Optical communications – Optical repeater system – Regenerative
Reexamination Certificate
2001-06-26
2004-11-23
Phan, Hanh (Department: 2633)
Optical communications
Optical repeater system
Regenerative
C398S176000, C398S178000, C398S140000, C398S141000, C398S147000, C398S150000, C398S173000
Reexamination Certificate
active
06823144
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of optical communications, and in particular, to a method and apparatus for optical single-sideband (SSB) modulation that incorporates one or more mid-span analog (non-optical) SSB regenerators.
BACKGROUND OF THE INVENTION
Optical single sideband (SSB) modulation offers some advantages over conventional double sideband (DSB) modulation; in particular, the signal spectrum is reduced by a factor of two, enabling smaller channel spacing and increased immunity to dispersion effects. Furthermore, square-law detection of SSB signals is notably different from square-law detection of DSB signals: for SSB transmission, the phase information in the recovered baseband signal contains unambiguous information about the phase response of the optical transmission channel. This is not true for square-law-detected DSB signals, where phase information of the transmission channel is destroyed in the detection process (due to a spectrum backfolding effect described by M. Sieben, J. Conradi, D. E. Dodds, in an article entitled “Optical single sideband transmission at 10 Gb/s using only electrical dispersion compensation”,
J. Lightwave Tech
. 17, 1742-49 (1999).
Optical SSB transmission is not without its pitfalls, however. The first challenge is how to generate these signals. One can incorporate SSB generation into the optical modulation by appropriately biasing and driving a lithium niobate Mach-Zender (MZ) modulator (see the M. Sieben et al. article cited above), or one can do the SSB generation post-modulation in the optical domain, using very precise optical filtering to eliminate one of the sidebands. See, for example, an article by K. Yonenaga and N. Takachio entitled “A fiber chromatic dispersion compensation technique with an optical SSB transmission in optical homodyne detection systems”,
IEEE Phot. Tech. Lett
. 5, 949-951 (1993).) At the moment the former method seems preferable, though with pending improvements in optical filtering technologies and ever increasing modulation bandwidths, one should not rule out latter method. It is also important in all cases to minimize the degree of chirp introduced by the modulation process.
Another issue that must be addressed in relation to optical SSB transmission is the inherent distortion produced by square-law detection of an SSB signal. See, for example, page 178 of the textbook by B. P. Lathi entitled
Modern Digital and Analog Communication Systems
(Oxford University Press, New York, 1998). This distortion can be rather severe, and can be avoided by using smaller modulation depths, which undesirably results in a power penalty at the receiver. M. Sieben et al. (see article cited above) have proposed a novel modulation scheme for eliminating second-order distortion in SSB transmission. This scheme fails, however, in the presence of chromatic dispersion.
Another area of study relating to the current invention is a technique known as optical phase conjugation (OPC). See, for example, an article by A. Yariv, D. Fekete, D. M. Pepper, entitled “Compensation for channel dispersion by nonlinear optical phase conjugation”,
Opt. Lett.
4, 52-54 (1979) and an article by A. H. Gnauck, R. M. Jopson, and R. M Derosier, entitled “10-Gb/s 360-km transmission over dispersive fiber using midsystem spectral inversion”,
IEEE Phot. Tech. Lett
. 5, 663-666 (1993). In OPC, nonlinear optical interactions are used to optically phase-conjugate a signal at the midpoint of a transmission span. When this is done, all even orders of chromatic dispersion are automatically compensated in the link. In addition, if the optical power distribution in the link is made symmetric about the mid-span point, all nonlinear signal degradations relating to the Kerr effect are also compensated. However, OPC requires nonlinear optical devices to accomplish the phase conjugation, and these devices are not readily commercially available.
SUMMARY OF THE INVENTION
An optical transmission system utilizes optical single-sideband (SSB) modulation and incorporates a novel mid-span analog SSB regenerator. The regeneration process, referred to as complementary sideband regeneration (CSR), causes a phase conjugation of the transmitted signal. This phase conjugation is accomplished in the electrical domain, is imprinted on the retransmitted optical signal, and requires no nonlinear optical techniques. If the CSR is performed at the mid-point between two identical spans of optical fiber, all even orders of chromatic dispersion are automatically compensated.
In one embodiment of the invention, a mid-span analog SSB regenerator is interposed between the first and second interconnected optical transmission line spans. An input signal is applied to an input transmitter, and modulates a carrier such that either the upper or lower sideband is transmitted on the first optical transmission line span to the mid-span regenerator. The SSB signal is square-law detected at the mid-span location, and is subsequently retransmitted over the second optical transmission line span. This retransmission also incorporates SSB modulation, with the transmitter arranged to modulate a carrier such that the other (i.e., lower or upper) complementary sideband is utilized for transmission. The process of regenerating the SSB optical signal with a sideband reversal causes a phase conjugation of the signal. As a result, at the receiver end of the second optical line span (which is assumed to be matched to first line span), all even orders of chromatic dispersion are compensated.
In another embodiment of the invention, the inherent distortion introduced by square-law detection of the SSB signals can be partially mitigated. This is accomplished by including signal inversion at the mid-span regenerator. For optical signals, which are always “positive” in nature, signal inversion implies that, after detection, large received signal voltages are retransmitted as small optical powers, and small received signal voltages are retransmitted as large optical powers, and that the time derivative of the received signal is reversed in sign. One method of accomplishing signal inversion is by suitably arranging the bias voltage of the MZ modulator in the regenerator transmitter. Upon square-law detection of the inverted signal by the receiver at the end of the second optical transmission span, the second-order distortion terms introduced by the SSB modulation format are cancelled. Note that the signal inversion technique does not require the two fiber spans to be matched to accomplish the distortion cancellation. However, this embodiment of distortion cancellation fails in the presence of chromatic dispersion.
In a further embodiment of the invention, cancellation of distortion terms caused by square-law detection of SSB signals can be accomplished even in the presence of chromatic dispersion, by using a four-span transmission with both CSR and signal inversion. In this embodiment, three mid-span analog SSB regenerators are interposed between four interconnected optical transmission line spans. The three regenerators are (a) spaced essentially evenly along the entire length of the optical transmission medium, such that the four fiber spans are matched and (b) arranged with electrical receivers and transmitters that (i) receive an SSB signal on one sideband (upper or lower) (ii) transmit the SSB signal on the other sideband (lower or upper, respectively), and (iii) in the first and third regenerators only, do signal inversion as described above. Again, the signal inversions can be conveniently accomplished by adjusting the modulation bias. This embodiment of the invention simultaneously accomplishes both the dispersion compensation and the SSB distortion cancellation described above.
REFERENCES:
patent: 5365362 (1994-11-01), Gnauck et al.
patent: 6304348 (2001-10-01), Watanabe
Sieben, M et al, “Optical Single Sideband Transmission at 10 Gb/s Using Only Electrical Dispersion Compensation”,J. Lightwave Technology, vol. 17, No. 10, Oct. 1999, pp. 1742-1749.
Yonenaga, K., et al
Freedman Barry H.
Lucent Technologies - Inc.
Phan Hanh
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