JPH04340525A - Wavelength conversion element - Google Patents

Abstract

PURPOSE:To provide the wavelength conversion element which has high efficiency and can make short wavelength conversion relating to the structure of the wavelength conversion element for which a nonlinear optical effect is utilized. CONSTITUTION:Polarity inversion layers of a width W are formed at every polarity inversion period A2 of second order on an LiTaO3 substrate 1 of a -C plate which is the wavelength conversion element. A proton exchange optical waveguide 5 is formed across these polarity inversion layers 4. The element is so constituted as to satisfy the relation of the polarity inversion period LAMBDA2=lambda/(N2omega-Nomega) for the execution refractive index Nomega to basic light 6 (having lambda wavelength) propagating in the waveguide 5 and the execution refractive index N2omega to SHG light 7 (having lambda/2 wavelength). The max. value of the conversion efficiency is obtd. if the width W of the polarity inversion layers 4 is between LAMBDA2/2<W<LAMBDA2 by this constitution.

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 used in the fields of optical information processing and optical applied measurement control using a coherent light source.

0002

[Background Art] Polarization inversion, which forcibly inverts the polarization of a dielectric material, uses an optical frequency modulator that uses surface acoustic waves and polarization inversion of nonlinear polarization by forming a periodic polarization inversion layer on the dielectric material. It is used for wavelength conversion elements, etc. In particular, if it becomes possible to periodically invert the nonlinear polarization of a nonlinear optical material, a second harmonic generating element with extremely high conversion efficiency can be produced. By converting light from semiconductor lasers and other sources, it is possible to create compact, short-wavelength light sources, which can be applied to fields such as printing, optical information processing, and optical applied measurement and control, and are therefore being actively researched.

[0003] As a conventional wavelength conversion element using such polarization inversion, for example, (Electron. Lett., 1989, No. 25, P.
731)E. J. There is a polarization inversion type wavelength conversion element by Mr. Lim. In this method, a periodic polarization inversion layer is formed on the LiNbO3 substrate by periodically diffusing Ti on the surface of the LiNbO3 substrate, and an optical waveguide is formed across this periodic polarization inversion layer to form a wavelength conversion element. It consists of FIG. 10 is a configuration diagram of this conventional wavelength conversion element. In FIG. 10(a), 21 is a LiNbO3 substrate,
22 is a proton exchange waveguide, 23 is a polarization inversion layer, 24 is a fundamental light, 25 is a second harmonic wave (hereinafter referred to as SHG light), 26
is the first period Λ1 of the polarization inversion layer, and 27 is the width of the polarization inversion layer. FIG. 10(b) shows how this wavelength conversion element is connected to the waveguide 22.
This is a cross section taken along the . In FIG. 10(b),
22 is a proton waveguide, 23 is a polarization inversion layer, the optical waveguide 22 has a depth D, and the polarization inversion layer 23 has a period Λ1,
It has a triangular shape with a width W and a depth d. The first period Λ1 of this polarization inversion layer is the effective refractive index Nω for the fundamental light (wavelength is λ) and the SHG guided through the optical waveguide.
The relationship Λ1=λ/2/(N2ω−Nω) is satisfied for the effective refractive index N2ω for light (wavelength is λ/2). Fundamental light propagating within the waveguide is converted into SHG light by periodically formed polarization inversion layers. When the element length is 1 mm, the SHG conversion efficiency is 37%/W/cm2. Further, the wavelength dependence of the fundamental light is a full width at half maximum of 0.5 nm. This full width at half maximum is inversely proportional to the element length.

0004

[Problems to be solved by the invention] LiNbO3 crystals have the problem of optical damage, and increasing the optical power density causes a change in the refractive index within the waveguide, making it difficult to stabilize the output at high outputs. be. Furthermore, as shown in FIG. 10(b), the depth d of the polarization inversion layer is approximately 1 with respect to the depth D of the waveguide.
/2 or less, because the ratio of the depth d and the width W of the polarization inversion layer becomes a constant similarity. However, in order to generate blue light, a polarization inversion period Λ1 of approximately 3 μm is required, and therefore, the polarization inversion layer depth d is regulated by the polarization inversion width W, and the polarization inversion layer is deep enough for the optical waveguide. It becomes impossible to form. Wavelength conversion by nonlinear optical effect is fundamental light, SHG
Since it depends on the overlapping integral of three factors: light and polarization inversion layer depth, if only such a shallow polarization inversion layer can be formed, the conversion efficiency will be extremely low, and there is a problem that highly efficient wavelength conversion cannot be achieved. .

Therefore, the present invention sets the period of polarization inversion to Λ2(
=2Λ1), and thereby the purpose is to manufacture a highly efficient wavelength conversion element by obtaining a sufficiently deep polarization inversion layer with respect to the waveguide.

[0006]

[Means for Solving the Problems] A dielectric substrate, a polarization inversion layer having a width W in which nonlinear polarization is inverted with a second-order period Λ2 formed near the surface of the substrate, and comprising an optical waveguide extending perpendicularly to the polarization inversion layer, and an input part and an output part formed on both end surfaces of the optical waveguide,
and the period Λ of the polarization inversion layer is the effective refractive index Nω for the fundamental light (wavelength is λ) guided through the optical waveguide and the SHG light (wavelength is λ/2) guided through the optical waveguide.
Λ2=λ/(N2
ω−Nω), and the width W of the polarization inversion layer
is a wavelength conversion element that satisfies the relationship Λ2/2<W<Λ2.

[0007]

[Operation] The present invention constructs a wavelength conversion element using a second-order polarization inversion period with the above-described structure. Here, the first and second
The next polarization inversion will be explained.

FIG. 8(a) shows the relationship between the element length of the wavelength conversion element and the SHG output converted within the element. FIG. 8(b) shows the state of polarization in the first-order polarization inversion, and FIGS. 8(c) and (d) show the state of polarization in the second-order polarization inversion. Figure 8(b)(c)(d)
), the arrow indicates the direction of polarization.

Now, as shown in FIG. 8(b), if the direction of polarization is alternately reversed with a period Λ1, the SHG output will be as shown in FIG. 8(a).
As shown by the curve b), it increases in proportion to the square of the distance.

[0010] When this period becomes twice Λ2 (=Λ1), it is a second-order polarization inversion. However, as shown in Fig. 8(c), if the direction of polarization is alternately reversed with a period of Λ2, Fig. 8(a)
As shown in (c), the SHG output does not increase with distance, and a highly efficient SHG output cannot be obtained. However, even with the same period Λ2, the ratio of polarization reversal within one period Λ2 is 1
:3, the SHG output increases in proportion to the square of the distance, as shown in (d) in FIG. 8(a), allowing highly efficient conversion.

[0011] As a result, the period of the domain-inverted layer is expanded to twice the first-order period Λ1, increasing the tolerance for forming the domain-inverted layer and facilitating its manufacture. In addition, since the depth of the polarization inversion layer depends on the width of the polarization inversion layer, by making the width of the polarization inversion layer larger than 1/2 of the polarization inversion period, it is possible to form a deeper polarization inversion layer, thereby forming an optical waveguide. This increases the overlap between the two and improves the conversion efficiency. Furthermore, by adjusting the ratio between the polarization inversion layer and the non-inversion layer, the conversion efficiency can be further increased and a high output wavelength conversion element can be constructed.

0012

Embodiment FIG. 1 shows a configuration diagram of a wavelength conversion element in a first embodiment. 1 is a -C plate LiTaO3 substrate, 4 is a polarization inversion layer, 5 is a proton exchange waveguide, and 6 is a wavelength of 800n.
7 is the fundamental light of m, and SHG light with a wavelength of 400 nm. In FIG. 1, the polarization inversion layer 4 is a portion in which the direction of polarization is reversed with respect to the substrate. The period Λ2 of the polarization inversion layer of the wavelength conversion element of this example is quadratic, and is 2nd-order of the first-order period Λ1.
It has twice the period (Λ2=Λ1*2). The second-order period Λ2 of this polarization inversion layer is determined by the effective refractive index Nω for the fundamental light (wavelength is λ) and the effective refractive index N2ω for the SHG light (wavelength is λ/2) guided through the optical waveguide. for,
The relationship Λ2=λ/(N2ω−Nω) is satisfied. The effective refractive index is the refractive index that light actually feels. L
In the case of iTaO3, when the fundamental light wavelength is 800 nm, 1
The next period Λ1 is approximately 4 μm, and the second period is approximately 8 μm (the phase matching between the fundamental light and the SHG light is achieved and the SHG light is generated because the period of the polarization inversion layer is an integer multiple of the first polarization inversion period Λ1). ). FIG. 2 shows a process diagram of a method of manufacturing a wavelength conversion element in the first embodiment, which is a method of forming a polarization inversion layer. In Figure 2, 1 is -C
2 is a Ta mask, 3 is a proton exchange layer, 4 is a polarization inversion layer, 8 is a mask width W, and 9 is a second-order period Λ2. The manufacturing method will be explained below. (a)-C plate LiT
A 30% Ta film is deposited on the aO3 substrate 1 by sputtering.
0A is formed. (b) After coating a photoresist on the Ta film, stripes with a width W are formed in the Y propagation direction of the substrate at intervals of Λ by a normal photolithography method (the period Λ is set to 7 μm). (c) Transfer the resist pattern to the Ta mask by dry etching in a CF4 atmosphere. (d) A proton exchange layer 3 is formed by heat treatment in pyrophosphoric acid at 260° C. for 20 minutes. (e) LiTa with heating device
The O3 substrate was heated.

[0013] Here, the polarization inversion treatment refers to LiTaO3
This is a process in which the temperature of the crystal is raised to near the Curie temperature, and the polarization of the crystal, which is aligned in a certain direction, is partially reversed. The possible mechanism for this polarization reversal is
First of all, by proton exchange, Li in the proton exchange layer is
The concentration decreases, which lowers the Curie point of the proton exchange layer. Therefore, if the substrate is heat-treated between the Curie point of the LiTaO3 crystal and the Curie point of the proton-exchanged LiTaO3, only the proton-exchanged portion can be brought into the Curie state. Further, the polarization of the proton exchange layer in the Curie state is reversed by an electric field generated between ions in the proton exchange layer and ions in the substrate, thereby forming a polarization inversion layer.

FIG. 3 shows a method for producing an optical waveguide formed on the produced polarization inversion. In Figure 3, (a) Figure 2 (
The Ta mask 2 for polarization inversion formation shown in e) is removed. (b) A Ta film of 340 A is formed on the surface of the LiTaO3 substrate 1 by sputtering. (c) Form a mask pattern by photolithography and dry etching. (d) 1 in pyrophosphoric acid at 260°C
When the heat treatment is performed for 0 minutes, a proton exchange optical waveguide 5 is formed. The Ta mask was removed with acid, and both end faces of the waveguide were optically polished to form a wavelength conversion element.

Next, the operation of the wavelength conversion element will be explained. The fundamental wave incident on the optical waveguide is converted into SHG light in the polarization inversion layer, and when the fundamental light with a wavelength of 800 nm is incident, blue SHG light with a wavelength of 400 nm can be generated. Theoretically, when a domain-inverted layer deep enough for an optical waveguide can be formed, the inversion period affects the conversion efficiency, and the efficiency of the second-order domain-inverted layer is approximately 1/1 of the efficiency of the first-order domain-inverted layer.
It becomes 4. However, in the first-order polarization inversion, since the polarization inversion period is short, a sufficiently deep polarization inversion layer cannot be formed in the optical waveguide. This is because the polarization inversion layer does not depend on the size.
Because of the similar shapes, a deep domain-inverted layer cannot be formed in a domain-inverted layer with a short period and a small domain-inverted layer width. On the other hand, since the period of the second-order polarization inversion layer is twice the period of the first-order period, the width of the polarization inversion layer is expanded and a deep polarization inversion layer can be formed. This will be explained using FIGS. 4 and 5.

FIG. 4(a) is a cross section of a wavelength conversion element having a first-order polarization inversion period taken along the waveguide 5. 1 is a -C plate LiTaO3 substrate, 4 is a polarization inversion layer, 5 is a proton exchange waveguide, 10 is an electric field distribution of fundamental light with a wavelength of 800 nm, 11 is an electric field distribution of SHG light with a wavelength of 400 nm, 12 is the fundamental light and SHG This is the overlap of the electric fields within the optical polarization inversion layer. FIG. 5(a) is a cross section of a wavelength conversion element having a second-order polarization inversion period taken along the waveguide 5. 1 is a -C plate LiTaO3 substrate, 4 is a polarization inversion layer, 5 is a proton exchange waveguide, and 10 is a wavelength of 800 nm.
11 is the electric field distribution of the SHG light with a wavelength of 400 nm, and 13 is the overlap of the electric fields in the polarization inversion layer of the fundamental light and the SHG light. The SHG output is proportional to the overlap 12, 13 of the electric fields of the fundamental light 6 and the SHG light 7 within the polarization inversion layer. Therefore, as shown in FIG. 5(b), in the case of a wavelength conversion element with a second-order polarization period, the electric field overlap 13 of the fundamental light 6 and SHG light 7 in the polarization inversion layer is shown in FIG. 4(b). This is much larger than the electric field overlap 12 of a wavelength conversion element with a first-order polarization inversion period, so highly efficient wavelength conversion can be performed.

For example, the fundamental light 6 of a semiconductor laser having a wavelength of 800 nm and an output of 40 mW was focused by a focusing optical system and made incident on a fabricated wavelength conversion element. The fundamental wave and SHG light emitted from the waveguide were collimated with a lens and measured with a power meter. The results are shown in FIG. Figure 6 shows the basic light output and the SHG light output, where the horizontal axis is the basic light output;
The vertical axis is the SHG output. The output of the SHG light in the wavelength conversion element with the second-order polarization inversion period was 2 mW, and the conversion efficiency at this time was 60%/W·cm 2 , which was very high efficiency wavelength conversion. Further, this value was approximately twice the SHG output in the first polarization inversion period. Furthermore, although the power of the fundamental light was increased to 200 mW, no fluctuation due to optical damage was observed. As a result of the above, a stable and highly efficient wavelength conversion element was fabricated.

The relationship between the width of the polarization inversion layer and SHG will be explained. FIG. 7 shows the relationship between the width W of the polarization inversion layer and the SHG output in a wavelength conversion element with a second-order polarization inversion period. In FIG. 7, the horizontal axis is the width W of the polarization inversion layer, and the vertical axis is SH
It is G output. If the second-order polarization inversion period is Λ2, then SH
The G output is as shown in FIG. The SHG output reaches its maximum when the width W of the polarization inversion layer is near Λ2/4 and 3Λ2/4, but
The SHG output becomes maximum not between 0<W<Λ2/2 but between Λ2/2<W<Λ2.

The reason for this will be explained using FIG. 9. Figure 9 (
a) shows the relationship between the element length of the wavelength conversion element and the SHG output. Figures 9(b), (c), (d), and (d) show the waveguide and the polarization inversion layer; the direction of polarization in the waveguide is upward, and the direction of polarization in the inversion layer is downward. be.

In FIGS. 9(b), (c), and (d), the period of polarization inversion is Λ2, the width and depth of the polarization inversion layer are W and d, respectively, and the depth of the waveguide is D. The SHG output is determined by the overlap between the polarization inversion depth d and the waveguide depth D.

FIG. 9(b) shows that the width W of the polarization inversion layer is 0<W<
Λ2/2, and in this case the SHG output increases with distance, but the polarization inversion depth d with respect to the waveguide depth D
Since the SHG output is small, only a low SHG conversion efficiency can be obtained as shown in (b) in FIG. 9(a).

FIG. 9(c) shows that the width W of the polarization inversion layer is W=Λ2
/2. At this time, the SHG output becomes (c) in FIG. 9(a), and the SHG output becomes almost zero.

However, as shown in FIG. 9(d), Λ2/2
When <W<Λ2, the width W of the polarization inversion layer becomes large, so the depth d of the polarization inversion layer becomes sufficiently deep with respect to the waveguide depth D.

[0024] As a result, high conversion efficiency of the SHG output can be obtained as shown in (d) in Fig. 9(a).

On the other hand, in the range Λ2/2<W<Λ2, the width of the polarization inversion layer is wide, so the depth is sufficiently deep to the waveguide, and there is a point where the SHG output is maximum, resulting in a very high conversion. Efficiency can be achieved.

In this example, the polarization inversion layer was fabricated in the Y propagation direction, but a similar device can also be fabricated in the X propagation direction.

Although a LiTaO3 substrate was used as the substrate in this embodiment, a similar device can also be fabricated using a LiTaO3 substrate doped with MgO, Nb, Nd, or the like.

In this example, pyrophosphoric acid was used for ion exchange, but other materials such as orthophosphoric acid, benzoic acid, and sulfuric acid can also be used.

In this example, a Ta film was used as an ionization-resistant mask, but other materials such as Ta2O5, Pt, and Au
Any film having acid resistance, such as, can be used.

Although a proton exchange waveguide was used as the optical waveguide in this embodiment, other optical waveguides such as a Ti diffusion waveguide, a Nb diffusion waveguide, and an ion implantation waveguide can also be used.

[0031]

[Effects of the Invention] As explained above, by constructing a polarization-inverted wavelength conversion element using a second-order polarization inversion period, the polarization inversion period becomes twice that of the first-order period, and fabrication becomes easier. becomes easier. In addition, since the width of the polarization inversion layer is increased, it is possible to form a sufficiently deep polarization inversion layer with respect to the optical waveguide, which increases the overlap between the electric field distributions of the fundamental light and SHG light within the polarization inversion layer, resulting in conversion to SHG light. Efficiency increases. Furthermore, by making the width of the polarization inversion layer larger than 1/2 of the polarization inversion period Λ, a deeper polarization inversion layer can be formed, and the SHG conversion efficiency can be increased. SHG of next polarization reversal period
Since the maximum output value can be obtained, it is possible to construct a polarization-inverted wavelength conversion element that is highly efficient, capable of short wavelength conversion, and easy to manufacture, and has great practical effects.

[Brief explanation of the drawing]

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

[
Figure 2: Cross-sectional view of the manufacturing process of the polarization inversion layer of the wavelength conversion element of the present invention

[Fig. 3] A perspective view of the manufacturing process of the proton exchange waveguide of the wavelength conversion element of the present invention.

[Figure 4] (a) Structural cross-sectional view of a wavelength conversion element with a first-order polarization inversion period (b) Diagram showing the overlap of electric field distributions of fundamental light and SHG light in a wavelength conversion element with a first-order polarization inversion period

[Figure 5] (a) Structural cross-sectional view of a wavelength conversion element with a second-order polarization inversion period (b) Diagram showing the overlap of electric field distributions of fundamental light and SHG light in a wavelength conversion element with a second-order polarization inversion period

[Figure 6] Diagram showing SHG output for fundamental light

[Figure 7] Diagram showing SHG output versus width W of polarization inversion layer

[Figure 8] (a)
Diagram showing the SHG output versus element length for the first-order polarization inversion period and the second-order polarization inversion period (b) Diagram showing the first-order polarization inversion period Λ1 of the wavelength conversion element (c) Second-order polarization of the wavelength conversion element Diagram (d) showing the inversion period Λ2 (the ratio of the width W of the non-polarization inversion layer and the polarization inversion layer is 1:1). The ratio of W is 1
:3) Diagram showing

FIG. 9 (a) A diagram showing the SHG output relationship of the second-order polarization inversion layer width with respect to the device length. (b) Polarization inversion with respect to the polarization inversion layer width W when the polarization inversion layer width W satisfies 0<W<Λ2. Cross-sectional view of the wavelength conversion element showing the depth d of the layer (c) Cross-sectional view of the wavelength conversion element showing the depth d of the polarization inversion layer with respect to the polarization inversion layer width W when the polarization inversion layer width W is W=Λ2/2 Cross-sectional view (d) Cross-sectional view of the wavelength conversion element showing the depth d of the polarization inversion layer with respect to the polarization inversion layer width W when the polarization inversion layer width W satisfies Λ2/2<W<Λ2

FIG. 10 (a) Configuration diagram of a conventional wavelength conversion element (b) Cross-sectional diagram of a conventional wavelength conversion element

[Explanation of symbols]

1 -C plate LiTaO3 substrate 2 Ta mask 3 Proton exchange layer 4 Polarization inversion layer 5 Proton exchange waveguide 6 Fundamental light with a wavelength of 800 nm 7 Second harmonic (SHG light) with a wavelength of 400 nm 8
Mask width W 9 Period Λ 10 Electric field distribution of fundamental light with a wavelength of 800 nm 11 Electric field distribution of SHG light with a wavelength of 400 nm 12 Fundamental light and S
Overlap of electric fields in the polarization inversion layer of HG light 13 Overlap of electric fields in the polarization inversion layer of fundamental light and SHG light 21 LiNbO3 substrate 22 Proton exchange waveguide 23 polarization inversion layer 24 Fundamental light 25 Second harmonic (SHG) (light) 26 Period of polarization inversion layer 27 Width of polarization inversion layer 28 Electric field distribution of basic light 29 Electric field distribution of SHG light

Claims (1)

[Claims] 1. A dielectric substrate, a domain-inverted layer having a width W in which non-linear polarization is inverted with a second-order period Λ2 formed near the surface of the substrate, and the domain-inverted layer formed near the surface of the substrate. an optical waveguide that is perpendicular to the optical waveguide, and an input section and an output section formed on both end surfaces of the optical waveguide, and the second-order period Λ2 of the polarization inversion layer is such that the fundamental light (wavelength is λ) guided through the optical waveguide is provided. satisfies the relationship Λ2=λ/(N2ω−Nω) with respect to the effective refractive index Nω for the harmonic (wavelength is λ/2) guided in the optical waveguide; and the width W of the polarization inversion layer satisfies Λ2/2<W
A wavelength conversion element characterized by satisfying the relationship <Λ2.