Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-03-18
2001-01-09
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06172781
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical transmission systems.
BACKGROUND OF THE INVENTION
Optical transmission systems in which optical pulses are transmitted over optical fibers, typically of low loss silica, are becoming of increasing importance. Generally these systems use wavelength division multiplexing (WDM) to increase the channel capacity of transmission thereby to reduce the unit cost per channel.
The lowest cost transmitters for WDM networks presently are directly modulated lasers, for example distributed feedback (DFB) lasers. However, the pulses produced by such lasers are generally characterized by chromatic dispersion in that the leading edge of the pulse typically includes frequency components that are changing from high to low whereas the trailing edge typically includes frequency components that are changing from low to high. Such pulses when transmitted over an optical fiber experience a phenomenon described as chirping, which leads to a reduction in the distance a train of closely spaced pulses can be transmitted without overlap between pulses. Such overlap impairs the fidelity of transmission and the recovery of the signal information at the receiver.
One technique that has been used to improve the quality of pulses from a directly modulated laser has been to pass the pulses through a narrow pass band filter to remove these unwanted frequencies at the leading and trailing edges of the pulses. However, in a WDM system using many channels of different wavelengths, this approach requires a separate filter for each channel and adds to the complexity and cost of the system.
Additionally, it has been known that following a non-return-to-zero (NRZ) electro-absorption modulator with a soliton pulse shaper attenuates the regions of highest transient chromatic dispersion generated by the NRZ modulator for soliton transmission systems. However, such a scheme has not previously been proposed for attenuating the region of highest chromatic dispersion in directly modulated lasers.
SUMMARY OF THE INVENTION
The present invention, in one aspect, is a method for increasing the effective distance a train of optical pulses from directly modulated lasers can be transmitted in WDM networks. Basically, the method involves providing a plurality of lasers, each modulated at a different wavelength independently as a separate channel but at the same bit rate in an NRZ format. The separate pulse trains are then multiplexed on the same optical fiber in the usual WDM fashion. The combined pulse train is then passed through a narrow-band amplitude modulator that is driven with a sine wave at the bit rate frequency and timed for transforming each channel from the NRZ format to a return-to-zero format. Moreover, in the process there are attenuated the portions of each pulse corresponding to the leading and trailing edges, effectively narrowing each pulse and removing the portions most vulnerable to phase dispersion as the pulses move along the optical fiber to the receiving station.
In accordance with another aspect, the invention is a WDM network that comprises at the transmitting end a plurality of optical lasers each tuned to a different wavelength for providing a plurality of different wavelength channels. Separate modulators impress independent data in each channel but all at the same bit rate and the separate channels are then routed onto a single optical fiber. The optical fiber is connected to a suitable narrow-band modulator-attenuator, for example, a March-Zehender modulator driven in a push-pull configuration with a 2.5 GHz sine wave synchronized with the NRZ drive to the lasers. Moreover, the sine wave is synchronized so that the peak of the sine wave coincides essentially with the center of the optical pulse. The modulator-attenuator serves both to convert the NRZ pulse format to the return-to-zero pulse format, while attenuating substantially the leading and trailing portions of the pulses, the portions most vulnerable to the chromatic dispersion that distorts the shape of the pulses during transmission along the fiber.
REFERENCES:
patent: 5946119 (1999-08-01), Bergano et al.
L.F. Mollenauer et al. “Solitons in High Bit-Rate Long-Distance Transmission” Optical Fiber Telecommunications III A Academic Press, San Diego, 1997, pp. 438-439.
Chan Jason
Lieu Vu
Lucent Technologies - Inc.
LandOfFree
Wave division multiplexed optical network does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Wave division multiplexed optical network, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Wave division multiplexed optical network will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2498034