JPH05289130A - Second harmonic generating element - Google Patents

Abstract

PURPOSE:To obtain a element radiating SHG in which allowable accuracy for variation of a fundamental wave is wide and radiation intensity is high. CONSTITUTION:This element has periodic structure in which two regions n1, n2 provided in a light waveguide path 2 are alternately arranged, also each refraction index of these adjacent regions n2, n2 is same for a fundamental wave or at least same grade. On the other hand, this refraction index is clearly different for harmonics, also these two regions n1, n2 are constituted as periodic structure in which elements are arranged in the perpendicular direction to the direction of light propagation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a second harmonic generating element (hereinafter simply referred to as "SHG element"), and particularly to an SHG element having a large permissible accuracy with respect to wavelength fluctuation of a fundamental wave and excellent in practical use. Is.

[0002]

2. Description of the Related Art An SHG element is a laser having an incident wavelength of λ, which is λ / wavelength by utilizing the nonlinear optical effect of a nonlinear optical material.
It is an element that converts to 2 wavelengths and outputs, and when this element is used, the wavelength of output light is converted to 1/2,
When applied to an optical disk memory, a CD player, etc., the recording density can be improved (about 4 times). Further, it is an optical material that has recently been particularly attracting attention because it can obtain a high resolution when applied to a laser printer, photolithography and the like.

Conventionally, a bulk single crystal of a nonlinear optical crystal using a high-output gas laser as a light source has been used as the SHG element. However, in the fields of optical disc devices, laser printers, and the like, which are the main applications of this SHG, there is a strong demand for downsizing the entire device. Moreover, the gas laser requires an external modulator for optical modulation, and it is necessary for downsizing. There was a problem that it was not suitable.

Therefore, in recent years, an SHG element has attracted attention, which can be used in place of the above-mentioned gas laser and can use a semiconductor laser which can be directly modulated and which is cheaper and easier to handle than a gas laser. Among them, in the case of using a semiconductor laser as a light source, the output thereof is as low as several mW to several tens mW, and therefore, there are great expectations for the SHG element having a waveguide structure capable of obtaining particularly high conversion efficiency.

As such an SHG element, a conventional type in which a film thickness phase matching method, a temperature phase matching method, an angle phase matching method, a Cherenkov radiation method and the like are used to extract the harmonics by aligning the phases of the oscillating harmonics However, various proposals have been made. However, the above film thickness phase matching method is not practical because the film thickness of the waveguide must be adjusted on the order of several tens of liters. Further, both the temperature phase matching method and the angle phase matching method are methods capable of oscillating a harmonic only under the condition that the ordinary light refractive index and the extraordinary light refractive index are close to each other. There were many restrictions on the fundamental wavelength.

Further, in the above Cherenkov radiation method, the emitted light is crescent-shaped, and it is difficult for the lens optical system to focus the light within the diffraction limit. It was a big problem. Therefore, at present, it is desired that the second harmonic be emitted from the waveguide, and in order to meet this demand, an SHG element having a periodic domain inversion structure formed in the waveguide has been conventionally proposed. It was.

As a conventional technique that can meet such a demand, for example, a periodic domain inversion structure portion and / or a periodic refractive index structure portion are formed in a waveguide as described in Japanese Patent Application Laid-Open No. 2-93624. There is an SHG element formed by However, in this conventional technique, it is necessary that the effective refractive index of the waveguide in the fundamental wave is larger than the refractive index of the substrate in the harmonic, and in the case of using the single mode waveguide in order to improve the power efficiency of the fundamental wave. However, this region is considerably limited or does not exist, and even in a multimode waveguide, this condition may not exist, which is a large limitation.

As a new method different from this, B
The PM method has been proposed (Appl. Phys. Lett. 57 (20),
1990). This conventional technique can be applied to a material in which it is difficult to form a domain-inverted layer, but in any case, the permissible accuracy for wavelength fluctuations by the quasi-phase matching method is (several Å to several nm), while semiconductor lasers, etc. In the case of, in general, the wavelength always fluctuates unstablely (there is no stability), so it is extremely difficult to achieve phase matching.

[Problems to be Solved by the Invention]

An object of the present invention is to overcome the problems of the above-mentioned conventional SHG element and to develop and propose an element that emits SHG having a high permissible accuracy with respect to the fluctuation of the fundamental wave and a high emission intensity.

[0010]

[Means for Solving the Problems] In the research for realizing the above-mentioned object, the inventors of the present invention have S of the following summary structure.
HG device was developed. That is, this SHG element is a second harmonic generation element in which an optical waveguide having nonlinear optical crystal characteristics is formed on a substrate, and a periodic structure in which regions n ₁ and n ₂ defined by the optical waveguide are arranged alternately. And adjacent regions n ₁ and n ₂ have an index of refraction that is the same or at least about the same as that of the fundamental wave, and that this index of refraction is higher than that of the harmonic wave. There is a clearly different relationship, and these two regions n ₁ and n ₂ are
The periodic structure is arranged in a direction orthogonal to the light propagation direction.

In particular, the two adjacent regions n ₁ and n
The relationship of ₂ requires that the refractive index difference in the fundamental wave is 5/1000 or less, and the refractive index difference in these harmonics is 10/1000 or more.

[0012]

The feature of the present invention is that the optical waveguide formed on the substrate has two regions n ₁ and n ₂ defined alternately in a direction orthogonal to the light propagation direction as shown in FIG. The regions n ₁ and n ₂ have a periodic structure (grating structure) arranged side by side, and the refractive index characteristics of these regions n ₁ and n ₂ have a wavelength dispersion curve as shown in FIG. In particular, these adjacent regions n ₁ and n ₂ are constructed so that their refractive indices are the same as or at least close to the same level for the fundamental wave, and this refractive index is clear for the harmonics. It has a periodic structure in which the different relationships are maintained. The reason for adopting such a waveguide configuration is that it is configured based on the following knowledge.

In a waveguide such as that shown in FIG.
Consider a case in which a waveguide having a periodic structure portion in which regions n ₁ and n ₂ having a refractive index characteristic as shown in FIG. Now, when the light on the substrate having the propagation constant of K ₀ is guided to the above waveguide, the second harmonic wave is generated in the region n ₁ . The generated harmonic has a propagation constant K ₁ . Generally, when light is guided to the periodic structure portion (grating portion), and when the difference in refractive index between the region n ₁ and the region n ₂ is large, the light is emitted into the air or the substrate side by the grating. However, in the above waveguide structure,
Since the region n ₁ and the region n ₂ are the same as or close to the fundamental wave, they propagate as they are without being affected by the grading portion composed of the defined regions n ₁ and n ₂ . However, in the case of harmonics generated by propagation,
Since the regions n ₁ and n ₂ are different from each other, they are emitted into the air or the substrate side while satisfying the phase matching condition.

As described above, as the preferable period of the periodic structure (grading) composed of the alternating arrangement of the regions n ₁ and n ₂ , any value satisfying the following equation is adopted in consideration of the emission angle. Where n _C : refractive index of air n _S : refractive index of substrate N: effective refractive index of higher harmonic Λ: period θ ₁ : angle of emission to air θ ₂ : angle of emission to substrate λ: wavelength of higher harmonic wave q = ± 1, ± 2, ± 3 ……

In the above equation, the constant q is preferably q = ± 1. This is because if q> 1 or q <-1, there are many radiation modes, and the generated harmonics cannot be collected efficiently. The emission angle θ is not particularly limited, but θ = 0 is preferable. It is easy to use when collecting light later. Furthermore, the period Λ is given by the following equation; Λ filled with is advantageous. This is because when Λ becomes larger than the Λ value satisfied by this equation, the generated harmonics interfere with each other and weaken the harmonics. On the other hand, if Λ is too small, processing becomes difficult, which is not preferable. It should be noted that if Λ is changed, it is possible to focus the harmonics generated in this grading section in space. Further, regarding the ratio of the grading portion (periodic structure portion), it is advantageous to take a larger nonlinear constant.

The periodic structure of the waveguide exhibiting such emission characteristics may be a periodic structure (grading) of a relief structure composed of regions n ₁ , n ₂ and n ₃ shown in FIG. In this case, the areas n ₁ , n ₂ , and n ₃ may satisfy the relationship of the following equation. That is, the average refractive index n ₂ is Is.

In the SHG element of the present invention having the above-mentioned waveguide structure, the periodic structure part n ₁ for the fundamental wave,
The refractive index difference between the n ₂ regions is preferably 5/1000 or less. The reason for this is that if the refractive index difference at this time is 5/1000 or more, the fundamental wave is also radiated and attenuated in the guided wave, so that efficient harmonics are not generated. On the other hand, the difference in refractive index between the periodic structure portions n ₁ and n ₂ in the harmonic is preferably 10/1000 or more. The reason is that the difference in refractive index at this time is 10/1000.
This is because in the following, the emission efficiency of the harmonics is poor and the harmonics cannot be collected efficiently.

As the optical waveguide used in the present invention,
Obtained by epitaxial growth on a substrate or substrate
Such a thin film can be used. For example, S of the present invention
In the HG device, a combination of the thin film optical waveguide and the substrate
As a thin film optical waveguide, LiNbO₃, Α-quartz, K
TiOPO_Four(KTP), β-BaB₂O_Four(BB
O), KH₂PO_Four(KDP), KD₂PO_Four(KD^*
P), NH_FourH₂PO_Four(ADP), C_FiveH₂AsO_Four
(CDA), C_FiveD ₂AsO_Four(CD^*A), RbH₂
PO_Four(RDP), RbH₂AsO_Four(RDA), Be
SO_Four・ 4H₂O, LiClO_Four・ 3H₂O, LiCl
O₃, Α-LiCdBO₃, LiB₃O_Five(LBO),
Urea, Y₃Fe_FiveO₁₂, (Bi, Y) ₃(Al, Fe)
_FiveO₁₂Garnet such as, Ba₂NaNb_FiveO₁₅(BN
N), (Sr, Ba) Nb₂O₆Etc. can be used as a substrate used in combination with this thin film optical waveguide
Is LiTaO₃, LiTaO₃Thin film formed Li
NbO₃, SiO₂, Α-sapphire, BeO, ZnO,
Ga₃Ga_FiveO₁₂Garnet, KTP, BBO, etc.
LBO, KDP and similar compounds, soda glass, pie
Rex glass or the like is used. Especially thin film optical waveguide
As LiNbO₃Using LiTaO as the substrate
₃The combination using is the most desirable form. It
Are similar in crystal structure to each other.
This is because the lattice matching is easy.

As a method of forming the above optical waveguide,
It is formed by using an ion exchange method, a diffusion method, or an epitaxial growth method.

Next, a method of manufacturing the SHG element of the present invention will be described with reference to FIG. First, the optical waveguide 2 is formed on the substrate 1 (FIG. 4a). Next, in order to form a grating (periodic structure portion) in this optical waveguide 2, a photoresist is applied to the surface of the thin film optical waveguide 2 and exposed,
It is developed to obtain the required pattern 3 (FIG. 4b). afterwards,
A Ti film 4 is obtained by sputtering Ti and lifting off (FIG. 4c). Then, the Ti film 4 is used as a mask for ion beam etching (IBE), or similarly, Al is used as a mask for reactive ion etching (RIE) to form a groove 5 (FIG. 4).
d). In this way, a relief type grating is formed. Next, in this state, another material is formed in this groove by sputtering or CVD (FIG. 4e). As this material, a material having different wavelength dispersion as a waveguide material is selected according to the original optical waveguide. For example, it is glass 6, a composite of glass and TiO ₂ , or the like. After that, the metal used as the mask is lifted off, and if there is an excess material, it is polished to complete the SHG element of the present invention having a periodic structure in which so-called two regions n ₁ and n ₂ are alternately arranged in the light propagation direction. (Fig. 4f). The example of FIG. 5 is a method of manufacturing an SHG element whose surface is protected by a glass film.

[0021]

【Example】

Example A surface of a Z plate having a length of 10 mm in the y direction and a thickness of 1 mm of a KTP crystal was polished and used as a surface for forming a waveguide. RbNO ₃
A mixture of Ba and NO ( ₃ ) was immersed in a melted material at 330 ° C. for 70 minutes to form a channel waveguide having a depth of 2 μm in the y direction on the surface. When the refractive index of this waveguide was measured by observing the guided light, nx = 1.
It was 7644. Also, nx = 1.804 at a wavelength of 0.532 μm. A commercially available photoresist was applied to the above waveguide and exposed and developed to form a pattern with a period of 0.29 μm. Aluminum was formed thereon to a thickness of 0.5 μm by sputtering, and an aluminum mask was formed by the lift-off method. Next, using CF ₄ gas, a 1 μm groove was formed in the waveguide by reactive ion etching. Then, LaF ₉ glass having a wavelength of 1.064 μm and a refractive index of 1.762 and a wavelength of 0.532 μm and a refractive index of 1.825 was sputtered and lifted off. The surface of the waveguide was polished to remove excess glass and used as a second harmonic generation element.

The SHG device thus obtained has 100
When a laser beam with a wavelength of 1.064 μm having a power of mmW was made incident, a harmonic wave having a wavelength of 0.532 μm was emitted at an angle of 88.9 ° from the device surface. When the power of this harmonic was measured, it was 1 mW, which was sufficiently practical. Further, when the wavelength was scanned from 1.0 μm to 1.1 μm using a dye laser, a harmonic wave having a wavelength of 0.5 μm to 0.55 μm was emitted while changing from 92.2 ° to 87 ° with respect to the substrate surface. At this time, the output light intensity did not change. Furthermore, the temperature of the device was changed from 0 ° C. to 60 ° C. by using a Peltier device, but the output angle was changed from 89.1 ° to 88.4 °, and the output light intensity was not changed. As shown above,
It was found that this device is an excellent second harmonic wave generating device with practicality never before seen.

Comparative Example A surface of a Z plate having a length of 10 mm and a thickness of 1 mm of a KTP crystal was polished to form a surface for forming a waveguide. After forming a pattern having a period of 4 μm and a width of 6 μm on the surface of this substrate by a normal photolithography, Ti was sputtered and lifted off to obtain a periodic pattern of Ti. Next, this substrate
A mixture of RbNO ₃ and Ba (NO ₃ ) melted at 330 ° C.
After being immersed for 60 minutes, Ti was taken out with hydrofluoric acid to form a periodic waveguide having a depth of 5 μm to obtain a second harmonic generation element. This element has a wavelength of 1.064μ with a power of 100 mW.
When a laser beam of m is incident, the wavelength is from the opposite end face.
A harmonic wave of 0.532 μm was emitted. The power of this harmonic was measured and found to be 1.3 mW. When the wavelength was changed from 1.064μm using a dye laser for this device.
The harmonics were observed only between 1.063 μm and 1.065 μm. When the wavelength was fixed at 1.064 μm and the temperature of the element was changed, higher harmonics were observed only when the element temperature was 24.6 ° C to 28.2 ° C, but could not be observed in other cases. As described above, a device having only a normal refractive index change was an element having a very narrow permissible width.

[0024]

As described above, according to the SHG element of the present invention, even if the wavelength of the fundamental wave changes a little, the second harmonic wave having a large intensity can be generated by only slightly changing the emission angle. .. Further, according to the present invention, the temperature allowable width of the element is large, the shape of the emitted beam is a spot, and it is easy to collect the light. By selecting the grating period,
It is possible to provide a practical SHG element that can be emitted in an arbitrary direction and can be condensed in space by changing the grating period.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a periodic structure of an SHG element of the present invention.

FIG. 2 is a graph showing the relationship between the wavelength and the refractive index of defined regions n ₁ and n ₂ provided in the optical waveguide.

FIG. 3 is a schematic diagram showing another example of the SHG element of the present invention.

FIG. 4 is a manufacturing process drawing of the SHG element of the present invention.

FIG. 5 is a manufacturing process drawing of an SHG element whose surface is protected by a glass film.

[Explanation of symbols]

 1 substrate 2 optical waveguide 3 pattern 4 Ti film 5 groove 6 glass film

Claims (3)

[Claims] 1. A second harmonic generation element comprising an optical waveguide having a nonlinear optical crystal characteristic formed on a substrate, wherein the second harmonic generation element has a periodic structure in which regions n ₁ and n ₂ defined by the optical waveguide are alternately arranged. And the areas n ₁ and n ₂ adjacent to each other
Has the same or at least the same degree of refraction index for the fundamental wave, and has a clearly different relation for the harmonic wave, and these two regions n ₁ , n ₂ is a second harmonic generation element, which is configured as a periodic structure arranged in a direction orthogonal to a light propagating direction. _2. A second harmonic generation element, wherein the difference in refractive index between the _two regions n ₁ and n ₂ with respect to the fundamental wave is 5/1000 or less. 3. A second harmonic generation element, wherein the difference in refractive index between the _two regions n ₁ and n ₂ with respect to harmonics is 10/1000 or more.