Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2002-05-30
2004-09-21
Healy, Brian (Department: 2874)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S129000, C385S046000
Reexamination Certificate
active
06795631
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an optical waveguide apparatus for use in a wavelength division multiplex (WDM) optical communication system as well as a method of producing the same. More specifically, this invention relates to a planar optical waveguide device for implementing a wavelength selecting function, such as an arrayed waveguide grating (AWG) used as an optical signal multiplexer or demultiplexer, as well as a method of producing the same.
In recent years, a wavelength division multiplex (WDM) transmission system becomes widely used in optical transmission. In the WDM transmission system, a number of signals different in wavelength from one another are multiplexed and transmitted through a single optical fiber. As a greater number of signals are multiplexed, a transmission capacity is increased. Most recently, 100 or more signals different in wavelength are multiplexed. As a consequence, a separation or spacing between different wavelengths is narrowed. For example, in a system of a 100 GHz grid, the spacing between two adjacent wavelengths must be equal to 0.8 nm. The WDM transmission system is initially used in a long-distance network but is growing wider applications covering a periphery of a terminal.
In the above-mentioned WDM transmission system, a device having a wavelength selecting function of selecting a particular signal among a number of signals different in wavelength is essential and indispensable. Such wavelength selecting function is provided by a planar optical waveguide device as an integrated device.
As an example of the planar optical waveguide device having such a wavelength selecting function, an arrayed waveguide grating (AWG) is disclosed in Japanese Patent No. 2599786 (JP 2599786 C). The arrayed waveguide grating is used as an optical multiplexer/demultiplexer. Referring to
FIG. 1
, a waveguide pattern of silica-based glass is formed on a substrate
1
. The waveguide pattern includes at least one optical input waveguide
2
, an input-side slab waveguide
3
as a first slab waveguide, a plurality of patterned or arrayed waveguides (channel waveguides)
4
different in length from one another, an output-side slab waveguide
5
as a second slab waveguide, and at least one optical output waveguide
6
(in the illustrated example, a plurality of optical output waveguides
6
are shown) which are successively connected in this order. A combination of the arrayed waveguides
4
forms a diffraction grating
14
so that the arrayed waveguide grating is provided. For simplicity of illustration, only a small number of waveguides are shown in FIG.
1
. In an actual device, the arrayed waveguides are equal in number to about 100. The number of the optical output waveguides corresponds to the number of output channels.
The optical input waveguide
2
is connected to an optical fiber (not shown) so as to introduce a wavelength-multiplexed light beam. The light beam introduced through the optical input waveguide
2
into the input-side slab waveguide
3
is spread due to a diffracting effect of the input-side slab waveguide
3
to be incident to the respective arrayed waveguides
4
as split beams which propagate through the respective arrayed waveguides
4
. The split beams propagating through the respective arrayed waveguides
4
reach the output-side slab waveguide
5
. The split beams reaching the output-side slab waveguide
5
are condensed or focused as a focused beam which propagates into the optical output waveguides
6
to be outputted therefrom.
In the arrayed waveguide grating described above, the arrayed waveguides
4
are different in length from one another. Therefore, after the split beams delivered from the input-side slab waveguide
3
propagate through the respective arrayed waveguides
4
, the split beams are shifted or differed in phase from one another. Depending upon the magnitude (quantity) of the phase shift or difference, the wavefront of the focused beam is tilted. A focusing position is determined by the tilting angle of the wavefront of the focused beam. Therefore, by forming the optical output waveguides
6
at that position, output light beams different in wavelength from one another can be produced from the optical output waveguides
6
corresponding to the different wavelengths, respectively.
In the arrayed waveguide grating, the diffraction grating
14
has a wavelength resolution proportional to a difference (&Dgr;L) in length between the arrayed waveguides
4
forming the diffraction grating
14
. Therefore, by designing the diffraction grating
14
with a greater value of &Dgr;L, it is possible to carry out optical multiplexing and demultiplexing for multiple light beams at a narrower wavelength spacing.
However, in the above-mentioned arrayed waveguide grating, the patterened waveguides
4
are different in length from one another. This means that variations in length (optical path length) of the arrayed waveguides
4
in response to the variation in device temperature are different from one another. Therefore, in response to the variation in device temperature, filtered wavelengths, i.e., wavelengths demultiplexed by the arrayed waveguides
4
are greatly changed.
In order to solve the above-mentioned problem, it is proposed to introduce a temperature control mechanism into the optical multiplexer/demultiplexer. The temperature control mechanism comprises a Peltier device for cooling and a temperature control circuit and carries out temperature control of the arrayed waveguide grating so that the temperature variation itself is eliminated. However, introduction of such a temperature control mechanism results in an increase in size of the apparatus, an increase in cost, and an increase in power consumption.
As another approach without using the Peltier device, proposal is made of a method which will hereinafter be described in conjunction with FIG.
1
. In order to cancel the temperature dependence of the arrayed waveguides
4
of the arrayed waveguide grating, a trapezoidal groove is formed across the arrayed waveguides
4
, as depicted by a broken line in FIG.
1
. The arrayed waveguides
4
comprise silica-based glass cores having a positive temperature coefficient of refractive index. A temperature compensating part
9
is formed by filling the trapezoidal groove with silicone resin having a negative temperature coefficient of refractive index.
By canceling the variation in optical path length due to the temperature-dependent variation in refractive index of each arrayed waveguide
4
, it is possible to remove the temperature dependence of the arrayed waveguide grating (see “Athermal silica-based arrayed-waveguide grating (AWG) multiplexer”, ECOC '97 Technical Digest, pp. 33-36, 1997). In this approach, the temperature dependence of the transmission wavelength of the arrayed waveguide grating is reduced to a small value equal to 0.001 nm/° C. or less.
However, with the above-mentioned structure, optical mismatch is caused between the arrayed waveguides and the temperature compensating part filled with silicone resin. Furthermore, it is difficult to form a cladding layer on the temperature compensating part
9
of a trapezoidal shape formed in the arrayed waveguide grating. This brings about occurrence of excessive loss in a region of the temperature compensating part
9
of a trapezoidal shape. As a consequence, the optical transmission loss characteristic of the arrayed waveguide grating as a whole device is degraded.
As a still another approach, EP 0849231 A1 discloses a method of improving the temperature characteristic by selecting a material of the waveguide. This method aims to improve the temperature characteristic of the device resulting from the difference in temperature dependence between the waveguides different in material. By exactly matching the optical path length temperature-dependent variation rate of two waveguides, the temperature characteristic of the wavelength control function is improved.
However, the above-mentioned method is not applicable to a device such that th
Kobayashi Satoshi
Noro Yoshihiko
Suzuki Hisanori
Takahashi Shiro
Healy Brian
Hoya Corporation
Sughrue & Mion, PLLC
Wood Kevin S.
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