JPH0820336B2 - Refractive index distribution measuring method and measuring apparatus therefor - Google Patents

Description

The present invention relates to a method for measuring a refractive index distribution and a measuring apparatus therefor, and more particularly to a nondestructive, simple and highly accurate measuring method.

[Conventional technology]

Conventionally, a method using an interference microscope is widely known as a method for measuring the refractive index distribution. However, in this method, it is necessary to thin the measurement medium and further optically polish it. Therefore, it takes a lot of time to prepare the sample.
Moreover, since it is a destructive inspection, there is a drawback that the sample cannot be regenerated. In addition, as a method for measuring the optical constants of thin films,
Ellipsometry and reflection spectroscopy are known. But,
None of the methods has a drawback that it cannot be applied as a method for measuring the refractive index distribution.

On the other hand, with the recent progress of research on semiconductor lasers, research on an optical integrated circuit in which a light emitting / receiving element such as the above laser and various waveguide type optical elements are integrated on one substrate has been actively conducted. An optical waveguide is one of the most basic components of this optical integrated circuit. As a method of measuring the effective refractive index of an optical waveguide, Applied Optics, 10, 11 (1971
2395 to 2413 (Applied Optics Vol10, No.
11 (1971) PP2395-2413) and the like, the prism coupler method is generally widely known.

[Problems to be solved by the invention]

In the above-mentioned conventional technique, a prism having a refractive index of np is brought into close contact with an optical layer on an optical waveguide. Then, a light beam is made incident on the bottom surface of the prism at a predetermined angle θ to achieve phase matching with the optical waveguide to excite the guided light, and the guided light is extracted to the outside by the second prism. In this case, the effective refractive index of the optical waveguide is calculated using the principle that the angle of the emitted light differs depending on the waveguide mode of the optical waveguide. However, this method also has a drawback that it cannot be applied when the refractive index of the optical waveguide continuously changes.

The object of the present invention is to eliminate the above-mentioned drawbacks of the prior art,
It is an object of the present invention to provide a non-destructive, simple and highly accurate refractive index distribution measuring method and a measuring apparatus therefor.

[Means for solving problems]

The above-mentioned object is a refractive index distribution measuring method of a medium in which the refractive index is changing in the depth direction, a prism is brought into close contact with the medium, the prism is irradiated with laser light, and the incident angle of the laser light is changed, The reflected light intensity from the bottom surface of the prism is measured, at least two incident angles of the laser light that minimize the reflected light intensity are measured, the effective refractive index of the medium is calculated from the incident angle of the laser light, and the effective refraction is calculated. The refractive index change of the medium is obtained from the calculated value of the index by the inverse WKB method.

Further, a prism that is in close contact with a medium whose refractive index changes in the depth direction, a light source for irradiating the prism with laser light, a light receiving element for measuring the intensity of reflected light from the bottom surface of the prism, Refraction provided with a prism, a medium and a holding / rotating table for the light receiving element, and a computer control module for calculating the refractive index distribution of an optical waveguide whose refractive index continuously changes in the depth direction by using the inverse WKB method or the like. This can be achieved by a rate distribution measuring device.

[Action]

When the refractive index in the depth direction of the medium changes continuously, the light beam propagating through the medium draws an arc locus, and a different locus depending on the propagation mode. Therefore, each propagation mode is individually excited by changing the incident angle of the light beam to the prism integrated with the light receiving element, and the incident angle θi at which the intensity of reflected light from the bottom of the prism is minimized is read to correspond to the propagation mode. The effective refractive index N is calculated by the equation (1).

Here, α is the prism apex angle, and np is the refractive index of the prism.

Next, the wave equation of the TE mode in the two-dimensional optical waveguide whose refractive index distribution is n (y) is Is represented.

Here, β: propagation constant in the z direction, μ0: magnetic permeability in vacuum,
ω: angular frequency of light, h0: wave number (wavelength / 2π).

Using the above-mentioned equivalent refractive index N, equation (3) becomes Becomes When n (y) is known, finding an equivalent refractive index that satisfies the expression (6) is a unique value problem. Where WKB
According to the approximation method of, the following private equation is obtained.

Where ytm is the transition point of the m-th mode, and n0 is the surface refractive index.

Next, inverse WKB method, the effective refractive index N _1, N ₂ ...... Nm of each propagation mode described above, each of the transition points yt _1, yt _2, ......
This is a method of obtaining ytm and obtaining point sequences (Nt ₁ , yt ₁ ) and (Ntm, ytm). The refractive index distribution can be obtained by connecting these point sequences.

〔Example〕

 Hereinafter, examples of the present invention will be described in detail.

FIG. 1 shows an embodiment of the present invention, in which LiNbO _{3 is applied to} the substrate 1.
Crystals were used. The optical waveguide 2 whose refractive index continuously changes in the depth direction of the substrate 1 was formed by the thermal diffusion method of metallic Ti. The preparation conditions were such that Ti was deposited on the substrate 1 by the 200 A sputtering method, and then thermal diffusion was performed at 1000 ° C. in an O ₂ atmosphere for 4 hours. Next, the prism 3 is brought into close contact with the optical waveguide 2 via an air layer. Here, the prism 3 is made of TiO ₂ (rutile), the apex angle is 45 degrees, and the prism refractive index np = 2.854. The light beam is incident on the prism 3 by using a He-Ne laser having a wavelength of 0.633 μm as the light source 4, and the incident angle is adjusted by a rotary table 5 driven by a pulse motor (not shown).
I went in. FIG. 2 shows a method of supporting the prism 3. The prism 3 is fixed to the holder 6, and the degree of contact with the substrate 1 is adjusted by the push screw 7.

FIG. 3 shows trajectories of light rays which are incident on the prism 3 at various angles and which propagate through the optical waveguide 2 and are reflected by the bottom surface of the prism. Here, as described above, the light beam propagating through the optical waveguide 2 has a discrete value with respect to the incident angle of the light beam. Therefore, the incident angle of the light beam and the light intensity reflected by the bottom surface of the prism 3 are continuously measured by the light receiving element 8 integrated with the prism 3 and the holder 9. FIG. 4 shows an example of measurement data. The horizontal axis represents the incident angle θ of the light beam, and the vertical axis represents the relative brightness of the light receiving element. In the figure, the incident angle at which the relative luminance shows the minimum value corresponds to the effective refractive index N of each propagation mode, and (1)
The value is calculated by the formula. Next, using the measurement data of the effective refractive index corresponding to the above-mentioned propagation mode, the point sequence of the effective refractive index and the transition point is calculated by the equation (7). FIG. 5 shows an example of the calculation result of the refractive index distribution. The theoretical curve (solid line) assuming that the refractive index distribution is Gaussian distribution corresponded well with the calculated values, and the effectiveness of this method could be confirmed. The reproducibility by repeated measurement was also good at 3α = 0.001.

The drive control of the rotary table 5 and the calculation of the effective refractive index N and the refractive index distribution can be performed by a computer control module (not shown) including a microcomputer.

〔The invention's effect〕

As described above, according to the present invention, there is an effect that the measurement of the refractive index distribution, which could not be conventionally measured, can be performed nondestructively, easily and highly accurately.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of a refractive index distribution measuring apparatus according to the present invention, FIG. 2 is a front view showing a prism holding portion, and FIG. 3 is a ray trace to a medium through a prism according to the present invention. 4 is a front view showing a measurement example of a characteristic showing a relationship between a prism incident angle and a relative luminance by a light receiving element according to the present invention, and FIG. Is. 1 ... Substrate, 2 ... Optical waveguide, 3 ... Prism, 4 ...
Light source, 5 ... Rotary table.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazutomi Kawamoto 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Inside the Institute of Industrial Science, Hitachi, Ltd. (56) Reference JP-A-56-43529 (JP, A) Kai 62-12840 (JP, A)

Claims (2)

[Claims] 1. A method of measuring a refractive index distribution of a medium in which the refractive index changes in the depth direction, in which a prism is brought into close contact with the medium, the prism is irradiated with laser light, and the incident angle of the laser light is changed. The intensity of reflected light from the bottom surface of the prism is measured, the incident angle of the laser light at which the intensity of reflected light is minimized is measured at least two points, and the effective refractive index of the medium is calculated from the incident angle of the laser light. A method for measuring a refractive index distribution, characterized in that a change in refractive index of a medium is obtained by an inverse WKB method from a calculated value of a refractive index. 2. A prism in close contact with a medium whose refractive index changes in the depth direction, a light source for irradiating the prism with laser light, and a light receiving for measuring the intensity of reflected light from the bottom surface of the prism. An element, a holding / rotating table for the prism, the medium, and the light receiving element, and a computer control module for calculating the refractive index distribution of the optical waveguide whose refractive index continuously changes in the depth direction using the inverse WKB method or the like. A refractive index distribution measuring device comprising: