JPH06265955A - Wavelength converting element - Google Patents

Abstract

PURPOSE:To provide a simple ultraviolet light source by efficiently generating the fourth harmonic wave by one fundamental wave light source and one external double resonator. CONSTITUTION:A resonator 2a having a high reflection factor only for a second harmonic wave 2omega is constituted, and a first nonlinear optical crystal 3 for second harmonic wave generation and a second nonlinear optical crystal 4 for fourth harmonic wave generation are arranged on the optical axis in this resonator 2a, and a fundamental wave omega is made incident on the resonator 2a from the outside, and the generated second harmonic wave 2omega is allowed to resonate to increase the internal electric field intensity of the second harmonic wave, and a fourth harmonic wave 4omega is generated from the second harmonic wave 2omega.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion element that uses a non-linear optical crystal to generate a fourth harmonic from a fundamental wave.

[0002]

2. Description of the Related Art Conventionally, photolithography technology has been widely adopted, for example, in the manufacture of semiconductor integrated circuits. In this type of photolithography technique, g-line (436 nm) from an ultra-high pressure mercury lamp is mainly used in an exposure process that influences the degree of miniaturization. Excimer lasers as ultraviolet lasers have been considered to be effective because narrowing the wavelength of exposure light and narrowing the band are effective for thinning semiconductor integrated circuits. However, since the excimer laser has a defect that a stable output cannot be obtained for a long period of time, it has not been put to practical use for stable mass production.

On the other hand, as a short wavelength laser light source and a near infrared variable wavelength light source, a wavelength conversion element using a nonlinear optical crystal is widely used. The non-linear optical effect means that polarization induced by a strong electromagnetic field shows a non-linear response that is not proportional to the strength of the electromagnetic field. As a technique utilizing this non-linear optical effect, a technique for generating a second harmonic is typical, and a ruby laser, a He-Ne laser, a semiconductor laser and a second harmonic are used. Experiments have been conducted to generate

This wavelength conversion element converts the wavelength of incident laser light by utilizing the non-linearity of a non-linear optical crystal, and normally uses a resonator in order to perform wavelength conversion with high efficiency. It is being appreciated. Examples of this type of resonator include a fundamental wave resonator capable of confining only the fundamental wave, a second harmonic resonator capable of confining only the second harmonic wave, and both a fundamental wave and a second harmonic wave. There are three ways, a double resonator designed to be confined.

Since the absorption coefficient of the second harmonic of the nonlinear optical crystal is larger than that of the fundamental wave, the above-mentioned fundamental wave resonator is generally used. In order to generate the fourth harmonic, ultraviolet light, it is necessary to perform wavelength conversion twice. One way to do this is 2
One that performs wavelength conversion, one that performs second harmonic generation (SHG) in two stages by an external resonator, and one that combines SHG in a laser resonator and SHG by an external resonator. There is.

FIG. 9 shows an outline of a conventional fourth harmonic generator of a YAG laser. In the figure, reference numeral 30 is a rod-shaped YAG laser oscillation medium, 31 is an excitation lamp as an excitation means, 32 is a total reflection mirror, 33 is an emission mirror, and 34 is a second YAG laser. Reference numeral 35 denotes a first nonlinear optical element for generating a harmonic wave, and 35 denotes a second nonlinear optical element for generating a fourth harmonic wave of a YAG laser.

In the figure, a fundamental wave resonator 36 for resonating a fundamental wave ω (wavelength 1064 nm) is constituted by a total reflection mirror 32 and an emission mirror 33, and an excitation lamp (excitation means) 31 such as a krypton lamp is used. Excited and YA
The fundamental wave ω emitted from the G laser oscillation medium 30 is a fundamental wave resonator 3 including a total reflection mirror 23 and an emission mirror 33.
6 resonates in 6 and the 1064 nm fundamental wave ω is stimulated to be emitted and emitted from the emission mirror 33.

The fundamental wave ω of the YAG laser emitted from the fundamental wave resonator 36 is incident on the first nonlinear optical element 34 for generating the second harmonic wave, and the second harmonic wave 2ω (532).
nm). The generated second harmonic 2ω is
It is adapted to be incident on the second nonlinear optical element 35 for generating a harmonic wave to generate a fourth harmonic wave 4ω.

This is basically a one-pass wavelength conversion to the 4th harmonic, and the conversion efficiency is low,
It is usual to perform Q conversion on the AG laser and perform wavelength conversion with a pulse having an increased peak power of the fundamental wave.

FIG. 10 shows an outline of another conventional fourth harmonic generator. In this conventional example, a YAG laser oscillation medium 30 and a first harmonic generating second harmonic wave are provided between the resonator mirrors 38 and 39 forming the linear resonator 37 of the YAG laser.
Of the second harmonic 2ω with high efficiency by using the fundamental wave electric field inside the laser resonator 37.
Generate. Then, the output of the generated second harmonic wave 2ω such as green is made incident on the ring resonator 40 of the next stage,
By circulating the harmonic wave 2ω in the ring resonator 40, the electric field strength of the second harmonic wave 2ω in the ring resonator 40 is increased, and the fourth harmonic wave generation device arranged in the ring resonator 40 is generated. The second nonlinear optical crystal 35 of
A harmonic 4ω is generated.

FIG. 11 shows an outline of still another conventional fourth harmonic generator. In this conventional example, the fundamental wave ω (wavelength 106 is generated by the total reflection mirror 32 and the emission mirror 33.
A fundamental wave resonator 36 that resonates 4 nm) is constituted, and the fundamental wave ω emitted from the YAG laser oscillation medium 30 after being excited by the excitation lamp (excitation means) 31 such as a krypton lamp is totally reflected by the mirror 32. Resonating is performed by the resonator 36 composed of the emitting mirror 33 and the fundamental wave ω of 1064 nm is induced and radiated to be emitted from the emitting mirror 33.

Then, the fundamental wave ω from the YAG laser resonator 36 is made incident on the first ring resonator 41 in which the first nonlinear optical crystal 34 for generating the second harmonic is arranged. By circulating the fundamental wave ω in the ring resonator 41, the electric field strength of the fundamental wave ω in the ring resonator 41 is increased to efficiently generate the second harmonic wave 2ω. The second harmonic wave 2ω generated in the first ring resonator 41 is further internally included in the second nonlinear optical crystal 35 for generating the fourth harmonic wave.
Is injected into the second ring resonator 40 in which
By circulating the harmonic wave 2ω in the ring resonator 40, the electric field strength of the second harmonic wave 2ω in the ring resonator 40 is increased so that the ultraviolet light, which is the fourth harmonic wave 4ω, is efficiently generated. It was done.

[0013]

However, in the above-mentioned conventional example, basically, two resonators are connected in series to perform wavelength conversion by fundamental wave resonance in two stages. Instability is amplified. Moreover, since the wavelength conversion efficiency at each stage is low, not only a strong fundamental wave light source is required, but also the frequency matching between the input frequency and the resonator frequency and the input mirror reflectivity are optimized at each stage. There was a problem that it was necessary to perform matching, and this became quite complicated.

In view of the above, the present invention provides one that uses one fundamental wave light source and can perform wavelength conversion from the fundamental wave to the fourth harmonic with a simple structure and high efficiency. The purpose is to

[0015]

In order to achieve the above object, the wavelength conversion element according to the present invention is arranged such that the fundamental wave incident from the light source on the optical axis of the resonator for the second harmonic is the second harmonic. And a second nonlinear optical crystal for generating a fourth harmonic from the second harmonic, and a second nonlinear optical crystal for generating a fourth harmonic from the second harmonic. It is what

Further, the fundamental wave incident from the light source is incident on the optical axis of the ring resonator in which at least three second harmonic reflection mirrors for multiple reflection of the second harmonic are arranged in a closed ring shape. A first nonlinear optical crystal for generating a second harmonic for converting into a wave, and a second nonlinear optical crystal for generating a fourth harmonic for generating a fourth harmonic from the second harmonic are arranged. It is a feature.

Further, the fundamental wave incident from the light source is converted into the second harmonic wave on the optical axis of the linear resonator in which a pair of second harmonic wave reflection mirrors for multiply reflecting the second harmonic wave are linearly arranged. A first non-linear optical crystal for generating a second harmonic and a second non-linear optical crystal for generating a fourth harmonic that generates a fourth harmonic from the second harmonic. It is a thing.

Here, the first nonlinear optical crystal for generating the second harmonic and the nonlinear optical crystal for the second harmonic for generating the fourth harmonic are arranged to face each other, and the first nonlinear optical A linear resonator is formed by forming a second harmonic reflection mirror that reflects the second harmonic on the end faces of the crystal and the second nonlinear optical crystal that are opposite to the end faces that face each other and generate the second harmonic. For the first nonlinear optical crystal for KNbO ₃ and β-Ba for the second nonlinear optical crystal for generating the fourth harmonic.
It is also possible to use B ₂ O ₄ respectively.

[0019]

According to the present invention configured as described above, the second harmonic obtained by making the fundamental wave incident on the first nonlinear optical crystal for generating the second harmonic is reflected by multiple reflection in the resonator. Resonance enhances the electric field strength of the second harmonic in the resonator, thereby increasing the conversion efficiency to the second harmonic, and the second harmonic having the strong electric field strength is arranged in the resonator.
The fourth harmonic can be efficiently generated by entering the second harmonic nonlinear optical crystal for harmonic generation.

[0020]

Embodiments of the present invention will be described below with reference to FIGS. In each of the following embodiments, in the figure, the solid line indicates the optical path (optical path) of the fundamental wave ω, the broken line indicates the optical path (optical path) of the second harmonic 2ω, and the dashed-dotted line indicates the fourth path.
The respective paths (optical paths) of the light of the harmonic 4ω are shown.

FIG. 1 shows a first embodiment. This embodiment uses a pair of second harmonic reflection mirrors 1 and 1'which multiple-reflects the second harmonic, so that a straight line for the second harmonic is generated. The resonator 2a is formed, and the first nonlinear optical crystal 3 for generating the second harmonic and the second nonlinear optical crystal for generating the fourth harmonic are arranged on the optical axis inside the linear resonator 2a. 4 and 4 are arranged in series.

That is, the transmittance t of the nonlinear optical crystals 3 and 4 with respect to the fundamental wave ω is set to 99.7%, and the reflection of the second harmonic reflection mirrors 1ω on the light source side and the output side of the second harmonic reflection mirrors 1 and 1 '. Assuming that the rates are r ₁ (2ω) and r ₁ ′ (2ω), these reflectances are, for example, r ₁ (2ω) = 99.9%, r ₁ ′ (2
ω) = 99.9%.

Further, the reflectance r for the fourth harmonic 4ω
(4ω) is 0% (r
(4ω) = 0%). As a result, the fundamental wave ω entering the linear resonator 2a from the light source is multiple-reflected by the second harmonic reflection mirrors 1 and 1'and the electric field strength thereof is strengthened, and the second harmonic wave of this strong electric field strength is increased. 2ω is incident on the second nonlinear optical crystal 4 for generating the fourth harmonic wave arranged in the linear resonator 2a, and the fourth harmonic wave 4ω is obtained with high efficiency.

Here, KNbO ₃ or the like is used as the first nonlinear optical crystal 3 for generating the second harmonic, and β-BaB ₂ O ₄ or the like is used as the second nonlinear optical crystal 4 for generating the fourth harmonic. it can.

FIG. 2 shows the end faces of the first nonlinear optical crystal 3 and the second nonlinear optical crystal 4 without providing the second harmonic reflection mirrors 1 and 1'in the embodiment shown in FIG. By applying an optical coating to the first non-linear optical crystal 3 and the second non-linear optical crystal 4, the second harmonic reflection mirrors 1, 1'are integrally formed,
This is a configuration of the linear resonator 2b for the higher harmonic wave 2ω.

That is, the end faces of the first nonlinear optical crystal 3 and the second nonlinear optical crystal 4 opposite to the end faces facing each other are totally transmitted with respect to the fundamental wave ω and the second harmonic wave 2ω.
Optical coating with high reflectance is applied to
The second harmonic reflection mirrors 1, 1'are formed here.

In this case, the second harmonic reflection mirror 1, 1 '
It is preferable that the end faces of the nonlinear optical crystals 3 and 4 on which the ridges are formed are processed or polished into spherical surfaces. FIG. 3 shows an embodiment applied to a ring resonator, in which four second harmonic reflection mirrors 5, 6, 7, 8 which reflect the second harmonic 2ω are arranged in a closed ring shape to form a ring. The resonator 9a is formed, and the first nonlinear optical crystal 3 for generating the second harmonic and the second nonlinear optical crystal 4 for generating the fourth harmonic are arranged on the optical axis inside the ring resonator 9a. It is arranged.

These second harmonic reflection mirrors 5, 6,
7 and 8 show the reflectance for the second harmonic 2ω as r ₅ (2
ω), r ₆ (2ω), r ₇ (2ω), r ₈ (2ω)
These reflectances are, for example, r ₅ (2ω) = r ₆ (2ω) =
r ₇ (2ω) = r ₈ (2ω) = 99.9% and the fourth
The reflectance with respect to the harmonic 4ω is set to 0% at least for the mirror 8 on the emission side.

In the embodiment shown in FIG. 4, the ring resonator 9b is constructed by arranging three second harmonic reflection mirrors 5, 6 and 10 which reflect the second harmonic 2ω in a closed ring shape. The first nonlinear optical crystal 3 for generating the second harmonic and the second nonlinear optical crystal 4 for generating the fourth harmonic are arranged on the optical axis inside the ring resonator 9b.

These second harmonic reflection mirrors 5, 6, 1
0 is the reflectance for the second harmonic 2ω, r ₅ (2ω),
Let r ₆ (2ω) and r ₁₀ (2ω) be, for example, r ₅ (2ω) = r ₆ (2ω) = r ₁₀ (2ω) =
It is set to 99.9%, and the reflectance with respect to the fourth harmonic 4ω is at least 0% with respect to the emitting side mirror 6.

In the above embodiment, the first nonlinear optical crystal 3 and the second nonlinear optical crystal 4 may be arranged somewhere on the optical axis in the ring resonators 9a and 9b.
It is desirable to arrange them at two beam weight positions.

Here, a KNbO ₃ crystal is considered as the nonlinear optical crystal 3 for generating the second harmonic. The KNbO ₃ crystal can be phase-matched with respect to the wavelength (860 nm) of the semiconductor laser. In order to bring the generated second harmonic into the resonance state, it is necessary to match the phase after one round in the resonator with the original phase. However, the phase matching and the resonance condition are adjusted by adjusting the temperature of the KNbO ₃ crystal. Can be satisfied at the same time.

That is, assuming that the crystal length of the KNbO ₃ crystal is 1 mm, the temperature tolerance at a wavelength of 860 nm is 4 ° C.
Refractive index n _c of c-axis for second harmonic of KNbO ₃ crystal
The temperature coefficient of (2ω) is δn _c (2ω) /δT=0.0
Since it is about 001 / ° C., if the crystal length is 1 mm, the resonance condition is satisfied in a cycle of about 2.1 ° C. Therefore, when the temperature is changed as shown in FIG. 5, there is a point where the second harmonic resonance is established about three times within the temperature tolerance.

In the case of a crystal having a strict phase matching tolerance, or when it is necessary to strictly satisfy the resonance condition at the phase matching center, the position of the mirror is adjusted by an actuator such as PZT or the inside of the resonator is adjusted. A non-linear optical crystal for phase modulation is arranged in the to adjust the phase, or, preferably, by finely adjusting the temperature of the non-linear optical crystal 4 for generating the fourth harmonic, which is arranged at the same time, within the temperature tolerance of the second harmonic. A resonance state can be realized.

The three second harmonic reflection mirrors shown in FIG.
In the case of the ring resonator 9b composed of 5, 6 and 10, the reflectance of the second harmonic reflection mirrors-5, 6 and 10 for the second harmonic 2ω is 99.9%, and the KNbO ₃ crystal at the second harmonic 2ω is used. 3 absorption coefficient 3% / cm, conversion efficiency γ _SH = 0.02
When the internal power of the second harmonic 2ω is calculated by changing the crystal length of the KNbO ₃ crystal 3 as W · cm, the result is as shown in FIG.

From this figure, for example, when the crystal length is 5 mm,
The second harmonic of 5 W is stored in the resonator 9b with respect to the input of the fundamental wave of 200 mW. When the crystal length is 1 mm, the power inside the resonator 9b of the second harmonic increases up to about 12 W with respect to the input of the fundamental wave of 200 mW, and the power inside the resonator of the second harmonic 2ω increases. It is found that it is more advantageous to shorten the crystal length of the KNbO ₃ crystal to reduce the absorption loss and increase the Q factor of the resonator.

When the resonator internal power of the second harmonic increases, the sample temperature rises due to the absorption of the second harmonic, and the thermal lens effect and the temperature distribution in the propagation direction occur. In order to avoid this as much as possible, it is necessary to use a nonlinear optical crystal with low absorption or to shorten the crystal length as much as possible to avoid heat generation.

Β-BaB ₂ O ₄ is used as the nonlinear optical crystal 4 for generating the fourth harmonic, and the fourth harmonic conversion efficiency is γ.
Assuming that _SH = 0.00002 / W, it is possible to generate ultraviolet light, which is the fourth harmonic of about 3 mW, from the internal power of the second harmonic 2ω of 12 W.

Next, the first embodiment for the embodiment shown in FIG.
An example of the use of is shown in FIG. In this usage example, a SANYO LD is used as a light source LD (laser diode) 11 for a fundamental wave ω (wavelength 860 nm), and a collimating lens 12 is used to obtain a parallel light flux, and then a Faraday isolator is provided to prevent return of the fundamental wave ω. 13 and the focal length f = 10
The fundamental wave ω is condensed by the 0 mm condenser lens 14 and is incident on the ring resonator 9a.

The reflectance for the fundamental wave ω (wavelength 860 nm) and the second harmonic 2ω (wavelength 430 nm) of the second harmonic reflection mirrors 5, 6, 7, 8 of the ring resonator 9a and the fourth harmonic 4ω (wavelength) 215 nm) for the transmittance r
₅ (ω), r ₅ (2ω), T ₅ (4ω), r ₆ (ω), r ₆ (2
ω), T ₆ (4ω), r ₇ (ω), r ₇ (2ω), T ₇ (4
ω), r ₈ (ω), r ₈ (2ω), T ₈ (4ω)
₅ (2ω) = 99.9%, T ₅ (4ω) = 85%, r ₆ (2
ω) = r ₇ (2ω) = r ₈ (2ω) = 99.9%, T ₆ (4
ω) = 85%.

A KNbO ₃ crystal (a-axis crystal, crystal length 1 mm) is used as the nonlinear optical crystal 3 for generating the second harmonic, and antireflection coating (r (ω) = 0.2%,
r (2ω) = 0.2%, r (4ω) = 6%). The KNbO ₃ crystal 3 used was after crystal growth, and the absorption coefficient at the second harmonic 2ω was 3% / cm.

Β-BaB ₂ O ₄ is used as the nonlinear optical crystal 4 for generating the fourth harmonic, and antireflection coatings (r (ω) = 0.2%, r (2ω) = 0. 2%, r
(4ω) = 6%). As a β-BaB ₂ O ₄ crystal, a type 1 (θ = 70.5) in which a fundamental wave ω whose direction is linearly polarized is incident and a second harmonic wave 2ω orthogonal thereto is obtained.
3 °, φ = 0 °) and a crystal length of 3 mm was used.
The transmittance in one pass including the coating of β-BaB ₂ O ₄ was 99.5%.

The temperature of the KNbO ₃ crystal 3 was finely adjusted so that the blue emission of the fundamental wave ω was the strongest, and β-BaB ₂ O ₄
When the temperature of the crystal 4 was adjusted by a Peltier temperature element to achieve phase matching and at the same time, the phase was adjusted so that the resonance condition was satisfied. Some ultraviolet light (wavelength 215
nm) was obtained.

A second use example is shown in FIG. In this usage example, a semiconductor excitation YAG laser is used as a light source of a fundamental wave ω (wavelength 1064 nm), and two semiconductor lasers 15,
15 'is cross-pumped by the polarization beam splitter 16 to excite the YAG crystal 17. Then, the quarter wave plate 18 is used to obtain a single frequency, the fundamental wave emitted from the output mirror 19 is condensed by the condenser lens 20 having the focal length f = 100 mm, and is incident on the ring resonator 9a. It was done like this.

Here, the second harmonic wave of the ring resonator 9a
The fundamental wave ω of the reflecting mirrors 5, 6, 7, and 8 (wavelength 1064n
m) and the second harmonic 2ω (wavelength 532 nm) reflection
Rate and 4th harmonic 4ω (wavelength 266nm) transmission
Rate r_Five(ω), r_Five(2ω), T _Five(4ω), r₆(ω), r
₆(2ω), T₆(4ω), r₇(ω), r₇(2ω), T₇(4
ω), r₈(ω), r₈(2ω), T₈If (4ω), then r
_Five(2ω) = 99.9%, T_Five(4ω) = 85%, r₆(2
ω) = r₇(2ω) = r₈(2ω) = 99.9%, T₆(4
ω) = 85%.

KTP (crystal length 2 mm) is used as the first nonlinear optical crystal 3 for generating the second harmonic, and antireflection coating (r (ω) = 0.2%, r (2ω) is applied to both end faces thereof. =
0.2% and r (4ω) = 6%). Β-BaB ₂ as the second nonlinear optical crystal 4 for generating the fourth harmonic
O ₄ is used, and antireflection coating (R (ω) =
0.2%, R (2ω) = 0.2%, R (4ω) = 6%)
It is carried out. As the β-BaB ₂ O ₄ crystal, one having the above-mentioned type 1 (θ = 47.63 °, φ = 0 °) and a crystal length of 3 mm was used.

The temperature of KTP3 is finely adjusted so that the blue emission of the second harmonic wave 2ω becomes the strongest, and the temperature of the β-BaB ₂ O ₄ crystal 4 is adjusted by the Peltier temperature element to achieve phase matching and at the same time, the resonance condition is obtained. When the phase adjustment was performed so as to satisfy the condition, ultraviolet light (wavelength 266 nm) that was the fourth harmonic 4ω of 30 mW was obtained with respect to the input of the fundamental wave ω of 200 mW.

[0048]

Since the present invention has the above-mentioned structure,
A first non-linear optical crystal for generating a second harmonic, and a fourth
By arranging the second non-linear optical crystal for generating a harmonic inside the resonator for the second harmonic, the high internal electric field strength of the second harmonic in the resonator is utilized,
The fourth harmonic can be generated from one resonator with high conversion efficiency.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a second embodiment.

FIG. 3 is a schematic diagram showing a third embodiment.

FIG. 4 is a schematic diagram showing a fourth embodiment.

FIG. 5 is a graph showing the relationship between crystal temperature and phase matching characteristics and resonance characteristics in KNbO ₃ .

6 is a graph showing the relationship between the fundamental wave input power and the second harmonic internal power when the crystal length of the KNbO ₃ crystal in the example shown in FIG. 4 is changed.

FIG. 7 is a schematic diagram showing a first usage example of the embodiment shown in FIG.

FIG. 8 is likewise a schematic diagram showing a second usage example.

FIG. 9 is a schematic diagram showing a conventional example.

FIG. 10 is a schematic diagram showing another conventional example.

FIG. 11 is a schematic diagram showing still another conventional example.

[Explanation of symbols]

1,1 ′, 5,6,7,9,10 Second harmonic reflection mirror 2a, 2b Linear resonator 3 First nonlinear optical crystal 4 Second nonlinear optical crystal 9a, 9b Ring resonator

Claims (5)

[Claims] 1. A first nonlinear optical crystal for generating a second harmonic for converting a fundamental wave incident from a light source into a second harmonic on the optical axis of a resonator for the second harmonic, and a second nonlinear optical crystal. A wavelength conversion element, comprising: a second nonlinear optical crystal for generating a fourth harmonic that generates a fourth harmonic from the harmonic. 2. At least 3 for multiple reflection of a second harmonic
A first nonlinear for generating a second harmonic that converts a fundamental wave incident from a light source into a second harmonic on the optical axis of a ring resonator in which two second harmonic reflection mirrors are arranged in a closed ring shape. Optical crystal and a fourth harmonic that generates a fourth harmonic from the second harmonic
A wavelength conversion element, wherein a second nonlinear optical crystal for generating harmonics is arranged. 3. A fundamental wave incident from a light source on the optical axis of a linear resonator in which a pair of second harmonic reflection mirrors for multiple reflection of the second harmonic are linearly arranged is converted into a second harmonic. A first nonlinear optical crystal for generating a second harmonic, and a second nonlinear optical crystal for generating a fourth harmonic from the second harmonic.
And a non-linear optical crystal of the above are arranged. 4. A first nonlinear optical crystal for generating a second harmonic and a nonlinear optical crystal for a second harmonic for generating a fourth harmonic are arranged to face each other, and the first nonlinear optical crystal is arranged. 4. A linear resonator is formed by forming a second harmonic reflection mirror that reflects the second harmonic on the end faces of the second nonlinear optical crystal and the opposite end faces of the second nonlinear optical crystal. The wavelength conversion element described. 5. A KNbO ₃ crystal is used as the first nonlinear optical crystal for generating the second harmonic, and a β-BaB ₂ O ₄ crystal is used as the second nonlinear optical crystal for generating the fourth harmonic. The wavelength conversion element according to any one of claims 1 to 4,