JP2862840B2 - Optical waveguide polarizer and method of manufacturing the same - Google Patents

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 polarizer using an optical waveguide formed on a substrate and a method for manufacturing the same. In particular, the present invention relates to an optical waveguide polarizer in which an optical waveguide used for a waveguide optical modulator or the like has polarization dependency, and a method of manufacturing the same.

[0002] The present invention relates to a technique for imparting a polarizing effect to an optical waveguide formed on a substrate in an optical waveguide polarizer. On an optical waveguide formed on the surface of a crystal substrate having the characteristic of optical anisotropy, the refractive index in one direction is higher than that of the substrate, and the other is higher than that of the substrate. An optical waveguide type in which a separate member (hereinafter, referred to as “light-inducing member”) having the same crystal structure as the substrate whose chemical composition is adjusted so that the refractive index has the same or lower value as that of the substrate is installed and joined. It relates to a polarizer. By using a material having the same crystal structure as the substrate for the light-inducing member, the structure can be stabilized and the reliability can be improved. Also,
By using materials having the same crystal structure, the substrate and the light-inducing member can be completely integrated by applying an epitaxial film growth technique or the like.

[0003]

2. Description of the Related Art In recent years, optical fibers have been used as transmission media.
The development and commercialization of long-distance ultra-high-speed communication is being actively pursued, and in this, the waveguide type optical device plays an important role as a key device of the system. In particular, lithium niobate (LiNbO ₃ ) (hereinafter referred to as “LiNbO _3” ) having a large electro-optic effect
LN. ) Is thermally diffused with a different element such as Ti on the surface of a ferroelectric substrate to locally increase the refractive index, form a waveguide, and combine this with a signal wave electrode. It is a device that forms the core of a high-speed communication system.

A ferroelectric crystal such as lithium niobate (LiNbO ₃ ) generally has a large optical anisotropy (dependence of the refractive index on the crystal axis), and is therefore formed along a specific crystal axis direction. The refractive index felt by the propagating light varies depending on the polarization direction of the light wave propagating through the waveguide, and the propagation speed changes. As a result, characteristics such as the driving voltage and the extinction ratio of the modulator greatly change. Therefore, in order to stabilize the characteristics of the modulator and achieve higher performance, it is preferable that the polarization of the light wave incident on the waveguide of the modulator is aligned in one direction.

As a simple method of polarizing incident light and guiding it to the waveguide element, a method of coupling an optical fiber to the incident end face of the waveguide via a polarizer has been put to practical use. In particular, the mounting method of bonding the optical fiber to the end face of the waveguide with a thin-film polarizer using an ultraviolet-curing optical adhesive is combined with the case sealing technology to achieve a highly reliable waveguide-type modulation. Can be realized. However, in order to construct a waveguide modulator with higher performance, the loss of the propagation light intensity caused by the insertion of the polarizer in the optical path cannot be ignored.

Therefore, as a method without using an external polarizer, a separate member is provided on the surface of the waveguide, and a polarized wave in an unnecessary direction is led out from the waveguide or absorbed by the member to remove the member. The method has been taken in the past. For example, as shown in FIG.
- cut LN substrate surface, an optical waveguide type polarizer depositing a niobium oxide (Nb ₂ O ₅₎ film has been used. At this time,
LN of the extraordinary refractive index n _e (z-axis direction), the ordinary refractive index n _o
(X-y plane), and the refractive index n _c of the niobium oxide film, n _o
> Since the relation of n _c> n _e, perpendicular TM polarization in the substrate surface of the polarization of the incident light, when passing through the lower niobium oxide film, felt refractive index n _e and n _c, the refractive It leaks to the niobium oxide side where the rate is high. On the other hand, TE polarized parallel to the substrate surface, feel the refractive index n _o, be passed under niobium oxide film,
Since the refractive index on the waveguide side is larger, the light propagates while being confined in the waveguide. As a result, only TE polarization is guided at the output side. By forming such a waveguide polarizer portion, for example, near the entrance side of the modulator element, a polarizer / modulator integrated waveguide is created.

Further, as shown in FIG. 2, a metal film is deposited on the surface of the waveguide to make use of the fact that the electric field of the TM polarized wave of the propagating light penetrates deeply into the metal film and receives large absorption. Waveguide polarizers that pass polarized waves have also been used.

[0008]

The conventional method proposed to suppress the loss of the intensity of propagating light by using an external polarizer, that is, a method of loading a member completely separate from the substrate on the surface of the waveguide. Has the following disadvantages.

In the method of absorbing the TM polarization by the metal film, the electric field of the TE polarization, though slightly, penetrates and is absorbed in the metal film, which leads to a loss of the intensity of the propagating light.

In the method of loading a dielectric film or a bulk body whose refractive index has been adjusted using the refractive index anisotropy of the substrate on the surface of the waveguide, the elastic constant, A significant shift occurs in the coefficient of thermal expansion, and internal stress and strain are introduced into the substrate and the waveguide. When stress or strain is applied to a ferroelectric substrate such as LN, the refractive index changes due to the elastic optical effect, and the speed of propagating light is disturbed, so that the element characteristics change.

Further, the difference between the material of the substrate and the material of the loading member makes it difficult to obtain a high bonding strength for the same reason, and there is a problem in the mechanical reliability of the constituent elements.

[0012]

SUMMARY OF THE INVENTION In order to solve the fundamental problem of the conventional method of forming an optical waveguide type polarizer by joining a separate member made of a different material from the substrate to the substrate. The refractive index is changed or adjusted by adding a heterogeneous element to the substrate material without adding a separate member to the surface of the substrate waveguide, and a conductive carrier is introduced. A material to which an electroabsorption effect was imparted, but whose crystal structure, lattice constant, mechanical and elastic properties, and the like were substantially the same as the substrate material was used.

In the optical waveguide polarizer, a substrate having an optical waveguide on the surface of which an optical wave is guided, and a light having the same crystal structure as the substrate containing at least one element not contained in the substrate. An attraction member, at least one
A plurality of light-guiding members are bonded on the optical waveguide of the substrate.

The refractive index of the light-guiding member joined on the waveguide,
As an additional element having a size (ionic radius) that can adjust the electric conductivity and easily perform substitution doping on the light-inducing member, particularly, transition metals 3A, 4A, 5A, 6A, 7A, 8, 1
Selected from the B and 2B families.

In particular, as one of the principles of adjusting the refractive index of the light-inducing member, a strain caused by the addition of a different element to the bonding structure of atoms of the light-inducing member is exerted to induce a change in the refractive index. Select one with a relatively large ion radius. In particular, the element is preferably Zn or Ni.

In order to realize complete bonding and integration of the substrate and the light-inducing member at the atomic level and to effectively bring out the effects of the present invention, the bonding between the substrate and the light-inducing member is performed on the substrate portion. It is desirable to use a liquid phase growth method for solidifying the melt of the light-inducing member.

[0017] As another method for achieving the above object, the joining of the substrate and the light-inducing member may be performed by growing the light-inducing member on the substrate portion by a sol-gel method.

The joining of the substrate and the light-inducing member may be performed by bonding the substrate and the light-inducing member at a high temperature and integrating them. This method can be achieved because the crystal structures of the substrate and the light-inducing member are almost the same.

In addition, when it is not necessary to particularly enhance the reliability such as the bonding strength of the constituent elements, that is, when it is not necessary to obtain complete integration, as a simpler bonding method, the substrate and the light-inducing member are connected with each other. May be configured with an adhesive bonding member interposed between the substrate and the light-inducing member. In this case, since both members are of the same quality, peeling of the bonding interface due to a difference in the amount of thermal expansion between the two members hardly occurs even when a thermal cycle is applied to the element.

In the case of joining with a joining member, that is, an adhesive, the refractive index of the joining member is determined so that the light wave guided through the optical waveguide on the surface of the substrate is attracted into the light-inducing member through the joining member. Has been adjusted.

As an element to which the present invention is applied, the substrate is preferably LiNbO ₃ .

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention comprises a substrate having an optical waveguide on the surface of which an optical wave is guided, and one or more light-guiding members disposed so as to cover at least a part of the optical waveguide. In an optical waveguide polarizer comprising:
The substrate is formed of a member having at least one kind of element not included in the substrate and having the same crystal structure as the substrate, and is joined to the substrate so as to match the crystal orientation of the substrate.

The present invention is characterized in that the element contained in the light-inducing member is an element selected from transition metals. The present invention is characterized in that the element is Zn or Ni. The present invention is characterized in that a joining member is provided between the substrate and the light-inducing member.

The present invention is characterized in that the substrate is LiNbO ₃ . The present invention provides an optical waveguide type polarizer comprising: a substrate on which an optical waveguide on which a light wave is guided is formed; and one or more light guiding members disposed so as to cover at least a part of the optical waveguide. In the manufacturing method, the light-inducing member is made to contain at least one element not contained in the substrate.
It is characterized by comprising a member having the same crystal structure as the substrate to which at least one kind is added, and joining to the substrate so that the crystal orientation matches the substrate crystal orientation.

The present invention is characterized in that the joining of the substrate and the light-inducing member is performed on the substrate by a liquid phase growth method of solidifying a melt of the light-inducing member. The present invention is characterized in that the bonding between the substrate and the light-inducing member is performed by growing the light-inducing member on the substrate by a sol-gel method. The present invention is characterized in that the substrate and the light-inducing member are joined together by bonding the substrate and the light-inducing member at a high temperature. The present invention is characterized in that the bonding between the substrate and the light-inducing member is performed by sandwiching an adhesive bonding member between the substrate and the light-inducing member.

[0026]

FIG. 3 shows an embodiment of the present invention. An optical waveguide 2 is formed on the surface of a lithium niobate (LiNbO ₃ ) substrate 1 by thermal diffusion of Ti. The light-guiding member 3 is joined by a liquid phase growth method so as to cover a part of the optical waveguide 2 and a part of the surface of the substrate.

Here, the light-inducing member 3 is obtained by adding Zn to the same lithium niobate crystal as the substrate 1, and exhibits a refractive index as shown in Table 1 according to the amount of addition.

[0028]

[Table 1]

Lithium niobate (LN) with Zn added
In this case, the ordinary light refractive index becomes larger, and conversely, the extraordinary light refractive index becomes smaller, as compared with the non-added harmonic composition lithium niobate.
In the Ti diffusion waveguide, the refractive index of both ordinary light and extraordinary light increases in accordance with the amount of diffusion. Therefore, Ti diffusion waveguide diffusion conditions (diffusion amount of Ti, i.e., the diffusion temperature, diffusion time, etc.) and, by selecting the addition amount of Zn into the optical attractant _{member, n o (LN) <n} o ( _{Ti: LN) <n o (} Zn: L
N) and _ne (Zn: LN) < _ne (LN) < _ne (Ti: L
N) or _{n o (LN) <n o} (Ti: LN) ~n o (Zn: L
N) and _ne (Zn: LN) < _ne (LN) < _ne (Ti: L
N) can be created.

In any of the above cases, from the magnitude relation of the extraordinary light refractive index ( _ne ), the polarization component of the light wave propagating through the Ti diffusion waveguide, which senses the extraordinary light refractive index, is more abnormal light refraction than the waveguide. The light is not attracted to the LN substrate and the Zn: LN light attracting member having a small ratio, but is strongly confined in the waveguide and propagates.

On the other hand, a polarized component that senses ordinary light is not attracted to the LN substrate side having an ordinary light refractive index smaller than that of the waveguide, but is a Zn: LN light having an ordinary light refractive index larger than or similar to that of the waveguide. Only the polarization component which becomes more likely to be attracted into the attracting member and consequently feels extraordinary light selectively propagates through the waveguide.

When the above structure is applied to a z-plane cut LN substrate, as shown in FIG. 7, an extraordinary light refractive index (n _e ), which is a refractive index component parallel to the z axis, is sensed. Is the polarization component parallel to (by definition perpendicular to the surface)
Only M polarized waves can be guided. In the LN, since the electro-optic constant parallel to the z-axis is the maximum value, by adopting such a configuration, a polarized wave component (TM in this case) interacting with the largest electro-optic constant is propagated, and unnecessary By cutting the polarization component, the SN (extinction ratio) of the element output light intensity can be improved.

The bonding of the Zn-added LN, which is a light-inducing member, to the substrate surface can be performed by growing the light-inducing member from a liquid phase under the following conditions. In order to uniformly melt the raw material at a lower temperature, a Li ₂ O—V ₂ O ₅ system flux is used, and Li ₂ O: V ₂ O ₅ : N ₂ O ₅ = 5.
ZnO (Li) was added to the melt adjusted to a composition ratio of 0:40:10.
0.56 mol) is added to the NbO ₃ composition and melted. A predetermined portion of the LN substrate is placed on this melt at a temperature of 850 ° C.
After being immersed for 15 minutes, it is pulled up and the melt is solidified and crystallized on the substrate. At this time, the melt of the light-guiding member whose basic component is equivalent to that of the substrate grows epitaxially (z-plane cut L
On the surface of the N substrate, the z-plane of the light-inducing member crystal grows).

At this time, the light-inducing member does not need to be a single crystal, but may be a polycrystal (film) in which desired crystal axes are preferentially oriented. Therefore, it is difficult to grow a single crystal film, such as the sol-gel method, but a light-inducing member may be grown on a substrate using a technique characterized by growing a preferentially oriented crystal film at a low temperature. In the case of LN, at least 600 ° C by the sol-gel method
Thus, a crystal film can be grown.

As an additive element of the light-inducing member, for example, Ni can be used in addition to Zn. N in LN crystal
Also when i is added, the ordinary light refractive index increases and the extraordinary light refractive index decreases as compared with the substrate similarly to the case of Zn addition.
n: The same effect as when LN is used for the light-inducing member can be obtained. Ni metal is used for the LN crystal of the light-inducing member.
Table 2 shows the relationship between the refractive index of the crystal surface and the diffusion temperature when nm was vapor-deposited and thermally diffused in oxygen for 10 hours.

[0036]

[Table 2]

In the embodiment shown in FIG. 3, the light guiding member 3 is provided at one end. However, as shown in FIG. 4, it can be arranged at any place other than the end. FIG.
In the case where a plurality of light-inducing members are provided as shown in (1), they can be arranged at any positions including the other end.

An X-cut LN is applied to the light-guiding member by a liquid phase growth method.
An example of characteristics of an optical waveguide polarizer manufactured by bonding to a substrate will be described. A part of an X-cut LN substrate (waveguide is in the Y-axis direction) in which a Ti diffusion waveguide capable of guiding light having a wavelength of 1.5 μm in a single mode is formed in advance using ZnO,
In the LN melt (850 ° C.) to which 10 mol% was added, 3
After being immersed for 0 minutes, it was pulled up, and the melt was quenched and solidified on the substrate to be crystallized. In this case, Li ₂ O-V ₂
The O ₅ system LiVO ₃ of using the flux (flux amount, 80 wt%).

The thickness of the Zn: LN light guiding member is about 20 μm.
m, the substrate was epitaxially grown. Next, both ends of the substrate were cut and polished to obtain a shape as shown in FIG. The length of the substrate in the waveguide direction is 40 mm, and the length of the light-guiding member is 10 mm. This waveguide, TE / TM extinction ratio measured by guided from the side without the optical attractant member lightwave wavelength 1.5μm is 33 dB, TE polarization component transmission feel extraordinary refractive index n _e Was obtained.

FIG. 6 shows another embodiment of the present invention. LN
As a method of forming an optical waveguide polarizer using the light-inducing member 3 in which the refractive index is adjusted by adding Ni to the crystal as described above, a substrate having the optical waveguide 2 as shown in FIG. The light-inducing member 3 was bonded to the desired surface portion 1 via a bonding member 4 having an adhesive layer. At this time, the light wave propagating through the waveguide is sufficiently attracted into the light guiding member 3 via the joining member 4, and a polarized component having ordinary refractive index is efficiently guided from the waveguide into the light guiding member 3. The thickness of the adhesive layer was selected to be 0.5 μm. In this case, it is desirable that the thickness be 0.5 μm or less.

Further, it is desirable that the refractive index of the adhesive layer is adjusted. In general, the refractive index of an adhesive material is often isotropic, and it is difficult to act only on a desired polarization component, so that the waveguide does not cause a large loss to a light wave propagating through the waveguide. It is preferable that the refractive index is increased within a range lower than the refractive index of the above. However, since the loss amount of the guided light also depends on the bonding area between the bonding layer and the waveguide,
This is not the case when the bonding area is small.

As an additional element for adjusting the light-inducing member, a transition metal element having an equal or smaller ionic radius can be used in addition to Zn and Ni. For example, V (4
When an element having unbonded electrons as free electrons is added in an oxide state such as (valent), the light-guiding member is given an electrical conductivity close to that of a metal, so that the conventional electro-absorption type polarized light shown in FIG. An integrated waveguide polarizer corresponding to the function can be configured.

In the case of substances having the same crystal structure, the crystals can be joined by solid-phase thermal diffusion at the contact interface by heating the two in close contact so that the in-plane crystal orientations are aligned. It is. About 800 ° C for LN crystal
Therefore, it is possible to directly join a light-guiding member of the same quality separately prepared on a substrate by using this technique.

[0044]

According to the prior art, an entirely different member is joined to the surface of the waveguide by forming an optical waveguide type polarizer by joining a light-inducing member of the same quality as the substrate material to the surface of the waveguide. Compared with the optical waveguide type polarizer which has been used, the mechanical reliability can be improved, and an integrated element can be formed by applying a completely integrated junction such as an epitaxial junction technique.

[Brief description of the drawings]

FIG. 1 shows an optical waveguide polarizer using an anisotropic optical crystal (conventional example).

FIG. 2 shows an optical waveguide polarizer using a metal cladding (conventional example).

FIG. 3 shows an embodiment of an optical waveguide polarizer according to the present invention.

FIG. 4 shows another embodiment of the optical waveguide polarizer according to the present invention.

FIG. 5 shows another embodiment of the optical waveguide polarizer according to the present invention.

FIG. 6 shows still another embodiment of the optical waveguide polarizer according to the present invention.

FIG. 7 is a diagram illustrating the relationship between a light refractive index and a polarization component.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Optical waveguide 3 Light attraction member 4 Joining member

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kaoru Hikuma 585 Tomimachi, Funabashi-shi, Chiba Sumitomo Osaka Cement Co., Ltd. (72) Inventor Hirotoshi Nagata 585 Tomimachi, Funabashi-shi, Chiba Sumitomo Osaka Cement Co. Company New Technology Research Laboratory (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B 6/126 G02B 6/13

Claims (10)

(57) [Claims] 1. An optical waveguide type polarized light comprising: a substrate having a surface on which an optical waveguide through which a light wave is guided is formed; and at least one light guiding member disposed so as to cover at least a part of the optical waveguide. In one embodiment, the light-inducing member is formed of a member having the same crystal structure as the substrate including at least one or more elements not included in the substrate, and is bonded to the substrate so as to match the substrate crystal orientation. An optical waveguide polarizer characterized by the following. 2. The optical waveguide polarizer according to claim 1, wherein the element included in the light-inducing member is an element selected from transition metals. 3. The optical waveguide polarizer according to claim 1, wherein the element is Zn or Ni. 4. The optical waveguide polarizer according to claim 1, further comprising a bonding member between the substrate and the light-inducing member. 5. The optical waveguide polarizer according to claim 1, wherein the substrate is LiNbO ₃ . 6. An optical waveguide type polarized light comprising: a substrate having a surface on which an optical waveguide through which a light wave is guided is formed; and at least one light guiding member disposed so as to cover at least a part of the optical waveguide. In the method for manufacturing an element, the light-inducing member is formed of a member having the same crystal structure as that of the substrate to which at least one element not contained in the substrate is added, and the crystal orientation is matched with the crystal orientation of the substrate. A method of manufacturing an optical waveguide polarizer, wherein the optical waveguide polarizer is bonded to the substrate as described above. 7. The bonding between the substrate and the light-inducing member,
7. The method of manufacturing an optical waveguide polarizer according to claim 6, wherein the method is performed by a liquid phase growth method of solidifying a melt of the light-inducing member on the substrate. 8. The bonding between the substrate and the light-inducing member,
7. The method of manufacturing an optical waveguide polarizer according to claim 6, wherein the method is performed by growing the light-inducing member on the substrate by a sol-gel method. 9. The bonding between the substrate and the light-inducing member,
The method for manufacturing an optical waveguide polarizer according to claim 6, wherein the substrate and the light-inducing member are bonded together at a high temperature and integrated. 10. The method according to claim 6, wherein the bonding between the substrate and the light-inducing member is performed by sandwiching an adhesive bonding member between the substrate and the light-inducing member.
A method for producing an optical waveguide polarizer according to the above.