Process for producing buried waveguide device

A buried wave guide device is produced by immersing a phosphate glass containing exchangeable ions in a molten salt. Ions are exchanged from the phosphate glass to give a wave guide whose refractive index increases sharply from a first surface to a maximum at a depth below the first surface then decreases gradually in the direction of the opposite surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for producing a buried wave 
guide device which is used for branching or coupling of a light in fields 
such as optical communication and the like. 
2. Prior Art 
Compared with optical circuits of microoptics type produced by combination 
with a micro lens, prism or optical fiber, optical circuits of wave guide 
type produced by allowing high refractive index ion(s) to diffuse into a 
portion of a substrate to form a wave guide or a lens in the substrate, 
are advantageous in that they require no alignment and are resistant to 
vibration because the wave guide and the lens are formed in one substrate. 
Accordingly, the wave guide type optical circuits are finding wide 
applications. As one of the wave guide type optical circuits, there is a 
branching circuit using a wave guide (a slab wave guide), such as 
presented at the 4th Optical Meeting on Gradient Index Optical Imaging 
System (Jul. 4-5, 1983, Kobe) by K. Sano et al. The wave guide device 3 
presented has a structure as shown in FIG. 5, in which a wave guide 4 is 
formed in the vicinity of the upper surface of a substrate, fiber arrays 5 
and 6 are provided at the both ends of the wave guide 4 and thereby an 
optical signal emitted from one optical fiber at one end of the wave guide 
4 can be divided into a plurality of optical fibers at the other end of 
the wave guide 4. This wave guide 3 has hitherto been produced by the 
following two steps. That is, a plate-like glass containing large amounts 
of alkali ion(s) (e.g. Na.sup.+, K.sup.+) is produced, a metal film is 
formed on the four side surfaces and the bottom surface of the plate-like 
glass according to a known method to mask the surfaces, then the 
plate-like glass is immersed in a molten salt containing compound(s) of 
ion(s) (e.g. Ag.sup.+, Tl.sup.+, Li.sup.+) capable of giving high 
refractive index to the glass (said ion(s) being hereinafter referred to 
as "high refractive index ion(s)"), to allow these ion(s) to diffuse into 
the plate-like glass (the first step); subsequently, the plate-like glass 
is immersed in a molten salt containing compound(s) of ion(s) (e.g. 
Na.sup.+, K.sup.+ ) capable of giving low refractive index to the glass 
(said ion(s) being hereinafter referred to as "low refractive index 
ion(s)"), to ion exchange, at the peripheral portion of the glass, the 
ion(s) of high refractive index with the ion(s) of low refractive index 
(the second step); the first and second steps are effected while applying, 
if necessary, an electric field, whereby is obtained a buried wave guide 
device whose refractive index is maximum at the center of the wave guide 
and decreases gradually from the center to the peripheral portion of the 
wave guide. The above two-step method is explained further by referring to 
FIG. 6 showing the changes at each step, of the concentrations of ions in 
the plate-like glass. A plate-like starting glass having a uniform 
concentration 7a of alkali ion from the surface throughout the thickness 
d.sub.0 as shown in FIG. 6(A) is masked at the four side surfaces and the 
bottom surface, then the glass is immersed in a molten salt containing 
compound(s) of high refractive index ion(s), whereby ion exchange takes 
place between the alkali ion(s) and the high refractive index ion(s) in 
the portion of the glass ranging from the upper surface to a desired depth 
and there is obtained a plate-like glass in which the alkali ion(s) 
concentration 8a and the high refractive index ion(s) concentration 8b 
vary from the surface toward the depth direction as shown in FIG. 6(B) 
(the first step); the plate-like glass is then immersed in a molten salt 
containing compound(s) of low refractive index ion(s), whereby ion 
exchange takes place between the high refractive index ion(s) and the low 
refractive index ion(s) in the vicinity of the upper surface of the glass 
and there is obtained a buried wave guide device in which the alkali 
ion(s) concentration 9a, the high refractive index ion(s) concentration 9b 
and the low refractive index ion(s) concentration 9c vary from the upper 
surface toward the depth direction as shown in FIG. 6(C) and accordingly 
whose refractive index is maximum at a certain depth from the upper 
surface and gradually decreases from the depth to the peripheral portion 
(the second step). 
As stated above, the conventional process for producing a buried wave guide 
device requires the two steps, i.e., the first step of allowing high 
refractive index ion(s) to diffuse into a plate-like starting glass and 
the second step of subjecting the high refractive index ion(s) already 
present in the glass to ion exchange with low refractive index ion(s). 
Therefore, the process is disadvantageous in requiring complex operation. 
Hence, the object of the present invention is to provide a novel process 
for producing a buried wave guide device, which retains the advantages of 
the conventional diffusion and migration process (i.e., the resulting 
glass stably contains large amounts of high refractive index ion(s) and 
has a large difference in refractive index between the center and the 
peripheral portion of the glass), but is free from the complexity of the 
conventional diffusion and migration process and requires only one step of 
immersing a starting glass in a molten salt. 
SUMMARY OF THE INVENTION 
The present inventors made research in order to achieve the above object. 
As a result, it was found that a buried wave guide device consisting of a 
gradient refractive index type glass whose refractive index is maximum at 
a desired depth from the surface and decreases gradually from the depth to 
the peripheral portion, can be produced in one step by immersing a 
starting glass containing ion-exchangeable first ion(s) in a molten salt 
comprising not only compound(s) of second ion(s) capable of giving higher 
refractive index to the glass than the first ion(s) and capable of 
diffusing into the glass but also compound(s) of third ion(s) capable of 
giving lower refractive index to the glass and lower diffusion rate than 
the second ion(s). The above finding has led to the completion of the 
present invention. 
Hence, the present invention provides a process for producing a buried wave 
guide device, which comprises immersing a starting glass containing 
ion-exchangeable first ion(s) in a molten salt comprising (a) compound(s). 
of second ion(s) capable of giving higher refractive index to the glass 
than the first ion(s) and capable of diffusing into the glass and (b) 
compound(s) of third ion(s) capable of giving lower refractive index to 
the glass than the second ion(s) and capable of diffusing into the glass 
at a lower rate than the second ion(s), to effect ion exchange in the 
starting glass and thereby to obtain a buried wave guide device consisting 
of a gradient refractive index type glass whose refractive index is 
maximum at a desired depth from the surface and decreases gradually from 
the depth to the peripheral portion. 
The preferred embodiments of the present invention are as follows. 
(1) The ion-exchangeable first ion(s) contained in the starting glass are 
alkali ion(s) such as Na.sup.+, K.sup.+, Li.sup.+ and the like. 
(2) Ag.sup.+ is used as the second ion(s) capable of giving higher 
refractive index to the glass than the first ion(s) and capable of 
diffusing into the starting glass. 
(3) In the above (2), as the third ion(s) capable of giving lower 
refractive index to the glass than the second ion(s) and capable of 
diffusing into the starting glass at a lower rate than the second ion(s), 
there are used divalent ion(s) (e.g., Mg.sup.++, Ca.sup.++, Ba.sup.++, 
Cd.sup.++, Sr.sup.++, Pb.sup.++, Zn.sup.++ and particularly Sr.sup.++, 
Pb.sup.++), or alkali ion(s) (e.g. Na.sup.+, K.sup.+, Li.sup.+, Cs.sup.+ 
and particularly K.sup.+, Cs.sup.+) same as or different from the first 
ion(s) contained in the starting glass. 
(4) Tl.sup.+ is used as the second ion(s). 
(5) In the above (4), as the third ion(s), there are used divalent ion(s) 
(e.g., Mg.sup.++, Ca.sup.++, Ba.sup.++, Cd.sup.++, Sr.sup.++, Pb.sup.++, 
Zn.sup.++). 
(6) The molten salt is a nitrate, a sulfate, a chloride, a carbonate or the 
like, particularly a nitrate. 
(7) The first ion(s) are alkali ion(s) and the third ion(s) are also alkali 
ion(s). 
(8) In the above (7), the first ion(s) and the third ion(s) are same alkali 
ion(s). 
(9) In the above (7), the first ion is Na.sup.+ and the third ion is 
K.sup.+ or Cs.sup.+.

DETAILED EXPLANATION OF THE INVENTION 
The present invention is described in detail by referring to the drawings. 
FIGS. 1(A) and 1(B) show the concentration distributions of ions in the 
plate-like glass having a thickness d.sub.0 and a rectangular 
parallelepiped shape as shown in FIG. 2(A), before and after the treatment 
of the present invention. 
In the present process, a plate-like glass containing large amounts of 
first ion(s) produced according to the melting process is used as a 
starting glass. In concentration of the first ion(s) in the starting glass 
is uniform throughout the glass thickness d.sub.0 including the glass 
surface, as shown by the dotted line 1a of FIG. 1(A). 
The four side surfaces and the bottom surface of the plate-like starting 
glass having a thickness d.sub.0 is covered with a metal film according to 
a given method to mask said surfaces and then ion exchange is effected 
only at the upper surface of the starting of glass, whereby is produced a 
buried wave guide device 3 having a wave guide 4 in the vicinity of the 
upper surface of the plate-like glass, as shown in FIG. 2(A). 
(Incidentally, the above masking is optional and therefore the process can 
be effected without conducting the masking. An example of the process 
without conducting the masking is described later as Example 2.) 
In the conventional two-step process, a plate-like glass containing first 
ion(s) is immersed in a molten salt comprising compound(s) of second 
ion(s) capable of giving higher refractive index to the glass than the 
first ion(s) and capable of diffusing into the glass at a high rate, to 
effect ion exchange between the first ion(s) in the glass and the second 
ion(s) in the molten salt at temperatures around the transition 
temperature of the glass, whereby is obtained in an appropriate time a 
plate-like glass in which the second ion(s) concentration 8b varies from 
the surface toward the direction of the depth as shown in FIG. 6(B) (the 
first step); and in order to produce a buried wave guide device from the 
plate-like glass, it is required to conduct the second step of immersing 
the glass in a molten salt comprising compound(s) of third ion(s) capable 
of giving of lower refractive index to the glass than the second ion(s). 
Meanwhile in the process of the present invention, there is used a molten 
salt comprising not only the above mentioned second ion(s) but also third 
ion(s) capable of giving lower refractive index to the glass than the 
second ion(s) and capable of diffusing into the starting glass at a lower 
rate than the second ion(s) (an example of the third ion(s) is an alkali 
ion other than the alkali ion(s) contained in the starting glasses; for 
instance, when the alkali ion contained in the starting glass is Na.sup.+, 
the third ion is K.sup.+ or Cs.sup.+), in the form of compounds such as 
sulfate, nitrate, chloride, carbonate and the like; consequently, there 
occur, during the immersion of the starting glass in the above molten 
salt, an ion exchange between the first ion(s) and the second ion(s) and, 
at a delayed timing, an ion exchange between the first ion(s) and the 
third ion(s) or between the previously diffused second ion(s) and the 
third ion(s) and there is obtained in one step a buried wave guide device 
consisting of a gradient refractive index type glass in which, as shown in 
FIG. 1(B), the concentration 2b of the second ion(s) increases sharply 
from the upper surface of the glass toward the depth direction, reaches a 
peak at a desired depth and thereafter decreases gradually toward the 
depth direction and the concentration 2c of the third ion(s) is high at 
the upper surface and decreases sharply toward the depth direction. In 
FIG. 1(B), 2a is the concentration of the first ion(s) remaining in the 
buried wave guide device. As a result, there is obtained a buried wave 
guide device consisting of a plate-like glass whose refractive index, as 
shown in FIG. 2(B), increases sharply from the upper surface of the glass 
toward the depth direction, reaches a peak at a desired depth and 
thereafter decreases gradually toward the depth direction. This buried 
wave guide device 3 has a wave guide 4 in the vicinity of the upper 
surface, as shown in FIG. 2(A). 
In the above, there was explained a case the alkali ion(s) in the 
plate-like starting glass and the alkali ion(s) in the molten salt are 
different. However, the third ion(s) can be same as the ion(s) originally 
present in the starting glass and it gives no problem for the practice of 
the present process. The amount of the ions contained in the molten salt 
are controlled so that the ions can diffuse into the glass according to a 
chemical equilibrium. 
The present invention is explained in more detail on the basis of the 
following Examples. 
EXAMPLE 1 
As a shaped starting glass, there was used a plate-like glass of 5 mm in 
thickness, 2 mm in width and 50 mm in length, consisting of a phosphate 
glass containing 40 mole % of Na.sub.2 O and 10 mole % of SrO and having a 
glass transition temperature of 510.degree. C. The concentration of 
Na.sub.2 O in the plate-like glass in the depth direction was uniform at 
40 mole % as shown by the straight line 10a of FIG. 3(A). The 
concentration of SrO was also uniform at 10 mole % as shown by the 
straight line 10c. The bottom surface and the four side surfaces of the 
plate-like glass was covered with a titanium film having good acid 
resistance and good alkali resistance, according to the vacuum deposition 
method to mask the surfaces. Then, the plate-like glass was immersed in a 
molten salt consisting of 5% by weight of AgNO.sub.3, 20% by weight of 
Sr(NO.sub.3).sub.2 and 75% by weight of CsNO.sub.3 , at a temperature 
(435.degree. C.) around the strain point to the transition temperature of 
the glass for 2 hours, whereby Ag.sup.+ diffused into the glass from the 
upper surface to the depth of about 100 .mu.m. As shown by the curve 11b 
of FIG. 3(B), the Ag.sub.2 O concentration was 30 mole % at the glass 
upper surface, reached a peak of 40 mole % (the depth at this 
concentration was about 10 .mu.m), thereafter decreased gradually, and was 
almost 0 mole % at around a depth of 100 .mu.m. 
Meanwhile, Sr.sup.++ having a lower diffusion rate than Ag.sup.+ diffused 
into the glass from the upper surface to the depth of 10 .mu.m. As shown 
by the curve 11c of FIG. 3(B), the SrO concentration was 20 mole % at the 
glass upper surface, decreased sharply up to around a depth of 10 .mu.m, 
and thereafter was constant at 10 mole % toward the depth direction. 
Cs.sup.+ having a lower diffusion rate than Sr.sup.++ did not 
substantially take part in ion exchange in 2 hours. The Na.sup.+ 
originally present in the portion of the starting glass from the upper 
surface to the depth of about 10 .mu.m was substituted with Ag.sup.++ and 
Sr.sup.++ almost completely and the Na.sub.2 O concentration in said 
glass portion became substantially zero, as shown by the curve 11a of FIG. 
3(B). In the wave guide device thus formed, the center of the wave guide 
(i.e., the depth of 10 .mu.m from the glass upper surface) had a 
refractive index of 1.88 and the lower refractive index layer formed in 
the vicinity of the glass upper surface has a refractive index of 1.80. 
Refractive index decreased from the center of the wave guide toward the 
depth direction and was 1.68 at a depth of 100 .mu.m. 
EXAMPLE 2 
As a shaped starting glass, there was used a plate-like glass of 5 mm in 
thickness, 42 mm in width and 50 mm in length, consisting of the same 
phosphate as in Example 1. Without being masked, the plate-like glass was 
imersed in the same molten salt as in Example 1 at the same temperature as 
in Example 1 for the same length of time as in Example 1. The resulting 
plate-like glass had the same refractive index distribution as that of 
Example 1, in the depth direction and, as shown in FIG. 4(A), had a low 
refractive index layer 12 in the vicinity of the upper surface, a high 
refractive index layer 13 below the layer 12 and a low refractive index 
layer 14 below the layer 13. The plate-like glass was cut along the dotted 
lines of FIG. 4(A) and the surfaces of each resulting rectangular 
parallelepiped were ground and polished to obtain 16 buried wave guide 
devices each of 2 mm in thickness, 2 mm in width and 50 mm in length as 
shown in FIG. 4(B). 
As described in detail above, according to the present invention there can 
be produced, only in one step of immersing a particular glass in a 
particular molten salt, a buried wave guide device consisting of a glass 
stably containing large amounts of high refractive index ion(s) and having 
such a great refractive index difference between the center and the 
peripheral portion of the wave guide.