Optical waveguides – Planar optical waveguide
Reexamination Certificate
2000-06-16
2003-10-07
Healy, Brian (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S130000, C385S131000
Reexamination Certificate
active
06631235
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a hybrid integrated-type planer lightwave circuit platform to be used for optical transmissions, and its method for manufacturing.
BACKGROUND OF THE INVENTION
As a method to improve mass-productivity of an optical module to be used in the field of optical transmissions, and reduce the cost thereof, the mounting method by which passive parts such as an optical coupler and active parts such as a light receiving/emitting element are hybrid-integrated has been widely noticed. Particularly, the method for mounting active parts on a platform having planar lightwave circuits (PLC) has been noticed in terms of its excellent mass-productivity and the integration of various optical circuits and a light receiving/emitting element enabled by the method.
A prior-art hybrid integrated-type planar lightwave circuit platform has the structure shown in
FIG. 4
, for example, and the PLC platform has a silica-based waveguide formed on a Si substrate, and is manufactured by the following processes. That is, in FIG.
4
:
1) Uneven steps having a predetermined planar patterns are formed on the Si substrate
1
to form a terrace portion
1
a
. This terrace portion
1
a
is loaded with optical elements later.
2) On the Si substrate
1
, silica-based soot is deposited by means of flame hydrolysis deposition method (FHD), and transparent-vitrified at a high temperature, whereby a lower cladding layer
2
made from silica-based glass is formed. As gases for the material of the silica-based soot, SiCl
4
to become silica glass SiO
2
and PCl
3
and BCl
3
to become dopants P
2
O
5
and B
2
O
3
for adjusting the glass softening temperature are used. P
2
O
5
and B
2
O
3
are dopants having the effect of lowering the glass softening temperature of silica glass. The sintering temperature for vitrification to change the soot condition into the transparent glass condition is set to be higher than the glass softening temperature of the lower cladding layer
2
.
3) Then, the surface of the covered Si substrate
1
is mechanically ground to expose the terrace portion
1
a
and make the surface flat.
4) Next, height adjusting layer
3
made from silica-based glass with the same refractive index as that of the lower cladding layer
2
is formed. Dopants P
2
O
5
and B
2
O
3
which have the effect of lowering the glass softening temperature are added to the height adjusting layer
3
. The amount of dopants to add is four times the amount of dopants added to the lower cladding layer
2
. Furthermore, the temperature for transparent-vitrification of the height adjusting layer
3
is set to be higher than the glass softening temperature of the height adjusting layer
3
. This height adjusting layer
3
is provided in order to adjust activation layer
6
a
of optical element
6
to be loaded later and core layer
4
of the waveguide to be equal in height.
5) Then, the core layer
4
made from silica-based glass is formed. To the core layer
4
, dopants P
2
O
5
and B
2
O
3
, which have the function of lowering the glass softening temperature of quartz glass, are added by an amount being approximately 1.1 times of the amount added to the lower cladding layer
2
, and furthermore, GeO
2
is added so that the refractive index of the core layer becomes approximately 0.5% greater than those of the lower cladding layer
2
and height adjusting layer
3
. The temperature for transparent-vitrification of the core layer
4
is set to be higher than the glass softening temperature for the core layer
4
.
6) Next, the core layer
4
is etched by means of reactive ion etching (RIE) so as to have core patterns having a predetermined function.
7) Next, upper cladding layer
5
made from silica-based glass is deposited, and the core layer
4
is buried therein. To the upper cladding layer
5
, dopants P
2
O
5
and B
2
O
3
which have the effect of lowering the glass softening temperature are added by an amount being approximately three times the amount of dopants added to the lower cladding layer
2
. In addition, the temperature for transparent-vitrification of the upper cladding layer
5
is higher than the glass softening temperature thereof, and the upper cladding layer is transparent-vitrified at a temperature lower than the glass softening temperature of the core layer
4
.
8) The height adjusting layer
3
, core layer
4
, and upper cladding layer
5
on the terrace portion
1
a
are removed.
9) A predetermined pattern of electrode
7
is formed on the terrace portion
1
a.
The height adjusting layer
3
is used to make the activation layer
6
a
of the optical element
6
loaded on the terrace portion
1
a
and core layer
4
to be equal in height, and is a thin-film with a thickness of 10 &mgr;m or less.
In the case where a glass film is formed by means of FHD, the silica-based glass film has properties whereby if the silica-based glass film is thin, transparent-vitrification by means of sintering becomes difficult, and also, if the amount of added dopants P
2
O
5
and B
2
O
3
increases, transparent-vitrification becomes easier.
Therefore, since the height adjusting layer
3
is thin (thickness: 10 &mgr;m or less), in order to make transparent-vitrification thereof easier, dopants are added to the height adjusting layer
3
by a larger amount than that of dopants added to the core layer
4
and upper cladding layer
5
.
The quartz glass has a property whereby if the dopant amount is increased, the glass softening temperature lowers. Therefore, when the glass softening temperatures of the lower cladding layer
2
, height adjusting layer
3
, core layer
4
, and upper cladding layer
5
are T
U
, T
T
, T
C
, and T
O
, the relationship among them is as shown in expression (1). The glass softening temperature of the height adjusting layer
3
is lower than that of the core layer
4
and upper cladding layer
5
.
T
U
>T
C
>T
O
>T
T
(1)
Since the temperatures for transparent-vitrification of the core layer
4
and upper cladding layer
5
are higher than the glass softening temperatures T
C
and T
O
of the core layer
4
and upper cladding layer
5
, when sintering and transparent-vitrifying the core layer
4
and upper cladding layer
5
, the height adjusting layer
3
whose glass softening temperature is lower than that of the core layer
4
and upper cladding layer
5
is softened, and the core pattern of the core layer
4
formed on the height adjusting layer is deformed, and therefore, it becomes difficult to stably obtain desired optical performance.
OBJECT AND SUMMARY OF THE INVENTION
The present invention is made in order to solve the abovementioned problems, and the object of the invention is to provide a planar lightwave circuit platform constructed by forming a silica-based lower cladding layer, height adjusting layer, core layer, and upper cladding layer in order on a substrate, wherein the glass softening temperature of the height adjusting layer is higher than that of the core layer and upper cladding layer.
Herein, the height adjusting layer is provided in order to adjust the activation layer of a loaded optical element and core layer to be equal in height.
In another aspect of the planar lightwave circuit platform of the invention, the temperature for transparent-vitrification of each layer is higher than the glass softening temperature of the same layer, and the glass softening temperature of the height adjusting layer is higher than the temperatures for transparent-vitrification of the core layer and upper cladding layer.
The planar lightwave circuit platform of the invention is used for an optical module as one of purposes for use.
Another object of the invention is to provide a method for manufacturing a planar lightwave circuit platform which has a process to form a silica-based lower cladding layer, height adjusting layer, core layer, and upper cladding layer in order on a substrate, wherein silica-based soot having a sufficient thickness for transparent-vitrification by means of sintering is deposited by means of FHD, and tran
Kawashima Hiroshi
Nakamura Shiro
Nara Kazutaka
Watanabe Kazunori
Healy Brian
Knobbe Martens Olson & Bear LLP
The Furukawa Electric Co. Ltd.
Wood Kevin S
LandOfFree
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