JPH1048444A - Optical waveguide device - Google Patents

Abstract

(57) [Problem] To provide an excellent optical waveguide device capable of minimizing power loss of guided light as much as possible. SOLUTION: An optical waveguide device in which a metal element is diffused into a substrate 1 having an electro-optical effect to form a waveguide on one side and a branch on the other side, wherein the width of the waveguide is Gradually increases in a region (transition region 6) located before and after the branch point of the guided light propagating through the waveguide,
Alternatively, it gradually decreases in a region (transition region 6) located before and after the combining point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type optical waveguide device used in the field of optical communication and optical information processing, and more particularly to a branch waveguide in which a single waveguide branches into a plurality of waveguides. The present invention relates to an optical waveguide device provided.

[0002]

2. Description of the Related Art In recent years, as optical communication systems and optical information processing systems have been put into practical use, a system capable of processing a larger amount of optical signals and having higher functions is required. It has become to. In order to realize these systems, an optical integrated circuit in which optical functional elements are integrated is indispensable. For example, using a planar technology, a straight line is formed on a substrate of a single crystal material such as lithium niobate (LiNbO3: LN). The integration of waveguides, bent waveguides, branch waveguides, control electrodes, and others (optical integrated circuits) has been actively studied.

[0003] In this type of optical waveguide type optical waveguide device, how to reduce the power loss of guided light is one of the major issues for practical use. Further, the loss (loss due to branching or multiplexing of guided light) of an optical waveguide device provided with a so-called branch waveguide having a branch portion in which the waveguides form an included angle is very much smaller than the loss of the entire device. Since it accounts for a large proportion, an optical waveguide device in which such a loss is minimized has been desired.

For example, as shown in FIG. 4, an optical waveguide device J1 having a branch waveguide 52 obtained by diffusing titanium (Ti) in a surface layer portion of an LN base 51 having an electro-optic effect is known. ing. This is a configuration often used for a so-called Mach-Zehnder interferometer or the like in which a modulation electrode is disposed on the branch waveguide 52 and driven by applying a predetermined voltage to the modulation electrode. 53 and two linear waveguides 54, 55, etc. connected to it, and these three linear waveguides intersect with each other to form a Y-shape.

Here, considering an ideal Y-branch waveguide (the Y-branch angle θ is infinitely small), from the straight waveguides 54 and 55,
When single-mode guided lights having the same intensity are incident in the same phase (in the case of multiplexing of guided lights), the linear waveguide 53 and the linear waveguides 54 and 55 are formed by the incident waveguide lights of the linear waveguides 54 and 55.
In the taper portion 56 (portion where the width of the waveguide changes) and its vicinity, the even mode becomes in-phase and strengthens each other, while the odd mode cancels out. Since the even mode can be guided through the straight waveguide 53 as it is, there is no loss due to the Y branch.

However, since the practical Y-branch angle θ is generally about 1 °, the light incident from the linear waveguides 54 and 55 has a sudden change in the guided light path and mode shape at and near the taper portion 56. And the primary mode is excited.

Generally, since the linear waveguide 53 is designed as a single-mode waveguide, the excited first-order mode light is converted into a radiation mode, leaks out of the waveguide, and becomes a Y-branch loss. . The inventors studied the state of the propagating light by simulation using the beam propagation method. As a result, a sharp change in the mode shape occurred near the transition region, the primary mode was excited, and the light shifted to a single mode waveguide. At the time, it was found that the radiation mode was converted.

[0008] The above problem arises when the guided light is branched.
In other words, the same can be said for the case where single-mode guided light enters from the straight waveguide 53 and branches to the straight waveguides 54 and 55.

In order to solve the above problem, a metal thin film having a rectangular cut-out region is formed and adhered at a position where the tapered portion 56 is formed, and the metal thin film is thermally diffused to form a waveguide. A technology to do this has been proposed. Accordingly, a waveguide that gradually changes the mode shape can be formed from the vicinity of the branch point or the multiplexing point 57 of the guided light to the terminal end 58 of the tapered portion 56, and the power due to the rapid change of the mode shape can be formed. It is intended to reduce the loss (see, for example, Japanese Patent Publication No. Hei 6-21889).

However, it is difficult to sufficiently reduce the Y-branch loss, and in order to suppress a sudden change in the mode shape occurring near the tapered portion 56, a notch having a sufficient width must be provided. No.

On the other hand, providing a notch having a sufficient width causes a discontinuous change in the refractive index at the end of the notch, and consequently causes a sharp mode shape change. Due to factors such as reflection and scattering, a sufficient loss reduction effect cannot be expected.

In addition, the present inventors have proposed a Y-branch waveguide structure (hereinafter referred to as a comb-like Y-branch structure) in which a metal thin film having a comb-like structure is thermally diffused near the intersection of the Y-branch waveguides. And
An optical waveguide device capable of achieving a gradual mode shape change and almost completely suppressing the excitation of the first-order mode can be provided (Japanese Patent Application No. 7-14809).

However, in order to realize a gradual change in the mode shape, on the other hand, the tapered portion must be as low as possible to the vicinity of the terminal end 58 (the start end at the time of splitting). For this reason, the metal to be subjected to thermal diffusion is diffused in a high concentration into the surface layer of the base body 51, so that there is no problem if the confinement of the guided light is strong, but if the confinement is weak,
In the vicinity of the terminal end 58 of the tapered portion 56, it was practically unavoidable that part of the guided light energy was converted into a radiation mode and leaked.

Accordingly, an object of the present invention is to solve such a problem and to provide an excellent optical waveguide device which can reduce the power loss of guided light as much as possible.

[0015]

In order to achieve the above object, an optical waveguide device according to the present invention is characterized in that a metal element is diffused into a substrate having an electro-optic effect so that one side is single and the other side is branched into a plurality. An optical waveguide device formed by forming a divided waveguide, wherein the width of the waveguide is located before and after a branch point of guided light propagating through the waveguide (transition region).
, Or gradually decreases in a region (transition region) located before and after the combining point.

Further, the region located after the branch point of the guided light propagating through the waveguide or the region located before the combining point is characterized in that the metal element is partially diffused.

For example, in a waveguide that forms an angle between the transition region and a partial region along the propagation direction of the guided light in a single waveguide, or along a propagation direction of the guided light in a branched waveguide. By preventing the diffusion source from being attached to a part of the region and making the change amount of the waveguide width near the intersection of the waveguide center line in the propagation direction of the guided light smaller than the change amount of the transition region, the thermal diffusion is performed. In this case, the diffusion concentration distribution can be made smooth, and the refractive index near the intersection of the branch waveguide center line can be slightly increased.

Thus, the metal thin film before thermal diffusion can change the width and the length of the peripheral cut portion and the central cut portion independently of each other, and can be applied to a waveguide manufactured under all conditions. In addition, it is possible to provide an optical waveguide device that can form a waveguide under optimum conditions, realize a good and gradual mode shape change, and suppress generation of a first-order mode.

Further, when the guided light propagating through a single optical waveguide moves to a multi-branched waveguide, or when the guided light propagating through a multi-branched waveguide moves to a single optical waveguide, Even at the narrowed portion formed between the waveguides, the energy of the guided light is efficiently split or multiplexed into the branched waveguide, and the power loss of the guided light can be reduced as much as possible.

[0020]

Next, specific embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a plan view showing a part of an optical waveguide device A, in which a metal element is diffused into a substrate 1 having an electro-optic effect, so that one side is a single part and the other side is branched into a plurality of parts. It is a thing. Actually, a single linear waveguide 3, a branch point 7 of guided light, a plurality of linear waveguides 4, 5, a parallel linear waveguide (not shown), a plurality of linear waveguides (not shown). (Not shown), a waveguide light combining point (not shown), and a single linear waveguide (not shown). The electrodes and the like are not shown for simplicity. In FIG. 1, the waveguide shows a region where the metal element concentration is high by a straight line.
Actually, the metal element is also diffused in other parts.

First, a method of manufacturing the optical waveguide device A shown in FIG. 1 will be described. Here, the base 1 is an optical grade LN single crystal having a thickness of about 1.0 mm, both surfaces of which are optically polished, and the main surface of which is Z-cut (cut surface is (0, 0, 1) plane). What it does.

As shown in FIG. 2, a Ti thin film 13 serving as a diffusion source is formed on the main surface of the substrate 1 by a lift-off method or an etching method, and then thermally diffused. As described above, the branch waveguide 2 having a high Ti concentration is formed. Thereafter, a modulation electrode (not shown) is formed on the branch waveguide 2 by vapor deposition or the like to complete the optical waveguide device A.

Here, a specific method of forming a thin film pattern of a metal serving as a diffusion source of a waveguide is performed as follows.
On the substrate 1, a Ti thin film 13 having a width of 6 to 14 μm and a thickness of about 1000 ° is formed at a portion to be a branch waveguide. Further, a single straight waveguide 3 shown in FIG.
The width of the waveguide between the straight waveguides 4 and 5 gradually increases (or decreases) before and after the branching point 7 (or combining point) of the guided light in the traveling direction of the guided light. Transition area 6
Is formed in at least a part of the transition region 6 (the region located after the branch point of the guided light propagating through the waveguide or the region located before the combining point) in the propagation direction of the guided light. A plurality of fine rectangular cutout patterns 13a are formed at a pitch of about 1 μm in such a manner. Outside the waveguide, there is provided a divergent region 14 that spreads at an angle θ ′ (approximately θ / 3) smaller than the original branch angle θ of the branch waveguide. This fine pattern is produced by, for example, UV or Deep-UV contact exposure. With such a thin film pattern formed,
The thermal diffusion is performed for about several hours in a furnace heated to about 1000 ° C., and the branch waveguide 2 is formed in the portion where Ti has diffused.

The refractive index of the branch waveguide changes smoothly in the propagation direction of the guided light in and around the transition region 6. This prevents a part of the power of the guided light from being converted into the radiation mode by the mode shape of the guided light propagating through the branch waveguide changing sufficiently slowly. However, when providing a comb-shaped pattern in the transition region 6 and its vicinity, the degree of the comb-shaped cutout (the distance from the branch point 7 to the cutout end), the size of the angle θ ′ of the divergent region, and the conduction of the divergent region The position of the starting point in the wave light propagation direction is related to the magnitude of the loss due to the branch portion (hereinafter, referred to as branch loss) when the guided light propagates through the branch waveguide 2.

The effect of such a comb-shaped Y-branch structure is that the waveguide light is originally strongly confined, for example, the thickness of the deposited metal thin film before thermal diffusion is set to be large, or the thermal diffusion time is shortened. This is particularly remarkable when the diffusion metal element is diffused to a high concentration in the vicinity of the substrate surface by lowering the thermal diffusion temperature.

However, if metal is diffused at a high concentration in the vicinity of the surface of the base 1 without any limitation, the surface of the waveguide is roughened. Therefore, in order to have good waveguide characteristics and to suppress branch loss, the mode shape change is performed sufficiently slowly by the comb-shaped Y-branch structure, and the divergent region 1 is formed.
It is important to minimize the generation of the radiation mode near the start of the transition region 6 (the end at the time of multiplexing).

Next, the dependence of the Y-branch loss of the comb-like Y-branch structure on the comb-like shape when the flared region 14 is not provided will be examined. In this case, for the sake of simplicity, a comb-shaped pattern having three cuts is disposed in the transition region 6 and its vicinity, and the cut is formed (L1 in FIG. 2; the first formed on the extension of the center line of the linear waveguide 3). Distance from the branch point 7 of the notch 10, L2; distance from the branch point 7 of the second and third cuts 11, 12 formed on both sides of the first notch 10)
A description will be given of the result of the investigation of the relationship between the branch loss.

The results shown below show that a single waveguide passes through a branch, and two parallel waveguides (not shown) from this branch.
When the distance between the two waveguides is equal to 30 μm, the length until the point where the single waveguide branches and the two branched waveguides is parallel is 1500 μm. .
As for the branch loss at this time, even when L1 and L2 were optimized, only a result of 0.61 dB was obtained per one stage of the Y branch (data without the divergent region in FIG. 3).

Providing a fine notch pattern in the above-mentioned transition region 6 and a thin film pattern serving as a diffusion source near the transition region 6 focuses on the production of a waveguide in which a waveguide is formed by thermal diffusion of a diffusion element. The fact that a very slow mode shape change can be realized in and around the transition region 6 by adopting the above structure is in good agreement with the calculation result using the beam propagation method. However, it was also found that some loss occurs near the branch point 7 (or the multiplexing point).

On the other hand, when the divergent region 14 is present near the branch point 7 (or the multiplexing point), it can be seen that the Y-branch loss is greatly reduced, and the comb-shaped Y-branch shape and the divergent region are provided. It has been found that the Y-branch loss can be reduced as much as possible for optical waveguide devices manufactured under all waveguide manufacturing conditions (minimum value: 0.12 dB /
Stage).

From the above results, it can be understood that the provision of the divergent region greatly contributes to the reduction of the branch loss. It should be noted, however, that it does not make sense to adopt this flared region separately from the comb-shaped Y-branch and adopt it alone. When only the divergent region is provided without the comb-shaped Y-branch shape, on the contrary, the amount of metal diffusion near the branch point (or multiplexing point) of the Y-branch waveguide changes rapidly, that is, the change of the refractive index distribution sharply. , Which causes a sudden change in the shape of the propagating light mode. That is, a large amount of energy is transferred to the excited first-order mode, and as a result, the Y-branch loss increases. Therefore, it is necessary for the Y-branch loss to be reduced that the comb-shaped Y-branch waveguide exists and the divergent region exists simultaneously.

In this embodiment, an example in which the optical waveguide is formed on the surface layer of the LN single crystal substrate has been described as an example. However, the present invention is not limited to this, and the substrate may be, for example, lithium tantalate or the like. It goes without saying that the present invention can be applied to optical waveguides of various well-known forms such as those having an optical waveguide formed on ribs on a substrate. Further, the present invention can be applied to an optical branch which does not have an electrode and does not originally have an electro-optic effect. Further, the above-described method for slowly changing the mode shape of guided light is implemented as described above. The present invention is not limited to the examples, and may be appropriately modified and implemented without departing from the gist of the present invention.

[0033]

As described above, according to the optical waveguide device of the present invention, a single waveguide is branched into a plurality of waveguides.
Alternatively, in the transition region where a plurality of waveguides merge into a single waveguide, the mode shape change during guided light propagation is made as slow as possible, whereby the energy of the guided light is efficiently branched, and An optical waveguide device having excellent device characteristics capable of minimizing power loss can be provided.

Further, since the transition region has a divergent region that is smaller than the divergence angle of the branched waveguide and a region where the metal element is partially diffused, the power loss of the guided light can be further reduced. Optical waveguide device can be provided. In particular, this transition region can be manufactured very easily and excellently by forming a metal thin film pattern having a plurality of cuts substantially parallel to the propagation direction of the guided light by thermal diffusion. .

[Brief description of the drawings]

FIG. 1 is a plan view illustrating an example of an optical waveguide device according to the present invention.

FIG. 2 is a plan view illustrating a shape of a metal thin film pattern formed on a substrate having an electro-optical effect.

FIG. 3 is a diagram illustrating loss measurement results of a Y-branch structure improved by the present invention.

FIG. 4 is a plan view illustrating a conventional optical waveguide device.

[Description of Signs] 1 ・ ・ ・ Base 2 ・ ・ ・ Branch waveguide 3,4,5 ・ ・ ・ Linear waveguide 6 ・ ・ ・ Transition region 7 ・ ・ ・ Branch point A ・ ・ ・ Optical waveguide device

Claims (2)

[Claims] 1. An optical waveguide device comprising: a substrate having an electro-optic effect; and a metal element being diffused into a substrate having one side formed into a single waveguide and the other side formed into a plurality of branched waveguides. An optical waveguide device, wherein a width gradually increases in a region located before and after a branch point of guided light propagating through a waveguide, or gradually decreases in a region located before and after a combining point. 2. A metal element is partially diffused in a region located after a branch point of a guided light propagating through the waveguide or in a region located before a multiplexing point. Item 2. An optical waveguide device according to item 1.