JP2897366B2 - Optical waveguide device - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an optical waveguide device, and more particularly to optical second harmonic generation (second ha) by Cherenkov radiation.
rmonic generation, SHG).

[Summary of the Invention]

The present invention relates to an optical waveguide device, comprising: a nonlinear optical crystal substrate; and a dielectric thin film formed on the nonlinear optical crystal substrate, the dielectric thin film being transparent to at least light having an angular frequency ω and having a larger refractive index than the nonlinear optical crystal substrate. When light of angular frequency ω is incident on one end of the optical waveguide, the second harmonic light of Cerenkov radiation of angular frequency 2ω is emitted at a Cerenkov radiation angle of 1 ° or less, and the Cherenkov radiation angle And the divergence angle of the Cerenkov radiated second harmonic light in the width direction of the optical waveguide is about the same as the Cherenkov radiation angle. As a result, it is possible to remarkably improve the light collection characteristics of the Cherenkov radiated second harmonic light.

[Conventional technology]

The Cherenkov radiation light SHG generates light having an angular frequency of 2ω, that is, SHG light in a Cherenkov radiation mode by introducing light (fundamental wave) having an angular frequency ω. The Cherenkov synchrotron radiation SHG is extremely practical since the phase matching condition is always satisfied, and has attracted attention in recent years.

Conventionally, this Cherenkov synchrotron radiation SHG device
As shown in the figure, a substrate is known in which a proton-exchanged LiNbO ₃ optical waveguide 102 is formed on a substrate 101 composed of a LiNbO ₃ single crystal z-plate (a single crystal plate whose plane is orthogonal to the c-axis of the crystal) ( For example, Japanese Unexamined Patent Publication No. 61-18952, 48th Annual Meeting of the Japan Society of Applied Physics, Proceedings, 19p-ZG-1,2,
3,4).

[Problems to be solved by the invention]

However, in the conventional Cherenkov synchrotron radiation SHG device shown in FIG. 7, the Cherenkov angle θ is about 1
It is as large as 6 ゜. In this case, although the emitted SHG light is collimated in the thickness direction of the substrate 101, the divergence angle (hereinafter, the width direction of the optical waveguide 102) in the direction perpendicular to the thickness direction of the substrate 101 (hereinafter, the width direction of the optical waveguide 102). , The lateral divergence angle) is about the same as the Cherenkov radiation angle θ, that is, as large as about 16 °.
By the way, a condensing optical system for condensing the emitted SHG light is provided on the output side of the Cherenkov radiation light SHG device, and the lateral divergence angle of the emitted SHG light is 16 ° as described above. Since it is as large as this, the SHG light condensing characteristics are poor. For this reason, there is a problem in that the condensing optical system becomes complicated, such as when a special conical lens is required to condense the emitted SHG light to the diffraction limit.

Accordingly, it is an object of the present invention to provide an optical waveguide device capable of significantly improving the light-gathering characteristics of the second harmonic light radiated by Cherenkov radiation.

[Means for solving the problem]

In order to achieve the above object, the present invention provides an optical waveguide device, comprising: a non-linear optical crystal substrate (1); and at least a light having an angular frequency ω formed on the non-linear optical crystal substrate (1). And an optical waveguide (2) made of a dielectric thin film having a refractive index larger than that of the nonlinear optical crystal substrate (1), and light (a) having an angular frequency ω is provided at one end of the optical waveguide (2).
Is incident, the Cerenkov radiation second harmonic light (b) having an angular frequency of 2ω is emitted at a Cherenkov radiation angle of 1 ° or less, and the spread of the Cherenkov radiation angle and the Cherenkov radiation second width in the width direction of the optical waveguide (2). Harmonic light (b)
Is about the same as the Cherenkov radiation angle.

Here, the optical waveguide (2) is preferably transparent also to the Cerenkov-radiated second harmonic light (b) having an angular frequency of 2ω.

In order to effectively confine light and improve SHG efficiency, a cladding layer is preferably formed on the optical waveguide (2).

[Action]

According to the optical waveguide device of the present invention configured as described above, since the Cherenkov radiation angle is 1 ° or less, the divergence angle of the Cherenkov-radiated second harmonic light (b) in the width direction of the optical waveguide (2). That is, the lateral divergence angle is as small as about the Cherenkov radiation angle, that is, about 1 ° or less. Also, the spread of the Cherenkov radiation angle is about 1 ° or less. For this reason, the spot of the Cherenkov radiated second harmonic light (b) has a substantially circular shape. As a result, it is possible to remarkably improve the light-collecting characteristics of the Cherenkov-emitted second harmonic light (b). In addition, the conical lens which has been required conventionally becomes unnecessary, and the light collecting optical system can be simplified.

In general, the refractive index of the nonlinear optical crystal substrate for the second harmonic light having an angular frequency of 2ω is Assuming that the effective refractive index of the optical waveguide with respect to the light having the frequency ω (fundamental wave) is N ^ω and the Cherenkov radiation angle θ, the following relationship is established.

From equation (2), that θ is 1 ° or less as described above means that Is equivalent to That is, in this case Is very close to 1. (1)
From the formula Does not exceed 1.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows Cerenkov radiation according to one embodiment of the present invention.
FIG. 2 is a perspective view showing the SHG device, and FIG. 2 is a cross-sectional view along the optical waveguide of the Cherenkov radiation light SHG device according to one embodiment of the present invention.

As shown in FIGS. 1 and 2, in the Cherenkov synchrotron radiation SHG device according to this embodiment, for example, KTiOPO
_{4 A} striped channel optical waveguide 2 made of, for example, a Ta ₂ O ₅ film is formed on a substrate 1 made of a ₄ (KTP) single crystal a plate (a single crystal plate whose plane is orthogonal to the a axis of the crystal). .

In this case, the width W, the thickness h, and the length L of the channel optical waveguide 2 made of a Ta ₂ O ₅ film are such that the Cherenkov radiation angle θ is 1
さ れ Optimized to be: Specifically, the width W of the channel optical waveguide 2 is, for example, 5 μm, the thickness h is, for example, 2380 °, and the length L is, for example, 2.5 mm.

Next, a method for manufacturing the Cherenkov radiation SHG device according to the present embodiment configured as described above will be described.

First, for example, tantalum pentaethoxide Ta (OC ₂ H ₅ )
For example, an amorphous Ta ₂ O ₅ film is formed on the substrate 1 made of a KTP single crystal a plate by a CVD method using ₅ as a source gas. Next, a resist pattern having a shape corresponding to the shape of the optical waveguide by photolithography to this the Ta ₂ O ₅ film on, by the resist pattern the Ta ₂ O ₅ film, for example, reactive ion etching as a mask (RIE) method Etch. Thus, the channel optical waveguide 2 made of the Ta ₂ O ₅ film is formed.

As shown in FIG. 2, as shown in FIG. 2, a 865 nm wavelength T optical waveguide is connected to one end of the channel optical waveguide 2 of the Cherenkov radiation light SHG device according to the present embodiment constructed as described above.
When the E-mode laser beam a was incident, the wavelength was 432.5 (= 86
5/2) nm SHG light b was generated at a Cerenkov radiation angle of about 0.6 ° (center value). When this SHG light b was condensed by a microscope objective lens having a numerical aperture of NA = 0.80, a circular spot of about 0.6 μm was formed. In FIG. 2, c indicates light traveling in the channel optical waveguide 2.

As described above, the central value of the Cherenkov radiation angle in this case is about 0.6 °, but its spread also exists about 1 ° or less. On the other hand, the lateral divergence angle of the SHG light is also less than 1 °.

In this situation, the far field pattern of the SHG light is as shown in FIG. FIG. 3 also shows, for comparison, a far-field pattern (dashed-dotted line) when the Cherenkov radiation angle is about 16 °. From FIG. 3, it can be seen that the far field pattern of the SHG light generated by the Cherenkov radiation SHG device according to the present embodiment has an almost circular shape with a small size. On the other hand, when the Cherenkov radiation angle is 16 °, the far field pattern has a flat crescent shape, and its lateral spread is extremely large.

As described above, according to this embodiment, a striped channel optical waveguide 2 made of a Ta ₂ O ₅ film is formed on a substrate 1 made of a KTP single crystal a plate, and the width of the channel optical waveguide 2 is further increased. By optimizing W, thickness h, and the like, the Cherenkov radiation angle can be extremely reduced to about 0.6 °. Thus, the spread of the Cherenkov radiation angle and the lateral divergence angle of the SHG light can be reduced to approximately the same level as the Cherenkov radiation angle, that is, 0.6 °. For this reason, the far field pattern of the SHG light has a substantially circular shape as shown in FIG.
Therefore, the SHG light collecting characteristics can be significantly improved as compared with the related art. For this reason, it is not necessary to use a conical lens to collect the SHG light as in the related art, and it is possible to sufficiently collect the SHG light to the diffraction limit with a normal lens such as a convex lens.

Although there is a report that the lateral divergence angle of the SHG light is uniquely determined by the width W of the channel optical waveguide 2, the above result shows that the lateral divergence angle of the SHG light is determined by the width W of the channel optical waveguide 2. It can be seen that it varies depending on other parameters, for example, the thickness h and the refractive index of the channel optical waveguide 2 and cannot be determined only by the width W of the channel optical waveguide 2.

Although the above description is for the case where the wavelength of the incident laser light is 865 nm, by changing the width W and the thickness h of the channel optical waveguide 2 variously, other wavelengths can be used.
It is possible to achieve a Cherenkov radiation angle of about 0.6 °.

FIG. 4 shows S generated at a Cherenkov radiation angle θ of about 0.6 ° by the Cherenkov radiation SHG device according to the present embodiment.
Shows an example of the relationship between power and theta _c of HG light. Where θ
_c is defined as shown in FIG. 6, and is a projection of the Cherenkov radiation angle θ onto a yz plane parallel to the plane of the optical waveguide 2. In FIG. 6, the OC direction indicates the traveling direction of the SHG light. For comparison, the Cherenkov radiation angle θ is
The power of SHG light generated by a conventional Cherenkov radiation SHG device having a proton exchanged LiNbO ₃ optical waveguide 102 formed on a substrate 101 made of a LiNbO ₃ single crystal z-plate as shown in FIG. an example of the relationship between the theta _c shown in Figure 5.

As shown in FIG. 4, the Cherenkov radiation of the present embodiment
It can be seen that the power of the SHG light generated by the SHG device at a Cherenkov radiation angle of about 0.6 ° is concentrated in a range where _θc is small, and thus the brightness of the SHG light is extremely high.
On the other hand, as shown in FIG. 5, the power of the SHG light generated at a Cherenkov radiation angle of about 16 ° by the conventional Cherenkov radiation light SHG device is widely spread up to a large θ _c , It can be seen that the brightness of the light is much lower than in this embodiment.

On the other hand, the refractive indices n _s and n _f of the substrate 1 made of the KTP single crystal a plate and the channel optical waveguide 2 made of the Ta ₂ O ₅ film with respect to, for example, light having a wavelength of 840 nm are about 1.84 and 2.18, respectively, and n _f > n _s
It can be seen that the relationship is satisfied. In this case, the refractive index difference _{Δn = n f -n s = 2.18-1.84} = 0.34, an extremely large value. Therefore, the effect of confining light is large, and high SHG efficiency can be obtained. Incidentally, the proton-exchanged L was placed on the substrate 101 made of a LiNbO ₃ single crystal z-plate shown in FIG.
conventional Cherenkov radiation to form the LiNbO ₃ optical waveguide 102
In the case of an SHG device, a substrate 101 made of a LiNbO ₃ single crystal z-plate is used.
The refractive indexes n _s and n _f of the proton-exchanged LiNbO ₃ optical waveguide 102 with respect to light having a wavelength of 840 nm are 2.175 and 2.315, respectively, and the difference in refractive index is as small as Δn = 2.315−2.175 = 0.14. Therefore, in this case, the light confinement effect is small, and the SHG efficiency is low.

Note that the Ta ₂ O ₅ film constituting the channel optical waveguide 2, for example, be doped with TiO _2, thus when doped with TiO ₂ can be increased more the difference in the refractive index delta, SHG efficiency can be further improved. In this case, when the ratio of the number of Ti atoms to the sum of the number of Ti atoms and the number of Ta atoms in the TiO ₂ -doped Ta ₂ O ₅ film is Ti / (Ti + Ta), for example, 0 ≦ Ti / (Ti + Ta) ≦ 60 %.

In order to further increase the SHG efficiency, a cladding layer may be formed on the channel optical waveguide 2 of the Cherenkov radiation SHG device according to the present embodiment. When the clad layer is formed in this manner, a sufficiently high SHG efficiency can be obtained by optimizing the refractive index of the clad layer. Specifically, the cladding layer is (SiO ₂ ) _1-x (Ta ₂ O ₅ ) _x formed by, for example, a sputtering method.
A film or the like can be used. The cladding layer also plays a role of protecting the channel optical waveguide 2.

Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments, and various modifications based on the technical idea of the present invention are possible.

For example, it is possible to use a substrate other than a KTP single crystal a plate as a nonlinear optical crystal substrate, and to use Ta as an optical waveguide.
It is also possible to use a film other than the ₂ O ₅ film.

〔The invention's effect〕

As described above, according to the present invention, since the spread of the Cherenkov radiation angle and the divergence angle of the Cerenkov radiation second harmonic light in the width direction of the optical waveguide are substantially equal to the Cherenkov radiation angle, the Cherenkov radiation second harmonic light Can be significantly improved.

[Brief description of the drawings]

FIG. 1 shows a Cherenkov radiation SH according to an embodiment of the present invention.
FIG. 2 is a perspective view showing the G device, FIG. 2 is a cross-sectional view along the optical waveguide of the Cerenkov radiation light SHG device according to one embodiment of the present invention, and FIG. 3 is a diagram showing the Cherenkov radiation light SHG device according to one embodiment of the present invention. FIG. 4 is a graph showing a far-field pattern of the generated SHG light, and FIG. 4 is a graph showing an example of the relationship between the power of the SHG light generated by the Cherenkov radiation light SHG device according to one embodiment of the present invention and θ _c . FIG. 5 is a graph showing an example of the relationship between the power of SHG light generated by the conventional Cherenkov radiation light SHG device and θ _c, and FIG. 6 is θ _c.
FIG. 7 is a sectional view showing a conventional Cherenkov radiation light SHG device. Description of main reference numerals in the drawings: 1: substrate composed of KTP single crystal a-plate, 2: channel optical waveguide composed of Ta ₂ O ₅ film.

Claims (1)

(57) [Claims] 1. A nonlinear optical crystal substrate, comprising: a dielectric thin film formed on the nonlinear optical crystal substrate and transparent to at least light having an angular frequency of ω and having a larger refractive index than the nonlinear optical crystal substrate. When light having an angular frequency ω is incident on one end of the optical waveguide, the second harmonic light of Cerenkov radiation having an angular frequency of 2ω is 1
光 The light guide is characterized in that the light is emitted with the following Cherenkov radiation angle, and the spread of the Cherenkov radiation angle and the divergence angle of the Cherenkov radiation second harmonic light in the width direction of the optical waveguide are substantially the same as the Cherenkov radiation angle. Wave device.