JP2705790B2 - Optical waveguide and method for forming the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a waveguide having a small radius of curvature formed on an electro-optic crystal substrate. As optical waveguides, optical fiber cables (optical transmission fibers) with transparent quartz as the core and cladding (sheath) of glass with a lower refractive index than this, transparent polymers with different refractive indices (polymer organic compounds) ) For a plastic optical fiber cable using a core and a clad, an optical waveguide in which metal is diffused on an electro-optic crystal substrate to form a line with a higher refractive index than the substrate, or a mask pattern consisting of a metal film on a glass substrate After the formation, there is an optical waveguide having a higher refractive index than the substrate obtained by immersing in a molten salt and performing ion exchange. Here, an optical fiber cable is mainly used as a waveguide used for optical communication, and is used in combination with an optical element such as an optical switch, a polarization separation element, and a lens. However, since these waveguides and optical elements are formed separately, they need to be finely aligned with each other, which not only inevitably increases the loss due to mismatching but also increases the man-hours for adjustment and is expensive. Attached to In order to avoid this, an optical circuit element in which a waveguide and an optical element are integrated and integrated has been proposed. Among them, an optical circuit element formed using a crystal substrate having a large electro-optical effect, such as lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ), is promising because a device with a low driving voltage can be realized. Among them, LiNbO ₃ is excellent as a substrate of an optical circuit element because a low-loss waveguide can be easily formed particularly by thermal diffusion. [Prior Art] LiNbO ₃ is used as a substrate, on which a plurality of optical elements are formed and integrated. In this case, an optical waveguide for interconnecting optical elements such as switches formed in an integrated manner needs to be curved and patterned to the optical element in order to shorten the connection length. However, since the waveguide formed by thermal diffusion has a relatively small change in the refractive index, a radius of curvature of a certain value or more is required for low-loss transmission, and when the radius of curvature is small, light is out of the waveguide. However, there is a problem that the loss is abnormally increased due to the escape. In addition, as a method of forming a bent waveguide, there is a method using total reflection. However, in this case as well, loss is increased due to scattering on a reflection surface. From these facts, it is necessary to maintain the radius of curvature of the optical waveguide in the range of 30 to 40 mm in order to transmit light with low loss, and therefore, there is a limit in improving the degree of integration. [Problems to be Solved by the Invention] As described above, the waveguide connected to the optical element formed integrally on the LiNbO ₃ substrate needs to be formed in a curved shape, but with low loss. There is a problem in that the radius of curvature is limited for transmitting light, so that sufficient integration cannot be performed. [Means for Solving the Problems] To solve the above problems, according to the present invention, LiNb
A curved optical waveguide for transmitting extraordinary light is formed on the O ₃ crystal substrate, the optical waveguide has a higher refractive index than the substrate, and magnesium oxide is formed along the outer periphery of the curved optical waveguide substantially parallel thereto. An optical waveguide structure is provided, wherein a diffusion region is formed. Further, according to the method for forming an optical waveguide according to the present invention, Li
In forming a curved extraordinary light propagation optical waveguide having a higher refractive index than the substrate on a NbO ₃ crystal substrate, magnesium oxide (MgO 3) is formed along the outer periphery of the extraordinary light propagation curved optical waveguide substantially parallel to the outer periphery. ) Is diffused. [Operation] The present invention diffuses magnesium oxide, which lowers the refractive index of (extraordinary light), around the curved portion of the optical waveguide on the LiNbO ₃ substrate to give the substrate a refractive index gradient. In the portion, the difference in the refractive index from the optical waveguide becomes large, so that light, especially extraordinary light, is prevented from escaping from the waveguide. That is, if MgO is diffused along the outer periphery of the curved portion of the waveguide, the refractive index is reduced only by the extraordinary optical mode due to the diffusion of MgO. The waveguide passes through the waveguide without loss due to the substantial increase in the refractive index difference, whereas the refractive index difference between the waveguide and the outer peripheral portion does not substantially change for the ordinary mode that is not affected by MgO. Escape outside. Thus, it functions as a filter for ordinary light. [Embodiment] FIGS. 1 and 2 show an embodiment of the present invention, in which LiNb is used.
A first titanium (Ti) metal pattern 4 is formed on the O ₃ substrate 3 and thermally diffused to form a first diffusion region for forming an optical waveguide 5. Next, a second magnesium oxide (MgO) pattern 7 is formed on the outer periphery of the curved portion of the optical waveguide 5 generated as described above, and thermal diffusion is performed to form a second diffusion region 8. FIG. 3 is a sectional view (A) specifically illustrating the formation of the first diffusion region and a refractive index profile (B) generated by thermal diffusion. Are diffused as shown by a broken line to form a first diffusion region 5 (optical waveguide), and a refractive index profile as shown in FIG. In the present invention, as shown in FIG. 4, a second MgO pattern 7 is provided at an outer peripheral position of the waveguide curved portion, and the second MgO pattern 7 is thermally diffused to form a second diffusion region 8. As a result, a refractive index gradient in which the refractive index on the outer peripheral side is reduced is formed on the substrate of the waveguide curved portion, and the difference in the optical path length between the inner peripheral portion and the outer peripheral portion of the waveguide is compensated, so that light escapes at the curved portion. Can be prevented. FIG. 5 is an illustrative view showing a concentration gradient of a refractive index of the optical waveguide according to the present invention. The second diffusion region 8 is an arc-shaped pattern having substantially the same curvature as the curvature of the curved portion of the optical waveguide 5.
And extend substantially in parallel. Patterns 4 and 7 of first and second diffusion regions
Can be made using, for example, an electron beam evaporation method and a lift-off method. Since Mg has a larger diffusion constant than Ti
MgO pattern is lower temperature (800-900 ° C) than Ti (about 1000 ° C)
Spread with. Therefore, if the second diffusion region is formed by diffusion after forming the optical waveguide by diffusion, the concentration distribution of the optical waveguide is hardly affected, which is advantageous. However, in practicing the present invention, even if both the first and second diffusion regions are thermally diffused simultaneously,
Alternatively, the optical waveguide 5 may be formed by thermal diffusion after forming the second region 8 by thermal diffusion in advance. Also, the refractive index is reduced only by the extraordinary light mode due to MgO diffusion and does not affect the ordinary light mode at all, so that it has a function as a filter of the ordinary light mode which leads to deterioration of switch characteristics. The distance between the first Ti metal pattern 5 and the second MgO metal pattern 8 is three types: 2 μm, 0 (when both are in contact), and −2 μm (when the patterns overlap by 2 μm). As a result of experiments, it was confirmed that the optical loss was the lowest when both patterns were formed in contact (the optical loss was 1.5 to 2 d when the radius of curvature of the optical waveguide was 5 mm).
B). The heat diffusion conditions (temperature, time, etc.) has an optical path length L ₂ of the light passing through the optical path length L ₁ and the outer periphery side of the light passing through the inner peripheral side of the optical waveguide 5 becomes the same (L ₁ = L ₂₎ Is selected as follows. That is, the outer peripheral length 1 ₁ of the optical waveguide 5, the outer peripheral side refractive index n _1, an inner peripheral length 1 _2, the optical path length L when the inner peripheral side refraction index and n ₂ because is given by L = 1 × n, N ₁ and n ₂ may be selected so as to satisfy 1 ₁ × n ₁ = 1 ₂ × n ₂ . [Effects of the Invention] As described above, by implementing the present invention, it is possible to make the radius of curvature 30 to 40 mm, which is conventionally required, much smaller (5 mm or less), and it is possible to improve the degree of integration of optical circuit elements. Becomes

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view for explaining an embodiment of the present invention, FIG. 2 is a sectional view taken along the line II-II of FIG. Sectional view for explaining formation of first diffusion region (A)
FIG. 4 is a sectional view (A) illustrating the formation of a second diffusion region.
FIG. 5 is a graph showing a concentration gradient of the refractive index. In the figure, 4 is a first Ti metal pattern, 5 is an optical waveguide, 7 is a second MgO pattern, and 8 is a second diffusion region.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Naoyuki Megada               Fujitsu, 1015 Ueodanaka, Nakahara-ku, Kawasaki-shi               Inside the corporation                (56) References JP-A-63-147111 (JP, A)

Claims (1)

(57) [Claims] A curved optical waveguide for transmitting extraordinary light is formed on the LiNbO ₃ electro-optic crystal substrate, and the optical waveguide has a higher refractive index than the substrate, and is oxidized along the outer periphery of the curved optical waveguide substantially parallel thereto. An optical waveguide structure wherein a magnesium diffusion region is formed. 2. When forming a curved extraordinary light propagation optical waveguide having a higher refractive index than the substrate on a LiNbO ₃ electro-optic crystal substrate, magnesium oxide is formed along the outer periphery of the extraordinary light propagation curved optical waveguide substantially parallel thereto. A method for forming an optical waveguide, comprising: