JP3031740B2 - 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 harmonic generator for converting a fundamental wave emitted from a laser light source into a harmonic in a resonator having a nonlinear optical material.

[0002]

2. Description of the Related Art In recent years, various devices have been proposed for obtaining a second harmonic or a third harmonic 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 reflection surfaces, and a fundamental wave is confined in the resonator and amplified, so that harmonics are efficiently generated. As the resonator, a monolithic resonator in which a reflection film is provided on the end face of the nonlinear optical material and resonates inside the resonator, and a plurality of mirrors are arranged to form a resonator, and the nonlinear optical An external resonator in which a material is arranged is known.

Recently, in order to reduce the size of the device and to improve the efficiency of conversion to harmonics, an external resonator type has been used.
The mainstream is shifting to a monolithic type that resonates a fundamental wave inside a nonlinear optical material.

FIG. 3 shows an example of a second harmonic generator using such a monolithic resonator.

The second harmonic generation device 1 is composed of a semiconductor laser (hereinafter referred to as LD) 2, a collimating lens 3, a mode matching lens 4, and a nonlinear optical material 5 made of a KNbO ₃ crystal or the like. The LD 2 emits a fundamental wave 6 having a wavelength of 860 nm, for example. The left and right surfaces of the nonlinear optical material 5 in the figure are polished into a spherical shape.

[0006] The surface on the left side in the figure forms the incident surface of the fundamental wave 6,
On this surface, a spherical mirror 8 partially transmitting the fundamental wave 6 and reflecting the second harmonic 7 is formed. The surface on the right side in the figure constitutes an emission surface of the second harmonic 7, on which a spherical mirror 9 reflecting the fundamental wave 6 and transmitting the second harmonic 7 is formed. Further, the lower surface in the figure of the nonlinear optical material 5 forms a plane mirror 10 that reflects both the fundamental wave 6 and the second harmonic 7.

The fundamental wave 6 having a wavelength of 860 nm emitted from the LD 2 passes through the collimator lens 3 and the mode matching lens 4 and enters from the spherical mirror 8 of the nonlinear optical material 5. In this case, the fundamental wave 6 incident from the spherical mirror 8
However, the fundamental wave 6 is made incident on the crystal axis a at a specific angle θ so as to travel in the nonlinear optical material 5 in a direction parallel to the crystal axis a.

The fundamental wave 6 is reflected, resonated and amplified in a ring resonator composed of the spherical mirror 9, the plane mirror 10, and the spherical mirror 8. 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 7 having a wavelength of 430 nm, and is transmitted through the spherical mirror 9 and output. If such a monolithic resonator is used, conversion to the second harmonic can be performed efficiently, and the size of the second harmonic generator can be reduced.

[0009]

However, in the conventional monolithic resonator, even if the incident position and incident angle of the fundamental wave 6 are slightly deviated, the reflection points and the reflection angles of the mirrors 8, 9, and 10 are changed. Since both are shifted, there is a problem that the optical path shifts more and more from the resonance path each time the light is reflected in a ring shape, and the resonance condition is not satisfied. Since the nonlinear optical material 5 constituting the resonator is very small and the area of the light incident surface is, for example, 10 mm ² or less, it is difficult to adjust the alignment for realizing the resonance state, and the processing accuracy is reduced. Due to the low tolerance, this was a serious problem in practical use.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a harmonic generation device which facilitates alignment adjustment by increasing the tolerance for the incident position and incident angle of a fundamental wave for realizing a resonance state. .

[0011]

In order to achieve the above-mentioned object, the present invention provides a harmonic generator comprising a light source for generating a fundamental wave and a resonator having a nonlinear optical material, wherein the resonator comprises: the nonlinear two convex curved mirror formed of two opposing end faces of the optical material itself, a triangular ring between one of the mirrors arranged outside of the non-linear <br/> type optical material Constituting the resonance path, the mirror disposed outside, the incident position of the fundamental wave to the nonlinear optical material and
The non-linear optical material is made movable in accordance with the shift of the incident angle .

In a preferred embodiment of the present invention,
A pair of end faces located at both ends of the nonlinear optical material in the crystal axis direction where phase matching can be achieved are two spherical mirrors on the light input / output side, and a notch groove is formed in one face parallel to the crystal axis of the nonlinear optical material. Is formed, a plane mirror is arranged outside the notch groove in parallel with the crystal axis, and a fundamental wave resonates in a triangular ring shape between the two spherical mirrors and the plane mirror. . In addition, the two convex curved mirrors may be aspherical mirrors such as an elliptical mirror other than a spherical mirror and an elliptical mirror.

[0013]

In the harmonic generation device of the present invention, a triangular ring-shaped resonance path is formed between two mirrors formed on the end face of the nonlinear optical material and a mirror disposed outside the nonlinear optical material. . And the mirror arranged outside ,
For deviation of incident position and incident angle of fundamental wave to nonlinear optical material
Accordingly, since it is possible to move with respect to the nonlinear optical material,
Even if the incident position and the incident angle of the fundamental wave on the nonlinear optical material are shifted, it can be corrected so that the mirror disposed outside is moved to cause resonance. For this reason, the tolerance for the incident position and incident angle of the fundamental wave to the nonlinear optical material is increased, and alignment adjustment can be facilitated. In addition, deviations in dimensions and parallelism in processing accuracy can be corrected by the external mirror. Hereinafter, the incident position and incident angle of the fundamental wave are referred to as nonlinear optical materials.
Means the position and angle of incidence of the fundamental wave on the sample.

In a preferred embodiment of the present invention, a pair of end faces located at both ends in the direction of the crystal axis at which phase matching of the nonlinear optical material can be obtained are two spherical mirrors on the light input / output side, and are parallel to the crystal axis of the nonlinear optical material. A notch groove is formed on one surface. In the case where a plane mirror is arranged outside the notch groove in parallel with the crystal axis, the fundamental wave incident from the spherical mirror on the light incident side of the nonlinear optical material is reflected by the spherical mirror on the light emitting side, and cut off. The light is emitted from the inner surface of the notch groove, is reflected by the outer plane mirror, enters the nonlinear optical material again from the inner surface of the notch groove, is reflected by the spherical mirror on the light incident side, and is again a spherical mirror on the light emitting side. , A triangular ring resonance is made.

Further, even when the incident position and the incident angle of the fundamental wave are shifted, the optical path can be corrected so as to resonate by moving the plane mirror with respect to the nonlinear optical material. The plane mirror may be moved in parallel or rotated to change the inclination angle.

In the above, the movement of the plane mirror is preferably performed by a mechanism for finely moving the plane mirror while keeping it parallel to the crystal axis of the nonlinear optical material. This movement amount is, for example, 1
mm or less, and the tolerance for the incident position and incident angle of the fundamental wave can be sufficiently increased in practical use.

Further, in the above preferred embodiment, the notch groove is formed on one surface parallel to the crystal axis of the nonlinear optical material because the fundamental wave reflected by the spherical mirror on the light emission side is formed of the nonlinear optical material. If the light is incident on a plane parallel to the crystal axis as it is, the light may be totally reflected without being emitted to the outside. Therefore, a notch groove is provided so that the light can be emitted to the outside with an incident angle. It is more preferable that the notched groove has a curved shape with an inner surface formed with an appropriate curvature.

[0018]

1 and 2 show an embodiment in which the present invention is applied to a second harmonic generator.

The second harmonic generator 11 includes an LD 13 as a light source for generating a fundamental wave and a collimator lens 15.
, A mode matching lens 17, and a resonator 19.

The LD 13 has a wavelength of 860 in this embodiment.
One that emits a fundamental wave 33 with small astigmatism in nm, single longitudinal and single transverse modes is used. Note that light emitted from a solid-state laser medium such as YAG or YLF excited by an LD can be used as a light source. The collimating lens 15 and the mode matching lens 17
This is for irradiating the resonator 19 with the fundamental wave 33 as a predetermined beam in a mode matched to the mode.

The resonator 19 has a spherical mirror 23 formed by processing the entrance surface of the fundamental wave 33 and the exit surface of the second harmonic 35 of the nonlinear optical material 21 into a spherical shape having a radius of curvature of 5 mm and depositing a reflective film. 25 and a plane mirror 31 arranged near the outside of the nonlinear optical material 21. The reflection film deposited on the spherical mirror 23 side is a fundamental wave 33.
That reflects 93% of the spherical mirror 2
The reflective film deposited on the fifth side reflects 99.9% of the fundamental wave 33 and transmits 90% of the second harmonic 35.

The nonlinear optical material 21 is made of a KNbO ₃ crystal having a length of 7 mm in the direction of the crystal axis a. In addition to the KNbO ₃ crystal, a KTiOPO ₃ crystal is used.
₄ , various non-linear optical crystals such as KH ₂ PO ₄ and LiNbO ₃ can also be used. Further, a material in which a transparent base material such as a glass block is joined to both ends of the nonlinear optical crystal may be employed. The nonlinear optical material 21 is phase-matched in a direction parallel to the crystal axis a. When the fundamental wave 33 advances in the direction of the crystal axis a, a part of the
It is converted to a harmonic 35.

The bottom surface 27 of the nonlinear optical material 21 is cut in parallel with the crystal axis a, and a notch groove 29 having a depth of 0.4 mm and a width of 1.8 mm is formed in the center of the bottom surface 27. . The left and right inner side surfaces 29a and 29b of the notch groove 29 in the figure are each formed into a curved surface with a radius of curvature of 1 mm. The inner surfaces 29a and 29b form the input and output surfaces of the fundamental wave 33 in the resonance path.

The reason why the notch groove 29 is provided is that when the fundamental wave 33 is directly incident on the bottom surface 27, it is totally reflected and cannot be emitted to the outside. In addition, an antireflection (AR) coat for preventing reflection of the fundamental wave 33 is applied to the inner side surfaces 29 a and 29 b of the notch groove 29. Further, in this embodiment, the inner side surfaces 29a and 29b of the notch groove 29 are formed in a curved shape, but may be cut in a tapered shape.

In the vicinity of the outside of the notch groove 29, a plane mirror 31 is arranged parallel to the crystal axis a of the nonlinear optical material 21. The plane mirror 31 is a total reflection surface that reflects both the fundamental wave 33 and the second harmonic 35. The plane mirror 31 can be finely moved with respect to the nonlinear optical material 21 by a mechanism (not shown) while being kept parallel to the crystal axis a of the nonlinear optical material 21. The plane mirror 31 constitutes the resonator 19 together with the spherical mirrors 23 and 25, and resonates and amplifies the incident fundamental wave 33 in a triangular ring shape.

Using this second harmonic generator 11, an LD
A fundamental wave 33 having a wavelength of 860 nm is emitted from 13 and is incident on a point A of a spherical mirror 23 of the nonlinear optical material 21 at a predetermined incident angle with respect to a crystal axis a through a collimating lens 15 and a mode matching lens 17.

The incident fundamental wave 33 is generated by the nonlinear optical material 2
1 propagate along the crystal axis a and face the opposite spherical mirror 2
It is reflected at point B of No. 5. Further, the fundamental wave 33 is emitted from the inner surface 29 a of the nonlinear optical material 21, is reflected by the plane mirror 31 disposed outside, and is incident again from the inner surface 29 b of the nonlinear optical material 21. And the spherical mirror 23
Then, the light is reflected by the spherical mirror 23 and travels to the point B of the spherical mirror 25, so that the light is resonated and amplified in a triangular ring shape.

Then, the fundamental wave 33 is applied to the nonlinear optical material 21.
When the light propagates between the points A and B in the direction of the crystal axis a, a part thereof is converted into a second harmonic 35 having a wavelength of 430 nm, and the second harmonic 35 is emitted from the spherical mirror 25.

As shown by the dotted line in FIG. 1, when the incident position of the fundamental wave 33 is shifted and radiated to the point D of the spherical mirror 23, the fundamental wave 33 moves in the nonlinear optical material 21 to the crystal axis a. Point E of the spherical mirror 25 which propagates along
At the inner surface 29a of the nonlinear optical material 21. In this case, the plane mirror 31 has the fundamental wave 33 at the point A.
If it is at a position set so as to resonate when it is incident on the plane mirror 31, the fundamental wave 33 reflected by the plane mirror 31 cannot return to the point D, and does not resonate.

However, when the plane mirror 31 is moved to the position shown by the dotted line in the figure while keeping the plane mirror 31 parallel to the crystal axis a of the nonlinear optical material 21, the fundamental wave 33 reflected by the plane mirror 31 It is possible to return to the point D and resonate in a triangular ring shape.

Actually, the fundamental wave 33 having a wavelength of 860 nm is incident at an arbitrary angle of 5 to 10 ° with respect to the crystal axis a of the nonlinear optical material 21 using the second harmonic generator 11 of this embodiment. When the external flat mirror 31 was
By finely moving the crystal in the following range while keeping it parallel to the crystal axis a, output light of the second harmonic 35 having a wavelength of 430 nm could be easily obtained. At this time, it was found that the incident point of the fundamental wave 33 on the spherical mirror 23 can be selected within a range of about 0.8 mm in a direction perpendicular to the crystal axis a.

As described above, by adjusting the position of the plane mirror 31, it becomes possible to correct the incident position and the incident angle of the fundamental wave 33 and resonate. Incidentally, the deviation of the resonance path, not only when the incident position and incident angle of the fundamental wave 33 is shifted, the shape of the nonlinear optical material 21, but also caused by Re <br/> not the dimensions, if such Also, by adjusting the position of the plane mirror 31, the resonance path can be corrected. For this reason, the alignment adjustment for setting the incident position of the fundamental wave 33 can be facilitated, the allowance in designing and processing the device can be increased, and the yield can be improved.

[0033]

As described above, according to the present invention,
Form a resonance path between the mirror formed on the end face of the nonlinear optical material and the mirror located outside the nonlinear optical material, and move the mirror located outside the nonlinear optical material with respect to the nonlinear optical material by which enables, it is possible to widen the incident position and incident angle of the fundamental wave, Ala <br/> stamp face preparative adjustment is facilitated. In addition, the tolerance of the design and processing of the device can be increased, and the yield can be improved.

[Brief description of the drawings]

FIG. 1 is a side view showing an embodiment in which the present invention is applied to a second harmonic generator.

FIG. 2 is a bottom view of a resonator of the second harmonic generation device.

FIG. 3 is a side view showing a conventional second harmonic generator.

[Explanation of symbols]

 11 Second Harmonic Generator 13 Semiconductor Laser (LD) 15 Collimator Lens 17 Mode Matching Lens 19 Resonator 21 Nonlinear Optical Material 23 Spherical Mirror 25 Spherical Mirror 29 Notch Groove 29a Inner Surface 29b Inner Surface 31 Planar Mirror 33 Fundamental Wave 35 2nd harmonic

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/35-1/39 H01S 3/083 H01S 3/108-3/109 JICST file (JOIS)

Claims (2)

(57) [Claims] And 1. A light source for fundamental wave generated in the harmonic generator and a resonator having a nonlinear optical material, wherein the resonator is formed on two opposite end faces of the nonlinear optical material itself and two convex curved mirror, said to constitute a triangular ring resonant path between one of the mirrors arranged outside of the non-linear optical material, the located outside mirror, to said nonlinear optical material Of the fundamental wave
A harmonic generation device characterized by being movable with respect to the non-linear optical material in accordance with a shift in the angle of incidence. 2. A pair of end faces located at both ends of the nonlinear optical material in the direction of a crystal axis at which phase matching can be attained.
Two spherical mirrors, a notch groove is formed on one surface parallel to the crystal axis of the nonlinear optical material, and a plane mirror is arranged outside the notch groove in parallel with the crystal axis. 2. The harmonic generator according to claim 1, wherein a fundamental wave resonates in a triangular ring shape between the spherical mirror and the plane mirror.