JPH06265953A - Wavelength converting element - Google Patents

Abstract

PURPOSE:To perform efficient wavelength conversion by satisfying the condition in which a fundamental wave and the second harmonic wave resonate and the condition in which the second harmonic wave power is efficiently accumulated in a resonator. CONSTITUTION:This wavelength converting element which is provided with a double resonator 4a, where a fundamental wave omega and a second harmonic wave 2omega are shut, and incorporates at least one nonlinear optical crystal 5 for second harmonic wave generation in this double resonator 4a is characterized by providing the double resonator 4a with a phase difference control part 6 which controls the phase difference between the fundamental wave omega and the second harmonic wave 2omega on each incidence end face of the nonlinear optical crystal to, for example, about pi/2 at the time of double resonance.

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 using a non-linear optical crystal, and more particularly to a wavelength conversion element having a double resonator so that high wavelength conversion efficiency can be obtained.

[0002]

2. Description of the Related Art Wavelength conversion elements using nonlinear optical crystals are widely used as short wavelength laser light sources, near infrared variable wavelength light sources, and the like. This wavelength conversion element is adapted to convert the wavelength of incident laser light by utilizing the nonlinearity of a nonlinear optical crystal.

In order to perform this wavelength conversion with high efficiency, it is usual to use a resonator, and as this type of resonator, a fundamental wave resonator capable of confining only a fundamental wave, a second harmonic There are three types: a second harmonic resonator that can confine only waves, and a double resonator that can confine both the fundamental wave and the second harmonic.

Since the absorption coefficient of the second harmonic of the nonlinear optical crystal is larger than that of the fundamental wave, the fundamental wave resonator is generally used. But,
There is a trade-off between the loss due to the absorption of the second harmonic of the nonlinear optical crystal and the effect of increasing the power of the resonator. However, when the above double resonator is used, the second harmonic is larger than that when the fundamental resonator is used. It is known that wave output may be obtained (JAArm-stroong, N. Bloembergen, J. Ducuin
g, and PSPershan ； Physical Review, 127 (6) p.1918 (S
See ept.1962)).

Therefore, for example, Japanese Patent Laid-Open No. 3-219215
As a light source for injecting a fundamental wave into a nonlinear optical crystal,
A non-linear optical crystal for converting a fundamental wave into a second harmonic and a double resonator for both the fundamental and the harmonic (double resonator) are provided, and a double resonator for the fundamental and the second harmonic is provided. In order to improve the wavelength conversion efficiency by doing so, Japanese Patent Laid-Open No. 4-263231 discloses that a nonlinear optical medium, a fundamental wave and a second harmonic wave are arranged on an optical axis in a ring resonator. It has been proposed to provide a phase shifter that shifts the phase difference between and by about π so that the phases of the fundamental wave and the second harmonic wave can be matched to receive a highly efficient amplifying action.

[0006]

However, in the above conventional example, in the case of the linear double resonator, the inside of the resonator is set to 1
The second harmonic generated after the cycle and the second generated from the fundamental wave
Harmonics are not necessarily additive just by satisfying the longitudinal mode condition (double resonance condition) of the fundamental wave and the second harmonic, and when the second harmonic power does not build up in the resonator. There is. Further, in both the case of the linear double resonator and the case of the ring double resonator, the double resonance frequency is determined from the reflection phase shift amount that the fundamental wave and the second harmonic wave receive from the mirror when the circuit makes one round in the resonator. , Not necessarily in the phase matching curve,
There is a problem that double resonance cannot be performed because of good wavelength conversion efficiency.

This will be described with reference to FIGS. This figure qualitatively shows the power accumulation state in the case of a linear double resonator. For simplicity, the phase matching is perfectly achieved in the nonlinear optical crystal 1, and the fundamental wave ω and The ideal mirror 2 having the same reflecting surface for the second harmonic 2ω is considered.

The fundamental wave electric field in the nonlinear optical crystal 1 and the second harmonic electric field excited from the fundamental wave propagate in a phase relationship as shown in FIG. And the fundamental wave ω and the second
When the harmonic 2ω is reflected by the ideal mirror 2, the phase of both is shifted by π. That is, as shown in FIG.
For example, what was the peak of the electric field becomes the valley of the electric field after being reflected by the ideal mirror 2. At this time, the fundamental wave ω and the second harmonic wave 2
The relative phase of ω changes by π.

Therefore, the state of light wave propagation in the return path is
As shown in FIG. 19, the second harmonic 2 generated in the outward path
ω and the second harmonic 2ω ′ newly generated on the return path cancel each other.

When the resonator makes one revolution, the fundamental wave ω
And the second harmonic 2ω both have a phase shift of 2π.
Therefore, if the longitudinal mode condition of the fundamental wave ω is satisfied, the second
The longitudinal mode condition is also satisfied for the higher harmonic wave 2ω. However, as described above, the second harmonic wave 2ω that has made one round and the newly generated second harmonic wave 2ω ′ cancel each other out, and the second harmonic power does not build up in the resonator. .

The above description is a special case.
The case where the reflection at the ideal mirror 2, that is, the phase is inverted is considered, but there are various changes in the relative phase difference between the fundamental wave and the second harmonic due to the actual reflection from the mirror.

Then, the fundamental wave electric field and the second harmonic are respectively described by a cosine function, and the phases thereof are φ ₁ for the fundamental wave ω and φ _{2 for} the second harmonic wave 2ω, and the fundamental wave ω and the second harmonic wave are given. If the phase difference with 2ω is φ = 2φ ₁ −φ ₂ ,
The following relationship holds.

B) When the incident light on the end face of the nonlinear crystal is only the fundamental wave and there is no second harmonic wave In this case, the power unilaterally shifts from the fundamental wave to the second harmonic wave as the light wave propagates. .

B) When the phase difference φ = 2φ ₁ −φ ₂ = π / 2 The mode amplitude for the second harmonic at this time is a tanh function, and the period is infinite. Then, as the light wave propagates, the power unilaterally shifts from the fundamental wave to the second harmonic.

C) When the phase difference φ = 2φ ₁ −φ ₂ = −π / 2 At this time, the amplitude of the second harmonic decreases, and the power transfer from the second harmonic to the fundamental wave is unidirectional. Occur. When the light wave propagates in the nonlinear optical crystal for a long enough time, once the power of the second harmonic wave is completely transferred to the fundamental wave, the phase difference is inverted,
Conversely, power transfer from the fundamental wave to the second harmonic occurs unilaterally.

D) When the phase difference φ = 2φ ₁ −φ ₂ ≠ ± π / 2 At this time, the mode amplitude of the second harmonic changes cyclically between the upper limit value and the lower limit value determined by the initial value. , The power alternates between the fundamental and the second harmonic. This cycle length becomes shorter as the power in the resonator becomes larger.

In the stationary field inside the resonator, both the fundamental wave and the second harmonic wave enter the nonlinear optical crystal. That is, the above b)
~ D). In the case of (c), the transition of the power from the second harmonic to the fundamental wave occurs once, and in the case of (d), it becomes a function in which the mode amplitude changes periodically.

From the above, in order to efficiently perform the power conversion from the fundamental wave to the second harmonic, it is important to control the phase difference between the fundamental wave and the second harmonic, and this phase difference is controlled by the nonlinear optical crystal. It can be seen that it is desirable to set π / 2 (φ = 2φ ₁ −φ ₂ = π / 2) at the incident end face.

Since the second harmonic does not exist when the fundamental wave first enters the resonator, the above case (a) occurs, and the phase difference φ between the fundamental wave and the generated second harmonic is inevitable. It becomes π / 2.

Therefore, in the ring double resonator, when the resonator is constructed so that the fundamental wave and the second harmonic wave that circulate on the incident end face of the nonlinear optical crystal have the same phase as the wave of the previous cycle,
The phase difference φ in the nonlinear optical crystal in the resonator must be π / 2
Therefore, the power of the second harmonic can be efficiently accumulated in the resonator.

On the other hand, when a light wave is incident on the nonlinear optical crystal a plurality of times during one round in the resonator like a linear double resonator, the incident fundamental wave is generated at each incident end face of each nonlinear optical crystal. It is necessary to configure the resonator so that the phase difference φ of the incident second harmonic becomes π / 2.

Although the case where the phase matching is achieved has been described above, the same applies to the case where the phase matching is not achieved, and the phase difference φ between the fundamental wave and the second harmonic wave at each incident end face of the nonlinear optical crystal. It is desirable to oscillate at π / 2.

Next, the double resonance frequency in the linear double resonator will be considered. Here, the resonator is a linear double resonator composed only of a nonlinear optical crystal having a length of 1 and a mirror, and the mirrors are arranged in the same manner on the left and right sides.

When the reflection phase of the light wave on the equivalent reflection surface is δ ₁ for the fundamental wave and δ _{2 for} the second harmonic, and the phase mismatch amount in the nonlinear optical crystal is Δkl (Δk; wave number mismatch amount). The double resonance point is a frequency at which 2Δkl + 2 (2δ ₁ −δ ₂ ) = 2Nπ Δk = 2k ₁ −k ₂ . Where k ₁ is the wave number of the fundamental wave, k
₂ is the wave number of the second harmonic, and N is an integer.

That is, when one of the reflection phase difference δ (= 2δ ₁ −δ ₂ ) between the fundamental wave mirror and the second harmonic mirror or the phase mismatch amount Δkl is determined, the double resonance point is determined, and the double resonance point is determined. It can be seen that there are a plurality of phase mismatch amounts Δkl at intervals of π.

FIG. 20 shows the calculation result of this state by changing the value of the reflection phase difference δ = 2δ ₁ −δ ₂ between the fundamental wave and the second harmonic wave at the mirror. In the case of the linear double resonator, since the light wave passes through the nonlinear optical crystal twice during one round, the phase matching curve is affected by the reflection phase difference δ.

Here, FIG. 9A shows a phase matching curve using the reflection phase difference δ as a parameter, and FIG. 8B shows the movement of the double resonance point. The horizontal axis is half of the amount of phase mismatch between the fundamental wave and the second harmonic, and the vertical axis is normalized by the maximum value η ₀ of one pass when the nonlinear optical crystal 2l is used.

When the reflection phase difference δ is π, the phase matching curve is bimodal, and when the reflection phase difference δ is 0, the nonlinear optical crystal 2
It matches the phase matching curve when l. When the reflection phase difference δ approaches 0, the phase matching curve shifts from left to right. Also, the double resonance point shifts from left to right when the reflection phase difference δ approaches 0.

Although not shown, on the contrary, when the reflection phase difference δ approaches −π to 0, both the phase matching curve and the double resonance point shift from right to left. From this, if the reflection phase difference δ is −π / 2 <δ <π / 2, that is, if the phase difference φ at the incident end face of the nonlinear optical crystal is 0 <φ <π, the efficiency decrease is small. I understand. To oscillate at a specific frequency, the reflection phase difference δ between the fundamental wave and the second harmonic wave at the mirror
And it is necessary to control the wave number mismatch Δk.

Next, the ring double resonator will be described. As described above, when one nonlinear optical crystal for generating the second harmonic is built in the resonator, at the double resonance point, the fundamental wave and the second harmonic are generated at the incident end face of the nonlinear optical crystal. The phase difference φ of is π / 2. Further, when a plurality of nonlinear optical crystals are built in, it is desirable that the phase difference φ between the fundamental wave and the second harmonic be π / 2 at each incident end face of each nonlinear optical crystal. This is similar to a linear resonator. However, the behavior of the double resonance point and the reflection phase difference δ of the phase matching curve is different from that of the linear double resonator.

A ring double resonator constituted by only one nonlinear optical crystal of length 1 and a mirror will be described below as an example. At the double resonance point, the reflection phase at the equivalent reflection surface of the light wave is δ ₁ for the fundamental wave and δ _{2 for} the second harmonic, and the phase mismatch amount in the nonlinear optical crystal is Δkl (Δk; wave number mismatch amount). Then, the frequency is Δkl + Σ (2δ ₁ −δ ₂ ) = 2Nπ. Here, Σ represents the total sum for each mirror.

From this, it can be seen that the double resonance points have a plurality of phase mismatch amounts Δkl at intervals of 2π. This spacing is twice that of a linear resonator. The phase matching curve is a one-pass phase matching curve, and the sum δ (= Σ (2
It is expressed as shown in FIG. 21 (A) regardless of δ ₁ −δ ₂ )). In the same figure (A), the double resonance point at the sum δ = π of the reflection phase differences is also shown.

The double resonance point shifts from the left side to the right side by a half of the change in the total reflection phase difference δ as shown in FIG. 7B, as the total reflection phase difference δ decreases. Therefore, it can be seen that if −π / 2 <δ <π / 2, the efficiency decrease is small. Further, like the linear double resonator, in order to oscillate at a specific frequency, it is necessary to control the sum δ of the reflection phase differences between the fundamental wave and the second harmonic at the mirror and the wave number mismatch Δk. .

After all, in order to efficiently construct the double resonator for generating the second harmonic for both the linear double resonator and the ring double resonator, a) In one round in the resonator, The phase difference φ between the fundamental wave and the second harmonic is 2πn <φ <(2n + 1) at each incident end face of each nonlinear optical crystal so that the generated second harmonic is additively superimposed.
It is necessary to adjust to π (n; integer), and b) to set the reflection phase at the mirror so that the double resonance point can be set at an efficient point in the phase matching curve.

In view of the above, the present invention achieves efficient wavelength conversion by satisfying the condition that the fundamental wave resonates with the second harmonic and the condition that the second harmonic efficiently accumulates in the resonator. The purpose is to provide the things that can be done.

[0036]

In order to achieve the above object, a wavelength conversion element according to the present invention comprises a double resonator for confining both a fundamental wave and a second harmonic, and the double resonator is provided in the double resonator. In a wavelength conversion element having at least one nonlinear optical crystal for generating a second harmonic, the phase difference between the fundamental wave and the second harmonic at each incident end face of the nonlinear optical crystal at the time of double resonance is adjusted. The phase difference adjusting section is provided inside the double resonator.

Further, at least one second harmonic wave is generated on the optical axis of the linear double resonator in which a pair of reflecting mirrors for multiply reflecting both the fundamental wave and the second harmonic wave are linearly arranged. A non-linear crystal is arranged, and a phase difference adjusting unit for adjusting the phase difference between the fundamental wave and the second harmonic wave after being reflected here is provided on the reflection surfaces of the pair of reflection mirrors facing each other. To do.

Further, at least one second harmonic wave is generated in the optical axis of the ring double resonator in which at least three reflecting mirrors for multiply reflecting both the fundamental wave and the second harmonic wave are arranged in a closed ring shape. And a phase difference adjusting unit for adjusting the difference between the sum of the phase changes of all the mirrors of the fundamental wave and the second harmonic wave are arranged on the optical axis. To do.

At least one mirror forming the double resonator is formed on the end face of the nonlinear optical crystal or the end face of the phase difference adjusting section, or the phase difference adjusting section is formed of an optical element. Optical elements can be arranged on both sides of the nonlinear optical crystal on the optical axis. Further, the double resonator is composed of a pair of fundamental wave reflecting mirrors that reflect only the fundamental wave and a pair of second harmonic wave reflecting mirrors that reflect only the second harmonic wave. At least one of the second harmonic high-frequency reflection mirrors is configured to be movable along the optical axis to form the phase difference adjusting section, or the phase difference adjusting section is made of a crystal exhibiting an electro-optical effect. A fundamental wave reflection film that reflects only the fundamental wave is formed on one end surface, and a second reflection layer is formed on the other end surface.
A crystal that forms a double resonator by forming a second harmonic reflection film that reflects only the harmonic, or changes the optical lengths of the fundamental wave and the second harmonic by adjusting the temperature of the phase difference adjusting unit. It can also be configured with.

[0040]

According to the present invention configured as described above, the phase difference between the fundamental wave and the second harmonic is adjusted via the phase adjusting section so that the phase difference between the fundamental wave and the second harmonic becomes approximately π / 2 at each input end face of the nonlinear optical crystal. This makes it possible to satisfy the double resonance condition in which the power of the second harmonic in the double resonator is efficiently accumulated.

[0041]

Embodiments of the present invention will be described below with reference to the drawings. In each of the following examples, a semiconductor laser SDL-8030-101 made by SANYO is oscillated at a wavelength of 860 nm to serve as a fundamental wave light source, and a nonlinear optical crystal is used, and both end faces thereof have a fundamental wave of 860 nm wavelength and 430 n.
A KNbO ₃ crystal having a length of 5 mm and having an AR (non-reflection) coating with a reflectance of 0.5% for the second harmonic of m is used. Then, the light emitted from the laser diode (LD) passes through a collimating lens having an NA (numerical aperture) to 0.4 and an isolator, and is focused on the nonlinear optical crystal (KNbO ₃ crystal) by a focusing lens having a focal length of 100 mm. Let me
An example in which this is the fundamental wave input is shown. Also, wavelength 8
The path (optical path) of light of the fundamental wave ω of 60 nm is shown by a solid line, and the path (optical path) of light of the second harmonic wave 2ω of a wavelength of 430 nm is shown by a broken line.

First, FIG. 1 shows a first embodiment. In this embodiment, a pair of reflecting mirrors 3 and 3'which multiple-reflect both the fundamental wave ω and the second harmonic 2ω are arranged at predetermined intervals. A linear double resonator 4a for confining both the fundamental wave ω and the second harmonic wave 2ω is formed by arranging them in a straight line while arranging them so that the concave surfaces, which are the reflecting surfaces, face each other, and further, The nonlinear optical crystal (KNbO ₃ crystal) 5 for generating the second harmonic is arranged on the optical axis inside the double resonator 4a.

The respective reflection mirrors 3 and 3'multiple-reflect both the fundamental wave ω and the second harmonic 2ω, and the reflection surfaces of the respective reflection mirrors 3 and 3'after being reflected here. A phase difference adjusting unit 6 is provided to make the phase difference φ between the fundamental wave ω and the second harmonic wave 2ω have a predetermined phase relationship.

That is, each of the phase difference adjusting sections 6 has a fundamental wave reflection film 7 which is highly reflective for the fundamental wave ω on the reflective surface of each of the reflection mirrors 3 and 3'and a high reflection of the second harmonic wave 2ω. The second harmonic reflection film 8 is multilayer-coated, and the distance L between the equivalent reflection surface 7a of the fundamental wave ω and the equivalent reflection surface 8a of the second harmonic wave 2ω is equal to the wavelength λ of the second harmonic wave 2ω in vacuum. It is configured by setting an optical length corresponding to about 1/4 (L≈1 / 4λ).

Here, one reflection mirror 3 on the incident side is provided.
Is 95% for the fundamental wave ω, 99.8% for the second harmonic wave 2ω, and the reflectance of the other reflecting mirror 3'on the exit side is 99% for the fundamental wave ω, for example. .5%,
It is set to 95% with respect to the second harmonic 2ω.

As a result, the phase adjusting section 6 causes the phase φ ₁ for the fundamental wave ω and the phase φ ₂ for the second harmonic wave 2ω.
And the phase difference φ (= 2φ ₁ −φ ₂ ) with the second harmonic double resonator is adjusted to be approximately π / 2 at the incident end face of the nonlinear optical crystal 5 for generating the second harmonic. It is configured to satisfy the double resonance condition in which the power inside the stack efficiently accumulates.

The equivalent reflection surface 7a of the fundamental wave and the second
The distance L between the harmonic and the equivalent reflection surface 8a is 2πn + λ / 8.
<L <2πn + 3λ / 8 (n; integer), the condition 2πn <φ <(2n + 1) π of the phase difference φ is set.
Can satisfy the relationship.

As shown in FIG. 2, KNbO ₃ crystals 9 for fine adjustment made of, for example, KNbO ₃ crystals are arranged on both sides of the nonlinear optical crystal 5 so that the nonlinear optical crystal 5 has the second harmonic. KN for temperature adjustment and fine adjustment for wave generation
It is also possible to remove the bO ₃ crystal 9 from the phase matching temperature and adjust the temperature for phase adjustment so as to finely adjust the phase difference. With such a configuration, a fundamental wave input of 100 mW gives an output of 3 mW. It has been confirmed that a second harmonic output can be obtained.

Further, as shown in FIG. 3, a fundamental wave reflection film 7 having high reflection of the fundamental wave ω on both end faces of the nonlinear optical crystal 5 for generating the second harmonic wave, and a high reflection of the second harmonic wave 2ω. The second harmonic reflection film 8 is coated in a multilayer structure to form the reflection mirrors 3 and 3'that form the linear double resonator 4b integrally with the nonlinear optical crystal 5, and further, the equivalent reflection surface 7a of the fundamental wave is formed. It is also possible to use this as the phase difference adjusting unit 6 by setting the distance L between the equivalent reflection surface 8a of the second harmonic wave and the equivalent reflection surface 8a to a predetermined value.

In the embodiment shown in FIG. 4, a pair of fundamental wave reflecting mirrors 10 and 10 'for multiply reflecting the fundamental wave ω to the outside are provided.
The second harmonic reflection mirrors 11 and 11 ′ that multiple-reflect the second harmonic 2ω inside are arranged in a straight line by arranging them so that their concave surfaces, which are reflection surfaces, face each other, and are aligned at a predetermined distance while aligning the optical axes. In addition to constituting the resonator 4c, the nonlinear optical crystal 5 for generating the second harmonic is arranged inside the linear double resonator 4c, and the nonlinear optical crystal 5 is incident on both sides sandwiching the nonlinear optical crystal 5. The phase adjusters 12 and 12 are arranged so that the phase difference between the fundamental wave ω and the second harmonic wave 2ω on the end face has a predetermined phase relationship.

The phase adjusting section 12 can be composed of, for example, a crystal having a photoelectric optical effect, a temperature adjusted crystal, a crystal having a variable angle with respect to the optical axis in order to make the optical path length variable.

Then, the phase adjusting unit 12 is set to KNbO.
_In the case of being composed of _three crystals, the nonlinear optical crystal (KN
bO ₃ crystal) 5 is temperature-adjusted to generate the second harmonic, and this phase adjustment unit (KNbO ₃ crystal for phase adjustment) 12 is removed from the phase adjustment temperature to obtain a phase φ ₁ and a second phase φ related to the fundamental wave ω.
Phase difference between the phase phi ₂ regarding harmonic 2ω φ (= 2φ ₁ -
The temperature is adjusted so that φ ₂ ) becomes approximately π / 2 at the incident end face of the nonlinear optical crystal 5, and thereby the double resonance condition that the power of the second harmonic in the double resonator is efficiently accumulated is set. To be satisfied.

Here, as shown in FIG. 5, a pair of fundamental wave reflection mirrors 10 and 1 that multiple-reflect the fundamental wave ω inside.
0'is a linear double resonator in which second harmonic reflection mirrors 11 and 11 'that multiple-reflect the second harmonic 2ω to the outside are arranged at predetermined intervals while facing the concave surfaces that are reflection surfaces. 4d can also be configured.

Further, as shown in FIG. 6, instead of the second harmonic reflection mirror 11 shown in FIG. 4, the phase difference adjusting section 12 has a second face on the end face opposite to the face facing the nonlinear optical crystal 5. The linear double resonator 4e is formed by coating the second harmonic reflection film 8 that is highly reflective of the second harmonic 2ω, or as shown in FIG.
In place of the fundamental wave reflection mirror 10 shown in FIG. 1, the fundamental wave reflection film 7 having a high reflection of the fundamental wave ω is formed on the end face of the phase difference adjusting unit 12 opposite to the face facing the nonlinear optical crystal 5.
By coating the linear double resonator 4f
Can also be configured.

Further, as shown in FIG. 8, the phase adjusting unit 12
On the end face opposite to the end face facing the non-linear optical crystal 5 of FIG.
The second harmonic reflection film 8 that is highly reflective of the second harmonic 2ω is multilayer-coated to configure the double resonator 4g, or as shown in FIG. 9, the nonlinear optical crystal 5 of the phase adjuster 12 is used. Double resonance is formed by forming a second harmonic reflection film 8 which is highly reflective of the second harmonic 2ω on the end surface facing the and a fundamental wave reflective film 7 which is highly reflective of the fundamental wave ω on the opposite end surface. As shown in FIG. 10, in contrast to FIG. 9, the end face of the phase adjusting unit 12 facing the nonlinear optical crystal 5 is provided with a fundamental wave reflection film 7 highly reflecting the fundamental wave ω, as shown in FIG. It is also possible to form the double resonator 4i by forming the second harmonic reflection films 8 that are highly reflective of the second harmonic 2ω on the opposite end faces.

In the embodiment shown in FIG. 2, the fundamental wave reflection film 7 and the second harmonic reflection film 8 are coated at appropriate intervals, and the fine adjustment KNbO ₃ crystal 9 is used for the phase adjusting portion. It can also be 12.

The embodiment shown in FIG. 11 has a fundamental wave ω on the outside.
A pair of fundamental wave reflecting mirrors 10 and 10 ', and a second harmonic wave reflecting mirror 1 for reflecting the second harmonic wave 2ω inside.
The linear double resonator 4j is formed by arranging 1 and 11 'at concave surfaces, which are reflecting surfaces, so as to oppose each other and keeping their optical axes coincident with each other with a predetermined gap therebetween.
The non-linear optical crystal 5 is arranged on the optical axis inside j, the second harmonic mirrors 11 and 11 'are fixed, and the fundamental wave reflecting mirrors 10 and 10' are attached to the optical axis using, for example, a piezoelectric actuator. The phase adjusting unit is configured so as to be movable along.

That is, the fundamental wave reflecting mirrors 10 and 10 'are moved so that the distance 10, between the fundamental wave reflecting mirrors 10 and 10' and the second harmonic reflecting mirrors 11 and 11 'facing each other,
By adjusting the optical lengths of 11, 10 ', 11' and, by extension, the reflecting surfaces of both, the phase φ1 with respect to the fundamental wave ω and the second
Phase difference φ (= 2φ1−
φ 2) is adjusted to be approximately π / 2 at the incident end surface of the nonlinear optical crystal 5, and the double resonance condition that the power of the second harmonic 2ω in the double resonator 4j is efficiently accumulated can be satisfied. Is configured.

Here, as shown in FIG.
A pair of second harmonic reflection mirrors 1 for reflecting the harmonic 2ω
1, 11 ′ are arranged inside the fundamental wave reflection mirrors 10 and 10 ′ that reflect the fundamental wave ω with a predetermined space therebetween to form a linear double resonator 4 k.
The nonlinear optical crystal 5 is arranged inside k, the fundamental wave mirrors 10 and 10 'are fixed, and the second harmonic reflection mirrors 11 and 11' are movable along the optical axis to form a phase adjusting unit. It can also be configured.

In the embodiment shown in FIGS. 11 and 12, one of the mirrors is fixed and the other mirror is movable along the optical axis to constitute the phase adjusting section.
It is also possible to make both phase mirrors movable along the optical axis to form the phase adjusting unit.

Further, as shown in FIG. 13, instead of the second harmonic reflection mirrors 11 and 11 'shown in FIG. 11, the second harmonic wave 2ω is highly reflected on the end face of the nonlinear optical crystal 5. The linear double resonator 4l is formed by coating with the harmonic reflection film 8 or, as shown in FIG. 14, the fundamental wave reflection mirrors 10 and 1 shown in FIG.
Instead of 0 ', the linear double resonator 4m can be constructed by coating the end face of the nonlinear optical crystal 5 with a fundamental wave reflection film 7 that highly reflects the fundamental wave ω.

In the embodiment shown in FIG. 15, four reflection mirrors 13 and 1 which multiple-reflect the fundamental wave ω and the second harmonic wave 2ω.
4, 15, 16 are arranged in a substantially quadrangular closed ring shape to form a ring double resonator 17a capable of confining both the fundamental wave ω and the second harmonic wave 2ω, and the entrance side of the ring double resonator 17a Difference between the total amount of phase change in the total reflection mirrors 13, 14, 15, 16 of the fundamental wave ω and the second harmonic 2ω on the optical axis sandwiched between the pair of reflection mirrors 13 and 14 on the reflection side. In addition to arranging the same phase adjuster 12 as in the above-mentioned embodiment for adjusting the above, the nonlinear optical crystal (KNbO ₃ crystal) 5 is arranged on the optical axis sandwiched by the other pair of reflecting mirrors 15 and 16. is there.

Here, for example, with respect to the fundamental wave ω, the reflectance of the reflection mirror 13 on the incident side is 95%, the reflectances of the other reflection mirrors 14, 15, 16 are 99.8%, and the second harmonic wave 2ω. The reflectance of the reflection mirror 14 on the output side is 95
%, The reflectance of the other reflection mirrors 13, 15, 16 is 99.
It is set to 8% respectively.

The phase difference adjusting section 12 includes the reflection mirrors 1
Phase change amount δ of the fundamental wave ω at 3, 14, 15, and 16
Sum of the differences between ₁ and the phase change amount δ ₂ of the second harmonic 2ω δ
The phase difference φ between the phase φ ₁ regarding the fundamental wave ω and the phase φ ₂ regarding the second harmonic wave 2ω is adjusted so that (= Σ (2δ ₁ −δ ₂ )) becomes approximately 0 (δ = 0). (= 2φ ₁ −φ ₂ )
Is approximately π / 2 at the incident end surface of the nonlinear optical crystal 5 for generating the second harmonic, and the double resonance condition that the power of the second harmonic in the double resonator is efficiently accumulated is satisfied. The purpose is to be able to do it.

It should be noted that the difference δ in the sum of the phase change amounts is
By setting π / 2 + 2πn <δ <π / 2 + 2nπ (n; integer), the condition 2πn <φ <of the phase difference φ is set.
The relationship of (2n + 1) π can be satisfied.

In the case of this embodiment, a KNbO ₃ crystal is used as the phase adjuster 12, and a nonlinear optical crystal (KNbO ₃ crystal is used.
(Crystal) 5 for temperature control for second harmonic generation, phase control section (KNbO ₃ crystal) 12 is removed from the phase matching temperature for temperature control for phase control, and a double resonance state is realized.
With the fundamental wave input of 100 mW, the second harmonic output of 5 mW was obtained.

In the embodiment shown in FIG. 16, three multi-reflection mirrors 13 for multiply reflecting the fundamental wave ω and the second harmonic wave 2ω,
14 and 18 are arranged in a substantially triangular closed ring shape to form a ring double resonator 17b capable of confining both the fundamental wave ω and the second harmonic 2ω. The phase adjusting unit 12 and the nonlinear optical crystal 5 are sequentially arranged from the incident side on the optical axis sandwiched by the pair of reflecting mirrors 13 and 14 on the emitting side.

[0068]

Since the present invention has the above-described structure, the second harmonic generated in one round in the resonator is additively added through the phase adjusting section arranged in the double resonator. It is possible to adjust the phase difference between the fundamental wave and the second harmonic at each incident end face of each nonlinear optical crystal so as to overlap with each other, and to set the double resonance point at an efficient point in the phase matching curve. Thus, it is possible to satisfy the condition that the fundamental wave and the second harmonic resonate and the condition that the second harmonic efficiently accumulates in the resonator, and perform efficient wavelength conversion.

[Brief description of drawings]

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

FIG. 2 is a schematic diagram showing a basic configuration of a second embodiment.

FIG. 3 is a schematic diagram showing a basic configuration of a third embodiment.

FIG. 4 is a schematic diagram showing a basic configuration of a fourth embodiment.

FIG. 5 is a schematic diagram showing a basic configuration of a fifth embodiment.

FIG. 6 is a schematic diagram showing a basic configuration of a sixth embodiment.

FIG. 7 is a schematic diagram showing a basic configuration of a seventh embodiment.

FIG. 8 is a schematic diagram showing a basic configuration of an eighth embodiment.

FIG. 9 is a schematic diagram showing a basic configuration of a ninth embodiment.

FIG. 10 is a schematic diagram showing a basic configuration of a tenth embodiment.

FIG. 11 is a schematic diagram showing a basic configuration of an eleventh embodiment.

FIG. 12 is a schematic diagram showing the basic configuration of a twelfth embodiment.

FIG. 13 is a schematic diagram showing a basic configuration of a thirteenth embodiment.

FIG. 14 is a schematic diagram showing the basic structure of a fourteenth embodiment.

FIG. 15 is a schematic diagram showing the basic structure of a fifteenth embodiment.

FIG. 16 is a schematic diagram showing the basic structure of a sixteenth embodiment.

FIG. 17 is a diagram showing the relationship of the phase difference in the nonlinear optical crystal between the fundamental wave in the linear double resonator and the second harmonic wave excited from the fundamental wave.

FIG. 18 is a diagram showing a phase difference relationship when reflected by an ideal mirror.

FIG. 19 is a diagram showing a phase difference relationship between a second harmonic wave that makes one revolution of the resonator and a second harmonic wave that is newly generated from the fundamental wave that makes one revolution of the resonator.

FIG. 20 is a diagram showing a relationship between a phase matching curve and a phase mismatch amount in a linear double resonator.

FIG. 21 is a diagram showing a relationship between a phase matching curve and a phase mismatch amount in a ring double resonator.

[Explanation of symbols]

3, 3 ', 13, 14, 15, 16, 18 Reflecting mirrors 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h, 4
i, 4j, 4k, 4l, 4m linear double resonator 5 a nonlinear optical crystal (KNbO ₃ crystals) 6,12 phase difference adjusting section 7 fundamental frequency reflection film 8 and the second harmonic wave reflection layer 10, 10 'fundamental frequency reflection mirror 11, 11 'Second harmonic reflection mirror 17a, 17b Ring double resonator

Claims (8)

[Claims] 1. A wavelength conversion element comprising a double resonator for confining both a fundamental wave and a second harmonic, and at least one nonlinear optical crystal for generating the second harmonic is built in the double resonator. A phase difference adjusting unit for adjusting a phase difference between the fundamental wave and the second harmonic at each incident end face of the nonlinear optical crystal at the time of double resonance is provided inside the double resonator. Wavelength conversion element. 2. A nonlinear for generating at least one second harmonic on the optical axis of a linear double resonator in which a pair of reflecting mirrors that multiple-reflect both the fundamental wave and the second harmonic are linearly arranged. The crystal is arranged, and the fundamental wave after being reflected here by the reflecting surfaces of the pair of reflecting mirrors facing each other and the second
A wavelength conversion element comprising a phase difference adjusting unit for adjusting a phase difference with a harmonic. 3. For generating at least one second harmonic on the optical axis of a ring double resonator in which at least three mirrors for multiple reflection of both the fundamental wave and the second harmonic are arranged in a closed ring shape. And a phase difference adjusting unit for adjusting the difference between the sum of the phase changes of all the mirrors of the fundamental wave and the second harmonic is provided on the optical axis. Wavelength conversion element. 4. The wavelength conversion element according to claim 1, wherein at least one mirror forming the double resonator is formed on an end face of the nonlinear optical crystal or an end face of the phase difference adjusting section. 5. The wavelength converter according to claim 2, wherein the phase difference adjusting section is composed of an optical element, and the optical elements are arranged on both sides of the nonlinear optical crystal on the optical axis. element. 6. The double resonator comprises a pair of fundamental wave reflection mirrors for multiple reflection of a fundamental wave and a pair of second harmonic reflection mirrors for multiple reflection of a second harmonic wave. 3. The wavelength conversion element according to claim 2, wherein at least one of the wave reflection mirror and the second harmonic high frequency reflection mirror is configured to be movable along the optical axis to configure the phase difference adjusting unit. 7. The phase difference adjusting section is composed of a crystal exhibiting an electro-optic effect, one end face of the crystal is provided with a fundamental wave reflection film for multiple reflection of a fundamental wave, and the other end face thereof is multiple reflection of a second harmonic wave. The wavelength conversion element according to claim 1, wherein a double resonator is formed by forming respective second harmonic reflection films. 8. The wavelength conversion element according to claim 1, wherein the phase difference adjusting section is made of a crystal that changes the optical lengths of the fundamental wave and the second harmonic by adjusting the temperature.