JP2855676B2 - 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 formed. 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 waveguide due to the radiation of the bent portion, and this attenuation constant α is equal to 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, 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. 5 is a perspective view showing an example of a conventional bent waveguide, and has a structure in which magnesium oxide is further diffused outside the Ti diffusion bent waveguide. 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. 5, reference numeral 31 denotes a lithium niobate crystal substrate (Li
NbO ₃ ) and 32 are Ti diffusion bending waveguides, 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 plate) is described in B. SUHPPERT's ELECTRONICS LETTERS, Vol. 23, 1
No. 5, 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 polarization direction perpendicular to the substrate surface.
The light is separated into a TM mode and a TE mode having a polarization component orthogonal to the TM mode. For example, in a bent waveguide in a LiNbO ₃ Z plate that is usually used, in the TM mode, the refractive index difference Δn _e for extraordinary light is involved, and in the TE mode, the refractive index difference Δn ₀ for ordinary light is involved. Because of the difference (Δn _e
> Δn ₀ ), and the radiation loss when each of the TM mode and TE mode light is guided is Loss ^(TM) and Loss ^(TE) , respectively.
^(TM) <Loss ^(TE) , and the optical waveguide has a drawback that the polarization dependence of loss occurs. In the additional diffusion of magnesium oxide, the refractive index n _e for extraordinary light and the refractive index n ₀ for ordinary light both decrease, and both Δn _e and Δn ₀ increase, so that the polarization dependence of loss cannot be reduced.

An object of the present invention reduces the refractive index n ₀ of the substrate surface, by increasing the refractive index n _e, it 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 a portion adjacent to the curved portion of the optical waveguide along the curved portion is subjected to ion exchange from the substrate surface. Is provided.

[Action]

In crystals having optical anisotropy, such as lithium niobate and lithium tantalate,
Refractive index n ₀ of the substrate is reduced with respect to ordinary light, the refractive index n _e for extraordinary light is increased. For this reason, in the present invention, the light confinement in the TE mode is enhanced, and
The difference in radiation loss between the mode and the TM mode is reduced, and the loss polarization dependence of the optical waveguide is reduced.

FIG. 6 shows a comparison of polarization polarization dependence between the optical waveguide of the present invention and the conventional example, and shows the refractive index difference and radiation loss characteristics of the substrate surface when TE mode and TM mode are guided in the optical waveguide. Each is shown. Here, the horizontal axis indicates the refractive index difference (Δn) on the substrate surface, and the vertical axis indicates the radiation loss (Loss). That is, the curves 21a and 21b correspond to T
E mode (a), TM mode (b) guided refractive index difference
In the radiation loss characteristics, the curves 22a and 22b are according to the present invention.

As is clear from FIG. 6, the present invention makes it possible to improve the polarization dependence of the loss as compared with the conventional example.

〔Example〕

FIG. 1 is a perspective view of one embodiment of the present invention. 1 is a lithium niobate crystal substrate (LiNb) whose Z axis is perpendicular to the substrate surface.
O ₃ ), 2 is a Ti diffusion bending waveguide, and 3 is a proton-exchange layer (proton-exchange layer). The Ti diffusion bending waveguide 2 is prepared by depositing a Ti film of about 450 ° on NiNbO ₃ by sputtering and heat-treating it at about 1000 ° C. The proton exchange layer 3 can be formed by heating a melt obtained by mixing about 3 to 5% of lithium benzoic acid with benzoic acid to about 220 ° C., immersing the crystal substrate 1, and annealing the substrate at about 400 ° C. By the proton exchange, the refractive index n ₀ of the substrate (proton exchange layer) with respect to ordinary light decreases by about 2 × 10 ^−3, and the surface refractive index difference Δn ₀ between the Ti diffusion bending waveguide and the proton exchange layer.
Becomes larger. Refractive index n _e for extraordinary ray 6 × 10 ^-3
And Δn _e decreases.

FIG. 2 shows the surface refractive index difference Δn with respect to the Ti film thickness. By performing the proton exchange, Δn ₀ is large and Δn _e is small, and the degree of light confinement for the TE and TM modes can be made closer.

In FIG. 2, the horizontal axis represents the Ti film thickness, and the vertical axis represents the surface refractive index difference Δn.
Is the refractive index difference between the substrate surface and the Ti diffused optical waveguide, straight line 12a,
12b is a refractive index difference between the proton exchange layer and the Ti diffused optical waveguide.

In this way, the radiation loss of the TE mode refractive index n ₀ is involved is reduced, by the radiation loss of the TM mode n _e is involved is increased, polarization dependency involved in the loss of the optical waveguide improved Is done.

FIG. 3 is a perspective view in which proton exchange is performed on the entire outer surface of the Ti diffusion bending waveguide, and FIG. 4 is a perspective view in which proton exchange is performed on both sides of the waveguide. 3 and 4, the same effect of the invention as in FIG. 1 can be obtained.

〔The invention's effect〕

As described above, the present invention provides a TE mode by performing ion exchange from a substrate surface to an adjacent portion along a channel waveguide formed by diffusing impurities into a crystal substrate having optical anisotropy. Refractive index for the involved ordinary ray
n ₀ reduces, also increasing the refractive index n _e for extraordinary ray which is involved in the TM mode, 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 characteristic diagram of Ti film thickness / surface refractive index difference of the optical waveguide in FIG. FIG. 4 is a perspective view in which proton exchange is performed on the entire outside of the Ti diffusion bending waveguide, FIG. 4 is a perspective view in which proton exchange is performed on both sides of the Ti diffusion bending waveguide, and FIG. 5 is a perspective view of a conventional bending waveguide. FIG. 6 and FIG. 6 are graphs showing the refractive index difference and radiation loss characteristics showing the operation of the present invention. 1,31 ...... lithium niobate crystal substrate (LiNbO _3), 2,32 ...
... Ti diffusion bending waveguide, 3 ... Proton exchange layer, 11a ...
… Ordinary light refractive index difference from substrate, 11b… extraordinary light refractive index difference from substrate, 12a… ordinary light refractive index difference from proton exchange layer, 12b…
... Anomalous optical refractive index difference from proton exchange layer, 21a ... TE mode radiation loss in conventional example, 21b ... T in conventional example
M mode radiation loss, 22a: TE mode radiation loss in the present invention, 22b: TM mode radiation loss in the present invention, 33: magnesium oxide additional diffusion layer.

Claims (1)

(57) [Claims] In a channel optical waveguide including a curved shape formed by diffusing impurities into a crystal substrate having optical anisotropy, ions are formed from a substrate surface at least in an adjacent portion along a curved portion of the optical waveguide. subjected to replacement, reduces the refractive index n ₀ for ordinary light in the ion exchange has been applied region, by increasing the refractive index n _e for extraordinary light,
An optical waveguide, wherein the polarization dependence of loss generated when light propagates along a curved portion of the optical waveguide is reduced.