JPH0462644B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for manufacturing an embedded single-mode optical waveguide using an ion exchange method, and particularly to a method for manufacturing a single-mode optical waveguide that does not require the application of an electric field. [Prior Art] Optical transmission systems and optical sensor systems using single-mode fibers require various single-mode optical devices for branching, merging, demultiplexing, and multiplexing. Several ion exchange methods are known as methods for producing these devices on glass substrates. However, in consideration of practical effects, it is desirable that the glass substrate waveguide be of a buried type, and that the electric field distribution be approximately the same so that the direct coupling with the single mode fiber is efficient. A two-step electric field application ion exchange method is a method that is close to the method for producing the above-mentioned waveguide using conventionally known techniques. In this method, a mask film made of metal or the like is laminated on a glass substrate, then patterned into the shape of a waveguide, and brought into contact with molten salt while an electric field is applied to form a high refractive index part near the surface of the substrate. After the mask is removed, a second ion exchange is performed. For the above method, for example, Springer-
It is explained in detail in the paper by H. Lilienhof et al., published on page 71 of "Integrated Optics" published by Verlag. Furthermore, Japanese Patent Publication No. 47-5975 discloses a method of creating a photoelectric transmission path in a glass substrate in which the refractive index gradually decreases from the center to the periphery in a cross section by performing two-stage ion exchange. ing. [Problems to be solved by the invention] In the waveguide fabrication method using electric field application, it is necessary to apply an electric field to the molten salt that is brought into contact with the front and back surfaces of the glass substrate, which poses the problem of complicating the ion exchange process. Ta. Moreover, Japanese Patent Publication No. 48-5975 does not specifically indicate the conditions for making the cross section of the buried waveguide circular or elliptical, nor does it specifically indicate the conditions for forming a single mode waveguide. The present invention aims to provide a novel method for manufacturing a buried single-mode optical waveguide having a circular or nearly elliptical cross section using a simple thermal ion exchange method that does not require the application of an electric field. [Means for Solving the Problems] In order to solve the above problems, the present invention uses thermal ion exchange in both the first and second stages of ion exchange, and furthermore, the cross section of the implantable mold is circular or oval. The ion exchange conditions required to realize a waveguide close to the above were clearly defined for the first and second stages. That is, the product D ₁ t 1 of the effective ion exchange constant D ₁ during the first thermionic exchange and the ion exchange time t ₁ is 3D _{1 t 1} 30 , and the effective ion exchange constant during the second thermionic exchange
Let the product D ₂ t ₂ of D ₂ and ion exchange time t ₂ be 0.6D ₂ t ₂ 0.75×D ₁ t ₁ . Ions for ion exchange that can be used in the present invention include monovalent alkali ions, such as Li, Na, K, Rb, and Cs.
In addition to ions, there are monovalent Tl ions and Ag ions. In addition to Ti and Al, thin films of dielectrics such as SiO ₂ can be used as masks for ion exchange. [Operation] Ion exchange between the glass substrate and the molten salt is the product of the effective exchange constant D of ions and the effective exchange time t.
It can be specified by Dt. The effective exchange constant referred to here is an apparent diffusion constant when the movement of one ion is described by a diffusion equation when two types of ions are exchanged. Since the effective exchange constant D is generally a function of temperature, the Dt product is strictly determined by integration. By setting the Dt product in the first and second stage ion exchange treatments within the range mentioned above,
A single-mode optical waveguide with a nearly circular cross section can be obtained. [Embodiment] FIG. 1 is a cross-sectional view showing an embodiment of the present invention step by step. 1 is a glass substrate containing a total of 13 mole% of Na and K as monovalent ions for ion exchange;
Consists of a polished high-quality glass plate with SiO ₂ and B ₂ O ₃ as the main network-forming oxides. 1 μm thick on one side of glass substrate 1 by sputtering method
A coating film 2a made of a Ti film of about 100% is deposited, and openings 2c of a predetermined waveguide pattern are formed in the coating film 2a by photolithography and etching steps to form a mask film 2b. The glass substrate 1 coated with the above-mentioned mask film 2b is immersed in the first molten salt 4a to perform a first-stage thermal ion exchange treatment. The first molten salt 4a held in a crucible 3 placed in an electric furnace (not shown) is
It is used to increase the refractive index near the opening 2c provided in the mask film 2b, and is a molten salt containing at least one first ion selected from the group consisting of monovalent ions. This molten salt 4a
In addition to sulfate or nitrate, chloride salt is added as necessary to make ion exchange uniform and to prevent damage to the glass substrate. After completing the first stage thermal ion exchange treatment, the mask film 2b is removed from the surface of the substrate 1, and the substrate 1 from which the mask film has been removed is immersed in a second molten salt 4b to perform a second stage thermal ion exchange process. Perform ion exchange treatment. The second molten salt 4b is used to reduce the refractive index near the surface of the glass substrate 1, and is a molten salt containing at least one second ion selected from the group consisting of monovalent ions. . Here, the first ion and the second ion must have at least opposite effects on the glass substrate 1 to change the refractive index. After completing the second stage thermionic exchange treatment described above,
A buried waveguide section 5 is formed within the glass substrate 1. In the above embodiment, as the first ion,
Tl ions were used and K ions were used as the second ion. The first molten salt 4a is a nitrate containing Tl ions, and the second molten salt 4b is a sulfate containing K ions. The temperature at which ion exchange is performed is
The temperature was set near the glass transition point at which the ion exchange constant was large and the glass substrate 1 did not undergo any deformation such as warping. Next, the ion exchange conditions in the present invention will be explained in detail. Ion exchange between the glass substrate and the molten salt can be specified by the product Dt of the effective exchange constant D of ions and the effective exchange time t. The effective exchange constant referred to here is an apparent diffusion constant when the movement of one ion is described by a diffusion equation when two types of ions are exchanged. Since the effective exchange constant D is generally a function of temperature, the Dt product is strictly determined by integration. In the examples, the first combination shown in Table 1 was used.
The ion exchange conditions for the stage and second stage were tested (conditions are expressed as Dt product). As shown in Table 1, in samples No. 1 to 5, the dimensions of the waveguide cross section are 14 to 21 μm on the long axis and 8 to 15 μm on the short axis.
You get what you want. With this waveguide dimension, the maximum refractive index difference is 0.004
If the waveguide is set to a certain degree, the waveguide becomes a single mode, and the coupling efficiency can be made relatively good with respect to a single mode fiber. In sample No. 6, the waveguide dimensions are too large, making it difficult to form a single mode waveguide. In samples Nos. 7 to 10, the ion exchange in the second stage progressed too much, and the waveguide dimensions became smaller, while at the same time

〔Effect of the invention〕

According to the present invention, it is possible to realize an embedded single mode waveguide using thermionic exchange method, which was previously impossible. Since the method of the present invention does not require the application of an electric field, it has the advantage that ion exchange is very simple, and it also has the advantage that damage to the glass surface and non-uniformity of the waveguide that tend to occur with the electric field application method can be reduced. . Therefore, the method of manufacturing a buried single-mode optical waveguide according to the present invention is extremely suitable for inexpensive mass production.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing step-by-step an embodiment of the present invention, FIG. 2 is a cross-sectional view showing an example of the ion concentration distribution of an ion exchange waveguide obtained by the method of the present invention, and FIG.
The figure shows the product of the effective ion exchange constant and ion exchange time in the first stage and the second stage, and the suitability and unsuitability of the waveguide dimensions. 1... Glass substrate, 2b... Mask film, 2c...
...Waveguide pattern opening, 4a...first molten salt,
4b... second molten salt, 5... waveguide.

Claims (1)

[Claims] 1. A mask film for ion exchange control is provided on a glass substrate containing monovalent ions selected from the group consisting of monovalent alkali ions, monovalent Tl ions, and Ag ions as ion exchange ions. a step of forming;
forming a predetermined waveguide pattern on the mask film; and immersing the glass substrate at high temperature in a molten salt containing at least one first monovalent ion that changes the refractive index of the glass substrate. The mask is removed during a step of thermal ion exchange and into a molten salt containing at least one second monovalent ion that changes the refractive index of the glass substrate in a direction opposite to that of the first ions. The glass substrate is immersed at high temperature and the second
This is a two-step thermionic exchange method consisting of _a step of thermionic exchange _of
Let D ₁ · t ₁ be 3D ₁ t ₁ 30, and the product D _{2 t 2} of the effective ion exchange constant D 2 during the second thermionic exchange and the ion exchange time t ₂ is 0.6D _{2 t 2 0.75} ×D ₁ t _1. Manufacturing method of embedded single mode optical waveguide. 2 the first monovalent ion is an ion containing at least one of Tl or Cs ions, and the second monovalent ion is an ion containing at least one of Tl or Cs ions;
at least one monovalent ion of Na or K ion
2. The method of manufacturing a buried single-mode optical waveguide according to claim 1, wherein the ions include: