Optical waveguides – With optical coupler
Reexamination Certificate
2001-10-09
2004-12-28
Font, Frank G. (Department: 2883)
Optical waveguides
With optical coupler
C385S016000, C385S018000
Reexamination Certificate
active
06836580
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength dispersion compensation system for compensating even for higher-order dispersion.
2. Description of the Related Art
In optical communications, there is a great demand for a wavelength dispersion compensator for solving the problem that when an optical pulse is transmitted over a long distance, the signal transmitted over a long distance degrades.
FIG. 1
shows a conventional fiber-optical communications system for transmitting information by light.
In
FIG. 1
, a transmitter
1
transmits a pulse to a light receiver
6
through an optical fiber
2
. However, the wavelength dispersion, which is also called “chromatic dispersion”, of the optical fiber degrades the signals of the system. More specifically, the propagation velocity of a signal in an optical fiber depends on the wavelength of the signal due to wavelength dispersion. For example, the phenomenon that a pulse with a long wavelength (for example, a pulse
3
with a wavelength representing a red color pulse) propagates faster than a pulse with a short wavelength (for example, a pulse
4
with a wavelength representing a blue color pulse) is generally called “normal dispersion”. Conversely, the phenomenon that a pulse with a short wavelength (for example, a blue color pulse) propagates faster than a pulse with a long wavelength (for example, a red color pulse) is generally called “abnormal dispersion”. Therefore, in the case that a pulse
5
, including both red and blue color pulses is transmitted from the transmitter
1
, the pulse
5
is separated into the red and blue color pulses when the pulse
5
is transmitted through the optical fiber
2
. The light receiver
6
receives the separated red and blue color pulses at respective different times. In this case, the case where the red color pulse propagates faster than the blue color pulse is “normal dispersion”. For an example of other pulse propagation, if a pulse has consecutive wavelength components ranging from blue to red, the pulse is expanded in an optical fiber since the red and blue components propagate at different speeds, and is distorted due to the wavelength dispersion. Since all the pulses include limited numbers of wavelength range, such wavelength dispersion often occurs in fiber-optical communications. Therefore, to obtain a high transmitting power in a fiber-optical communications system, wavelength dispersion must be compensated for.
To compensate for chromatic dispersion, a diffraction grating pair, chirp fiber grating, dispersion compensation fiber and the like has traditionally been used to compensate for chromatic dispersion, in particular, the diffraction grating pair being not only for chromatic dispersion in a fiber. Japanese Patent Laid-open Nos. 10-534450 and 11-513133 have proposed a device, including a “Virtually Imaged Phased Array” (VIPA) as an inverted-dispersion component.
FIGS. 2A and 2B
show the operation of a VIPA.
As shown in
FIG. 2A
, input light, including a plurality of wavelength components are collected by a cylindrical lens and are inputted to a VIPA plate. The inputted light is reflected a umber of times on reflection films provided on the each side of the VIPA while expanding. Light is outputted in small amounts from the VIPA plate during multi-reflection. The plurality of outputted light interfere each other and generate a plurality of parallel light rays propagate in different directions for each wavelength. The operation of this VIPA plate is understood as shown in FIG.
2
B. In other words, since light is reflected a multiple number of times on a plurality of reflection planes on a VIPA plate, it can be virtually considered that light is outputted from a plurality of virtual images. Specifically, a plurality of light a to e are linearly arrayed. All the distances between the two virtual images are equal. Since it can be virtually considered that virtual images are phase-matched and arrayed, the component is named as a VIPA plate.
FIG. 3
shows a dispersion compensation system using a transmission type diffraction grating with the function equivalent to a VIPA plate.
How to compensate for dispersion is described later with reference to a VIPA plate. The operation of a transmission type diffraction grating is described here.
In a transmission type diffraction grating, stair-type steps are formed on the surface of a transparent material, and a plurality of light a to e are outputted from the surfaces of these steps. Since the plurality of light a to e are outputted after being transmitted over distances
1
to
5
, respectively, there are phase differences between the plurality of light a to e after emission. After emission the plurality of outputted light a to e interfere with each other and generate a plurality of parallel light propagating in different directions for each wavelength. In a transmission type diffraction grating, a plurality of light a to e directly transmitted from a real light source are arrayed, while in a VIPA plate, a plurality of light a to e transmitted from virtual images are arrayed. However, the substantial operations of both the transmission type diffraction grating and VIPA plate are the same.
FIGS. 4 and 5
show the operational principle of a dispersion compensator using a VIPA plate.
FIG. 4
shows the appearance of a dispersion compensator using a VIPA plate. A plurality of light inputted from an optical fiber are converted into a plurality of parallel light by a collimation lens and are collected on a VIPA plate by a cylindrical lens. As described earlier, in the VIPA plate, a plurality of parallel light propagating in different directions for each wavelength are generated and outputted. The plurality of output light are collected on a mirror by a lens. The plurality of light inputted to the mirror are reflected in different directions and are inputted to the lens again. The plurality of light inputted to the lens are converted into a plurality of parallel light, are returned to the VIPA plate and are outputted to the cylindrical lens. The cylindrical lens generates a plurality of parallel light and inputs the plurality of parallel light to the collimation lens. The collimation lens collects the plurality of inputted parallel light and combines the plurality of parallel light in an optical fiber.
FIG. 5
shows a mechanism for compensating for wavelength dispersion by the configuration shown in FIG.
4
.
As described earlier, a plurality of light outputted from a VIPA plate can be virtually regarded as a plurality of light outputted from virtual images a to d. For example, light outputted from a virtual image a (in this case, a real image) propagates toward a lens, is defracted by the lens and hits the surface of a mirror. The mirror reflects the inputted light in a different direction and inputs the light at a different position of the lens. The lens converts the inputted light into parallel light and returns to the VIPA plate. In this case, the light returned to the VIPA plate is inputted to a position different from the position from which the light is outputted initially. As shown in
FIG. 5
, it can be virtually considered that the light is returned to a different virtual image c. However, since virtual images a and c are separate from each other, the light is transmitted additional by distance
1
and is returned. Therefore, such transmitted light is returned with a delay caused by the extra transmission distance. Therefore, if a blue color pulse is delayed from a red color pulse due to wavelength dispersion, the propagation delay between the red and blue color pulses can be avoided by configuring the system so that the red pulse is transmitted along the route shown in FIG.
5
and delaying the propagation of the red pulse. The propagation time can also be reduced by configuring the system so that the light route is short. Therefore, if a blue color pulse propagates faster than a red color pulse, the propagation delay can also be avoided by making the transmission distance of the red co
Izumi Hirotomo
Yoshida Setsuo
Font Frank G.
Fujitsu Limited
Kianni K. Cyrus
Staas & Halsey , LLP
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