JP3282221B2 - Laser light 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 laser beam generator, and more particularly to a laser beam generator for generating a laser beam by irradiating a neodymium solid laser resonator with an excitation beam from an excitation light source element such as a laser diode. Related to the device.

[0002]

2. Description of the Related Art It has been conventionally proposed to efficiently perform wavelength conversion using a high power density inside a resonator. For example, an external resonance type SHG (second harmonic generation) has been proposed.
Also, SHG using a nonlinear optical element inside a laser resonator has been attempted.

As an example of the second harmonic generation type in a laser resonator, there is known a laser resonator in which a laser medium and a nonlinear optical crystal element are arranged between at least one pair of reflectors constituting a resonator. In the case of this type of laser beam generator, the second harmonic laser beam is efficiently matched by phase matching the second harmonic laser beam with the fundamental laser beam in the nonlinear optical crystal element inside the resonator. Can be taken out.

As a method for realizing the above-mentioned phase matching, a type I or type II phase matching condition is established between the fundamental laser light and the second harmonic laser light. In other words, type I phase matching is based on the principle that the ordinary ray of the fundamental laser light is used to generate a phenomenon in which two photons polarized in the same direction produce one photon whose frequency is doubled. Is what you do. On the other hand, type II phase matching is such that two fundamental wave eigenpolarizations orthogonal to each other are incident on a nonlinear optical crystal element so that phase matching conditions are satisfied for the two eigenpolarizations, respectively. The fundamental wave laser light is divided into an ordinary ray and an extraordinary ray inside the nonlinear optical crystal element, and phase-matches to the extraordinary ray of the second harmonic laser beam.

However, when the second harmonic laser light is to be generated using the type II phase matching condition, each time the fundamental laser light repeatedly passes through the nonlinear optical crystal element, the characteristic polarization of the fundamental laser light is changed. Since the phase is changed, there is a possibility that the generation of the second harmonic laser light cannot be stably continued.

That is, each time the fundamental laser light generated in the laser medium repeatedly passes through the nonlinear optical crystal element by the resonance operation, the phases of the orthogonal natural vibrations (that is, the p-wave component and the s-wave component) are shifted. Eventually, in each part of the resonator, a fundamental state cannot be obtained in which the fundamental laser beams efficiently reinforce each other, so that a strong resonance state (strong standing wave) cannot be formed,
As a result, the conversion efficiency of the fundamental laser light into the second harmonic laser light may be degraded, and noise may be generated in the second harmonic laser light.

Accordingly, the present applicant has disclosed in Japanese Patent Application Laid-Open No. 1-220.
No. 879, a birefringent element such as a quarter-wave plate is inserted in a resonance optical path of a fundamental laser light in a laser light source configured to generate a second harmonic laser light by a nonlinear optical crystal element. By doing so, a laser light source that stabilizes the second harmonic laser light emitted as output laser light has been proposed.

FIG. 5 shows an example of the laser light source disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 1-220879, that is, a laser light generator. The laser light generator shown in FIG. 5 is a laser medium (laser rod) using Nd: YAG.
The resonator 101 includes a reflecting surface (dichroic mirror) 103 formed on the incident surface of the light emitting device 102 and a reflecting surface (dichroic mirror) inside the concave concave mirror 104 for output. : A YAG laser medium 102, a nonlinear optical crystal element 106 made of KTP (KTiOPO ₄ ), and a birefringent element 107 which is a quarter-wave plate made of, for example, a quartz plate.

The laser medium 102 in the resonator 101
The excitation laser light emitted from the excitation semiconductor laser 111 is incident on the incidence surface 103 of the collimator lens 11.
2. By being incident through the objective lens 113,
A fundamental laser light LA (ω) is generated. This fundamental laser light LA (ω) passes through the nonlinear optical crystal element 106 and the birefringent element 107 and is reflected by the reflecting surface of the concave mirror 104,
Again through the birefringent element 107, the nonlinear optical crystal element 106, and the laser medium 102, the incident surface (reflective surface) 10
It is reflected at 3. Therefore, the fundamental laser light LA (ω)
Will resonate so as to reciprocate between the reflection surface 103 of the laser medium 102 of the resonator 101 and the reflection surface inside the concave concave mirror 104 for output.

A birefringent element 10 such as the above-mentioned quarter-wave plate
7, in the plane perpendicular to the direction of light propagation, as shown in FIG. 6, the direction of the extraordinary light direction refractive index n _{e (7)} is extraordinary light direction refractive index n _e of the nonlinear optical crystal element 106 _{(6 )} Is set at an optical axis position inclined by a predetermined azimuth angle θ with respect to the direction of, for example, θ = 45 °.

In the above configuration, the fundamental laser light LA
(Ω) generates the second harmonic laser light LA (2ω) when passing through the nonlinear optical crystal element 106 through the resonance optical path, and this second harmonic laser light LA (2ω) is generated by the concave mirror 10.
4 and transmitted as output laser light.

In this state, the fundamental laser light LA
Each light beam forming (ω) is applied to the nonlinear optical crystal element 106
Through the birefringent element (a quarter-wave plate of a fundamental wave) 107 set at an azimuth angle θ = 45 ° with respect to the laser beam, the power of the laser beam in each part of the resonator 101 becomes a predetermined level. Is stabilized. This is the laser medium 1
02 is operated by resonance so that the fundamental laser light LA (ω) generated in
When the II second harmonic laser light LA (2ω) is generated, the birefringent element 107 suppresses the coupling due to the generation of the sum frequency between the two eigenpolarization modes orthogonal to each other of the fundamental laser light LA (ω). This stabilizes the oscillation.

[0013]

When an isotropic solid-state laser medium is used, the １ ／ wavelength plate is placed on the pumping light side, and an element having a phase delay of π / 2 is placed on the opposite side of the resonator. Promising method to reduce spatial hole burning in solid-state laser medium, reduce the number of longitudinal modes, and stabilize SHG laser in cavity by arranging 45 degrees around optical axis with each other Have been watched.

At this time, since the excitation light source is linearly polarized light, when the excitation light is incident on the solid-state laser medium, the excitation light is partially reflected on the front surface, the back surface, and the surface of the solid-state laser medium. When the reflected light returns to the light source, the light source becomes unstable, and the noise level of the solid-state laser medium at low frequencies also increases. At this time, it is known that the influence is small if the polarization of the return light is orthogonal to the outgoing light. However, in the case of a conventionally used quarter-wave plate, the surface reflection is the same as that of the incident light. Where the reflected light from the back surface and the reflected light from the solid-state laser medium are 1 /
Since the light is reciprocated in the four-wavelength plate and becomes elliptically polarized light, PB
It has not been possible to simultaneously remove the light with an S (polarizing beam splitter), a polarizing prism, or the like, or to combine it with another polarizing element to make orthogonally polarized light if necessary. Also,
Although it is possible in principle to control the polarization state of all return light with a bonded wave plate, it is disadvantageous in terms of durability of the adhesive and optical loss.

The present invention has been made in view of such circumstances, and it is possible to remove or reduce the influence of return light from a laser resonator, to reduce the optical loss and to increase the durability. The purpose is to provide.

[0016]

According to the present invention, there is provided a laser light generating apparatus, comprising: a quarter-wave plate in a resonator used for stabilization; a polarization state of excitation return light from a neodymium-based solid-state laser resonator; In order to keep the constant and reduce the influence on the return light or the excitation light source, a quartz crystal that operates exactly as a 1/4 wavelength plate at the solid-state laser oscillation wavelength and approximately 0, 1 or 1/2 wavelength plate at the excitation wavelength. By using a veneer, the above-mentioned problem is solved.

Here, the laser medium of the neodymium-based solid-state laser resonator is Nd: YAG, Nd: YV
O ₄ , LNP, Nd: BEL or the like is used. In order to generate the second harmonic, a nonlinear optical crystal element, for example, KTP, LN, BBO, LBO, or the like may be arranged in the laser resonator. As a laser resonator serving as a second harmonic generation (SHG) laser light source, a fundamental laser beam generated in a laser medium excited by pump light passes through a nonlinear optical crystal element provided inside the resonator. In addition to generating the second harmonic laser light of type II by performing the resonance operation, the coupling due to the generation of the sum frequency between the two polarization modes of the fundamental laser light is suppressed, and stable oscillation in two modes is performed. For this purpose, the above-described intra-cavity quarter-wave plate is used.

As the quarter-wave plate in the resonator, a single crystal plate in which a relational expression between a cutting direction and a thickness is individually designated according to the excitation light wavelength and the oscillation wavelength of the solid-state laser medium is used. Is preferred. Preferably, a nonlinear optical crystal element for generating a harmonic laser light of the oscillation laser light of the solid laser medium is provided in the laser resonator.
Further, a polarizing beam splitter is provided between the pump light source element and the laser resonator, and a 1 /
It is preferable to arrange a return light control means including a four-wavelength plate.

[0019]

The most part of the excitation light (return light) returning from the laser resonator to the excitation light source element can be made to have the same polarization, and the return light can be provided by providing return light control means using another polarization element or the like. Can be removed or the effect can be reduced. Also,
By forming the quarter-wave plate in the resonator with a single crystal plate, optical loss is small and the durability against laser power in the resonator is excellent.

[0020]

FIG. 1 is a configuration diagram showing a schematic configuration of an embodiment of a laser beam generator according to the present invention. In FIG. 1, laser light as excitation light is emitted from a laser diode 11 which is a semiconductor laser element as an excitation light source element. This excitation laser light is transmitted through a collimator lens 12C.
Are collimated by a condensing (focusing) lens 12F via a return light control unit 20, which will be described later, and then the concave mirrors 14 and 1 of the neodymium solid laser resonator 13
The light is incident on a neodymium-based solid-state laser medium 16 using, for example, Nd: YAG via a quarter-wave plate 15. The concave mirror 14 on the excitation light incident side of the laser resonator 13 has an inner concave surface serving as a reflection surface 14R.

The quarter-wave plate 15 is exactly a quarter-wave plate with respect to the fundamental laser light wavelength oscillating in the laser resonator, as will be described later.
About 0, 1 or 1/2 for the wavelength of the excitation light from
It is a wave plate element that operates as a wave plate, and is formed of a single quartz plate. The laser medium 16 generates a fundamental laser beam LA (ω) in accordance with the incidence of the excitation light, and the fundamental laser beam LA (ω) is, for example, KTP (KTiOPO).
₄ ) The plane mirror 1 passes through the nonlinear optical crystal element 17 using
8 and is reflected by the reflecting surface 18R.

Nonlinear optical crystal element 17 such as KTP
Is the fundamental wave laser light L
A (ω) is a second harmonic laser light LA (2
ω). For example, if the wavelength λ of the fundamental laser light LA (ω) is 1064 nm, the second harmonic laser light LA
The wavelength of (2ω) is 532/2 of λ / 2. Concave mirror 1
4 reflects the excitation light (for example, wavelength 810n).
m) and has a characteristic of reflecting the fundamental laser light LA (ω) generated by the laser medium 16,
Has a characteristic of reflecting the fundamental laser light LA (ω) and transmitting the second harmonic laser light LA (2ω). These reflecting surfaces 14R, 18R
It can be formed by a so-called dichroic mirror. Therefore, the fundamental laser light LA generated in the laser medium 16
(Ω) denotes the reflection surface 14R of the laser resonator 13 and the reflection surface 1
8R, the laser beam oscillates.
The quarter-wave plate 15 is used for the nonlinear optical crystal element 1.
The azimuth angle is set at an azimuth θ = 45 ° with respect to 7, and as described later, is for stabilizing the power of the laser beam in each part of the resonator 13.

The applicant of the present invention has disclosed the above-mentioned quarter-wave plate 15 in the specification and drawings of Japanese Patent Application Laid-Open No. Hei 1-220879 and Japanese Patent Application No. 2-125854, and Japanese Patent Application No. 3-17 / 1990.
No. 068, which is a birefringent element used based on the technology disclosed in the drawings and the like, and is a 波長 wavelength plate set to an azimuth θ = 45 ° with respect to the nonlinear optical crystal element 17. 15, the resonator 13
The purpose of this is to stabilize the power of the laser light in each part of the above.

That is, by inserting the quarter-wave plate 15, (i) non-linear coupling between the polarization modes due to the generation of the sum frequency is eliminated, and mode competition between the polarization modes can be prevented. (Ii) Since the spatial phase difference between the two polarization modes is 90 °, the two polarization modes oscillate, so that the spatial hole burning effect can be suppressed, and a stable longitudinal two-mode (polarization two mode) oscillation can be obtained. Can be The operation and effect described above can be obtained.

Next, the return light control unit 2 disposed between the collimator lens 12C and the focusing lens 12F
0 is 1 / to the excitation light from the laser diode 11.
It comprises a four-wavelength plate 21 and a polarizing beam splitter 22. The polarizing beam splitter 22 is arranged on the collimator lens 12C side, and the quarter-wave plate 21 is arranged on the focusing lens 12F side. The polarization beam splitter 22 has a high transmittance in the polarization direction of the excitation light traveling from the laser diode 11 toward the laser resonator 13,
It is positioned so that the reflectance is high for orthogonally polarized light.

FIG. 2 shows the optical path of the excitation light when the excitation light from the laser diode 11 having the structure shown in FIG. 1 is incident on the laser resonator 13. The concave mirror 14, 1/4 This is for describing reflection and transmission at the surfaces a to f of the optical components of the wavelength plate 15 and the laser medium 16.

As described above, the excitation light incident on the solid-state laser medium 16 is efficiently absorbed, and the generated fundamental laser light is reflected by the reflection surface 14 of the laser resonator 13 shown in FIG.
It oscillates between R and 18R, and if necessary, is converted into SHG light by the nonlinear optical crystal element 17 in the same resonator. In particular, when generating SHG light in the resonator, the reflection surface 14R and the solid-state laser medium 1 are used for stabilizing the laser.
6, a quarter-wave plate 15 in the resonator for the fundamental wave is inserted and arranged. In this case, (1) If the polarization direction of the pump laser diode 11 and one of the main axes of the quarter-wave plate 15 match, the return light control unit 20 as shown in FIG. Light returning from the surfaces a to f can be prevented. (2) Even if the polarization direction of the pump laser diode 11 does not coincide with one of the principal axes of the quarter-wave plate 15, FIG.
The polarization of the reflected light on the surfaces a to c is the same, and the polarization of the reflected light on the surfaces df is the same.
Is a zero-wave plate, a one-wave plate, and a half-wave plate with respect to the excitation light, the polarization states of the reflected light from all the surfaces a to f match, and the return light control unit in FIG. 20 prevents return light from each of the surfaces a to f in FIG.

In the case of the above (1), there is no condition to specify the azimuth of the quarter wave plate 15 except that the fundamental wave is a quarter wave plate. In the case of the above (2), a single plate having a thickness of, for example, about 0.5 mm (0.2 mm to 1.0 mm) is taken into consideration in consideration of specific conditions required for the １ ／ wavelength plate 15. Suppose. As described above, the solid-state laser medium 16 is set to N
d: When YAG is used, particularly when quartz is selected as the material, when the relationship between the thickness (plate thickness) and the cutout angle (the angle between the cutout surface normal and the crystal c axis) as shown in FIG. 3 is satisfied, A ４ wavelength plate for an oscillation (fundamental wave) laser beam wavelength of 1.064 μm and a 0 wavelength plate for an excitation light wavelength of 0.810 μm. FIG. 4 shows the amount of phase delay with respect to the cutout angle of quartz when the thickness is 0.5 mm. The arrow in FIG. 4 indicates that when the thickness is 0.5 mm,
It is shown that when the cutout angle is set to about 25.4 degrees, the phase delay amount of the pump light becomes 2π and the phase delay amount of the fundamental wave becomes 1.5π. When the thickness changes, the specified cutout angle also changes, but the angle margin is up to ± 2 degrees,
The thickness margin is about ± 5 μm.

In the case of the configuration as shown in FIG.
It can be said that it is relatively easy to make the polarization direction of the laser diode 11 coincide with the relative orientation of the axis of the quarter-wave plate 15 as described above. However, when the attachment of the laser diode or the quarter-wave plate is fixed, particularly when the rotation around the optical axis is fixed, the above-mentioned (1) is used by using the half-wave plate as necessary. Although it is possible to satisfy the condition of (2), there is a case where the above (2) is more advantageous. The condition (2) and the return light control unit 20 shown in FIG.
With such a configuration, the influence of the return light to the laser diode 11 is reduced.

Therefore, most of the return light from the resonator can be made to have the same polarization, so that the return light can be removed or its influence can be reduced by combining with some other polarization elements. In addition, since it is made of a single crystal plate, the optical loss is small and the durability of the laser power in the resonator is high.
Further, measures taken at three wavelengths also improve the stability of the SHG output. In this case, it is possible to reduce the influence of the return light of the excitation light source having polarization such as the SHG laser in the stabilized semiconductor laser excitation resonator, and to further reduce the laser output and the noise level at a low frequency during the SHG output. It becomes possible.

In the configuration of the return light controller 20 shown in FIG. 1, the polarizing beam splitter 22 can be replaced by a polarizing element such as a Rochon prism, a Wollaston prism, or a Glan-Taylor prism. Further, such an element including the polarizing beam splitter is difficult to apply when only one finite magnification lens is used between the laser diode and the solid-state laser resonator 13. in this case,
If the polarization of the return light is made nearly orthogonal to the laser diode output light without using a polarization beam splitter or the like, the unstable operation state of the laser diode due to the return light is reduced.

[0032]

As is clear from the above description, according to the laser light generating apparatus of the present invention, the laser light is arranged in the laser resonator, and the oscillation wavelength of the solid laser medium in the resonator is 1/4.
1/4 in a resonator that functions as a wave plate and operates as a substantially 0, 1 or 1/2 wave plate at the pump light wavelength from the pump light source element.
Since the wavelength plate is provided, most of the return light from the resonator can be made to have the same polarization, and the return light can be removed or its influence can be reduced by combining with another polarizing element or beam splitter. Further, since the intra-cavity quarter-wave plate is made of single-crystal quartz, optical loss is reduced, and durability against laser power in the resonator can be increased.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a schematic configuration of a basic embodiment of a laser light generator according to the present invention.

FIG. 2 is an enlarged view of a main part used for describing the embodiment.

FIG. 3 is a graph showing a relationship between a cut-out angle and a thickness of a single crystal plate used in an example of the present invention.

FIG. 4 is a graph showing the relationship between the cutout angle and the amount of phase delay when the thickness of a single crystal plate is specified.

FIG. 5 is a configuration diagram showing a schematic configuration of a conventional example of a laser light generator.

FIG. 6 is an explanatory diagram of an azimuth of a birefringent element used in the conventional example of FIG.

[Explanation of symbols]

11 Laser diode 13 Laser resonator 14 Concave mirror 15 1/4 (to the oscillation fundamental wave laser light)
Wave plate 16 Laser medium 17 Non-linear optical crystal element 18 Plane mirror 20 Return light control unit 21 (For excitation light) 1 / 4 wavelength plate 22 ..... polarizing beam splitter

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01S 3/00-3/30

Claims (4)

(57) [Claims] 1. An excitation light source element, a neodymium-based solid laser medium excited by a light beam from the excitation light source element, and reflection means that is positioned before and after the solid laser medium to form a laser resonator, A resonator 1 disposed in a laser resonator and serving as a quarter-wave plate at the oscillation wavelength of the solid-state laser medium and operating as an approximately 0, 1 or 1/2 wavelength plate at the excitation light wavelength from the excitation light source element. A laser light generator comprising a wavelength plate. 2. The single-quarter wavelength plate in a resonator, wherein a relational expression between a cutting direction and a thickness is individually specified according to the excitation light wavelength and the oscillation wavelength of the solid-state laser medium. 2. The laser beam generator according to claim 1, wherein the laser beam generator is formed by using a laser beam. 3. The laser light generator according to claim 1, wherein a nonlinear optical crystal element for generating a harmonic laser light of the oscillation laser light of the solid-state laser medium is provided in the laser resonator. 4. A return light control means comprising a polarization beam splitter and a quarter-wave plate for the excitation light is arranged between the excitation light source element and the laser resonator. The laser light generator according to claim 1.