JP2945107B2 - Optical wavelength conversion element and method of manufacturing the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion element used as a second harmonic generation element, a blue light emitting element, and the like, and a method for manufacturing the same.

2. Description of the Related Art A second harmonic generation (SHG = second harmonic generation) element utilizes a nonlinear optical effect of an optical crystal material having a nonlinear optical effect to convert a laser beam having a wavelength of λ into a light beam having a wavelength of λ / 2. Is an element for converting to Therefore, the wavelength of the emitted light becomes の of that of the incident light, and the recording density can be quadrupled, so that it is being applied to optical discs, CD players, laser printers, photolithography and the like.

Here, semiconductor lasers are becoming the mainstream as an input light source in order to meet the requirements of such SHG elements such as small size, direct modulation and the like. When such a semiconductor laser is used as a light source, it is necessary to obtain high conversion efficiency.
SHG elements are used. As an optical crystal material having a nonlinear optical effect capable of forming a high-quality optical waveguide, LiNb is generally used.
O ₃ is considered optimal. However, in the LiNbO ₃ single crystal, phase matching is performed at a light source wavelength of a semiconductor laser of 0.82 to 0.84 μm (phase matching is a refractive index of an incident laser light in an optical waveguide = an effective refractive index and an effective refractive index of a second harmonic light). (Meaning that the refractive index matches the refractive index) is reported to be impossible. This is the ordinary refractive index n _o ^ω = 2.253 at the frequency omega of the case where the incident light of LiNbO _3, extraordinary refractive index n _e ^2ω = 2.282 next at frequency 2 [omega of the emitted light, the requirement for phase matching (n
This is because _{the ^o ω} ⁼ n _{e 2ω)} does not meet the.

For this reason, for example, according to JP-A-2-12135, a laser beam having a wavelength of 0.82 to 0.84 μm is used as a basic light, and a 3.7 to 9.0 μm LiNbO ₃ waveguide layer is formed on a LiTaO ₃ single crystal substrate. There is a structure in which the SHG is phase-matched by making the light incident at an angle of 0 to 35 ° with respect to the crystal axis.

IEICE Technical Report MW89-144
According to the “Output Characteristics of Waveguide SHG Device Using LD Light Source”, a three-layer structure in which a LiNbO ₃ thin film is formed on a LiTaO ₃ single crystal substrate, and a MgO-doped LiNbO ₃ layer is further formed thereon The figure shows a structure in which semiconductor laser light having a fundamental wavelength of 0.83 μm is subjected to SHG conversion.

According to the former publication, the effective refractive index of the waveguide can be changed by changing the thickness of the LiNbO ₃ waveguide layer. At the time of guiding light, a state where the phase matching condition is satisfied exists by injecting light at an angle capable of phase matching with respect to the z axis (optical axis), and SHG is realized. However, strict angular alignment accuracy is required,
There is a disadvantage that it is weak against fluctuations in temperature, wavelength, and the like.

On the other hand, in the case of the latter literature method, the MgO-doped LiNbO ₃ layer is a waveguide originally manufactured to satisfy the phase matching condition, and has SHG characteristics. However, it is very difficult to control the film thickness to satisfy the phase matching condition and its accuracy. Therefore, as in the former case, there is a drawback that it is weak against fluctuations in temperature, wavelength, and the like.

Therefore, in the case of these conventional methods, since the SHG conversion efficiency is greatly affected by various fluctuations, the conversion efficiency is actually insufficient, having only about several%.

Means for Solving the Problems In an optical wavelength conversion device using an optical waveguide having a nonlinear optical effect, a LiTa _x Nb _1-x O ₃ single crystal substrate (where 0 ≦ x
≦ 1) on the first layer, LiTa _y Nb _1-y O ₃ having different composition ratios
Second single crystal (0 ≦ y ≦ 1 and x <y)
Layer and LiTa _z Nb _1-z O ₃ single crystal (provided that 0 ≦ z ≦ 1 and z
<Y) has a three-layer structure in which a waveguide layer is laminated with a third layer, the ordinary light refractive index at the frequency ω of the incident light is n _o ^ω, and the extraordinary light refractive index at the frequency 2ω of the outgoing light is n. when the _e ^{2 [omega,} the second layer and a third layer ^{_{0 <(n e 2ω -n o}} ω) ( second ^{_{layer) ≒ (n o ω -n e}} 2ω) ( third layer) ≦ 0.01 It was formed so as to satisfy the following conditions.

In this case, at least MgO is doped into each of the first layer, the second layer, and the third layer, and Li, Be, Na, and N are added to the third layer.
At least one of the elements i, Ti, V, Nd, Cr, K, Ca, Sr, Ce and Ba was doped.

As a method for manufacturing such an optical wavelength conversion element, at least a second layer and a third layer are formed on a first layer by a liquid phase growth method, and an ion implantation method, a sputtering method, and a diffusion method are performed on the third layer. The second element and the third layer are doped with a desired element so that the second layer and the third layer have a desired refractive index distribution relationship by any one of a doping method of a laser ablation method or a laser ablation method.

Action The second layer of LiTa _y Nb _1-y O ₃ single crystal and the LiTa _y Nb _1-y O ₃ single crystal having different composition ratios with the first layer of LiTa _x Nb _1-x O ₃ single crystal substrate
_In a three-layer structure in which a waveguide layer composed of a third layer of zNb _1-z O ₃ single crystal is laminated, the ordinary refractive index at the frequency ω of the incident light and the emergent light When the extraordinary light refractive index at the frequency 2ω satisfies the predetermined relational expression, the phase matching condition for generating the second harmonic is satisfied. It is easy to finely control the refractive index of the second and third layers so as to satisfy such a condition by a doping method such as an ion implantation method as described in the third aspect of the present invention. Therefore, it is possible to realize a highly reliable second harmonic generation element that is easy to manufacture and has high conversion efficiency and is resistant to fluctuations in temperature and the like, and does not require direct alignment such as angle matching with a semiconductor laser having a wavelength of about 0.8 μm. Two-harmonic generation is also possible.

In particular, as described in claim 2, each layer is doped with MgO to reduce the propagation loss, and by doping a predetermined element to form the third layer, the thickness of the third layer in the thickness direction is reduced. It is easy to realize that the refractive index distribution satisfies the phase matching condition for generating the second harmonic.

Embodiment An embodiment of the present invention will be described with reference to the drawings. The optical wavelength conversion element 1 of this embodiment is basically composed of a LiTa _x Nb _1-x O ₃ single crystal substrate (where 0 ≦ x ≦, as shown in FIG. 1A).
On the first layer 2 according to 1), LiTa _y Nb _1-y O ₃ having different composition ratios
Second single crystal (0 ≦ y ≦ 1 and x <y)
Layer 3 and LiTa _z Nb _1-z O ₃ single crystal (provided that 0 ≦ z ≦ 1 and
It has a three-layer structure in which a waveguide layer formed by the third layer 4 with z <y) is laminated. The frequency ω is applied to such a thin-film waveguide layer.
N _o ^ω, SHG the refractive index of ordinary light when is incident fundamental wave of
N _e ^{2 [omega} refractive index of extraordinary light of coming frequency 2 [omega that issued by
When defining a generally, n _o ^omega <since it is n _e ^{2 [omega,} the phase matching condition ⁿ _o ω ⁼ n _e 2ω is not satisfied. Also, LiTa _X N
In the case of a crystal such as b _1-X O ₃ , even if the composition ratio x is changed, the above-mentioned phase matching condition is not satisfied, and λ = 0.
When light such as a semiconductor laser of 83 μm is used, SHG cannot be realized.

Therefore, in this embodiment, the first to third layers 2 to 2 shown in FIG.
4 of each layer n _o ^omega, the n _e ^{2 [omega,} changing the composition ratio x, y, and z, and,
By adding an appropriate impurity (dopant), the phase matching condition is satisfied in a wide range with respect to temperature, wavelength, and film thickness.

First, with respect to the LiTa _X Nb _1-X O ₃ single crystal of the first layer 2,
The composition of the LiTa _y Nb _1-y O ₃ single crystal of the layer 3 is made to approach the LiTaO ₃ in order to lower the refractive index. For this purpose, the second layer 3 is formed under the condition of x <y. _{Next, LiTa z Nb 1-z O} 3 of the third layer 4 single ^crystal, n _o ω <perform the introduction of dopants such as to reverse the n _e ^{2 [omega} following condition, n _o
^ω> and ⁿ _e 2ω made conditions. However, ambient temperature, for a stable phase matching condition is satisfied with respect to fluctuation of the semiconductor laser wavelength and the waveguide layer thickness, the second layer 3 third layer ^{_{4, 0 <(n e 2ω -n}} o ^omega) (second ^{_{layer) ≒ (n o ω -n e}} 2ω) ( which is formed so as to satisfy the third layer) ≦ 0.01 relational expression.

FIG. 2A shows a mode dispersion curve characteristic showing a state of phase matching when this condition is satisfied. By the way,
FIG. 1B shows a mode dispersion curve characteristic according to the conventional method. As can be seen from the figure, the phase matching point between the incident light TM ^ω and the SHG light TE ^{2ω in} the conventional method of FIG. According to FIG. 7 (a), they intersect gently, indicating that the phase matching condition is wide for each of the above-mentioned fluctuations.

FIG. 1B shows the refractive index distribution of each layer when viewed as a three-layer structure as a whole. The second layer 3 by changing the composition ratio of Ta and Nb with respect to the first layer ^2, n _o ω, n _e
The refractive index of ^2ω is reduced. For the third layer 4 n _o
elements such as ^ω come greater than n _e ^{2 [omega,} for example Na ^+, K ^+, C
a ⁺ , Ni ⁺ , Ti ⁺ , Nd ⁺ , Li ^{+ and the} like are doped as dopants. As a method of introducing these dopants, a liquid phase epitaxial growth method and a sputtering method,
And, in recent years, n arbitrarily by the ion implantation method _{o ^ω,} n _e
^2ω can be controlled. Further, a laser ablation method may be used. In addition, it is necessary to entirely dope MgO into each layer in order to reduce propagation loss and the like as a whole. As other dopants, for example, elements such as Be, Ba, Ce, Sr, Cr, V, etc. are promising, in addition to those described above mainly for alkali-based ones. Also, when viewed third layer 4 as a wave ^layer, n _o ω, n _e 2ω second layer 3
Needless to say, it is preferable to have a value larger than that of. Also in the relationship between the crystal orientation and the mode of the incident light, x
For cut and y-cut crystals, input in TM mode,
Of course, the z-cut (c-axis cut) is basically based on the incidence in the TE mode.

By the way, a practical SHG based on such a basic configuration
The element 1 may be configured as shown in FIG. 3, for example. This is a ridge-type waveguide structure for improving light use efficiency. The basic structure or a specific method of manufacturing the SHG element as shown in FIG. 3 will be described with reference to FIG.

Specific Example 1 First, as shown in FIG. 4 (a), MgO-doped LiNbO
₃ on the first layer 2 of single crystal substrate, LIT by liquid phase growth method
a _y Nb _1-y O ₃ (where 0 ≦ y ≦ 1, y = 0.5 in this case) is epitaxially grown to form a second layer 3 having a thickness of 5
It was formed to a thickness of μm. Further, as the third layer 4, MgO-doped L
After iNbO ₃ was grown to a thickness of 5 μm by liquid phase growth, Na ⁺ was added to 1 × 10 ¹² by ion implantation as shown in FIG.
[/ Cm ² ] was dosed. At this time, the ion implantation is performed in two steps of energy of 20 KeV and 60 KeV so that the ion implantation is uniform over the entire third layer 4 having a thickness of 5 μm. Thereby, the third layer is formed as a doped layer.
Layer 4 is formed.

Specific Example 2 LiTa _y Nb _{1 -y} by MgO-doped LiTa _x Nb ₁ -x O ₃ (where 0 ≦ x ≦ 1, where x = 0.1) O ₃ (However, 0 ≦ y ≦
1. Here, a single-crystal thin film of y = 0.9) was formed to a thickness of 3 μm, and then annealed (at 600 ° C. for 2 hours in an Ar inert gas) to improve the crystallinity. After that, the element V is vapor-deposited by an electron beam to form a film having a thickness of 100 °.
The third layer 4 was formed by performing diffusion at 0 ° C. for 2 hours.

Specific Example 3 In order to use the SHG element 1 having a three-layer structure formed in the specific example 1 (or specific example 2) as a waveguide, a waveguide processing as shown in FIG. Is applied. That is, a resist (OFPR) 5 having a predetermined dimension, for example, W = 5 μm
Was formed on the third layer 4 and then etched by 2 μm with an ion beam etcher (Ar ion) to obtain a ridge-type waveguide as shown in FIG. This resist is removed as shown in (d) by O _z ashing 30 minutes, further, annealing (500 ° C., 30 minutes in O _z) by performing, thereby completing the SHG element 1.

FIG. 3 shows an example of a configuration as a blue light emitting element 7 obtained by combining the semiconductor laser 6 with the SHG element 1 manufactured as described above. 8 is a heat sink.

According to such a specific example, the effective refractive index of the waveguide layer is SH
Satisfies the G condition and the wavelength of the semiconductor laser, λ = 0.83μ
Thus, SHG can be directly performed with respect to m, and a blue light-emitting element with high efficiency of 10 to several tens%, which has not been achieved in the past, has been realized.

Effect of the Invention As described above, the present invention provides a LiTa _y Nb _1-y having a different composition ratio with respect to the first layer of the LiTa _X Nb _1-X O ₃ single crystal substrate.
Second layer of O ₃ single crystal and _third layer of LiTa _z Nb _1-z O ₃ single crystal
In a three-layer structure in which waveguide layers are
The ordinary light refractive index at the incident light frequency ω and the extraordinary light refractive index at the output light frequency 2ω in the layer and the third layer satisfy a predetermined relational expression, and the phase matching condition for the generation of the second harmonic Therefore, a highly reliable second harmonic generation element having a high conversion efficiency and a high resistance to fluctuations in temperature and the like can be realized with a three-layer structure that is easy to manufacture as in the invention according to claim 3. Unnecessary direct second harmonic generation such as angle matching with a semiconductor laser having a wavelength of about 0.8 μm is also possible, and in particular, according to the invention of claim 2, since each layer is doped with MgO, the propagation loss is reduced. In addition, since the third layer is formed by doping a predetermined element, the refractive index distribution in the thickness direction of the third layer can be adjusted to a phase matching condition for generating the second harmonic. The subtle control that satisfies the condition can be easily realized.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention. FIG. 1 is an explanatory view showing the relationship between the basic structure and the refractive index distribution. FIG. 2 shows the relationship between the thickness of the waveguide layer and the effective refractive index. FIG. 3 is a characteristic diagram shown in comparison with the example, FIG. 3 is a perspective view showing a specific configuration example of a blue light emitting element, and FIG. 4 is a process diagram showing a manufacturing process. 2 1st layer 3 2nd layer 4 3rd layer

Claims (3)

(57) [Claims] 1. An optical wavelength conversion device using an optical waveguide having a nonlinear optical effect, wherein a composition ratio is formed on a first layer of a LiTa _x Nb _1-x O ₃ single crystal substrate (0 ≦ x ≦ 1). Different Li
Ta _y Nb _1-y O ₃ single crystal (however, 0 ≦ y ≦ 1 and x <y)
And the LiTa _z Nb _1-z O ₃ single crystal (where 0 ≦ z ≦
1, and, z <a three-layer structure in which the third layer and by waveguide layer is laminated by y), the ordinary refractive index at the frequency omega of the incident light and n _o ^omega, abnormality in the frequency 2ω of the emitted light Light refractive index
when the n _e ^{2 [omega,} wherein the second layer and a third layer ^{_{0 <(n e 2ω -n o}} ω) ( second ^{_{layer) ≒ (n o ω -n e}} 2ω) ( third layer) ≦ An optical wavelength conversion element formed to satisfy the condition of 0.01. 2. The method of claim 1, wherein each of the first, second and third layers is doped with at least MgO and the third layer is composed of Li, Be, Na, N
2. The optical wavelength conversion device according to claim 1, wherein at least one of the elements i, Ti, V, Nd, Cr, K, Ca, Sr, Ce, and Ba is doped. 3. A LiTa _x Nb _1-x O ₃ single crystal substrate (where 0 ≦ x ≦
On the first layer according to 1), at least LiTa having a different composition ratio
_y Nb _1-y O ₃ single crystal (where, 0 ≦ y ≦ 1, and, x <y) a second layer according to the LiTa _z Nb _1-z O ₃ single crystal (where, 0 ≦ z ≦ 1,
And a third layer with z <y) is formed by a liquid phase epitaxy method, and an ion implantation method, a sputtering method,
The second layer and the third layer have an ordinary light refractive index at the frequency ω of the incident light as n _o ^ω and an extraordinary light refractive index at the frequency of 2ω of the outgoing light by a doping method of either the diffusion method or the laser ablation method. when was a ^{_{n e 2ω, 0 <(n}} e 2ω -n o ω) ( second ^{_{layer) ≒ (n o ω -n e}} 2ω) ( third layer) ≦ 0.01 becomes satisfying refractive index distribution relationship A method for manufacturing an optical wavelength conversion element, characterized in that a desired element is doped into a holding state.