JPH02202079A - Laser device - Google Patents

Abstract

PURPOSE:To improve a laser device in wavelength conversion efficiency by a method wherein a pair of non-linear crystals of a type II is arranged in a resonant light path in series, and the linearly polarized light separated by one of the non-linear crystals is recombined again in the other crystal. CONSTITUTION:A non-linear crystal pair 13 composed of a pair of non-linear crystals 13a and 13b of a type II is used. The crystals 13a and 13b are arranged in a resonant light path in series, and linearly polarized light separated by the crystal 10a is recombined in the other crystal 10b. Therefore, the phenomenon, in which two linearly polarized light rays are separated through the non- linear crystals 10a and 10b, is restricted to a part between the crystals 10a and 10b. Therefore, the disturbance of a mode caused by the separation of light rays into two linearly polarized light rays can be prevented, and a means 11a which converges light rays is provided to a laser medium 11 to enable light rays converge to the insides of the non-linear crystals 10a and 10b. By this setup, the laser device can be improved in wavelength conversion efficiency.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is directed to converting the wavelength of a fundamental laser beam emitted from an excited laser medium to produce a wavelength-converted laser beam having a frequency different from that of the fundamental laser beam. This invention relates to a laser device that obtains light.

[Prior Art] Conventionally, a laser device as shown in FIG. The device shown in Fig.
(See Publication No. 189783).

In FIG. 2, reference numeral 1 denotes a laser medium, and a reflective film 1a is partially formed on the end surface of the laser medium 1 on the excitation light supply side (the left end surface in the figure). This partially reflective film 1
a is such that the excitation light 2 emitted from a semiconductor laser device (not shown) is transmitted, but the resonant beam 3, which is the fundamental laser beam excited by the excitation light 2, and the wavelength-converted laser beam 6 are totally reflected. It is something.

The end surface of the laser medium 1 on the resonant beam emission side (the right end surface in the figure) is formed into a convex spherical surface, and the resonant beam 3 emitted from the laser medium 1 is converted into a convergent beam to improve the oscillation efficiency. The structure is designed to improve the performance of the user.

A partially transmitting mirror 4 is disposed at the front of the resonant beam emission side of the laser medium l. In this partially transmitting mirror 4, a reflecting surface 4a facing the laser medium 1 has a concave spherical surface, and a laser resonance optical path is formed by the reflecting surface 4a of this transmitting mirror 4 and the partially reflecting film 1a of the laser medium 1. It is configured.

Furthermore, between the laser medium 1 and the partially transmitting mirror 4,
A nonlinear crystal 5 is arranged, which emits light including wavelength-converted laser light when the fundamental laser light generated by the laser medium 1 is incident thereon.

When the fundamental laser beam is incident on the crystal 5, the nonlinear crystal 5 converts a part of the fundamental laser beam into a wavelength-converted laser beam having a frequency that is an integral multiple of the frequency of the fundamental laser beam. (Harmonic) Set to 6.

Further, the fundamental wave laser beam has frequencies ω1 and ω, for example.

When linearly polarized light having . .

Specifically, for example, if ω1 and ω are the same frequency, a second harmonic having twice the frequency of these is obtained.

Generally, when a wavelength-converted laser beam is obtained from a fundamental laser beam using a nonlinear crystal, the conversion efficiency is proportional to the product of the power densities of two linearly polarized lights (frequency ω1, ω,) incident on the nonlinear crystal. do.

Therefore, if the nonlinear crystal 5 is placed in the laser resonant optical path as in the laser device shown in FIG. 2, the laser beam with high power density in the resonant optical path can be used as the fundamental laser beam. Wavelength-converted laser light can be obtained efficiently.

By the way, in order to obtain the second harmonic wavelength converted laser beam, it is necessary to perform phase matching.

This phase matching is to make the difference between the sum of the wave number vector of ω1 and the wave number vector of ω, and the wave number vector of (ω1+ω,) to zero.

There are two types of crystals used as the nonlinear crystal 5, as shown below, which differ in their methods of achieving phase matching.

■Type I ω1 with the same polarization plane. A type in which linearly polarized light with a frequency of ω is incident from a direction forming a predetermined angle to the crystal axis, and the wavelength of these linearly polarized lights is converted without being separated within the crystal.

■Type■ ω1 with different polarization planes. Linearly polarized light with a frequency of ω is incident on the crystal axis from a direction forming a predetermined angle different from that in the case of Type I, and these linearly polarized lights are separated from each other within the crystal, resulting in two linearly polarized lights. A type that converts wavelengths where they overlap.

The laser device shown in FIG. 2 uses a type I crystal as the nonlinear crystal 5. In this type of laser device using a type I crystal, it is necessary to use a long nonlinear crystal in the direction of the laser resonant optical path. The wavelength conversion efficiency can be increased by

Further, since the fundamental laser beam generated in the laser medium 1 is focused by the convex spherical surface 1b of the laser medium 1 and enters the nonlinear crystal 5, the power density of the fundamental laser beam in the nonlinear crystal 5 is reduced. This can increase the wavelength conversion efficiency within the crystal 5.

Furthermore, since the reflecting surface 4a of the partially transmitting mirror 4 is made concave, a laser resonant optical path with a stable mode can be obtained. Laser light 6 can be extracted with the expected efficiency.

Incidentally, there may be a large difference in wavelength conversion efficiency between the type I crystal and the type (II) crystal described above.

For example, when the nonlinear crystal is KTP (KT i OPO,), which is a crystal with a relatively high conversion efficiency, the conversion efficiency in type (II) is about 50 times that in type I.

Therefore, in order to obtain wavelength-converted laser light with high efficiency, type (2) crystals are often used.

Furthermore, in such a case, it is desirable to take advantage of the characteristics of the type (2) crystal to ensure as high a conversion efficiency as possible and to obtain wavelength-converted laser light with a stable mode.

[Problems to be Solved by the Invention] However, for example, in the laser device shown in FIG. 2, if a crystal of type 2 is used as the nonlinear crystal 5, there is a problem that the expected high wavelength conversion efficiency cannot be obtained. Ta.

One of the causes of this problem is that in the case of type H crystals, the entire nonlinear crystal cannot be used effectively for wavelength conversion.

As shown in FIG. 3, in the case of a type H crystal, the incident fundamental wave laser beam 3 is separated into linearly polarized light 3a and linearly polarized light 3b within the nonlinear crystal 5. Then, the overlapping part of the linearly polarized light 3a and the linearly polarized light 3b (the shaded part in the figure)
A wavelength-converted laser beam is obtained from the wavelength-converted laser beam, but this overlapping portion is determined by the angle between the linearly polarized lights 3a and 3b and the beam diameter of the linearly polarized lights 3a and 3b, and is unrelated to the length dimension of the crystal 5 itself. Therefore, even if the length of the crystal is set large, the wavelength conversion efficiency cannot be increased.

Another reason why the expected wavelength conversion efficiency cannot be obtained when using the crystal of type (1) is that a laser resonant optical path with a stable mode cannot be formed.

This means that in the nonlinear crystal 5 of type (2), the incident fundamental wave laser beam 3 is linearly polarized in the crystal 5.
However, as shown in FIG. 4, the separated laser beams 3a and 3b are partially reflected in different directions by the concave spherical reflecting surface 4a of the transmitting mirror 4. This is due to this.

Therefore, an optical system shown in FIG. 5 or 6 was investigated as an optical system capable of forming a laser resonant optical path with a stable mode using a type (III) nonlinear crystal.

In the optical system shown in FIG. 5, the reflective surface of the laser medium 1 located on the side where the laser light is separated is made flat.

In the optical system shown in FIG. 6, a part of the reflecting surface 4a of the transmitting mirror 4 is made flat.

However, with the device shown in FIG. 5, it is not possible to efficiently focus the resonant beam, which is the fundamental laser beam, into the nonlinear crystal 5, and in the end, the characteristics of type H cannot be utilized.
There was a problem that high wavelength conversion efficiency could not be obtained.

In addition, in the apparatus shown in FIG. 6, the beam waist (<fin) of the resonant beam 3 is located at point P in the figure, so it cannot be sufficiently focused within the crystal 5, and as shown in FIG. As in the case of , there was a problem that the characteristics of type H could not be fully utilized and high wavelength conversion efficiency could not be obtained.

The present invention was made against the above-mentioned background, and even though a type H nonlinear crystal is used, it is possible to obtain a resonant optical path with a stable mode, and moreover, the laser beam can be focused within the nonlinear crystal. To provide a laser device that can increase the generation area of wavelength-converted laser light in a crystal, and can therefore obtain wavelength-converted laser light with a stable mode with higher efficiency. With the goal.

[Means for Solving the Problems] A laser device according to the present invention wavelength-converts a fundamental laser beam emitted from an excited laser medium by a nonlinear crystal provided in a resonant optical path of the fundamental laser beam. In order to obtain a wavelength-converted laser beam having a frequency different from that of the fundamental wave laser beam, a so-called type (III) crystal is used as the nonlinear crystal. , are devising ways to utilize the nonlinear crystal.

That is, the nonlinear crystal provided in the resonant optical path for wavelength conversion is not used alone, but is used by forming a nonlinear crystal pair by forming a pair of nonlinear crystals.

The paired nonlinear crystals are arranged in series in the resonant optical path, so that the linearly polarized light separated by one crystal is recombined within the other crystal.

[Function] In the laser device according to the present invention, nonlinear crystals of type II are used in pairs, and the paired nonlinear crystals are arranged in series in the resonant optical path, and separated by one nonlinear crystal. Since the linearly polarized light is recombined within the other nonlinear crystal, the phenomenon of separation into two linearly polarized lights by the nonlinear crystal is limited between the pair of nonlinear crystals.

Therefore, it is possible to prevent mode disturbance due to separation into two linearly polarized lights, and it is also possible to provide a means to focus the light on the laser medium and focus it within the nonlinear crystal, improving power density through focusing. With the aim of
- It is also possible to increase the layer wavelength conversion efficiency.

In addition, nonlinear crystals are used in pairs, and the part involved in wavelength conversion, that is, the part where separated linearly polarized lights overlap,
Since the wavelength conversion efficiency can be obtained in each of the paired nonlinear crystals, the wavelength conversion efficiency can be increased accordingly.

Therefore, by making full use of the original characteristics of the nonlinear crystal of type (2), wavelength-converted laser light with a stable mode can be obtained with even higher efficiency.

[Embodiment] Figs. 1 and 7 show a first embodiment of a laser device according to the present invention, in which Fig. 1 is a longitudinal cross-sectional view and Fig. 7 is a cross-sectional view.

This laser device includes a laser medium 11, a partially transmitting mirror 12,
It is equipped with a nonlinear crystal pair 13 and the like.

The laser medium 11 is a Nd: YAG laser rod, and the excitation light 15 is a laser beam with a wavelength of 808 nm (
When it is irradiated with a laser beam (provided by a semiconductor laser device or the like), it generates a fundamental laser beam with a wavelength of 1.06 μm. A reflective film 11a is partially formed on the end face of the laser medium 11 on the excitation light supply side (the left end face in the figure).

This partially reflective film 11a transmits excitation light 15 (wavelength: 808 nm) emitted from a semiconductor laser device (not shown), but fundamental laser light 16 generated within the laser medium 11
(a so-called resonant beam with a wavelength of 1.06 μm) and the second harmonic 17 (a so-called wavelength conversion laser beam with a wavelength of 5320 m) are of the nature of total reflection.

Further, the laser medium 11 has an end face (
The right end surface )llb in the figure is formed into a flat surface.

The partially transmitting mirror 12 is placed ahead of the laser medium 11 on the resonance beam emission side. In this - portion reflecting mirror 12, a reflecting surface 12a facing the laser medium 11 is a concave spherical surface, and the reflecting surface 12a of the partially transmitting mirror 12 and the partially reflecting film 11a of the laser medium 11 basically A laser resonant optical path for wave laser light is configured.

The reflecting surface 12a of this partially transmitting mirror 12 has a radius of curvature of 10
It is set to 0mm.

When the fundamental wave laser beam (hereinafter referred to as a fundamental wave or a resonant optical path) 16 emitted from the laser medium 11 is incident on the nonlinear crystal pair 13, the nonlinear crystal pair 13 converts a part of the fundamental wave 16 into a wavelength-converted laser beam. It emits light including light (second harmonic) 17, and includes the laser medium 11 and a partially transmitting mirror 12.
is located between.

This nonlinear crystal pair 13 is formed by a pair of nonlinear crystals 13a and 13b that are paired with each other.

The nonlinear crystals taa and 13b are of the aforementioned type ■
KTP crystals with the same length dimension (L=
5mm).

In addition, these crystals 13a and 13b are such that the resonant beam 16 is φ-26 with respect to the crystal axis of these crystals 13a and 13b.
° θ=90° (However, X, Y, Z that are orthogonal to each other
When the axis is the crystal axis, the angle between the incident light and the Z axis is θ
, where the angle that the projected image of the incident light on the XY plane makes with the X-axis is φ).

Therefore, in the plane parallel to the plane of the paper in Figure 1,
The fundamental wave resonant light 16 is linearly polarized light 16a and linearly polarized light 1
6b.

The separated linearly polarized light 16a and linearly polarized light 16b have almost the same beam diameter, and the distance between the beams is approximately 90 μm.
becomes.

Further, the relationship between the nonlinear crystals taa and 13b will be explained as follows.

The crystals 13a and 13b are arranged to face each other by carefully setting the directions of their crystal axes so that the linearly polarized lights 16a and 16b separated by one crystal are recombined by the other crystal.

To explain more specifically, these crystals 13a, 13
b are arranged in series on the resonant optical path, the beam separation widths on mutually opposing surfaces (parallel to each other) are approximately equal, and in addition, one crystal contains two separated beams. and the plane containing the two separated beams in the other crystal are located on the same plane.

Specifically, such an arrangement can be determined by the following procedure.

First, each of the crystals 13a and 13b is arranged in series on the resonant optical path, and the position of one crystal is fixed.
The other crystal is rotated around the resonant optical path. and,
Observe changes in mode stability and power in response to this rotation, and stop the rotation of the other crystal at a position where the power is maximum and the mode is stable, and this is the optimal positional relationship for both. .

According to the device of the first embodiment, the wavelength-converted laser beam 17 containing the second harmonic (wavelength: 532 nm) is placed on the right side of the partially transmitting mirror 12 in the diagram (FIG. 1 or FIG. 7). Obtainable.

In this case, the nonlinear crystal pair 13 involved in wavelength conversion has a configuration in which the linearly polarized light separated by one nonlinear crystal is recombined within the other nonlinear crystal, so that the mode due to separation when using a nonlinear crystal is Disturbance can be prevented and a mode-stable resonant optical path can be obtained.

Further, overlapping portions 18a and 18b of linearly polarized light 16a and 16b are formed on both of the nonlinear crystals 13a and 13b, and a wavelength conversion laser 17 is generated from both of these overlapping portions 18a and 18b.

Therefore, for example, compared to the case where wavelength conversion is performed using a single nonlinear crystal that has the same output surface as the crystal 13a and is twice the length, the wavelength conversion efficiency is doubled, By making full use of the original characteristics of the type (III) nonlinear crystal, wavelength-converted laser light with a stable mode can be obtained with even higher efficiency.

In addition, since the phenomenon of separation into two linearly polarized lights by a nonlinear crystal is limited between a pair of nonlinear crystals, it is also possible to provide a means for focusing the light in the laser medium and focus it within the nonlinear crystal. It is also possible to increase the -layer wavelength conversion efficiency by increasing the power density.

An example of focusing within a nonlinear crystal is shown below.

FIG. 8 and FIG. 9 show a second laser device according to the present invention.
An embodiment is shown, and FIG. 8 is a longitudinal cross-sectional view of the device, and FIG. 9 is a cross-sectional view thereof.

This embodiment is based on the laser medium 1 in the first embodiment described above.
1, a laser medium 21 of a new configuration is introduced.

This laser medium 21 has an end surface 21b facing the nonlinear crystal pair 13 side formed into a convex spherical surface, and the radius of curvature of this convex spherical surface is set to 5 mm.

When such a laser medium 21 is introduced, the fundamental wave 16 emitted from within the laser medium 2 can be made into a focused beam by the end surface 21b, which is a convex spherical surface, and made to enter the nonlinear crystal pair 13. Compared to the first embodiment described above, the power density of the resonant beam 16 can be made higher, the fundamental wave excitation efficiency can be increased, and the second harmonic conversion efficiency can be further increased.

10 and 11 show a third embodiment of the laser device according to the present invention. FIG. 10 is a longitudinal cross-sectional view of the third embodiment, and FIG. 2 is a cross-sectional view thereof.

In this embodiment, a new nonlinear crystal pair 33 is introduced in place of the nonlinear crystal pair 13 in the first embodiment.

In this nonlinear crystal pair 33, a new nonlinear crystal 33a is introduced in place of the nonlinear crystal 13a in the first embodiment.

This nonlinear crystal 33a is obtained by improving the shape of the end surface (left end surface in FIG. 1) on the laser medium 11 side of the nonlinear crystal 13a to a convex spherical surface;
Same as a.

The convex spherical surface 34 in this nonlinear crystal 33a has a light condensing function, and as in the case of the second embodiment described above, the resonant light beam 1
6 can be increased, thereby increasing the fundamental wave excitation efficiency and further increasing the second harmonic conversion efficiency.

In each of the above embodiments, the case of end-face excitation using a semiconductor laser has been described, but the present invention is not limited to these examples, and the present invention is not limited to these examples, but may also include end-face excitation using a method other than a semiconductor laser, or
It can also be applied to side excitation using a flash lamp, for example, other than end excitation.

Furthermore, the laser medium is not limited to Nd:YAG, but may include other laser media such as Nd:YL.
F, Nd:BEL, Nd:Cr:GSGG, Nd:
YALO,Er :YAGS Er :YLF,E
r: YSGG, E r: YALO, H
o: YAG, Ho: YLF, Nd Nigarasu,
It may be an Erni glass or the like, or even a gas laser.

Furthermore, for the purpose of increasing wavelength conversion efficiency or improving output stability, a Brewster plate, a quarter-wave plate, a temporary etalon, a Q-switch element, etc. may be placed in the resonant optical path. By applying stress to the medium,
The deflection of the fundamental wave may also be controlled.

In addition, nonlinear crystals are not limited to KTP, such as type ■ β-BaB, O, type ■ KNbO, etc.
KDPSUrea etc. may also be used.

However, in this case, the crystal must be transparent and phase-matched to the fundamental laser beam and the wavelength-converted laser beam.

Furthermore, in each of the above embodiments, the case where two identical crystals are used as a nonlinear crystal pair was given as an example, but the present invention is not limited to this (KTP of type ■ and β of type ■
- A combination of different crystals such as BaB, O, etc. may be used.

However, in that case, the length of the crystal must be determined so that the distance between the two separated linear deflections is equal in both crystals.

Furthermore, although the number of nonlinear crystal pairs provided in the resonant optical path is one in each of the above embodiments, the number is not limited to this, and a plurality may be provided depending on conditions such as required conversion efficiency.

Furthermore, it is not necessary to provide a gap between a pair of nonlinear crystals arranged in series (it is also possible to make them abut each other).

[Effects of the Invention] As is clear from the above description, in the laser device according to the present invention, type H nonlinear crystals are used in pairs, and the nonlinear crystals in the pairs are connected in series in the resonant optical path. Since the linearly polarized light separated by one nonlinear crystal is recombined within the other nonlinear crystal, the phenomenon of separation into two linearly polarized lights by the nonlinear crystal is restricted between the pair of nonlinear crystals. Ru.

Therefore, it is possible to prevent mode disturbance due to separation into two linearly polarized lights, and it is also possible to provide a means to focus the light on the laser medium and focus it within the nonlinear crystal, improving power density through focusing. With the aim of
- It is also possible to increase frequency conversion efficiency.

In addition, nonlinear crystals are used in pairs, and the part involved in wavelength conversion, that is, the part where separated linearly polarized lights overlap,
Since the wavelength conversion efficiency can be obtained in each of the paired nonlinear crystals, the wavelength conversion efficiency can be increased accordingly.

Therefore, by making full use of the original characteristics of the type H nonlinear crystal, wavelength-converted laser light with a stable mode can be obtained with even higher efficiency.

[Brief explanation of the drawing]

Fig. 1 is a longitudinal cross-sectional view of the first embodiment of the present invention, Fig. 2 is an explanatory diagram of a conventional laser device, Fig. 3 is an explanatory diagram of a type ■ nonlinear crystal, and Fig. 4 is a partial illustration of a transmitting mirror. Nearby optical path diagrams, FIGS. 5 and 6 are explanatory diagrams of an optical system using a conventional type (III) nonlinear crystal, FIG. 7 is a cross-sectional view of the first embodiment of the present invention, and FIG. 8 is a diagram of the present invention. Longitudinal sectional view of the second embodiment of the invention, No. 9
10 is a longitudinal sectional view of the third embodiment of the invention, and FIG. 11 is a transverse sectional view of the third embodiment of the invention. 1.11.21... Laser medium, 2,15...
...Excitation light, 3,16...Resonance light beam, 4,
12...- part transmission mirror, 5.13a, 13b, 3
3a-=-・Nonlinear crystal, 6, 17...Output light.

Claims (1)

[Claims] A fundamental wave laser beam emitted from an excited laser medium is wavelength-converted by a nonlinear crystal pair provided in a resonant optical path of the fundamental wave laser beam, and the fundamental wave laser beam has a frequency different from that of the fundamental wave laser beam. A laser device that obtains wavelength-converted laser beams with different wavelengths, wherein the nonlinear crystal pair separates linearly polarized lights with different planes of polarization into two directions within the crystal, and during this separation, the two linearly polarized lights overlap. This is a pair of nonlinear crystals that perform wavelength conversion in one part.The nonlinear crystals in the pair are arranged in series in the resonant optical path, and the linearly polarized light separated by one crystal is transferred into the other crystal. A laser device that is characterized by being recombined.