JPH05346602A - Wavelength conversion element - Google Patents

Abstract

PURPOSE:To modulate a refractive index along a wavelength direction and to relieve a phase matching condition by changing the doping quantity of a dopant, such as Mg, along the waveguiding direction of a waveguide. CONSTITUTION:An additive layer 5 consisting of magnesium oxide gradually increased in film thickness along the Z-axis direction is formed by a sputtering method on the +C surface of an LiNbO3 substrate 1. The substrate 1 is then heated and is subjected to a heat treatment to thermally diffuse the MgO into the LiNbO3 substrate. The LiNbO3 substrate having the region 6 doped with the MgO on the +C face is produced. Consequently, the concn. of the MgO increases and the refractive index of the region 6 near the +C face of the substrate decreases gradually with an increase in the distance from the incident end face of the substrate to the Z direction. The region of plural polarization inversion domains 4 is then formed on the +C face of the substrate by the diffusion of Ti and a heat treatment near the Curie point. Three-dimensional waveguide 3 are then formed by a proton exchange method on the +C face of the substrate, by which the wavelength conversion element is obtd.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a quasi-phase matching (QP) having a waveguide and injecting a fundamental wave into the waveguide.
A wavelength conversion element (hereinafter,
QPM element).

[0002]

2. Description of the Related Art Second Harmonic Gener
is known as a wavelength conversion element using quasi-phase matching. The QPM element has a polarization inversion structure composed of a plurality of polarization inversion domains in which the polarization of the waveguide is periodically inverted along the extension direction of the waveguide.

Quasi-phase matching utilizes the fact that the second harmonic output from the waveguide into which the fundamental wave is injected cyclically repeats the maximum and minimum for each coherence length lc as it propagates.
This is a matching method in which the sign of the polarization wave generated for each coherence length is alternately inverted and the output is increased by adding the outputs of the second harmonics. In order to periodically invert the sign of the polarization wave, the sign of the nonlinear coefficient in the waveguide may be inverted, and the inversion characteristic of the domain can be used in the ferroelectric substance.
For example, on the surface of a LiNbO ₃ crystal, a domain-inverted domain is likely to be generated due to external factors such as impurities, strain stress, heat and electric field, so that the crystal is used as a substrate of a QPM element. L
A QPM element is obtained by forming a waveguide and a domain-inverted domain which is periodic in the extension direction on the main surface of the substrate of the iNbO ₃ crystal. The domain inversion domain structure formation method includes heat treatment near the Curie point, high voltage treatment with a comb-shaped electrode, electron beam drawing treatment, and the like.

The half period Λ of the domain inversion domain may be represented by the following equation as long as it is an odd multiple of the coherence length lc.

[0005]

[Number 1] Λ = (2m + 1) lc = (2m + 1) λ 0/4 (| n (2ω)
−n (ω) |) where m is an integer, λ ₀ is the wavelength of the fundamental wave, n (2ω) is the refractive index of the waveguide for the second harmonic of frequency 2ω, and n (ω) is the fundamental wave of frequency ω The refractive index of the waveguide for waves is shown.

Therefore, in order to obtain a QPM device with high conversion efficiency, a stable laser and an accurate domain inversion domain structure are required.

[0007]

However, as is clear from the above equation, the temperature of the QPM element itself changes to change the refractive indices n (ω) and n (2ω), and the basic incident light from the laser is changed. If the wavelength λ _{0 of the} wave fluctuates, the coherence length lc changes and quasi-phase matching cannot be achieved.
The QPM element has small tolerance to temperature and wavelength, for example, the coherence length lc is on the order of μm, and the refractive index difference | n (2ω) −n (ω) | is on the order of 10 ⁻² . Therefore, the allowable wavelength fluctuation range of the fundamental wave is as small as 0.2 nm, the phase matching condition of the QPM element is strict, and
The same order is required for the processing accuracy of the domain-inverted domain, and there is a problem that high fineness is required in manufacturing the device.

Therefore, an object of the present invention is to provide a wavelength conversion element having a structure in which the quasi-phase matching condition is relaxed.

[0009]

The wavelength conversion element of the present invention is a wavelength conversion element having a waveguide made of a non-linear optical material, and the waveguide is gradually increased or decreased along the extension direction of the waveguide. And a plurality of domain-inverted domains in which the polarization of the waveguide is periodically inverted along the extension direction of the waveguide.

[0010]

In the wavelength conversion element of the present invention, since the refractive index of the waveguide is gradually changed along the waveguide direction, even if the wavelength of the fundamental wave is changed, either of the wavelengths in the extending direction of the waveguide can be changed. Quasi-phase matching is achieved in the region. That is, by changing the doping amount of the dopant such as Mg along the waveguide direction of the waveguide, the refractive index is modulated along the waveguide direction, and the phase matching condition can be relaxed.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A wavelength conversion element according to an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows the wavelength conversion element of this embodiment. As shown, the substrate 1 is made of lithium niobate (LiNbO ₃ ) crystal. In the substrate 1, the spontaneous polarization of crystals is aligned in the c-axis direction, that is, in the thickness direction of the substrate 1 (the upward arrow direction in the drawing). + C side 2 of substrate 1
On the (main surface), a waveguide 3 is formed across the main surface 2 by, for example, a proton exchange method. This waveguide 3
Has one end face as the incident end face 3a of the fundamental wave, and the other end face is the second end face.
It is the emitting end face 3b of the harmonic.

The waveguide 3 has a refractive index distribution that gradually increases (or decreases) along the extension direction (the guiding direction of the fundamental wave). For example, as shown in the drawing, the portion of the waveguide 3 having many points has a low refractive index. Further, in the waveguide 3, a plurality of domain-inverted domains 4 (in the direction of the downward arrow in the figure) in which the spontaneous polarization is periodically inverted along the waveguide direction of the fundamental wave are formed by, for example, the Ti diffusion method. The polarization period of this polarization inversion structure is determined based on the coherence length lc and the wavelength of the fundamental wave, and is constant in this embodiment.

Further, a method of manufacturing such a wavelength conversion element will be described. First, as shown in FIG. 2 (a), LiNbO
₃ On the + C surface of the substrate 1, the additional layer 5 made of magnesium oxide (MgO) whose film thickness temporarily increases along the Z-axis direction is formed by the sputtering method. The additional layer 5 has the thinnest film thickness on the incident end face 3a of the waveguide 3 and the thinnest film on the outgoing end face 3b as the film thickness increases along the waveguide direction. The additional layer 5 is formed so that the film thickness changes with respect to the waveguide direction by depositing a dopant from above using a shielding plate that has an opening only near the emission end face 3b during sputtering deposition.

Next, as shown in FIG. 2 (b), the substrate 1 is placed in a furnace and heated to perform a heat treatment for thermally diffusing MgO in the LiNbO ₃ substrate, and the region 6 doped with MgO is + C.
A LiNbO ₃ substrate is prepared near the plane. At this time,
The MgO concentration and the substrate refractive index in the vicinity of the + C plane of this substrate have the distribution shown in FIG. 2 (b) with respect to the distance in the substrate Z direction. That is, as the distance from the shooting end face 3a of the substrate in the Z direction increases, the MgO concentration increases and the refractive index of the region 6 near the + C plane of the substrate gradually decreases.

Next, as shown in FIG. 2 (c), the +
On the C-plane 2, regions of a plurality of domain inversion domains 4 are formed by, for example, Ti diffusion and heat treatment near the Curie point at a constant polarization period determined based on the coherence length lc and the wavelength of the fundamental wave. Next, as shown in FIG. 2D, a three-dimensional waveguide 3 is formed on the + C plane 2 of the substrate by, for example, the proton exchange method to obtain the device of this example.

As a second embodiment, a substrate 1 whose MgO concentration gradually increases in the extending direction shown in FIG. 2 (b) is prepared. First, as shown in FIG. 3 (a), in the melting furnace apparatus with a rotating pulling rod denoted by the species of LiNbO ₃ crystal, during the pulling operation of the LiNbO ₃ crystal rod 11 from the melting furnace 10, a dopant such as MgO The material is mixed with molten LiNbO ₃ in a furnace to grow MgO-LiNbO ₃ bulk crystals.

The characteristics of the MgO concentration in the length direction of the obtained crystal rod 11 are as shown in FIG. 3 (b). Next, the obtained crystal rod 11 is heated to apply an electric field to make the C-axis direction perpendicular to the MgO concentration gradient direction, and as shown in FIG. 3 (c), the length direction of the crystal rod 11 is the waveguide. The substrate is cut out from the crystal rod so that the + C
The same domain inversion domain and 3
A three-dimensional waveguide is formed and an element is formed.

When a fundamental wave emitted from, for example, a semiconductor laser (not shown) is incident on the incident end face 3a of the waveguide 3 with respect to the produced wavelength conversion element, the fundamental wave is guided through the waveguide 3. Along with this, the second harmonic is excited and this second harmonic is output from the emission end face 3b. At this time, the waveguide 3
Of the fundamental wave and the second wave in the waveguide 3 decrease from the incident end face 3a toward the emitting end face 3b along the waveguide direction.
Since the refractive indices n (ω) and n (2ω) of the harmonics are similarly decreased along the waveguide direction, the refractive index of the fundamental wave and the second harmonic and the fundamental wave are changed by the temperature change of the substrate 1. The propagation constant of the waveguide 3 can be gradually changed in advance along the waveguide direction in consideration of variations in wavelength and the like. Therefore, in the wavelength conversion element of the present embodiment, the allowable wavelength range for exciting the second harmonic is set to be wider than the conventional allowable wavelength fluctuation range. Even if the wavelength variation occurs, the quasi-phase matching condition can be satisfied in any region in the waveguide 3.

The substrate 1 is not limited to LiNbO ₃ and can be made of an appropriate nonlinear optical crystal that generates a second harmonic by quasi-phase matching such as lithium tantalate (LiTaO ₃ ) and has the same effect. Further, although the inversion period of the polarization inversion structure is constant in the above-mentioned embodiment, if the inversion period is gradually changed from the incident end face 3a to the emission end face 3b, a synergistic effect of further relaxing the quasi-phase matching condition is obtained. can get.

Further, the film thickness of the additional layer is not limited to that in the above-mentioned embodiment, and the light emitting end face is formed thinner than the light incident end face, or the film thickness along the waveguiding direction changes periodically. If it is appropriately changed in the waveguiding direction, the same effect as that of the above embodiment can be obtained.

[0021]

According to the present invention, since the refractive index is changed along the waveguide direction of the fundamental wave and the waveguide parameter of the waveguide is changed in the waveguide direction, the second harmonic is excited. The wavelength tolerance can be set wider than the conventional wavelength variation tolerance.
Therefore, even if the wavelength variation exceeds the conventional wavelength variation allowable range, the phase matching condition by the quasi phase matching can be satisfied in any region in the waveguide, and the phase matching condition can be relaxed. ..

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a wavelength conversion element according to an embodiment of the present invention, taken along the waveguide extension direction.

2 is a cross-sectional view of constituent members in a manufacturing process of the wavelength conversion element shown in FIG.

FIG. 3 is a schematic side view of a crystal rod used in a manufacturing process of a wavelength conversion element according to another embodiment.

[Explanation of symbols for main parts]

 1 substrate 3 waveguide 4 polarization inversion domain 5 additional layer

Claims (1)

[Claims] 1. A wavelength conversion element having a waveguide made of a non-linear optical material, wherein the waveguide has a refractive index distribution that gradually increases or decreases along an extension direction of the waveguide,
A wavelength conversion element having a plurality of domain-inverted domains in which the polarization of the waveguide is periodically inverted along the extension direction of the waveguide.