JPS61241706A - Production of optical branching and optical coupling circuit - Google Patents

Abstract

PURPOSE:To make possible the adjustment of a optical branching ratio and optical coupling ratio with high accuracy and to obviate the exertion of an influence to the operating characteristic of the other optical circuits on the same substrate by executing an ion exchange in part of the optical branching circuit or optical coupling circuit after thermal diffusion of a metal and adjusting the refractive index distribution by ion exchange conditions. CONSTITUTION:The directional coupler type optical branching circuit 22 is first formed on the LiNbO3 substrate 1 by thermal diffusion of Ti. Guided light is thereafter excited by prism or end face coupling to a piece of the incident waveguides 21 of the Ti- diffused LiNbO3 directional coupler type optical branching circuit 22. The exit light to two exit waveguides 23 is prism-coupled and the branching ratio of the light is measured. The ion exchange is executed only in the part of the inter-waveguide space of the directional coupler to change the refractive index distribution of the directional coupler part in the case of the optical branching ratio is different from a prescribed value. The coupling length is adjusted and the optical branching ratio is adjusted by changing the coupling stage of the light wave between the two optical waveguides of the directional coupler in the above-mentioned manner.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method for manufacturing an optical branching circuit that branches the energy of incident light into a fixed ratio and an optical coupling circuit that couples the energy of incident light at a fixed ratio. The present invention relates to a method of manufacturing an optical branching/coupling circuit in which the optical branching ratio and optical coupling ratio can be adjusted with high precision by forming an optical waveguide by diffusing heat into a dielectric substrate.

As single-source communication systems have come into practical use in recent years, systems with even higher capacity, higher speed, more functionality, and higher accuracy are required. New functions such as the generation of high-speed optical signals and the switching of optical transmission lines are required to be added. Switches are expected to meet these demands due to their features of small size, high speed, and high efficiency. In particular, LiNbO3
A waveguide optical device formed by thermally diffusing T1 onto a strongly crystalline material as a substrate has low light absorption and low loss, and has a large electro-optic effect, making it highly efficient. Various types of optical switches are available, including directional coupler type optical modulators or switches, branching interference type optical switches or modulators, and total internal reflection type optical switches. The application of this to light control devices has been reported.

When applying such a waveguide type optical control element to an actual optical communication system, low loss and high speed are required as mentioned above, but in order for the waveguide type optical control element to operate at high speed, It is important that the operating voltage of the element be low and that the characteristic impedance of the electrode be easily matched to the characteristic impedance of the voltage-driven electric circuit system. Among the various types of optical control elements mentioned above, the branching interference dimmer switch or modulator is 17J3 (58% ), and because there is a large degree of freedom in designing the characteristic impedance of the electrodes, it has recently been attracting attention as it is suitable for increasing speed.

This branching interference type optical switch or modulator uses a 3dB optical branching circuit to split the incident light into two optical paths, applies a phase difference to the light waves propagating in the two optical paths using electro-optic effect or thermo-optic effect, and then splits the incident light into two optical paths using the electro-optic effect or thermo-optic effect. The light waves propagating through the optical path are combined into a t-3 dB optical coupling circuit to perform switching and modulation of the light. At this time, if the branching ratio/coupling ratio between the 3 dB optical branch circuit and the 3 dB optical coupling circuit is not exactly 1=1, crosstalk deterioration in optical switching and extinction ratio deterioration in optical modulation will occur. Therefore, in order to improve the characteristics of a branching interference type optical switch or modulator, it is necessary to adjust the branching ratio and coupling ratio of the 3 dB optical branch circuit and the 3 dB optical coupler with high precision.

[Prior art and its problems]

LiNb0. When forming an optical branching/optical coupling circuit by thermally diffusing metal such as TL onto a ferroelectric substrate such as
The coupling length of the directional coupler is determined by the width and thickness of the film, the distance between the two strip-shaped Ti films, the diffusion temperature, and the diffusion time. The branching ratio and the one-component combination ratio are determined.

Therefore, conventionally, the optical branching ratio and optical coupling ratio of a directional coupler of a certain length are determined in advance by calculation or experiment based on the width, thickness, and diffusion conditions of the T1 film that diffuses. The width, thickness, and diffusion conditions of these Tl films are set so that the desired optical branching ratio and -yt coupling ratio can be obtained, and the desired optical branching ratio and one-component coupling ratio are obtained by - times of thermal diffusion. A manufacturing method was used to obtain the desired result. However, in general KTi diffusion waveguides, the diffusion temperature is about 900 to 1100°C and the diffusion time is several to ten hours, so there are temporal changes and non-uniformity in the temperature distribution inside the diffusion lamp, and there are problems with temperature rise. Due to the difficulty of keeping the temperature gradient constant at all times and the fact that the temperature is not always set exactly to the set value, the direction after diffusion may vary even with the same Tl film shape and diffusion conditions. There was variation in the length of the sexual connective tissue. Therefore, it has been difficult to accurately set the branching ratio and coupling ratio of the directional coupler type optical branching/coupling circuit due to thermal diffusion.

As a manufacturing method to avoid this problem, 1. If the coupling length is measured and the branching ratio and coupling ratio are not the desired values, Ti is again thermally diffused at about 900 to 1100°C to change the refractive index distribution of the waveguide to obtain the desired branching ratio and coupling ratio. There is a method of continuing heat diffusion until the However, with this method, when several other optical circuits are integrated on the same substrate, rather than just a directional coupling type optical branching/coupling circuit, the operating characteristics of the other optical circuits change. This is a problem because it causes

For example, a conventional branching interference type optical switch as shown in FIG. The directional coupler type optical coupling circuit section 43 is composed of three parts. In this optical phase modulator section 42, a voltage V7c for adding a difference in phase of light propagating through two optical paths by 1,
The smaller the energy distribution of the propagating light is confined under the electrode, the smaller the half-wave voltage Vz becomes. However, in the method of obtaining the desired branching ratio/coupling ratio by repeating the above-mentioned re-diffusion, the directional coupler type optical branching circuit section 41. Not only the optical coupling circuit section 43 but also the refractive index distribution of the optical waveguide of the optical phase modulator section 420 changes.

In optical waveguides formed by thermal diffusion of T1, the longer the diffusion time, the larger the diffusion spread, and the smaller the maximum refractive index in the refractive index distribution, so the energy of propagating light is confined. becomes weaker. Therefore, there is a problem that the half-wave voltage of the optical phase modulator section 42 becomes large.

A manufacturing method that eliminates this problem involves attaching electrodes to a directional coupler-type optical branching/coupling circuit.

There is a method of adjusting the light branching ratio and one-component coupling ratio by controlling the coupling length by controlling the electro-optical effect depending on the voltage applied to the electrode. However, when this method is used, an extra voltage driving electric circuit and energy are required after manufacturing the optical branching/optical coupling circuit, and applying a DC voltage between the electrodes for a long period of time generates charges on the substrate surface, etc. There is also the possibility that the DC drift may accumulate and cause a so-called DC drift phenomenon. It should be noted that these problems occur in exactly the same way in optical branching/coupling circuits of other configurations, such as cross-type branching/coupling circuits.

The purpose of the present invention is to eliminate the drawbacks of such conventional manufacturing methods, to be able to adjust the optical branching ratio and optical coupling ratio with high precision, and to be able to adjust the operating characteristics of other optical circuits on the same substrate. It is an object of the present invention to provide a method for manufacturing an optical branching one-party coupling circuit that does not cause any interference.

[Structure of the invention]

A first configuration of the present invention is formed by thermally diffusing metal onto a ferroelectric substrate, and an incident optical waveguide.

In a method of manufacturing an optical branching circuit or optical coupling circuit in which at least one of the output optical waveguides has a plurality of optical waveguides, after thermally diffusing the metal, a part of the optical branching circuit or optical coupling circuit is subjected to ion exchange, and the The optical branching ratio or the optical coupling ratio of the optical branching circuit or the optical coupling circuit is adjusted by adjusting the refractive index distribution of the optical branching circuit or the optical coupling circuit according to ion exchange conditions.

In a second configuration of the present invention, in the same method for manufacturing an optical branching circuit or optical coupling circuit, after thermally diffusing the metal, ion exchange is performed on a part of the optical branching circuit or optical coupling circuit, and then the metal is heated again. It is characterized in that only the ion exchange portion is thermally diffused at a temperature lower than the thermal diffusion temperature, and the refractive index distribution of the optical branch circuit or the optical coupling circuit is adjusted by the thermal diffusion time of the ion exchange portion.

[Principle and operation of the invention]

According to the present invention, an optical branching circuit and an optical coupling circuit are formed by thermally diffusing a metal such as T1 on a ferroelectric substrate such as tzL i N b Oa, and the optical branching circuit.

If the optical branching ratio 1-coupling ratio of the optical coupling circuit is different from the desired value, perform ion exchange on a part of the single branch circuit and optical coupling circuit until the desired value is obtained. ,
Or after performing ion exchange for a certain period of time, T1
The optical splitting ratio and the optical coupling ratio can be adjusted with high precision by thermally diffusing only the ion exchange part at a sufficiently low temperature until the desired optical branching ratio of 1 and the optical coupling ratio are obtained. A method for manufacturing a well-organized optical branching/coupling circuit can be obtained.

By using this manufacturing method, ion exchange into a ferroelectric substrate such as LtNbo3 is possible at a temperature sufficiently lower than the thermal diffusion temperature of the T1 metal, and the exchanged ions can also be diffused into a ferroelectric substrate such as Ti. This is possible at a sufficiently low temperature compared to the thermal diffusion of metals, so even if optical circuits other than optical branching and optical coupling circuits are integrated on the same substrate, the operating characteristics of other optical circuits will not be affected. In this way, it is possible to obtain an optical branching/coupling circuit in which only the optical branching ratio and optical coupling ratio of the optical branching/coupling circuit can be adjusted with high precision.

The present invention will be described in detail below with reference to the drawings.

[Example 1] Figures 1(a) to (jJ are cross-sectional views explaining step-by-step an embodiment of a method for applying a moxibustion method for an optical bifurcation coupling circuit according to the present invention. As shown in a), LiN
A 7-photoresist 2 is applied onto the bO3 substrate 1, and a groove having the same shape as the waveguide pattern is formed on the photoresist 2 using a photomask using a conventional 7-photoresist technique (FIG. 1(b)).

From above, a Ti film 3 of about 300~tzooA is deposited over the entire surface by vapor deposition or sputtering, as shown in Fig. 1 (C17
When the photoresist is dissolved, an optical waveguide pattern of T1 as shown in FIG. 1(d) is formed.

FIG. 2 is a perspective view of a LiNbO3 substrate 1 on which T1 optical waveguide patterns 11 to 13 of a directional coupler restoration branch circuit are formed. After forming the Ti optical waveguide patterns 11 to 13, the substrate 1 is
By heating in a furnace at 0°C for about 5 to 10 hours,
A, Tl diffusion source waveguide 4 as shown in FIG. 1 (el) is formed. After forming this Ti diffusion source waveguide 4, as shown in FIG.
A mAt film 5 is formed on the entire substrate 1 as shown in f). After that, a photoresist film 6 is formed thereon (Fig. 1(g)).
, 7o) Figure 1 Cb using IJ Soge, 7F technique
), if only the part of the directional coupler waveguide gap of the photoresist film 6 is dissolved and the At film 5 is etched only in that part, ◆ it is better, as shown in Fig. 1(i). Atm masks for ion exchange (two are formed.

FIG. 3 is a perspective view of the substrate 1 at this time. That is, a KAt mask 24 is formed over the entire surface of the substrate 1.

Ion exchange windows 25 are opened only in the waveguide gaps of the optical branch circuits 22 of the directional couplers (2) to 23). Thereafter, the entire substrate is immersed in benzoin melt at 125 to 250°C, and ion exchange 7 of protons (H+) and Ll+ is performed through the ion exchange window of the At mask (Fig. 1(j)
)).

In the present invention, first, by thermal diffusion of Ti, LiNbO
A directional coupler type optical branching circuit 22 is formed on the third substrate l. After that, this Ti-diffused LiNbO3 excites the guided light to one of the input waveguides 2) of the combinatorial branch circuit by prism or end face coupling, and the output light to the two output waveguides 23 is transmitted to the prism. Combine and measure the original branching ratio. If this optical branching ratio is different from the desired value, use the method described above to perform ion exchange only on the waveguide gap portion of the directional coupler to change the refractive index distribution of the directional coupler. The coupling state of the light waves between the two optical waveguides of the isotropic coupler is changed to adjust the coupling length and the optical branching ratio.

When performing ion exchange of H+ and L1+ by immersing a LiNbO3 substrate in a melt of benzoic acid, it is possible to adjust the ion exchange time, ion exchange temperature, and depth of the waveguide. The maximum amount of refractive index change can be adjusted depending on the amount of L1+ ions added therein.

In other words, in ion exchange using benzoic acid, the refractive index distribution in the thickness direction becomes step-like in the thickness direction of the substrate, but the amount of change in the surface refractive index depends on the L added to the benzoic acid.
It can be controlled by the amount of i9on, and as the amount of Li+cD added increases, the surface refractive index change 1- can be linearly reduced.
It can be controlled by the ion exchange time.

Therefore, if the optical branching ratio of the Ti diffusion directional coupler type optical branching circuit 22 is different from the desired value.

First, ion exchange is performed for a short period of time in the gap between the two optical waveguides of the directional coupler with a large amount of Li+ ions added in benzoic acid. A shallow change region is formed and the optical branching ratio is measured again. If the optical branching ratio is still not at the desired value, the ion exchange described above is repeated and the ion exchange time is increased until the desired optical branching ratio is obtained.

In addition, when the amount of change in bond length is small, L in benzoic acid
By reducing the amount of i+4 added and increasing the amount of increase in the refractive index of the directional coupler optical waveguide gap, the amount of change in the coupling length can be increased. However, at this time, if the refractive index change region becomes too deep and the refractive index change 1 in the waveguide gap portion becomes too large, the refractive index distribution in the directional coupler section will change as shown in the characteristic diagram of Fig. 4 (aJ). This increases the thickness at the ion exchange part, and there is a risk that the propagation mode of the light wave will be set up at the waveguide gap and be scattered into the substrate, but if the amount of increase in the refractive index at the waveguide gap is increased.

The amount of Li+ ions added and the ion exchange time were determined,
The waveguide is designed so that only the vicinity of the substrate surface is a high refractive index distribution region, and the refractive index distribution in the directional coupler part is effectively reduced in the ion exchange part, as shown in the characteristic diagram of FIG. 4(b). It is possible to prevent propagation modes from forming in the gap.

In this example, first, a directional coupling@type branch circuit is formed by thermally diffusing T1 into the LiNbO3 crystal, and if the desired optical branching ratio cannot be obtained, a directional coupler By performing ion exchange in the waveguide gap, changing the refractive index distribution of the directional coupler, and changing the coupling length of the directional coupler, the optical branching ratio of the one-element branch circuit can be adjusted.
Arrange. This ion exchange is performed by immersing the LiNbO3 substrate in benzoic acid to exchange H+ ions and Lth+ ions, but the melting point of benzoic acid is 12) °C and the boiling point is 250 °C at atmospheric pressure, so this temperature range proton exchange within f
Since the thermal diffusion temperature of %TI can be 900~1
The temperature is sufficiently low compared to 100 DEG C., and even if other elements other than the optical branch circuit are integrated on the same substrate, the operating characteristics of the other optical circuits will not be affected.

[Embodiment 2] FIG. 5 shows a configuration diagram of a shoulder structuring device for questioning a second embodiment of the present invention. In the figure, a substrate holder 35 is placed on a heater 38 such as a hot plate, and a substrate holder 35 is placed on top of the heater 38 using the same manufacturing method as in the first embodiment. 5. A Ti-diffusion directional coupler type optical branching circuit is formed, and the LiN is subjected to proton ion exchange in the gap between two equal optical wave paths of the directional coupler.
A bO3 substrate 1 is mounted. The optical fiber 32 to which the output light of the semiconductor laser 31 is coupled is end face-coupled to the input optical waveguide end face of the directional coupler restoration branch circuit, and the two output waveguide propagating lights of the directional coupling type optical branch circuit are connected to the prism. 3
3, the 9LiNb03 is emitted to the outside of the substrate 13, and 2
It is guided to one of the seven detectors 34. A temperature sensor 36 such as a thermocouple is attached to the substrate holder 35 so that the heating temperature can be monitored using a thermometer 37.

In this example, a Ti diffused directional coupler type original branching circuit was formed using the manufacturing method shown in FIG. Proton ion exchange is performed in the gap between the two optical waveguides of the directional coupler using a method similar to the manufacturing method. The metal film formed for the ion exchange mask was removed by etching.
The LiNbO3 substrate 1 was heated to 400°C while monitoring the branching ratio of the directional coupler type optical branching circuit using the apparatus shown in Fig. 5.
Only the ion exchange part is thermally diffused at a temperature of

As a result, the ions in the ion exchange section diffuse into the LiNbO3 substrate 1, so that the refractive index distribution of the directional coupler section changes with time. Therefore, the coupling length of the directional coupler changes with the substrate heating time, and the optical branching ratio changes. Therefore, the substrate l is heated while monitoring the one-element branching ratio with the photodetector 34.

If the heating is stopped when the desired optical branching ratio is obtained, a directional coupling type optical branching circuit having the desired optical branching ratio can be obtained. Immediately after ion exchange, the refractive index distribution of the optical waveguide due to proton ion exchange becomes step-like in the depth direction of the substrate, as shown in A of the 4-character diagram in Figure 6, but when the substrate is heated, H+ ions easily diffuses into the substrate, and B in Figure 5fL6
As shown in Figure 2, the maximum refractive index decreases immediately after ion exchange, resulting in a refractive index distribution that spreads in the depth direction. It is known that when heating is continued, the maximum refractive index further decreases, resulting in a Sigauss distribution, which is a refractive index distribution that spreads further in the depth direction. Therefore, when the substrate 1 is heated after ion exchange, the elemental branching ratio changes moment by moment due to the change in the refractive index distribution, and if heating is stopped when the desired optical branching ratio is reached, the desired optical branching ratio is obtained. A directional coupling type optical branching circuit is obtained.

Further, since the H+ ions that have entered the crystal through proton exchange easily diffuse into the substrate at a temperature of about 400°C, the temperature may be sufficiently lower than the thermal diffusion temperature of T1.

Therefore, according to this manufacturing method, the single branching ratio can be easily controlled in real time, and even if other optical circuits are integrated on the same substrate, the operating characteristics of the other optical circuits will not be affected. A manufacturing method is obtained in which only the branching ratio of the optical branching circuit is adjusted.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to adjust the optical branching ratio 1 optical coupling ratio with high precision, and the ion exchange and thermal diffusion of the exchanged ions are also sufficient compared to the thermal diffusion of T1. Because the temperature is low, even if optical circuits other than optical branching, -yt, and coupling circuits are integrated on the same substrate, the operating characteristics of other optical circuits are not affected, and the optical branching and coupling circuits are A method for manufacturing an optical branching/coupling circuit is obtained in which only the optical branching ratio and the optical coupling ratio can be adjusted with high precision.

Note that the optical branching/coupling circuit obtained by the present invention is not limited to the directional coupler type as shown in this embodiment, but may also be a cross-type optical branching/coupling circuit.

A Y-shaped branch circuit, a Y-shaped coupling circuit, etc. may be used, and ion exchange is performed on a part of these optical branching/optical coupling circuits, and the ion exchange time or the thermal diffusion time of the exchanged ions is set as in this example. Controlled optical branching/optical coupling circuit optical branching 1
The binding ratio can be adjusted. Furthermore, the substrate forming the optical branching/coupling circuit is not limited to LiNbO3, but may also be LiTaO3, etc., and ion exchange is not limited to only proton exchange in a benzoic acid solution (Ag
Exchange of Ag+ ions in N0g vinegar solution, Tt+ ions in TtNO3 solution, etc. may also be used. The metal used as a mask when performing this ion exchange is not limited to At, but may also be %Tt, o-, etc. Further, in this embodiment, only the method for manufacturing an optical branch circuit has been described, but an optical coupling circuit can also be manufactured using the same manufacturing method as that for an optical branch circuit.

[Brief explanation of the drawing]

FIGS. 1(a) to 1(j) are cross-sectional views showing an embodiment of the method for manufacturing an optical branching/optical coupling circuit according to the present invention in the order of steps. 2 is a perspective view of the Ti pattern of the Ti-diffused LiNbO3 directional coupler type optical branch circuit obtained from FIG. 1;
The figure is a perspective view showing an At mask for ion exchange only into the waveguide gap of the directional coupler section after Ti diffusion.
Kl, (b) is a characteristic diagram showing the refractive index distribution in the horizontal direction of the substrate obtained by ion exchange in FIG. 1, FIG. The figure is a characteristic diagram showing changes in the refractive index distribution in the direction of the substrate before and after thermal diffusion in the ion exchange section, and FIG. 7 is a perspective view showing the configuration of a branching interference type optical switch according to the prior art. In the figure 1
...LINb03 board, 2.6...7
Otoresist, 3...T1 film, 4...
T1 diffusion waveguide% 5...At film, 7...
...W° ion, 11...Incoming optical waveguide Ti pattern, 12...Directional coupler type optical branch circuit T
i pattern, 13...output optical waveguide T1 pattern, 2)...input optical waveguide, 22...
Directional coupler type optical branch circuit, 23... output optical waveguide. 24...At mask, 25...Ion exchange bay window. 31... Semiconductor laser module, 3
2... Optical fiber s 33... Prism, 34... Photodetector, 35...
...Substrate holder, 36...Temperature sensor, 37.
...Thermometer, 38...Heater. 41... Directional coupler type optical branch circuit section, 42...
...Optical phase modulator section, 43... Directional coupler type original coupling bud 2 Figure $6I! I Kaya 7 times

Claims (2)

[Claims] (1) In a method for manufacturing an optical branching circuit or an optical coupling circuit formed by thermally diffusing metal onto a ferroelectric substrate, at least one of an input optical waveguide and an output optical waveguide has a plurality of optical waveguides, After thermally diffusing the optical branching circuit or optical coupling circuit, a part of the optical branching circuit or optical coupling circuit is subjected to ion exchange, and the refractive index distribution of the optical branching circuit or optical coupling circuit is adjusted according to the ion exchange conditions. A method for manufacturing an optical branching/coupling circuit, which comprises adjusting the optical branching ratio or optical coupling ratio of the optical branching/coupling circuit. (2) In a method for manufacturing an optical branching circuit or an optical coupling circuit formed by thermally diffusing metal onto a ferroelectric substrate, at least one of an input optical waveguide and an output optical waveguide has a plurality of optical waveguides, After thermally diffusing a part of the optical branching circuit or optical coupling circuit, ion exchange is performed on a part of the optical branching circuit or optical coupling circuit, and then only the ion exchange part is thermally diffused again at a temperature lower than the thermal diffusion temperature of the metal, and the optical branching circuit or optical coupling circuit is heated. Optical branching/light, characterized in that the optical branching ratio of the optical branching circuit or the optical coupling ratio of the optical coupling circuit is adjusted by adjusting the refractive index distribution of the optical coupling circuit by the thermal diffusion time of the ion exchange part. A method of manufacturing a coupling circuit.