Optical waveguides – Planar optical waveguide
Reexamination Certificate
2001-03-07
2003-03-04
Nasri, Javaid (Department: 2839)
Optical waveguides
Planar optical waveguide
Reexamination Certificate
active
06529667
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical guide waveguide element, a manufacturing method for optical waveguide elements, an optical deflection element and an optical switching element, and more particularly to an optical waveguide element capable of being coupled with an optical fiber at high coupling efficiency and a manufacturing method for such optical waveguide elements, an optical deflection element and an optical switching element to which the optical waveguide element according to the invention is applied.
2. Description of the Related Art
Conventionally, glass such as silica, oxide ferroelectrics and electro-optical materials such as LiNbO
3
, magneto-optical materials such as Y
3
Ga
5
O
12
, polymers such as PMMA, and GaAs-based chemical compound semiconductors are used for planar optical waveguides. Among these materials, oxide ferroelectrics are known to manifest particularly satisfactory acousto-optical and electro-optical effects, but most of the actually produced acousto-optical elements and electro-optical elements use LiNbO
3
.
There are a wide variety of oxide ferroelectrics including LiNbO
3
, BaTiO
3
, PbTiO
3
, Pb
1-x
La
x
(Zr
y
Ti
1-y
)
1-x/4
O
3
(which may be PZT, PLT or PLZT depending on the values of x and y), Pb(Mg
⅓
Nb
⅔
)O
3
, KNbO
3
, LiTaO
3
, Sr
x
Ba
1-x
Nb
2
O
6
, Pb
x
Ba
1-x
Nb
2
O
6
, Bi
4
Ti
3
O
12
, Pb
2
KNb
5
O
15
, and K
3
Li
2
Nb
5
O
15
, and many of them are superior in characteristics to LiNbO
3
. Especially, Pb
1-x
La
x
(Zr
y
Ti
1-y
)
1-x/4
O
3
is known as a material having a much higher electro-optical coefficient than LiNbO
3
. The electro-optical coefficient of LiNbO
3
monocrystals is 30.9 pm/V whereas that of PLZT (8/65/35:x=8%, y=65%, 1-y=35%) ceramics is as high as 612 pm/V.
The reason why most of the elements actually produced use LiNbO
3
or LiTaO
3
, in spite of the availability of many ferroelectrics with better characteristics than LiNbO
3
, is that monocrystal growth technology and waveguide formation technology by Ti diffusion to wafers or proton exchange are well established for LiNbO
3
and LiTaO
3
, while thin films need to be formed by epitaxial growth for other materials than LiNbO
3
and LiTaO
3
, and conventional vapor-phase growth cannot provide thin film optical waveguides of high enough quality for practical use.
The present inventors, with a view to solving this problem, proposed a solid phase epitaxial growth technique capable of providing thin film optical waveguides of high enough quality for practical use (Japanese Published Unexamined Patent Application No. Hei 7-78508), but an epitaxially grown thin film optical guide often has to be thinner than the mode field diameter of the optical fiber on account of the requirement for singleness of the mode or that to reduce the drive voltage, inviting an increased loss in coupling with the optical fiber.
Previously, regarding semiconductor optical waveguides and silica-based waveguides, techniques to provide a flared optical waveguide in the position of connection to the optical fiber to reduce the coupling loss between the optical waveguide and the optical fiber were proposed in Japanese Published Unexamined Patent Application No. Hei 9-61652 and Japanese Published Unexamined Patent Application No. 5-182948 among others.
However, there is no technique available for producing a fine pattern in the epitaxially grown oxide thin film optical waveguide, making it difficult to fabricate the optical waveguide in a flared shape. For LiNbO
3
monocrystalline wafers for example, a fabrication method for three-dimensional optical waveguides to which Ti diffusion and proton exchange are described by Nishihara, Haruna and Suhara in a publication on optical integrated circuits by Ohmsha (1993) pp. 195-230, but no method for other elements or exchanging ions is known for other materials, especially Pb
1-x
La
x
(Zr
y
Ti
1-y
)
1-x/4
O
3
. For silica-based optical waveguides and the like, a method for producing channel optical waveguides by reactive ion etching is disclosed by Kawachi in NTT R&D, 43 (1994)1273 and elsewhere, but it is difficult to accomplish selectively etch a monocrystalline epitaxial ferroelectric thin film optical waveguide without roughing its surface, which would invite scattering loss or damaging the substrate, which is an oxide of the same kind as the thin film optical waveguide. For this reason, no report is found on the successful fabrication of any relatively loss-free channel optical waveguide into an epitaxial ferroelectric thin film optical waveguide. Furthermore, there is another problem that merely flaring an epitaxially grown oxide thin film optical waveguide can hardly prevent the waveguide mode from becoming multi-mode.
SUMMARY OF THE INVENTION
The present invention, therefore, provides an optical waveguide element which can be coupled with an optical fiber at high coupling efficiency. The invention also provides a manufacturing method for optical waveguide elements which can be coupled with optical fibers at high coupling efficiency. The invention further provides an optical switching element and an optical deflection element enabled to achieve coupling with an optical fiber at high coupling efficiency by applying the optical waveguide element according to the invention.
In order to achieve one of these intentions, an optical waveguide element according to an aspect of the invention is provided with an optical waveguide layer having an optical waveguide, and a cladding layer which is provided over at least one of the incidence end and the emission end of the optical waveguide on the surface of the optical waveguide layer, has a lower refractive index than that of the optical waveguide layer, and gradually increases in thickness towards the end(s) in a flared shape.
According to another aspect of the invention, there is provided an optical waveguide element manufacturing method for manufacturing optical waveguide elements by forming, over the surface of an optical waveguide layer provided with an optical waveguide, an amorphous thin film whose refractive index is smaller than that of the optical waveguide layer, shaping the amorphous thin film over at least one of the incidence end and the emission end of the optical waveguide to leave a flared part whose thickness increases towards the end(s), and forming the shaped amorphous thin film into a cladding layer by solid phase epitaxial growth.
Another manufacturing method for optical waveguide elements according to another aspect of the invention includes an optical waveguide formation step to shape an amorphous thin film formed over the surface of a monocrystalline substrate into a prescribed channel pattern, form a buffer layer by subjecting the amorphous thin film so shaped to solid phase epitaxial growth, and form a channel optical waveguide by solid phase epitaxial growth of an optical waveguide layer over the buffer layer, and a cladding layer formation step to form an amorphous thin film whose refractive index is smaller than that of the optical waveguide layer over the surface of the optical waveguide layer provided with the optical waveguide, shape the amorphous thin film over at least one of the incidence end and the emission end of the optical waveguide to leave a flared part whose thickness increases towards the end(s), and form a cladding layer by subjecting the amorphous thin film so shaped to solid phase epitaxial growth by heating.
An optical deflection element includes an optical waveguide layer having an epitaxial or single-oriented electro-optical effect, provided over an electroconductive or semi-electroconductive monocrystalline substrate to serve as a lower electrode or over a substrate over the surface of which is formed an electroconductive or semi-electroconductive monocrystalline substrate to serve as a lower electrode; a light beam controlling electrode which is arranged over the optical waveguide layer and forms, between the optical waveguide layer and the monocrystalline substrat
Fuji 'Xerox Co., Ltd.
Nasri Javaid
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