JPH0553023A - Optical element having circular waveguide - Google Patents

Abstract

PURPOSE:To fetch more light quantity from light propagated through the inside of a waveguide, as a luminous flux of a circularly polarized light having a center symmetrical intensity distribution. CONSTITUTION:An optical coupler is provided with a circular waveguide 12, and a straight waveguide 14 for leading light therein. When light is made incident on the straight waveguide 14 in the direction as indicated with an arrow, as for a part of light propagated through the inside of the straight waveguide 14, in the vicinity of the part in which the straight waveguide 14 and the circular waveguide 12 are adjacent to each other, light spread out of the straight waveguide 14 goes into the inside of the circular waveguide 12. When a radius of the circular waveguide 12 is denoted as (r), light of whose wavelength is 2pir/n turns continuously in the direction as indicated with an arrow in the circular waveguide 12. In this case, (n) is a certain natural number. On the surface of the circular waveguide 12, a grating 12a is formed at a pitch of 2pir/(n+1).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element having a waveguide. More specifically, it relates to an optical coupling element using a waveguide.

[0002]

2. Description of the Related Art As an optical coupling element using a waveguide, one having a grating on the surface of a linear waveguide is well known.

[0003]

In such an optical coupler, the guided light entering from one end of the waveguide is emitted from the other end while being attenuated exponentially. At this time, if the coupling efficiency is increased to extract more light, the difference in the light intensity between the input end and the emission end becomes large, and the light emitted from the grating region is strongly emitted to the emission end near the input end. The intensity distribution becomes weaker as it gets closer. A light flux having such an intensity distribution is not preferable. On the contrary, if the intensity of the light emitted from the grating region is set to be substantially constant, the coupling efficiency will be significantly reduced.

The light extracted by such an optical coupler is limited to linearly polarized light. This is a constraint on the optical design. Circularly polarized light is often used in an optical system, and therefore it is desired to provide an optical element capable of extracting light propagating in a waveguide as circularly polarized light.

An object of the present invention is to provide an optical coupler capable of extracting a larger amount of light as a light flux having a centrally symmetric light intensity. Another object of the present invention is to provide an optical element capable of extracting light propagating in the waveguide as circularly polarized light.

[0006]

SUMMARY OF THE INVENTION The present invention is an optical element having a circular waveguide, on the surface of which the position of the light and the grating when the light used is half-circulated around the circular waveguide. A grating having a period in which the phase difference is substantially deviated by π is provided.

[0007]

The light (guided light) that has entered the circular waveguide continues to circulate in it, during which the light is always emitted from the grating. Since the phase difference between the guided light and the waveguide is deviated by π at two points on the circular waveguide that is symmetric with respect to the center, when the light with the maximum amplitude is radiated at one point on the waveguide, Light with the maximum amplitude is also emitted from the points located at the central symmetry positions. These two points move with time according to the mutual relationship between the wavelength of light and the pitch of the grating. In addition, the light emitted from the grating generally has a plane of polarization in a fixed direction with respect to the tooth, and each tooth of the grating on the circular waveguide is directed in a different direction, so the light emitted from each tooth Have different polarization directions. Therefore, the polarization plane of the light emitted from the circular waveguide rotates with the passage of time. Therefore, when the coupling efficiency of the grating is constant, the emitted light is circularly polarized light.

[0008]

FIG. 1 shows an optical coupler according to an embodiment of the present invention. In the figure, (A) is a top view and (B) is a side view. The figure schematically shows the structure of the optical coupler, and the relative sizes of the circular waveguide and the period of the grating are shown differently from the actual size for convenience of explanation.

The optical coupler of this embodiment includes a circular waveguide 12 and
The linear waveguide 14 for introducing light into this is provided. The circular waveguide 12 and the linear waveguide 14 have a width of about the wavelength of the light to be used, and these waveguides have a support substrate 16 such that the gap between the closest portions is equal to or less than the wavelength.
Is provided above. When light is incident on the linear waveguide 14 in the direction of the arrow, a part of the light propagating in the linear waveguide 14 is a portion where the linear waveguide 14 and the circular waveguide 12 are close to each other, that is, the reference numeral in the drawing. Circular waveguide 1 near A
Enter the inside of 2. The circular waveguide 12 has a radius / width / thickness / refractive index distribution so that light having a wavelength of 2πr / n (r is a radius of the circular waveguide, n is a natural number) continues to rotate in the direction of the arrow inside the circular waveguide 12. It is set.

On the surface of the circular waveguide 12, 2πr / (n +
The grating 12a is formed at the pitch of 1). Points A are selected on the circular waveguide, and points obtained by rotating the points A by π / 2 are referred to as B, C, and D, respectively. Then, the phase differences between the guided light and the grating at points A, B, C, and D are Δφ (A) and Δφ, respectively.
(B), Δφ (C), and Δφ (D). When Δφ (A) = 0 at time t = 0, Δφ (A), Δφ (B),
FIG. 2 shows changes in Δφ (C) and Δφ (D) with time.

Here, on the axis perpendicular to the support substrate 16 passing through the center O of the circular waveguide 12, points A, B, C,
Consider a case where radiation light from the vicinity of each point D is observed. When the emitted light from the vicinity of the point A has the maximum amplitude at t = 0, the emitted light from the vicinity of the point C also has the maximum amplitude. This is because the traveling directions of the guided light at the points A and C are opposite to each other, and Δφ (A) and Δφ (C) always differ by π. At this time, the radiated light from the vicinity of the point B and the vicinity of the point D has the minimum amplitude. This is because Δφ (B) and Δφ (D) are Δφ
This is because they are different from (A) by π / 2 and −π / 2, respectively.
Since the polarization direction of the light emitted from the grating is constant with respect to the extending direction of the grating, linearly polarized light having a specific polarization direction is emitted at the moment t = 0.

The point where the emitted light has the maximum amplitude moves with the passage of time. That is, the point at which the amplitude of the radiated light becomes maximum at t = 0 is point A, but such a point moves in the direction of the arrow with the passage of time, and moves sequentially to point B, point C, and point D. After that, it returns to point A again. Along with this, the plane of polarization of the emitted light also rotates. As a result, when the coupling efficiency of the grating is constant regardless of the angle direction with respect to the center O, the emitted light becomes circularly polarized light. Further, elliptically polarized light can be easily obtained by changing the coupling efficiency of the grating according to the angle direction with respect to the center O.

As described above, in the optical coupler according to the present invention, the light that has entered the circular waveguide continues to be radiated by the grating while rotating around the waveguide, so that the coupling efficiency is increased and a large amount of light can be extracted. Moreover, since the emitted light is emitted from the annular grating, it becomes a preferable one having a centrally symmetric intensity distribution. Further, desired circularly polarized light or elliptically polarized light can be obtained by adjusting the coupling efficiency of the grating according to the purpose of use.

The present invention can be modified in various ways without departing from the spirit of the invention. The means for introducing light into the circular waveguide, the shape of the grating, the difference in the period of the guided light and the grating, etc. are not limited to the above-described embodiments.

[0015]

According to the present invention, light continues to be emitted by the grating as it travels through the circular waveguide. Therefore, when used as an optical coupler, the coupling efficiency becomes high. Moreover, circularly polarized light or elliptically polarized light can be easily obtained by appropriately adjusting the coupling efficiency of the grating. Further, since the emitted light is emitted from the annular grating, it has a centrally symmetric intensity distribution, and the non-uniformity like the emitted light from the linear grating is eliminated.

[Brief description of drawings]

FIG. 1 shows an optical coupler according to an embodiment of the present invention,
(A) is a top view and (B) is a side view.

2 shows the phase difference between the guided light and the grating at each point A, B, C, D of the waveguide of FIG.

[Explanation of symbols]

 12 ... Circular waveguide, 12a ... Grating.

Claims (1)

[Claims] 1. A grating having a circular waveguide, the surface of which is provided with a grating whose period is such that the phase difference between the light and the grating is substantially π when the light used half-circulates the circular waveguide, An optical element having a circular waveguide.