JP3402401B2 - Coupling method between guided light and external light - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of outputting light guided in an optical waveguide formed on a substrate to the outside of the optical waveguide by a diffraction grating, or diffracting external light. The present invention relates to a method of inputting light into an optical waveguide by using a grating, and more particularly, to prevent the input efficiency from being reduced due to a change in substrate temperature or to prevent the light emission angle from the end face of the substrate from fluctuating. The present invention relates to a method for coupling guided light and external light. [0002] When light is guided in an optical waveguide,
Conventionally, a diffraction grating (grating coupler) is provided on the surface of an optical waveguide to input external light into the optical waveguide or output guided light to the outside of the optical waveguide. Is considered to be combined. Such a diffraction grating is advantageous in reducing the size and weight of the optical waveguide element as compared with other light input / output means such as a prism coupler. However, on the other hand, the diffraction efficiency of this diffraction grating is reduced when used for light input, and the diffraction grating is reduced when used for light output, depending on fluctuations in ambient temperature. Has a problem that the light emission angle fluctuates sensitively and, consequently, the light emission angle from the optical waveguide element fluctuates. As a method of coupling guided light and external light to solve such a problem, a method disclosed in Japanese Patent Application Laid-Open No. Hei.
A method disclosed in Japanese Patent Publication No. 7-107 is known. In this method, external light is passed through the end face of the substrate of the optical waveguide, where it is refracted in the same direction as the direction of diffraction by the diffraction grating, and the angle formed by the end face of the substrate with respect to the optical waveguide is set to a specific value. Therefore, in the case of external light input, the incident angle of external light to the diffraction grating is adjusted to the phase-matching incident angle (this Is changed according to the substrate temperature). In the case of a guided light output, external light emitted from the diffraction grating is passed through the end face of the substrate set at the above-mentioned angle, thereby compensating for a change in the emission angle from the diffraction grating due to a change in the substrate temperature. The external light emission angle from the end face becomes constant. Although this conventional method is certainly effective, it still cannot sufficiently prevent a decrease in input efficiency or a change in light emission angle due to a change in substrate temperature, and there is room for improvement in this respect. It had been. The present invention has been made in view of the above circumstances, and it is possible to reduce the light input efficiency or to minimize the fluctuation of the light emission angle from the substrate end face even if the substrate temperature changes. It is an object of the present invention to provide a method for coupling wave light and external light. According to the present invention, there is provided a method of coupling guided light and external light according to the present invention, as described above. In the method of coupling the guided light and the external light by the diffraction grating provided in the optical waveguide, the external light is passed through the end face of the substrate of the optical waveguide, where it is refracted in the same direction as the direction of diffraction by the diffraction grating. The angle α formed by the end face of the substrate with respect to the optical waveguide is given by: [0010] However, phi is in the optical wavelength n is any temperature t ₀ in a vacuum is the incident angle or exit angle Λ external light grating period λ at any temperature t ₀ for the diffraction grating at any temperature t ₀ The substrate refractive index a is set to a value that substantially satisfies the relationship of the substrate refractive index temperature coefficient b and the linear expansion coefficient of the optical waveguide. [0011] The above "refraction in the same direction as the direction of diffraction" means that the direction of diffraction in the diffraction grating is, for example, on the right side with respect to the traveling direction of light when viewed from a certain direction. For example, it means that the light is refracted so that the direction of refraction on the end face of the substrate is also on the right side with respect to the traveling direction of light when viewed from the same direction. According to the study of the present inventors, the conventional method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2-213807 discloses an optical waveguide in consideration of the phase matching incident angle with respect to a diffraction grating. It has been found that since the change in the period of the diffraction grating due to the thermal expansion is not taken into account, it is not possible to sufficiently prevent the reduction in input efficiency or the change in the light emission angle due to the change in the substrate temperature. The method of coupling guided light and external light according to the present invention takes into account the change in the period of the diffraction grating due to the thermal expansion of the optical waveguide, and the reduction in the input efficiency or the change in the light emission angle due to the change in the substrate temperature. Is to prevent,
Hereinafter, the operation and effects will be described in detail with reference to FIG. The following description will be made on the case where external light 17 is input to the optical waveguide 12 via the diffraction grating 13 as shown in the figure. First, the temperature dependence of the phase matching incident angle φ in the optical input diffraction grating 13 will be considered. When the external light 17 is incident from the substrate 11 side as in this method, the phase matching condition in the diffraction grating 13 on the surface of the optical waveguide is usually used.
For the primary light, nk sin φ = Nk−K (2) where n is the substrate refractive index N is the effective refractive index k of the optical waveguide k is the wave vector magnitude of light in vacuum K is the wave number of the diffraction grating The size of the vector. Here, the light wavelength in vacuum is λ, and the diffraction grating period is Λ
Then, k = 2π / λ and K = 2π / Λ. The effective refractive index N of the optical waveguide is shown as N = n (1 + δ), and the effective refractive index N of the optical waveguide is regarded as having the same temperature coefficient as the refractive index n of the substrate. ## EQU3 ## ## EQU1 ## In this equation, parameters having temperature dependency are the substrate refractive index n and the diffraction grating period Λ. That is, the temperature coefficient of the substrate refractive index n is a,
Assuming that the linear expansion coefficient of the optical waveguide 12 is b, the phase matching incident angle φ is a function of temperature as follows: ## EQU2 ## Here, t is a temperature change amount, Λ ₀ is a diffraction grating period at t = 0, and n ₀ is a substrate refractive index at t = 0. Then, when φ is differentiated with respect to t, t =
The temperature coefficient dφ / dt of the phase matching incident angle φ at 0 is: Is obtained. Next, consider the temperature dependence of the incident angle φ due to the refraction of the external light at the end face of the substrate. As shown in FIG. 1, the incident angle and the outgoing angle of the external light 17 on the end face 11a of the substrate are set to θ ₁ and θ ₂ respectively, and the refraction of air as the surrounding medium is set to 1
Then, And θ ₂ = α−φ (6). Therefore, the incident angle φ of the external light 17 to the diffraction grating 13 is given by: ## EQU2 ## In this case, since the only parameter having temperature dependency is the substrate refractive index n, the incident angle φ is given by the following equation as a function of temperature. ## EQU1 ## Then, when φ is differentiated with respect to t, the temperature coefficient dφ / dt of the incident angle φ at t = 0 is given by: Is obtained. Next, the temperature dependence of the incident angle φ after the refraction of the substrate end surface shown in the equation (7) and the temperature dependence of the phase matching incident angle φ shown in the above equation (4) match. Then, the angle α of the substrate end face is obtained. Since these two temperature dependencies match, the following equation is obtained. From the above equations (5) and (6), sin θ ₁ = n ₀ sin (α−φ) (9) From these equations (8) and (9), the following equation is obtained. ## EQU4 ## In the above equation showing the case where the temperature change amount t = 0, if Λ ₀ and n ₀ are generally values Λ and n at an arbitrary temperature t ₀ , respectively,
Equation (1) is derived. As described above, if the angle α formed by the end face of the substrate with respect to the optical waveguide is set to the value defined by the equation (1), the incident angle φ becomes the refractive index of the substrate due to a change in the substrate temperature. Due to the change (that is, due to the change of the emission angle θ _{2 at} the end face of the substrate), the phase matching incident angle is automatically set to change according to the change of the substrate refractive index and the change of the diffraction grating period caused by the thermal expansion of the optical waveguide. As a result, the light input efficiency in the diffraction grating is kept high. As described above, the case of light input has been described. By passing the external light output from the diffraction grating through the end face of the substrate set at the angle α as described above, the fluctuation of the emission angle from the diffraction grating is compensated. It is apparent from the reciprocity theorem in the case of light input and light output that the external light emission angle from the end face is constant. The above description is based on the assumption that the ratio (1 + δ) of the effective refractive index N of the optical waveguide to the refractive index n of the substrate is constant even if the refractive index n of the substrate changes. In fact, that is the case. However, the present invention is also effective for an optical waveguide device in which the ratio (1 + δ) slightly varies according to the change in the substrate refractive index n. In other words, even in such an optical waveguide element, if the angle α formed by the substrate end face and the optical waveguide is set to the value defined by the above equation (1), when the substrate temperature changes, if the optical input is used, Although the phase matching in the diffraction grating may be deteriorated to some extent, the light input efficiency can be maintained higher as compared with the case where no measure is taken. In the case of the light output, similarly, the fluctuation of the light emission angle from the end face of the substrate can be further reduced. Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. 1 and 2 show an example of an optical waveguide device in which external light is input into an optical waveguide by the method of the present invention. The optical waveguide element 10 includes a slab optical waveguide 12 formed on a transparent substrate 11 and a light input diffraction grating (hereinafter referred to as LG) provided on the surface of the optical waveguide 12 so as to be separated from each other.
C) 13 and a light output LGC 14. In this embodiment, as an example, an optical waveguide 12 is formed by using a LiNbO ₃ wafer for the substrate 11 and providing a proton exchange layer on the surface of the wafer. For example, a light source such as an argon laser 15 is arranged to emit a light beam (laser beam) 17 from the substrate 11 toward the LGC 13. The light beam 17 passes through the substrate end surface 11a cut obliquely, and is
It is incident on the portion of C13. The light beam 17 is diffracted by the LGC 13 and enters the optical waveguide 12, and travels in the optical waveguide 12 in the waveguide mode in the direction of arrow A. This guided light 17 'is LGC
The light is diffracted at 14 and exits from the optical waveguide 12 to the substrate 11 side. Light beam 1 emitted from optical waveguide 12 to become external light 1
7 "is emitted out of the device from the substrate end face 11b. In this embodiment, the optical waveguide device is used.
It is assumed that a reference temperature t ₀ = 15 ° C. in designing each of the ten elements. At this temperature, the substrate refractive index n = 2.24 and the effective refractive index of the optical waveguide N = 2.245.
Length L of GC13 = 2000 μm, grating period of LGC13Ｌ = 2.
1 μm. And the temperature coefficient a of the substrate refractive index n = 5.3
× 10 ⁻⁵ , the coefficient of linear expansion b of the optical waveguide 12 is 1.5 × 10 ⁻⁵ , and the wavelength λ of the light beam 17 in vacuum is 514.5 nm. From the above values, the phase-matching incident angle φ at the reference temperature t ₀ = 15 ° C. and the substrate end face 11a indicate that the optical waveguide
When the angle α to be set to 12 is obtained based on the above-described equations (3) and (1), φ = 63.23 ° and α = 80.54 °. In this way, in order to make the incident angle φ = 63.23 ° when α = 80.54 °, the light beam 17 is incident so that the direction of refraction on the substrate end face 11a is the same as the direction of diffraction on the LGC13. And the light beam 17 against the end face 11a.
Incident angle θ ₁ is 41.80 based on the equations (5) and (6).
Set to °. When the respective conditions are set as described above, the substrate temperature changes and the substrate refractive index n and the grating period of the LGC 13 Λ
Is maintained, the light input efficiency in the LGC 13 is maintained substantially equal to that in the case where t ₀ = 15 ° C. for the reason described above. FIGS. 3 and 4 show how the light input efficiency changes according to the amount of change in the substrate temperature. These two figures show the same measurement results, and FIG. 4 is an enlarged view of a part of FIG. The light input efficiency on the vertical axis in these figures is
The efficiency at the reference temperature t ₀ = 15 ° C. is defined as I ₀ , and the efficiency is shown as a relative value I / I ₀ , while the temperature change Δt on the horizontal axis
Is a value obtained by subtracting the reference temperature t ₀ = 15 ° C. from the substrate temperature. Contrary to this case, even when the value obtained by subtracting the reference temperature t ₀ = 15 ° C. from the substrate temperature becomes a negative value, if the absolute value of the difference is taken as the temperature change Δt, basically, FIG. And the same characteristics as those in FIG. In the figure, the curve a is that of the above embodiment (α = 8
0.54 °), and the curve b has an angle α with respect to that of the above embodiment.
Is reduced by 1 ° (α = 79.54 °), curve c is obtained by setting the angle α by the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2-213807 (α = 76.88 °), and curve d is obtained.
But without any correction for the substrate temperature change
11a in which the light beam 17 is vertically incident (α = 63.23
°) are shown. 3 and 4, it is clear that according to the present invention, even when the substrate temperature changes, the light input efficiency decreases only slightly. As clearly shown in FIG. 4, even if the angle α slightly deviates from the value defined by the above equation (1), the effect of clearly preventing the decrease of the light input efficiency can be obtained. . Therefore, the angle α is 1 from the value defined by the equation (1).
A configuration deviating by about 2 ° is also included in the present invention. The embodiment in which the present invention is applied to light input has been described above. However, as described above, the present invention is also applicable to the case of light output. The effect of preventing is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view showing a part of an apparatus for carrying out a method of the present invention. FIG. 2 is a perspective view showing an entire configuration of the apparatus shown in FIG. 1. FIG. Graph showing the relationship with light input efficiency [FIG. 4] Graph showing a part of the graph of FIG. 3 enlarged [Explanation of reference numerals] 10 Optical waveguide element 11 Substrate 12 Optical waveguide 13 LGC for optical input 14 LGC for optical output 17 Light beam 17 'Guided light 17 "Output light

Claims (1)

(57) [Claim 1] A method of coupling guided light guided through an optical waveguide formed on a substrate and external light by a diffraction grating provided on the surface of the optical waveguide. The external light passes through the end face of the substrate, where it is refracted in the same direction as the direction of diffraction by the diffraction grating, and the angle α formed by the end face of the substrate with respect to the optical waveguide is given by: However, phi is a substrate refractive index at a given temperature t ₀ is the incident angle or exit angle Λ external light optical wavelength n in vacuum grating period λ at any temperature t ₀ for the diffraction grating at any temperature t ₀ a is a temperature coefficient of the refractive index of the substrate; b is set to a value that substantially satisfies the relationship of the linear expansion coefficient of the optical waveguide;