Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2000-06-21
2002-04-30
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S049000, C385S050000, C385S036000
Reexamination Certificate
active
06381389
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to an optical coupling device and an optical coupling method used in optical communication, particularly in wavelength division multiplex communication and ultra high speed optical communication.
2. Detailed Description of the Related Art
A conventional technique as disclosed in Japanese Patent Laying-Open Publication No. 8-234062 entitled “Optical Coupling Device and Optical Coupling Method” is known.
FIG. 1
is a schematic view showing this conventional technique. A core
101
is formed on a semiconductor substrate
102
, and the core
101
has its upper surface and side surfaces inclined in the propagation direction of light, so that the cross sectional size thereof gradually changes along this light propagation direction. A first cladding layer
103
is formed to surround the core
101
. The refractive index of the first cladding layer
103
is smaller than that of the core
101
, and larger than that of the semiconductor substrate
102
. Here, the thickness t
2
of the first cladding layer
103
is so designed that the spot size at the end plane for optical coupling matches the large spot size of an optical waveguide device, at least in the vicinity of the end plane for optical coupling with the optical waveguide device.
The optical coupling device and optical coupling method permit the spot size to be changed for optically coupling optical waveguide devices having different spot sizes.
The optical coupling device and optical coupling method described above are however encountered with the following disadvantages. More specifically, by the optical coupling device and optical coupling method described above, it is hard to convert the spot size of a light beam at the incident side into an incredibly different size at the emitting side. Particularly in order to reduce the spot size of the emitting side beam to the level of the wavelength, the taper of the upper surface and side surfaces of the core
101
must be extremely steep. The resultant shape might no longer be defined as a taper or inclined surface. As a result, conversion with low losses would be difficult. Particularly in the case of a waveguide using photonic crystal as shown in
FIG. 2
, the waveguide size will become 1 &mgr;m or less, i.e., less than the wavelength, and it would be impossible to convert the spot size into this level by the conventional technique as described above.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical coupling device and an optical coupling method permitting the spot size of a light beam at the incident side to be converted into an incredibly different size at the emitting side.
The present invention is particularly directed to a technique of optically coupling an optical fiber to a waveguide using the photonic crystal as shown in FIG.
2
. More specifically, the spot size of light emitted from such an optical fiber is about 10 &mgr;m, while the width of the photonic crystal waveguide is 1 &mgr;m or less, and therefore the spot size of light should be changed into a size at most 1/10 as large.
An optical coupling device according to the present invention couples optical waveguide devices having different spot sizes and uses photonic crystal having a modulation structure having a refractive index periodic at about the same interval as the wavelength of light used by the optical waveguide devices. As a result, a spot size of the light at an emitting end of said photonic crystal can be a different size from the spot size at an incident end of said photonic crystal.
In the optical coupling device, crystal arrangement and light incident direction thereof may be set so that a dispersion plane of the photonic crystal is inclined when viewed from the light incident direction, whereby a spread width of the light propagating in a photonic crystal in wave number space can be larger than a spread width of the incident light in the wave number space. As a result, a spot size thereof in a real space in inverse proportion to the spread width can be relatively reduced.
Also, in the optical coupling device, curvature of the dispersion plane of the photonic crystal may be set larger than curvature of an light cone of the incident light. Thus, an effective refractive index in the photonic crystal can be increased, the product of the spread width in the wave number space and the spot size in the real space can be reduced, and the spot size of light propagating in the photonic crystal in the real space can be effectively reduced.
In the optical coupling device, the curvature of the dispersion plane of the photonic crystal can be set smaller than the curvature of the light cone of the incident light, so that the spot size of the light propagating in the photonic crystal in the real space can be effectively increased.
An optical coupling method according to the present invention, for an optical coupling device coupling optical waveguides having different spot sizes to one another, uses photonic crystal having a modulation structure having a refractive index periodic about at the same interval as the wavelength of light used by the optical waveguides, so that the spot size at the emitting end of said photonic crystal is converted into a size different from the spot size at the incident end of said photonic crystal.
According to the present invention, the photonic crystal is defined as crystal having a modulation structure having a refractive index periodic at about the same interval as the wavelength of light passing therethrough. According to the present invention, the photonic crystal is applied to an optical coupling device as a lens. Upon the application, the relation between the periodic interval of the photonic crystal and the wavelength and incident angle of light is defined. The modulation structure with the periodic refractive index can be formed, for example, by providing a number of holes
26
in a Si layer
21
sandwiched between a pair of SiO
2
layers
22
and
23
as shown in FIG.
3
. The effect of the photonic crystal is brought about by the holes
26
.
In the photonic crystal used in the present invention, the crystal arrangement and the light incident direction are set so that its dispersion plane (equi-energy plane in a wave number surface) is inclined when viewed in the incident direction of light or so that the curvature of the dispersion plane is larger than the curvature of the light cone of the incident light. In an embodiment, a light incident plane
24
and a light emitting plane
25
form an angle of 70°, while light emitted from an optical fiber forms an angle of 7° with respect to the normal direction of the incident light. Such angles depend on the periodic intervals of holes, the arrangement pattern of the holes and the wavelength of the incident light, rather than limited to 70° and 7°, and these numerical values are determined to enhance the lens effect.
REFERENCES:
patent: 5526449 (1996-06-01), Meade et al.
patent: 6317554 (2001-11-01), Kosaka et al.
patent: 8-234062 (1996-09-01), None
patent: 2001-4877 (2001-01-01), None
Enoch et al., Numerical evidence of ultrarefractive optics in photonic crystals. Optics Communications (161), pp. 171-176. Mar. 1999.*
Mekis et al., Tapered couplers for efficient interfacing between dielectric and photonic crystal waveguides. Journal of Lightwave Technology, vol. 19 No. 6, pp. 861-865. Jun. 2001.
Bovernick Rodney
NEC Corporation
Stahl Michael J.
LandOfFree
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