Optical communications – Transmitter and receiver system – Including optical waveguide
Reexamination Certificate
2000-11-01
2004-07-27
Hgo, Hung N. (Department: 2633)
Optical communications
Transmitter and receiver system
Including optical waveguide
C398S081000, C398S180000, C398S159000
Reexamination Certificate
active
06768874
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a device for compensation of chromatic dispersion in optical fiber communication systems.
BACKGROUND OF THE INVENTION
Most high-speed fiber optic communication systems today use externally modulated lasers to minimize laser ‘chirp’ and reduce the effects of chromatic dispersion in the fiber. Even with external modulation, there is a certain amount of ‘chirp’ or broadening of the laser spectrum, because any modulated signal must contain frequency ‘sidebands’ which are roughly as wide as the modulation rate. Higher bit rate transmission systems consequently have broader frequency sidebands, and at the same time can tolerate less phase delay because of the shorter bit period. Next-generation high bit rate systems are consequently very sensitive to chromatic dispersion of the optical fiber and any components such as WDM's within the system.
Chromatic dispersion of optical fiber is roughly constant over the 1550 nm communication window, and can be compensated by several techniques including dispersion compensating fiber, FBG's, etc. However, certain wavelength filtering components such as WDM's can have significant dispersion characteristics due to a fundamental Kramers-Kronig type relationship between transmission spectrum and dispersion characteristics. This type of dispersion characteristic typically varies substantially over the narrow WDM passband, and therefore is difficult to compensate using conventional techniques such as dispersion compensating fiber. It is one objective of the present invention to compensate for the dispersion from WDM devices, including multiplexers, demultiplexers, and interleavers. Conventional laser systems are known to utilize directly modulated semiconductor lasers. In combination with chromatic dispersion characteristics of single-mode optical fiber, chirping of these lasers contributes to the spread of optical pulses and results in intersymbol interference and overall degradation in transmission. Current and “next-generation” high speed systems employ transmitters which use narrow linewidth lasers and external modulators in a window or range of wavelengths about 1550 nm. These external modulators generally have a very low chirp; some designs have a zero or negatively compensating chirp. Nevertheless, transmission distance is still dispersion-limited to about 80 kilometers at transmission rates of 10 Gb/s using conventional single mode fibers.
One solution to this problem is in the use of dispersion shifted fiber which has little dispersion in the 1550 nm window. However, replacing existing fiber with dispersion shifted fiber is costly. Other approaches have been proposed such as optical pulse shaping to reduce laser chirp, using a semiconductor laser amplifier to impose a chirp on the transmitted signal that counteracts the chirping of the directly modulated semiconductor laser.
Approaches that are more consistent with the teachings of this invention attempt to reduce the intersymbol interference at or near the receiver, or intermediate the transmitter and the receiver. Essentially any medium capable of providing a sufficient dispersion opposite to that of the optical fiber can serve as an optical pulse equalizer. For example it is known to use a special optical fiber having an equal chromatic dispersion at a required operating wavelength but opposite in sign to that of the transmitting fiber. Other methods include the use of fiber Bragg gratings as disclosed in U.S. Pat. No. 5,909,295 in the name of Li et al., and disclosed by Shigematsu et al., in U.S. Pat. No. 5,701,188 assigned to Sumitomo Electric Industries, Ltd., and the use of planar lightwave circuit (PLC) delay equalizers. Unfortunately, special compensating fiber has a very high insertion loss and in many applications is not a preferable choice. Fiber gratings are generally not preferred for some field applications due to their narrow bandwidth, and fixed wavelength. PLCs are also narrow band, although tunable devices; fabricating a PLC with large dispersion equalization remains to be difficult. Shigematsu et al. disclose a hybrid of both of these less preferred choices; dispersion compensating fibre with chirped Bragg gratings.
In a paper entitled “Optical Equalization to Combat the Effects of Laser Chirp and Fiber Dispersion” published in the Journal of Lightwave Technology. Vol. 8, No. 5, May 1990, Cimini L. J. et al. describe an optical equalizer capable of combating the effects of laser chirp and fiber chromatic dispersion on high-speed long-haul fiber-optic communications links at 1.55 &mgr;m. Also discussed is a control scheme for adaptively positioning the equalizer response frequency. Cimini et al. describe a device having only one common input/output port at a first partially reflective mirror and a second 100% reflective mirror together forming a cavity. The control scheme described attempts to track signal wavelength by obtaining feedback from a receiver. The amplitude response of the equalizer is essentially flat with wavelength at the input/output port, and thus, the proposed control scheme is somewhat complex requiring processing of high speed data at the optical receiver. As well, the proposed control method is stated to function with RZ signals but not with NRZ signals, a more commonly used data format. Although the equalizer described by Cimini et al. appears to perform its intended basic dispersion compensating function, there exists a need for an improved method of control of the position of the equalizer frequency response, and as well, there exists a need for an equalizer that will provide a sufficient time shift over a broader range of wavelengths. U.S. Pat. No. 5,023,947 in the name of Cimini et al., further describes this device.
A Fabry-Perot etalon having one substantially fully reflective end face and a partially reflective front face is known as a Gires-Tournois (GT) etalon. In a paper entitled Multifunction optical filter with a Michelson-Gires-Turnois interferometer for wavelength-division-multiplexed network system applications, by Benjamin B. Dingle and Masayuki Izutsu published 1998, by the Optical Society of America, a device is described which is hereafter termed the MGT device. The MGT device as exemplified in
FIG. 1
serves as a narrow band wavelength demultiplexor; this device relies on interfering a reflected E-field with an E-field reflected by a plane mirror
16
. The etalon
10
used has a 99.9% reflective back reflector
12
r
and a front reflector
12
f
having a reflectivity of about 10%; hence an output signal from only the front reflector
12
f
is utilized.
In an article entitled “Optical All-Pass Filters for Phase Response Design with Applications for Dispersion Compensation” by C. K. Madsen and G. Lenz, published in
IEEE Photonics Letters
, Vol 10 No. 7, July 1998, a coupled reflective cavity architecture in optical fibre, as shown in
FIG. 19
, is described for providing dispersion compensation. Cavities are formed in the optical fiber between fiber Bragg grating reflectors. However a multi-cavity filter in fiber has a limited free spectral range (FSR) insufficient for a telecommunications system. For a typical 100 GHz FSR required in the telecommunications industry, the cavity length is about 1 mm. A Bragg grating reflector, if manufactured using commonly available grating -writing techniques, would need to be longer than 1 mm, and hence the two reflector cavity structure would be too long to achieve the necessary FSR. Another draw back to this prior art solution is the requirement for an expensive optical circulator to separate the input and output signals.
As of late, interleaving/de-interleaving circuits are being used more widely. These specialized multiplexor/demultiplexors serve the function of interleaving channels such that two data streams, for example a first stream consisting of channel
1
,
3
,
5
,
7
, and so on, is interleaved, or multiplexed with a second stream of channels,
2
,
4
,
6
,
8
, and so on, for form single signal consisting of
Colbourne Paul
Fu Xiaoli
Xie Jimin
Hgo Hung N.
JDS Fitel Inc.
Teitelbaum Neil
Teitelbaum & MacLean
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