizer simultaneously equalizes one or more received signals modulated at a preselected modulation bit rate in a multiwavelength system, the equalizer comprising
a first coupler with a variable coupling ratio for splitting the light into two or more portions;
a controllable interferometer unit having two or more arms, each arm receiving one portion, at least one arm having an additional delay which is equal to an integer multiple of 1/&Dgr;f, where &Dgr;f is the channel spacing of the multiwavelength system;
at least one arm having a controllable delay unit for adjusting the relative phase of the light passing through that arm;
and a second coupler for combining the signals from the arms.
In another embodiment, the optical signal equalizer for equalizing a received optical signal modulated at a preselected modulation bit rate is implemented using two or more equalizer units
According to our method of operating an optical equalizer, we equalize a received multiwavelength optical signal modulated at a preselected modulation bit rate using the steps of:
splitting the light into two or more variable portions;
creating a differential delay between the two or more of the portions, which is approximately equal to an integer multiple of 1/&Dgr;f, where &Dgr;f is the channel spacing of the multiwavelength system,
adjusting the phase of the light in one of the two or more portions,
combining the signals into a single output, and
whereby the variable adjustments are made so as to mitigate intersymbol interference impairments in the received optical signal.
REFERENCES:
patent: 6351586 (2002-02-01), Krol et al.
patent: 6396607 (2002-05-01), Cao
patent: 6400870 (2002-06-01), Hill et al.
patent: 6463186 (2002-10-01), Li
M. Bohn, G. Mohs, C. Scheerer, C. Glingener, C. Wree, and W. Rosenkranz, “An adaptive optical equalizer concept for single channel distortion compensation,” Proc. 27th Eur. Conf. on Opt. Comm., paper Mo.F.2.3, 2001.
IBM-Zurich Research Lab., Tunable Dispersion Compensator in High-Index Contrast SiON Technology (Preliminary Product Info.).
Chandrasekhar Sethumadhavan
Chraplyvy Andrew Roman
Doerr Christopher Richard
Winzer Peter J.
Doan Jennifer
Lucent Technologies - Inc.
Ullah Akm Enayet
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