JPH05100274A - Harmonic generator - Google Patents

Abstract

(57) [Abstract] [Purpose] 1 without compromising total reflection or high reflection conditions
The generation of scratches and dirt on the flat reflecting surface is prevented, and harmonic output is generated efficiently and stably. [Structure] The monolithic resonator 16 uses a KNbO ₃ crystal as a nonlinear optical material, and a protective material 17
Is BK7 which is an optical glass. 860
The refractive index of the KNbO ₃ crystal for the fundamental wave 18 of nm is n ₁ = 2.278, while the refractive index of BK7 is n ₂ = 1.51 and the critical angle of total reflection is θ = 41. It becomes 0.5 ° and does not impair the actual total reflection condition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a harmonic generator for converting a fundamental wave emitted from a light source such as a semiconductor laser into a harmonic within a monolithic resonator.

[0002]

2. Description of the Related Art In recent years, various devices have been proposed for obtaining a second harmonic wave or a third harmonic wave whose wavelength is converted by passing a fundamental wave emitted from a semiconductor laser or the like through a nonlinear optical material. In these devices, a nonlinear optical material is arranged in a resonator composed of a plurality of reflecting surfaces, and a fundamental wave is confined in the resonator to be amplified so that harmonics are efficiently generated.

As the resonator, a reflection film is provided on the end face of the nonlinear optical material, and a monolithic resonator for resonating inside thereof and a plurality of mirrors are arranged to form the resonator. An external resonator in which a non-linear optical material is arranged is known. Recently, in order to downsize the device and improve the conversion efficiency to higher harmonics, the mainstream is shifting from the external resonator type to the monolithic type that resonates the fundamental wave inside the nonlinear optical material. I am doing it.

FIG. 2 shows, as an example of a conventional harmonic generator, a second harmonic generator using a monolithic resonator. The second harmonic generation device 1 includes a semiconductor laser (hereinafter referred to as LD) 2, a collimator lens 3,
It is composed of a mode matching lens 4 and a nonlinear optical material 5 made of KNbO ₃ crystal or the like. LD2
Emits a fundamental wave 6 having a wavelength of 860 nm, for example. Two surfaces of the nonlinear optical material 5 facing each other in the figure are polished into a spherical shape. Of these, the surface on the left side of the drawing is the incident surface of the fundamental wave 6, and a spherical mirror 8 that partially transmits the fundamental wave 6 is formed on this surface. In addition, the surface on the right side in the drawing forms the exit surface of the second harmonic wave 7, and a spherical mirror 9 that is highly reflective of the fundamental wave 6 and highly transmissive of the second harmonic wave 7 is formed on this surface. .. Further, the lower surface of the nonlinear optical material 5 in the figure constitutes a plane mirror 10 that totally reflects both the fundamental wave 6 and the second harmonic wave 7.

In the above structure, the fundamental wave 6 having a wavelength of 860 nm emitted from the LD 2 is collimated by the collimator lens 3, passes through the mode matching lens 4, and passes from the point A of the spherical mirror 8 of the nonlinear optical material 5. Incident. At this time, the fundamental wave 6 is incident on the crystal axis a at a specific angle θ so that the fundamental wave 6 incident on the point A travels in parallel with the crystal axis a of the nonlinear optical material 5. The fundamental wave 6 is reflected in a ring shape at points A, B, and C in the ring resonator composed of the two spherical mirrors 8 and 9 and the plane mirror 10 to be amplified.

When the fundamental wave 6 passes through the nonlinear optical material 5 in the direction of the crystal axis a, a part of the fundamental wave 6 is converted into a second harmonic wave 7 having a wavelength of 430 nm, and the B of the spherical mirror 9 is reflected.
Emitted from a point. In order to meet the phase matching condition and stabilize the conversion efficiency to harmonics, the nonlinear optical material 5
Is temperature controlled by the Peltier element 11 and the like. By using such a harmonic generator, the fundamental wave can be efficiently converted into a harmonic.

[0007]

However, in the above-described second harmonic generator of the related art, due to scratches and dirt attached during use, a part of the resonance light to be totally reflected by the plane mirror 10 is scattered or scattered. There is a problem in that the second harmonic wave output may be significantly reduced because it may be lost due to absorption or the resonance state may not be obtained.

Therefore, an object of the present invention is to provide the plane mirror 1
It is an object of the present invention to provide a harmonic wave generating device capable of efficiently and stably generating a harmonic wave output by preventing scratches and stains on the total reflection surface without impairing the total reflection condition at 0.

[0009]

In order to achieve the above object, the harmonic generator of the present invention is a nonlinear device which internally causes a fundamental wave to ring-resonate in a triangular shape by three reflecting surfaces of two curved surfaces and one flat surface facing each other. A higher harmonic wave generating device provided with a monolithic resonator including an optical material is characterized in that a reflective protective material is provided on a reflecting surface of the non-linear optical material consisting of the one plane.

As the above-mentioned reflective protective material, a total reflective protective material made of a transparent material such as optical glass, plastic, transparent resin or the like having a lower refractive index for the fundamental wave than the nonlinear optical material can be preferably used. Alternatively, a metal plate having a high reflectance of the fundamental wave may be bonded to the non-linear optical material, or a metal thin film having a high reflectance of the fundamental wave may be adhered by means such as vapor deposition and the surface thereof may be a plastic plate or a plastic resin. It may be covered with a protective material. Further, a dielectric multilayer high reflection film in which thin films of SiO ₂ and TiO ₂ are laminated by means such as vapor deposition and CVD may be provided as a reflective protective material. In this case, 99.5 for the fundamental wave with a wavelength of 860 nm.
% Or more of the high reflection film can be obtained.

As the nonlinear optical material, KNbO is used.
Non-linear optical crystals such as ₃ crystals, organic non-linear optical materials, etc. are used.

[0012]

In the harmonic generator of the present invention, the monolithic resonator has a reflecting surface formed of one plane and a reflective protective material is provided on the reflecting surface. This protective material prevents the harmonic output from being lowered due to scratches and stains on the reflecting surface.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment in which the present invention is applied to a second harmonic generation device. The present invention is not limited to the second harmonic generation device, and can be applied to the third harmonic generation device and the like.

The second harmonic generator 12 comprises an LD 13 as a laser light source, a collimator lens 14, a mode matching lens 15, a monolithic resonator 16 and a protective material 17 for the reflecting surface, which are sequentially arranged.

In this embodiment, the LD 13 has a wavelength of 860 nm,
Basic wave with little astigmatism in single longitudinal and single transverse modes 18
The one that emits is used. As the light source, light emitted from a solid-state laser medium such as YAG or YLF excited by an LD can be used. The collimator lens 14 collimates the fundamental wave 18 emitted from the LD 13 into a parallel beam, and the mode matching lens 15 narrows the beam to match the resonance mode in the monolithic resonator 16 with the incident beam.

In this embodiment, the monolithic resonator 16 uses KNbO ₃ crystal as a nonlinear optical material, and the protective material 17 is BK which is optical glass.
7 is used. In the case of this embodiment, the refractive index of the KNbO ₃ crystal for the fundamental wave 18 of 860 nm is n ₁ = 2.
On the other hand, the refractive index of BK7 is n ₂ = 1.51 and the critical angle of total reflection is θ = 41.5 °, which does not impair the actual total reflection condition.

The end face of one monolithic resonator 16 located on the incident side of the fundamental wave 18 is formed in a spherical shape, and a reflective film for reflecting the fundamental wave by 93% is vapor-deposited on this surface to form a spherical mirror 20. It is said that. The end face of the resonator 16 located on the emission side of the second harmonic is also formed into a spherical shape, and 99.9% of the fundamental wave is reflected and 90% of the second harmonic is transmitted to this face. The film is vapor-deposited to form the spherical mirror 21. Further, the lower surface of the non-linear optical material in the figure is cut into a plane along the crystal axis a and polished, and a plane mirror 22 that totally reflects the fundamental wave 18 and the second harmonic wave 19 together.
There is.

The non-linear optical material forming the monolithic resonator 16 is a block having a length of 7.0 mm in the direction of the crystal axis a and a spherical radius of curvature of 5.0 mm. As the protective material for the reflecting surface, a plate-shaped material having a thickness of 0.5 mm and a side length of 2 mm is used. When the second harmonic generator 12 is used to emit a fundamental wave having a wavelength of 860 nm from the LD 13,
The fundamental wave 18 is made into a parallel beam by the collimator lens 14, is condensed by the mode matching lens 15, and enters the monolithic resonator 16 from the point A of the spherical mirror 20.

The fundamental wave 18 that has entered the resonator 16 propagates in the nonlinear optical material along the crystal axis a, is reflected at the point B of the facing spherical mirror 21, and is reflected at the point C of the plane mirror 22.
Towards the point A of the plane mirror 22, the light is totally reflected at the point C of the plane mirror 22, returns to the point A of the spherical mirror 20, is reflected at the point A, propagates again along the crystal axis a, and overlaps with the original light to cause traveling wave type resonance. Is done. In this way, the fundamental wave 18 resonates along the triangular ring-shaped resonance path in the monolithic resonator 16 and is amplified.

When the fundamental wave 18 thus amplified propagates through the nonlinear optical material in the direction of the crystal axis a, a part of the fundamental wave 18 is converted into a second harmonic wave 19 having a wavelength of 430 nm. The second harmonic wave 19 is emitted from the spherical mirror 21. Note that the temperature of the monolithic resonator 16 is controlled by the Peltier element 23 or the like in order to stabilize the conversion efficiency to harmonics by adapting to the phase matching condition. Therefore, when the harmonic generator 12 of the present invention is used as a light source for information detection to construct an information reading device for an optical recording medium, a device having a high recording density can be obtained.

[0021]

As described above, according to the present invention,
By providing a reflective protective material on the reflecting surface of one plane of the monolithic resonator, it is possible to prevent scratches and stains on the reflecting surface without impairing total reflection or high reflection conditions, and to efficiently output higher harmonics. It can be generated stably.

[Brief description of drawings]

FIG. 1 is a side view showing an embodiment of a second harmonic generation device of the present invention.

FIG. 2 is a side view showing an example of a conventional second harmonic generator.

[Explanation of symbols]

 12 Second Harmonic Generator 13 LD 14 Collimating Lens 15 Mode Matching Lens 16 Monolithic Resonator 17 Protective Material 18 Fundamental Wave 19 Second Harmonic Wave 20 Spherical Mirror 21 Spherical Mirror 22 Plane Mirror 23 Peltier Element

Claims (5)

[Claims] 1. A harmonic generator comprising a monolithic resonator including a non-linear optical material in which a fundamental wave is triangularly ring-resonated inside by three reflecting surfaces of two curved surfaces and one flat surface facing each other. A higher harmonic wave generating device, characterized in that a reflective protective material is provided on a reflecting surface formed of the one plane of the material. 2. The harmonic generator according to claim 1, wherein the reflective protective material is a total reflective protective material made of a transparent material having a lower refractive index for a fundamental wave than a nonlinear optical material. 3. The harmonic generator according to claim 1, wherein the reflective protective material is a metal plate having a high reflectance of a fundamental wave. 4. The reflective protective material comprises a metal thin film having a high reflectance of a fundamental wave and a protective material covering the surface thereof.
Harmonic generator. 5. The harmonic generation device according to claim 1, wherein the reflective protective material is a dielectric multilayer highly reflective film.