JP2843196B2 - Second harmonic generator - Google Patents

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 to light having a wavelength of 1/2, and more particularly to a device for converting light of a semiconductor laser (LD) into light having a wavelength of 1/2. The present invention relates to a two-harmonic generation (SHG) device.

[0002]

2. Description of the Related Art Conventionally, regarding a second harmonic generator,
For example, References (I) JP-A-61-239231 (II) Proceedings of the 51st Annual Meeting of the Japan Society of Applied Physics 29p
-P-7 p.988 (1990) (III) SPIE Vol.1148, Nonlinear Optical
Properties of Materials "Nonlinear Optica
l Properties of Materials "(1989) pp. 207-212.

[0003] First, a second harmonic generation (SHG) element disclosed in Document (I) will be described. FIG. 4A is a schematic perspective view of a known Cherenkov radiation type second harmonic generation element, and FIG.
FIG. 4A is a cross-sectional view of the SHG element of FIG. This SHG element 10
An optical waveguide 14 is formed on a LiNbO ₃ substrate 12 by a proton exchange method. This optical waveguide 14
When light 16 having a fundamental wavelength λ (ω) (where ω is an angular frequency) (hereinafter simply referred to as a fundamental wave light or a fundamental wave) 16 is made to propagate through the waveguide, an angle satisfying the Cerenkov condition is obtained. Light 18 having a wavelength λ (2ω) that is Ｓ of the fundamental wavelength in the direction of θ (referred to as Cherenkov angle) (hereinafter, simply referred to as a second harmonic (SH wave or SH light)) 18 is extracted. ((A) and (B) of FIG. 4). The Cherenkov condition is given by the following equation (1), where the effective refractive index of the fundamental wave 16 propagating through the optical waveguide 14 is N (ω), and the refractive index of the substrate 12 with respect to the SH wave 18 is n (2ω).

N (2ω) cos θ = N (ω) (1) The SHG element 10 of this method has a feature in that phase matching can be easily achieved, and recently, an excellent element having extremely high conversion efficiency has been manufactured. Is now available. But S taken out
Since the H wave is a Cherenkov radiant light beam (a light beam emitted in a cone-shaped radiation pattern, hereinafter simply referred to as Cherenkov radiated light), it is difficult to collect light with a normal lens. The light beam is also called a light beam.

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

FIGS. 5 (A) and 5 (B) show documents (I)
Having the same structure as the SHG element 10 disclosed in
It is the perspective view and sectional drawing which show the principal part of the SHG apparatus using an element schematically. S emitted from this SHG element 10
The H wave, and thus the Cherenkov radiation, is converted into a plane wave by the grating axicon lens 20 and is condensed as substantially one point on the screen 22 by the action of a normal lens. The grating axicon lens 2
Reference numeral 0 denotes a grating type (zone plate type) compound lens having the functions of an axicon lens described below and a normal lens. As is well known, the axicon lens is a special lens, and its shape is formed by combining a cone with a vertical angle α and a cylinder as shown in FIG. Shape. The axicon lens has a function of collimating the Cherenkov radiation (making it a plane wave). Therefore, if there is capacity in axicon lens collimates against Cherenkov angle theta ₁ of the light beam,
This lens does not have the ability to collimate light beams having a different Cherenkov angle θ ₂ . Therefore,
When 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 is not so wide (large). Here, the tolerance is a degree of the degree of blurring of the condensed spot with respect to a change in θ. The higher the tolerance, technically the S
The H wave and thus the Cherenkov radiation light can be easily collected. As described above, one of the objectives of shortening the wavelength by using the SHG element is to convert light having a large beam diameter into a spot as small as possible or light as narrow as possible. It causes a serious problem in practical use.

Another problem is that the complete form of the Cherenkov radiation can be focused by the axicon lens, and the planar SHG used in FIGS.
Since the element has a 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 on the substrate side) can be obtained. Therefore, even if θ is appropriately set, it is ideal. Is that it cannot be focused on a typical spot. This state is shown on the screen 22 shown in FIG. It can be seen that only the lower half of the fundamental wave light 16 is projected on the screen.

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

FIG. 6 is a configuration diagram schematically showing a main part of a conventional SHG device for obtaining spot-shaped convergent light. The optical fiber type SHG element 30 includes a core 32a and a clad 32b, and the core 32a is formed of flint glass S
The F4 and the cladding 32b are made of an organic nonlinear optical crystal of 4- (N, N-dimethylamino) -3-acetamidonitrobenzene (4- (N, N-dimethylamino) -3-ac
Etamidonitrobenzene (DAN)) is used as a material to form a single crystal fiber. Core 32a
The refractive index for the fundamental wave 16 is N (ω), and the cladding 32
b with respect to the SH wave (SHG light) 18 is represented by n (2
ω), the SH light 18 is extracted as Cherenkov radiation light in which θ that satisfies the expression (1) is half the vertex angle. The SH light 18 is the former planar type SH light.
Unlike the Cherenkov light from the G element, it has a circular band shape without any missing portions. For this reason, as shown in FIG. 6, light can be condensed to an ideal spot using the axicon lens 34 (vertical angle α). This 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. The SH wave 18 is an axicon lens 34
After this, the Cherenkov radiation is converted into a plane wave (parallel light), and then focused on the objective lens 36 in the form of a spot.
Assuming that the refractive index of the material of the axicon lens 34 with respect to the SH wave is n _A (2ω), if the relationship between the angles θ and α satisfies the following expression (2), the Cerenkov radiation light as shown in FIG. The light is converted into parallel light by the conlens 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 Cherenkov light is focused on an ideal spot
Only when this equation (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 a very serious obstacle in practical use. In particular, it becomes an obstacle in applications using a semiconductor laser (LD) as a fundamental light source. Usually, an axicon lens is formed with the apex angle α determined in advance. For this reason, it is not easy to find a semiconductor laser (LD) that oscillates at a wavelength satisfying Expressions (2) and (3) for this lens.
In addition, 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 SH wave wavelength are prepared. In addition, it is necessary to replace the axicon lens every time the wavelength changes, but such a procedure is practically impossible, and it is necessary to prepare individual axicon lenses for all wavelengths. Is also not realistic.

Normally, a short wavelength light source using an SHG element is used to convert the second harmonic light into a parallel light beam having the maximum light intensity on the optical axis, or to collect the second harmonic light at one point. This is to make it light. As shown in FIG. 6, the SH light beam is obtained as a parallel light beam behind the axicon lens. As described above, the SH light beam is a cylindrical light beam with a central portion where the light intensity on the optical axis is 0 is omitted. is there. However, in industrial applications, a so-called Gaussian beam having the maximum light intensity on the optical axis is most widely used. In addition, the light beam obtained behind the axicon lens propagates while gradually increasing the radius of the light beam even though it is a parallel light beam due to the diffraction effect of light.

By the way, the reference (IV) “Physical Review Letters (PHYSICAL REVIEW)
LETTERS), Vol. 58, No. 15 (19
87), p. 1499-1501 ", by combining an annular slit and a convex lens with respect to a parallel light beam having a uniform light intensity distribution in a cross section perpendicular to the optical axis, a diffraction-free mode is achieved, and A technique has been disclosed in which a parallel light beam having a radius of about one wavelength is obtained. FIG. 7 shows this combination structure. In the figure, a 40-wave slit plate is used.
For example, a glass plate is provided with an opaque light-shielding film so that light can pass only through the slit 40a. The slit 40a is formed in an annular shape, the average radius is d, and the slit width is Δd. The slit plate 40 is provided on the optical axis separated by the focal length f of the convex lens 42 so as to be perpendicular to the optical axis so that the center of the diameter of the annular slit 42a is located.
According to this document, that the scope of bundle of parallel rays of very small diameter of no diffraction mode is obtained, in the range of the longest propagation distance Z _MAX from the convex lens. However, the light intensity of the obtained non-diffracting mode parallel light beam is 1/100 of the light intensity of the incident light L.
The following is not practical.

Therefore, instead of using the objective lens 36 in the configuration shown in FIG. 6, if a combined structure of an annular (ring) -shaped slit 40 and a convex lens 42 is used,
It is considered that it is possible to obtain a parallel light beam having a small diameter and a large light intensity. FIG. 8 shows the configuration. But,
In this case, the optical fiber type SHG is used as the SHG element.
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 beam passing through the axicon lens 34 is not used. , The slit 40
It is difficult to make a match with a, and therefore, the configuration shown in FIG. 8 is not practical.

[0015]

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

In the case of a conventional SHG element using a planar optical waveguide and a Cerenkov phase matching, a part of the Cherenkov radiation light is lost, so that it cannot be focused on one point in principle.

In the case of a SHG element based on a Cerenkov-type phase matching using a fiber-type optical waveguide, a cylindrical Cherenkov light in which light is distributed in a circular band (annular shape) after passing through an axicon lens is obtained. However, only the Cherenkov radiation with a radiation angle determined by the shape of the axicon lens, which is one of the components required for light collection, can be collected only.

Further, a conventional Cherenkov radiation type SHG
The SH light beam obtained from the device is a cylindrical parallel light beam having no light intensity on the optical axis, and the radius of the light beam gradually widens and becomes large due to the diffraction effect, so that there is a problem in practical use. There is fear.

Further, even if an attempt is made to obtain a non-diffraction mode parallel light beam using a conventional fiber-type SHG element, a stable non-diffraction mode parallel light beam cannot be obtained at all times. poor.

An object of the present invention is to provide a parallel beam of a second harmonic light which has a stable oscillation wavelength and can be easily focused on one point of the second harmonic light or has a 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 generator of the present invention, (a) a semiconductor laser in which both end surfaces are formed as low reflection surfaces, and (b)
A Cerenkov radiation type second harmonic generation (SHG) element for converting a fundamental wave light from the semiconductor laser into a second harmonic light, and (c) condensing light emitted from the semiconductor laser on an optical waveguide of the SHG element. And an optical system for inputting return light from the optical waveguide to the semiconductor laser,
(D) a second harmonic generator including an axicon lens for converting the Cherenkov radiation as the second harmonic into parallel light flux, which is emitted from the 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 optical waveguide is characterized in that the outer periphery other than the input / output end faces is formed of a material layer transparent to the second harmonic light.

In practicing the present invention, preferably, the periodic refractive index distribution structure is provided with a first wave with respect to the aforementioned fundamental wave.
And the second refractive index regions are formed alternately and periodically arranged, and when the period is denoted by Λ, it is preferable that 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 for the fundamental wave, and n (ω) is the aforementioned The refractive index of the material layer for the second harmonic light, n _A (2ω), is the refractive index of the axicon lens for the second harmonic light, and α is the apex angle of the axicon lens.

In practicing the present invention, it is preferable that an objective lens is provided after the axicon lens.

A ring-shaped slit for passing the above-mentioned Cherenkov radiated light converted into a parallel light beam is provided at the subsequent stage of the axicon lens, and a lens separated by a focal length from a plane including the slit at the latter stage of the slit. It is preferable to adopt a configuration including

Further, a Fresnel zone plate may be used instead of the axicon lens.

[0027]

According to the second harmonic generator of the present invention, SH
Since the optical waveguide of the G element is formed of at least one material layer that is transparent to the second harmonic light on the outer periphery other than the input / output end faces thereof, the generated SH light has no circles. 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 into a cylindrical parallel light beam having a circular light distribution in a cross section in a plane perpendicular to the optical axis by the axicon lens. You can do it. By condensing this parallel light beam using an appropriate objective lens, it can be condensed as a spot having substantially no blur.

Further, according to the above configuration, since the optical waveguide is configured as a periodic refractive index distribution structure, the fundamental wave from the semiconductor laser (LD) is transmitted to the SHG element by
A fundamental wave that matches the phase matching condition returns to the LD by Bragg reflection. Since the low reflectivity coating is applied to both end faces of the LD, the threshold current is large.
For this reason, if the period of the periodic refractive index structure is designed so as to match the light collecting condition of the SH light, the LD automatically and stably oscillates at the oscillation wavelength as designed. As a result, the SH light as the Cherenkov radiation light is emitted from the SHG element at a stable Cherenkov angle θ, and 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 set to this angle. θ, so that a stable, parallel ray bundle of Cherenkov radiation is
Always get. Therefore, if this parallel light beam is condensed by the objective lens, it can be condensed as a minute spot with no blur and a large light intensity.

Further, by providing an annular slit at the subsequent stage of the axicon lens and using a lens having this slit positioned on the focal plane, a light beam in a non-diffraction mode and a maximum light on the optical axis can be obtained. A collimated light beam having a high intensity is always stably obtained.

[0030]

Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the shapes, sizes, and arrangements of the components are only schematically shown so that the present invention can be understood.

FIG. 1 is a block diagram showing an embodiment of the second harmonic generator according to the present invention. FIGS. 2A and 2B are a perspective view and a sectional view showing an embodiment of the structure of the SHG element used as one component of the present invention.
FIG. 3 is a configuration diagram showing another embodiment of the present invention.

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

Next, each component of the SHG device will be described, but the detailed description of the components already described will be omitted unless particularly necessary. First, a semiconductor laser (LD) 100 used in the present invention.
Are formed as low reflection surfaces at both end surfaces. By forming both end surfaces of the semiconductor laser 100 as low reflection surfaces (AR surfaces), the threshold current can be made sufficiently larger than that of a normal semiconductor laser. Therefore, the returned light from the SHG element 120 to be described later, oscillates at the fundamental L _F beyond the threshold value. This low-reflection surface is
The input and output end faces of the light emitting device may be coated with a low-reflection film, and this coating technique has been conventionally used.

The optical system 110 includes the semiconductor laser 100 and S
Since this is a means for optically coupling with the HG element 120, the light emitted from the semiconductor laser 100 is focused on the optical waveguide 122 of the SHG element 120, and
What is necessary is just to have a configuration in which the feedback light from 22 is input to the semiconductor laser 100. In this embodiment, from the semiconductor laser 100 side, a rolimate lens 112, an anamorphic prism pair 114 and a condenser lens 116 are configured. Naturally, both 100 and 120 may be optically coupled using an optical fiber or other optical means (including an air space and / or a vacuum space).

The second harmonic generation of the Cherenkov radiation type (SH
G) The 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.

The SHG element 120 has 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 (FIG. 2A). In this embodiment, the SHG element 120 has a structure in which a clad 128 is provided on the upper surface of a conventionally known planar SHG element including a substrate 126 and a nonlinear optical waveguide 122 provided thereon. The optical waveguide 122 is formed as a non-linear optical waveguide formed as a periodic refractive index distribution structure by a conventionally known proton exchange method (Li-
It is formed on the substrate 126 by the H ⁺ exchange method).

Although not shown in FIG. 2A as a periodic refractive index distribution structure, the 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 used.
Two different refractive index portions are alternately and periodically arranged along the waveguide direction. In this structure, the original shape of the optical waveguide is once formed on the substrate 126 using the material of the low-refractive-index portion 122b by using the ion exchange method, and then the high-density is formed at a constant period and a constant width by using the ion exchange method. Refractive index section 122a
I just need to make it.

Next, a cladding 128 made of the same material as the substrate is formed on the entire surface of the substrate 126 on which the optical waveguide 122 is formed.
Is provided. The reason why the substrate and the clad are formed of the same material is to make the Cherenkov angle the same on the substrate side and the clad side. As the material, it is preferable to use, for example, LiNbO ₃ which is transparent to the second harmonic. Therefore, in this embodiment, the material layer 124 is constituted by the substrate 126 and the clad 128 which is in optical contact with the substrate and the optical waveguide.

The difference between the refractive indices of the high refractive index portion 122a and the low refractive index portion 122b is preferably small. In this way, the emission modes of the Cherenkov radiation can be made substantially the same. If the mode of the Cherenkov radiation is different,
The SH light extracted from the SHG element 120 enters a mode in which it cannot be condensed by an axicon lens (130 in FIG. 1) at the subsequent stage, resulting in an SHG device with poor practicality.

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

Now, one period of the periodic refractive index structure is denoted by 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 the layer 124 for the second harmonic light, n
_A (2ω) is the refractive index of the axicon lens 130 for the second harmonic light, and α is the apex angle of the axicon lens. Equation (6) corresponds to equation (1).
Equation (7) corresponds to equation (2). The effective refractive index is given by the arithmetic average of the refractive indices of the high and low refractive index portions. Since the refractive index values of both the refractive index portions are very close to each other, either of the refractive index portions May be approximately given by the refractive index.

Hereinafter, this condition will be described. Light refractive index portion 122a and low refractive index portion 122b of optical waveguide 122
Refractive index for the fundamental wave L _F of each and Na (omega) and Nb (omega). Further, the respective refractive indexes for the SH wave L _S are set to n _SA (2ω) and n _SB (2ω).
At this time, the shift angle △ θ 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 ) Is ideal, the refractive index differences [Na (ω) −Nb (ω)] and [n _SA (2ω) −n _SB (2ω)] are preferably small. However, setting the refractive index difference to zero does not achieve the effect of the present invention, so it is necessary to evaluate how long the refractive index difference should be reduced.

Therefore, in the structure shown in FIG. 2B, the length D in the waveguide direction of the SHG element 120 will be considered as 5 mm. The period of the refractive index periodic structure lambda, the wavelength of the fundamental wave L _F lambda the (omega) and 830nm, Na (ω) = 2 .
172. Assuming that the periodic refractive index distribution structure causes ninth-order Bragg reflection, P is 9. In this case, the period Λ given by Expression (5) is Λ = 9 × 0.83
/(2×2.172)=1.72. Now, D = 5
mm, the SHG element 120 has a 2900-period Bragg reflection grating.

Next, if [Na (ω) -Nb (ω)] =
Assuming 0.001, [n _SA (2ω) −n _SB (2ω)]
Is about 0.001, and the Bragg reflectance R is calculated to be approximately 94.3% by using the well-known formula (9), which means that the Bragg reflectance R is sufficient. Note that n ₁ and n ₂ are _{1 as} the refractive index 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 and Nb (ω) = 2.172
When equation (9) is calculated as follows, R = 0.943.

On the other hand, in this case, the Cherenkov radiation angle θa
The difference between θa = COS ⁻ and θb is given assuming that the refractive indices of the high and low refractive index portions 122a and 122b with respect to the wavelength 415 nm of the second harmonic are n _SA = 2.312 and n _SB = 2.311, respectively. ¹ [Na (ω) / n _SA (2ω)] = 19.9688 ° θb = COS ⁻¹ [Nb (ω) / n _SB (2ω)] = 19.9688 °. Therefore, from Expression (8), it is a very small value of Δθ = 0.0043 °. This value can be understood by those skilled in the art as a value sufficiently smaller than that of the SHG device having the conventional structure when the semiconductor laser is not oscillated stably. The SH wave L _S1 radiated at the deviation angle △ θ having a small value of about 4/5 ° to about 1000/1000 is not hindered by the axicon lens 130 provided at the subsequent stage.
The light is condensed, so that a parallel light beam L _S2 having a donut-shaped light intensity distribution can be emitted (FIG. 1). Further, since the optical waveguide 122 giving such a difference in refractive index can be formed by the current ion exchange technology, a practically usable Cerenkov radiation type SHG element can be formed.

As described above, in the configuration example shown in FIG. 1, both ends of the semiconductor laser 100 are AR-coated, and the oscillation threshold value is set sufficiently large. Then, the optical waveguide 122 having the periodic refractive index distribution structure of the SHG element 120 can be configured so as to generate the Bragg reflection satisfying the expression (5). Since the semiconductor laser 100 receives the feedback of the light satisfying the condition 5), once the oscillation starts, the fundamental wave automatically and stably oscillates at a constant wavelength, and the oscillation wavelength changes depending on the ambient temperature and the injection current. Hardly affected by value changes. Therefore, at least equations (5) and (6)
If the SHG element is configured so as to satisfy (7) and (7), the fundamental wave that is oscillating stably is efficiently converted into Cerenkov radiation light as the second harmonic, and the Cherenkov radiation light is converted to the second harmonic. It can be understood that the radiation angle can be radiated within a range that can be regarded as 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 downstream of the axicon lens 130. According to this configuration, the cylindrical parallel light beam L _S2 from the axicon lens 130 can be efficiently converged to one point by the objective lens 140, and the spot-shaped converging point 150 can be obtained.

FIG. 3 is a diagram showing the configuration of another preferred embodiment of the present invention. According to this embodiment, after the axicon lens 130, a ring-shaped slit 160a for passing the Cerenkov radiation light (second harmonic light: SH wave) L _S2 converted into a parallel light beam, and a rear stage of the slit 160a The lens 162 is separated from the surface including the slit (the surface of the slit plate 160) by a focal length f. In this embodiment, the optical system 110 is a simple air gap. Further, the slit plate 160 can be formed in the same manner as the slit plate described with reference to FIG. Further, the combination of 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, and thus the detailed description is omitted.

According to the configuration of the SHG apparatus shown in FIG. 3, the SH light L, which is made into a parallel light beam by the axicon lens 130 and whose light intensity is annularly distributed in a plane orthogonal to the optical axis.
By efficiently condensing _S2 , it is possible to form a non-diffracting mode, fine-diameter parallel light beam 164 on the optical axis behind the lens 162 and within the range from the lens 162 to the longest propagation distance. Since the longest propagation distance is usually several tens of cm, there is no practical problem. Further, according to this configuration, since the slit 160a is provided only in a region where the parallel SH light L _S2 from the axicon lens 130 substantially exists, the light intensity of the SH light is maximized. Will pass through the slit 160a. Therefore, the light intensity of the parallel light beam 164 collected by the slit 160 and the lens 162 is not substantially reduced unlike the case of the conventional configuration. For this reason,
A parallel light beam having the maximum light intensity without blurring on the optical axis can be obtained. Note that the SH of the SHG element 120 is
When the distance S from the end face on the emission side of the wave L _S1 to the axicon lens 130 changes, the diameter between the centers of the parallel light beam L _S2 changes correspondingly. It can be set in advance in correspondence.

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

As a material for forming the SHG element, L
Although iNbO ₃ was used, instead of using iNbO ₃ , LiT
It can also be formed by using aO _3, KTP (KTiOPO ₄ ), or other non-linear optical materials.

[0055]

As is apparent from the above description, the second harmonic generator according to the present invention has the following advantages.

Semiconductor laser and Cherenkov radiation type SH
Since the G elements are combined, the size of the device can be reduced.

Also, 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 reflection surfaces, they are not affected by the drive current value of the semiconductor laser or the ambient temperature. In addition, stable oscillation can be performed at the designed fundamental wave oscillation wavelength.

Further, since the SHG element is surrounded by a layer made of the same material on the outer periphery of the optical waveguide, the refractive index of the layer with respect to the second harmonic is configured to be uniform. The emitted light is always emitted at a constant emission angle over 360 ° around the optical waveguide.
Therefore, if the wavelength of the fundamental wave is determined, the second harmonic is SH
Automatically at a constant radiation angle in a ring from the G element, and
Since the light is radiated stably, a parallel light beam can always be obtained by the axicon lens at the subsequent stage. Therefore, it is sufficient to prepare an axicon lens having an apex angle corresponding to the wavelength of the fundamental wave according to the design, so that waste can be reduced.

As a result of the above, the second harmonic can be condensed as a spot with a large light intensity and a small diameter without blur by disposing the condenser lens at the subsequent stage of the axicon lens. Further, by disposing the combination structure of the slit and the objective lens at the subsequent stage of the axicon lens, it is possible to obtain a light beam with a large light intensity on the optical axis without blurring and with a small diameter.

[Brief description of the drawings]

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

FIG. 2A shows an SHG used in the SHG device of the present invention.
It is a perspective view which shows one Example of a HG element schematically,
FIG. 2B 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 view 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 shows a conventional SHG for obtaining spot-shaped convergent light.
It is a block diagram which shows an apparatus schematically.

FIG. 7 is a configuration diagram showing a conventional combination structure of a slit and an objective lens for converting a parallel light beam having a uniform light intensity distribution into a parallel light beam on the optical axis.

FIG. 8 shows a conventional SHG for obtaining a parallel light beam on the optical axis.
It is a block diagram which shows an apparatus schematically.

[Explanation of symbols]

100: semiconductor laser (LD), 110: optical system 112: rolimate lens 114: anamorphic prism pair, 116: condenser lens 120: SHG element, 122: optical waveguide 122a: high refractive index portion, 122b:
Low refractive index portion 124: Material layer, 126: Substrate 128: Cladding, 130: Axicon lens 140: Condensing lens, 150: Spot 160 Slit plate, 160a:
Slit 162: objective lens, 164:
L _F : Fundamental light, L _S2 : Cerenkov radiation L _S2 : Cherenkov radiation (circular in light intensity distribution).

Claims (5)

(57) [Claims] 1. A (a) semiconductor laser having both end faces formed as low reflection surfaces; and (b) a Cerenkov radiation type second harmonic generation (SH) for converting a fundamental wave light from the semiconductor laser into a second harmonic light.
G) an element; and (c) an optical system for condensing outgoing light from the semiconductor laser on an optical waveguide of the SHG element and inputting return light from the optical waveguide to the semiconductor laser; (E) the optical waveguide includes a periodic index of refraction that includes an axicon lens for converting the Cerenkov radiation as the second harmonic light emitted from the SHG element into a parallel light flux. (F) the SHG element is formed of a material layer that is transparent to second harmonic light around the outer periphery of the optical waveguide other than the input and output end faces. A second harmonic generator. 2. The second harmonic generation device according to claim 1, wherein the periodic refractive index distribution structure is arranged to have a first refractive index with respect to the first fundamental wave.
And the second refractive index region is alternately and periodically arranged. When the period is defined as 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, θ is the Cherenkov angle, and n (2ω) is the material. The refractive index of the layer for the second harmonic light, n _A (2ω) is the refractive index of the axicon lens for the second harmonic light, and α is the apex angle of the axicon lens. 3. The second harmonic generator according to claim 1, further comprising an objective lens provided at a stage subsequent to the axicon lens. 4. The second harmonic generation device according to claim 1, wherein a rear end of the axicon lens has a ring-shaped slit through which the parallel beam of Cherenkov radiation passes, and a rear end of the slit. A lens which is separated by a focal distance from a plane including the slit.
Harmonic generator. 5. A second harmonic generator using a Fresnel zone plate instead of the axicon lens according to claim 1.