Optical waveguides – With optical coupler
Reexamination Certificate
2002-04-03
2004-05-11
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
C385S147000
Reexamination Certificate
active
06735354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device used for optical communication. In particular, the present invention relates to an optical device such as an optical separator, an optical filter, a light transmitting/receiving module for WDM (wavelength division multiplexing), an optical inductor, a bend waveguide, and an optical deflector.
2. Description of the Related Art
FIG. 17
shows an example of an optical separator utilizing a Y-separation waveguide, which is a conventional optical device. Light is incident upon a Y-separator
184
having an optical waveguide structure through an ingoing optical fiber
181
. Light propagating through a Y-shaped core
186
is separated to outgoing optical fibers
182
and
183
. The Y-separator
184
has a configuration in which a Y-shaped core
186
is formed on a substrate
185
.
In a conventional optical device, in order to couple light in the ingoing optical fiber
181
, the Y-separator
184
with an optical waveguide structure and the outgoing optical fibers
182
and
183
, it is necessary to conduct the alignment of optical axes and matching in mode shapes with high precision, which requires a high skill for assembling such an optical separator. In addition, since a separation angle of the Y-separator
184
is at most about 4°, when the length of the Y-separator
184
is set to be too small, there is insufficient light separation, which makes it difficult to miniaturize the optical separator.
A conventional transmitting/receiving module for WDM will be described with reference to FIG.
18
. The transmitting/receiving module for WDM is composed of an optical waveguide and a multi-layer filter.
On a substrate
191
, an optical waveguide
197
, a photodiode (1.3 &mgr;m)
193
, a laser diode (1.55 &mgr;m)
194
, a photodiode (1.55 &mgr;m)
195
, and a optical fiber
192
are placed.
The optical waveguide
197
is provided with a cladding
197
d
, a first core
197
a
, a second core
197
b
, a third core
197
c
, and a WDM dielectric multilayer filter (1.3/1.55 &mgr;m)
198
. The first core
197
a
, the second core
197
b
, and the third core
197
c
constitute a Y-shaped core, and the WDM dielectric multi-layer filter (1.3/1.55 &mgr;m)
198
is formed so as to divide these cores.
The photodiode (1.3 &mgr;m)
193
is disposed on the substrate
191
so as to be coupled to the first core
197
a
. Furthermore, the optical fiber
192
is fixed in a V-groove
196
formed on the substrate
191
so as to be coupled to the second core
197
b
. Furthermore, the laser diode (1.55 &mgr;m)
194
and the photodiode (1.55 &mgr;m)
195
are disposed on the substrate
191
so as to be coupled to the third core
197
c.
When signal light of 1.3/1.55 &mgr;m WDM is incident upon the second core
197
b
from the optical fiber
192
, the signal light is separated by the multi-layer filter
198
. Then, light (1.3 &mgr;m) propagates to the first core
197
a
, and light (1.55 &mgr;m) propagates to the third core
197
c
. The light propagating to the first core
197
a
is received by the photodiode (1.3 &mgr;m)
193
. Similarly, the light propagating to the third core
197
c
is received by the photodiode (1.55 &mgr;m)
195
. Furthermore, signal light emitted from the laser diode (1.55 &mgr;m)
194
propagates to the third core
197
c
. Then, the signal light is guided to the second core
197
b
by the multi-layer filter
198
and sent to the optical fiber
192
. An arrow
199
a
represents a propagation direction of the light (1.3 &mgr;m), and an arrow
199
b
represents a propagation direction of the light (1.55 &mgr;m).
As described above, by using the WDM transmitting/receiving module, bidirectional communication can be conducted with light (1.55 &mgr;m), and communication of receiving only can be conducted with light (1.3 &mgr;m).
However, the conventional WDM transmitting/receiving module requires the optical waveguide
197
having a Y-shaped core and the multi-layer filter
198
for separation of a wavelength. This increases the number of components, making it difficult to achieve a low cost.
In order to solve the above-mentioned problem, constituting an optical device such as an optical separator and an optical filter with a photonic crystal has drawn attention. For example, JP11(1999)-271541 discloses a wavelength separating filter using a photonic crystal with a two-dimensional triangular lattice.
In the present specification, the term “photonic crystal” refers to an artificial multi-dimensional periodic structure substantially having a period of a light wavelength.
FIGS. 19A and 19B
show a configuration of the wavelength separating filter using a photonic crystal disclosed by JP11(1999)-271541. In this configuration, materials with different refractive indexes are arranged periodically, whereby strong deflection dispersion characteristics (which are not found in general optical crystal) are obtained to control wavelength deflection. Specifically, as shown in
FIG. 19A
, the wavelength separating filter has a configuration in which a substrate
200
, which has atomic media
204
embedded in a background medium
203
in a two-dimensional triangular arrangement, is interposed between a first cladding
201
and a second cladding
202
. As shown in
FIG. 19B
, an incident surface
208
of a light signal is tilted at a predetermined angle with respect to an incident direction
207
of the light signal, and the light signal is output from an output surface
209
. The interval between the adjacent atomic media
204
is designed in accordance with the wavelength of a light signal. The thickness of the substrate
200
is designed in such a manner that a light signal is confined sufficiently in the substrate
200
, and a light traveling direction does not deviate largely from the surface of the substrate
200
.
The above-mentioned photonic crystal with a two-dimensional triangular lattice has a structure in which lattice vectors are matched with reciprocal lattice vectors. Even if light is incident upon a photonic crystal with such a structure in a lattice vector direction, strong deflection dispersion characteristics cannot be obtained. In order to obtain strong deflection dispersion characteristics, it is required to set a light incident surface of the photonic crystal so as to be non-vertical to a lattice vector direction or to tilt the light incident surface with respect to an incident surface vertical to the lattice vector direction, thereby allowing light to be incident upon the photonic crystal. Therefore, the incident surface
208
is tilted at a predetermined angle with respect to the incident direction
207
of a light signal in FIG.
19
B.
Next, a relationship between primitive lattice vectors (a
1
, a
2
) and basic reciprocal lattice vectors (b
1
, b
2
) will be described.
FIGS. 20A
to
20
C respectively show a relationship between a lattice and a Brillouin zone.
FIG. 20A
shows a tetragonal lattice, and
FIG. 20B
shows a triangular lattice. In each of
FIGS. 20A
to
20
C, the upper stage shows a lattice space, whereas the lower stage shows a reciprocal lattice space. Reference numeral
211
denotes atomic media constituting a lattice, and
212
denotes a Brillouin zone. The tetragonal lattice and the triangular lattice respectively have a symmetric structure (for example, an interior angle equal to or smaller than 90° between the primitive lattice vectors is 45°, 60°, 90°, or the like). Important symmetric points of the Brillouin zone
212
in the tetragonal lattice and the triangular lattice shown in
FIGS. 20A and 20B
are two points (X, M) and (M, K), respectively. With such a structure, incident light to the primitive lattice vectors (a
1
, a
2
) does not exhibit deflection characteristics because the direction of the incident light is matched with the direction of the important symmetric point of the Brillouin zone
212
.
On the other hand, in the case of an oblique lattice with low symmetry as shown in
FIG. 20C
, for example, when an interior angle &thgr; between the primitive
Merchant & Gould P.C.
Ullah Akm Enayet
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