JPH07318994A - Harmonic generating element - Google Patents

Abstract

PURPOSE:To obtain such a harmonic generating element having a discrete type waveguide structure that loss due to scattering of light is decreased, the wavelength conversion efficiency is high, and the element is produced in a simple process and easily designed by forming almost curved faces at the border between a polarization inversion region and a region without inversion. CONSTITUTION:The Z-plane of a KTP single crystal is used as a substrate 27. A Ti thin film is formed on the Z-plane 28 of the substrate 27. Then a photoresist is applied, exposed to light through a photomask having a form for a polarization inversion region 29, and developed to form the pattern in the resist. Further, the pattern is formed on the Ti film by chemical etching and the substrate is heat treated in a mixture soln. of Ba(NO3)2 and RbNO3 to replace part of K ion in KTP ion with Rb to form a cylindrical polarization inversion region 29. In this process, the border of the polarization inversion region is made almost cylindrical surface. If a material having different conditions for ion exchange, for example, lithium tantalate is used, a spherical plane is formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a harmonic generating element that uses a stable and high-power coherent light source such as a laser to obtain coherent light of a short wavelength and reduces the wavelength to half or less.

[0002]

2. Description of the Related Art In order to obtain a harmonic generating element having a high conversion efficiency, for example, an SHG (second harmonic generating) element, it is necessary to match the phase between incident light and SHG light generated in the element. It is important to. In the case of a bulk device, phase matching is achieved by adjusting the angle of the SHG crystal with respect to the optical axis by utilizing the anisotropy of refractive index of the crystal.

In order to further increase the SHG generation efficiency, a method of increasing the light intensity by a waveguide structure is adopted.
Since the conversion efficiency of the harmonic generating element increases in proportion to the light intensity in the element, the light intensity can be increased by focusing the light inside the element by the waveguide structure. Various methods have been proposed as a method of phase matching in a waveguide structure. Among them, as a practical one, for example, PK
Tien et al .: Appl. Phys. Lett. 14 (1970) 291 using Cherenkov radiation and, for example, S. Somekh et a.
l.:Appl.Phys.Lett.21(1972)140 uses a periodic structure shown in FIG. The former has a simple waveguide structure, but since SHG light is radially generated with a certain angle with respect to the waveguide, there is a problem that a complicated optical system is required for condensing. Has a problem that the structure is complicated and the manufacturing process is long, but since SHG light is propagated in the waveguide mode, it has a feature that it is easy to collect light.

In the quasi-phase matching method, the periodic structure for quasi-phase matching and the waveguide structure for guiding light are generally provided separately, which makes the structure of the device more complicated. . However, JDBierlein et al .: Appl.Phys.Let
The method published in t.56 (1990) 1725 is KTP (KTiOPO ₄ ).
An SHG device having a quasi-phase matching structure, which has a structure in which a polarization inversion structure is periodically formed in a crystal by ion exchange and the polarization inversion portion forms a discrete waveguide, and the process is simple. As one of the most practical. In this method, the condition of phase matching is loosened by using the balanced phase matching method in which the inversion of polarization is subdivided into segments shorter than the coherent length to obtain phase matching.
In addition, the conventional SHG device has the advantages that the difference in the refractive index of the polarization inversion portion is small and the reflection loss is small, and that the diffraction loss can be suppressed to a small value by making the interval between the inversion portions sufficiently small with respect to the Fresnel length. We have succeeded in creating one of the most efficient devices.

[0005]

As described above, the element which uses the balanced phase matching by the periodic polarization reversal and the discrete type optical waveguide together is one of the most practical SHG elements. However, it is still not enough to make a truly practical device. In the discrete optical waveguide structure, the diffraction loss is reduced by making the interval of the polarization inversion portion shorter than the Fresnel length, but in reality, the loss due to the reflection at the boundary surface and the diffraction cannot be made zero. Normally, in an SHG element, since more than 100 discrete structures are provided in the light propagation direction, even a slight loss
By accumulating it, the total loss increases, the SHG light that propagates in the waveguide and can be effectively used decreases, the overall SHG conversion efficiency does not increase, and the loss light from each discrete structure is diffracted. Then, there is a problem in that the output light cannot be converged to one point because it overlaps the optical output. In addition, since it is not possible to freely select the spacing between the polarized portions because it is necessary to reduce the loss, there is a problem that the conditions for phase matching are restricted and the degree of structural freedom is low.

Another problem is that a very accurate process is required to form the waveguide. Generally, in an SHG element having a waveguide structure, the loss of the waveguide largely reflects the surface accuracy of the boundary between the substrate and the optical confinement region. Therefore, in order to reduce the loss, the interface accuracy of the waveguide is reduced to a fraction of a wavelength. It is necessary to raise to the following. Since the current waveguide formation process is a method of transferring the pattern of the photomask to the photoresist and performing ion exchange or the like in that portion, there is a problem that it is difficult to satisfy the requirement for the accuracy of the boundary surface. .

Another problem related to the waveguide structure is that the SHG conversion efficiency is proportional to the square of the electric field strength of the propagating light, so that the narrower the width of the waveguide, the higher the efficiency. In the process, it was difficult to form a waveguide with a narrow wavelength order.

Further, the harmonic generating element using the optical waveguide needs to propagate light in a single transverse mode inside the waveguide in order to increase the harmonic conversion efficiency. In this case, the width of the optical waveguide is equal to that of the optical waveguide. Is on the order of several .mu.m.
When focusing the incident laser light on the end face of the waveguide using a lens, it is necessary to match the position of the focusing point with the position of the end face of the waveguide on the order of μm, so high mechanical accuracy and stability are required, It is difficult to efficiently enter light into the optical waveguide, and there is a problem that a loss due to coupling of incident light is large.

A common problem of all SHG elements using the quasi phase matching method is that the allowable width for efficiently generating SHG light is extremely narrow. The permissible wavelength is generally 0.1 nm or less for normal visible light to near infrared light. For this reason, the wavelength of the incident laser light can be accurately adjusted to S
There is a problem in that it is necessary to match and stabilize the allowable wavelength of the HG element, and it is difficult to use a general and inexpensive semiconductor laser whose output wavelength changes with temperature and driving current as a light source.

Therefore, the present invention solves the above-mentioned problems in an element using both a periodic polarization inversion structure and a discrete optical waveguide, reduces the loss due to light scattering, has a high wavelength conversion efficiency, and It is an object of the present invention to provide a harmonic wave generating element having a discrete waveguide structure which is easy to manufacture and easy to design.

It is another object of the present invention to provide a harmonic wave generating element that facilitates optical coupling to a waveguide and enhances the efficiency of optical coupling.

[0012]

In order to solve the above-mentioned problems, the present invention forms a periodic polarization inversion part and a non-inversion part in a non-linear optical material so that the phase of incident light and the nth harmonic wave is pseudo. In a higher harmonic wave generation element having a structure that achieves proper matching and forms a discrete type optical waveguide, the boundary between the polarization inversion portion and the non-inversion portion is a substantially curved surface.

Particularly, KTP is used as the non-linear optical material of the above harmonic generating element, and a means for forming polarization inversion is provided by substituting a part of potassium (K) ions with other metal ions. Is.

In the above harmonic generating element, the curved surface is made convex when the polarization inversion part has a higher refractive index than the non-inversion part and concave when the polarization inversion part is low, and the light is converged by the discrete waveguide. It was done so.

Further, since the light is converged by the discrete type waveguide, the length of the polarization inversion portion is L ₁ , the length of the polarization non-inversion portion is L ₂ , and the polarization inversion portion and the non-inversion portion are the same. the focal length of the lens structure formed by a substantially curved surface at the boundary is f, the case of ^{_{L 1 ≧ L 2, 4k 2}} × D 4 / (L 1 + L 2) ≧ f ≧ L 1/2 or L _2/2 ≧ f ≧ L ₁ × L ₂ / [2 (L ₁ +
L _2)] For ^{_{L 2 ≧ L 1, 4k 2}} × D 4 / (L 1 + L 2) ≧ f ≧ L 2/2 or _{L 1/2 ≧ f ≧ L} 1 × L 2 / [2 (L 1 +
L ₂ )] is satisfied. Here, k is the propagation coefficient and D is the width of the optical waveguide.

Furthermore, the present invention is to solve the above-mentioned problems.
In a harmonic wave generating element having a waveguide structure, a lens-shaped light focusing mechanism for coupling incident light to the waveguide is provided on the element end face.

In particular, a light focusing mechanism is constructed by providing a portion having a high refractive index and having a convex shape with respect to the periphery in the vicinity of the end face of the element to give a lens effect.

Further, in combination with the above-mentioned light focusing mechanism, the direction of nonlinear polarization with respect to the traveling direction of light inside the optical waveguide is reversed at a cycle of coherence length to pseudo-phase-match the incident light and harmonics. In addition, the discrete optical waveguide structure is provided by utilizing the fact that the refractive index of the polarization inversion portion is higher than that of the nonlinear optical material substrate.

Furthermore, by making the boundary between the inversion part and the non-inversion part of the non-linear polarization in the harmonic generating element a substantially curved surface,
A discrete optical waveguide having a light focusing function is formed.

In the harmonic generating device having such a waveguide structure and a light focusing mechanism for optical coupling of the incident end face, the light focusing mechanism having a lens function and the optical waveguide for generating the harmonic are processed by the same impurity diffusion. Can be formed by.

[0021]

According to the harmonic generating element of the present invention, the light is converged, the loss due to the scattering of the light is reduced, and the conversion efficiency is improved.

According to the harmonic generating element of the present invention,
The efficiency of the optical coupling between the incident light and the harmonic generating element is improved, the alignment of the incident light and the harmonic generating element is facilitated, and the allowable wavelength range for high frequency conversion is widened, so that a laser light source with a short wavelength can be obtained. Will be easier.

[0023]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 shows a conventional SHG element having a discrete waveguide structure. The hatched portion (1) is a portion where the polarization of the part of the substrate (2) is inverted by ion exchange and the refractive index is increased with respect to the substrate. Incident light (3) is converted to SHG light (4) while passing through the discrete waveguide structure. FIG. 3 shows how light propagates in the waveguide having such a discrete structure. 3A is a cross-sectional structure of the substrate viewed from the lateral direction, and FIG. 3B is a cross-sectional view of the substrate viewed from above. Since the refractive index of the domain-inverted portion (5) is generally only about several percent higher than that of the substrate (6), the light intensity (7) of the propagating light seen from the lateral direction is asymmetrical in the vertical direction and is considerably intruded into the substrate. It is in a state. When viewed from above, the light intensity (8) of the propagating light spreads symmetrically.

Since there is no structure for confining light in the region between the polarization inversion parts, the propagating light is emitted into the substrate at the boundary part. FIG. 4 shows a spread state (10) of the light emitted from such a waveguide (9).
If the width (11) of the waveguide is 2a and the wavelength of the propagating light is λ, the light spreads at an angle θ = λ / a in the distance (12).
However, at a site close in distance, particularly at a distance of a ² / λ or less defined as Fresnel length, the spread of light is small, and most of the light is coupled to the next waveguide structure (13). This is the principle of the discrete waveguide. However, as is clear from FIG. 4, there is a slight spread of light even below the Fresnel length,
That is a loss.

At the boundary surface (14) of the waveguide, the light is confined inside the waveguide at a critical angle determined by the refractive index difference between the waveguide and the substrate, and the roughness of the boundary surface of the waveguide directly affects the waveguide. Affect the loss of light propagating through.

Next, the principle of the present invention will be described with reference to FIG. FIG. 5 is an enlarged top view of the periodic element of the discrete element structure according to the present invention. The feature of the present invention is that in the discrete structure, the polarization inversion parts (15) and (26) are substantially curved at the boundary parts (16) and (17) with respect to the light propagation direction.
For example, to make it a nearly spherical surface. An ellipse as a curved surface,
There are paraboloids, spherical surfaces, and cylindrical surfaces. Polarization reversal part (15)
When the refractive index of (26) is higher than that of the substrate (18), the curved surfaces of the boundary portions (16) and (17) are formed on the substrate (18).
By making it convex with respect to this, this boundary portion functions as a lens and has an effect of converging the propagating light (19). Since the domain-inverted structure exists periodically, light propagates while being converged by the array of periodically arranged convex lenses, and the propagation loss due to diffraction is reduced despite the discrete structure, and the light is confined by the lens structure. Therefore, the waveguide side surface (20) (2
The effect that the accuracy of 1) does not affect the propagation loss can be obtained.

Here, the conditions for confining light in such a converging type waveguide structure will be obtained. Let us analyze the state of light propagating in this structure by the ray matrix method assuming Gaussian beam propagation. First, the central part (2
In 2), the wavefront of the light propagating through the waveguide is P. The focal points of the lenses formed by the spherical structure of the boundary surface (17) (23) between the polarization inversion part and the non-inversion part are f ₁ and f ₂ respectively.
If the lengths of the polarization inversion part and the non-inversion part are L ₁ and L ₂ , the light in the center (22) of the polarization inversion part propagates to the same part (25) of the polarization inversion part of the next cycle. The change in the state of light until reaching it is calculated by the ray matrix method.

The ray matrix [M] associated with the propagation is given by the following equation (where j ² = -1).

[Equation 1]

In equation (1), each matrix on the right side propagates in the polarization inversion part (15), refracts in the spherical surface (17), propagates in the polarization non-inversion part (24), and refracts in the spherical surface (23). ,
Propagation in the polarization inversion section (26) is shown. Where k ₁ , k
₂ each polarization inversion unit is a propagation coefficient of the non-inversion unit, if each refractive index of n _1, n _2, represented by the following formula. k ₁ = 2πn ₁ / λ, k ₂ = 2πn ₂ / λ

The wavefront P at the start point (22) of propagation and the wavefront at the end point (25) are given by the following equation. (M ₁₁ × P + M ₁₂ ) / (M ₂₁ × P + M ₂₂ ) ... Formula (2)

If the condition that this becomes a plane wave equal to the original wavefront P is obtained, the light is confined in the periodic lens structure and propagates without diffraction loss.

Next, a method for more simply and approximately obtaining the value of f will be described. f ₁ and f ₂ are f ₁ = {n ₂ / (n ₁ −n) from the general equation of geometrical optics, where R ₁ and R ₂ are the radii of curvature of the curved surface that is the boundary between the polarization inversion part and the non-inversion part. ₂ )} × R ₁ f ₂ = {n ₁ / (n ₁ −n ₂ )} × R ₂ ... Equation (3).

The change in the refractive index due to the polarization reversal for performing the quasi-phase matching of SHG is generally very small because of ion exchange, and n ₁ ≈n ₂ . If the polarization inversion part has a symmetric structure and R ₁ = R ₂ = R, then f ₁ ≈f ₂ = f, k ₁
By setting ≈k ₂ , M ₁₁ = M ₂₂ , or in this case, the spot diameter S at the center of the domain-inverted portion is

[Equation 2] Given in. As a condition for the presence of significant spot diameter S from equation _{(4) (f-L 1} /2) × (f-L 2/2) × [f-L 1 × L 2 / {2 (L 1 + L _2)}] ≧ 0 ··· equation (5) than the L _1> L ₂ which f ≧ L _1/2 or _{L 2/2 ≧ f ≧ L} 1 × L 2 / {2 (L 1 + L 2) } L _2> for L ₁ f ≧ L _2/2 or _{L 1/2 ≧ f ≧ L} 1 × L 2 / {2 (L 1 + L 2)} ··· equation (6) is applied. If there is no much difference n ₁ and n ₂ from equation _{(3), f»L 1/2} , L 2/2 condition is satisfied, the formula (4)

[Equation 3] Indicated by.

As another condition for the light to propagate without loss, it is necessary that the spot diameter S for propagation is smaller than the width D of the optical waveguide, and the following condition is derived from the equation (7). D ⁴ × 4k ² / (L ₁ + L ₂ )> f Equation (8) Equations (6) and (8) are used in the present invention to propagate light without loss in the SHG element having the discrete optical waveguide structure. It is a condition for.

Next, a first embodiment of the structure of the element of the present invention will be described with reference to FIG. KTP as the substrate (27)
Using a single crystal Z plate, a cylindrical polarization inversion part (29) was formed from the Z surface (28) by an ion exchange method. It is desirable that the boundary surface of the polarization inversion portion is a spherical surface with respect to the light propagation direction, but for the sake of simplicity, the curvature radius is infinite in the direction perpendicular to the Z plane, and the curvature is provided only in the Z plane direction. In this case, it is impossible to satisfy the formula (8) in the direction perpendicular to the Z plane, but only in the Z plane direction, the formula (6),
The loss was reduced by setting the condition to satisfy the formula (8).

FIG. 6 shows a method of manufacturing the device of the first embodiment shown in FIG. A thin film of Ti (3
1) is formed. Next, the photoresist (32) is coated, and the photoresist is exposed and developed by the photomask (33) having the shape of the domain-inverted portion to form a pattern on the photoresist (34). Further chemical etching Ti
A pattern (35) is formed on the surface of Ba (NO ₃ ) ₂ and RbN
By heat treatment in a mixed solution of O ₃ , a part of the K ions of KTP is replaced with Rb (rubidium), and the polarization inversion structure (36) is formed. The device is completed by removing the Ti thin film (37).

Table 1 shows the dimensions and the refractive index of each part in the first embodiment of FIG. Here, the process is simplified by making the pattern of the polarization inversion portion circular.

[0038]

[Table 1]

Under such conditions, the wavelength is 1.06 μm, S
The propagation state of light in HG light 0.53 μm is shown. Wavelength 1.
For 06 μm, from the formula (3), f = 108 μm k = 10 μm ^{−1 In} the condition of the formula (6), when L ₁ > L ₂ , f> L ₁ /2=1.5 μm is established. . Further, under the condition of the expression (8), D ⁴ × 4k ² / (L ₁ + L ₂ ) = 7862 μm> f is established. In this case, the spot diameter of the propagating light is S = 1 μm from the equation (7). Similarly, for the SHG light, the conditions that satisfy the equations (6) and (8) are satisfied.

An apparatus for obtaining an SHG light output with the device in the first embodiment of FIG. 1 and the results are shown in FIGS.
It shows in (c). FIG. 7A shows a measurement system in which the output light (39) of the YAG laser having a wavelength of 1.06 μm is focused on the SHG element (38) through the lens (40). The output light is 0.53 after cutting 1.06 μm with the lens (41).
It is projected on a screen (43) through a filter (42) that transmits μm. In the experiment, an element having a conventional rectangular segment structure and an element having the structure of the first embodiment of the present invention shown in FIG. FIG. 7 (b) is a pattern on the screen by the element of the conventional structure, and FIG. 7 (c) is a pattern on the screen of the element structure of the present invention. In the conventional structure, scattered light (45) from each polarization inversion structure around the converged spot (44)
In contrast, in the structure of the present invention, only the spot (46) was obtained. Further, the conversion efficiency of incident light to SHG light was about 10 times or more that of the conventional structure.

Although KTP is used as the substrate in the present invention, it is also possible to use non-linear optical materials such as lithium niobate and lithium tantalate, or organic non-linear materials as other examples.

In the first embodiment of the present invention, the periodic polarization inversion structure is used as the condition for the quasi-phase matching of the incident light and the SHG light, but as another embodiment, the periodic polarization inversion is not accompanied. Even in the structure that performs quasi phase matching only by changing the refractive index,
Highly efficient SH by adopting a similar convergent waveguide structure
It is possible to obtain a harmonic generating element such as G, THG (third harmonic).

In the present invention, the boundary of polarization inversion is a substantially cylindrical surface, but as another embodiment, if a material having different ion exchange conditions for polarization inversion, such as lithium tantalate, is used, a spherical surface is formed. It is possible to obtain a device with higher efficiency.

Next, FIG. 9 shows a state of optical coupling to a conventional waveguide type SHG element. FIG. 10 shows a conventional waveguide type SHG.
The state of optical coupling in the vicinity of the incident end face of the device is enlarged and shown. Deviation between the optical axis (51) of the incident light and the optical axis (52) of the waveguide (5
3) causes the coupling loss. The theory of the increase in loss due to the shift of the optical axis in such optical coupling has been studied in detail in the case of optical fiber connection, for example, in the Optical Communication Manual (Hiroshi Hirayama et al., Published by Kagaku Shimbun, 1984).
As shown on page 226, the loss exceeds 1 dB when the axial displacement is about 1/3 of the width of the waveguide. That is, in the case of a single transverse mode waveguide such as the SHG element, the width of the waveguide is about several μm, and therefore the axis deviation limit is also several μm, and a very highly accurate alignment mechanism is required. .

The second embodiment of the present invention will be described with reference to FIG. The element is provided with an optical waveguide (54) except for the incident end face portion, and is provided with a periodic polarization inversion structure (55) for phase matching. SHG element substrate is KTP
And the phase inversion structure is RbNO ₃ and Ba (NO ₃ ) ₂
The heat treatment is performed in the mixed solution of (1) to replace K ions of the KTP substrate with Rb ions and to diffuse Ba (barium) as an impurity. Waveguide structure is RbN
By heat-treating only O ₃ and substituting K for Rb, only the refractive index is increased. The optical coupling portion (56) on the end face increases the refractive index by injecting N ions into a spherical semicircle to form a lens-like structure.

FIG. 11 is an enlarged view of the element end face portion which is the second embodiment of the present invention. If the focal length of the lens-like structure formed by the optical coupling portion (56) of the end face portion is f, then all the light incident from the end face is focused at one point at the focal length f. The refractive indexes of the substrate and the optical coupling part (56) are n ₀ and n, respectively.
_{If the} radius of curvature (58) of the optical coupling portion is R, the focal length f of the lens formed by the optical coupling portion (56) is f.
Has a _{f = {n 1 / (n} 1 -n 0)} × R ··· formula (9).

By setting the distance (60) between the optical waveguide (54) and the optical coupling portion (56) to be substantially equal to the above focal length f,
Even if the optical axis of the incident light has a deviation (57) from the optical axis of the waveguide, the light is guided to the optical waveguide (54) according to the optical axis (59) by the lens effect of the optical coupling portion (56). .

As shown in FIG. 12, even when the beam diameter of the incident light has a diameter (61) larger than the width of the optical waveguide, the incident light can be efficiently guided by the lens effect of the optical coupling portion (56). Can be combined with. Therefore, the end face portion (56) is also useful in a non-discrete optical waveguide structure.

A third embodiment of the present invention is shown in FIG. The portion of the optical waveguide of the harmonic generating element of this embodiment is the first
This is similar to the optical waveguide of the example. That is, in the quasi-phase matching type harmonic generating device having a discrete waveguide, by making the boundary of the polarization inversion part a substantially curved surface, the light propagating in the waveguide is focused, and the harmonic can be generated with high efficiency. Becomes

As in the second embodiment, the device substrate is made of KTP, and the optical coupling portion (62) and the optical waveguide and quasi phase matching portion (63) are formed by substituting some ions with other ions. Is forming.

In this embodiment, since the waveguide itself has a light focusing function, the distance between the optical coupling part (62) and the optical waveguide part (63) is the focal length of the lens structure formed by the optical coupling part (62). There is no need to exactly match with.

However, as shown in FIG. 14, in order to allow the incident light having a very large diameter (64) and the large axis deviation, it is formed by the optical coupling portion (62) as in the second embodiment. The focal length of the lens structure and the distance (65) between the optical coupling portion (62) and the optical waveguide (63) are made substantially equal.

The fourth embodiment of the present invention is shown in FIG. In this embodiment, the structure of the optical coupling part is composed of a plurality of lens structures (66).
Are formed from. As shown in FIG. 14 of the third embodiment,
When a large axis deviation is allowed, the distance from the waveguide (6
5) also becomes large, and it is necessary to lengthen the entire device. In this embodiment, the optical coupling portion is substantially shortened by the plurality of lens structures.

FIG. 16 shows a fifth embodiment of the present invention. In each of the second, third and fourth embodiments, the change in the refractive index of the optical coupling portion and the polarization inversion portion are achieved by substituting different substances. In this embodiment, the optical coupling portion and the phase inversion portion are achieved by the same ion substitution process made of the same substance. Although FIG. 16 is an example applied to the fourth embodiment, the same method can be applied to the third embodiment.

Optical coupling parts (56) (62) as described above.
Since the allowable wavelength of SHG conversion is widened by using, it is possible to use an inexpensive laser.

FIG. 17 shows an outline of the device manufacturing process for the fifth embodiment shown in FIG. 16 among the above embodiments. This process is almost the same as the process of the first embodiment shown in FIG. That is, first, Z of the KTP substrate (67)
A Ti thin film (68) is formed on the surface. Next, a photoresist (69) is coated, and the photoresist is exposed and developed by a photomask (70) having a shape of a domain inversion part,
A pattern is formed on the photoresist (71). Further, a pattern (72) is formed on Ti by chemical etching, and Ba
By heat treatment in a mixed solution of (NO ₃ ) ₂ and RbNO ₃ , part of K ions of KTP is replaced with Rb, and a polarization inversion structure (73) is formed. The device is completed by removing the Ti film (74).

As a pattern, the domain inversion portion has a diameter of 3 μm,
A semicircular lens structure having a center interval of 4.5 μm and a diameter of 6 μm only on the incident end face was formed.

FIG. 18A shows an experimental system for measuring the optical coupling efficiency in the example of the present invention. The output light of the wavelength-stabilized semiconductor laser (75) is SH by the lens (76).
Focus on the G element (77). The output light of the SHG element is focused by the lens (78), passes through the optical filter (79) that transmits only the SHG wave but not the fundamental wave, and the photodetector (8
Focus on 0). The SHG element finely moved in the direction perpendicular to the optical axis to adjust the axis shift, and the output light of the SHG element was measured.
As shown in FIG. 18B, in the case of this embodiment (81), the conventional SHG element (82) having no optical coupling structure on the end face is used.
It can be seen that the allowable value of the axis deviation is wider than that of, and the coupling efficiency is improved at the same time.

[0059]

As described above, according to the present invention, the problems of the conventional harmonic generating element having the discrete polarization inversion structure, such as low conversion efficiency and non-convergence of light, are solved. It is possible to obtain a high efficiency harmonic generating element. Further, by combining the harmonic generating element according to the present invention with the semiconductor laser, it is possible to inexpensively obtain a small light source of blue-green wavelength, which has been difficult in the past.

Further, according to the present invention, the problem of high mechanical precision required for optical coupling and the low efficiency of optical coupling, which is present in the conventional harmonic generating device having the waveguide structure, is solved. By combining the element and the semiconductor laser, a highly efficient short wavelength light source in the green-blue region can be easily obtained.

[Brief description of drawings]

FIG. 1 is a structural diagram of a harmonic wave generating element showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a structure of a conventional SHG element having a discrete waveguide structure.

FIG. 3 is a schematic diagram showing a light propagation state of a conventional SHG element having a discrete waveguide structure.

FIG. 4 is a diagram showing a situation in which optical loss occurs in a conventional SHG element having a discrete waveguide structure.

FIG. 5 is a diagram showing a principle of light propagation of the harmonic wave generating element of the present invention.

FIG. 6 is a diagram showing a manufacturing process of an element of an example of the present invention.

FIG. 7A is a diagram of an SHG generator using an SHG element according to an embodiment of the present invention, FIG. 7B is a diagram showing the convergence state of SHG light obtained by a conventional SHG element, and FIG. [Fig. 4] is a diagram showing a convergence state of SHG light obtained by the SHG element of the present embodiment.

FIG. 8 is a diagram showing a second embodiment of the present invention.

FIG. 9 is a diagram showing a conventional waveguide type SHG element.

FIG. 10 is an enlarged view of the vicinity of the incident end face of the conventional waveguide type SHG element.

FIG. 11 is an enlarged view of a light incident end surface of the second embodiment.

FIG. 12 is a diagram showing a case where the beam system of incident light is wider than the width of the waveguide in the second embodiment.

FIG. 13 is a diagram showing a third embodiment.

FIG. 14 is a diagram showing a case where the beam system of incident light is large in the third embodiment.

FIG. 15 is a diagram showing a fourth embodiment.

FIG. 16 is a diagram showing a fifth embodiment.

FIG. 17 is a diagram showing a manufacturing process of the SHG element of the fifth embodiment.

FIG. 18 (a) is a diagram showing an apparatus for measuring SHG conversion efficiency with respect to an axis shift between the focused light and the waveguide in the fifth embodiment, and FIG. 18 (b) is a diagram showing the measurement result.

[Explanation of symbols]

1 polarization inversion part 2 SHG element substrate 3 SHG element incident light 4 SHG output light 5 polarization inversion part 6 SHG element substrate 7 SHG element intensity distribution of propagation light seen from the cross-sectional direction 8 intensity of propagation light seen from the top surface of the SHG element Distribution 9 Polarization inversion part 10 Light emitted by the polarization inversion part 11 Width of the optical waveguide 12 Light emission angle from the optical waveguide 13 Next polarization inversion part in the periodic structure 14 Boundary part of the optical waveguide with the substrate 15 Polarization inversion part 16 Boundary between polarization inversion part and non-inversion part in light propagation direction 17 Boundary between polarization inversion part and non-inversion part in light propagation direction 18 SHG element substrate 19 Light propagating in waveguide structure 20 Waveguide side surface part 21 Waveguide Side part 22 Central part of polarization inversion part 23 Boundary between polarization inversion part and non-inversion part in the light propagation direction in the next element 24 Non-inversion part between polarization inversion parts 25th polarization Central part of inversion part 26 Next polarization inversion part 27 Substrate of SHG element in the embodiment of the present invention 28 X plane of KTP 29 Polarization inversion part in which KTP is ion-exchanged 30 KTP substrate 31 Ti film 32 Photoresist 33 Photomask 34 Development Subsequent photoresist 35 Ti thin film pattern 36 Ion-exchanged element 37 Completed element structure 38 SHG element of the embodiment of the present invention 39 YAG laser light 40 lens 41 lens 42 optical filter 43 screen 44 Convergence in conventional SHG element structure Spot 45 Diffracted and scattered light in the conventional SHG element structure 46 Converging spot in the SHG element structure of the embodiment of the present invention 51 Optical axis of incident light to the conventional SHG element 52 Optical axis of waveguide of the conventional SHG element 53 Optical axis Deviation 54 Optical Waveguide of Second Example 55 Second Example Example phase inversion structure 56 Optical coupling portion of end face portion of second embodiment 57 Optical axis shift of incident light and waveguide 58 Radius of curvature of optical coupling portion 59 Optical axis 60 at exit of optical coupling portion Waveguide and light Distance of coupling portion 61 Diameter of incident light 62 Optical coupling portion of the third embodiment 63 Waveguide portion of the third embodiment 64 Very large incident light diameter of the third embodiment 65 Optical coupling portion of the third embodiment Waveguide interval 66 Multiple lens structures of the fourth embodiment 67 KTP substrate 68 Ti thin film 69 Photoresist film 70 Photomask 71 Photoresist pattern 72 Ti pattern 73 Polarization inversion structure 74 Completed device 75 Semiconductor laser 76 Lens 77 SHG device 78 lens 79 optical filter 80 photodetector 81 change of SHG output with respect to optical axis shift in the case of embodiment 82 SHG output with respect to optical axis shift in the conventional example Change

Claims (9)

[Claims] 1. A harmonic generation element for injecting coherent light into a non-linear optical material and converting the incident light into an n-th harmonic having a wavelength of half or less, wherein: By inverting the direction of the nonlinear polarization of the nonlinear material with the period of the coherence length,
It has a mechanism for pseudo matching the phase of the incident light and the phase of the generated nth harmonic, and utilizes that the refractive index of the inversion part of the periodic nonlinear polarization is different from the refractive index of the non-inversion part. A harmonic generation element having a structure that constitutes a discrete type optical waveguide, wherein the boundary between the inversion part and the non-inversion part of the nonlinear polarization is a substantially curved surface. 2. The method according to claim 1, wherein KTP is used as the nonlinear optical material, and periodic nonlinear polarization is achieved by exchanging a part of K ions in KTP with other metal ions. The harmonic generating element described. 3. When the refractive index of the inversion part of the periodic nonlinear polarization is higher than that of the non-inversion part, the polarization inversion part has a substantially curved surface which is convex with respect to the non-inversion part, or When the refractive index of the non-inverted part of the nonlinear polarization is lower than that of the non-inverted part, the domain-inverted part has a substantially curved surface that is concave with respect to the non-inverted part, and is propagated to the discrete optical waveguide. The harmonic generation element according to claim 1 or 2, wherein the harmonic generation element has a condition that the light is converged. The 4. A length of the optical waveguiding direction of the inverting section of the periodic nonlinear polarization and L _1, the length of the light propagation direction of the non-inverting section and L _2, are made by the substantially curved surface Assuming that the focal length of the lens structure is f, the propagation constant k of light in the nonlinear material, and the width of the inversion part of the nonlinear polarization is D, in the case of L ₁ ≧ L ₂ , 4k ² × D ⁴ / (L ₁ + L ₂ ) ≧ f ≧ L _1/2 or _{L 2/2 ≧ f ≧ L} 1 × L 2 / [2 (L 1 +
L _2)] For ^{_{L 2 ≧ L 1, 4k 2}} × D 4 / (L 1 + L 2) ≧ f ≧ L 2/2 or _{L 1/2 ≧ f ≧ L} 1 × L 2 / [2 (L 1 +
The harmonic generating element according to claim 3, wherein the condition that L ₂ )] is satisfied so that the light propagating through the optical waveguide is converged. 5. A harmonic generating element for injecting coherent light into a non-linear optical material and converting the incident light into an nth harmonic having a wavelength of half or less, wherein the non-linear optical material has an optical waveguide structure. And a structure in which incident light is converted into an nth harmonic inside the optical waveguide, and a lens-like shape for optically coupling the incident light to the optical waveguide is provided on a light incident end face of the harmonic generating element. A harmonic generation element characterized by being provided with a light focusing mechanism. 6. The optical waveguide of the higher harmonic wave generating element does not exist near the light incident end face, and there is a portion having a higher refractive index than the nonlinear optical material near the light incident end face. The harmonic generating element according to claim 5, wherein the light focusing mechanism is configured by having a substantially curved surface whose boundary is convex with respect to the periphery. 7. The phase of the incident light is generated by inverting the direction of nonlinear polarization of the nonlinear material with respect to the traveling direction of the incident light at a cycle of a coherence length in the harmonic generation element. By utilizing the fact that the phase of the n-th harmonic is pseudo-matched and the refractive index of the inversion part of the periodic nonlinear polarization is higher than that of the non-inversion part, a discrete optical waveguide is used. 7. The harmonic generating element according to claim 6, having a structure that constitutes a waveguide. 8. A discrete optical waveguide having a light converging function is formed by forming a boundary between the inversion part and the non-inversion part of the non-linear polarization in the harmonic generation element to be a substantially curved surface. The harmonic generation element according to claim 7. 9. In the harmonic generating element, whether the light focusing mechanism of the light incident end face portion and the optical waveguide replace a part of the nonlinear material from a surface of the nonlinear optical material with another substance. Alternatively, it is simultaneously formed by diffusing another substance as an impurity.
The harmonic generating element described.