JPH0682859A - Wavelength conversion device - Google Patents

Abstract

PURPOSE:To enable the conversion of a basic wave having a wavelength approximate to a wavelength lambdaC0 to a second harmonic wave by providing two ion implantation layers apart from each other within a nonlinear optical material and forming the part between these two ion implantation layers as a waveguide layer. CONSTITUTION:The two ion implantation layers 3, 4 are provided apart from each other within the nonlinear optical material 1 and the part between these two ion implantation layers is formed as the waveguide layer 2. The second harmonic wave 27 is generated from the basic wave 26 by using the waveguide layer 2. As a result, the symmetrical waveguides having the equal refractive index of the layers adjacent to this waveguide layer 2, i.e. the two ion implantation layers 3, 4 are formed. Consequently, the thickness of cut off layers for both of the basic wave 26 and the higher harmonic wave is decreased to 0mum and, therefore, the basic wave of the wavelength shorter than the wavelength of the conventional wavelength conversion device having the single ion implantation layer, i.e. of the value approximate to lambdaC0 possessed by the nonlinear optical crystal is converted to the second harmonic wave.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength converter for converting the wavelength of light into a short wavelength.

[0002]

2. Description of the Related Art Recently, densification of information has become active, and researches for shortening wavelength of light used for optical information equipment are being actively conducted. In addition to this, short-wavelength coherent light is also required in various fields such as the medical and chemical industries.

As a method of obtaining short wavelength coherent light,
A wavelength conversion device that generates a second harmonic using a nonlinear optical material has been actively studied.

In particular, a wavelength conversion device having an optical waveguide formed by ion implantation has attracted attention because of its ease of manufacture. FIG. 4 is a sectional view showing such a wavelength conversion device.

In FIG. 4, 21 is a nonlinear optical crystal KNbO ₃
It is a b-axis oriented substrate which is made of (potassium niobate) and whose direction perpendicular to the substrate surface is the b-axis direction. 22
Is He ⁺ (helium monovalent positive ion) in the substrate 21.
Is an ion-implanted layer formed by injecting the ion implantation layer 22. The ion-implanted layer 22 is in a state close to amorphous because the crystallinity is destroyed by the ion implantation.

As shown in FIG. 5 showing the refractive index distribution along the broken line A-D in FIG. 4, the ion implantation layer 22 has a lower refractive index than the portion of the substrate 21 where the ion implantation layer 22 is not formed. Has become. Therefore, the upper region of the substrate 21 above the ion implantation layer 22 is the air layer 2 serving as a clad layer.
3 and the ion implantation layer 22 are sandwiched between two low refractive index layers,
It becomes the waveguide layer 24.

By the way, since the waveguide layer 24 has a second harmonic generation (SHG) function, the fundamental wave 26 incident on the waveguide layer 24 and the generated and output in the waveguide layer 24 are output. The layer thickness of the waveguide layer 24 is optimized so that the phase matching of the second harmonic 27 can be performed, that is, the effective refractive index of the waveguide layer 24 with respect to the fundamental wave 26 and the second harmonic 27 becomes equal. ing.

The phase matching in this case will be described in more detail below.

As shown in FIG. 6, the waveguide layer 24 and the non-linear optical crystal KNbO ₃ which has not been ion-implanted.
Is a broken line showing the main refractive index n _a (d), a broken line showing the main refractive index n _b (d), and a main refractive index n _c (d) in the a-axis direction, the b-axis direction, and the c-axis direction, respectively. Has a chromatic dispersion characteristic A represented by a broken line, a solid line showing _a main refractive index n _a , a solid line showing a main refractive index n _b, and a wavelength dispersion characteristic B shown by a solid line showing a main refractive index n _c . Each main refractive index indicated by the wavelength dispersion characteristic A has a value slightly different from each corresponding main refractive index of the wavelength dispersion characteristic B due to a slight decrease in crystallinity due to ion passage during ion implantation. .

As shown in FIG. 6, the waveguide layer 2
4 is the main refractive index n for a certain wavelength λ _bValue of (d) and the wave
Principal refractive index n for wavelength λ / 2 which is half the length_c(D)
Value that makes the values of λ equal to λ_chave. In addition, the ion injection
Non-filled nonlinear optical crystal KNbO₃Is
Principal refractive index n for wavelength λ_bAnd half of the wavelength
Principal refractive index n for wavelength λ / 2_cValue λ at which the values of are equal
= Λ_c0have.

Further, as shown in FIG. 7, the ion-implanted layer 22 is formed.
Is the broken line showing the main refractive index n _a (i), the broken line showing the main refractive index n _b (i), and the main refractive index n _c (i) in the a-axis direction, the b-axis direction, and the c-axis direction, respectively. It has a chromatic dispersion characteristic C represented by a broken line. In the figure, the wavelength dispersion characteristic B is the same as that in FIG. Each of the main refractive indices indicated by the wavelength dispersion characteristic C has substantially the same ratio as the corresponding main refractive index of the wavelength dispersion characteristic B due to the deterioration of crystallinity due to ion implantation, and the wavelength dispersion of FIG. Compared with the absolute value of the change amount of the characteristic A, that is, the absolute value of the change amount in the waveguide layer 24 is greatly reduced. As shown in FIG. 7, the ion implantation layer 22 has a main refractive index n for a certain wavelength λ.
_A value λ = λ _cs (λ _cs ) at which the value of _b (i) is equal to the value of the main refractive index n _c (i) for the wavelength λ / 2 that is half the wavelength.
<Λ _c ).

When a TM mode fundamental wave having a wavelength longer than λ _c is propagated in the waveguide layer 24 in the a-axis direction, the layer thickness of the waveguide layer 24 is optimized to achieve phase matching, and TE mode The second harmonic of can be generated.

The fact that this phase matching is possible can be explained from the mode dispersion characteristics of the fundamental wave having a wavelength longer than λ _c and its harmonics in the wavelength converter shown in FIG. In the figure, the horizontal axis represents the layer thickness of the waveguide layer 24, and the vertical axis represents the effective refractive index. The curve S _w is the dispersion characteristic of the fundamental wave of the TM mode, and S _2w is the dispersion of the second harmonic of the TE mode. It is a curve.

That is, in general, modal dispersion curves S _w , S
_2w is a predetermined thickness t _bw ion implantation layer 22 near the respective waveguide layer 24, t _c2w (cutoff layer thickness) in the effective refractive index _{n b w (i), n} c2w (i) are each ion principal refractive index n _b of the fundamental wave of the injection layer 22 (i), equal to the principal refractive index n _c (i) for the second harmonic, the waveguide layer 24
The effective refractive index n _bw (d) increases as the layer thickness increases.
(= Principal refractive index n of the waveguiding layer 24 with respect to the fundamental wave
_b (d)), the effective refractive index n _c2w (d) (= the main refractive index n _c (d) for the second harmonic) approaches asymptotically.

By the way, in the nonlinear optical crystal KNbO ₃ , when the fundamental wave of TM mode has a longer wavelength λ _l than λ _c , the wavelength of the fundamental wave is λ _l −λ, as can be seen from the wavelength dispersion characteristic A of FIG. than the reduction of n _b (d) with respect to increases _c, since towards the reduction of n _c (d) with respect to _{λ l / 2-λ c /} 2 increasing wavelength of the harmonic is large, the waveguide layer The principal refractive index n _{bw in the} b-axis direction for the fundamental wave at an infinite layer thickness
(D)> It can be seen that the relationship of the main refractive index n _c2w (d) in the c-axis direction with respect to the second harmonic at an infinite layer thickness of the waveguide layer holds. Furthermore, in an asymmetric waveguide layer in which the refractive indices of two layers adjacent to the waveguide layer are different from each other, the shorter the wavelength of light in the waveguide layer, the smaller the cutoff layer thickness, and the rise of the mode dispersion curve. Since it becomes steep, the relationship of cut-off layer thickness t _c2w for harmonics <cut-off layer thickness t _bw for fundamental waves holds. As a result, as shown in FIG. 8, the dispersion curve S _w of the fundamental wave and the harmonic wave S _2w show a certain layer thickness t.
You will cross at _pm . Therefore, this layer thickness t _pm
The phase matching can be performed by selecting the depth position of the ion implantation layer 22 so that

However, in such a wavelength conversion device, when the fundamental wave has a wavelength shorter than λ _c , n _bw (d) <n
_Since the relationship is _c2w (d), there is a problem that phase matching cannot be performed and light of a shorter wavelength cannot be obtained. In particular, the wavelength λ _c becomes larger than λ _c0 = 0.86 μm (in the case of KNbO ₃ without ion implantation) as the ion implantation dose increases (for example, the dose amount is 1 × 10 ^16).
In the case of ion / cm ² , it was difficult to convert a fundamental wave having a wavelength close to the wavelength λ _c0 into the second harmonic.

[0017]

The present invention has been made in view of the above problems, and provides a wavelength conversion device capable of converting a fundamental wave having a wavelength close to the wavelength λ _c0 to a second harmonic. The purpose is to provide.

[0018]

A wavelength conversion device of the present invention is provided with two ion-implanted layers separated from each other in a non-linear optical material, and the two ion-implanted layers are waveguiding layers. It is characterized in that the fundamental wave is made incident from one end face of the one and the second harmonic is generated from the other end face of the waveguide layer.

[0019]

When a structure in which a region sandwiched between two ion-implanted layers is used as a waveguide layer, a layer adjacent to the waveguide layer, that is, 2
Use a symmetric waveguide in which the refractive indices of the two ion-implanted layers are equal, that is, cutoff layer thicknesses of both the fundamental wave and the harmonic wave are 0.
Since it can be made to be μm, phase matching can be performed even when a fundamental wave having a shorter wavelength is used.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A wavelength converter according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of this wavelength conversion device, and FIG. 2 is a diagram showing a refractive index distribution in a direction along a broken line E-H in FIG. The same parts as those of the conventional example and corresponding parts are designated by the same reference numerals.

In FIG. 1, 1 is a nonlinear optical crystal KNbO.
It is a b-axis oriented substrate made of ₃ (potassium niobate), and the direction perpendicular to the substrate surface is the b-axis direction, and the main surface is the (010) plane. Reference numeral 2 denotes a first ion-implanted layer 3 formed by implanting He ⁺ (monovalent positive ions of helium) into the substrate 1, and the first ion-implanted layer 3 separated from the substrate 1 in the same manner as above. A second ion-implanted layer 4 having a refractive index substantially equal to that of the first ion-implanted layer 3, which is formed by injecting He ⁺ into the ion-implanted layer 3, is formed between the ion-implanted layers 3 and 4. The waveguide layer.

Here, as shown in FIG. 2 as the refractive index distribution along the broken line E-H in FIG. 1, the waveguide layer 2 is crystallized by ion implantation and the first and second ion implantation layers 3 and 4 are crystallized by ion implantation. In this state, the refractive index is lower than the refractive index of the substrate 1 itself, and the first and second ion implantation layers 3 and 4 have the same refractive index. It becomes a waveguide.

The first and second ion-implanted layers 3 and 4 are formed by ion-implanting twice with different implantation energies, that is, in the present embodiment, the respective implantation energies 485 KeV and 2
It was formed by injecting into the substrate 1 at 20 KeV. Where the first,
The depth control of the second ion implantation layers 3 and 4 is controlled by the implantation energy, and in order to make the refractive indexes of the first and second ion implantation layers 3 and 4 equal, the ion implantation layers 3 and 4 are controlled. The same dose amount. Both ion implantation conditions are an implantation off angle (angle with respect to the b-axis direction) of 7 ° and a dose amount of 1 × 10 ¹⁶ ion / cm ² .

In the wavelength conversion device produced under such implantation conditions, the first and second ion implantation layers 3 and 4 have depths of 0.5 μm and 1.1 μm, respectively. The principal refractive indices n _a (i) and n _b (i) of 4 are both about 7%.
The main refractive index n _c (i) was decreased by about 9%. In addition, the waveguide layer 2 has an effective layer thickness of about 0.2 μm, its main refractive index n _a (d) and n _b (d) are reduced by about 1%, and the main refractive index n _c
(D) was about 1% increase.

In such a wavelength conversion device, the output power from the semiconductor laser device is 100 mW and the room temperature oscillation wavelength λ _m =
When 0.88 μm TM mode light (fundamental wave) 26 is incident from one end face of the waveguide layer 2 along the a-axis direction of the waveguide layer 2, the temperature of the semiconductor laser device is changed to By finely adjusting the wavelength of light, the output of the other end face of the waveguide layer 2 is 0.2 mW and the wavelength is 0.441 μm.
The coherent second harmonic 27 of was observed.

The reason why the second harmonic can be generated even in the light having the wavelength λ _m = 0.88 μm closer to λ _c0 = 0.86 μm will be described with reference to FIG.

When the fundamental wave λ _m having a wavelength shorter than λ _c is converted into the second harmonic wave, the effective refractive indices n _bw (d) and n _c2w (d) of the waveguide layer 2 are as shown in FIG. , N _bw (d) <n _c2w
Regarding the relationship of (d), as described above, in a symmetric waveguide in which two layers sandwiching the waveguide layer have the same refractive index, the cutoff layer thicknesses for the fundamental wave and the second harmonic are both 0 μm, and Since the wavelength λ _{m of the} wave is larger than λ _cs , as can be seen from FIG. 7, the effective refractive index n _bw (i), n at the cut-off layer thickness is
_{The relationship of} n _bw (i)> n _c2w (i) holds for _c2w (i). Therefore, as described above, the phase matching can be achieved by the dispersion curve C _w of the fundamental wave of the TM mode and the dispersion curve C _2w of the second harmonic of the TE mode, as shown in FIG. _This is because they intersect at _pm '.

[0028] In the above embodiment was used b-axis orientation substrate of KNbO _3, may be used c-axis alignment substrate of KNbO _3, by the case of using the coherent light of TE mode as the fundamental wave, The second harmonic that is coherent at the short wavelength of the TM mode can be extracted.

As a nonlinear optical crystal, KNbO ₃
However, other nonlinear optical crystals such as KTP (KTiOPO ₄ , potassium titanate phosphate), LiNbO ₃ (lithium niobate) and the like can be appropriately used.

Further, in order to form the ion implantation layer,
Ions other than He ⁺ can be appropriately used.

As described above, the two ion-implanted layers are provided in the nonlinear optical material so as to be separated from each other, and the two ion-implanted layers are used as the waveguide layer.
By generating harmonics, it is possible to make a layer adjacent to the waveguide layer, that is, a symmetric waveguide in which two ion-implanted layers have the same refractive index. As a result, the cutoff layer thickness of both the fundamental wave and the harmonic wave can be set to 0 μm, which is shorter than that of the conventional wavelength converter having a single ion-implanted layer, that is, λ _{c0 of the} nonlinear optical crystal. It is possible to convert the fundamental wave having a wavelength close to to the second harmonic.

Further, in this structure, since the two ion-implanted layers are low refractive index layers sandwiching the waveguide layer, the cladding layer above the waveguide layer, that is, the external atmosphere (air or Nitrogen gas). Therefore, the device of the present invention can be used in the same shape and size regardless of the external atmosphere.

[0033]

Since the wavelength conversion device of the present invention has a structure in which the region sandwiched between two ion implantation layers is used as the waveguide layer, the refractive index of the layer adjacent to the waveguide layer, that is, the two ion implantation layers. Can be symmetrical waveguides. Therefore,
A fundamental wave having a shorter wavelength than that of a conventional wavelength conversion device formed of a single ion-implanted layer can be converted into a second harmonic.

[Brief description of drawings]

FIG. 1 is a sectional view of a wavelength conversion device according to an embodiment of the present invention.

FIG. 2 is a diagram showing a refractive index distribution along a broken line E-H in FIG.

FIG. 3 is a chromatic dispersion characteristic diagram of the wavelength conversion device of the above embodiment.

FIG. 4 is a cross-sectional view of a conventional wavelength conversion device.

5 is a diagram showing a refractive index distribution along the broken line A-D in FIG.

FIG. 6 is a diagram showing wavelength dispersion characteristics of a waveguide layer formed by ion implantation.

FIG. 7 is a diagram showing wavelength dispersion characteristics of an ion implantation layer.

FIG. 8 is a diagram showing wavelength dispersion characteristics of the wavelength conversion device of the conventional example.

[Explanation of symbols]

 1 substrate (non-linear optical material) 2 waveguiding layer 3 first ion-implanted layer 4 second ion-implanted layer 26 fundamental wave 27 second harmonic

Front page continued (72) Inventor Hideyuki Nonaka 2-18 Keihan Hondori, Moriguchi City, Osaka Sanyo Electric Co., Ltd.

Claims (1)

[Claims] 1. A non-linear optical material having two ion-implanted layers separated from each other, the two ion-implanted layers serving as a waveguide layer, and a fundamental wave incident from one end face of the waveguide layer. A wavelength conversion device, wherein a second harmonic is generated from the other end face of the waveguide layer.