JPH0485523A - Optical waveguide device - Google Patents

Abstract

PURPOSE:To improve the condensing characteristic of Cherenkov radiation second harmonic wave light by providing at least an optical waveguide consisting of a thin dielectric film which is transparent to light of angular frequency omega and has the refractive index larger than the refractive index of a nonlinear optical crystal substrate on this substrate. CONSTITUTION:The striped channel optical waveguide 2 consisting of a Ta2O3 film is formed on the substrate 1 consisting of a single crystal (a) plate of KTiOPO4 and the width (w) and thickness (h) thereof are optimized, by which the Cherenkov radiation angle is extremely decreased to about 0.6 deg.. The spread of the Cherenkov radiation angle and the transverse diversion angle of SHG light are decrease to about the same extent, by which a far field pattern is formed to a nearly circular shape and the condensing characteristic of the SHG light is extremely improved. The need for using a circular conical lens is, therefore, eliminated and the sufficient condensing down to about the diffraction threshold with an ordinary convex lens is possible.

Description

[Detailed Description of the Invention] 5. Industrial Application Field] The present invention relates to an optical waveguide device, and in particular, the present invention relates to an optical waveguide device.
Optical second harmonic generation (s) by Cherenkov) radiation
econd harn+onic generaeio
n, SHG) and is suitable.

[Summary of the Invention] The present invention provides an optical waveguide device including a nonlinear optical crystal substrate, which is transparent to light at least at an angular frequency ω, and has a refractive index lower than that of the nonlinear optical crystal substrate. and an optical waveguide made of a dielectric thin film with a large
The light is emitted at a Cherenkov radiation angle of less than 0.degree., and the spread of the Cherenkov radiation angle and the divergence angle of the second harmonic Cherenkov radiation in the width direction of the optical waveguide are approximately the same as the Cherenkov radiation angle. Thereby, the focusing characteristics of the Cerenkov radiation second harmonic light can be significantly improved.

[Conventional technology]

The Cherenkov radiation SHG generates light with an angular frequency of 2ω, that is, SMC light, in Cherenkov radiation mode by introducing light (fundamental wave) with an angular frequency of ω. This Cherenkov synchrotron radiation SHG always satisfies the phase matching condition, so it is extremely practical and has attracted attention in recent years.

Conventionally, as this Cherenkov synchrotron radiation SHG device, the seventh
As shown in the figure, a proton-exchanged LiNb0. A device in which an optical waveguide 102 is formed is known (for example,
Publication No. 4, ■ Proceedings of the 48th Japan Society of Applied Physics Academic Conference, lecture number 19p-ZG-1, 2, 3, 4).

Problem 8 to be Solved by the Invention S] However, in the conventional Cherenkov synchrotron radiation SHG device shown in FIG. 7 described above, the Cherenkov angle θ is as large as about 16°. In this case, the emitted S
Although the MC light is collimated in the thickness direction of the substrate 101, the divergence angle in the direction perpendicular to the thickness direction of the substrate 101 (width direction of the optical waveguide 1020) (hereinafter referred to as lateral divergence angle) is the Cerenkov radiation angle. The same degree as θ, i.e. 16°
degree and large. By the way, Cherenkov synchrotron radiation 5KIG
A condensing optical system is provided on the output side of the device to condense the emitted SMC light, but since the lateral divergence angle of the emitted SMC light is as large as about 16° as mentioned above,
This SMC light has poor focusing characteristics. Therefore, when trying to condense the emitted SMC light up to the diffraction limit, a special conical lens is required, resulting in a complicated condensing optical system.

Therefore, an object of the present invention is to provide an optical waveguide device that can significantly improve the focusing characteristics of Cerenkov radiation second harmonic light.

[Means to solve the problem]

In order to achieve the above object, the present invention provides an optical waveguide device including a nonlinear optical crystal substrate (1) and a nonlinear optical crystal substrate (1) formed on the nonlinear optical crystal substrate (1). an optical waveguide (2) made of a dielectric thin film that is transparent and has a higher refractive index than the nonlinear optical crystal substrate (1), and allows light (a) with an angular frequency ω to be incident on one end of the optical waveguide (2). Then, the second Cerenkov radiation with angular frequency 2ω
Harmonic light (b) is emitted at a Cherenkov radiation angle of less than l°, and the Cherenkov radiation angle spreads and the optical waveguide (2)
The divergence angle of the Cerenkov radiation second harmonic light (b) in the width direction is approximately the same as the Cerenkov radiation angle.

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

Further, in order to effectively perform optical confinement and improve SHO efficiency, a landing layer is preferably formed on the optical waveguide (2).

[Effect]

According to the optical waveguide device of the present invention configured as described above, since the Cerenkov radiation angle is 1° or less, the Cerenkov radiation second harmonic light (
The divergence angle b), that is, the lateral divergence angle, is extremely small, about the same as the Cherenkov radiation angle, that is, about 1° or less. Further, the spread of the Cerenkov radiation angle is also about 1° or less.

Therefore, the spot of this Cherenkov radiation second harmonic light (b) has a nearly circular shape.

As a result, the focusing characteristics of the Cerenkov radiation second harmonic light (b) can be significantly improved. Further, the conical lens that was conventionally required is no longer necessary, and the condensing optical system can be simplified.

By the way, in general, the refractive index of a nonlinear optical crystal substrate for second harmonic light with an angular frequency of 2ω is n? N1 is the effective refractive index of the optical waveguide for light (fundamental wave) of frequency ω, and when θ is the Cerenkov radiation angle, the following relationship holds true.

nn〉N''' (1) From equation (2), the fact that θ is 1° or less as described above corresponds to N7 n1'':≧0.99985. That is, it can be seen that N7nlW is a value very close to 1 in this case. In addition, (1) N7n from 2
Since 〒<1, N7n:u never exceeds 1.

〔Example〕

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

FIG. 1 shows Cerenkov synchrotron radiation S according to an embodiment of the present invention.
FIG. 2 is a perspective view showing the MC device, and FIG. 2 is a cross-sectional view along the optical waveguide of the Cerenkov synchrotron radiation SMC device according to an embodiment of the present invention.

As shown in FIGS. 1 and 2, in the Cerenkov synchrotron radiation SMC device according to this embodiment, for example, KTiO
For example, Ta
Striped channel optical waveguide 2 made of zOs film
is formed.

In this case, a channel optical waveguide 2 made of a Taxes film is used.
The width W, thickness h, and length of are optimized so that the Cerenkov radiation angle θ is 1° or less. Specifically: For example, the width W of the channel optical waveguide 2 is 5 μm, and the thickness h
is, for example, 2380A, and the length is, for example, 2,5@.

Next, a method of manufacturing the Cerenkov synchrotron radiation SMC device according to this embodiment configured as described above will be explained.

First, for example, tantalum pentaethoxy)-Ta(OCz
H5) From the CVD method using S as a raw material gas, K
For example, an amorphous TazOs film is formed on the base Fit made of a TP single crystal a-plate. Next, a resist pattern having a shape corresponding to the shape of the optical waveguide is formed on this TazOs film by lithography, and using this resist pattern as a mask, the Ta2O film is etched by, for example, reactive ion etching (RTE). As a result, a channel optical waveguide 2 made of Ta205 film is formed.

According to this embodiment configured as described above, Cherenkov radiation 5F (G) is transmitted to one end of the channel optical waveguide 2 of the device through an optical system (not shown) at a wavelength of 865 as shown in FIG.
When the TE mode laser beam a of nm was incident, the wavelength was 4.
32.5 (-865/2)n, m SMC light intensity is approximately 0
．． It occurred at a Cerenkov radiation angle of 6° (center value).

When this SMC beam was focused with a microscope objective lens having a numerical aperture of NA=0.80, a circular spot of about 0.6 μm was obtained. Note that in FIG. 2, C indicates light traveling within the channel optical waveguide 2.

As mentioned above, the central value of the Cerenkov radiation angle in this case is approximately Q, 6°, but its spread is also I.

Exists to a weak degree. On the other hand, the lateral divergence angle at the SMC tip is also 1°
It is only weak.

In this situation, the far field pattern after 5ING is as shown in FIG. For comparison, FIG. 3 also shows a far field pattern (-dotted chain line) when the Cherenkov radiation angle is about 166. From FIG. 3, it can be seen that the far field pattern of the SMC light generated by the Cerenkov synchrotron radiation SHG device according to this embodiment has a small size and approximately circular shape. On the other hand, when the Cerenkov radiation angle is 16°, the far field pattern has a flat crescent shape,
Its lateral spread is extremely large.

As described above, this embodiment has a structure in which a striped channel optical waveguide 2 made of a TazOs film is formed on a substrate 1 made of a KTP single crystal a-plate, and furthermore, the width W of this channel optical waveguide 2 is By optimizing the thickness h etc., the Cherenkov radiation angle can be made extremely small (about 0.6°).This allows the spread of the Cherenkov radiation angle and the lateral divergence angle of the SMC light to be reduced to the Cherenkov radiation angle. The angle, that is, Q, can be made as small as 6°.As a result, the far-field pattern of the SMC light becomes almost circular as shown in Figure 3, and therefore, the SMC light can be focused more easily than before. The optical characteristics can be significantly improved.Therefore, there is no need to use a conical lens to condense SMC light as in the past, and it is possible to sufficiently improve the SMC light to the diffraction limit using a normal lens such as a convex lens. It can focus light.

There is also a report that the lateral divergence angle of the SMC light is uniquely determined by the width W of the channel optical waveguide 2; It can be seen that it varies depending on other parameters such as the thickness h and refractive index of the channel optical waveguide 2, and is not determined only by the width W of the channel optical waveguide 2.

Furthermore, although the above description is for the case where the wavelength of the incident laser beam is 865 nm, by varying the width W and thickness h of the channel optical waveguide 2, the wavelength of the incident laser beam is 865 nm. It is possible to realize Cerenkov radiation angles.

FIG. 4 shows an example of the relationship between θ and the power of the SMC light generated by the Cerenkov radiation SHG device according to this embodiment at a Cerenkov radiation angle θ of about 0.6°. Here, θ. is defined as shown in Figure 6, so to speak, the Cherenkov radiation angle θ is the waveguide! 3. In Fig. 6, the OC direction is SM.
C indicates the traveling direction of the light. Also, for comparison, the L
iNbO3? Proton-exchanged LiNb0. The power and θ of the SMC light generated by the conventional Cherenkov radiation 5HGI arrangement forming the optical waveguide 102. An example of the relationship is shown in FIG.

As shown in FIG. 4, the Cerenkov synchrotron radiation S of this example
The power of the SMC light generated by the HG device at a Cerenkov radiation angle of about 0.6° is θ.

It can be seen that the brightness of the SMC light is concentrated within a small range, and therefore the brightness of the SMC light is extremely high. On the other hand, as shown in FIG. 5, the power of the SMC light generated at a Cerenkov radiation angle of about 16° by a conventional Cerenkov synchrotron radiation SHG device is large θ.

Therefore, it can be seen that the brightness of the SMC light is extremely low compared to this example.

On the other hand, the substrate l consisting of KTP single crystal furnace plate and Taz05
For example, the wavelength 840 of the channel optical waveguide 2 made of a film
The refractive indexes n, , n, for light of nm are approximately 1.8, respectively.
4,2.18, and it can be seen that the relationship nf>ns is satisfied. In this case, the refractive index difference Δn”nr n
s = 2.18-1, . 84=0.34, which is an extremely large value. Therefore, the light confinement effect is large,
High SHG efficiency can be obtained. Incidentally, in the case of a conventional Cerenkov synchrotron radiation SHG device in which a proton exchange LiNbO3 optical waveguide 102 is formed on a substrate 101 made of a LiNb0:+ single crystal Z plate shown in FIG.
A group consisting of Nb0□ single crystal Z plate @101 and proton-exchanged LiNb0. Optical waveguide 'jI1102 wavelength 840nm
The refractive indexes n, , n, for light are respectively 2,175.
2.315, and the refractive index difference is Δn=2.315-2.
175=0.14, which is small. Therefore, in this case, the light confinement effect is small and the SHG efficiency is low.

Note that the TazOs film constituting the channel optical waveguide 2 may be doped with, for example, TiO□;
When doped with iO□, the refractive index difference Δn can be further increased, and the SHO efficiency can be further increased. In this case, when the ratio of the number of Ti atoms to the phase of the number of Ti atoms and the number of Ta atoms in this TiO□-doped TazOs film is defined as Ti / (Tl + Ta), for example, 0
≦Ti/(Ti+Ta)560%.

Furthermore, in order to further increase the SHO efficiency, a cladding layer may be formed on the Cerenkov synchrotron radiation SHG device 1 (7) + the yambling optical waveguide 2 according to this embodiment. When forming the cladding layer in this way, a sufficiently high SHG efficiency can be obtained by optimizing the refractive index of the cladding layer.

Specifically, this cladding layer is made of (SiOz)+-x (TazOs) formed by sputtering, for example.
) x film etc. can be used. Note that this cladding layer also serves to protect 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 can be made based on the technical idea of the present invention.

For example, it is possible to use a nonlinear optical crystal substrate other than KTP single-crystal plate, and as an optical waveguide, it is possible to use a T
azO=, it is also possible to use something other than the membrane plate.

〔Effect of the invention〕

As described above, according to the present invention, since 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 approximately the same as the Cherenkov radiation angle, the Naerenkov radiation second harmonic light can significantly improve the light focusing characteristics of

[Brief explanation of the drawing]

FIG. 1 shows Cerenkov synchrotron radiation S according to an embodiment of the present invention.
2 is a sectional view along the optical waveguide of a Cerenkov synchrotron radiation SMC device according to an embodiment of the present invention, and FIG. 3 is a perspective view showing a Cerenkov synchrotron radiation SMC device according to an embodiment of the present invention. FIG. 4 is a graph showing an example of the relationship between the power at the SMC destination and θ6 generated by the Cerenkov synchrotron radiation SMC device according to the embodiment of the present invention. Figure 5 is a graph showing an example of the relationship between the power of the SMC light generated by the conventional Cerenkov synchrotron radiation S I (G device and θ), and Figure 6 is an explanatory diagram for explaining the definition of θ. Fig. 7 is a sectional view showing a conventional Cherenkov synchrotron radiation SMC device. Explanation of main symbols in the drawing 1: Substrate made of KTP single crystal furnace plate, 2: Channel optical waveguide made of TazOs film.

Claims (1)

[Scope of Claims] A nonlinear optical crystal substrate; and a dielectric thin film formed on the nonlinear optical crystal substrate that is transparent to at least light with an angular frequency ω and has a refractive index higher than that of the nonlinear optical crystal substrate. When light with an angular frequency ω is incident on one end of the optical waveguide, a second harmonic of Cerenkov radiation with an angular frequency of 2ω is emitted at a Cerenkov radiation angle of 1° or less, and the Cherenkov radiation is An optical waveguide device characterized in that an angular spread and a divergence angle of the Cherenkov radiation second harmonic light in the width direction of the optical waveguide are approximately the same as the Cherenkov radiation angle.