JPH06120597A - Ld excitation solid laser device - Google Patents

Abstract

PURPOSE:To make a laser oscillation of less second harmonic noise possible by providing, on the end faces of a graded index type focusing lens, optical thin films consisting of a first layer of a material having a refractive index of n1 and a second layer of a material having a refractive index of n2 sequentially from the surface side, and setting the values of the optical film thickness of the first layer and the optical film thickness of the second layer under specific conditions. CONSTITUTION:The values of the optical film thickness n.d1 of the first layer and the optical film thickness n2.d2 of the second layer are set by satisfying formulas I-IV which minimize the reflectance of an excitation light vertically incident from a semiconductor laser upon the optical axis plane of a graded index type focusing lens including optical thin films 21 and 22, and by the conditions provided by formulas V-X for minimizing the reflectance of an excitation light incident from the semiconductor laser with an incidence angle of theta0 upon the effective radius portion of the graded index type focusing lens including the optical thin films. Thus, it is possible to substantially reduce the reflection of the excitation light from the whole end face of the focusing lens. Accordingly, the fundamental wave stabilizes, and the SH wave noise is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, a graded index type condensing lens for condensing pumping light from this semiconductor laser, and a converging pumping light to generate a fundamental wave. A solid-state laser element and a second harmonic generation element that outputs a second harmonic (hereinafter referred to as SH wave) upon incidence of the fundamental wave are provided, for example, for recording and reproduction of an optical disc, which is a high-density recording medium. L applicable to light source
The present invention relates to a D-pumped solid-state laser device, and more particularly, to an improvement of an LD-pumped solid-state laser device capable of reducing the SH wave noise.

[0002]

2. Description of the Related Art This type of LD-pumped solid-state laser device includes a semiconductor laser a and optical thin films b1 and b2 on both end faces thereof as shown in FIG. A graded index type condensing lens b represented by SML) and a solid-state laser device c in which optical thin films c1 and c2 are provided on both end surfaces.
And a second harmonic generation element d having optical thin films d1 and d2 provided on both end surfaces thereof, and an output mirror e having an optical thin film e1 provided on the end surface on the second harmonic generation element d side. What constitutes a part is known.

The wavelength of the excitation light from the semiconductor laser a is temperature-tuned to the absorption peak of about 809 nm of the solid-state laser element c, and after being condensed by the graded index condenser lens b, The solid-state laser device c is irradiated and excited. The fundamental wave generated by the solid-state laser device c is the optical thin film c1 and the optical thin film e.
The SH wave that resonates with the first harmonic conversion element 1 and is converted by the second harmonic generation element d toward the output mirror e side is directly output from the optical thin film e1, while the SH wave toward the solid-state laser element c side is the optical thin film. It is reflected by c1 and again passes through the second harmonic generating element d to be output from the optical thin film e1.

By the way, when this LD pumped solid-state laser device is applied to the recording and reproducing light source of the above-mentioned optical disk or the like, noise due to the fluctuation of the output of the SH wave becomes a problem. The return light of the semiconductor laser is one of the causes of the noise of the SH wave.

That is, since the excitation light emitted from the semiconductor laser a is reflected by both end surfaces of the graded index type condenser lens b and the end surface of the solid-state laser element c on the semiconductor laser a side, the reflected excitation light is It returns to the semiconductor laser a again and makes the laser oscillation of this semiconductor laser a unstable. When the oscillation of the semiconductor laser a, which is the excitation light, becomes unstable, the fundamental wave of the solid-state laser element c being excited becomes unstable, and the SH wave causes noise.

Therefore, in order to prevent the above-mentioned returning light of the semiconductor laser, an antireflection film for excitation light is conventionally provided on both end faces of the graded index type condenser lens b and the end face of the solid-state laser element c on the semiconductor laser a side. For example, provision is made, the end face of the graded index type condensing lens b on the solid-state laser element c side is obliquely processed, or an isolator is incorporated between the semiconductor laser a and the graded index type condensing lens b.

However, the method of obliquely processing the end face of the graded index type condensing lens b on the side of the solid-state laser element c has a problem that the optical axis adjustment work for laser oscillation becomes complicated because the optical axis is not a straight line. On the other hand, in the method of incorporating the isolator, it becomes difficult to efficiently enter the pumping light of the semiconductor laser a into the solid-state laser element c, so that the output is reduced and the LD-pumped solid-state laser device is increased in size and cost. was there.

Therefore, in most cases, a method of providing an antireflection film for pumping light on both end faces of the graded index type condenser lens b and the end face of the solid-state laser element c on the semiconductor laser a side is adopted.

Here, as a conventional example of the graded index type condensing lens provided with this antireflection film, the one described in JP-A-2-67155 is known.

That is, although this graded index type condensing lens was developed for an LED printer, the graded index type condensing lens is made of a low-refractive index material from the surface side of the incident surface of the graded index type condensing lens. The film thickness n · d (n: refractive index, d: physical film thickness) is ¼ · λ _o
(Λ _o : wavelength of excitation light emitted from semiconductor laser) and the optical film thickness n · d of the high refractive index material was set to ¼ · λ _o An antireflection film composed of a second layer is provided, and with respect to the refractive index on the optical axis of the graded index type condensing lens and for the case of vertical incidence (incident angle of 0 degree). It was designed so that its reflectance would be zero in theoretical calculation.

When the graded index type condenser lens provided with the antireflection film is incorporated in the LD pumped solid state laser device, the graded index type condenser lens not provided with the antireflection film is assembled. Since the return light can be prevented as compared with a device having a built-in device, it has an advantage that the SH wave noise can be reduced.

[0012]

However, Japanese Unexamined Patent Publication (Kokai) No. 2-6.
The graded index type condensing lens described in Japanese Patent No. 7155 is theoretically calculated only for the refractive index on the optical axis of the graded index type condensing lens and for the case of vertical incidence as described above. Only designed so that the reflectance becomes 0, and the reflection of excitation light that is obliquely incident with respect to the refractive index of the circumferential portion that is radially displaced from the optical axis of the graded index type condenser lens is prevented. Since no consideration has been given to the above, the residual reflection from the circumferential portion cannot be ignored, and the LD pumped solid-state laser device incorporating the graded index type condensing lens provided with the antireflection film still remains. 1
There is a problem that a noise component is included in the range of 0 k to 1 MHz.

In this case, the film thickness of each layer of the antireflection film composed of the first layer and the second layer is continuously changed from the optical axis of the graded index type condensing lens toward the circumferential portion. By doing so, it is theoretically possible to reduce the reflectance to 0 with respect to the refractive index of the circumferential portion of the graded index type condensing lens which is radially displaced from the optical axis and with respect to the obliquely incident excitation light. It is possible.

However, the diameter of the graded index type condensing lens is usually 3 mm or less, and a shadow is formed by complicatedly combining a mask or the like on the end face of the graded index type condensing lens, and the vacuum evaporation method is used. It is extremely difficult to continuously change the film thickness of each layer due to factors such as the above, and productivity is extremely poor and it is practically difficult to apply.

The present invention has been made by paying attention to such a problem, and an object thereof is to prevent the film thickness of the antireflection film from being continuously changed and the radius from the optical axis. An object of the present invention is to provide an LD pumped solid-state laser device in which the SH wave noise is small by incorporating a graded index type condensing lens capable of minimizing the reflectance of the circumferential portion deviated in the direction.

[0016]

That is, the invention according to claim 1 is directed to a semiconductor laser, a graded index type condenser lens for condensing the excitation light from the semiconductor laser, and irradiation with the condensed excitation light. Based on the LD pumped solid-state laser device provided with a solid-state laser element that generates a fundamental wave and outputs a second harmonic wave when the fundamental wave is incident, the graded index type laser An optical thin film including a first layer made of a material having a refractive index n ₁ and a second layer made of a material having a refractive index n ₂ is provided on at least one end surface of the optical lens from the surface side, and the optical thin film is provided. In addition to satisfying the conditions of the following formulas (1) to (4) for minimizing the reflectance of the excitation light vertically incident from the semiconductor laser on the optical axis surface of the graded index type condensing lens provided with a thin film, The reflectance R of the excitation light incident from the semiconductor laser at the incident angle θ _o on the effective radius portion of the graded index type condensing lens provided with the optical thin film obtained by the following mathematical formulas (5) to (10) is minimum. The values of the optical film thickness n ₁ · d ₁ of the first layer and the optical film thickness n ₂ · d ₂ of the second layer are set under the following conditions.

[0017]

[Equation 3]

[Equation 4] (Where δ ₁ is the phase difference of the first layer, δ ₂ is the phase difference of the second layer, n _o is the refractive index of air, n _s is the refractive index on the optical axis of the graded index type condensing lens, and n is _s' is the physical thickness of the gray refractive index of the effective radius portion of the dead-index focusing lens, d ₁ is the physical thickness of the first layer, d ₂ is the second layer, lambda _o is excitation from the semiconductor laser The wavelength of light, θ _o is the incident angle of the excitation light from the air to the first layer, θ ₁ is the refraction angle from the air to the first layer, θ ₂ is the refraction angle from the first layer to the second layer, and θ _s indicates the refraction angle from the second layer to the graded index type condensing lens.) In such technical means, the semiconductor laser, the graded index type condensing lens, which constitutes the above LD pumped solid state laser device, Conventionally applied materials for solid-state laser elements, second harmonic generation elements, etc. Can be used as it is. For example, as the solid-state laser device, the oscillation wavelength is around 1.06 μm and Nd-doped YVO ₄ , N
An Nd-doped solid-state laser crystal such as d-doped YAG can be used.

Further, as the constituent material of the first layer provided on the end surface of the graded index type condensing lens, an oxide film having high hardness and good durability against humidity is desirable, for example, a low refractive index material. Is SiO ₂ ,
Alternatively, a material in which Al ₂ O ₃ or the like is mixed with this SiO ₂ as a base can be used.

The invention according to claim 2 is constructed for such a technical reason.

That is, the invention according to claim 2 is claim 1
Based on the LD pumped solid-state laser device according to the first aspect, the first layer of the optical thin film is composed of a SiO ₂ thin film of a low refractive index material, and the second layer is composed of a HfO ₂ thin film of a high refractive index material. It is characterized by that.

[0021]

According to the first aspect of the present invention, at least one end surface of the graded index type condensing lens has a first layer composed of a material having a refractive index n ₁ from the surface side and a material having a refractive index n ₂ . An optical thin film composed of a second layer is formed, and the reflectance of the excitation light vertically incident from the semiconductor laser is minimized on the optical axis surface of the graded index type condensing lens provided with this optical thin film. In addition to satisfying the conditions of the above formulas (1) to (4),
To (10), the reflectance R of the excitation light incident from the semiconductor laser at the incident angle θ _o on the effective radius portion of the graded index type condensing lens provided with the optical thin film is minimized under the above condition. The values of the optical film thickness n ₁ · d _{1 of one} layer and the optical film thickness n ₂ · d ₂ of the second layer are set, and each of the first layer and the second layer has a uniform film thickness. However, the reflectance of the excitation light vertically incident on the optical axis surface of the graded index condensing lens and the reflectance of the excitation light incident on the effective radius part at the incident angle θ _o are minimized. Therefore, the reflection of the excitation light from the entire end surface of the graded index type condensing lens can be significantly reduced.

According to the second aspect of the invention, the first layer of the optical thin film is formed of a SiO ₂ thin film which is a low refractive index material having high hardness and good durability against humidity, and Since the two layers are composed of HfO ₂ thin film of high refractive index material, it is possible to reduce the reflection of excitation light from the entire end face of the graded index type condensing lens and to improve the physical resistance of the optical thin film. Is possible.

[0023]

Embodiments of the present invention will now be described in detail with reference to the drawings.

The LD-pumped solid-state laser device according to this embodiment is similar to the conventional device, as shown in FIG. 1, a semiconductor laser (hereinafter referred to as LD) 1 and optical thin films 2 on both end faces thereof.
Cell-Hoc microlenses (hereinafter referred to as SML) 2 manufactured by Nippon Sheet Glass Co., Ltd. provided with Nos. 1 and 22, and a solid-state laser element 3 made of Nd-doped YVO ₄ crystal provided with optical thin films 31 and 32 on both end surfaces. Optical thin film 4 on both ends
The second harmonic wave generating element 4 made of KTP crystal provided with Nos. 1 and 42, and the output mirror 5 having the optical thin film 51 provided on the end face on the second harmonic wave generating element 4 side, constitute the main part thereof. There is.

Further, the optical thin film 21, 22 provided on both end faces of the SML2, the optical thickness n ₁ · made of SiO ₂ thin film having a refractive index n ₁ is 1.43, as shown in FIG. 2
The first layer 6 having d _{1 set} to 0.222 · λ _o (λ _o : wavelength of excitation light emitted from the applied LD1 = 0.809 μm) and HfO having a refractive index n ₂ of 1.85. The second layer 7 is composed of _two thin films and has an optical film thickness n ₂ · d ₂ set to 0.330 · λ _o .

The first layer 6 and the second layer 7 provided on both end faces of the SML 2 have the above-mentioned set values based on the following technical analysis.

First, FIG. 3 shows the optical path of the excitation light from the LD1 condensed by the SML2. The excitation light from the LD1 enters the end face of the SML2 with a divergence angle.
The excitation light is incident on this end face in the vicinity of the optical axis on the optical axis, as shown in FIG. The above S
The exit side end face of ML2 is the same as that on the excitation light incident side. In particular, as shown in FIG. 4, the stripe 10 of LD1
On the other hand, the divergence angle in the horizontal direction is about 5 degrees, which can be considered to be substantially equivalent to the vertical incidence, but the divergence angle in the vertical direction is about 10 degrees as shown in FIGS. On the other hand, LD
Since the polarization direction of 1 is parallel to the stripe 10 of LD1, when designing an optical thin film for reflection prevention, SML
The incident angle of the LD1 on the two end faces may be about 0 to 10 degrees of S-polarized light (direction perpendicular to the incident surface).

FIG. 5 is a graph showing the radial refractive index distribution of the SML 2 applied to this embodiment. However, the effective radius of the SML 2 is a portion about 70% from the center where the influence of aberration is small. Therefore, from FIG.
In the case of ML2, the refractive index of SML is 1.599 to 1.56.
It is in the range of 5.

The refractive index of the thin film material forming the first layer 6 of the optical thin films 21 and 22 provided on both end faces of the SML 2 is n ₁ , and the optical film thickness thereof is n ₁ · d ₁ . When the refractive index of the thin film material forming the layer 7 is n ₂ and the optical film thickness thereof is n ₂ · d ₂ , the optical axis plane of the SML ₂ is L.
In order to minimize the reflectance of the excitation light vertically incident from D1, it is necessary to satisfy the conditions of the following mathematical formulas (1) to (4).

[0030]

[Equation 5] (However, [delta] ₁ is a phase difference of the first layer, the [delta] ₂ phase difference of the second layer, n _o is the refractive index of air = 1, n _s is a refractive index on the optical axis of the SML = 1.599, d ₁ is the physical thickness of the first layer, d ₂
Is the physical film thickness of the second layer, and λ _o is the wavelength of the excitation light from the LD = 0.809 μm, respectively. On the other hand, when the physical resistance of the optical thin films 21 and 22 is taken into consideration, As the thin-film material constituting 6, SiO ₂ having a refractive index of 1.43 at a wavelength of 0.80 μm is preferable.

Therefore, as the thin film material forming the first layer 6, the refractive index n _{1 at the} wavelength of 0.80 μm is 1.43.
When the SiO ₂ thin film of No. ₂ is applied, the refractive index n ₂ of the second layer 7 satisfying the expressions (1) and (2) with respect to the refractive index of the SML 2 on the optical axis of 1.599 is expressed by the expression (1). It is necessary from equation (2) to be 1.81 or more.

Generally, among the high refractive index material films of oxide films, the highest refractive index is 2.10 of the TiO ₂ film. Therefore, the refractive index n ₂ of the second layer 7 is 1.81 to
It is limited to the range of 2.10.

Next, a method for obtaining the optical film thickness n ₁ · d _{1 of} the first layer 6 and the optical film thickness n ₂ · d ₂ of the second layer 7 will be described below.

SML2 is added to n _s in the equations (1) and (2).
Substituting the refractive index of 1.599 on the optical axis and the refractive index of 1.43 of the SiO ₂ thin film into the refractive index n ₁ of the first layer 6, the second layer in the range of 1.81 to 2.10. determined 7 and the phase difference [delta] ₁ of the first layer 6 for each refractive index n ₂ second layer 7 of the phase difference [delta] _2, respectively, and, (3) and (4) from the refraction of the second layer 7 The optical film thickness n ₁ · d ₁ and the optical film thickness n ₂ · d ₂ for the rate n ₂ are obtained.

Then, a group of numerical values of the optical film thickness n ₁ · d ₁ and the optical film thickness n ₂ · d ₂ for each refractive index n ₂ obtained for the excitation light perpendicularly incident on the optical axis of the SML ₂ From the inside
Refractive index of 1.565 at a portion of SML2 with an effective radius of 70%
Lowest reflectance (R _s ) for S-polarized light with incident angle of 10 degrees
The refractive index n ₂ of the second layer 7 is calculated from the following mathematical formulas (5) to (10).

[0036]

[Equation 6] (However, [delta] ₁ is a phase difference of the first layer, [delta] ₂ is the second layer phase difference, n _o is the refractive index of air = 1, n _s' is the refractive index at the site of the effective radius 70% SML = 1 .565, d ₁ is the physical film thickness of the first layer, d ₂ is the physical film thickness of the second layer, λ _o is the wavelength of the excitation light from the LD = 0.809 μm, and θ _o is FIG.
As shown in, the incident angle of the excitation light from the air to the first layer 6, θ ₁ is the refraction angle from the air to the first layer 6, θ ₂ is the refraction angle from the first layer 6 to the second layer 7, θ _s is SML from the second layer 7
As a result, the refractive index n ₂ of the second layer 7 is 1.85, and the optical film thickness n ₁ · d _{1 of} the first layer 6 and the second layer 7 are shown. When the optical film thicknesses n ₂ · d ₂ of the SML2 are 0.222 · λ _o and 0.330 · λ _o , respectively, the reflectivity (R _s = 0.006%) of the SML2 at the effective radius 70% ) Was calculated to be the lowest. The high-refractive index material film having a refractive index of 1.85 includes the above-mentioned HfO ₂ thin film.

By the way, the SML2 provided with the above optical thin film
Strictly speaking, the reflectance of was calculated from the refractive index of the SML at a certain position X in the diametrical direction of the SML2, the reflectance obtained by calculation from the incident angle of the excitation light from the LD1, and the intensity distribution of the excitation light of the LD. The product of the intensities of the excitation light incident on a certain position X has to be integrated for all SML effective end faces. Approximately, the optimum refractive index n ₂ of the second layer 7 and the first layer 6 are calculated by the method described above. The optical film thicknesses n ₁ · d ₁ and the optical film thicknesses n ₂ · d ₂ of the second layer 7 may be obtained.

N obtained by such a technical analysis
₁ = 1.43, its optical film thickness n ₁ · d ₁ = 0.222
・ The first layer 6 composed of a SiO ₂ film of λ _o and n ₂ =
1.85, its optical film thickness n ₂ · d ₂ = 0.330 · λ
_A second layer 7 composed of a HfO ₂ film of _o was formed on both end faces of the SML 2 by a vacuum vapor deposition method to obtain the SML 2 of the LD pumped solid-state laser device according to the example. In addition, FIG.
FIG. 6 is a graph showing the spectral reflection characteristics on the optical axis of the SML 2 provided with the optical thin films 21 and 22 composed of the first layer 6 and the second layer 7.

Incidentally, Japanese Patent Laid-Open No. 2-67 mentioned above as a comparative example.
According to the method of Japanese Patent Publication No. 155, the refractive index n ₁ is 1.38 and the optical film thickness n ₁ · d ₁ is ¼ · λ on both end faces of the SML.
_a first layer made of a MgF ₂ film having a refractive index n ₂ of 1.63 and an optical film thickness n ₂ · d ₂ of 1/4 · λ _{o
A two-} layer antireflection film composed of a second layer composed of a ₂ O ₃ film was formed.

FIG. 8 shows an SML according to an example in which the optical thin films 21 and 22 including the first layer 6 and the second layer 7 are provided, and an SML according to a comparative example in which the two-layer antireflection film is provided. FIG. 7 is a graph diagram in which each end face is irradiated with excitation light from an LD and the reflectance corresponding to the distance in the radial direction from the optical axis is plotted.

From this graph, it can be confirmed that the reflectance at the radial end of the SML is extremely reduced in the example compared with the comparative example.

Further, the SML according to the embodiment provided with the optical thin films 21 and 22 and the SML according to the comparative example provided with the two-layer antireflection film are respectively incorporated in an LD pumped solid-state laser device, and output from each device. Number of SH waves 10k-1
Noise in the MHz range was observed using a spectrum analyzer.

As a result, it was confirmed that the LD-pumped solid-state laser device according to the example reduced the noise in the range of several 10 k to 1 MHz by about 50% as compared with the comparative example.

[0044]

According to the invention of claim 1, each of the first layer and the second layer formed on at least one end surface of the graded index type condensing lens has a uniform film thickness as a whole. Nevertheless, it minimizes the reflectance of the excitation light vertically incident on the optical axis surface of the graded index type condensing lens and the reflectance of the excitation light incident on the effective radius part at the incident angle θ _o. Therefore, it is possible to significantly reduce the reflection of the excitation light from the entire end surface of the graded index type condensing lens.

Therefore, the oscillation of the semiconductor laser which emits the excitation light is stabilized, and the oscillation of the fundamental wave is also stabilized accordingly.
This has the effect of easily providing an LD pumped solid-state laser device with low SH wave noise.

According to the second aspect of the invention, it is possible to reduce the reflection of the excitation light from the entire end surface of the graded index type condensing lens and to improve the physical resistance of the optical thin film. Therefore, there is an effect that an LD pumped solid-state laser device with low SH wave noise and good durability can be provided easily.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an LD-pumped solid-state laser device according to an embodiment.

FIG. 2 is a partially enlarged view of a graded index type condenser lens having an optical thin film formed thereon.

FIG. 3 is an explanatory diagram showing an optical path of excitation light from a semiconductor laser.

FIG. 4 is a schematic configuration diagram of a semiconductor laser that emits excitation light.

FIG. 5 is a graph showing a refractive index distribution in the radial direction of the condenser lens.

FIG. 6 is an explanatory diagram of an incident angle and a refraction angle of excitation light with respect to the condenser lens.

FIG. 7 is a graph showing the spectral reflection characteristics on the optical axis of a condenser lens provided with an optical thin film.

FIG. 8 is a graph showing the reflectance distribution in the radial direction of the graded index type condensing lenses according to the example and the comparative example.

FIG. 9 is a schematic configuration diagram of a conventional LD-pumped solid-state laser device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser (LD) 2 Selfoc microlens (SML) 3 Solid-state laser element 4 Second harmonic generation element 5 Output mirror 6 First layer 7 Second layer 21, 22 Optical thin film 31, 32 Optical thin film 41, 42 Optical Thin film 51 Optical thin film

Claims (2)

[Claims] 1. A semiconductor laser, a graded index type condensing lens for condensing pumping light from the semiconductor laser, a solid-state laser element for irradiating the condensed pumping light to generate a fundamental wave, and In a LD pumped solid-state laser device including a second harmonic generation element that receives a fundamental wave and outputs a second harmonic, a refractive index n is applied to at least one end face of the graded index type condensing lens from the surface side. The optical axis of the graded index type condensing lens provided with the optical thin film including the first layer made of the material of ₁ and the second layer made of the material of refractive index n ₂ is provided. Formula (1) below that minimizes the reflectance of the excitation light vertically incident on the surface from the semiconductor laser
~ In addition to satisfying the conditions of (4), the semiconductor laser is incident on the effective radius portion of the graded index type condensing lens provided with the optical thin film, which is obtained by the following mathematical formulas (5) to (10) at an incident angle θ _o. The values of the optical thickness n ₁ · d ₁ of the first layer and the optical thickness n ₂ · d ₂ of the second layer are set under the condition that the reflectance R of the excitation light is minimized. A characteristic LD-pumped solid-state laser device. [Equation 1] [Equation 2] (Where δ ₁ is the phase difference of the first layer, δ ₂ is the phase difference of the second layer, n _o is the refractive index of air, n _s is the refractive index on the optical axis of the graded index type condensing lens, and n is _s' is the physical thickness of the gray refractive index of the effective radius portion of the dead-index focusing lens, d ₁ is the physical thickness of the first layer, d ₂ is the second layer, lambda _o is excitation from the semiconductor laser The wavelength of light, θ _o is the incident angle of the excitation light from the air to the first layer, θ ₁ is the refraction angle from the air to the first layer, θ ₂ is the refraction angle from the first layer to the second layer, and θ _s indicates the refraction angle from the second layer to the graded index condenser lens) 2. The first layer of the optical thin film is S which is a low refractive index material.
_2. The LD pumped solid-state laser device according to claim 1, wherein the LD pumped solid-state laser device is composed of an iO ₂ thin film and the second layer is composed of a HfO ₂ thin film of a high refractive index material.