JPH0593932A - Production of wavelength conversion element - Google Patents

Abstract

PURPOSE:To prevent the lateral spread of the polarization inversion layer formed within proton exchange layers and to allow the formation of the polarization inversion layers in a short period by forming the periodic proton exchange layers on an LiTaO3 crystal and heat treating these layers to form the polarization inversion layers. CONSTITUTION:After the periodic proton exchange layer are formed on the LiTaO3 crystal 1, striped metallic films 10 are loaded on the proton exchange layers 3 and are subjected to the high-speed heating treatment by an IR heating device, by which the polarization inversion layers 4 are formed. Since the LiTaO3 substrate 1 hardly absorbs IR rays, only the metallic films 10 absorb IR rays to rapidly increase the temp. of the proton exchange layer 3 parts right under the metallic films. The heat treatment is rapidly completed by increasing the heating up speed, by which the diffusion of the proton exchange layers 3 is prevented. Since the proton exchange layers 3 are partially heated, the lateral diffusion of the proton exchange layers 3 is suppressed and the polarization inversion layers 4 of the short period are formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a wavelength conversion element using a coherent light source and used in the fields of optical information processing and optical measurement control.

[0002]

2. Description of the Related Art Polarization reversal for forcibly reversing the polarization of a dielectric material uses an optical frequency modulator utilizing surface acoustic waves or polarization reversal of nonlinear polarization by forming a periodic polarization inversion layer in the dielectric material. It is used as a wavelength conversion element. In particular, if it is possible to periodically invert the nonlinear polarization of the nonlinear optical material, it is possible to fabricate a second harmonic generation element with extremely high conversion efficiency. As a result, a compact short-wavelength light source can be realized by converting light from a semiconductor laser or the like, and the light source can be applied to the fields of printing, optical information processing, optical applied measurement control, and the like, and thus is actively researched.

FIG. 9 shows a block diagram of a conventional wavelength conversion element.
You Harmonic generation for the fundamental wave with a wavelength of 1.06 μm or less
(Wavelength 0.53 μm) will be described in detail with reference to the drawings.
(E.J.Lim, M.M.Fejer, R.L.Byer, "Second harmonicg
eneration of blue and green light in periodically-
poled planar lithium niobate waveguides ", IGW
O, 1988, see). As shown in Fig. 9, LiNbO ₃substrate
An optical waveguide 22 is formed on the optical waveguide 21, and further on the optical waveguide 2.
A layer 23 (polarization inversion layer) whose polarization is periodically inverted is formed.
Has been. Mismatch of propagation constants of fundamental wave and generated harmonics
Is compensated by the periodic structure of the domain inversion layer 23, the high efficiency
You can put out harmonics in the rate. Incident surface of optical waveguide 22
When the fundamental wave P1 is incident on the optical waveguide 24, the harmonics are emitted from the optical waveguide 22.
Wave P2 is efficiently generated and operates as an optical wavelength conversion element
To do. Such a conventional wavelength conversion element has a polarization inversion structure.
Was the basic component. The polarization inversion layer of this device
The manufacturing method will be described with reference to FIG.

In FIG. 1A, the nonlinear optical crystal LiN is used.
A pattern of Ti101 was formed on the bO ₃ substrate 100 by lift-off and vapor deposition with a period of several μm in width. Next, in FIG. 3B, heat treatment was performed at a temperature of about 1100 ° C. to form a polarization inversion layer 102 in which the polarization was inverted in the opposite direction to the LiNbO ₃ substrate 1. Next, in Fig. 3 (c), 3 in benzoic acid (200 ° C) was used.
After heat treatment for 0 minutes, annealing is performed at 350 ° C. to form the optical waveguide 103. The optical wavelength conversion element manufactured by the benzoic acid treatment has a length of the optical waveguide of 1 mm and a power of the fundamental wave P1 of 1 with respect to the fundamental wave P1 having a wavelength of 1.06 μm.
When the power was set to mW, the power of the harmonic P2 was 0.5 nW. If the fundamental wave enters 40 mW, 800 nW
It is possible to output higher harmonics. In this case, the conversion efficiency per 1 W in a 1 cm element is 5% / W · cm.

[0005]

In the method of manufacturing the domain inversion layer as described above, the domain inversion layer spreads not only in the depth direction but also in the lateral direction. Therefore, if the depth of the domain-inverted layer is made too deep with respect to the distance between the mask patterns, the domain-inverted layer is laterally connected during the heat treatment, and there is a problem that a periodic domain-inverted layer cannot be formed. Therefore, the overlap between the domain inversion layer and the optical waveguide becomes small, and there is a problem that a wavelength conversion element with high conversion efficiency cannot be formed. Specifically, since the wavelength conversion of 1.06 μm is performed, the cycle is 6 μm.
If an attempt is made to form a domain-inverted layer of m, the domain-inverted layer becomes 0.8 μm or less, and only a shallow inversion layer can be formed with respect to the optical waveguide depth of 1.5 μm. For this reason, 1 for a 1 cm element
The conversion efficiency per W was as low as 5% / W · cm ² .

[0006]

The present invention solves the above problems.
To solve, polarization based on proton exchange and heat treatment
The inversion layer can be created by adding new ideas to the inversion layer formation method.
Conversion with a deep polarization inversion layer controlling the lateral diffusion of light
An element is provided. In other words, C plate (C axis of crystal
Of the substrate cut out on a plane perpendicular to_(1-X)Ta
_(X-1)O₃(0 ≦ X ≦ 1) A stripe-shaped pattern is formed on the surface of the substrate.
A step of forming a roton exchange layer, and
Forming a striped metal film on the
b _(1-X)Ta_(X-1)O₃(0 ≦ X ≦ 1) Substrate as heating means
Therefore, the method includes a step of performing heat treatment to form a domain inversion layer.
Of.

Further, in the present invention, in order to manufacture a highly efficient wavelength conversion element, LiNb _(1-X) Ta _(X-1) O ₃ (of a C plate (a substrate cut out in a plane perpendicular to the C axis of the crystal ₎₎ 0 ≦ X ≦ 1) a step of forming a stripe-shaped proton exchange layer on the surface of the substrate, a step of forming a stripe-shaped metal film on the proton exchange layer, and the above LiNb _(1-X) Ta _{(X- 1)} O ₃ (0 ≦
X ≦ 1) a step of heat-treating the substrate by a heating means to form a domain inversion layer and the LiNb _(1-X) Ta _(X-1) O ₃ (0 ≦
X ≦ 1) a step of forming an optical waveguide on the substrate.

[0008]

According to the present invention, after the periodic proton exchange is formed in the LiTaO ₃ crystal by the above-mentioned method, a metal film is loaded on the proton exchange layer, and this is subjected to high-speed heat treatment by an infrared heating device to obtain a proton. By performing heat treatment only on the exchanged portion at high speed in a short time, it is possible to form a deep domain inversion layer with a short period which has not been realized conventionally. Then, by manufacturing a wavelength conversion element using this polarization inversion layer, it is possible to manufacture a wavelength conversion element capable of converting a short wavelength and having high efficiency.

The reason will be described below. The polarization inversion layer in the LiTaO ₃ crystal is formed by forming a proton exchange layer and heat-treating it. When proton exchange is performed, proton exchange LiTaO ₃ is formed, and the Curie point of this portion is lowered. This Heat treatment at LiTaO ₃ below the Curie point of the crystals and above the Curie point of the proton exchange LiTaO ₃ proton exchange LiTaO ₃ reaches the Curie point. At this time, since the electric field generated by the pyroelectric effect is applied to the entire substrate, the polarization of the proton exchange part is reversed. However, since the proton exchange layer spreads due to thermal diffusion, the proton exchange layer is diffused during heating and heating, and in the domain-inverted layer with a short cycle, the inversion layers are connected laterally and the periodic structure Was not formed.

Therefore, when a metal film is loaded on the proton exchange layer and heated by an infrared heating device, the LiTaO ₃ substrate hardly absorbs infrared rays, and only the metal film absorbs infrared rays, and the proton exchange layer portion directly below the metal film is absorbed. The temperature rises rapidly. By increasing the heating rate, the heat treatment can be completed in a short time, and the diffusion of the proton exchange layer can be prevented. Further, since the proton exchange layer is partially heated, lateral diffusion of the proton exchange layer is suppressed, and it becomes possible to form a domain-inverted layer with a short period.

[0011]

EXAMPLE FIG. 1 shows a block diagram of a wavelength conversion element. 1 is C
A LiTaO ₃ substrate on the −C surface of the plate, 4 is a polarization inversion layer, 5 is a proton exchange waveguide, 6 is a fundamental wave having a wavelength of 800 nm, and 7 is a second harmonic wave having a wavelength of 400 nm. In FIG. 1, the polarization inversion layer 4 is a portion in which the polarization direction is reversed with respect to the polarization inversion of the substrate. In the case of the LiTaO ₃ substrate 1 in which the polarization direction is + C, the polarization inversion layer The direction is the -C direction. The period of the polarization inversion layer differs depending on the wavelength of the fundamental wave and the refractive index of the waveguide, but when the wavelength of the fundamental wave is 800 nm, the first-order period is about 3.5 μm and the third-order period is about 10.5 μm (polarization inversion). The period of is an odd multiple of the first-order period.The phase matching condition is satisfied in polarization reversal and wavelength conversion is performed only when the period matches the odd-order period. As it increases, the conversion efficiency decreases to the square of its order).

FIG. 2 shows the outputs of the second harmonics of the non-polarized element, the first-order poled element and the third-order poled element. As shown in the figure, when the effective refractive index of the fundamental wave (wavelength λ) is Nω and the effective refractive index of the harmonic wave (wavelength λ / 2) is N2ω, the primary period Λ is Λ = λ / (2 (N2ω-N2ω)),
The third-order cycle is represented by a cycle three times as long as the first-order cycle Λ.
The effective refractive index is the refractive index that light actually feels. It can be seen that the smaller the order of the cycle, the higher the output.

In the wavelength conversion element shown in FIG. 1, the fundamental wave incident on the optical waveguide is converted into the second harmonic in the polarization inversion layer, and when the fundamental wave having the wavelength of 800 nm is incident, the second blue wave having the wavelength of 400 nm is emitted. Harmonics can be generated. At this time, the inversion period affects the conversion efficiency, and the efficiency of the third-order polarization inversion layer is about 1/9 of the efficiency of the first-order polarization inversion layer.
become.

FIG. 3 is a process chart of the method of manufacturing the wavelength conversion element according to the first embodiment, which is a method of forming the polarization inversion layer. In FIG. 3, 1 is a Li-TaO ₃ substrate of C plate, 2 is T
a film, 3 proton exchange layer, 4 domain inversion layer, 8 mask width W, 9 period Λ, and 10 Ta film. However, the polarization reversal process is a process in which the temperature of the LiTaO ₃ crystal is raised to near the Curie temperature and the polarization of the crystal aligned in a certain direction is partially reversed. The method of manufacturing the wavelength conversion element will be described below. (A) -C plate Li
A Ta film 2 of 300 A is formed on the TaO ₃ substrate 1 by the sputtering method. (B) After applying a photoresist on the Ta film,
A stripe having a width W is formed for every 10 mm in the Y propagation direction of the substrate by a normal photolithography method (the period Λ is 1 to 50 μm, and W is Λ / 2).
(C) The resist pattern is transferred to the Ta mask by dry etching in a CF ₄ atmosphere. (D) Heat treatment is performed in pyrophosphoric acid at 260 ° C. for 20 minutes to form a proton exchange layer. (E) Deposit 300 A of Ta film 10 on the proton exchange layer. (F) After applying a photoresist on the Ta film, the Ta film other than the proton exchange layer is removed by a normal photolithography method and dry etching. (G) The LiTaO ₃ substrate was heated by an infrared heating device. The infrared heating device is a device that heats a sample with an infrared lamp and can realize a high temperature rising rate. The heating device can control the temperature rise in a wide range. At this time,
The temperature was changed between 0.1 and 100 ° C./sec and the heating was performed. The samples were compared with those in which the Ta membrane was loaded on the proton exchange layer and those not loaded. The heat treatment temperature is 550 ° C and 570 ° C.
Met. The Curie temperature of the LiTaO ₃ substrate used was 60.
It was 4 ° C.

First, when the domain-inverted layer was formed by forming the domain-inverted layer at a temperature rising rate of 0.1 ° C./sec, only the domain-inverted layer having a period of 30 μm or more was formed. This is because the temperature at which the domain-inverted layer is formed is about 450 ° C to 600 ° C near the Curie temperature of the LiTaO ₃ substrate, but if the heating rate is slow, the proton exchange layer spreads by thermal diffusion before reaching this temperature. This is because the proton exchange layers formed periodically are connected in the lateral direction. This is shown in FIG. Reference numeral 1 is a LiTaO ₃ substrate, and 4 is a domain inversion layer.
FIG. 4A shows a case where a periodic domain inversion layer is formed.
(B) shows the case where the inversion layer is connected in the lateral direction. Figure 5
The relationship between the temperature rising rate and the minimum polarization inversion period that can be formed was obtained. The formable polarization inversion period decreased as the heating rate was increased, and by forming a metal film on the proton exchange layer, the formable polarization inversion period decreased.

FIG. 6 shows the relationship between the width and depth of the domain inversion layer. With a width of 2 μm, the inversion layer that can be formed has a depth of 0 μm at a temperature rising rate of 0.1 ° C./sec, 1.7 μm at a temperature increase of 100 ° C./sec without the Ta film 10, and the Ta film 10 is attached. It increased to 2.5 μm. In this method, a metal film is loaded on the proton exchange layer, and this is heated by infrared heating to increase the temperature rising rate and reduce the spread of the proton exchange layer diffused during the heat treatment. Further, since the infrared absorption coefficient of the metal film is significantly higher than that of the substrate, the metal film is partially and rapidly heated by infrared heating. Therefore, since the proton exchange layer immediately below it is partially heated, the lateral extension of the polarization inversion layer is further suppressed. As a result, it is possible to prevent the lateral inversion of the domain-inverted layer formed in the proton exchange layer from spreading and form a deeper inversion layer. This made it possible to form a domain-inverted layer with a short period.

On the other hand, as a short-wavelength light source, what is currently required for compact discs, optical memories, etc. is a wavelength of 48.
It is particularly important to form a wavelength conversion element that emits blue light having a wavelength of 0 nm or less and that can generate blue light. Considering the conversion efficiency of the wavelength conversion element, the conversion efficiency of the wavelength conversion of blue light is very low even if an element having a polarization inversion period of the fifth order or more is manufactured, and is not practically used. Is insufficient.

Therefore, a wavelength conversion element having a polarization inversion layer having high conversion efficiency is necessary. The polarization inversion period that can be realized by such a short-wavelength light source is 12 μm or less in the third-order polarization inversion period and 4 μm or less in the first-order polarization inversion period. From the relationship between the rate of temperature rise in FIG. 5 and the minimum polarization inversion period that can be formed, the required rate of temperature rise during the heat treatment is found to be 0.8 ° C./sec or more in the third order domain inversion layer Therefore, a speed of 15 ° C./sec or more is required. As a result, it is necessary to heat at a temperature rising rate of 0.8 ° C./sec or more in order to manufacture a wavelength converter with high output and high efficiency.

Next, a proton exchange waveguide was formed on the domain-inverted layer produced by the above method to produce a wavelength conversion element, and its characteristics were evaluated. FIG. 7 shows a method for manufacturing a proton exchange waveguide.
Shown in. In FIG. 7, 1 is a C-plate LiTaO ₃ substrate, 4 is a polarization inversion, 11 is a Ta mask, and 12 is a proton exchange waveguide. The manufacturing method is as follows. After forming a domain-inverted period of 4 μm (first-order domain-inverted layer) at a layer heating rate of 50 ° C./second,
(A) After removing the Ta film 10 for forming the polarization inversion, the Ta film is sputtered on the surface of the substrate by 340
A film is formed. (B) The mask pattern is formed by photo dry etching. (C) Proton exchange optical waveguide is formed by heat treatment in pyrophosphoric acid at 260 ° C. for 10 minutes. (D) The Ta mask was removed with an acid, and both end faces of the waveguide were optically polished to form a wavelength conversion element. Wavelength 8
Light 6 of a semiconductor laser having a wavelength of 00 nm and an output of 40 mW was condensed by a condensing optical system and made incident on a wavelength conversion element produced. The fundamental wave and the second harmonic wave emitted from the waveguide were collimated with a lens and measured with a power meter. As a result, the output of the second harmonic of wavelength 400 nm of 7 is 3.2 mW, and the conversion efficiency at this time is 20% / 100 mW · cm ^2.
That is a very high efficiency wavelength conversion. This is 1.6 times the conversion efficiency (12.5%) of the wavelength conversion element using the polarization inversion layer formed without metal loading. Further, the power of the fundamental light was increased and guided to 200 mW, but no fluctuation due to optical damage was observed. As a result, a stable and highly efficient wavelength conversion element could be manufactured.

In this embodiment, the direction of fabrication of the domain inversion layer is the Y propagation direction, but the same element can be fabricated in the X propagation direction.

In this example, a LiTaO ₃ substrate was used as the substrate, but LN doped with LiNbO ₃ or MgO, Nb, Nd, etc. was also used.
Similar devices can be fabricated on iTaO ₃ or LiNbO ₃ substrates.

Although an infrared heating device is used as the heating device in this embodiment, a flash lamp heating device,
A similar element can be manufactured by a heating device having a high temperature rising rate such as a CO2 laser heating device.

In this embodiment, pyrophosphoric acid is used for ion exchange, but orthophosphoric acid, benzoic acid, sulfuric acid, etc. can also be used.

In this embodiment, an ionization resistant mask is used.
Then, the Ta film was used, but₂O _FiveAcid resistance such as Pt, Au, etc.
Any film having a property can be used.

In this embodiment, as the metal layer loading film, T
Although the film a is used, any other film having heat resistance such as Ti, Pt, or Au can be used.

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

[0027]

As described above, when a periodic proton exchange layer is formed on a LiTaO ₃ crystal and heat treated to form a domain inversion layer, a metal film is loaded on the proton exchange layer. As a result, it was possible to increase the temperature rising rate, prevent the diffusion of the proton exchange layer until reaching the polarization inversion treatment temperature, and prevent the lateral spread of the polarization inversion formed in the proton exchange layer. Further, by partially heat-treating the proton exchange layer, the lateral extension of the inversion layer is further suppressed, and as a result, it becomes possible to form a deep domain inversion layer with a very short period, which has a large practical effect.

When a polarization inversion type wavelength conversion element is formed by using this inversion layer, a highly efficient inversion layer can be formed by a deep inversion layer, and its practical effect is great.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the relationship between the wavelength conversion element length and the second harmonic output.

FIG. 3 is a process cross-sectional view of a method of manufacturing a polarization inversion layer.

FIG. 4 is a cross-sectional view showing the lateral spread of the polarization inversion layer.

FIG. 5 is a diagram showing the relationship between the rate of temperature rise during heat treatment and the manufacturable polarization inversion period.

FIG. 6 is a diagram showing the relationship between the width and the depth of the domain inversion layer.

FIG. 7 is a process perspective view showing a method of manufacturing a waveguide of a wavelength conversion element.

FIG. 8 is a process sectional view showing a method of manufacturing a conventional wavelength conversion element.

FIG. 9 is a structural perspective view of a conventional wavelength conversion element.

[Explanation of symbols]

1 LiTaO ₃ Substrate 2 Ta Film 3 Proton Exchange Layer 4 Polarization Inversion Layer 5 Proton Exchange Waveguide 6 Fundamental Wave 7 Second Harmonic 8 Mask Width W 9 Period Λ 10 Ta Film 11 Ta Mask 12 Proton Exchange Waveguide 21 LiTaO ₃ Substrate 22 Proton Exchange Waveguide 23 Polarization Inversion Layer 24 Incident Part 100 LiNbO ₃ Substrate 101 Ti Pattern 102 Polarization Inversion Layer 103 Optical Waveguide

Claims (4)

[Claims] 1. A stripe-shaped LiNb _{(1-X )} Ta _(X-1) O ₃ (0 ≦ X ≦ 1) substrate of a C plate (a substrate cut in a plane perpendicular to the C axis of the crystal). Forming a stripe-shaped metal film on the proton exchange layer; and forming the LiNb _(1-X) Ta _(X-1) O
₃ (0 ≦ X ≦ 1) a step of heat-treating the substrate by a heating means to form a domain-inverted layer, and a method for manufacturing a wavelength conversion element. Wherein C plate (substrate cut in a plane perpendicular to the C axis of the _{crystal) LiNb (1-X) Ta} (X-1) O 3 (0 ≦ X ≦ 1) stripe on the surface of the substrate forming a proton exchange layer to form a stripe-shaped metallic film on the proton exchange layer, wherein the _{LiNb (1-X) Ta (} X-1) O 3 (0
≦ X ≦ 1) a step of heat-treating the substrate by a heating means to form a domain inversion layer, and the above LiNb _(1-X) Ta _(X-1) O ₃
(0 ≦ X ≦ 1), and a step of forming an optical waveguide on the substrate. 3. The method of manufacturing a wavelength conversion element according to claim 1, wherein an infrared heating device is used as the heating means. 4. A step of heating to a temperature T at a temperature rising rate S and a step of performing a heat treatment at a temperature T as a step of heat-treating by a heating means to form a domain inversion layer, and the value of the temperature rising rate S. _3. The wavelength conversion according to claim 1 or 2, wherein an infrared heating device is used as a heating means having a temperature of 0.8 ° C./sec<S and a heat treatment temperature T of not higher than the Curie temperature of the LiTaO ₃ substrate. Manufacturing method of device.