JPH0682862A - Solid-state laser device excited with semiconductor laser - Google Patents

Abstract

PURPOSE:To decrease residual reflections and to provide the LD excited solid- state laser device having high efficiency by providing three-layered antireflection films which have the relations between film thicknesses and refractive indices satisfying specific conditions. CONSTITUTION:The three-layered antireflection films are constituted in the order of an SiO2 layer, an Al2O3 layer and an SiO2 layer. In addition, the optical film thicknesses nLdL of the first and third layers and the optical film thickness nLdH of the second layer satisfy equations when the refractive indices of the SiO2 and the Al2O3 layer respectively at about 1.06mum wavelength are designated as nL, nH and the physical film thicknesses thereof as dL, dH. More particularly preferably the average grain size of the Al2O3 layer of the second layer is <=10nm, where ns denotes the refractive index near 1.06mum wavelength of the SHG element, deltaL, deltaH respectively denote the phase differences between the SiO2 layers of the first and third layers and the Al2O3 layer of the second layer; deltaO denotes the oscillation wavelength 1.06mum of the basic wave.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser pumped solid-state laser device using a second harmonic generation element.

[0002]

2. Description of the Related Art A semiconductor laser (hereinafter referred to as "LD") using a second harmonic wave generating element and a solid-state laser device are considered to be usable for recording and reproducing on an optical disc which is a high density recording medium. There is.

FIG. 1 shows a solid-state laser device, which is a typical edge-pumped LD-pumped solid-state laser made from an Nd-doped solid-state laser crystal such as Nd-doped YVO ₄ and Nd-doped YAG whose oscillation wavelength is around 1.06 μm. A schematic diagram of the apparatus and pumping light from the LD, fundamental wave, second harmonic (hereinafter referred to as “S
"H wave". ) Is the optical path. The LD pumped solid-state laser device includes an LD 1, a condenser lens 2, a solid-state laser element 3, and a second
A harmonic generation (hereinafter referred to as "SHG") element 4, an output mirror 5, an optical thin film 6 on the LD side of the solid-state laser element and an optical thin film 7 on the opposite side, and an optical thin film 8 on the solid-state laser element side of the SHG element. It is composed of an optical thin film 9 on the opposite side and an optical thin film 10 on the SHG element side of the output mirror. When the laser oscillated from the solid-state laser element typified by Nd-doped YAG is not linearly polarized, a polarization control element such as a Brewster plate may be arranged between the solid-state laser element and the SHG element.

The excitation light (wavelength 0.809 μm) condensed by the condenser lens 2 excites the solid-state laser element 3, and the fundamental wave generated there is on the condenser lens 2 side of the solid-state laser element 3. The optical thin film 6 and the optical thin film 10 on the solid-state laser element 3 side of the output mirror 5 resonate and are amplified. The Fabry-Perot interferometer composed of the optical thin film 6 and the optical thin film 10 is called a laser resonator. The fundamental wave is confined in the laser resonator and repeatedly amplified, and a part of the fundamental wave that has passed through the SHG element 4. Are converted into SH waves, and only the SH waves are emitted from the output mirror 5.

By the way, in the laser resonator, for example,
Loss to the fundamental wave, such as absorption by the solid-state laser element 3, reflection at the end face, absorption and scattering by the optical thin films 6, 7, 8, 9, 10 may cause the laser oscillation efficiency of the fundamental wave to be extremely reduced. Therefore, the SH wave output is also significantly reduced. Therefore, the solid-state laser element and the SHG element are provided with an antireflection film in order to prevent reflection of the fundamental wave and reduce the loss of laser light in the resonator.

Conventionally, the wavelength of the fundamental wave is 1.06 μm.
When a laser beam having a refractive index of 2.10 to 2.50 in the vicinity of a wavelength of 1.06 μm is used as an SHG element using the laser beam of 1., a SiO ₂ film having a refractive index of 1.44 in the vicinity of a wavelength of 1.06 μm or 1. The Al ₂ O ₃ film No. 60 has a single-layer antireflection film with an optical film thickness of 0.25λ ₀ (λ ₀ is the oscillation wavelength of the fundamental wave of 1.06 μm). Is a wavelength of 1.0
When a single-layer antireflection film having an optical film thickness of 0.25λ ₀ is provided on LiNbO ₃ having a refractive index of 2.23 in the vicinity of 6 μm, the single-layer antireflection film required for reducing residual reflection to zero. The theoretical refractive index n _F can be obtained by the antireflection conditional expression (6), where n ₀ and n _S are the air (n ₀ ≈1) and the refractive index near the wavelength of 1.06 μm of the SHG element, respectively. , 1.49, but there is no optical thin film having a refractive index of 1.49 among general optical thin films. Therefore, a single-layer antireflection film using SiO ₂ or Al ₂ O ₃ having a refractive index close to 1.49 has been adopted.
The single-layer antireflection film of the SHG element drastically reduced the reflectance of the laser entrance / exit end face of the SHG element, so that the fundamental wave in the laser resonator could amplify the laser energy while resonating.

N _F = (n ₀ n _S ) ^1/2 (6)

[0008]

In the single-layer antireflection film of the SHG element described above as the prior art, the refractive index of the antireflection film is provided on the laser entrance / exit end surface of the SHG element, even though the antireflection film is applied. Since the conditions are not perfect, there is a slight residual reflection, which is an obstacle to improving the efficiency of the LD pumped solid-state laser device. That is, the residual reflectance R of the conventional single-layer antireflection film is obtained by refracting n ₀ , n _S , and n _F in the air (n ₀ ≈1), the SHG element, and the single-layer antireflection film in the vicinity of the wavelength of 1.06 μm. The index of refraction is LiNbO having a refractive index of 2.23, which can be obtained by the formula (7).
Residual reflectance when an antireflection film of SiO ₂ film or an Al ₂ O ₃ film ₃ is 0.13%, respectively, 0.47%
Is. This residual reflection results in a loss of the laser energy of the fundamental wave in the laser resonator, which lowers the laser oscillation efficiency.

R = (n ₀ n _S −n _F ² ) ² / (n ₀ n _S + n _F ² ) ² (7)

The present invention has been made from the above viewpoint, and includes a semiconductor laser, a condenser lens, a solid-state laser element made of an Nd-doped solid-state laser crystal, and a wavelength of 1.06 μm.
In a semiconductor laser pumped solid-state laser device basically composed of an SHG element having a refractive index in the vicinity of 2.10 to 2.50 and an output mirror, a three-layer antireflection film capable of further improving efficiency, An object is to provide an SHG element provided with the antireflection film and an LD pumped solid-state laser device using the element.

[0011]

A three-layer antireflection film of the present invention for solving the above-mentioned problems is a SiO ₂ layer, an Al ₂ O ₃ layer, and a SiO ₂ layer.
The layers are arranged in order, and the wavelength of each film is 1.06.
The refractive index of SiO ₂ and Al ₂ O ₃ near μm is n _L ,
n _H and the physical film thicknesses d _L and d _H , the optical film thicknesses n _L d _{L of} the first and third layers and the optical film thickness n of the second layer
_H d _H is one that satisfies the following formulas (8) to (12), and preferably the average grain size (grain size) of Al ₂ O _{3 in} the second layer is 10 nm or less. . And
The SHG element of the present invention is provided with the three-layer antireflection film of the present invention on the laser entrance / exit end face thereof, and the LD pumped solid-state laser device of the present invention is an LD pumped solid state using the SHG element of the present invention. It is a laser device.

[0012]

[0013]

[0014]

[0015]

[0016]

However, n _S is the wavelength of the SHG element 1.06 μ
The refractive index near m, δ _L and δ _H indicate the phase difference between the first and third SiO ₂ layers and the second Al ₂ O ₃ layer, respectively, and λ ₀ is the oscillation of the fundamental wave. The wavelength is 1.06 μm.

[0018]

The antireflection film must have a low loss with respect to the fundamental wave. This is because if there is a loss in this antireflection film, it will cause a decrease in laser oscillation efficiency. The causes of the loss of the antireflection film with respect to the fundamental wave in the laser resonator are absorption, scattering and reflection. Absorption and scattering are directly related to the physical properties of the film materials that make up each layer, but since the material used as an antireflection film is required to have moisture resistance, oxide-based dielectrics with excellent moisture resistance are required. It is necessary to use a body optical thin film. From this viewpoint, Si is used as the film material.
O ₂ , Al ₂ O ₃ , ZrO ₂ , HfO ₂ , Ta ₂ O ₅ , TiO ₂
The suitability of was examined.

1) Selection of low absorption material For absorption of an optical thin film at a wavelength of 1.06 μm, the absorption of the optical thin film is determined by irradiating the optical thin film with a laser and analyzing the temperature rise characteristic thereof. Was measured by applying the laser calorimeter method. From the measurement results shown in Table 1, it is understood that the absorption of the SiO ₂ film and the Al ₂ O ₃ film is small.

Table 1 ─────────────────────────────────── Film material SiO ₂ Al ₂ O ₃ ZrO ₂ HfO ₂ Ta ₂ O ₅ TiO ₂ ─────────────────────────────────── Absorption (ppm) <10 < 10 20 15 30 40 ────────────────────────────────────

2) Selection of Low Scattering Material If scattering is TIS and surface roughness is σ, there is a relationship shown in equation (13) between them. TIS = (4πσ / λ _TIS ) ² (13) where λ _TIS is the measurement laser wavelength.

As shown in equation 13, scattering is a function of surface roughness.
Since it is proportional to the power, the surface roughness is required to reduce the scattering.
Must be small. Surface roughness for optical thin films
Decreasing the grain size reduces the grain size.
Is equivalent to. Therefore, in the present invention, the optical thin
Regarding the scattering of the film, the scattering of the laser light with a wavelength of 1.06 μm
Optical thin film using a scanning electron microscope without directly measuring
The grain size was measured and evaluated. Table 2 shows the measurement results
It was shown to. And from Table 2, SiO₂Membrane and Al₂O 3 membranes
It turned out that the grain size was small. The result of 1)
If combined, each layer constituting the multilayer antireflection film of the SHG element
Film material is SiO₂Membrane and Al₂O₃The best combination of membranes
I can say.

Table 2 ─────────────────────────────────── Film material SiO ₂ Al ₂ O ₃ ZrO ₂ HfO ₂ Ta ₂ O ₅ TiO ₂ ─────────────────────────────────── Grain 5 10 25 20 20 20 10 Size (nm) -15-20-20-30-30-30-30 ─────────────────────────────────── ─

3) Possibility of a two-layer antireflection film It is clear that the total thickness of the multilayer antireflection film may be reduced in order to reduce the loss of the fundamental wave in the laser resonator due to absorption and scattering. is there. However, there is a fixed relationship between the film thickness, the reflectance, and the refractive index of the material, and the purpose of reducing the reflectance cannot be achieved even if the thickness is simply reduced. Therefore,
The SiO ₂ film (refractive index: 1.44) and the Al ₂ O ₃ film (refractive index: 1.60) were combined to study the film thickness that can minimize the reflectance.

The refractive index of the material of the first layer is n ₁ from the air side.
And the physical film thickness is d ₁ , the refractive index of the material of the second layer is n ₂ , and the physical film thickness is d ₂ , the optical film thickness n ₁
In a two-layer antireflection film in which d ₁ and n 2d ₂ are 0.25λ ₀ , respectively, the formula (14) or the formula (15) must be satisfied.

N ₁ ² n _S = n ₂ ² n ₀ (14)

N ₁ n ₂ = n ₀ n _S (15) where n ₀ and n _S are air (n ₀ ≈1) and SHG, respectively.
It is the refractive index in the vicinity of the wavelength of the element of 1.06 μm.

Si is used as n ₁ and n ₂ in the equation (14) or the equation (15).
Substituting the refractive indexes of the O ₂ film and the Al ₂ O ₃ film, the expressions 14 and 15 are not satisfied. Therefore, as long as the SiO ₂ film and the Al ₂ O ₃ film are used, the optical film thickness n ₁ d ₁ of the first layer is 0.25.
lambda _0, 2-layer antireflection film optical thickness n ₂ d ₂ of the second layer is 0.25 [lambda ₀ is impossible design.

Next, in order for the two-layer film in which the optical thickness of each layer is not 0.25λ ₀ when the first layer and the second layer are arranged from the air side to function as an antireflection film, The refractive index n ₁ of the first layer and the refractive index n ₂ of the second layer must satisfy the expression (16). Substituting the refractive index of the Al ₂ O ₃ film for the refractive index n ₂ of the equation (16), the refractive index n ₁ becomes 1.07 or less,
It can be seen that SiO ₂ cannot be used as the material of the layer film. Therefore, the optical thickness of each of the first and second layer films is 0.
A two-layer antireflection film that is not 25λ ₀ cannot be designed.

N ₁ ≦ (n ₀ / n _S ) ^0.5 n ₂ (16)

4) Three-layer antireflection film As a result of various investigations, the present inventor found that the three-layer antireflection film was S.
It was found that the use of iO ₂ and Al ₂ O ₃ makes it possible to form an antireflection film that can achieve the object of the present invention. This will be described below.

That is, according to an optical calculation assuming that the first layer and the third layer are made of the same material and have the same thickness, a three-layer antireflection film, the SHG element and the first and third layers and the second layer are Wavelength 1.
The refractive indexes near 06 μm are n _S , n _L , and n _H , respectively.
And the phase difference between the first and third layers and the second layer is δ respectively.
_L and δ _H , the physical film thicknesses of the first and third layers and the second layer are d _L and d _H , respectively, and λ ₀ is the oscillation wavelength of the fundamental wave 1.
When set to 06 μm, the first that satisfies the following formulas (17) to (21)
And the optical thickness n _L d _{L of} the third layer and the optical thickness n of the second layer
_{If H} d _H is present as an understanding, a three-layer antireflection film having the solution as an optical thickness achieves the object of the present invention.

[0033]

[0034]

[0035]

[0036]

[0037] Here, n _S represents the refractive index of the SHG element near the wavelength of 1.06 μm, and δ _L and δ _H are the first layer and the third layer, respectively.
The phase difference between the SiO ₂ layer of the second layer and the Al ₂ O ₃ layer of the second layer is shown, and λ ₀ shows the oscillation wavelength of the fundamental wave of 1.06 μm.

In the present invention, for example, the first and third layers are made of Si.
In O _2, the second layer determining the optical film thickness of each layer of the three-layer anti-reflection film LiNbO ₃ made SHG element of Al ₂ O _3, LiN
The refractive index n _S of bO ₃ is 2.23, and from the above values, the optical film thickness n _L d _L of SiO ₂ of the first layer from the air side is 0.097.
λ ₀ , Al ₂ O ₃ optical film thickness n _H d _H of the second layer is 0.0
56λ ₀ , the optical film thickness n _L d _L of SiO ₂ of the third layer is 0.
It becomes 097λ ₀ . Therefore, a three-layer antireflection film in which the optical film thickness of each layer has the corresponding value described above is an example of the three-layer antireflection film of the present invention.

Further, equation (17) to the refractive index of n _L of Al ₂ O ₃ (21), the d _L is the physical thickness of the Al ₂ O _3, the n _H SiO ₂
When the refractive index, d _H, is calculated by replacing the physical film thickness of SiO _{2 with} the physical film thickness of SiO ₂ , the optical film thickness n _L d _L of Al ₂ O ₃ of the first layer from the air side is 0.068λ ₀ , The optical film thickness n _H d _H of SiO ₂ of the layer is 0.114λ ₀ , and the optical film thickness n _L d _L of Al ₂ O ₃ of the third layer is 0.068λ ₀ . Like the three-layer antireflection film, it can be the three-layer antireflection film of the present invention. However, it is necessary to use SiO ₂ having higher hardness and excellent moisture resistance for the first layer. Therefore, from this viewpoint, the film having this structure cannot be the three-layer antireflection film of the present invention. . Further, in order to reduce the loss of the fundamental wave in the laser resonator of the three-layer antireflection film, the second layer of Al ₂ O _{3 is used.}
The grain size of the layer is preferably 10 nm or less.

Next, the second harmonic generating element of the present invention has the three-layer antireflection film of the present invention having the above-described structure provided on the laser entrance / exit end face thereof. As a method that can be applied when the three-layer antireflection film is provided, there are a vacuum vapor deposition method, a sputtering method, and the like, which are not particularly particular,
In order to reduce the grain size of the Al ₂ O ₃ film of the second layer, it is preferable to use the ion assisted vapor deposition method of oxygen. By the way, if an Al ₂ O ₃ film is formed by a general electron beam evaporation method in which the substrate heating temperature is 300 ° C., the grain size is 10 to 20 nm, but oxygen is used as the ionizing gas, and the acceleration voltage is 100 V. , Acceleration current 1
If an Al2O3 film is formed by the ion assisted vapor deposition method with 0 mA, the grain size becomes 5 to 10 nm. Regarding the first and third layers of SiO ₂ film, the grain size does not change even if ion assist is applied, but there is no problem even if the ion assist method is applied for the convenience of film formation. At this time, it should be noted that the SiO ₂ film may be slightly colored when the ion acceleration voltage is higher than about 500V. Finally, the LD-pumped solid-state laser device of the present invention is an LD-pumped solid-state laser device using the SHG element of the present invention and requires no further explanation.

[0041]

EXAMPLES Next, examples of the present invention will be described. Example 1 An example of the present invention will be described in detail below.
Specifically, the three-layer antireflection film of the SHG element of LiNbO ₃ having a refractive index n _S of 2.23 used in the LD pumped solid-state laser device will be described.

First, the optical film thickness of each layer is obtained.
Substituting the refractive index of 1.44 of the SiO ₂ film and the refractive index of 1.60 of the Al ₂ O ₃ film into n _L and n _H of the equations (17) to (21), respectively, the first layer and the second layer are calculated. The optical film thickness n _L d _{L of the} third layer (SiO ₂ film) and the optical film thickness n _H d _{H of} the second layer (Al ₂ O ₃ film) are 0.097λ ₀ and 0.056λ ₀ , respectively.
Becomes

Next, a method of forming the three-layer antireflection film will be described. The SHG element was ultrasonically cleaned using a detergent, an organic solvent and the like. An ion-assisted vapor deposition apparatus was used to form the antireflection film. After setting the sample, the sample was heated to 300 ° C. and evacuated to 1 × 10 ⁻⁶ Torr, oxygen was used as the ionized gas, and the vapor deposition rate was 0.5 nm / An SiO ₂ film having a predetermined thickness was formed in sec. Then add 1x oxygen gas
0.3 nm / se by using a so-called ion assist method in which oxygen is introduced as an ionized gas, acceleration voltage is 100 V, and acceleration current is 10 mA by introducing up to 10 ⁻⁴ Torr.
An Al ₂ O ₃ film having a predetermined thickness was formed at the vapor deposition rate of c. Then, the gas was exhausted again to 1 × 10 ⁻⁶ Torr, oxygen gas was used as the ionization gas, and the deposition rate was 0.5 nm / sec.
Then, a SiO ₂ film having a predetermined thickness was formed. An optical interference monitor was used to control the optical film thickness of each layer.

FIG. 2 shows a schematic view of this antireflection film.
FIG. 3 shows the spectral reflection characteristics of this three-layer antireflection film. For the comparison, also creates LiNbO ₃ made SHG element subjected to single-layer anti-reflection film of a conventional LiNbO ₃ made SHG device was subjected to SiO ₂ single layer anti-reflective film Al ₂ O _3, of the present invention 3-layer 500mWLD excited with SHG element of the antireflection film-LiNbO ₃ Nd: YVO4 disposed in a laser to measure the respective SH wave output, the results are shown in Table 3. From the measurement results of SH wave output shown in Table 3, the results of 3 of the present invention were obtained.
LD using LiNbO ₃ SHG element with anti-reflection coating
The pumping solid-state laser device is a conventional LiN with a single-layer antireflection film.
and LD pumped solid-state laser apparatus than can be obtained SH wave output 1.5 times or more using the SHG element bO _3.

Table 3 ─────────────────────────────────── Film type Three-layer antireflection of the present invention Film Single-layer antireflection film SiO ₂ / Al ₂ O ₃ / SiO ₂ SiO ₂ Al ₂ O ₃ ──────────────────────────── ─────── SH wave output 25 15 6 (mW) ────────────────────────────────── ─

[0046]

Since the three-layer antireflection film of the present invention can further reduce the residual reflection, the three-layer antireflection film is provided on the SHG element having a refractive index of 2.10 to 2.50 at a wavelength of about 1.06 μm. LD constructed using SHG element created by
If the pumped solid-state laser device is used, high-output SH waves can be obtained more efficiently.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a typical LD-pumped solid-state laser device.

FIG. 2 is a schematic view of a three-layer antireflection film applied to the SHG element of the present invention.

FIG. 3 is a spectral reflection characteristic of a three-layer antireflection film applied to the SHG element of the present invention.

[Explanation of symbols]

1 --- LD, 2 --- condensing lens, 3 --- solid-state laser device, 4 --- SHG device, 5 --- output mirror, 6 ----
-Optical thin film, 7 --- Optical thin film, 8 --- Optical thin film, 9
--- Optical thin film, 10 --- Optical thin film, 11 --- Optical thin film, 12 --- Optical thin film, 13 --- SiO2 film, 1
4 --- Al2O3 film, 15 --- SiO2 film, 16 ---
-SHG element

Claims (4)

[Claims] 1. A semiconductor laser, a condenser lens, and Nd
A solid-state laser device composed of a doped solid-state laser crystal and having a refractive index of 2.10 to 2.50 in the vicinity of a wavelength of 1.06 μm.
In a semiconductor laser pumped solid-state laser device basically composed of a second harmonic generation element of
_Two layers, an Al ₂ O ₃ layer and a SiO ₂ layer are formed in this order, and the refractive index of SiO ₂ and Al ₂ O ₃ near the wavelength of 1.06 μm of each film is n _L , n _H When the film thickness is d _L and d _H , the optical film thickness n _{L of} the first and third layers
d _L and the optical thickness n _H d _{H of} the second layer are expressed by the following formula (1)
A three-layer antireflection film for a second harmonic generation element, characterized by satisfying (2) to (2). Here, n _S represents the refractive index in the vicinity of the wavelength of 1.06 μm of the second harmonic generation element, and δ _L and δ _H are the first and third layers of SiO ₂ layer and the second layer of Al ₂ O, respectively. The phase difference from the _three layers is shown, and λ ₀ shows the oscillation wavelength of the fundamental wave of 1.06 μm. 2. The three-layer antireflection film according to claim 1, wherein the average grain size (grain size) of Al 2 O _{3 in} the second layer is 10 nm or less. Three-layer antireflection film. 3. The three-layer antireflection film for a second harmonic wave generating element according to claim 1 or 2 is provided on a laser entrance / exit end face of a second harmonic wave generating element having a refractive index of 2.10 to 2.50. A second harmonic generation element, characterized in that 4. An LD-pumped solid-state laser device using the second harmonic generation element according to claim 3.