JP2847844B2 - Optical waveguide - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical waveguide formed in a channel shape on a surface of a substrate.

[Conventional technology]

The optical waveguide is formed as a high refractive index layer near the surface of the substrate by thermal diffusion, ion exchange, ion implantation, etc., confine the light in the waveguide, and integrate an optical switch, modulator and other functional elements. Can be configured. In particular, a bent waveguide is important for performing optical path conversion between elements of an optical integrated circuit. At this time, as the radius of curvature of the curved waveguide is smaller, the optical path can be converted with a shorter propagation distance, but the loss of the curved waveguide increases as the radius of curvature decreases. This means that when the light propagates in the bent waveguide, a part of the light protruding outside the bend is radiated in the radial direction, and the radiation loss increases as the radius of curvature decreases. One of the purposes of the optical integrated circuit is to reduce the size of the element. To achieve this, it was necessary to reduce the radius of curvature of the bent waveguide and to reduce the radiation loss.

In general, the radiation loss of a bent waveguide can be represented by the attenuation constant α of the guided light accompanying the radiation of the bent portion, and the attenuation constant α is the radius of curvature R and the refractive index difference ΔN (the effective refractive index N of the waveguide mode and Difference N− from the surface refractive index n _s near the waveguide
n _s ) is determined by the following equation.

αA · (N ² −n ² _s ▼) ⁻¹ · exp (−BR · R) (1) where A and B are constants determined by the waveguide. Conventionally, in order to reduce radiation loss of a bent waveguide. Then, magnesium oxide, which is known to reduce the refractive index of the substrate, is bent and diffused out of the waveguide, and the refractive index difference ΔN
It has been reported how to increase the size.

FIG. 4 is a perspective view showing an example of a conventional bent waveguide, and has a structure in which a magnesium oxide additional diffusion layer 33 is formed outside a Ti diffusion bent waveguide 32. FIG. Radiation loss is reduced by increasing the refractive index difference ΔN from the Ti diffusion waveguide by the additional diffusion of magnesium oxide and strengthening light confinement in the waveguide. In FIG. 4, 31 is a lithium niobate crystal substrate, 32 is a Ti diffusion bending waveguide, and 33 is a magnesium oxide additional diffusion layer. The use of the magnesium additional diffusion method for reducing radiation loss of a Ti diffusion bending waveguide (lithium niobate crystal substrate) is described in B. SCHUPPERT's ELECTRONICS LETTERS, Vol. 23, No. 15,
Issue, pages 797-798.

An optical integrated circuit based on this optical waveguide is generally inserted into an optical transmission line and is often used for signal processing of light propagated in an optical fiber. In a high-speed, large-capacity optical communication system, a single mode optical fiber is used as an optical fiber, and a semiconductor laser is used as a light source. A semiconductor laser emits linearly polarized light, but light propagated in a single mode optical fiber generally becomes elliptically polarized light,
Further, the polarization state also varies with time. That is, in order to directly connect the optical integrated circuit and the single mode optical fiber,
It is necessary to remove the polarization dependence of the characteristics of the optical integrated circuit.

[Problems to be solved by the invention]

The guided light of an optical waveguide formed on a crystal substrate generally has two independent modes, that is, a TM whose polarization direction is perpendicular to the substrate surface.
The light is separated into a mode and a TE mode having a polarization component orthogonal to the mode. For example, the refractive index difference Δn _e for the extraordinary light is involved in the TM mode in the bent waveguide in the LiNbO ₃ Z plate that is often used, and the refractive index difference Δn _o for the ordinary light is involved in the TE mode. Because of the difference (Δn _e >
Δn _o ), assuming that the radiation loss when guiding the light in the TM mode and the TE mode is Loss ^(TM) and Loss ^(TE) , respectively, Loss ^(TM) <Loss
s ^(TE) , and the optical waveguide has a drawback that the polarization dependence of loss occurs. Refractive index n _o In additionally diffusion of magnesium oxide for ordinary light refractive index n _e for extraordinary light are both decreased, delta
n _e, does not lead to reduce the polarization dependence of the losses for the [Delta] n _o are both increased.

An object of the present invention, by increasing the refractive index n _e of the substrate surface is to provide an optical waveguide with reduced polarization dependence loss.

[Means for solving the problem]

The present invention is directed to a channel optical waveguide including a curved shape formed by diffusing impurities into a crystal substrate having optical anisotropy, wherein at least an adjacent portion along the curved portion of the optical waveguide is diffused out of the substrate surface. An optical waveguide characterized in that (out diffusion) has been performed.

[Action]

The crystal having optical anisotropy represented by lithium niobate, the site that has been subjected to external diffusion,屈撤factor n _e for extraordinary light is increased. Therefore, in the present invention, lateral confinement of extraordinary light is weakened, and ordinary light and extraordinary light (TE
The difference between the radiation loss of the mode and the TM mode is reduced, and the loss polarization dependence of the optical waveguide is reduced.

Therefore, according to the present invention, the polarization dependence of the loss can be improved as compared with the conventional example.

〔Example〕

FIG. 1 is a perspective view of one embodiment of the present invention. 1 is Z
Lithium niobate crystal substrate whose axis is perpendicular to the substrate surface, 2
Is a Ti diffusion bending waveguide, and 3 is an outer diffusion layer. The Ti-bent waveguide is formed by first depositing a Ti film on a crystal substrate by sputtering or vapor deposition, and thermally diffusing the patterned Ti film through a fine processing step such as photolithography.

Diffusion is generally performed in an electric furnace, and the crystal substrate is heated to about 1000 ° C. in a vacuum or inert gas atmosphere, and Li ₂ O is released to the outside from the crystal surface to form a high refractive layer (waveguide). . By Li ₂ O that is released to the outside, the refractive index n _e is changed with respect to extraordinary light, the refractive index increment is about 10 ^-3. As a result, the lateral confinement of extraordinary light is weakened, the difference in radiation loss between ordinary light and extraordinary light is reduced, and the loss polarization dependence of the optical waveguide is reduced.

FIG. 2 and FIG. 3 show that the portion not requiring the outer diffusion layer is made of Li.
FIG. 2 is a perspective view of an embodiment formed by covering with ₂ CO ₃ , LiNbO ₃ or the like and performing heat treatment. FIG. 2 shows the case where the outer diffusion layer 3 is provided only on the outside of the Ti diffusion bending waveguide, and FIG. 3 shows the case where the outer diffusion layer 3 is provided near the outer periphery of the Ti diffusion bending waveguide. 2 and 3, the same effects as those of FIG. 1 can be obtained.

〔The invention's effect〕

As described above, the present invention provides an outer diffusion layer at an adjacent portion along a channel waveguide formed by diffusing impurities into a crystal substrate having optical anisotropy, thereby providing a refractive index for extraordinary rays. n _e increases the effect of reducing the polarization dependency regarding loss of the optical waveguide.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of an optical waveguide according to the present invention, FIG. 2 is a perspective view of an embodiment having an outer diffusion layer only outside a Ti diffusion bending waveguide, and FIG. FIG. 4 is a perspective view of an embodiment having an outer diffusion layer near the outer periphery of a bent waveguide. FIG. 4 is a perspective view showing a conventional optical waveguide. 1,31: Lithium niobate crystal substrate; 2,32: Ti diffusion bending waveguide; 3: outer diffusion layer; 33: magnesium oxide additional diffusion layer.

Claims (1)

(57) [Claims] 1. A channel optical waveguide including a curved shape formed by thermally diffusing an impurity into a crystal substrate having optical anisotropy, wherein at least a portion adjacent to the curved portion of the optical waveguide along a curved portion is formed from the substrate surface. An optical waveguide characterized by being subjected to external diffusion.