JPH04320221A - Higher harmonic generation device - Google Patents

Abstract

PURPOSE:To offer a 2nd higher harmonic generation device which has an incidence position for a fundamental wave over a wide range. CONSTITUTION:Both the end surfaces of nonlinear optical crystal 19 are formed into rotary parabolic surfaces which include an axis (a) as their common axis of rotation and have their common focus at a point A, and reflecting surfaces are provided to form curved surface mirrors 21 and 23. Further, a plane along the axis (a) is formed as a total reflecting plane mirror 25 to constitute a monolithic resonator 17. When the fundamental wave 27 is made incident on an optional point B on the curved surface mirror 21 from a semiconductor laser 13 through a collimator lens 15 and a mode matching lens 16 so as to become parallel to the axis (a) after the incidence, and resonance is caused through the optical path of points B, C, A, D, E, A, and B to convert the fundamental wave 27 into a 2nd higher harmonic 29, which is outputted from the curved surface mirror 23.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a harmonic generation device in which a nonlinear optical material is disposed within a resonator constituted by a plurality of reflecting surfaces.

0002

2. Description of the Related Art In recent years, research has been carried out to pass a fundamental wave emitted from a semiconductor laser or the like through a nonlinear optical material to obtain a second harmonic wave having half the wavelength. In order to efficiently generate the second harmonic, various conditions such as phase matching are required, but in general, the stronger the fundamental wave is, the more the second harmonic is generated.
Harmonics can be generated efficiently. For this reason,
Various devices have been proposed that efficiently generate second harmonics by constructing a resonator using mirrors, arranging nonlinear optical materials inside the resonator, and confining the fundamental wave inside the resonator and amplifying it. has been done.

Recently, in order to reduce the size of the device and improve the conversion efficiency to second harmonics, from an external resonator type that uses individual mirrors to a device that resonates the fundamental wave inside a nonlinear optical material. The mainstream is shifting to monolithic types that allow

FIG. 6 shows an example of a second harmonic generator using such a monolithic resonator.

The second harmonic generator 1 is composed of a semiconductor laser (hereinafter referred to as LD) 2, a collimator lens 3, a mode matching lens 10, and a nonlinear optical material 4 made of KNbO3 crystal or the like.

[0006] The LD 2 emits a fundamental wave 7 having a wavelength of 860 nm, for example. The two surfaces on the left and right in the figure of the nonlinear optical material 4 are
Polished into a spherical shape. The plane on the left side of the diagram is the fundamental wave 7
The fundamental wave 7 is partially transmitted through this surface,
A curved mirror 5 is formed to reflect the second harmonic 8. Further, the right side surface in the figure serves as an output surface for the second harmonic wave 8, and a curved mirror 6 that reflects the fundamental wave 7 and transmits the second harmonic wave 8 is formed on this surface. Furthermore, the lower surface of the nonlinear optical material 4 in the drawing forms a plane mirror 9 that reflects both the fundamental wave 7 and the second harmonic 8.

A fundamental wave 7 having a wavelength of 860 nm emitted from the LD 2 passes through a collimating lens 3 and a mode matching lens 10, and enters a curved mirror 5 made of a nonlinear optical material 4. This fundamental wave 7 is transmitted to the curved mirror 6 and the flat mirror 9.
It is reflected within a ring resonator constituted by a curved mirror 5, resonates, and is amplified. When the fundamental wave 7 passes through the nonlinear optical material 4 in a predetermined direction, a part of it is converted into a second harmonic wave 8 having a wavelength of 430 nm, which is transmitted through the curved mirror 6 and output. If such a monolithic resonator is used, conversion to the second harmonic can be performed efficiently, and the second harmonic generator can be downsized.

[0008]

[Problems to be Solved by the Invention] In the second harmonic generator 1 shown in FIG. 6, for example, the curved mirror 5 on the incident side
The curved mirror 6 on the output side forms a spherical surface with a radius of curvature of 4.5 mm centered on point A, and 0.3 mm from points A and B.
A plane mirror 9 is formed at a position of mm, and if the distance between curved mirrors 5 and 6 is 5.5 mm, the fundamental wave 7 is
Resonance can be achieved only when the beam is incident at a point 1 mm away from the central axis AB and at an angle such that it becomes parallel to the axis AB after refraction.

[0009] Even if the incident spot or angle of incidence shifts slightly, the optical path will gradually shift each time it is reflected in a ring shape, making it difficult to achieve resonance. In this way, in the conventional second harmonic generator, the fundamental wave 7
The problem is that the incident spot and angle of incidence are extremely strictly required, and alignment of component parts is time-consuming and difficult to manufacture.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a harmonic generation device that can widen the incident spot of the fundamental wave and increase the tolerance for assembly accuracy of component parts.

[0011]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a harmonic generation device in which a nonlinear optical material is disposed in a resonator constituted by a plurality of reflective surfaces. At least one of them is a paraboloid of revolution.

[0012] The present invention will be explained in more detail below by giving specific examples.

In the present invention, an LD is preferably used as the light source for generating the fundamental wave, but a solid-state laser such as YAG may also be used. Further, it is preferable that the fundamental wave is irradiated to the resonator through a collimating lens, a mode matching lens, etc. according to a conventional method. In addition, various optical materials may be combined depending on the purpose.

The resonator for amplifying the fundamental wave may be an external resonator in which a separate mirror is arranged outside the nonlinear optical material, but preferably the end face of the block containing the nonlinear optical material is A monolithic resonator is used that uses a mirror to reflect the light internally.

In the case of an external resonator, at least one mirror may form a paraboloid of revolution. In the case of a monolithic resonator, at least one end surface of a block containing a nonlinear optical material may be a paraboloid of revolution, and a reflective film may be provided on that surface to form a curved mirror. Here, the block containing a nonlinear optical material refers to a block made entirely of a nonlinear optical material, as well as a block in which a transparent base material such as glass or plastic is bonded to the end face of a nonlinear optical material.

In a preferred embodiment of the monolithic resonator, both end surfaces of the block containing the nonlinear optical material form a paraboloid of revolution having a reflective film and having a focal point and a rotation axis that coincide, and An example is one in which a plane mirror is formed along the rotation axis, and a ring-shaped resonator is constituted by the paraboloid of rotation and the plane mirror.

In another preferred embodiment of the monolithic resonator, both end surfaces of the block containing the nonlinear optical material form a paraboloid of revolution having a reflective film and having a focal point and a rotation axis that coincide, and the rotation of these One example is one in which a standing wave type resonator is configured by a paraboloid.

The harmonic generator of the present invention can also be applied to cases where harmonics other than the second harmonic are used.

[0019]

[Operation] A reflective surface forming a paraboloid of rotation focuses light incident parallel to the rotation axis to a single point (focal point) on the rotation axis, and reflects light incident through the focal point parallel to the rotation axis. It has an effect. Therefore, an oblique optical path passing through the focal point,
If a resonance path is formed by an optical path parallel to the axis of rotation, then if the fundamental wave is refracted at the incident surface and then incident at an angle parallel to the axis of rotation of the paraboloid of revolution, the incident position of the fundamental wave can be changed. It is possible to cause the fundamental wave to resonate even if it is set at any location on the incident surface. Therefore, alignment of incident light can be facilitated.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 show an embodiment of a second harmonic generator according to the present invention.

The second harmonic generator 11 is constructed by sequentially arranging an LD 13 as a laser light source, a collimating lens 15, a mode matching lens 16, and a monolithic resonator 17. LD13 has a wavelength of 860n
A fundamental wave 27 of m is emitted, and the collimator lens 15 and mode matching lens 16 convert this fundamental wave 27 into a predetermined beam and irradiate it onto the monolithic resonator 17 .

In the monolithic resonator 17, both end faces of a nonlinear optical crystal 19 are formed into a paraboloid of rotation with the a-axis of the crystal passing through point A as the axis of rotation and point A as the focal point. A reflective film that reflects 93% of the fundamental wave 27 is formed by vapor deposition on the surface on the incident side of the fundamental wave 27, thereby forming the curved mirror 21. Further, on the output side surface of the second harmonic wave 29, a reflective film that reflects 99.9% of the fundamental wave 27 and transmits 90% of the second harmonic wave 29 is formed by vapor deposition, thereby forming the curved mirror 23. Furthermore, the nonlinear optical crystal 19 is cut into a plane along the a-axis, and this plane is used as a plane mirror 25 that totally reflects both the fundamental wave 27 and the second harmonic wave 29.

As the nonlinear optical material 19, KNbO3 crystal was used in this example, but KTiOP
Various nonlinear optical crystals such as O4, KH2PO4, LiNbO3, organic nonlinear optical materials, etc. can be used.

Using this second harmonic generator 11, the LD
A fundamental wave 27 with a wavelength of 860 nm is emitted from the mirror 13 and is irradiated to point B of the curved mirror 21 of the monolithic resonator 17 through the collimating lens 15 and the mode matching lens 16. The fundamental wave 27 is refracted through the curved mirror 21 and enters the monolithic resonator 17, but the incident angle of the fundamental wave 27 is adjusted so that the incident light becomes parallel to the a-axis within the monolithic resonator 17. Set.

When the fundamental wave 27 is thus incident from point B on the curved mirror 21, the fundamental wave 27 hits point C on the opposing curved mirror 23, is reflected, and heads toward point A on the a-axis,
It is reflected by the plane mirror 25 and hits point D on the curved mirror 21. Since this light has passed through point A, it is reflected again parallel to the a-axis, and is reflected at point E of the opposing curved mirror 23.
corresponds to Furthermore, it is reflected by the curved mirror 23 and heads for point A on the a-axis, is reflected by the plane mirror 25, and returns to point B of the curved mirror 21. Since it overlaps with the original light in this way, traveling wave type resonance occurs and the fundamental wave 27 is amplified.

In this case, the beam of the fundamental wave 27 is at point B-
Since it has a waist near the middle between C and points D and D, if the crystal orientation of the nonlinear optical crystal 19 is adjusted so that the phase is matched in the a-axis direction, the fundamental wave 27 will be located between points B and C and D.
-E, a part of it is converted into a second harmonic wave 29 with a wavelength of 430 nm, and this second harmonic wave 29 is emitted from the curved mirror 23 on the output side.

In this way, the curved mirrors 21 and 23 form a paraboloid of revolution with a common focal point and rotation axis, and the flat mirror 25
forms a surface along the rotation axis, so no matter where the point B on which the fundamental wave 27 is incident is placed on the curved mirror 21, resonance is possible through the optical path as described above, and the incident spot of the fundamental wave 27 can be widened. This facilitates alignment for setting the direction of incidence of the fundamental wave 27. In addition, the fundamental wave 27 is
If the light is incident on a point along a plane perpendicular to the plane mirror 25, points B and D and points C and E will overlap, so
It becomes a resonator that returns to its original shape after three reflections.

FIG. 3 shows another embodiment of the second harmonic generator of the present invention. In the following embodiments, parts that are substantially the same as those in the embodiment shown in FIGS. 1 and 2 are given the same reference numerals, and their explanations will be omitted.

This embodiment basically has the same structure as the device of the embodiment shown in FIGS. 1 and 2, but monolithic resonance is achieved using a rectangular nonlinear optical crystal 19 and a glass block 33. The difference is that a container 31 is configured. That is, both end surfaces of the glass block 33 form a paraboloid of revolution having a common rotation axis and focal point, and these surfaces form curved mirrors 21 and 23 similar to those of the previous embodiment. Further, it has a plane along the rotation axis, and this plane forms a plane mirror 25 that totally reflects the fundamental wave 27 and the second harmonic 29.

A notch is formed in the center of the surface of the glass block 33 facing the plane mirror 25, and a rectangular nonlinear optical crystal 19 is fitted into this notch, and an optical adhesive 35, They are joined via 37. The optical adhesives 35 and 37 have the same refractive index as the glass block 33, and antireflection is achieved between the glass block 33 and the adhesive layer. Further, an anti-reflection film is formed on the bonding surface of the nonlinear optical crystal 19 to prevent reflection from the adhesive layer.

In the device of this embodiment, the fundamental wave 27 incident from the curved mirror 21 follows the same optical path B→C as in the previous embodiment.
→ A → D → E → A → B, resonates, and is amplified. For example, if the crystal orientation of the nonlinear optical crystal 19 is adjusted so that the phase is matched in the direction of point B-C, a part of the fundamental wave 27 is converted into a second harmonic wave 29 when passing through the optical path, and is output. The light is emitted from the curved mirror 23 on the side. In this embodiment, it is sufficient to cut the end face of the nonlinear optical crystal 19 into a flat surface, eliminating the need for curved surface polishing and reducing the amount of the nonlinear optical crystal 19 used, thereby reducing manufacturing costs.

FIG. 4 shows still another embodiment of the second harmonic generator of the present invention.

In this embodiment, a standing wave type resonator 39 is used as the monolithic type resonator. That is, both end faces of the nonlinear optical crystal 19 are
It is formed into a paraboloid of revolution with the axis as a common rotational axis and the point A on the a-axis as a common focus, and a part of the fundamental wave 27 is partially transmitted and a second A curved mirror 21 is formed by forming a reflective film that reflects the harmonic wave 29, and on the output side surface of the second harmonic wave 29, the fundamental wave 27 is reflected and the second harmonic wave 29 is transmitted. Curved mirror 2 by forming a film
Set it to 3.

In the device of this embodiment, when the fundamental wave 27 is made incident from point B of the curved mirror 21 on the incident side and then made incident parallel to the a-axis, the incident light changes from point C→A→D→
It resonates through the optical path of E→A→B. In this case, the point that goes straight without being reflected when passing through point A is shown in Figure 1.
This is different from the second embodiment. For example, if the crystal orientation of the nonlinear optical crystal 19 is adjusted so that the phase is matched in the a-axis direction, when the fundamental wave 27 passes between points B-C and D-E, a part of it will have a wavelength of 430 nm. The second harmonic wave 29 is converted into a second harmonic wave 29, and this second harmonic wave 29 is output from the curved mirror 2 on the output side.
It is emitted from 3.

FIG. 5 shows still another embodiment of the second harmonic generator of the present invention.

In this embodiment, the standing wave type resonator 39 shown in FIG. , a reflective plane mirror 43 for the second harmonic 29.
The surface forming a paraboloid of revolution facing this is the fundamental wave 27
The curved mirror 23 reflects the second harmonic wave 29 and transmits the second harmonic wave 29.

In the device of this embodiment, when the fundamental wave 27 is incident on an arbitrary point F of the plane mirror 43 parallel to the a-axis, the incident light moves from point G→A→I→H→I→A →G→F
→It passes through the optical path of G and causes so-called reciprocating vibration type resonance. For example, if the crystal orientation of the nonlinear optical crystal 19 is adjusted so that the phase is matched in the a-axis direction, the fundamental wave 27
passes between points FG and I-H, a part of it has wavelength 4
It is converted into a second harmonic wave 29 of 30 nm, and this second harmonic wave 29 is emitted from the curved mirror 23 and the plane mirror 43.

[0038]

[Effects of the Invention] As explained above, according to the present invention,
By making at least one of the plurality of reflecting surfaces constituting the resonator a paraboloid of revolution, it is possible to widen the incident position of the fundamental wave, and alignment for setting the incident position of the fundamental wave becomes easy. Therefore, assembly of the device is facilitated and yield is improved.

[Brief explanation of the drawing]

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

[Figure 2] Plan view of the second harmonic generator

FIG. 3 is a side view showing another embodiment of the second harmonic generator of the present invention.

FIG. 4 is a side view showing still another embodiment of the second harmonic generator of the present invention.

FIG. 5 is a side view showing still another embodiment of the second harmonic generator of the present invention.

[Fig. 6] Side view showing a conventional second harmonic generator

[Explanation of symbols] 11 Second harmonic generator 13 Semiconductor laser (LD) 15 Collimating lens 16 Mode matching lens 17 Monolithic resonator 19 Nonlinear optical crystal 21 Curved mirror 23 Curved mirror 25 Plane mirror 27 Fundamental wave 29 Second harmonic Wave 31 Monolithic resonator 39 Monolithic resonator 41 Monolithic resonator 43 Plane mirror

Claims (4)

[Claims] 1. A harmonic generation device in which a nonlinear optical material is disposed in a resonator constituted by a plurality of reflecting surfaces, wherein at least one of the reflecting surfaces forms a paraboloid of revolution. wave generator. 2. The resonator is a monolithic resonator, and at least one end face of the block containing a nonlinear optical material forms a paraboloid of revolution having a reflective film.
harmonic generator. 3. Both end surfaces of the block containing the nonlinear optical material form a paraboloid of revolution having a reflective film and having a focal point and a rotation axis coincident with each other, and a plane mirror is formed along the rotation axis, 3. The harmonic generation device according to claim 2, wherein a ring-shaped resonator is constituted by the paraboloid of revolution and the plane mirror. 4. Both end surfaces of the block containing the nonlinear optical material form paraboloids of rotation having a reflective film and whose focal point and rotation axis coincide, and these paraboloids of rotation constitute a standing wave type resonator. 3. The harmonic generator according to claim 2.