Process for Producing Buried Waveguide Device

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.

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.+.

FIGS. 1(A) and 1(B) are graphs showing the concentration distributions of ions in the plate-like glass before and after the treatment of the present invention.

FIG. 2(A) is a perspective view of the buried wave guide device obtained according to the present process and FIG. 2(B) is a graph showing the distribution of refractive index in the device.

FIGS. 3(A) and 3(B) are graphs showing the concentration distributions of ions in the plate-like glass before and after the treatment of Example 1.

FIGS. 4(A) and 4(B) are views showing the gradient refractive index type plate-like glass obtained in Example 2 and the buried wave guide device obtained from the glass, respectively.

FIG. 5 is a view showing a state in which a buried wave guide device is used.

FIGS. 6(A), 6(B) and 6(C) are graphs showing the concentration distributions of ions at each step, in the plate-like glass in the conventional two-step process.

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.

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.

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.