JP2973463B2 - Optical waveguide device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an optical waveguide device, particularly an optical waveguide type second harmonic (hereinafter referred to as SHG: Second Harmonic) by Cherenkov radiation.
Generator)).

[Summary of the Invention]

The present invention relates to an optical waveguide device, the KTiOPO ₄ single crystal substrate, by forming an amorphous optical waveguide TiO ₂ is doped with Ta ₂ O ₅ or Ta ₂ O _5,, for example, in the case of application to SHG High efficiency light output can be obtained,
The control of the Cherenkov radiation angle is possible to improve the characteristics.

[Conventional technology]

The optical second harmonic generator SHG generates a second harmonic light having a frequency of 2ω when light having a frequency of ω is introduced.
With this SHG, the wavelength range can be expanded, and accordingly, the range of use of the laser can be further expanded and the use of laser light in each technical field can be optimized. For example, optical recording and reproduction using laser light by shortening the wavelength of laser light,
In magneto-optical recording and reproduction, the recording density can be improved.

A Cherenkov radiation type SHG in which a linear optical waveguide is formed on a non-linear single crystal substrate, a fundamental wave is passed through the substrate, and a second harmonic is emitted from the substrate side as a radiation mode is, for example, Applied Physics Lez.
tters) 17 , 447 (1970). This SHG is obtained by forming a ZnS polycrystalline optical waveguide on a ZnO nonlinear single crystal substrate, and obtains a second harmonic of 0.53 μm using a 1.06 μm Nd: YAG laser. However, in this case, the propagation loss is large because the waveguide is polycrystalline,
Further, the d constant of ZnO of the substrate is small and the efficiency is considerably poor.

In the SHG disclosed in Japanese Patent Application Laid-Open No. 61-189524, a LiNbO ₃ (hereinafter referred to as LN) substrate is used, and an optical waveguide is formed by proton exchange. In this case, the SHG efficiency η has a high value of 1% or more.

In the case of Cherenkov radiation SHG, the efficiency η of the SHG has a relationship of η∝d ² · P ^w · l with respect to the d constant of the nonlinear optical material used, the fundamental wave power density P, and the interaction length l. Therefore, in order to obtain high efficiency, it is necessary to use a material having a large d constant, increase the fundamental power density P, and increase the interaction length l. Also, the value of d constant is the crystal orientation,
In the case of LN, which differs due to the geometric relationship of the polarization direction of the fundamental wave,
| d ₃₃ | = 34.4 × 10 ^-12 (m / V), which is the largest. For proton exchange, it is possible to increase only the refractive index n _e for extraordinary light, and X plate, LN as a nonlinear single crystal substrate for a surface subjected to etching in the proton exchange is coarse in Y plate
Can only use the Z plate (substrate having a plate surface orthogonal to the z axis in the direction along the c axis) and can only use the TM mode.
Therefore, the optical coupling of the light source with the semiconductor laser is complicated. The refractive index difference Δn between the waveguide and the substrate is about 0.14 at the maximum. Therefore, there is a limit to light confinement, that is, the maximum efficiency. Further, proton exchange is effective only for LN, and other LiTaO ₃ or KTiOPO ₄ (hereinafter referred to as KTP) cannot be used.

Furthermore, since the Cherenkov radiation angle at the maximum efficiency in the SHG using the LN substrate is a relatively large angle of about 16 °, the length in the light traveling direction of the substrate in order to increase the SHG efficiency η described above. If l is large, there is a problem that the SHG light is reflected at the interface between the back surface of the substrate and the outside, for example, air, and a plurality of output lights are generated. This reflected light is processed or the thickness of the substrate is increased. However, there is a drawback in that an SHG light collecting optical system becomes complicated.

On the other hand, for example, Applied Physics Letters (Ap
plied Physics Letters) 50 , 1216 (1987).
In SHG, a KTP nonlinear single crystal substrate is used, and an optical waveguide is formed on the KTP substrate by an ion exchange method. The SHG using the mode dispersion of the waveguide is possible.

KTP single crystals, nonlinear constant d ₃₃ is ^{_{d 33 = 13.7 × 10 -12 (}} m
V) is a crystal that is quite large and transparent in the blue wavelength region, and has a refractive index of n _{x at a} wavelength of 0.84 μm, for example.
_{= 1.748, n y = 1.755,} n z = 1.840 and LiNbO ₃ (LN) or LiTaO
_It is much smaller than ₃ mag.

However, when formed by this ion exchange method
The SHG efficiency was extremely sensitive to parameters such as the fundamental wavelength and the depth of the optical waveguide, and was not practical. Therefore, KT is a material that has a small refractive index n and can be expected to have a good light confinement effect in the optical waveguide formed thereon.
The practical application of P nonlinear single crystal substrates to SHG is desired.

[Problems to be solved by the invention]

The present invention is a problem of propagation loss when using a polycrystalline body as a waveguide as described above, a problem that can be used only TM mode,
In addition, the problems such as the limitation of the maximum efficiency due to the insufficient refractive index difference Δn are solved, and the above-mentioned optical waveguide device using the KTP substrate is practically applied to the SHG, and a highly efficient optical output is obtained. To improve the characteristics.

Incidentally, the applicant of the present invention has previously disclosed in Japanese Patent Application No. 63-265786 a second optical harmonic generation device (SHG) by Cherenkov radiation in which an optical waveguide is provided on a nonlinear optical material substrate.
Proposed an SHG in which the optical waveguide is composed of an amorphous optical waveguide in which TiO ₂ is doped in Ta ₂ O ₅ . According to this, a good SHG with high efficiency can be obtained.However, as a result of intensive studies, the present inventor has found that, especially in KTP, the refractive index difference Δn is increased to increase the light confinement effect, The aim is to improve the SHG efficiency and to simplify the condensing optical system by further reducing the Cherenkov radiation angle.

[Means for solving the problem]

One example of the optical waveguide device according to the present invention is shown in a schematic enlarged sectional view of FIG. In this case, a cross-sectional view along the traveling direction of light is shown.

The present invention, on the KTiOPO ₄ single crystal substrate (1) as shown in FIG. 1, Ta ₂ O _5, or Ta ₂ O ₅ to TiO ₂ to form a doped amorphous optical waveguide (2).

[Action]

As described above, in the optical waveguide device according to the present invention, when a KTP crystal is used as a substrate and a thin film made of, for example, Ta ₂ O ₅ is formed as a waveguide, the refractive index difference Δn has a large value of Δn = 0.33 to 0.42. Thus, SHG light could be obtained with high efficiency.

In addition, the use of a KTP substrate enabled the reduction of the Cherenkov radiation angle, and the simplification of the condensing optical system improved the characteristics.

In addition, SHG light in the blue wavelength region has not been obtained in the conventional SHG using an ion exchange optical waveguide configuration using a KTP crystal substrate.However, the optical waveguide device according to the present invention generates good SHG even in the blue wavelength region. It was possible to do.

〔Example〕

The present invention relates to a KTP single crystal substrate (1) as shown in FIG.
Amorphous optical waveguide (2) in which Ta ₂ O ₅ or Ta ₂ O ₅ is doped with TiO ₂ is formed thereon. When TiO ₂ is doped into Ta ₂ O ₅ , for example, the ratio of Ti to the sum of Ti and Ta, Ti / (T
i + Ta) is set to 0 ≦ Ti / (Ti + Ta) ≦ 60 (atomic%).

Hereinafter, each example of the optical waveguide device according to the present invention will be described in detail.

Example 1 As shown in FIG. 1, a KTP single crystal a-plate, that is, a substrate (1) having a surface orthogonal to the direction along the a-axis
O ₂ is undoped, that is, the ratio of Ti to the sum of Ti and Ta
By setting / (Ti + Ta) = 0, a so-called ridge-type optical waveguide (2) is formed to obtain an optical waveguide device (10).

An example of a method of forming the optical waveguide device (10) will be described with reference to manufacturing process diagrams of FIGS. First, FIG. 2A
As shown in (1), a substrate (1) made of a plate of KTP single crystal is prepared, and the temperature of the substrate (1) is reduced to a required temperature by CVD (chemical vapor deposition) over the entire surface. While holding, using tantalum pentaethoxide Ta (OC ₂ H ₅ ) ₅ as a raw material, an amorphous thin film of Ta ₂ O ₅ (2
0) is formed with a thickness of, for example, 2050 °.

Next, as shown in FIG. 2B, an etching resist (4) using, for example, a photoresist is finally obtained on the Ta ₂ O ₅ amorphous thin film (20). It is formed by a well-known technique in a stripe shape perpendicular to the plane of the drawing.

Then, as shown in FIG. 2C, using this etching resist (4) as a mask, an undoped Ta ₂ O ₅ thin film (20)
Is etched by, for example, RIE (Reactive Ion Etching) to form a ridge type amorphous waveguide (2) to obtain an optical waveguide device (10).

As shown in FIG. 1, TE mode light having a wavelength of 0.84 μm was introduced into the optical waveguide device (10) thus obtained. In FIG. 1, arrow a is incident light, arrow b is traveling light, and arrow c is SH
Indicates G light. In this case, the refractive index n of the KTP substrate (1) is 1.8396
The refractive index n of the Ta ₂ O ₅ optical waveguide (2) was 2.172, the refractive index difference Δn was about 0.33, and blue SHG light having a wavelength of 0.42 μm could be generated with high efficiency.

At this time, the Cherenkov radiation angle θ was 10 °, which was smaller than the conventional radiation angle of 16 ° described at the beginning.

Then, as shown in FIG. 3, a cross-sectional view orthogonal to the light traveling direction is shown. As described above, the ridge type undoped Ta ₂ O ₅ is formed on the substrate (1) formed of the KTP single crystal a plate. After forming the optical waveguide (2), the optical waveguide (2)
An upper cladding layer (3) made of SiO ₂ having a refractive index n of 1.45, that is, a refractive index smaller than that of the optical waveguide, was entirely formed thereon by, for example, a CVD method.

In such a configuration, a laser beam having a wavelength of 0.84 μm is used.
When input as TE mode, Cherenkov radiation angle θ
Was θ = 6 °, and blue SHG light having a wavelength of 0.42 μm was generated.

Example 2 As in Example 1, also in this case, the KTP single crystal a
A ridge-type optical waveguide (2) made of undoped Ta ₂ O ₅ is formed on a substrate (1) made of a plate in the same manner as in Example 1, and the refractive index is entirely formed on the optical waveguide (2). n is 1.6
The upper cladding layer (3) made of Al ₂ O ₃ was formed by, for example, a CVD method.

In such a configuration, a laser beam having a wavelength of 0.84 μm is used.
When incident as TE mode, the Cherenkov radiation angle θ
Generated a blue SHG having a wavelength of 0.42 μm in the direction of θ {2}.

As is clear from Examples 2 and 3 above, by providing the upper cladding layer (3) of an appropriate material such as Al ₂ O ₃ on the optical waveguide (2), the Cherenkov radiation angle can be further reduced. did it. This is presumably because the provision of the cladding layer (3) increased the propagation constant of the fundamental wave guided mode.

In each of the above-described examples, the optical waveguide (2) is formed of undoped Ta ₂ O _{5. In} this case, since Ta ₂ O ₅ is a simple one-component oxide, the film is formed on a KTP substrate. A rate-determining state is determined only by the temperature condition, and a film can be formed with remarkably high reproducibility. Further, for example, in the case of the third embodiment as well, since both the optical waveguide (2) and the upper cladding layer (3) are made of a simple one-component oxide, a film can be stably formed with good reproducibility.

Further, when forming the optical waveguide (2) using Ta ₂ O ₅ doped with TiO ₂ , the refractive index difference Δn can be further increased, and the SHG efficiency can be further increased.

FIG. 4 shows the measurement result of the relationship between the refractive index and the Ti doping amount at a wavelength of 0.6328 μm of the waveguide made of such a TiO ₂ -doped Ta ₂ O ₅ film, and the horizontal axis indicates the sum of Ti and Ta. Are plotted on the vertical axis, and the refractive index n is plotted. As is apparent from this, the refractive index n increases in proportion to the Ti amount. Even when the Ti amount is 0, the refractive index is 2.2, and the refractive index difference Δn can be made sufficiently large. In this case, crystallization was observed when Ti / (Ti + Ta) was more than 60 atomic%. Therefore, the Ti doping amount is set to 0 ≦ Ti / (Ti + T
By setting a) ≦ 60, a good amorphous optical waveguide (2) can be formed.

In each of the above-described examples, the case where the present invention is applied to SHG is particularly described.However, the present invention can also be applied to various other optical communication and optical waveguide devices in an optical integrated circuit. In this case, the refractive index difference Δn is also large. Because of the large size, the light is efficiently confined, and the optical waveguide efficiency can be increased.

〔The invention's effect〕

As described above, the present invention relates to KTiOPO as shown in FIG.
_{4 On the} (KTP) single crystal substrate (1), use Ta ₂ O ₅ or Ta ₂ O ₅
By forming the amorphous optical waveguide (2) doped with TiO ₂ , the refractive index difference Δn between the substrate (1) and the optical waveguide (2) can take a large value of 0.33 to 0.42, thereby achieving high efficiency. Was able to obtain SHG light.

In addition, SHG light in the blue wavelength region has not been obtained in the conventional SHG using an ion exchange optical waveguide configuration using a KTP crystal substrate.However, the optical waveguide device according to the present invention generates good SHG even in the blue wavelength region. It was possible to do.

In addition, the use of a KTP substrate enabled the reduction of the Cherenkov radiation angle. That is, the Cherenkov radiation angle becomes 10 ° or less, and Al ₂ O is further placed on the optical waveguide (2).
By providing the upper cladding layer (3) of a suitable material such as ₃ , the Cherenkov radiation angle could be made smaller. Therefore, for example, by increasing the waveguide length, the SHG efficiency η
Even if the length of the substrate (1) in the light traveling direction is increased in order to increase the thickness, the thickness of the substrate (1) is reduced in order to avoid the problem caused by the Cherenkov radiation reflected on the back surface of the substrate (1). It is possible to avoid the inconvenience of increasing the size or complicating the condensing optical system, improve the SHG characteristics, and simplify the structure when actually used in the optical recording / reproducing apparatus described above. It has many practical advantages.

Further, when the optical waveguide (2) is formed of undoped Ta ₂ O ₅ as described in the first to third embodiments, Ta ₂ O ₅
Is a simple one-component oxide, its film formation is rate-determined only by the temperature conditions of the KTP substrate, and the film can be formed with remarkably high reproducibility. Further, for example, in the case of the third embodiment, since the optical waveguide (2) and the upper cladding layer (3) are both simple one-component oxides, the reproducibility of film formation can be further stabilized.

[Brief description of the drawings]

1 and 3 are enlarged schematic cross-sectional views of each example of the optical waveguide device according to the present invention, FIGS. 2A to 2C are manufacturing process diagrams of an example of the optical waveguide device according to the present invention, and FIG. _2- doped Ta ₂ O ₅
FIG. 4 is a measurement curve diagram showing a relationship between a film refractive index and a Ti doping amount. (1) is a substrate, (2) is an optical waveguide, (3) is an upper cladding layer, (4) is an etching resist, (20) is a Ta ₂ O ₅ thin film, and (10) is an optical waveguide device.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02F 1/37 G02B 6/12 JICST

Claims (1)

(57) [Claims] To 1. A KTiOPO ₄ single crystal substrate, Ta ₂ O ₅ or Ta ₂ O ₅ to TiO ₂ is formed doped amorphous optical waveguide, the upper cladding layer is formed on the optical waveguide, Cerenkov An optical waveguide device having a radiation angle of 10 ° or less.