JPH055921A - Wavelength conversion element - Google Patents

Abstract

PURPOSE:To provide a wavelength conversion element capable of increasing the conversion efficiency of a 2nd harmonic light. CONSTITUTION:Assuming that the equivalent refractive indexes of 1st and 2nd waveguides 20, 40 to a reference wave are respectively N1(omega) and N2(omega), the equivalent refractive index of the 1st waveguide 20 to a secondary harmonic wave is N1(2omega) in the wavelength conversion element provided with the 1st waveguide 20 embedded in a base 10 consisting of a non-linear optical material to guide incident light and the 2nd waveguide 40 provided in the vicinity of the 1st waveguide 20 to execute phase matching, the phase matching can be attained by the interaction of the two waveguides 20, 40 while satisfying a condition N1(omega)>N2(omega) and a condition that the ratio of N1(2omega) to an arithmetic mean value (N1(omega)+N2(omega)/2 between N1(omega) and N2(omega) is 0.90:1 to 1.05:1.

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, and more particularly to a wavelength conversion element using a nonlinear optical material.

[0002]

2. Description of the Related Art Conventionally, various wavelength conversion elements have been proposed which form a waveguide in a nonlinear optical crystal, confine light in a minute region, and perform wavelength conversion with high efficiency.

As an example, the second harmonic light by the three-dimensional waveguide formed in the LiNbO ₃ crystal will be described.
As shown in FIG. 3, the optical axes X, Y, Z of the Y plate crystal are shown.
(Refractive index n ₀ , n ₀ , n _e ) is taken, and it is made to coincide with this, and the coordinate axis of the waveguide is taken. Incident light has an E of angular frequency ω
^{This is the y} fundamental mode, and the second harmonic light of the output light is the E ^z fundamental mode having an angular frequency of 2ω. The wavelength conversion element having such a configuration uses the tensor component of the nonlinear constant d ₃₁ .

In this case, the output P ₂ of the second harmonic light is expressed by the following equation using the output P _{1 of the} input light, the element length L and the nonlinear constant d ₃₁ .

[0005]

[Equation 1]Where g₁, G₂Is the normalized electric field distribution function of each guided mode.
Is a number, g₁ ²・ G ₂The area integral associated with
Is a superimposition integral of the waveguide, and the integration range is the waveguide cross section. △
β = β₂-2β₁And β₁, Β₂Is each waveguide mode
Represents the propagation constant of.

From the above equation, in order to obtain high conversion efficiency, it is necessary to satisfy the phase matching condition (Δβ = 0), to use a larger nonlinear constant, and to increase the superposition integral.

However, LiNbO ₃ , LiTaO
₃ , Non-linear constants showing large values such as KTP are diagonal components of the non-linear polarization tensor. Therefore, in order to utilize such a constant showing a large value, it is necessary to consider a case where the electric field vector components of the incident light and the emitted second harmonic light are the same. That is, in terms of a waveguide, the refractive index in the same direction of the nonlinear material forming the element (for example, when both the incident light and the output light are in the TM mode) is used.

[0008]

However, if the refractive index in the same direction is used, the refractive index for the second harmonic light is always larger than that of the incident light because of the refractive index dispersion, and the ordinary waveguide is used. The phase matching condition cannot be satisfied in the configuration form.

The present invention has been conceived in view of the above circumstances, and its purpose is to increase the nonlinear constant by utilizing the refractive index in the same direction and to satisfy the phase matching condition and to obtain the second harmonic light. Is to provide a wavelength conversion element having a large conversion efficiency.

[0010]

In order to solve the above problems, the present invention provides a phase matching with a first waveguide which is provided in a substrate of a nonlinear optical material and which guides incident light. A wavelength conversion element comprising a second waveguide provided in the vicinity of the waveguide, wherein the equivalent refractive indices of the first waveguide and the second waveguide with respect to the fundamental wave are respectively N ₁ (Ω), N ₂ (ω) and the equivalent refractive index of the first waveguide for the second harmonic light is N ₁ (2ω), the condition of N ₁ (ω)> N ₂ (ω) And N
The ratio of ₁ (2ω) and the arithmetic mean value (N ₁ (ω) + N ₂ (ω)) / 2 of N ₁ (ω) and N ₂ (ω) is 0.90:
The phase matching was achieved by the interaction of the two waveguides while satisfying the condition of 1 to 1.05: 1.

[0011]

In the light guided in the first waveguide, the propagation constant of the fundamental wave is perturbed by the coupling with the second waveguide,
Furthermore, the second harmonic generated in the first waveguide is also perturbed when propagating through this waveguide, and the propagation constant of the perturbed fundamental wave and the propagation constant of the perturbed second harmonic. Coincide with each other, phase matching can be achieved, and a constant involved in the output of the second harmonic light can be made large, and as a result, the conversion efficiency of the second harmonic light becomes large.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of the wavelength conversion element of the present invention will be described below with reference to FIGS. FIG. 1 is a schematic configuration perspective view showing one embodiment of a wavelength conversion element 1 (hereinafter, simply referred to as conversion element 1) of the present invention, and FIG. 2 is a partial sectional view of FIG. 1 seen from a light incident side (front side). is there.

The conversion element 1 of the present invention comprises a substrate 10 made of a non-linear optical material, a first waveguide 20 provided along the longitudinal direction (waveguide direction) of the substrate 10, and a waveguide 20 on the waveguide 20. The formed sealing thin film 30 and the second thin film provided thereon.
The waveguide 40 of FIG.

The substrate 10 has a substantially rectangular parallelepiped shape, and in this embodiment, an inorganic non-linear optical material crystal of LiTaO ₃ is used. In addition, LiNbO ₃ , KTP (KTiOPO
₄ ) Inorganic materials such as DMNP (3,5-dimethyl-1- (4-nitrophenyl) pyrazole), MNA
(2-methyl-4-nitroaniline), MBANP (2
Organic nonlinear optical materials such as-(α-methylbenzylamino) -5-nitropyridine) may also be used.

In the substrate 10 made of such a nonlinear optical material,
Particularly, on the one-plane side 10a, a first waveguide 20 is formed for receiving light and guiding light and for performing wavelength conversion. The waveguide 20 has a substantially rectangular cross section, and is formed by, for example, a so-called proton exchange method, a diffusion method of diffusing Ti, Rb, or the like.

A sealing thin film 30 is laminated on the first waveguide 20, that is, on the one plane side 10a of the substrate 10. The encapsulating thin film 30 has a role of enclosing the first waveguide 20 to form a channel-type waveguide and separating a second waveguide 40 described later from the first waveguide 20 at a suitable position. .. The sealing thin film 30 is made of a material having a smaller refractive index than the first waveguide 20 and a second waveguide 40 described later, for example, Ta ₂ O ₅ , ZnO, SiO ₂ or the like. The sealing thin film 30 is usually formed by a vacuum film forming method such as a vapor deposition method.

A second waveguide 40 is formed on the sealing thin film 30. This second waveguide is used for the sealing thin film 30.
And is formed along the first waveguide 20 via. The second waveguide 40 is provided in order to achieve phase matching by the interaction of the two waveguides, and the second waveguide 40 is set so as to satisfy the following conditions.

That is, when the equivalent refractive indices of the first waveguide 20 and the second waveguide 40 with respect to the fundamental wave are N ₁ (ω) and N ₂ (ω), respectively,
₁ (ω)> N ₂ (ω), especially N ₁ (ω) / N ₂ (ω)
= 1.01 to 1.20, and the second waveguide 4
0 is formed.

Further, when the equivalent refractive index of the _first waveguide for the second harmonic light is N ₁ (2ω), N ₁
The ratio of (2ω) and the arithmetic mean value (N ₁ (ω) + N ₂ (ω)) / 2 of N ₁ (ω) and N ₂ (ω) is 0.90:
1.00 to 1.05: 1.00, more preferably 0.9
The second waveguide is formed so as to satisfy the condition of 1: 1.00 to 1.01: 1.00. The value of N ₂ (ω) is N
If it exceeds the value of ₁ (ω) or N ₁ (2ω), N
Arithmetic mean value of ₁ (ω) and N ₂ (ω) (N ₁ (ω) + N
_If the ratio of ₂ (ω) / 2 is out of the above range,
The disadvantage that phase matching cannot be achieved occurs.

That is, the inside of the first waveguide 20 is guided.
The light to be transmitted is basically coupled by coupling with the second waveguide 40.
The propagation constant of the wave is perturbed, and it is generated in the first waveguide.
The generated second harmonic light is also perturbed when propagating in this waveguide.
If the propagation multiplier of the adjacent waveguide is selected appropriately, the phase adjustment
The second condition of the second harmonic light
It is possible to take a large constant related to the output. Second
The waveguide 40 is made of, for example, SiO or Li._xNb_1-xO₃，
Li_xTa_1-xO₃, KTiOP_xAs_1-xO _FourMaterial such as
It is composed of materials and is usually guided by a vacuum film-forming method such as vapor deposition.
It is formed along the direction.

A specific example of the case where LiTaO ₃ is used for the substrate 10 and the waveguide direction is in the plane perpendicular to the Z axis of this crystal, and the polarization of the incident fundamental wave is also substantially parallel to the Z axis. A design example is shown below.

The first waveguide 20 is proton-exchanged with pyrophosphoric acid and has N ₁ (ω) = 2.1.
612, N ₁ (2ω) = 2.2702. The sealing thin film 30 was formed by vapor deposition of Ta ₂ O ₄ to a film thickness of 0.9 μm. The second waveguide 40 is formed by vapor deposition of SiO.
Film thickness D3 = 1.5 μm, N ₂ (ω) = 2.02
And

As shown in FIG. 2, the first waveguide 1
The width of 0 is W1 and the depth is D1, and the second waveguide 40
When the width is W2 and the thickness is D2, W2 / W1 = 0.
9 to 1.2, more preferably 0.95 to 1.05, D
2 / D1 = 0.8 to 2.5, more preferably 1.0 to
W1, D1, W2 and D2 are set to be 1.5. The distance between the waveguide 20 and the waveguide 40, that is, the thickness D3 of the sealing thin film 30 is 0.8 to 2.5 μm.
m, preferably 0.88 to 1.5 μm. When the values of these values W2 / W1, D2 / D1 and D3 are out of the above range, the phase matching between the fundamental wave propagating in the waveguide and the second harmonic cannot be achieved and the conversion efficiency is not improved. By the way, examples of the examples of W1 and D1 are 2 to 3 respectively.
μm and 1.7 to 2.0 μm. In addition, examples of the examples of W2 and D2 are 2-3 μm and 1.8-2.2 μm, respectively.

As a result of a concrete experiment using such a configuration example, a harmonic output of about 1 mw was obtained from a fundamental wave of 40 mw with a waveguide length of 10 mm. Next, theoretical support for adopting the configuration of the present invention will be described.

With respect to the input fundamental wave, the electric field distribution functions when the first waveguide and the second waveguide exist independently are E ₁ (x, y), E ₂ (x, y), The propagation constants are expressed as β ₁ and β ₂ , respectively. Average propagation constant βav = (β
₁ and + beta ₂₎ / 2, the value determined by the size of the waveguide coupling gamma ^+, gamma ^- using a first waveguide in the case where there is a second waveguide adjacent to the first waveguide The guided mode that propagates

[0026]

[Equation 2] It is represented by. Similarly, the guided mode propagating in the second waveguide in this case is

[0027]

[Equation 3] It is represented by. As shown in the above equation, when two waveguides are close to each other, it can be expressed by the sum of the propagation constant when the waveguides exist independently and the mode having a slightly larger propagation constant. Where γ ⁺ and γ ⁻ are respectively
It is a power distribution constant determined by the conditions for injecting light into the waveguide.

The same applies to the second harmonic light, and the waveguide mode propagating in the first waveguide is

[0029]

[Equation 4] It is represented by. Similarly, the guided mode propagating in the second waveguide in this case is

[0030]

[Equation 5] It is represented by. In the above formula for the second harmonic light,
A ' ^{(1) +} , A' ^(1)- , A ' ^{(2) +} and A' ⁽²⁾ -are the product of the power distribution constant of the fundamental mode and the function that reflects the phase matching, respectively. Can be represented. Therefore, in order to obtain high-power second harmonic light in the first waveguide,
It is essential to achieve phase matching and increase A ' ^{(1) +} and A' ^(1)- .

By the way, since the two waveguides are adjacent to each other, the phase mismatch amount Δβ is

[0032]

[Equation 6]

[0033]

[Equation 7] Has been derived (here, Δβ ₁₁
And Δβ ₁₂ are the phase mismatching amounts of the first and second waveguides) and the propagation constants of the adjacent waveguides are appropriately selected, the phase matching condition Δβ _ii = 0 is achieved. can do.

Furthermore, A ' ^{(1) +} , A'which can be said to be constants that achieve phase matching and are involved in the output of the second harmonic light.
Increasing ⁽¹⁾ -is advantageous for increasing the output of the second harmonic light. Here, for example, A ' ^{(1) +} is

[0035]

[Equation 8] It is derived that it is desirable that the power distribution coefficient A ^{(1) +} A ^{(1) + of the} term that achieves phase matching and the superposition integral are large.

For this reason, phase matching is achieved,
In order to increase the above A ′ ^{(1) +} and A ′ ^{(1) −} , the structure of the second waveguide to be added needs to be the same as that of the present invention.

[0037]

According to the present invention, a first waveguide provided in a substrate made of a non-linear optical material for guiding incident light and a second waveguide provided in the vicinity of the waveguide for phase matching are provided. In the wavelength conversion element, wherein the equivalent refractive indices of the first waveguide and the second waveguide to the fundamental wave are N ₁ (ω) and N ₂ (ω), respectively. The first
Of the equivalent refractive index of the waveguide for the second harmonic light is N
If a _{1 (2ω), N 1 (} ω)> condition N ₂ (ω) and, N ₁ and _{(2ω), N 1 (ω} ) and the arithmetic mean value of the N ₂ (ω) (N ₁ The ratio with (ω) + N ₂ (ω)) / 2 is
While satisfying the condition of 0.90: 1 to 1.05: 1,
Since the phase matching is achieved by the interaction of the two waveguides, so-called phase matching can be achieved and the constant relating to the output of the second harmonic light can be made large. This has the effect of increasing the conversion efficiency of harmonic light.

[Brief description of drawings]

FIG. 1 is a schematic configuration perspective view showing an embodiment of a wavelength conversion element of the present invention.

FIG. 2 is a partial front view of FIG. 1 viewed from the light incident side.

FIG. 3 is a perspective view showing an example of a conventional wavelength conversion element.

[Explanation of symbols]

 10 ... Substrate 20 ... First Waveguide 30 ... Sealing Thin Film 40 ... Second Waveguide

Claims (1)

Claim: What is claimed is: 1. A first waveguide provided in a substrate of a non-linear optical material for guiding incident light and provided in the vicinity of the waveguide for phase matching. A wavelength conversion element comprising a second waveguide, wherein the equivalent refractive indices of the first waveguide and the second waveguide to the fundamental wave are N ₁ (ω) and N ₂ (ω), respectively. When the equivalent refractive index of the first waveguide with respect to the second harmonic light is N ₁ (2ω), the condition of N ₁ (ω)> N ₂ (ω) and N
The ratio of ₁ (2ω) and the arithmetic mean value (N ₁ (ω) + N ₂ (ω)) / 2 of N ₁ (ω) and N ₂ (ω) is 0.90:
A wavelength conversion element characterized in that phase matching is achieved by interaction of the two waveguides while satisfying the condition of 1 to 1.05: 1. 2. The first waveguide is formed on one plane side of a non-linear optical material substrate, and the second waveguide is formed through a sealing thin film formed on the waveguide. The wavelength conversion element according to claim 1, wherein the wavelength conversion element is formed along the waveguide. 3. The width of the first waveguide is W1 and the depth is D.
1, the width of the second waveguide is W2, the thickness is D2, and the thickness of the sealing thin film is D3, W2 / W1 =
0.9-1.2, D2 / D1 = 0.8-2.5, D3 =
The wavelength conversion element according to claim 1 or 2, wherein the wavelength conversion element has a thickness of 0.8 to 2.5 µm.