Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2000-11-22
2002-11-26
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S033000, C385S023000, C385S036000, C359S199200
Reexamination Certificate
active
06487342
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fiber optic networks, and more particularly to chromatic dispersion compensators utilized in fiber optic communications networks that carry wavelength division multiplexed information signals.
BACKGROUND OF THE INVENTION
Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Wavelength division multiplexing is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber.
In this specification, these individual information-carrying lights are referred to as either “signals” or “channels.” The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.”
The term “wavelength,” denoted by the Greek letter &lgr; (lambda) is used synonymously with the terms “signal” or “channel.” Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, &lgr;, and a plurality of n such channels are referred to as “n wavelengths” denoted &lgr;
1
-&lgr;
n
. Used in this sense, the term “wavelength” may be understood to refer to “the channel nominally comprised of light of a range of physical wavelengths centered at the particular wavelength, &lgr;.”
One common and well-known problem in the transmission of optical signals is chromatic dispersion of the optical signal. Chromatic dispersion refers to the effect wherein the various physical wavelengths comprising an individual channel either travel through an optic fiber or component at different speeds—for instance, longer wavelengths travel faster than shorter wavelengths, or vice versa—or else travel different length paths through a component. This is a particular problem that becomes more acute for data transmission speeds higher than 2.5 gigabytes per second. The resulting pulses of the signal will be stretched, will possibly overlap, and will cause increased difficulty for optical receivers to distinguish where one pulse begins and another ends. This effect seriously compromises the integrity of the signal. Therefore, for a fiber optic communication system to provide a high transmission capacity, the system must compensate for chromatic dispersion.
The chromatic dispersion characteristics of optical fibers and components are given by the quantity D (ps−km
−1
−nm
−1
) defined by the relationship of Eq. 1
D
=
ⅆ
ⅆ
λ
(
1
v
g
)
=
1
L
(
ⅆ
τ
g
ⅆ
λ
)
(
1
)
wherein &lgr; is the channel wavelength (nm), v
g
is the group velocity (km/ps), &tgr;
g
is the group delay time (ps), and L is the fiber length (km). If v
g
decreases with increasing wavelength (i.e., longer or “red” wavelengths travel slower than relatively shorter or “blue” wavelengths) then D is positive, otherwise D is negative.
Conventional apparatuses that can be used as dispersion compensating components include dispersion compensation fiber, chirped fiber Bragg gratings, and diffraction gratings.
A dispersion compensation fiber, which is used in-line within a fiber communications system, has a special cross-section index profile so as to provide chromatic dispersion that is opposite to that of ordinary fiber within the system. The summation of the two opposite types of dispersion negates the chromatic dispersion of the system. However, dispersion compensation fiber is expensive to manufacture, has a relatively large optical attenuation, must be relatively long to sufficiently compensate for chromatic dispersion and cannot compensate for periodically varying chromatic dispersion.
A chirped fiber Bragg grating is a special fiber with spatially modulated refractive index that is designed so that longer (shorter) wavelength components are reflected at a farther distance along the chirped fiber Bragg grating than are the shorter (longer) wavelength components. By causing certain wavelength components to travel longer distances than other wavelength components, a controlled delay is added to those components and opposite dispersion can be added to a pulse. Unfortunately, a chirped fiber Bragg grating has a very narrow bandwidth for reflecting pulses, and therefore cannot provide a wavelength band sufficient to compensate for light including many wavelengths, such as a wavelength division multiplexed light. A number of chirped fiber Bragg gratings may be cascaded for wavelength multiplexed signals, but this results in an expensive system.
A conventional diffraction grating has the property of outputting different wavelengths at different angles. By using a pair of gratings in a coupled spatial arrangement, this property can be used to compensate chromatic dispersion in a fiber communications system. In such a spatial grating pair arrangement, lights of different wavelengths are diffracted from a first grating at different angles. These lights are then input to a second grating that diffracts them a second time so as to set their pathways parallel to one another. Because the different lights travel with different angles between the two gratings, certain wavelength components are made to travel longer distances than those traveled by other wavelength components. Chromatic dispersion is produced in the spatial grating pair arrangement because the wavelength components that travel the longer distances incur time delays relative to those that travel the shorter distances. This grating-produced chromatic dispersion can be made to be opposite to that of the fiber communications system, thereby compensating the chromatic dispersion within the system. However, a practical spatial grating pair arrangement cannot provide a large enough dispersion to compensate for the relatively large amount of chromatic dispersion occurring in a fiber optic communication system. More specifically, the angular dispersion produced by a diffraction grating is usually extremely small, and is typically approximately 0.05 degrees
m. Therefore, to compensate for chromatic dispersion occurring in a fiber optic communication system, the two gratings of a spatial grating pair would have to be separated by a very large distance, thereby making such a spatial grating pair arrangement impractical.
Accordingly, there is a need for an improved chromatic dispersion compensator. The present invention addresses such a need.
SUMMARY OF THE INVENTION
The present invention provides an improved chromatic dispersion compensator. The compensator includes an input optical fiber; an output optical fiber; a collimator optically coupled to the input and output optical fibers; and an interferometer optically coupled to the collimator at a side opposite to the input and output optical fibers. In a preferred embodiment, the interferometer is a Gires-Tournois interferometer. A plurality of sequentially optically coupled chromatic dispersion compensators may also be used. The compensator in accordance with the present invention provides flexibility in producing periodically varying chromatic dispersion so as to compensate for unwanted periodic chromatic dispersion produced in an interferometric interleaved channel separator. Also, the compensator enables compensation of fiber optic chromatic dispersion.
REFERENCES:
patent: 4558950 (1985-12-01), Ulrich et al.
patent: 5557468 (1996-09-01), Ip
patent: 2001/0021053 (2001-09-01), Colbourne et al.
patent: 1098211 (2001-05-01), None
patent: 1098212 (2001-05-01), None
patent: 0025154 (2000-05-01), None
K. Li, W. Knox, N. Pearson, Broadband cubic-phase compensation with resonant gires-tournois interferometer, May 1, 1989, pp. 450-452.
C. Eldering, A. Dienes, S. Kowel.,
Cao Simon X. F.
Wu Shudong
Avanex Corporation
Palmer Phan T. H.
Sawyer Law Group LLP
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