JP2640452B2 - Optical wavelength conversion element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to an incident fundamental wave having a second half wavelength.
The present invention relates to a method for manufacturing an optical wavelength conversion element for converting to a harmonic, and more particularly to a method for manufacturing an optical waveguide type optical wavelength conversion element.

(Prior Art) Conventionally, various attempts have been made to convert the wavelength of laser light (to shorten the wavelength) by using the second harmonic generation by a nonlinear optical material. As an optical wavelength conversion element for performing wavelength conversion in this manner, specifically, for example, Japanese Patent Application Laid-Open No. Sho 51-2651
The bulk crystal type and the optical waveguide type as disclosed in, for example, JP-A-60-14222 are well known. When comparing a bulk crystal type optical wavelength conversion element and an optical waveguide type optical wavelength conversion element, the latter can confine light in a smaller area than the former, so that the power density can be increased, and therefore, Higher wavelength conversion efficiency can be obtained.

Therefore, recently, research on an optical waveguide type optical wavelength conversion element has been actively conducted. This optical waveguide type optical wavelength conversion element comprises a pair of substrates arranged so as to face each other, and an optical waveguide formed of a nonlinear optical material having a higher refractive index than the substrates and formed between these substrates. It is composed of In this optical wavelength conversion element, for example, phase matching is performed between the waveguide mode of the fundamental wave in the optical waveguide and the radiation mode of the second harmonic to the substrate (so-called Cherenkov radiation).

(Problems to be Solved by the Invention) However, the conventional optical waveguide type optical wavelength conversion element has a problem that it is difficult to input a fundamental wave and to output (extract) a second harmonic. That is, conventionally, in many cases, a coupler prism is closely attached to the surface of the substrate, and the fundamental wave is made to enter the optical wavelength conversion element depending on the coupler prism. -When the prism was strongly pressed against the optical wavelength conversion element (directly on the substrate), the force sometimes destroyed the element. The second harmonic is emitted from the end face of the optical wavelength conversion element. However, the phase of the extracted light beam varies,
Therefore, it becomes impossible to narrow the light beam to a desired beam diameter.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical wavelength conversion element that can easily enter the fundamental wave and emit the second harmonic.

(Means for Solving the Problems) An optical wavelength conversion element according to the present invention comprises a pair of substrates and an optical waveguide as described above, and a second harmonic in which an incident fundamental wave is phase-matched by Cherenkov radiation. In a light wavelength conversion element for converting into a wave, at least one of the substrates is provided with a diffraction grating for entering a fundamental wave on a surface in contact with the optical waveguide, and a diffraction grating for emitting a second harmonic is provided on a surface opposite to the surface. A grid is provided.

As the nonlinear optical material forming the optical waveguide described above,
It is preferable to use an organic nonlinear optical material having an extremely large nonlinear optical constant as compared with an inorganic material such as LiNbO ₃ or KDP (KH ₂ PO ₄ ). Examples of the organic nonlinear optical material include MNA (2-methyl-4-nitroaniline), mNA (metanitroaniline), and POM (3) described in JP-A-60-250334.
-Methyl-4-nitropyridine-1-oxide), urea and the like. For example, MNA has a wavelength conversion efficiency about 2000 times higher than that of LiNbO ₃ which is an inorganic nonlinear optical material. By generating the second harmonic using infrared laser light from a low-cost semiconductor laser as a fundamental wave, it becomes possible to obtain short-wavelength laser light in the blue region.

(Operation) If a diffraction grating is provided on the surface of the substrate in contact with the optical waveguide, the fundamental wave can be easily incident on the optical waveguide by irradiating the diffraction grating with the fundamental wave. On the other hand, if a diffraction grating is provided on the surface of the substrate opposite to the above-mentioned surface (ie, the outer surface of the substrate), the second harmonic that travels by repeating total reflection between the outer surface of the substrate and the outer surface of the substrate can be obtained. After the phases are aligned by the diffraction grating, it can be easily taken out of the light wavelength conversion element.

Outgoing beam (second harmonic) whose phase is thus aligned
Can be narrowed down to a desired beam diameter using an external lens, or if the diffraction grating for emitting the second harmonic is used as a condensing grating, the emitted beam can be focused and extracted. It becomes possible.

(Examples) Hereinafter, the present invention will be described in detail based on examples shown in the drawings.

FIG. 1 shows an optical wavelength conversion element 15 according to an embodiment of the present invention. As shown, this light wavelength conversion element
Reference numeral 15 denotes a pair of glass substrates 5a and 5b arranged so as to face each other with a small gap therebetween, and an optical waveguide 9 'formed between these substrates 5a and 5b. The optical waveguide 9 'is made of, for example, the above-mentioned MNA which is one of the organic nonlinear optical materials. Optical waveguide 9 'of one substrate 5b
A linear diffraction grating (Linear Grating Coupler; hereinafter, referred to as LGC) 7 for incidence of a fundamental wave is formed on the surface on the side in contact with.
Focusing Grating Coupler (hereinafter referred to as FGC) 8 for emitting the second harmonic at a sufficient distance from C7
Are formed.

Hereinafter, a method for producing the optical wavelength conversion element 15 having the above configuration will be described with reference to FIGS. First, second
As shown in the figure, two flat glass substrates 5a and 5b having a lower refractive index than the above-mentioned MNA are prepared. On one surface of one glass substrate 5a, a number of parallel grooves 6a are formed by a known photolithography technique. These grooves 6a are formed, for example, with a spacing of about 0.5 mm and a depth of about 5 μm, and are about one example with respect to the longitudinal direction A of the glass substrate 5a (this is a drawing direction from a furnace described later).
It is formed at an angle of 50 °. The other glass substrate 5b
A groove 6b similar to the above-mentioned groove 6a is also formed on one surface of the substrate.
These grooves 6b are formed at the same intervals and at the same angle as the grooves 6a so as to align with the grooves 6a when the glass substrates 5a and 5b are fixed to face each other as described later. On the other glass substrate 5b, an LGC 7 is formed at one end of the surface on which the groove 6b is formed (the lower surface in FIG. 2), and a surface opposite to this surface (the upper surface in FIG. 2). FGC8 is formed on the surface (i.e., the surface) at a sufficient interval from the LGC7. These LGC7 and FGC8 are also formed by a known photolithography technique, and the lattice aperture size is 2 × 2 mm as an example.
Degree.

The two glass substrates 5a and 5b are overlaid so that the surfaces on which the grooves 6a and 6b are formed face inward as shown in FIG. At this time, the directions of the substrates 5a and 5b are strictly aligned so that the grooves 6a and 6b are aligned with each other. After the two glass substrates 5a and 5b are overlapped in this way, they are bonded to each other. Then, between the two glass substrates 5a and 5b,
A minute gap of about 0.5 to 1.0 μm is formed.

Next, the above-mentioned MNA, which is a crystalline organic nonlinear optical material, is filled in the minute gap between the glass substrates 5a and 5b to form an optical waveguide. The filling of the MNA is performed as shown in FIG. That is, the MNA 9 is kept in a molten state in a furnace or the like, and one ends of the glass substrates 5a and 5b are immersed in the molten solution. Then, the MNA 9 in a molten state enters the gap S due to a capillary phenomenon. The temperature of the melt is slightly higher than its melting point (132 ° C.) in order to prevent decomposition of MNA9. Thereafter, when the glass substrates 5a and 5b are rapidly cooled, the MNA 9 that has entered the gap S is solidified.

The optical waveguide element 15 in which the layer of the MNA 9 is formed between the glass substrates 5a and 5b as described above is then placed in a furnace, and the MNA 9
Is remelted. In the present embodiment, as the above furnace,
As shown in FIG. 4, a brass heat bath 12 in which an electric heating means 11 is embedded in a brass block 10 is used. At the center of the brass heat bath 12, an elongated hole 14 having a square cross-sectional shape is provided. An electric current is supplied to the electric heating means 11 through a temperature control circuit 13, so that the temperature in the hole 14 is maintained at a desired temperature. The optical waveguide element 15 is housed in the hole 14.

The brass heat bath in which the optical waveguide element 15 is housed as described above
The temperature of 12 (more precisely, the temperature in the hole 14) is also maintained at about 141 ° C., which is slightly higher than the melting point of MNA9. Incidentally, if, for example, grease is injected into the hole 14 of the brass heat bath 12, heat conduction from the brass heat bath 12 to the optical waveguide element 15 becomes good, and the drawing speed described later is constant, which is preferable. . The hole 14 is open at one end face (the end face on the near side in FIG. 4) of the block 10, but the other side is closed. The closed portion is provided with a small hole (not shown) communicating with the inside of the hole 14, and a wire 16 is inserted into the small hole. This wire 16 is fixed to a rack 17,
The rack 17 has a pinion 19 rotated by a motor 18.
Are engaged. Therefore, when the motor 18 is driven and the pinion 19 is rotated in the direction of arrow B, the wire 16
In the direction to block the optical waveguide element 15 in the hole 14
Push out. The temperature outside the brass heat bath 12 is lower than the melting point of the MNA 9 (for example, about room temperature).

By keeping the optical waveguide element 15 in the brass heat bath 12 as described above, the MNA 9 between the glass substrates 5a and 5b is again in a molten state. Then, from this state, the motor 18 is rotated at a low speed, and the wire 16 is moved as described above. Thereby, the optical waveguide element 15 is gradually drawn out of the brass heat bath 12. The drawing speed of the optical waveguide element 15 is, as an example,
Set to about 1.5 to 2.0 cm / hour. When the optical waveguide element 15 is gradually pulled out of the brass heat bath 12 in this manner, an MNA single crystal grows little by little at the interface between the liquid phase and the solid phase of the MNA 9 (which is naturally located outside the brass heat bath 12). I will do it. Accordingly, in the optical waveguide element 15 drawn out from the brass heat bath 12, an optical waveguide 9 'made of MNA 9 having a single crystal state and a uniform crystal orientation is formed over a long distance (for example, several cm). This MNA single crystal has the grooves 6a, 6b
(Specifically, a region between the grooves 6a and 6a and a region between the grooves 6b and 6b), and the crystal orientation is different for each region. This state can be confirmed by arranging the optical waveguide element 15 between the orthogonal Nicols of the deflection microscope and observing the extinction. In this manner, the MNA in the single crystal state is changed to the optical waveguide 9 '.
The optical waveguide element 15 (optical wavelength conversion element) having the above is obtained.

Hereinafter, the operation of the optical wavelength conversion element 15 of the present embodiment formed as described above will be described with reference to FIG.
The optical wavelength conversion element 15 of this embodiment has an overall length of 1.2 c as an example.
m is formed. Then, a Q-switched YAG laser (wavelength: 1.06 μm) 30 is used as a fundamental wave generating means, and a laser beam (fundamental wave) 32 condensed by an objective lens 31 is applied to the LGC 7 to irradiate the laser beam 32. The light can be incident into the optical waveguide 9 '. The phase matching is Cherenkov radiation that generates a second harmonic as a radiation mode from the nonlinear polarization wave generated by the waveguide mode of the fundamental wave in the optical waveguide 9 ′ to the substrate. This second harmonic 32 '
Is taken out of the element 15 from the FGC 8 formed on the outer surface of the substrate 5b. At this time, the second harmonic wave 32 'is focused on one point after the phase is adjusted by the FGC 8.

As described above, in this optical wavelength conversion element 15,
The incidence of the fundamental wave (laser light 32) and the extraction of the second harmonic 32 'can be performed very easily. And the second harmonic 3
Since the phase of 2 'is taken out of the element after its phase is adjusted by the diffraction grating, it becomes possible to narrow the second harmonic 32' to a desired beam diameter. In the above embodiment, FGC8 is used as the second harmonic emission table diffraction grating. However, even if LGC is used instead, the phase of the second harmonic 32 'is made uniform as described above to form a parallel beam outside the element. If the phases are aligned, the second harmonic 32 'can be focused by an external lens.

In this embodiment, since the grooves 6a and 6b as described above are provided in the glass substrates 5a and 5b, the optical waveguide 9 'is divided into a number of elongated single crystal regions. And LGC
7, because the size of FGC8 is set sufficiently larger than the interval between the grooves 6a and 6a (6b and 6b), and therefore each single crystal region optically couples LGC7 and FGC8, respectively. An optical waveguide having a desired crystal orientation can be arbitrarily selected and used.

In the above embodiment, the grooves 6a and 6b are formed in both of the two glass substrates 5a and 5b. However, even if the grooves are formed in only one of the substrates, the organic nonlinear optical material can be monocrystallized to some extent satisfactorily. The effect which makes it do is obtained. In the case where a groove is provided only on one of the substrates, the groove and the diffraction grating can be easily formed by providing the groove on the substrate on which the diffraction grating is not provided as described above. Note that such grooves 6a and 6b do not need to be particularly provided. However, if provided, the organic nonlinear optical material can be favorably single-crystallized as described above, which is preferable from the viewpoint of increasing the wavelength conversion efficiency.

Further, in the above embodiment, the LGC7 for the fundamental wave incidence and the FGC8 for the second harmonic emission are provided, but one of them is omitted and the fundamental wave is incident from the end face of the substrate 5b instead. Alternatively, the second harmonic may be emitted.

As described above, the organic optical material is used as the nonlinear optical material.
Although the embodiment using the MNA has been described, it is also possible to form the optical waveguide using other organic or inorganic nonlinear optical materials.

(Effects of the Invention) As described in detail above, in the optical wavelength conversion element of the present invention, the incident and emission of the fundamental wave and the emission of the second harmonic are performed by the diffraction grating on the substrate surface. It can be performed reliably. When the second harmonic is emitted from the optical wavelength conversion element as described above,
Since the phase of the second harmonic is adjusted by the diffraction grating, the second harmonic can be sufficiently narrowed down to a desired beam diameter. Therefore, the optical wavelength conversion device of the present invention can be used in a wide range of fields. .

[Brief description of the drawings]

FIG. 1 is a perspective view showing an optical wavelength conversion device according to one embodiment of the present invention, and FIGS. 2, 3 and 4 are perspective views showing steps for manufacturing the optical wavelength conversion device of the above embodiment. 5a, 5b: glass substrate, 6a, 6b: groove in substrate 7: linear diffraction grating, 8: condensing diffraction grating 9: MNA, 9 ': optical waveguide 15: optical waveguide element (light Wavelength conversion element) 32: fundamental wave, 32 ': second harmonic

Claims (4)

(57) [Claims] A pair of substrates disposed so as to face each other;
An optical waveguide formed of a non-linear optical material formed between these substrates, wherein the optical wavelength conversion element converts an incident fundamental wave into a second harmonic phase-matched by Cherenkov radiation. At least one is provided with a fundamental wave incident diffraction grating formed on a surface in contact with the optical waveguide and / or a second harmonic emission diffraction grating formed on a surface opposite to the surface. An optical wavelength conversion element characterized by the above-mentioned. 2. The optical wavelength conversion device according to claim 1, wherein said optical waveguide is made of a crystalline organic nonlinear optical material. 3. An optical waveguide comprising a plurality of optical material single crystals arranged in parallel in a width direction of the two diffraction gratings, and the two diffraction gratings are optically coupled by each of the optical material single crystals. 3. The optical wavelength conversion element according to claim 2, wherein 4. The diffraction grating according to claim 1, wherein said diffraction grating for emitting the second harmonic is a light-gathering diffraction grating.
Item 4. The optical wavelength conversion element according to any one of Items 3 to 3.