Optical waveguide device and method for making such device

The present disclosure describes an optical waveguide device and a method for making such device. The device comprises a substrate; a waveguide embedded in the substrate, the waveguide having a refractive index higher than the refractive index of the substrate; and first and second optical mirrors placed respectively in two different positions along the waveguide. The method comprises steps of (a) cleaning a substrate by means of a cleaning agent; (b) embedding a waveguide in the substrate; and (c) placing first and second optical mirrors respectively at two different positions in the substrate, the steps (b) and (c) being performed in such a manner that the mirrors being positioned along the waveguide.

FIELD OF THE INVENTION 
The present invention relates to an optical waveguide device and a method 
for producing such device. More particularly, the present invention 
relates to an optical waveguide device which can be used to make 
non-linear optical devices such as logic gates, switches and modulators. 
The present device can also be used to make lasers and tap and combiner 
devices. The present invention may have many applications in optical 
communication, optical signal processing systems and optical sensors. 
BACKGROUND OF THE INVENTION 
There are many nonlinear optical devices such as a laser which comprises a 
waveguide disposed between two optical mirror devices. This kind of laser 
is very difficult to mass-produced. 
It is an object of the present invention to provide an optical waveguide 
device which can be mass-produced. 
It is another object of the present invention to provide an optical 
waveguide device which has a small size. 
It is another object of the present invention to provide an optical 
waveguide device which requires a very low-threshold light power. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided an optical waveguide 
device comprising: 
a substrate; 
a waveguide embedded in said substrate, said waveguide having a refractive 
index higher than the refractive index of said substrate; and 
first and second optical mirrors placed respectively in two different 
positions along said waveguide. 
According to the present invention, there is also provided a method for 
making an optical waveguide device, comprising steps of: 
a) cleaning a substrate by means of a cleaning agent; 
b) embedding a waveguide in said substrate, said waveguide having a 
refractive index higher than the refractive index of said substrate; and 
c) placing first and second optical mirrors respectively at two different 
positions in said substrate, said steps (b) and (c) being performed in 
such a manner that said mirrors being positioned along said waveguide. 
The objects, advantages and other features of the present invention will 
become more apparent upon reading of the following non restricted 
description of preferred embodiments thereof, given for the purpose of 
examplification only with reference to the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In FIG. 1, there is shown an optical waveguide device comprising a 
substrate 2 and a waveguide 4 embedded in the substrate 2. The waveguide 4 
has a refractive index higher than the refractive index of the substrate 
2. The device also comprises first and second optical mirrors 6 and 8 
secured respectively at both ends of said waveguide 4. 
In a first embodiment of the optical waveguide device shown in FIG. 1, the 
substrate 2 is made of glass and contains a rare-earth element. 
In a second embodiment of the optical waveguide device shown in FIG. 1, the 
substrate 2 is made of glass and free from rare-earth element, and the 
waveguide 4 contains a rare-earth element. 
In FIGS. 2 and 3, there are shown different embodiments of an optical 
waveguide device according to the present invention. These embodiments 
comprise a substrate 2 and a waveguide 4 embedded in the substrate 2. The 
waveguide 4 has a refractive index higher than the refractive index of the 
substrate 2. They also comprise first and second optical mirrors 6 and 8 
which are placed respectively in the two different positions along the 
waveguide 4. These first and second optical mirrors 6 and 8 are 
respectively first and second gratings. These gratings contain a 
rare-earth element. The gratings are defined by the Bragg resonant 
equation which is: 
##EQU1## 
where .LAMBDA. is the period of the gratings, .lambda. is the emission 
wavelength of the rare-earth element, and N is the effective index of the 
waveguide 4. The first and second gratings have respectively a refractive 
index different from the refractive index of the waveguide 4. 
In a first embodiment of the optical waveguide device shown in FIG. 2, the 
substrate 2 is made of glass and contains the rare-earth element, and the 
waveguide 4 contains the rare-earth element. 
In a second embodiment of the optical waveguide device shown in FIG. 2, the 
substrate 2 is made of glass and free from the rare-earth element, and the 
waveguide 4 contains the rare-earth element. 
In a third embodiment of the optical waveguide device shown in FIG. 2, the 
substrate 2 is made of glass and free from the rare-earth element, and the 
waveguide 4 is free from the rare-earth element. The grating 8 contains 
the rare-earth element. 
In the three embodiments of the optical waveguide device shown in FIG. 2, 
the refractive index of the first grating is substantially similar to the 
refractive index of the second grating. 
In the embodiment of the optical waveguide device shown in FIG. 3, the 
waveguide comprises two longitudinal waveguide sections. Each section has 
two extremities 10, 11, 13 and 15 adjacent to the edge of the substrate 2. 
One section has two distinct areas 12 adjacent to two distinct areas 14 of 
the other section. The two different positions of the gratings being 
situated respectively on each waveguide section 4 between its two distinct 
areas 12 and 14. The waveguide 4 is free from the rare-earth element, and 
the substrate 2 is made of glass and free from the rare-earth element. The 
grating 8 contains the rare-earth element. 
The method for making the first embodiment of the optical waveguide device 
shown in FIG. 1, comprises steps of cleaning a substrate by means of a 
cleaning agent, embedding a waveguide in the substrate, and securing first 
and second optical mirrors respectively at both ends of the waveguide. The 
substrate is made of glass and doped with a rare-earth element. The step 
of embedding comprises steps of depositing a film onto the surface of the 
substrate by an evaporation process; forming a mask by making openings in 
the film to expose parts of the surface; immersing the substrate in a bath 
of molten salt to form the waveguide by diffusion of the rare-earth 
element; and removing the mask from the surface. The mask is formed by a 
photolithographic process. 
The method for making the second embodiment of the optical waveguide device 
shown in FIG. 1, comprises steps of cleaning a substrate by means of a 
cleaning agent, embedding a waveguide in the substrate, and securing first 
and second optical mirrors respectively at both ends of the waveguide. The 
substrate is made of glass and free from rare-earth element. The embedding 
step comprises steps of depositing a film onto the surface of the 
substrate by an evaporation process; forming a mask by making openings in 
the film to expose parts of the surface; immersing the substrate in a bath 
of molten salt containing the rare-earth element to form the waveguide by 
diffusion of the rare-earth element; and removing the mask from the 
surface. The mask is formed by a photolithographic process. 
The method for making the first embodiment of the optical waveguide device 
shown in FIG. 2, comprises steps of cleaning a substrate by means of a 
cleaning agent, embedding a waveguide in the substrate, and placing first 
and second optical mirrors at two different positions along the waveguide. 
The embedding process comprises steps of depositing a first film onto the 
surface of the substrate by an evaporatin process, forming a first mask by 
making openings in the first film to expose first parts of the surface, 
immersing the substrate in a first bath of molten salt to form the 
waveguide by diffusion, and removing the first mask from the surface. The 
first mask is formed by a photolithographic process. 
The process for positioning the mirrors comprises steps of depositing a 
second film onto the surface by an evaporation process, forming a second 
mask by making openings in the second film to expose two second parts of 
the surface, and immersing the substrate in a second bath of molten salt 
to change the refraction index of the glass in the two second parts to 
form first and second gratings containing a rare-earth element. The second 
mask is formed by a photolithographic process. The two second parts are 
situated at two different positions along the waveguide. The first and 
second gratings constitute the first and second mirrors. The gratings are 
defined by the Bragg resonant equation which is: 
##EQU2## 
where .LAMBDA. is the period of the gratings, .lambda. is the emission 
wavelength of the rare-earth element, and N is the effective index of the 
waveguide. The first and second gratings have a refractive index different 
from the refractive index of the waveguide. The second bath of molten salt 
has a temperature lower than the temperature of the first bath to prevent 
modification in the dimensions of the waveguide. 
The substrate 2 is doped with the rare-earth element. The molten salt of 
the first bath is different from the molten salt of the second bath to 
produce respectively different refractive indexes in the gratings and in 
the waveguide. The openings of the second mask are perpendicular to the 
waveguide. The method also comprises, after the removing of the first mask 
from the surface and before the deposition of the second mask onto the 
surface, a step of immersing the substrate in a third bath of molten salt 
to bury the waveguide. 
The method for making the second embodiment of the optical waveguide device 
shown in FIG. 2, comprises steps of cleaning a substrate by means of a 
cleaning agent, embedding a waveguide in the substrate, and placing first 
and second optical mirrors at two different positions along the waveguide. 
The embedding process comprises steps of depositing a first film onto the 
surface of the substrate by an evaporation process, forming a first mask 
by making openings in the first film to expose first parts of the surface, 
immersing the substrate in a first bath of molten salt to form the 
waveguide by diffusion, and removing the first mask from the surface. The 
first mask is formed by a photolithographic process. 
This process of placing first and second optical mirrors comprises steps of 
depositing a second film onto the surface by an evaporation process, 
forming a second mask by making openings in the second film to expose two 
second parts of the surface, and immersing the substrate in a second bath 
of molten salt to change the refraction index of the glass in the two 
second parts to form first and second gratings containing a rare-earth 
element. The second mask is formed by a photolithographic process. The two 
second parts are situated at two different positions along the waveguide. 
The first and second gratings constitute the first and second mirrors. The 
gratings being defined by the Bragg resonant equation. The first and 
second gratings have a refractive index different from the refractive 
index of the waveguide. The second bath of molten salt has a temperature 
lower than the temperature of the first bath to prevent modification in 
the dimensions of the waveguide. The molten salt of the first bath 
contains the rare-earth element. The diffusion performed when the 
substrate is immersed in the first bath of molten salt, is a diffusion of 
the rare-earth element. 
The method for making the third embodiment of the optical waveguide device 
shown in FIG. 2, comprises steps of cleaning substrate by means of a 
cleaning agent, embedding a waveguide in the substrate and placing first 
and second optical mirrors (gratings) respectively at two different 
positions along the waveguide. The placing of the first and second 
gratings comprises steps of depositing a first film onto the surface of 
the substrate by an evaporation process, forming a first mask by making 
openings in the first film to expose two first parts of the surface, 
immersing the substrate in a first bath of molten salt to change the 
refractive index of said glass in the two first parts to form first and 
second gratings containing a rare-earth element, and removing the first 
mask from the surface. The first mask is formed by a photolithographic 
process. The two first parts are situated at two different positions in 
the substrate. The first and second gratings constitute the first and 
second mirrors. The gratings are defined by the Bragg resonant equation. 
The waveguide embedding step is performed in such a manner that the mirrors 
are positioned along the waveguide. This embedding step comprises steps of 
depositing a second film onto the surface by an evaporation process, 
forming a second mask by making openings in the second film to expose 
second part of the surface, and immersing the substrate in a second bath 
of molten salt to form the waveguide by diffusion. The second mask is 
formed by a photolithographic process. The waveguide has a refractive 
index different from the refractive index of the first and second 
gratings. The second bath has a temperature lower than the temperature of 
the first bath to prevent modification in the dimensions of the gratings. 
The substrate and the waveguide are free from the rare-earth element. 
The method for making the embodiment of the optical waveguide device shown 
in FIG. 3, comprises steps of cleaning a substrate by means of a cleaning 
agent, embedding a waveguide in the substrate and placing first and second 
optical mirrors (gratings) respectively at two different positions along 
the waveguide. The placing of the first and second gratings comprises 
steps of depositing a first film onto the surface of the substrate by an 
evaporation process, forming a first mask by making openings in the first 
film to expose two first parts of the surface, immersing the substrate in 
a first bath of molten salt to change the refractive index of said glass 
in the two first parts to form first and second gratings containing a 
rare-earth element, and removing the first mask from the surface. The 
first mask is formed by a photolithographic process. The two first parts 
are situated at two different positions in the substrate. The first and 
second gratings constitute the first and second mirrors. The gratings are 
defined by the Bragg resonant equation. 
The waveguide embedding step is performed in such a manner that the mirrors 
are positioned along the waveguide. This embedding step comprises steps of 
depositing a second film onto the surface by an evaporation process, 
forming a second mask by making openings in the second film to expose 
second part of the surface, and immersing the substrate in a second bath 
of molten salt to form the waveguide by diffusion. The second mask is 
formed by a photolithographic process. The waveguide has a refractive 
index different from the refractive index of the first and second 
gratings. The second bath has a temperature lower than the temperature of 
the first bath to prevent modification in the dimensions of the gratings. 
The substrate and the waveguide are free from the rare-earth element. The 
second parts exposed by the second mask comprise two longitudinal 
sections, each section having two extremities adjacent to the edge of the 
substrate. One section has two distinct areas adjacent to two distinct 
areas of the other section. The two first parts exposed by the first mask 
comprise two portions of the second parts, said two portions being 
situated respectively on each section of the second parts, between its two 
distinct areas. 
The optical waveguide device shown in FIG. 1 and in FIG. 2 can be used to 
produce a laser beam. These figures show a waveguide that is doped with a 
rare-earth element and provided with mirrors constituted by gratings. This 
waveguide can be pumped at one of the absortion wavelength of the 
rare-earth element, then, it can be used as a laser at the emission 
wavelength of the rare-earth element for which the mirrors are designed. 
This laser with mirrors is easier to fabricate and can be used as 
low-threshold individual laser. This laser is also simpler to integrate 
with other component on one substrate. 
The optical waveguide device shown in FIG. 3 can be used as a tap and 
combiner device. In operation, this tap and combiner device is pumped 
through port A 10 at one of the absortion wavelength of the rare-earth 
element. The gratings are designed to reflect at emission wavelength of 
the rare-earth element, .lambda..sub.1. Therefore, .lambda..sub.1 exits 
through port B 11, and .lambda..sub.0 exits through port D 13. 
Although, the present invention has been explained hereinabove by way of 
preferred embodiments thereof, it should be pointed out that any 
modifications to these embodiments, within the scope of the appended 
claims is not deemed to change or alter the nature and scope of the 
present invention.