JPS6116044B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is directed to a phase matching film of an optical waveguide device having an optical waveguide plate, in particular a transparent plate having a refractive index n ⁰ , in which a light beam is guided through repeated reflections, and the light beams generated by the reflections are guided. The present invention relates to a phase matching film of an optical waveguide device having an optical waveguide plate in which the phase shift between a P-polarized light component and an S-polarized light component is made substantially equal. Conventionally, electro-optic crystal optical waveguides have been used to construct optical modulators including, for example, optical switches and Faraday rotators. Fig. 1 shows its conceptual configuration, where 1 is a transparent body, 2 and 2' are electrodes provided on the top and bottom surfaces of the transparent body, 3 is a power source,
4 and 5 each represent a prism that can be provided as required. In such an optical modulator, the electrodes 2, 2'
Polarization is controlled by applying a voltage between the two, but in order to obtain the desired polarization by controlling the phase by a predetermined amount, the illustrated thickness d should be made small and the longitudinal distance l should be made large. , and it is desirable to select a large voltage for the power supply 3. However, when trying to obtain the desired polarized light while making the power supply voltage relatively low, for example, the thickness d is 200 μm and the distance l is 2 cm.
Therefore, the value l/d becomes very large. On the other hand, the light beam incident on the transparent body 1 is made to be incident at right angles to the cross section of the transparent body 1, but the light beam tends to expand in inverse proportion to the diameter of the light beam. If the value l/d is chosen to be large, the following problem arises. That is, if the value l/d is chosen to be large, the number of reflections that occur within the transparent body 1 will inevitably increase, since the expansion of the light beam is unavoidable. At this time, generally upon reflection, a phase shift occurs between the P-polarized light component and the S-polarized light component contained in the light beam. Figure 2 shows the P polarization component and the S polarization component with the incident angle shown on the horizontal axis.
It represents the phase shift between the incident light and the reflected light, respectively, with respect to the polarized light component. The reflection that occurs within the transparent body 1 has an incident angle of around 90°, but as can be seen from Fig. 2, even if the incident angle is about 87°, one reflection can reduce the amount of incident light. P in
The phase difference Δ between the polarized light component and the S-polarized light component changes in the reflected light. The above change becomes larger as the number of reflections increases. For this reason, there is a limit in selecting a large value l/d, in other words, it becomes necessary to increase the power supply voltage. The present inventors aimed to solve the above points by introducing a light beam into a transparent body given by the refractive index n ⁰ in a direction perpendicular to the cross section of the transparent body,
In the optical waveguide device that guides the light beam by repeating reflection at the boundary of the transparent body, the transparent body is configured as a transparent plate body in which the distance l in the optical waveguide direction is selected to be large relative to the thickness d; , a thin film having a refractive index n ¹ larger than the refractive index n ⁰ of the transparent plate, for example, on both upper and lower surfaces in the thickness direction of the transparent plate, and a refractive index n ¹ smaller than the first thin film on the thin film, respectively. refractive index n ²
An optical thin film means having a structure in which a substance having a An application has already been filed for an optical waveguide device in which the phase shift caused by the reflection is made substantially equal. The contents of this existing application will be explained below with reference to the drawings. As shown in Figure 3, a material with a refractive index of n ¹ and a material with a refractive index of n ²
The respective phase shifts Δp and Δs between the P-polarized light component and the S-polarized light component incident on the boundary 6 with the substance (n ¹ >n ² ) at an incident angle are expressed as follows. In addition, phase shifts occur due to multiple reflections within the thin film 7 having a thickness of t as shown in FIG. become that way.

[Table] (However, ξp is the phase shift angle of the P polarized light component, kp
is the coefficient in the linear approximation rp=kpθ-1 between the reflection coefficient rp of the P-polarized light component and θ=χ/2-φ≪1, δ
p is the phase shift of the P polarized light component caused by one reflection within the thin film 7, and ξs, ks, and δs are each ξ
Similar values are for the S polarization components corresponding to p, kp, and δp. ) Here, in order to make the phase shift ξp of the P-polarized light component and the phase shift ξs of the S-polarized light component equal to each other, the following equation must be satisfied. kp/ks=sin δs(1-cos δp)/sin
δp(1-cosδs)---(3) Now, the left side of equation (3) is determined by n ⁰ /n ¹ , and the right side is
It is determined by n ⁰ /n ¹ , n ² /n ¹ , n ⁰ , t, so for example
By selecting n ² and t, equation (3) can be satisfied. The following may be considered as an example.

[Table] FIG. 5 shows an optical waveguide device incorporating the principle explained in connection with FIG. 4 above. Reference numerals 1, 7, and 8 in the figure correspond to those in FIG. 4, respectively, 9 represents an input light beam, and 10 represents an output light beam. This optical waveguide device has a transparent body (transparent plate body) having a thickness d and a length l as shown in FIG.
A first thin film 7 having a thickness t is deposited on both the upper and lower surfaces in the thickness direction, and a second thin film 7 having a desired thickness is further applied thereon.
A thin film 8 of is deposited. When an input light beam 9 is incident on the optical waveguide device shown in FIG. 5, it is reflected repeatedly on both the upper and lower surfaces of the optical waveguide device as shown in FIG. 5, and is output as an output light beam 10. The Rukoto. Regarding this reflection, as explained with reference to FIG. 4, a condition is given such that the phase shift between the P-polarized light component and the S-polarized light component does not change when the incident angle φ is close to π/2. . In FIGS. 6A and 6B, by enlarging the inside of the optical waveguide device shown in FIG. 5, the applied power supply voltage can be set to a sufficiently low value, for example, about 20V. However, the phase matching film shown in FIG. 5 has the disadvantage that the phase shift changes greatly with changes in wavelength, and the phase shift changes significantly even when the reflection angle deviates from 90 degrees. Therefore, an object of the present invention is to provide a phase matching film in which such a change in phase shift is small. In the present invention, this purpose is to form a P-polarized light component of the light beam on the boundary surface of the transparent body perpendicular to the thickness direction to repeatedly reflect the light beam introduced in the direction substantially perpendicular to the thickness direction of the transparent body. The phase matching film that compensates for the phase shift between the and the S-polarized light component is composed of three layers of optical thin films, and the refractive index of the first and third layers counting from the transparent body side is smaller than the refractive index of the transparent body, This is achieved by a phase matching film characterized in that the refractive index of the second layer is greater than the refractive index of the transparent body. FIG. 7 is a diagram showing an embodiment of the present invention, FIG. 8 is a diagram showing the wavelength characteristics of the phase matching film in FIG. 7 and the same characteristics of the phase matching film in FIG. 5, and FIG. 9 is a diagram showing the same characteristics as in FIG. 7. FIG. 6 is a diagram showing the incident angle characteristics of the phase matching film of FIG. 5 and the same characteristics of the phase matching film of FIG. In the present invention, as shown in FIG. 7, the light beam directed from the transparent body 2 toward the first layer thin film 16 satisfies the condition of total reflection. That is, the refractive index n ¹ of the first layer thin film 16 is smaller than the refractive index n ⁰ of the transparent body 2. For example, as a transparent material, electro-optic crystal
When using Bi ₁₂ SiO ₂₀ , n ⁰ = 2.405, so
For example, silicon dioxide having a refractive index of 1.45 can be used as the first layer thin film 16. Incidentally, even if the relationship between the incident angle and the refractive index satisfies the total reflection condition as described above, the first layer thin film 16 does not have a large enough thickness to completely attenuate the surface waves. There is therefore a light beam passing through the first layer thin film 16, as is known as tunneling effect. In this way, the light beam that has reached the second layer thin film 17 then passes through the second layer thin film 17 and the third layer thin film 18.
The relationship between the magnitude of the refractive index (n ³ < n ⁰ <
n ² ) and is totally reflected. At this time, the thickness of the third layer thin film 18 is made sufficiently large. Specifically, the second layer may be a silicon layer obtained by decomposing silane gas and have a refractive index of 3.55, and the third layer may be a thin film of silicon dioxide with a refractive index of 1.45 and a thickness of 10,000 Å or more. Considering the behavior of the light beam at each boundary surface as a whole, it can be seen that a part of the light beam from the transparent body 2 passes through the first layer thin film 1 due to the tunnel effect.
6 and undergoes multiple reflections in a meandering manner within the second layer thin film 17, and these multiple reflected light beams return to the transparent body 2 again from the second layer 17 due to the tunnel effect. Through this series of processes, the light beam and the light beam simply reflected at the boundary between the transparent body 2 and the first layer thin film 16 are combined. Therefore, it is assumed that the phase shift between the P-polarized light component and the S-polarized light component in each of the two light beams is set to an appropriate value, that is, when a simple harmonic combination is performed, they cancel each other out and become zero. In this case, for example, by changing the film thickness of the first layer and the second layer, the phase shift between the P polarized light component and the S polarized light component in the light beam re-entered into the transparent body 2 through the first layer can be changed. It is sufficient to set the amplitude to an appropriate value that can change the amplitude and thereby cancel out the phase shift between the polarization components in the combined beam of both the beams. As an example, an electro-optic crystal Bi ₁₂ SiO ₂₀ , n ⁰ = 2.405 is used using a light beam with a center wavelength of 1.3 μm, the first layer is SiO ₂ , n ¹ = 1.45, and the thickness is 950 Å; the second layer is
Si (including H), n ² = 3.55, thickness 1820 Å; 3rd layer
When SiO ₂ , n ³ =1.45 and the thickness is 10000 Å (any thickness greater than 3000 Å), the characteristics shown in FIGS. 8 and 9 are obtained. That is, FIG. 8 shows the change in phase shift with respect to wavelength, and the solid line corresponds to the configuration shown in FIG. 7, and the broken line corresponds to the configuration shown in FIG. 5. As is clear from the figure, the one based on the present invention (solid line) can suppress the phase shift to a lower level over a wider wavelength band than the one before the improvement shown in FIG. 5 (broken line). Moreover, FIG. 9 shows the change in phase shift with respect to the incident angle (the angle when the light beam is made to enter from the transparent body 2 side with respect to the interface between the transparent body 2 and the first layer thin film 16). The solid line corresponds to the configuration shown in FIG. 7, and the broken line corresponds to the configuration shown in FIG. 5. From this figure, the one based on the present invention (solid line) is
Compared to the model before improvement (broken line), the phase shift can be suppressed to a low level over a wide range of incident angles. Note that the present invention is not limited to the transparent material and thin film material shown in the above embodiments, and any material can be used. For example, ferromagnetic materials such as YIG, which is a transparent material for Faraday rotation elements, Body crystals can also be used. In this case, since the size of the crystal can be reduced, the amount of magnetic field (amount of magnetic flux) applied to the crystal can also be reduced, and therefore the magnet for applying the external magnetic field can be downsized. As described above, according to the present invention, it is possible to widen the wavelength band and widen the range of incident angles, compared to the configuration shown in FIG. 5, which is an embodiment of the optical waveguide device filed earlier than the present application. The practical effect is extremely large.

[Brief explanation of the drawing]

Figure 1 is an example of an optical modulator, Figure 2 is an explanatory diagram illustrating the phase shift between the P-polarized light component and the S-polarized light component that occurs during reflection, and Figure 3 is a diagram illustrating the phase shift between the P-polarized light component and the S-polarized light component that occurs during reflection. 4 is an explanatory diagram illustrating the configuration of a reflecting section that prevents the phase shift. FIG. 5 is an optical waveguide device. A and B are explanatory diagrams respectively explaining the route of light propagating in the optical waveguide device shown in FIG. 5, FIG. 7 is a diagram showing the structure of a phase matching film improved based on the present invention, and FIG. is the fifth
FIG. 9 is a diagram showing the wavelength-phase shift characteristics of the components shown in FIGS. 5 and 7, and FIG. 9 is a diagram showing the incident angle-phase shift characteristics of the components shown in FIGS. 5 and 7. In the figure, 1 is a transparent body, 7 is a thin film, 8 is a substance, and 16
17 represents a first layer thin film, 17 represents a second layer thin film, and 18 represents a third layer thin film.

Claims (1)

[Claims] 1 Formed on the boundary surface of the transparent body perpendicular to the thickness direction that repeatedly reflects a light beam introduced in a direction substantially orthogonal to the thickness direction of the transparent body, and changes the phase of the P-polarized light component and the S-polarized light component of the light beam. The phase matching film for compensating for misalignment is composed of three layers of optical thin films, and the refractive index of the first and third layers counting from the transparent body side is smaller than the refractive index of the transparent body, and the refractive index of the second layer is lower than the refractive index of the transparent body. A phase matching film characterized in that the refractive index is larger than that of a transparent body.