JPH0445430A - Waveguide type wavelength converting element - Google Patents

Abstract

PURPOSE:To obtain the stable, high-productivity element which has no spherical aberration to SHG output light and high tolerance to design, manufacture, and temperature variation by structuring the element so that both fundamental wave light and secondary higher harmonic generating(SHG) output light become channel waveguide light. CONSTITUTION:A linear channel type waveguide 2 where light is transmitted in a (z)-axis direction is formed on the (x) (or (y)) surface of a crystal material which has a constant dxy (x, y=1,2, or 3, and xnot equal to y) contributing to the secondary nonlinear optical effect between orthogonal polarized light beams, a dielectric film 3 which has a lower refractive index than the channel type waveguide is provided on the channel type waveguide, and a metal film 4 is provided which varying in the covering width of the channel waveguide on the dielectric film in the light transmitting direction. The equivalent refractive indexes of the fundamental wave and secondary higher harmonic cross the propagation direction slantingly and the energy of the wave motion of one wave is converted into the other as the wave is propagated. Even if there is variation in equivalent refractive index due to variation in ambient temperature or a setting error due to a manufacture error to some extent, the conversion efficiency does not vary, so the element is rich in stability and productivity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a waveguide-type wavelength conversion element that makes it possible to realize a coherent short-wavelength compact light source.

[Conventional technology]

Wavelength conversion elements, especially second harmonic generation (SHG) elements, are extremely important in industry as devices that obtain coherent short wavelength light that is difficult to obtain with excimer lasers or the like.

Semiconductor lasers (LDs) are used in various optical communication devices and optical information devices as compact light sources that oscillate high-power coherent light. Currently, the wavelength of light obtained from this semiconductor laser is in the near-infrared region of 0.78 μm to 1.55 μm. In order to apply this semiconductor laser to a wider range of devices such as displays, light with shorter wavelengths such as red, green, and blue is required, but with current technology, it is difficult to quickly realize this type of semiconductor laser. If a wavelength conversion element that can efficiently convert wavelengths even with difficult input power as low as the output of a semiconductor laser could be realized, the effect would be enormous.

In recent years, semiconductor laser manufacturing technology has developed, and it has become possible to obtain higher output characteristics than ever before. By configuring a guided waveguide type SHG element, it is expected to increase the power density by confining the fundamental wave in the waveguide and avoid reducing the energy density due to light diffraction. , it is possible to realize a wavelength conversion element with a relatively high conversion rate. As such an example,
SH is a method in which an optical waveguide is formed in a lithium niobate crystal, near-infrared light is transmitted through the leading waveguide, and second harmonics are obtained from this light which is radiated into the crystal substrate (Cherenkov radiation).
There is an invention of a G element (Japanese Patent Laid-Open No. 60-14222, Japanese Patent Laid-Open No. 61-94031).

This type of SHG element has the advantage that it does not require precise temperature control because the phase matching condition (matching of phase velocity) between the fundamental wave and the SHG wave is automatically established.
Since the HG output is substrate radiation, light with a unique wavefront, severe aberrations, and an intensity distribution resembling a "thin eyebrow" is emitted from the end surface of the substrate. Therefore, converting this light into a normal, easy-to-use beam with a Gaussian intensity distribution requires a high-grade lens that corrects for this aberration. In addition, in this configuration, since the propagation direction of the SHG wave is angularly different from the fundamental wave, the conversion efficiency is simply proportional to the square of the length of the element, whereas in the case of the same direction, the conversion efficiency is proportional to the square of the length of the element. It is only proportional to the first power of , and the characteristic that the diffraction loss of the guided light is small is not utilized.

If the SHG wave and the fundamental wave are guided in the same channel, such a problem will not occur.

The phase matching condition, that is, the degree of matching of phase velocities is 10
Requires accuracy of -5 or less. The phase velocity of guided light, that is, the equipolarized refractive index, is affected not only by temperature changes in wavelength and refractive index, but also by changes in wavelength and refractive index.
It is greatly influenced by the refractive index, thickness, and width of the waveguide.
Even if various waveguide manufacturing techniques are used, it is difficult to achieve the above conditions with good reproducibility and productivity, and no industrial product has yet appeared in the world.

[Problem to be solved by the invention]

The purpose of the present invention is to eliminate the drawbacks of the conventional waveguide-type SHG element described above, and to provide a structure in which both the fundamental wave light and the 5H (J) light become channel-guided light, thereby achieving highly productive waveguide-type wavelength conversion. The purpose is to provide devices.

[Means to solve the problem]

The light transmission direction is 2 on the A straight channel-shaped waveguide in the axial direction is formed, a dielectric film having a refractive index lower than that of the waveguide is provided on the channel-shaped waveguide, and a dielectric film having a refractive index lower than that of the waveguide is provided. By providing a metal film with the width covering the channel waveguide changed in the light transmission direction, a waveguide-type wavelength conversion element that can stably generate second harmonics without adjustment with polarized light orthogonal to the fundamental wave can be obtained. It will be done.

〔Example〕

Hereinafter, the present invention will be described in detail based on examples and with reference to the drawings.

FIG. 1 is a diagram showing the structure of an embodiment of the waveguide type wavelength conversion element of the present invention. 1 is a crystal having a second-order nonlinear optical effect, for example, a β-BaB204 (beta barium oxalate crystal, commonly known as BBO) crystal plate, and the substrate orientation is the X plate (that is, the normal line to the substrate is the X axis). It is. 2 is BBO
It is a strip-shaped loaded waveguide made of, for example, MgO (magnesium oxide), which has a slightly higher refractive index than the BBO crystal formed on the X-plane of the crystal 1, and its light transmission direction is taken in the y-axis direction of the crystal. The surface of the BBO crystal having the MgO-loaded channel waveguide 2 is coated with a dielectric material having a refractive index lower than that of MgO, such as a Si○2 film 3 in this example, using a film forming method such as CVD or sputtering. It is covered completely. Furthermore, 5i
A metal film 4 made of gold or the like is provided on the 02 film 3. The metal film 4 is not formed on the entire surface of the S i 02 MB but on a portion thereof, and is set so that the width of a certain portion of the metal film gradually changes in the light transmission direction on the channel waveguide 2 .

A fundamental wave having a wavelength of 0.83 μm is made to enter from one end of the TMMB2 waveguide. As the fundamental wave 5 is confined near the boundary between the MgO channel waveguide 2 and the BBO crystal 1 and propagates, the second-order nonlinear optical constant d3 of the BBO crystal 1
1, the second harmonic of the TE wave whose polarization is orthogonal to the fundamental wave 5 is excited. Like the fundamental wave, this second harmonic also becomes a waveguide mode confined near the boundary between the channel waveguide 2 and the BBO crystal 1, propagates, and exits the crystal as an output light 6.

The BBO crystal has optical uniaxial anisotropy of noons. In Figure 1, assuming light propagation in a state where there is no S i 02 and metal film and only an MgO channel waveguide is placed on the BBO crystal, the fundamental wave propagation mode with respect to the MgO thickness, Figure 2 shows the change in the equipolarized refractive index of harmonic mode waveguide (the fundamental wave is set at a wavelength of 0.83
(μm semiconductor laser light). When the thickness t of MgO is set to be thinner than 0.19 μm, the fundamental wave (TM(ω)
) is the equipolarized refractive index of the second harmonic (TE (
2ω)) becomes larger. The thickness t of MgO is 0.18μ
5i02 and a metal film are provided thereon as explained in connection with the structure of FIG. 1 above.

As is well known, when a metal film is brought close to the top of the waveguide via a cladding layer, the equipolarized refractive index of the 7M mode decreases significantly, while the TE mode hardly changes (this phenomenon can be explained theoretically). A paper that has been experimentally and experimentally investigated is ``Mode cut-off type metal clad optical strip line structure mode filter'' by Masataka Ito et al., published in the Transactions of the Institute of Electronics and Communication Engineers C, paper number 1982-141).

As shown in the structure of the embodiment shown in FIG. As shown in Figure 3, the change in the equipolarized refractive index of each TE mode in the light transmission direction is such that the transmission refractive index of the fundamental wave 7M mode is located above the channel waveguide as the light travels. While it gradually decreases as the area of the metal film increases, the second harmonic TE mode remains almost constant, and the two equipolarized refractive indices intersect somewhere along the waveguide.

As is also well known, the phase constant (equipolarized refractive index) of the two coupled wave channel waveguides (in this case, the fundamental 7M mode and the second harmonic TE mode) is in the propagation direction. If they intersect diagonally, and the inclination angle is appropriate, the energy of one wave will be transferred to the other as it propagates, even if the length is not strictly determined, as long as the length of the channel waveguide is greater than or equal to the length of the channel waveguide. (theoretically 100%) (M, G, F, Wilson and G.

A paper co-authored by Mr. Qi “Wedge-shaped optical directional coupler (Tap
ered 0ptical Directiona!
Coupler ) JI E E E Transa
ction on Microwave Theory
andTechniques Volume 23, No. 1, 85
(pages 94, published January 1975)
That is, in the present embodiment shown in FIG. 1, if the thickness of the S i 02 film 3 and the slope and curve of the contour line on the channel waveguide of the metal film 4 are determined appropriately, the basic Complete conversion of waves to second harmonics is achieved. Furthermore, the intersection of equipolarized refractive indices occurs somewhere in the light transmission direction. Therefore, even if there is a change in the equipolar refractive index due to fluctuations in ambient temperature or a setting error due to manufacturing error, the conversion efficiency does not fluctuate, resulting in high stability and high productivity.

Such conditions are not limited to the BBO mentioned in the above embodiment as a nonlinear optical crystal. Other nonlinear optical crystal materials, such as LiB505 (lithium
triborate; lithium borate) crystals and LiN
b O3, (lithium niobate; lithium niobate) crystal, KNbO, (bothium niobate; potassium niobate) crystal, K T' i 0 P O4 (
It is also possible to use crystals such as botium titanyl phosphate.

In the case of these crystals, the second harmonic is in the 0.4 μm band (
The fundamental wave is in the wavelength range of semiconductor laser light (~0°8μm band))
Then, since n, > n, , n, it is sufficient to use two plates so that the two polarized light modes become TM.

Furthermore, organic nonlinear crystals, whose materials have been developed in recent years, can also be used in the same way.

In addition, we have described the case of loading a dielectric as a method for forming a waveguide, but thermal diffusion of atoms with a large atomic radius such as Ti, proton exchange method, and external diffusion method, which are well known for lithium niobate crystals, It is also possible to use waveguide formation techniques such as the following.

〔Effect of the invention〕

As explained above, according to the present invention, both the fundamental wave light and the SHG output light are channel guided light, and
Therefore, it is possible to obtain a waveguide type wavelength conversion element that has no wavefront aberration in the SHG output light, has high design tolerance, manufacturing tolerance, and tolerance to changes in ambient temperature, and is stable and highly productive.

[Brief explanation of drawings]

FIG. 1 is a perspective view illustrating the structure of a waveguide type wavelength conversion element according to an embodiment of the present invention, and FIGS. 2 and 3 are fundamental wave, secondary wave, and FIG. 3 is a diagram illustrating the behavior of the transmission refractive index of harmonics. ...B B○ crystal, 2...MgO, 3...Si○2 film, 4...metal film.

Claims (1)

[Claims] The light transmission direction is z in the x-plane (or y-plane) of a crystal material having a constant d_x_y (x, y=1, 2, 3, x≠y) that is involved in the second-order nonlinear optical effect between orthogonal polarizations. A straight channel-shaped waveguide in the axial direction is formed, a dielectric film having a refractive index lower than that of the waveguide is provided on the channel-shaped waveguide, and a dielectric film having a refractive index lower than that of the waveguide is provided. A waveguide-type wavelength conversion element characterized in that a metal film is provided with a width covering the channel waveguide varying in the light transmission direction.