JP2002350915A - Wavelength transformation element, optical waveguide device for wavelength transformation, harmonic component generating device and method for manufacturing wavelength transformation element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the wavelength transformation element of a quasi-phase matching system adaptive to a plurality of exciting light wavelengths by using the period polarization reverse structure of a specific period. SOLUTION: The wavelength transformation element 14A is provided with an optical waveguide 3 and period polarization reverse structures 2a, 2b and 12A formed in the optical waveguide 3. Light propagation directions B and C in the optical waveguide intersect an array direction A where polarization reverse parts are arrayed in the period polarization reverse structure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion element which can be used for a quasi-phase matching type harmonic generation device, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Quasi-phase-matched (Quasi-Phase-Matched) using an optical waveguide in which a periodically poled structure is formed in a single crystal of lithium niobate or lithium tantalate as a light source for a blue laser used in an optical pickup or the like. : QP
M) second harmonic generation (Second-Harmonic-Generati)
on: SHG) Devices are expected. Such devices can be used in a wide range of applications such as optical disk memories, medical applications, photochemistry applications, and various optical measurement applications.

[0003] For example, "Electronics Le"
ters, 24th April, 1997. Vo
l. 33, no. According to page 9, pages 806 to 807, a periodically poled structure is formed on a MgO-doped lithium niobate substrate, and a proton exchange optical waveguide is formed in a direction orthogonal to the structure. Thus, an optical waveguide type second harmonic generator is realized.
As a review, “LiNbO3 quasi phase matching SH
G-device ”(Toshiaki Suhara, Hiroshi Nishihara,“ Laser Research ”
Vol. 21, No. 11, pages 21 to 28).

The wavelength of the second harmonic depends on the period Λ of the domain-inverted portion, and the following relationship holds. λ = 1 × Λ / (N (2ω) -N (1ω) ・ ・ (1)

Here, λ is the wavelength of the pump light, Λ is the period of the domain-inverted portion, N (1ω) is the effective refractive index of the pump light, and N (2ω) is the second harmonic (SHG). ) Is the effective refractive index. The wavelength of the SHG light is λ / 2.

[0006]

As can be seen from the above equation (1), when the period 分 極 of the domain inversion is determined, S
There is a relationship that the wavelength value λ / 2 of HG is determined. When Λ is a constant value, the tolerance of the wavelength λ of the pump light that can generate SHG with high efficiency is extremely small.

If the excitation light wavelength λ is arbitrarily variable, SHG can be developed by changing the excitation light λ so as to satisfy the expression (1). However, the variable wavelength range of the excitation light is often limited or cannot be changed. In such a case, if the polarization inversion period 決定 is determined so as to satisfy the expression (1) in accordance with the predetermined excitation light wavelength λ, and the QPM-SHG device is not constructed,
SHG cannot be expressed.

Here, in an actual laser light emitting device, the wavelength λ of the excitation light is not strictly constant, but varies by about 1 nm due to a change in the surrounding environment such as temperature. In addition, a variation in oscillation wavelength of about ± 5 nm may occur due to a laser manufacturing error. On the other hand, the tolerance of the wavelength λ of the pumping light capable of generating SHG with high efficiency is, for example, 0.
It is about 1 nm. Therefore, in this case, when the pumping light wavelength λ slightly changes, a QPM-SHG device having a plurality of periods must be prepared and adjusted. In order to prepare a domain-inverted portion having a plurality of cycles, it is necessary to prepare each photolithography mask corresponding to each cycle.

"LiNbO3 quasi phase matching SHG device" (Toshiaki Suhara, Hiroshi Nishihara, "Laser Research"
According to the right column on page 27 of Vol. 21, No. 11, pages 21 to 28), a sector-shaped domain-inverted grating is formed, and an optical waveguide is formed perpendicular to the domain-inverted grating. Thus, a large number of optical waveguides having slightly different periods from each other are formed, and at least some of the many optical waveguides satisfy Expression (1) with respect to the wavelength λ of the excitation light. Then, after the sector-shaped periodically poled structure is formed, a plurality of optical waveguides that match the wavelength of the excitation light are selected from a large number of optical waveguides. However, in this case, it is necessary to form a large number of optical waveguides in order to form a small number of optical waveguides for SHG light emission. For this reason, the whole substrate becomes large.

Japanese Unexamined Patent Publication No. Hei 5-273623 proposes a device for improving the tolerance of the wavelength of the excitation light. In this device, a plurality of domain-inverted structures having a plurality of different periods are formed in a device, and the domain-inverted structures are separated from each other. The optical waveguide sequentially passes through a plurality of domain-inverted structures. At this time, since each period of each domain-inverted structure is different, the SHG emission range with respect to the excitation light wavelength is widened. However, when light passes through the optical waveguide,
Light loss occurs and light is attenuated. Therefore, Japanese Patent Application Laid-Open No.
In the configuration disclosed in Japanese Patent No. 73623, when the light emitted from the first-stage periodically poled structure is incident on the second-stage periodically poled structure, the output of the excitation light is attenuated more than when the light is incident on the first stage. are doing. For this reason, the wavelength dependence of the intensity distribution of the SHG may increase.

An object of the present invention is to provide a wavelength conversion element of a quasi-phase matching method which can cope with a plurality of excitation light wavelengths by using a periodically poled structure having a specific period.

[0012]

SUMMARY OF THE INVENTION The present invention comprises an optical waveguide,
A wavelength conversion element having a periodically poled structure formed in the optical waveguide, wherein a propagation direction of light in the optical waveguide and an arrangement direction in which the poled portions are arranged in the periodically poled structure. And intersect.

Further, the present invention comprises the wavelength conversion element and a substrate made of a ferroelectric material, wherein an optical waveguide and a periodically poled structure are formed on the substrate. The present invention relates to an optical waveguide device for conversion.

Further, the present invention is a method for manufacturing a wavelength conversion element having an optical waveguide and a periodically poled structure formed in the optical waveguide, comprising a ferroelectric material, When forming the optical waveguide on the substrate on which the domain-inverted structure is formed, the optical waveguide is guided so that the light propagation direction in the optical waveguide and the arrangement direction of the domain-inverted portions in the periodic domain-inverted structure intersect. A wave path is formed.

The present inventor has conceived of forming an optical waveguide such that the propagation direction of light in the optical waveguide intersects with the arrangement direction in which the domain-inverted portions are arranged in the periodically domain-inverted structure. Thereby, in the optical waveguide,
Since the arrangement direction of the domain-inverted portions intersects with the traveling direction of the light, when the interval (period) between the domain-inverted portions is 周期, the period that light actually feels is larger than Λ. That is, assuming that the angle of intersection between the light propagation direction and the arrangement direction of the domain-inverted portions is α, the period of the domain-inverted portion for light propagating in the optical waveguide is と / cosα. Therefore, the above equation (1) can be rewritten as follows.

Λ = 1 × Λ / cos α · (N (2ω) −N (1ω)) ·· (2)

Here, λ is the wavelength of the excitation light, Λ is the period of the domain-inverted portion, N (1ω) is the effective refractive index of the excitation light, and N (2ω) is the second harmonic (SHG). ) Is the effective refractive index. The wavelength of the SHG light is λ / 2.

Therefore, by changing the crossing direction (by changing α), it is possible to generate a high-output harmonic corresponding to the fluctuation of the wavelength λ of the pump light input to the optical waveguide. it can.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the drawings. FIG. 1 shows an embodiment of a QPM-SHG device according to one embodiment of the present invention. FIG. 2 is a cross-sectional view of the device of FIG. As shown in FIG. 2, the back surface 7 b of the wavelength conversion substrate 7 is bonded on the front surface 6 a of the fixing substrate 6 via the adhesive layer 5. The substrate 7 is made of a ferroelectric material. On the surface 7a side of the substrate 7, as shown in FIG. 1, a periodically poled structure 2 and an optical waveguide 3 are formed. The periodically poled structure 2 includes a large number of periodically poled portions 16 periodically formed with a constant period λ. A large number of domain-inverted portions 16 and non-inverted portions 15 are formed alternately.
Reference numeral 17 denotes a boundary between the two. Many polarization inversion units 16
They are formed parallel to each other and arranged so as to extend in the direction of arrow A.

On the surface 7a side of the substrate 7, a pair of grooves 4A, 4A
B is formed, and a ridge-type optical waveguide 3 is formed between the grooves 4A and 4B. The optical waveguide 3 includes straight portions 3a and 3b that extend substantially straight. In the linear portion 3a, the periodically poled structures 2a are arranged so as to intersect with the longitudinal direction of the optical waveguide. The longitudinal direction B of the linear portion 3a is the light propagation direction. Here, the direction of light propagation in the linear portion 3a is B, and the direction of arrangement of the domain-inverted portions in the periodically domain-inverted structure 2 is A. The arrangement direction of the domain-inverted portions 16 is a direction in which the domain-inverted portions 16 are arranged, and is a direction in which the periodic domain-inverted structure extends. Alternatively, the arrangement direction A is the boundary surface 17
It is a direction substantially perpendicular to. Arrangement direction A of polarization inversion part
And the angle of intersection between the linear portion 3a and the light propagation direction B is α1. In this case, assuming that the period in the periodically poled structure 2 is Λ, the period of the periodically poled structure 2a in the linear portion 3a is Λ / cosα1.

Similarly, the arrangement direction A of the domain-inverted portions
And the intersection angle between the light and the light propagation direction C in the linear portion 3b is α2. In this case, assuming that the period in the periodically poled structure 2 is Λ, the period of the periodically poled structure 2b in the linear portion 3b is Λ / cosα2.

The straight portions 3a and 3b are connected to the bent portion 1
1A. The bent portion 11A is bent (curved) linearly (linearly) between the linear portions 3a and 3b. Naturally, the period of the periodically poled structure 12A in the bent portion 11A smoothly decreases from Λ / cos α1 to Λ and increases smoothly from Λ to Λ / cos α2.

The wavelength conversion element 14A (or the wavelength conversion optical waveguide device 1A) includes a first conversion section 10A, a second conversion section 10B, and a bent portion. Each phase matching condition in the conversion units 10A and 10B has been described above.

According to such an element, the wavelength λ of the excitation light
Is slightly changed or fluctuated, the intersection angles α1 and α2 are changed and adjusted according to the change,
The phase matching condition can be satisfied by using the periodically poled structure 2 having a specific period.

In a preferred embodiment of the present invention, the arrangement direction of the domain-inverted portions is substantially constant over the entire length of the optical waveguide. That is, by changing the cross angle α from one type of periodically poled structure, the phase matching condition can be satisfied with respect to a plurality of types of excitation light wavelengths λ with high efficiency.

In a preferred embodiment, the optical waveguide comprises:
It has a bent portion where the light propagation direction changes.

In a preferred embodiment, the radius of curvature of the bent portion is 1 mm or more. That is, when the radius of curvature of the bent portion is small (that is, when the curvature is sharp), part of light is emitted from the bent portion, resulting in light loss. In order to suppress such light emission, it is effective to increase the radius of curvature of the bent portion and moderately curve the bent portion.

Further, from the viewpoint of suppressing an increase in the size of the element, the radius of curvature of the bent portion is preferably set to 50 mm or less.

In a preferred embodiment, the optical waveguide includes a plurality of linear portions, and adjacent linear portions are connected by a bent portion. FIG. 1 corresponds to this embodiment.

In such an embodiment, when the crossing angles α in the respective linear portions are substantially the same, the phase matching condition is satisfied for almost one excitation light wavelength throughout the entire length of the optical waveguide. However, the phase matching condition cannot be satisfied for a plurality of pump light wavelengths that are separated from each other to some extent.

On the other hand, the intersection angle α in each linear portion can be shifted. In this case, the pumping light wavelength λ that satisfies the phase matching condition in each linear portion is slightly different, so that the tolerance is improved.

In the embodiment of FIG. 1, two linear portions are provided. However, the number of linear portions is not particularly limited. For example, in the embodiment of FIG.
Two straight sections were provided. That is, the wavelength conversion element 14B (wavelength conversion optical waveguide device 1B) of FIG. 3 includes four wavelength conversion units 10A, 10B, 10C, and 10D. Each of the linear portions 3a, 3
b, 3c and 3d are provided. Bent portions 11A, 11B, 11 are provided between adjacent linear portions, respectively.
C is provided. Each bent portion is linearly (linearly) curved. Each linear portion 3a, 3
b, 3c, and 3d each include a periodically poled structure 2
a, 2b, 2c, and 2d are formed. In each of the bent portions 11A, 11B, and 11C, periodic domain-inverted structures 12A, 12B, and 12C are formed, respectively.
In each of the linear portions 3a, 3b, 3c, and 3d, the intersection angle between the light propagation direction and the arrangement direction of the domain-inverted portions is α1,
α2, α3, α4.

Although there is no particular upper limit for the intersection angle α in each linear portion, it is preferably 45 ° or less from the viewpoint of maintaining high conversion efficiency.

In a preferred embodiment, the crossing angle between the light propagation direction in the optical waveguide and the arrangement direction of the domain-inverted portions can be changed linearly. In this case, as described above, the latitude of the phase matching condition with respect to the pump light wavelength can be increased.

In this embodiment, the planar configuration of the optical waveguide is not particularly limited. However, it is preferred that the optical waveguide be curved linearly. Such a curved form is not particularly limited, and may be a higher-order function such as a quadratic function or a cubic function or a hyperbola. Particularly preferably, the shape of the optical waveguide is substantially sinusoidal. The sine curve here is synonymous with the cosine curve.

FIG. 4 is a plan view showing a wavelength conversion element 14C (wavelength conversion optical waveguide device 1C) according to this embodiment. On the front surface side of the substrate 7, a periodically poled structure 2 having a period Λ is formed. The domain-inverted portions are arranged in a certain direction (A direction). A pair of grooves 4C and 4D are formed on the substrate 7, and a ridge-type optical waveguide 3B is formed between the pair of grooves 4C and 4D. The optical waveguide 3B has a substantially sinusoidal shape in plan view.

Therefore, as schematically shown in the lower part of FIG. 4, in each part of the optical waveguide 3B, the intersection angle α between the light propagation directions B, C, and D and the arrangement direction A of the domain-inverted portions is equal to each other. Changes smoothly. Specifically, the intersection angle α is
From the left side to the right side of FIG. 4 (for example, from the input side to the output side of the excitation light), a predetermined maximum value αm from 0
increasing towards ax, then decreasing towards 0,
Next, it increases from 0 toward a predetermined maximum value αmax, and decreases again toward 0.

The value of the maximum value αmax is not particularly limited, but is preferably 45 ° or less from the viewpoint of maintaining high conversion efficiency.

The wavelength conversion element of the present invention is established as long as it includes at least the optical waveguide and the periodically poled structure in the optical waveguide. Therefore, portions other than the optical waveguide of the substrate 7 are not essential requirements.

Further, the harmonic generation device of the present invention is established as long as it includes at least the wavelength conversion element and input means for inputting excitation light to the wavelength conversion element. Examples of such input means include, but are not limited to, a semiconductor laser.

The processing of the groove for forming the ridge-type optical waveguide as described above can be performed by a laser ablation method, a dry etching method, or a mechanical processing method. When the laser ablation method is performed, usually, processing is performed after the substrate 7 is placed on an automatic stage.

As an important problem, after confirming the wavelength λ of the excitation light, it is necessary to set the intersection angle α in accordance with the accurate measured value of λ and process the groove. At this time, according to the laser ablation method, the intersection angle α can be easily changed by setting the moving pattern of the moving stage after confirming the wavelength λ of the excitation light, and therefore, the correspondence is easy. In this regard, the laser ablation method is particularly excellent.

On the other hand, in the case of the etching method,
It is necessary to manufacture each mask pattern one by one corresponding to each intersection angle. However, after confirming the wavelength λ of the excitation light,
It is difficult to set the intersection angle α in accordance with the accurate measurement value of λ and to manufacture a mask pattern corresponding to the intersection angle α one by one.

In the embodiment as described above, the substrate 7
Is adhered to the fixing substrate 6 by the adhesive layer 5. In this case, the refractive index of the adhesive layer is preferably lower than the refractive index of the substrate 7, and the adhesive layer is preferably amorphous. The difference between the refractive index of the adhesive layer and the refractive index of the substrate 7 is preferably 5% or more, preferably 10% or more.
% Is more preferable. The material of the bonding layer is preferably an organic resin or glass (particularly preferably low-melting glass). Examples of the organic resin include an acrylic resin, an epoxy resin, and a silicone resin. As the glass, a low-melting glass containing silicon oxide as a main component is preferable.

As a method of forming the optical waveguide, the following method is also available. (1) By altering the surface region of the substrate made of the nonlinear optical crystal and partially changing the composition, an altered layer having a high refractive index, for example, a titanium diffusion layer or a proton exchange layer is provided in the surface region of the substrate. (2) A single crystal film having a refractive index higher than that of the substrate is formed on the surface of the substrate made of the nonlinear optical crystal, and the single crystal film is processed into an elongated planar shape.

When the device of the present invention is used as a second harmonic generator, the wavelength of the harmonic is 330-1600 n.
m is preferable, and 400 to 430 nm is particularly preferable.

The nonlinear optical crystal constituting the optical waveguide is not particularly limited, but is preferably a ferroelectric single crystal which can easily form a periodically poled structure, and is preferably a lithium niobate (LiNbO.sub.3).
₃ ), lithium tantalate (LiTaO ₃ ), lithium niobate-lithium tantalate solid solution, K ₃ Li ₂ Nb
Each single crystals of ₅ O ₁₅ is particularly preferred.

In such a crystal, magnesium (M) is used to further improve the light damage resistance of the three-dimensional optical waveguide.
g), one or more metal elements selected from the group consisting of zinc (Zn), scandium (Sc) and indium (In), and magnesium is particularly preferred.

In the present invention, since the periodically poled structure is formed in the optical waveguide, the lithium niobate single crystal and the lithium niobate-lithium tantalate solid solution are considered from the viewpoint that the polarization reversal characteristics (conditions) are clear. Single crystals or those obtained by adding magnesium thereto are particularly preferable.

In the above-mentioned crystal, as a dope component,
A rare earth element can be contained. This rare earth element acts as an additional element for laser oscillation. As the rare earth element, in particular, Nd, Er, Tm, Ho, D
y and Pr are preferred.

The material of the fixing substrate is not particularly limited.
What is necessary is just to have predetermined structural strength. However, it is preferable that physical properties such as thermal expansion coefficient and the like are close to those of the optical waveguide, and lithium niobate (LiNbO ₃ ) and lithium tantalate (Li
TaO ₃ ), lithium niobate-lithium tantalate solid solution, and single crystals of K ₃ Li ₂ Nb ₅ O ₁₅ are particularly preferable.

The method for forming the periodically poled structure is not limited. As a method for forming a periodically poled structure on an electro-optic crystal substrate, for example, a lithium niobate substrate, a diffusion method in Ti, a diffusion method in Li ₂ O, a heat treatment method with SiO ₂ , a thermal oxidation method in Ti, a proton exchange heat treatment method, An electron beam scanning irradiation method, a voltage application method, and a corona charging method are preferred. Among these methods, the voltage application method is particularly preferable when an X-cut or Y-cut or its off-cut substrate is used from the viewpoint of forming a deep periodically poled structure with high accuracy. When a Z-cut substrate is used, it is particularly preferable to carry out the method by a corona charging method or a voltage application method.

The material for forming the mask pattern for forming the periodically poled structure includes resist, S
iO ₂ , Ta, etc. can be exemplified, and a photolithography method can be exemplified as a method of forming a mask pattern.

[0054]

As described above, according to the present invention, it is possible to provide a quasi-phase matching type wavelength conversion element which can support a plurality of pumping light wavelengths by using a periodically poled structure having a specific period.

[Brief description of the drawings]

FIG. 1 shows a wavelength conversion element 14A according to an embodiment of the present invention.
It is a top view which shows (wavelength conversion optical waveguide device 1A) roughly.

FIG. 2 is a cross-sectional view of the substrate 1A of FIG.

FIG. 3 shows a wavelength conversion element 14 according to another embodiment of the present invention.
FIG. 6B is a plan view schematically showing B (wavelength conversion optical waveguide device 1B).

FIG. 4 shows a wavelength conversion element 14 according to another embodiment of the present invention.
It is a top view which shows C (wavelength conversion optical waveguide device 1C) roughly.

[Explanation of symbols]

1A, 1B, 1C Optical waveguide device for wavelength conversion
2 periodically poled structure 2a, 2 on substrate 7
b, 2c, 2d Periodically domain-inverted structure in a linear portion
2e Periodically Poled Structure in Sinusoidal Curved Portion 3, 3A, 3B Optical Waveguide 4A, 4B, 4C,
4D groove 5 Adhesive layer 6 Fixing substrate 7 Substrate made of ferroelectric material 10A, 10B, 10C, 10D Wavelength conversion unit
Bent portion of 11A, 11B, 11C optical waveguide
12A, 12B, 12C Periodically domain-inverted structure in bent portion 14A, 14B, 14C Wavelength conversion element
15 Non-polarized part 16 Polarized part
A: Arrangement direction of domain-inverted portions 16 in periodic domain-inverted structure 2 B, C, D, E Propagation direction of light in linear portion α Intersection angle α1 between propagation direction of light in optical waveguide and said arrangement direction A, α2, α3, α4 Intersecting angle between the propagation direction of light in each linear portion of the optical waveguide and the arrangement direction A.
Period of periodically poled structure Λ / cosα Period of periodically poled structure in optical waveguide Λ / cosα
1, Λ / cosα2, Λ / cosα3, Λ / cosα4
Period of periodically poled structure in linear part

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H047 KA05 KA12 LA00 NA08 PA24 QA03 RA00 TA12 2K002 AA05 AB12 CA03 DA06 EA04 FA19 FA26 FA27 GA04 HA20

Claims (22)

[Claims] 1. A wavelength conversion element comprising: an optical waveguide; and a periodically poled structure formed in the optical waveguide, wherein: a direction of propagation of light in the optical waveguide; Wherein the arrangement direction in which the domain-inverted portions are arranged intersects with each other. 2. The wavelength conversion element according to claim 1, wherein said arrangement direction is substantially constant over the entire length of said optical waveguide. 3. The wavelength conversion element according to claim 1, wherein said optical waveguide has a bent portion. 4. The wavelength conversion element according to claim 3, wherein a radius of curvature of said bent portion is 1 mm or more. 5. The optical waveguide according to claim 3, wherein the optical waveguide includes a plurality of linear portions, and the adjacent linear portions are connected by the bent portion.
The wavelength conversion element as described in the above. 6. The wavelength conversion element according to claim 1, wherein an intersection angle between said propagation direction and said arrangement direction in said optical waveguide changes linearly. . 7. The wavelength conversion element according to claim 1, wherein said optical waveguide is linearly curved. 8. The wavelength conversion element according to claim 7, wherein said optical waveguide has a substantially sinusoidal shape. 9. A wavelength conversion device according to claim 1, further comprising a substrate made of a ferroelectric material, wherein said substrate is provided with said optical waveguide and said periodically poled structure. And an optical waveguide device for wavelength conversion. 10. The optical waveguide device according to claim 9, wherein a pair of grooves for contouring both sides in the width direction of said optical waveguide are formed in said substrate. 11. The optical waveguide device according to claim 10, wherein said groove is formed by a laser ablation method. 12. A wavelength conversion element according to claim 1, further comprising an input means for inputting light before wavelength conversion to the wavelength conversion element. , A harmonic generator. 13. A method of manufacturing a wavelength conversion element having an optical waveguide and a periodically poled structure formed in the optical waveguide, comprising a ferroelectric material, wherein the periodically poled structure is formed. When forming the optical waveguide on the formed substrate, the optical waveguide is formed such that the propagation direction of light in the optical waveguide intersects the arrangement direction in which the domain-inverted portions are arranged in the periodic domain-inverted structure. Forming a wavelength conversion element. 14. The method according to claim 13, wherein said arrangement direction is substantially constant in said substrate. 15. The method according to claim 13, wherein a bent portion is formed in the optical waveguide. 16. The method according to claim 15, wherein the radius of curvature of the bent portion is 1 mm or more. 17. The method according to claim 15, wherein the optical waveguide is provided with a plurality of linear portions, and the adjacent linear portions are connected by the bent portion. 18. The method according to claim 13, wherein an intersection angle between the propagation direction and the arrangement direction in the optical waveguide is changed linearly. 19. The method according to claim 13, wherein the optical waveguide is curved linearly. 20. The method according to claim 19, wherein the shape of the optical waveguide is substantially sinusoidal. 21. The method according to claim 13, further comprising forming a pair of grooves in the substrate for contouring both sides of the optical waveguide in the width direction. 22. The method according to claim 21, wherein said groove is formed by a laser ablation method.