Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-08-30
2004-05-04
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C385S024000, C385S027000, C385S028000, C398S081000
Reexamination Certificate
active
06731846
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a dispersion compensator suitable for use in an optical communication system, for compensating for wavelength dispersions of a transmission medium such as an optical fiber or the like in an optical pulse transmission path.
In an optical transmission system, a high-purity silica optical fiber is normally used as a transmission medium for a light signal. However, since the optical fiber has a wavelength dispersion, a pulse waveform is degraded when a light signal pulse having predetermined wavelength broadening is transmitted. The degradation of the light pulse waveform due to the dispersion of the optical fiber becomes a big factor that will restrict a transmission distance and transmission capacity of the optical transmission system. Therefore, the technology of canceling out such a wavelength dispersion becomes important for the large-capacity optical transmission system. If, for example, an optical system having the dispersion of the optical fiber and its reverse dispersion is inserted into an optical transmission path, then the dispersions of the optical fiber are canceled out so that the degraded waveform can be recovered or made up.
As the prior art, a technology for compensating for dispersions by use of a fiber (dispersion compensating fiber) having dispersions reverse in sign and large in absolute value has been put into practical use. The dispersion compensating fiber has been widely used because it has such characteristics that it is capable of realizing a desired characteristic with satisfactory reproducibility and has a wide compensable band, for example. However, a dispersion compensating value per unit length of the dispersion compensating fiber is as low as about −20 ps
m/km. If one attempts to obtain a desired dispersion value, then a very long fiber is needed. Therefore, a problem arises in that it cannot be brought into less size and becomes high in cost.
As a recent technology which aims to scale down a dispersion compensator, there has been proposed a dispersion compensator using a multidimensional structure of two or more types of mediums different in refractive index, i.e., a photonic crystal. It is known that light transmitted through the photonic crystal exhibits a peculiar dispersion characteristic. When a suitable lattice structure, a cycle or period, and the difference in refractive index between the mediums are selected with respect to light having a desired wavelength, a large dispersion can be obtained. A specific example thereof has been disclosed in Japanese Patent Application Laid-Open No. 2000-121987. The present dispersion compensator is one wherein a two-dimensional photonic crystal is formed on an Si (Silicon) substrate to compensate for dispersion. The dispersion compensator is capable of obtaining a few tens of +ps
m with a length of 5 mm.
However, the light that propagates through the two-dimensional photonic crystal, cannot avoid losses produced due to it scattering. The example disclosed in Japanese Patent Application Laid-Open No. 2000-121987 did not taken into consideration the insertion losses. Further, when the dispersion relations of the photonic crystal are used, a part of a complex dispersion curve is locally used. An extremely high degree of production accuracy is required to obtain desired performance, and the degree of freedom of design of the dispersion compensator is limited.
As described in the prior arts, there are known ones in which dispersion compensation for long-distance optical fiber communications has already been put into practical use. However, a dispersion compensator small in size and low in cost has not yet been realized.
With an increase in transmission capacity, there has been a need to use a large number of channels in higher density and over a wide range of wavelength regions or bands. Correspondingly, there is a need to assure wavelength dispersions more accurately. Further, a problem arises in that wavelength dispersions change with time due to a change in physical-property constant or the like with a variation in outside-air temperature. However, the conventional dispersion compensating system provides a fixed compensable dispersion value and is hence not capable of compensating for wavelength dispersions in an optical fiber, which change momentarily. In order to cope with such a problem, there has been a demand for a dispersion compensator capable of changing a compensation value flexibly and accurately according to conditions.
SUMMARY OF THE INVENTION
Thus, an object of the present invention is to provide a dispersion compensator which is ultra small in size and low in cost, and is capable of changing a dispersion compensating value, and an optical transmission system using the same.
In a dispersion compensator according to the present invention, defect modes of a photonic crystal are used to configure a waveguide having coupled microcavities or resonators, and a dispersion property of light that propagates through such a waveguide, is used to compensate for each waveform dispersion.
Upon the occasion of description of the principle and effects of the present invention, a description will first be made of the principle of allowing each defect in photonic crystal to function as a microcavity. Next, the concept and propagation characteristic of the waveguide having the coupled microcavities will be demonstrated and the principle of dispersion compensation using it will be explained.
The photonic crystal is a multidimensional periodic structure comprising combinations of two or more mediums different in refractive index.
FIG. 2
shows an example of one called a “two-dimensional photonic crystal” of photonic crystals.
FIG. 2
is a cross-sectional view of a structure which includes periodic structures as viewed in the direction horizontal to the sheet and vertically-extending structures are uniform. Columns each having a dielectric constant &egr;
2
are disposed in a medium having a dielectric constant &egr;
1
in a triangular lattice form (&egr;
1
>&egr;
2
). When each columnar portion is hollow, &egr;
2
=1. In
FIG. 2
, a indicates a lattice constant, and r indicates the radius of each column.
A diagram showing the relationship between a wave number of light propagating through a photonic crystal and the frequency thereof is called a photonic band chart.
FIG. 3
is a photonic band chart relative to a TM mode when &egr;
1
=3.5, &egr;
2
=1 and r/a=0.45 in the structure shown in FIG.
2
. Here, the TM mode indicates a mode in which an electric field is vertical to the sheet. The vertical axis indicates a normalized frequency (&ohgr;a/2&pgr;c), and the horizontal axis indicates a wave vector (ka/2&pgr;) normalized within a first brillouin zone. c indicates a light velocity in vacuum, &ohgr; indicates an angular frequency of the light and k indicates a wave number, respectively. The triangular lattice of
FIG. 2
corresponds to a hexagonal symmetry. A formed brillouin zone is an orthohexagonal structure shown in FIG.
3
. The apex of the orthohexagon is a point K, the midpoint of each side is a point M, and a point where a wave number is 0, is a point &Ggr;, respectively.
As diagonally shaded in
FIG. 3
, no band exists in a specific (normalized) frequency region over the whole region or band of the first brillouin zone. This means that light having a frequency corresponding to this band cannot propagate through a photonic crystal. Such a frequency band in which the propagation is prohibited, is called a “photonic bandgap”. For example, when light having a wavelength corresponding to a bandgap is launched into a crystal from outside, it is fully reflected.
Now consider where point defects, i.e., ununiform elements in periodic structures are introduced into a photonic crystal having a bandgap. Since the periodic structures are out of order at the defective portions, the band chart shown in
FIG. 3
is not applied and even light having a bandgap wavelength can exist. Since, however, the periphery of the
Hosomi Kazuhiko
Katsuyama Toshio
Lee Young-kun
Ojima Masahiro
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