JPH05257183A - Secondary higher harmonics generation device - Google Patents

Abstract

PURPOSE:To obtain a spot of secondary higher harmonics light or parallel luminous flux on an optical axis which has maximum intensity and is in focus by stabilizing the oscillation wavelength of a fundamental wave and thus stabilizing the angle of Cherenkov radiation light. CONSTITUTION:A semiconductor laser(LD) 100 which has an incidence and a projection low-reflection end surface and a Cherenkov radiation type 2nd higher harmonic generating element(SHG element) are combined. The light guide 122 of the SHG element is formed in periodic gradient index structure. The LD stably oscillates having the wavelength of the fundamental wave with feedback light from the light guide 122, and the Cherenkov radiation light is generated as the 2nd higher harmonic of the fundamental wave by the SHG element. This radiation light is converted by a rear-stage Axicon lens 130 into parallel luminous flux having an annular light intensity distribution and this luminous flux is converged by a condenser lens 140 to obtain the light spot.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for converting the wavelength of a fundamental wave light source into light of a half wavelength, and more particularly to an optical device for converting light of a semiconductor laser (LD) into light of a half wavelength. A second harmonic generation (SHG) device.

[0002]

2. Description of the Related Art Conventionally, regarding the second harmonic generator,
For example, Reference (I) Japanese Patent Laid-Open No. 61-239231 (II) Proceedings of the 51st Annual Meeting of the Applied Physics Society of Japan 29p
-P-7 p.988 (1990) (III) SPIE Vol.1148, Non-linear optical
Properties of Materials "Nonlinear Optica
l Properties of Materials "(1989) pp.207-212.

First, the second harmonic generation (SHG) element disclosed in Document (I) will be described. 4A is a schematic perspective view of a known Cherenkov radiation type second harmonic generation element, and FIG. 4B is a schematic perspective view of FIG.
FIG. 4A is a cross-sectional view of the SHG element of (A) including an optical waveguide and taken along the optical waveguide. This SHG element 10 is
The structure is such that the optical waveguide 14 is formed on the LiNbO ₃ substrate 12 by the proton exchange method. This optical waveguide 14
When light (hereinafter, simply referred to as a fundamental wave or a fundamental wave) 16 having a fundamental wavelength λ (ω) (where ω is an angular frequency) is made incident and propagates in this waveguide, an angle satisfying the Cherenkov condition is satisfied. A light (hereinafter, simply referred to as a second harmonic (SH wave or SH light)) 18 having a wavelength λ (2ω) that is ½ of the fundamental wavelength in the direction of θ (called Cherenkov angle) 18 is extracted. ((A) and (B) of FIG. 4). The Cherenkov condition is given by the following equation (1), where N (ω) is the effective refractive index of the fundamental wave 16 propagating in the optical waveguide 14 and n (2ω) is the refractive index of the substrate 12 for the SH wave 18.

N (2ω) cos θ = N (ω) (1) The SHG element 10 of this system is characterized in that phase matching can be easily performed, and recently, an excellent element having a very high conversion efficiency was produced. Came to be. But S taken out
The H-wave is a Cherenkov radial luminous flux (a luminous flux emitted in a cone-shaped radiation pattern, which is hereinafter simply referred to as Cherenkov radiation), so that it is difficult to condense with an ordinary lens. The light flux is also called a light flux.

Therefore, in order to collect the Cherenkov radiation, a structure in which a special optical system is combined with an SHG element has been conventionally proposed. Examples of such a structure are disclosed in the above-mentioned documents (II) and (III).

FIG. 5A and FIG. 5B are shown in reference (I).
SHG having the same structure as the SHG element 10 disclosed in US Pat.
It is a perspective view and a sectional view showing roughly the important section of the SHG device using an element. S emitted from this SHG element 10
The H-wave, and therefore Cherenkov radiation, is converted into a plane wave by the grating axicon lens 20, and is condensed as a single point on the screen 22 by the action of a normal lens. In addition, the grating axicon lens 2
Reference numeral 0 denotes a grating type (zone plate type) compound lens having the functions of an axicon lens and an ordinary lens described below. As is well known, the axicon lens is a special lens, and its shape is a combination of a cone having an apex angle α and a cylinder after that, as shown in FIG. It has a curved shape. The axicon lens has a function of collimating Cherenkov radiation (making it a plane wave). Therefore, if an axicon lens has the ability to collimate a light beam with a Cherenkov angle θ ₁ ,
This lens is not capable of collimating a light flux having a different Cherenkov angle θ ₂ . Therefore,
If it is formed as a grating type lens, the function as an axicon lens and the function as a normal lens can be realized by one component. However, even in this case, the tolerance of the light collecting ability with respect to the Cherenkov angle θ does not become so wide (large). The allowance here is the degree of blur of the focused spot with respect to the change of θ. The larger the allowance, the more technically S
Therefore, it becomes easy to collect the H-wave and therefore the Cherenkov radiation. As described above, one of the purposes of shortening the wavelength by using the SHG element is to make light having a large beam diameter into a spot as small as possible or as thin as possible, so that the blur of the beam spot is not increased. Remarkably hinders practical use.

Another problem is that the Cerenkov synchrotron radiation of a perfect shape can be condensed by the axicon lens, and the planar SHG used in FIGS. 4 and 5 is used.
Since the element has the structure in which the optical waveguide 14 is formed on the surface of the substrate 12, only the SH wave 18 of only the lower half of the Cherenkov light (only the substrate side) can be obtained. Therefore, even if θ is appropriately set, The point is that it cannot be focused on a specific spot. This state is shown on the screen 22 shown in FIG. It can be seen that the fundamental wave light 16 is also projected on the screen only in the lower half.

Therefore, a fiber type SHG element 30 as disclosed in Document (III) has been proposed. According to this element 30, SH light (wave) is obtained as Cerenkov radiant light in which light is distributed in the form of a belt-shaped ring in a plane having a Cerenkov angle of θ and orthogonal to the emission direction, and If the apex angle α of the axicon lens is set appropriately, the SH light can be focused on an ideal spot.
Hereinafter, this content will be described with reference to FIG.

FIG. 6 is a schematic diagram showing a main part of a conventional SHG device for obtaining a spot-shaped convergent light. The optical fiber type SHG element 30 is composed of a core 32a and a clad 32b, and the core 32a is made of flint glass S.
F4 and the clad 32b are formed of 4- (N, N-dimethylamino) -3-acetamidonitrobenzene (4- (N, N-dimethylamino) -3-ac, which is an organic nonlinear optical crystal.
It is formed as a single crystal fiber by using it as a material of etamidonitrobenzene (DAN). Core 32a
The refractive index of the fundamental wave 16 of the clad 32 is N (ω).
The refractive index of b for SH wave (SHG light) 18 is n (2
ω), the SH light 18 is extracted as Cerenkov radiant light with θ half the apex angle, which satisfies the above-described expression (1). This SH light 18 is the former planar type SH light.
Unlike the Cherenkov light from the G element, it has a circular band shape with no missing portion. Therefore, as shown in FIG. 6, it is possible to focus the light on an ideal spot using the axicon lens 34 (apex angle α). This point will be described in more detail.

The fundamental wave 16 is introduced from one end of the fiber type SHG element 30, and the SH light 18 and the fundamental wave 16 are emitted from the other end. Of these, the SH wave 18 is the axicon lens 34.
Thus, the Cherenkov radiation is converted into a plane wave (parallel light), and then is condensed in a spot shape on the objective lens 36.
Assuming that the refractive index of the material of the axicon lens 34 with respect to SH waves is n _A (2ω), if the relation between the angle θ and α satisfies the following equation (2), the Cherenkov radiant light as shown in the figure shows It is converted into parallel light by the con-lens 34.

COS [(α / 2) −θ)] = n _A · COS (α / 2) (2) That is, COS [(α / 2) −COS ⁻¹ {N (ω) / n (2ω)} ] = N _A (2ω) · COS (α / 2) (3) The Cerenkov light is focused on an ideal spot.
Only when the expression (3) is satisfied. Therefore,
If the wavelength of the fundamental wave, that is, the angular frequency ω is different, the focal point will be blurred. This is an extremely large obstacle in practical use. In particular, it becomes an obstacle in applications where a semiconductor laser (LD) is used as a fundamental wave light source. Usually, the apex angle α is predetermined and an axicon lens is formed. Therefore, it is not easy to find a semiconductor laser (LD) that oscillates at a wavelength that satisfies the expressions (2) to (3) for this lens.
Moreover, since the oscillation wavelength of the LD fluctuates depending on the injection current value and the ambient temperature, in order to be able to cope with this fluctuation, conventionally, all axicon lenses suitable for the expected wavelength of the SH wave have been prepared. It is necessary to replace the axicon lens every time the wavelength changes, but this kind of procedure is virtually impossible, and individual axicon lenses for all wavelengths must be prepared. Is not realistic.

Usually, the use of a short wavelength light source using an SHG element is used for converting the second harmonic light into a parallel light flux having the maximum light intensity on the optical axis, or collecting the second harmonic light at one point. This is to make it glow. As shown in FIG. 6, the SH light flux is obtained as a parallel light flux behind the axicon lens, but as already explained, it is a cylindrical light flux with the central portion missing, that is, the light intensity on the optical axis is 0. is there. However, in industrial applications, the so-called Gaussian beam, which has the maximum light intensity on the optical axis, is the most versatile. Also, the light flux obtained behind the axicon lens is a parallel light flux due to the diffraction effect of light, but the radius of the light flux gradually spreads and propagates.

By the way, reference (IV) “Physical Review Letters (PHYSICAL REVIEW)
LETTERS), Vol. 58, No. 15 (19
87), pp. 1499-1501 ", a parallel light flux having a uniform light intensity distribution in a cross section perpendicular to the optical axis is combined with an annular slit and a convex lens to obtain a non-diffraction mode, and A technique for obtaining a bundle of parallel rays having a radius of about one wavelength is disclosed. This combined structure is shown in FIG. In the figure, it is a 40-wave slit plate, which is usually a transparent material for the used light,
For example, a glass plate is provided with an opaque light-shielding film so that light can be transmitted only through the slit 40a. The slit 40a is formed in an annular shape, its average radius is d, and the slit width is Δd. The slit plate 40 is provided so as to be perpendicular to the optical axis such that the center of the diameter of the annular slit 42a is located on the optical axis separated by the focal length f of the convex lens 42.
According to this document, the range in which a non-diffraction mode minute-diameter parallel light flux is obtained is within the range of the longest propagation distance Z _MAX from the convex lens. However, the light intensity of the obtained parallel beam bundle in the non-diffraction mode is 1/100 of the light intensity of the incident light L.
It is not practical because it becomes the following.

Therefore, in the structure shown in FIG. 6, instead of using the objective lens 36, if a combination structure of a ring-shaped slit 40 and a convex lens 42 is used,
For the time being, it is considered possible to obtain a bundle of parallel rays of small diameter with a high light intensity. The structure is shown in FIG. But,
In this case, the SHG element is an optical fiber type SHG.
Since the element 30 is used, even if a semiconductor laser (LD) is used as a light source, the oscillation wavelength cannot be stabilized, so that the Cherenkov angle is not constant and the parallel light flux that has passed through the axicon lens 34. Always slit 42
It is difficult to match a, and therefore the configuration shown in FIG. 8 is not practical.

[0015]

As described above, the conventional SHG technique based on the Cherenkov radial phase matching method has the following various problems to be solved in order to put it into practical use.

In the case of a conventional SHG element using a Cherenkov type phase matching using a planar type optical waveguide, since a part of the Cherenkov radiated light is cut off, it cannot be focused at one point in principle.

In the case of an SHG element by Cherenkov type phase matching using a fiber type optical waveguide, cylindrical Cherenkov light in which light is distributed in a circular band (ring) after passing through an axicon lens is obtained. However, only Cerenkov radiation with an emission angle determined by the shape of the axicon lens, which is one of the components required for light collection, can be collected, that is, the conditions under which light can be collected are extremely narrow, which makes it extremely difficult to use in applications.

The conventional Cherenkov radiation type SHG
The SH light flux obtained from the device is a cylindrical parallel light flux having no light intensity on the optical axis, and the radius of the light flux gradually expands and increases due to the diffraction effect, which causes a practical problem. There is a fear.

Even if a conventional fiber type SHG element is used to obtain a parallel light flux in the non-diffraction mode, a stable parallel light flux in the non-diffraction mode cannot always be obtained. poor.

An object of the present invention is to stabilize the oscillation wavelength and, moreover, make it possible to focus light on one point of the second harmonic light easily, or to make parallel rays of the second harmonic light having the maximum intensity on the optical axis. An object of the present invention is to provide an SHG device capable of stably obtaining a bundle.

[0021]

In order to achieve this object, according to the second harmonic generation device of the present invention, (a) a semiconductor laser having both end surfaces formed as low reflection surfaces, and (b)
Cherenkov radiation type second harmonic generation (SHG) element for converting the fundamental wave light from this semiconductor laser into second harmonic light, and (c) the light emitted from this semiconductor laser is focused on the optical waveguide of the SHG element. And an optical system for inputting the feedback light from the optical waveguide to the semiconductor laser,
(D) A second harmonic generation device including an axicon lens for collimating Cherenkov radiation as second harmonic light emitted from this SHG element,
(E) This optical waveguide is a nonlinear optical waveguide formed as a periodic refractive index distribution structure, and (f) this SHG element is
The outer periphery of the optical waveguide other than the input / output end faces is formed of a material layer transparent to the second harmonic light.

In practicing the present invention, it is preferable that the periodic refractive index distribution structure should be firstly arranged with respect to the fundamental wave.
When the second refractive index regions are alternately arranged periodically and formed, and the period is Λ, it is preferable that Λ satisfy the following condition.

Λ = Pλ (ω) / 2N (ω) where COSθ = N (ω) / n (2ω) and COS [(α / 2) −θ] = n _A (2ω) COS (α / 2), P is a positive integer, λ (ω) is the wavelength of the fundamental wave, N (ω) is the effective refractive index of the optical waveguide with respect to the fundamental wave, and n (ω) is the above The refractive index of the material layer with respect to the second harmonic light, n _A (2ω) is the refractive index of the axicon lens described above with respect to the second harmonic light, and α is the apex angle of the axicon lens described above.

Further, in implementing the present invention, it is preferable that an objective lens is provided after the axicon lens described above.

Further, a ring-shaped slit for passing the Cherenkov radiated light made into the above-mentioned parallel light flux is provided in the rear stage of the axicon lens, and a lens which is separated from the surface including the slit by the focal length in the rear stage of this slit. It is preferable that the configuration includes and.

A Fresnel zone plate may be used instead of the axicon lens.

[0027]

According to the second harmonic generator of the present invention, the SH
The optical waveguide of the G element is formed by at least one material layer that is transparent to the second harmonic light on the outer periphery except the input / output end faces, so that there is no portion lacking the SH light that is generated. It can be extracted as Cherenkov radiation in the form of a ring. Therefore, the SH light generated from the SHG element is always converted by the axicon lens into a cylindrical parallel ray bundle in which the light intensity distribution in the cross section in the plane orthogonal to the optical axis is an annular distribution. You can do it. By condensing this parallel light flux using an appropriate objective lens, it can be condensed as a spot having substantially no blur.

Further, according to the above-mentioned structure, since the optical waveguide is structured as a periodic refractive index distribution structure, the fundamental wave from the semiconductor laser (LD) is
A fundamental wave matching the phase matching condition is returned to this LD by the Bragg reflection. Since both end faces of the LD are coated with low reflectance, the threshold current is large.
Therefore, if the period of the periodic refractive index structure is designed to match the SH light condensing condition, the LD automatically and stably oscillates at the oscillation wavelength as designed. As a result, since SH light as Cherenkov radiation is emitted from the SHG element at a stable Cherenkov angle θ, if the oscillation of the fundamental wave is determined in advance according to the design, the apex angle α of the axicon lens is also this angle. It can be set in correspondence with θ, and therefore a stable parallel beam bundle of Cherenkov radiation is
Always get. Therefore, when the parallel light flux is condensed by the objective lens, it can be condensed as a minute spot with a large light intensity without blurring.

Further, by using a lens having an annular slit in the subsequent stage of the axicon lens and arranging this slit on the focal plane, the light beam is in the non-diffraction mode and the maximum light is on the optical axis. A parallel ray bundle having strength is always obtained stably.

[0030]

Embodiments of the present invention will be described below with reference to the drawings. It should be noted that the drawings only schematically show the shapes, sizes, and arrangement relationships of the respective constituent components to the extent that the present invention can be understood.

FIG. 1 is a block diagram showing an embodiment of the second harmonic generation device of the present invention. 2A and 2B are a perspective view and a cross-sectional view showing one embodiment of the structure of the SHG element used as one component of the present invention.
FIG. 3 is a block diagram showing another embodiment of the present invention.

The second harmonic generation device (SHG device) of the present invention comprises a semiconductor laser 100 as a light source and an optical system 1.
10 and the Cherenkov radiation type second harmonic generation element (SH
The structure has at least a G element) 120 and an axicon lens 130 (FIG. 1).

Next, each component of this SHG device will be described, but the details of the components already described will be omitted unless there is a particular need. First, the semiconductor laser (LD) 100 used in the present invention
Has both end surfaces formed as low reflection surfaces. By making both end surfaces of the semiconductor laser 100 low reflective surfaces (AR surfaces), the threshold current can be made sufficiently larger than that of a normal semiconductor laser. Therefore, the feedback light from the SHG element 120, which will be described later, oscillates with the fundamental wave L _F exceeding the threshold value. This low reflection surface is the semiconductor laser 100.
It suffices to coat a low reflection film on the input / output end faces of the above, and this coating technique is conventionally used.

The optical system 110 includes the semiconductor laser 100 and S
Since it is a means for optically coupling with the HG element 120, the emitted light from the semiconductor laser 100 is condensed on the optical waveguide 122 of the SHG element 120 and the optical waveguide 1
It suffices that the feedback light from 22 is input to the semiconductor laser 100. In this embodiment, a laser beam lens 112, an anamorphic prism pair 114, and a condenser lens 116 are arranged from the semiconductor laser 100 side. Of course, an optical fiber or other optical means (including an air space and / or a vacuum space) or the like may be used to optically couple the both 100 and 120.

Cherenkov radiation type second harmonic generation (SH
The G) element 122 is an element that converts the fundamental wave light from the semiconductor laser 100 into the second harmonic light. This SHG
The element will be described with reference to FIGS. 2A and 2B.

The SHG element 120 includes an optical waveguide 122.
And a material layer 124 surrounding the outer periphery other than the input / output end faces and transparent to the second harmonic light ((A) in FIG. 2). In this embodiment, the SHG element 120 has a structure in which a cladding 128 is provided on the upper surface of a conventionally known planar type SHG element composed of a substrate 126 and a nonlinear optical waveguide 122 provided thereon. Then, this optical waveguide 122 is used as a non-linear optical waveguide formed as a periodic refractive index distribution structure, and a conventionally known proton exchange method (Li-
H ⁺ exchange method) is used to form the substrate 126.

Although not shown as a periodic refractive index distribution structure in FIG. 2A, this optical waveguide 122 is constituted by first and second refractive index regions 122a and 122b. In this embodiment, for example, as shown in FIG. 2B, two high refractive index portions 122a and low refractive index portions 122b are provided.
It is configured by alternately arranging two portions having different refractive indexes periodically along the waveguide direction. In this structure, once the original shape of the optical waveguide is formed on the substrate 126 by the material of the low refractive index portion 122b by using the ion exchange method, and then the ion exchange method is used to increase the height of the optical waveguide with a constant period and a constant width. Refractive index portion 122a
Just make it.

Next, on the entire surface of the substrate 126 on which the optical waveguide 122 is formed, the clad 128 is made of the same material as the substrate.
To provide. The reason that the substrate and the clad are made of the same material is that the Cherenkov angles are the same on the substrate side and the clad side. It is preferable to use, for example, LiNbO ₃ which is transparent to the second harmonic as the material. Therefore, in this embodiment, the material layer 124 comprises a substrate 126 and a cladding 128 which is in optical contact with the substrate and the optical waveguide.

The difference in refractive index between the high refractive index portion 122a and the low refractive index portion 122b is preferably small. By doing so, the emission modes of the Cherenkov radiation can be made substantially the same. If the modes of Cherenkov synchrotron radiation are different,
The SH light extracted from the SHG element 120 becomes a mode in which light cannot be collected by the axicon lens (130 in FIG. 1) in the subsequent stage, resulting in an SHG device with poor practicality.

Next, conditions to be satisfied by the SHG element 120 will be examined. The SHG element is a semiconductor laser 10
It is necessary to emit the feedback light for stably oscillating 0 with the fundamental wave and also to generate the second harmonic at a constant Cherenkov angle θ. Therefore, the optical waveguide 122 of the SHG element 120 needs to be configured to cause Bragg reflection.

Now, let one period of the periodic refractive index structure be Λ. At this time, it is preferable that Λ satisfy the following condition.

Λ = P · λ (ω) / 2N (ω) (5) where COSθ = N (ω) / n (2ω) (6) and COS [(α / 2) −θ] = n _A ( 2ω) · COS (α / 2) (7). Here, P is a positive integer given by the order of Bragg reflection, λ (ω) is the wavelength of the fundamental wave, N (ω) is the effective refractive index of the optical waveguide 122 for this fundamental wave, and n (ω) is the material. Refractive index of layer 124 for second harmonic light, n
_A (2ω) is the refractive index of the axicon lens 130 with respect to the second harmonic light, and α is the apex angle of this axicon lens. The equation (6) corresponds to the equation (1),
Expression (7) corresponds to expression (2). Further, the effective refractive index is given by the arithmetic mean value of the refractive indexes of the high and low refractive index portions, but the refractive index values of both refractive index portions are very close to each other, so that either refractive index portion The refractive index may be approximately given.

This condition will be described below. The optical refractive index portion 122a and the low refractive index portion 122b of the optical waveguide 122
Let Na (ω) and Nb (ω) be the refractive indices of the fundamental wave L _F , respectively. Further, the respective refractive indices for the SH wave L _S are n _SA (2ω) and n _SB (2ω).
At this time, the shift angle Δθ between the Cerenkov radiation angles θa and θb in both refractive index portions 122a and 122b is given by the following equation.

Δθ = COS ⁻¹ [Na (ω) / n _SA (2ω)] −COS ⁻¹ [Nb (ω) / n _SB (2ω)] (8) In this equation, Δθ is zero (0 It is ideal that the refractive index differences [Na (ω) -Nb (ω)] and [n _SA (2ω) -n _SB (2ω)] are small. However, since the effect of the present invention cannot be achieved by setting the difference in refractive index to zero, it is necessary to evaluate how long the difference in refractive index should be reduced.

Therefore, in the structure shown in FIG. 2B, the length D of the SHG element 120 in the waveguide direction will be 5 mm. When the wavelength λ (ω) of the fundamental wave L _F is 830 nm, the period Λ of the refractive index periodic structure is Na (ω) = 2.
It becomes 172. Assuming that the periodic refractive index distribution structure causes Bragg reflection of the 9th order, P is 9, and in that case, the period Λ given by the equation (5) is Λ = 9 × 0.83.
/(2×2.172)=1.72. Now, D = 5
Since it is assumed to be mm, this SHG element 120 has a Bragg reflection grating of 2900 cycles.

Next, suppose that [Na (ω) -Nb (ω)] =
If it is 0.001, [n _SA (2ω) −n _SB (2ω)]
Also becomes about 0.001, and the Bragg reflectance R is calculated to be approximately 94.3% by using the well-known equation (9), and it can be understood that the Bragg reflectance R has a sufficient reflectance. Note that n ₁ and n ₂ are the refractive indices of air in contact with the input / output end faces of the optical waveguide, and r is the number of repetitions of the period Λ, here 2900.

R = {[A] / [B]} ² [A] = 1− (Na (ω) / n ₁ ) × (Na (ω) / n ₂ ) × (Na (ω) / Nb (ω )) ^2r [B] = 1 + (Na (ω) / n ₁ ) × (Na (ω) / n ₂ ) × (Na (ω) / Nb (ω)) ^2r ... (9) Na (ω) = 2.173, Nb (ω) = 2.172
When equation (9) is calculated as, R = 0.943.

On the other hand, the Cerenkov radiation angle θa in this case
The deviations of θb and θb are θa = COS ⁻ , where n _SA = 2.312 and n _SB = 2.311 are the refractive indices of the high and low refractive index portions 122a and 122b for the second harmonic wavelength of 415 nm, respectively. ¹ [Na (ω) / n _SA (2ω)] = 19.9688 ° θb = COS ⁻¹ [Nb (ω) / n _SB (2ω)] = 19.9688 °. Therefore, from the formula (8), Δθ = 0.0043 °, which is an extremely small value. Those skilled in the art can understand that this value is a sufficiently small value as compared with the case of the SHG device having the conventional structure when the semiconductor laser is not stably oscillated. In this way, the SH wave L _S1 radiated with a small deviation angle Δθ of about 4 ° to 5 ° of about 1000 is not affected by the axicon lens 130 provided in the subsequent stage.
By condensing, a parallel light flux L _S2 having a donut-shaped light intensity distribution can be emitted (FIG. 1). Further, since the optical waveguide 122 that gives such a difference in refractive index can be formed by the current ion exchange technology, it is possible to form a Cherenkov radiating SHG element for practical use.

As described above, in the configuration example shown in FIG. 1, both end faces of the semiconductor laser 100 are AR-coated to set the oscillation threshold value sufficiently large. Since the optical waveguide 122 having the periodic refractive index distribution structure of the SHG element 120 can be configured so that the Bragg reflection satisfying the expression (5) is generated, the semiconductor laser 100 automatically calculates in the SHG element 122 by the expression ( Since the semiconductor laser 100 receives the feedback of the light satisfying the condition 5), once the oscillation starts, the fundamental wave automatically oscillates stably at a constant wavelength, and the oscillation wavelength depends on the ambient temperature and the injection current. Almost unaffected by changes in value. Therefore, at least equations (5) and (6)
If the SHG element is configured so as to satisfy (7) and (7), the stable oscillation fundamental wave is efficiently converted into Cherenkov radiation as the second harmonic, and this Cherenkov radiation is converted into It can be understood that the emission angles can also be emitted within a range that can be considered to be substantially the same.

In the embodiment shown in FIG. 1, as a preferred example,
As a part of the configuration of the SHG device of the present invention, an objective lens 140 is provided after the axicon lens 130. According to this configuration, the cylindrical parallel light flux L _S2 from the axicon lens 130 can be efficiently focused by the objective lens 140 at one point to obtain the spot-shaped focus point 150.

FIG. 3 is a diagram showing the configuration of another preferred embodiment of the present invention. According to this embodiment, in the subsequent stage of the axicon lens 130, a ring-shaped slit 160a for passing the Cherenkov radiation light (second harmonic light: SH wave) L _S2 formed into a parallel light flux, and in the subsequent stage of this slit 160a. In addition, the lens 162 is separated from the surface including the slit (the surface of the slit plate 160) by the focal length f. In this embodiment, the optical system 110 is simply an air gap. Further, the slit plate 160 can be formed in the same manner as the slit plate described in FIG. The lens 162 and the slit plate 160 may be combined in the same manner as in the case of FIG. The other components are the same as the components shown in FIG. 1, so detailed description thereof will be omitted.

According to the configuration of the SHG device shown in FIG. 3, the SH light L formed into a parallel light flux by the axicon lens 130 and having a light intensity distributed annularly in a plane orthogonal to the optical axis.
_It is possible to efficiently collect _S2 and form a parallel beam bundle 164 of a non-diffraction mode and a small diameter on the optical axis behind the lens 162 within the range from the lens 162 to the longest propagation distance. This longest propagation distance is usually several tens of cm, so there is no problem in practical use. Further, according to this configuration, since the slit 160a is provided only in the region where the parallel SH light L _S2 from the axicon lens 130 substantially exists, the light intensity of the SH light is maximum. Will pass through the slit 160a. Therefore, the light intensity of the parallel light bundle 164 condensed by the slit 160 and the lens 162 is not substantially weakened, unlike the case of the conventional configuration. For this reason,
It is possible to obtain a bundle of parallel rays having maximum light intensity without causing blurring on the optical axis. The SH of the SHG element 120
When the distance S from the end face on the exit side of the wave L _S1 to the axicon lens 130 changes, the diameter between the centers of the parallel ray bundle L _S2 also changes, so the diameter d of this slit 160a also changes to this distance S. Corresponding and can be preset.

It is clear that the invention is not limited to the embodiments described above, but that many variants or modifications can be made. For example, in the above-described embodiment, S
As the HG element, the clad is provided on the substrate on which the optical waveguide is formed, but any structure may be used as long as it has a material layer transparent to the same second harmonic around the optical waveguide. .. Therefore, for example, a fiber-shaped Cherenkov radiation type SHG element in which an optical waveguide having a periodic refractive index structure is formed may be used.

L is used as a material for forming the SHG element.
iNbO ₃ was used, but instead of using it, LiT
It can also be formed by using aO _3, KTP (KTiOPO ₄ ), or other nonlinear optical material.

[0055]

As is apparent from the above description, the second harmonic generation device of the present invention has the following advantages.

Semiconductor laser and Cherenkov radiation type SH
Since the G element is combined, the device can be downsized.

Further, since the optical waveguide of the SHG element has a periodic refractive index structure and the input / output end faces of the semiconductor laser are low-reflecting surfaces, they are not affected by the driving current value of the semiconductor laser or the ambient temperature. , It is possible to oscillate stably at the designed fundamental wave oscillation wavelength.

Further, since the SHG element is surrounded by the layer made of the same material on the outer periphery of the optical waveguide, the refractive index for the second harmonic of this layer is made uniform, and therefore the Cherenkov is formed. The emitted light is always emitted at a constant emission angle over 360 ° around the light guide.
Therefore, if the wavelength of the fundamental wave is determined, the second harmonic will be SH
From the G element to a ring shape automatically at a constant radiation angle, and
Since it radiates stably, a parallel ray bundle can always be obtained by the axicon lens in the latter stage. Therefore, it suffices to prepare an axicon lens having an apex angle corresponding to the wavelength of the fundamental wave according to the design, and waste can be saved.

As a result of the above items (1) to (4), by arranging the condenser lens at the rear stage of the axicon lens, it is possible to condense the second harmonic wave as a spot having a large light intensity and a fine diameter without blurring. Further, by arranging the combination structure of the slit and the objective lens in the subsequent stage of the axicon lens, it is possible to obtain a parallel light flux having a large light intensity on the optical axis, no blur, and a fine diameter.

[Brief description of drawings]

FIG. 1 is a second harmonic generation device (SHG device) of the present invention.
2 is a schematic configuration diagram for explaining the first embodiment of FIG.

FIG. 2A is an S used in the SHG device of the present invention.
It is a perspective view showing roughly one example of a HG element,
(B) is a cross-sectional view of FIG.

FIG. 3 is a schematic configuration diagram for explaining a second embodiment of the SHG device of the present invention.

FIG. 4A is a schematic perspective view of a conventional Cherenkov radiation type second harmonic generation element, and FIG. 4B is a diagram of FIG.
FIG.

5A is a perspective view schematically showing a main part of a conventional SHG device, and FIG. 5B is a cross-sectional view of FIG.

FIG. 6 is a conventional SHG for obtaining spot-shaped convergent light.
It is a block diagram which shows a device roughly.

FIG. 7 is a configuration diagram showing a conventional combined structure of a slit and an objective lens for converting a parallel ray bundle in which the light intensity is uniformly distributed into a parallel ray bundle on the optical axis.

FIG. 8: Conventional SHG for obtaining a bundle of parallel rays on the optical axis
It is a block diagram which shows a device roughly.

[Explanation of symbols]

100: Semiconductor laser (LD), 110: Optical system 112: Rolimate lens 114: Anamorphic prism pair, 116: Condensing lens 120: SHG element, 122: Optical waveguide 122a: High refractive index part, 122b:
Low refractive index portion 124: Material layer, 126: Substrate 128: Clad, 130: Axicon lens 140: Condensing lens, 150: Spot 160 slit plate, 160a:
Slit 162: Objective lens, 164:
Parallel light bundle (on the optical axis) L _F: fundamental light, L _S2: Cerenkov radiation light L _S2 :( light intensity distribution of annular) Cerenkov radiation light.

Claims (5)

[Claims] 1. A semiconductor laser having (a) both end surfaces formed as low reflection surfaces, and (b) Cherenkov radiation type second harmonic generation (SH) for converting fundamental wave light from the semiconductor laser into second harmonic light.
G) element, (c) an optical system for converging the light emitted from the semiconductor laser on the optical waveguide of the SHG element and inputting the return light from the optical waveguide to the semiconductor laser, (d) A second harmonic generation device including an axicon lens for collimating Cherenkov radiation as second harmonic light emitted from the SHG element, wherein: (e) the optical waveguide has a periodic refractive index. A nonlinear optical waveguide formed as a distributed structure, and (f) the SHG element is formed of a material layer transparent to the second harmonic light on the outer periphery of the optical waveguide except the input / output end faces. And a second harmonic generation device. 2. The second harmonic generation device according to claim 1, wherein the periodic refractive index distribution structure has a first refractive index distribution with respect to the fundamental wave.
And the second refractive index regions are alternately arranged periodically, and when the period is Λ, the Λ satisfies the following condition. Λ = P · λ (ω) / 2N (ω) where COSθ = N (ω) / n (2ω) and COS [(α / 2) −θ] = n _A (2ω) · COS (α / 2) Where P is a positive integer, λ (ω) is the wavelength of the fundamental wave, N (ω) is the effective refractive index of the optical waveguide with respect to the fundamental wave, and n (ω) is the second layer of the material layer. Refractive index for higher harmonic light, n _A (2ω) is the refractive index of the axicon lens for the second higher harmonic light, and α is the apex angle of the axicon lens. 3. The second harmonic generation device according to claim 1, further comprising an objective lens provided at a stage subsequent to the axicon lens. 4. The second harmonic wave generating device according to claim 1, wherein a ring-shaped slit for passing the Cherenkov radiation in the bundle of parallel rays and a post-stage of the slit are provided after the axicon lens. A lens separated from the surface including the slit by a focal length.
Harmonic generator. 5. A second harmonic generation device using a Fresnel zone plate instead of the axicon lens according to claim 1.