JPH0451124A - Waveguide type wavelength converting element - Google Patents

Abstract

PURPOSE:To allow the conversion with high efficiency in conversion to the same polarized light by providing a film of dielectrics having refractive index lower than the refractive index of a channel-type waveguide formed on the (z) face of a specific crystalline material on this waveguide and further, providing the film exhibiting a film having a specific conductive characteristic on this dielectric film at the width covering the channel waveguide varied in the optically equiv. direction. CONSTITUTION:The light transmission direction of the strip-shaped waveguide 2 provided on the surface of the (z) face of the crystal having a quadratic nonlinear optical effect, for example, an LiNbO3 (LN) crystal 1 is taken in the y-axis direction of the crystal. The surface of the LN crystal having the proton exchange channel waveguide 2 is put over the entire surface of an SiO2 film 3 having the refractive index lower than the refractive index of the LN crystal. Further, a thin film 4 consisting of ZnO (zinc oxide) is provided on the SiO2 film 3. The width of the thin film 4 is so set as to vary gradually in the light transmission direction on the channel waveguide 2. The waveguide type wavelength converting element having high productivity is obtd. stably with the same polarization for both of the basic wave light and second harmonic generation (SHG) output light.

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 small light sources that oscillate high-power coherent light with high efficiency.Currently, the wavelength of light obtained from this semiconductor laser is 0.78 μm. The wavelength is in the near-infrared region of ~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. Therefore, by configuring a guided waveguide type SHG element, it is expected that the power density will increase by confining the fundamental wave in the waveguide and the decrease in energy density due to light diffraction can be avoided. , it is possible to realize a wavelength conversion element with relatively high conversion efficiency. As an example of this, an optical waveguide is formed in a lithium niobate crystal, near-infrared light is transmitted through this optical waveguide, and then emitted (
S of the method to obtain the second harmonic caused by Cerenkov radiation
There is an invention of the HG element (Japanese Patent Application Laid-Open No. 60-14222, Japanese Patent Application Laid-Open No. 61-94031), and this type of SHG element has a phase matching condition (matching of phase velocity) between the fundamental wave and the second harmonic.
is automatically taken, so there is no need for precise temperature control.However, since the second harmonic output is substrate radiation, the wavefront is unique and has severe aberrations, resembling "thin eyebrows". Light with a uniform intensity distribution emerges from the edge 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, the propagation direction of the second harmonic is angularly different from that of the fundamental wave, and when the exchange efficiency is in the same direction, it is compared to the square of the length of the element, but it is simply Since L is only proportional to the first power, the characteristics of the waveguide type SHG element, such as being able to increase the element length and increasing the conversion efficiency, are not utilized. If the second harmonic and the fundamental wave are guided in the same channel, such a problem will not occur.

[Problem to be solved by the invention]

In addition to designing a highly efficient waveguide-type wavelength conversion element so that both the second harmonic and the fundamental wave are guided in the same channel, the requirements for designing a highly efficient waveguide-type wavelength conversion element are to design the maximum nonlinear optical constant of the material used. The goal is to make sure that they are used. The nonlinear optical constant is a so-called tensor quantity that differs depending on the propagation direction and polarization direction of light. In many nonlinear materials, including organic crystals, the polarization relationship between the fundamental wave and the second harmonic is the same, for example, d,3 (especially inorganic crystals), dos (especially organic crystals), and d33 are the largest. However, it has been difficult to satisfy the phase matching condition using these constants. In other words, the fundamental wave and the second harmonic are both extraordinary lights, or both are ordinary lights, and the condition that the refractive index (phase velocity) is equal, which is a phase matching condition, is because the refractive index generally has wavelength dispersion.
It is impossible to satisfy this requirement.

The degree of failure of the phase matching condition, ie the phase velocity (equivalent refractive index), requires an accuracy of 10@-5 or less. The equivalent refractive index of guided light is affected not only by temperature changes in wavelength and refractive index, but also by the refractive index, thickness, and width of the waveguide, and even if various waveguide manufacturing techniques are used, reproducibility and It is difficult to achieve the above conditions with good productivity, and waveguide-type wavelength conversion elements have not yet appeared in the world as industrial products, even if they utilize conversion to orthogonal polarized light.

An object of the present invention is to provide a configuration in which both the fundamental wave light and the SHG output light become channel guided light, which eliminates the drawbacks of the conventional waveguide type SHG element described above, and moreover achieves high conversion efficiency by converting to the same polarized light. It is an object of the present invention to provide a waveguide-type wavelength conversion element having a structure that enables the above-mentioned functions, and has a structure with higher productivity.

[Means to solve the problem]

The present invention relates to a crystalline material with a constant dzz (z=1.2.3) that is involved in second-order nonlinear optical effects between the same polarized light.
A straight channel-shaped waveguide is formed on the surface, a dielectric film having a refractive index lower than that of the waveguide is provided on the channel-shaped waveguide, and further on the dielectric film. A waveguide-type wavelength conversion element is provided with a film that does not exhibit electrical conductivity for the fundamental wave but exhibits electrical conductivity for the second harmonic, the width of which covers the channel waveguide is varied in the optical equivalent direction. It is.

〔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 LiNbO3 and lithium oxide crystal (commonly known as LN) crystal plate, and the substrate orientation is two plates (that is, the normal line to the substrate is two axes). Reference numeral 2 denotes a strip-shaped waveguide formed on two surfaces of the LN crystal 1 by, for example, a protane exchange method, and the light transmission direction thereof is taken in the y-axis direction of the crystal. The surface of the LN crystal having the proton exchange channel waveguide 2 is covered with a dielectric material having a refractive index lower than that of the LN crystal, in this example, a 5i02 film 3 using a film forming method such as CVD or sputtering. be. Furthermore, the S
A thin film 4 of Zn0 (zinc oxide) is formed on the i02 film 3.
is the provision.

This thin film 4 is not formed over the entire surface of the 5i02 film 3, but only partially, and is set so that the width of the film gradually changes in the light transmission direction above the channel waveguide 2, as shown in the figure.

A TM wave 5 having a wavelength of 0.83 μm is input as a fundamental wave from one end of the waveguide. As the incident fundamental wave 5 is confined near the channel waveguide 2 and advances, it becomes a second harmonic of the TM wave with the same polarization as the fundamental wave 5 via the second-order nonlinear optical constant d3s of the LN crystal 1. Like the fundamental wave, this second harmonic, which is excited, becomes a waveguide mode confined near the channel waveguide 2, propagates, becomes an output light 6, and is output from the LN crystal.

In order to efficiently convert the fundamental wave of the 7M mode into the second harmonic of the 7M mode, a phase matching condition (the phase constant of each mode, that is, the equivalent refraction ratio is the same) must be satisfied. The wavelength dispersion of the refractive index of the LiNbO3 crystal is as shown in FIG.
The equipolarized refractive index of the 7M mode of the plate is the extraordinary refractive index of the crystal n8
It is slightly larger (on the order of 0,003>), and its wavelength characteristics are almost the same as the behavior of ne. Therefore, simply building a waveguide on a crystal will result in the same behavior due to this refractive index dispersion. In the 7M mode, the fundamental wave and the second harmonic do not have the same polarized refractive index, so in order to realize the conversion, some kind of device is needed to make the equal polarized refractive index match. What constitutes this is a ZnO film 4 provided on the waveguide via a dielectric layer.

ZnO, G with an absorption edge around 0.5 to 0.6 μm
Semiconductors such as aP and Se generally exhibit conductive characteristics at short wavelengths of around 0.4 μm or less, whereas they exhibit mere dielectric characteristics at longer wavelengths.

On the other hand, as is well known, when a conductor is placed close to the waveguide through a low refractive index dielectric layer (cladding layer), the equipolarized refractive index of the 7M mode decreases significantly. On the contrary, TE
The mode hardly changes (papers that examine this phenomenon both theoretically and experimentally include the Institute of Electronics and Communication Engineers Transactions C, paper number 1982-141, and Masataka Ito et al.'s ``Mode cut-off metal clad optical strip line ), therefore, if the ZnO film 4 is on the waveguide 2 via the cladding layer (SiOz) 3, then 0.8
For the fundamental wave 7M mode with a wavelength of 3 μm, its equipolarized refractive index hardly changes, whereas at 0.415 μm
The equipolarized refractive index of the second harmonic TM mode of is significantly reduced (~1O-2) due to the conductive effect of this film. The magnitude of the decrease is determined by the thickness of the cladding layer interposed between the waveguide and the conductor. Therefore, if the thickness is appropriate, the fundamental wave 7M
In principle, it is possible to match the equipolarized refractive index of the mode with that of the second harmonic. However, setting the degree of coincidence to 1O-5 or less is close to a miracle and has little feasibility, as stated in the column ``Problem J to be Solved by the Invention.''

As shown in the structure of the embodiment shown in FIG. The change in the equipolarized refractive index of the wave 7M mode and the second harmonic TM mode in the light transmission direction is shown in Figure 3. The equipolarized refractive index of the second harmonic TM mode is In contrast, the second harmonic TE mode is almost constant, and there are two equal parts somewhere along the waveguide, whereas it gradually decreases as the area of the metal film located on the channel waveguide increases in the advancing direction. The polarized refractive indexes intersect.

As is also well known, the phase constant (equal polarized refractive index) of two uncoupled wave channel waveguides (in this case, the fundamental wave TM mode and the second harmonic TE mode) is If the angle of inclination is appropriate, the energy of one wave will be absorbed 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. converted (theoretically 100%) to the other (M, G, F, Mr. Wilson and G, A.

A paper co-authored in Qi magazine, “Wedge-shaped light directional coupling issue r Taper
ed 0ptical Directional Co
upler)” I E E E Transaction
n on Microwave Theory and
Techniques Vol. 23, No. 1, pp. 85-94, published January 1975. ).

That is, in this embodiment shown in FIG. 1, if the thickness of the 5i02 film 3 and the slope and curve of the contour line on the channel waveguide of Zn0M4 are appropriately determined, the second harmonic can be adjusted from the fundamental wave. Complete conversion to waves is achieved. Furthermore, the intersection of equipolarized refractive indices occurs somewhere in the light transmission direction. For this reason,
Even if there is a change in the equipolarized refractive index due to fluctuations in ambient temperature or a setting error due to manufacturing error, the conversion efficiency does not change, resulting in high stability and high productivity.

Such conditions are not limited to the LiNbO5 crystal used in the above embodiment as a nonlinear optical crystal. Other nonlinear optical crystal materials, such as KNbO3 (potassium
niobate/potassium niobate) crystal, K Ti
It is also possible to use OP 04 (botanium titanyl phosphate) crystal or the like.

Furthermore, organic nonlinear crystals with extremely large d-eye constants, which have been developed as materials in recent years, can also be used.

Furthermore, the photoconductor used is not limited to ZnO, and the various semiconductor materials described above can be used. Other organic photoconductors can also be used. Furthermore, in this example, the case where the fundamental wave is assumed to be in the 0.8 μm band has been described, but in the case of other wavelengths, a semiconductor material having an absorption edge between the fundamental wavelength and the second harmonic wavelength may be used. Bye.

In addition, as a method for forming a waveguide, the case of ion exchange method, which is typified by protons, was described, but the well-known T
Methods such as thermal diffusion of atoms with a large atomic radius such as i, out-diffusion of Li ions, or in the case of crystals for which such methods cannot be used, other waveguide formation methods such as loading a dielectric material can be used. Technology can also be used.

〔Effect of the invention〕

As explained above, according to the present invention, the fundamental wave light can also be
Both G output lights have the same polarization and are channel guided lights, so they are highly efficient, and the SHG output light has no wavefront aberration, and has excellent design tolerance, manufacturing tolerance, and tolerance to ambient temperature changes. A waveguide type wavelength conversion element that is expensive, stable, and highly productive can be obtained.

FIG. 3 is a diagram showing the dispersion characteristics of the bulk refractive index of the L i N b O3 crystal, and is a diagram illustrating the behavior of the equipolarized refractive index of the fundamental wave and second harmonic to provide a detailed explanation of the present invention. be.

DESCRIPTION OF SYMBOLS 1...LiNbO5 crystal, 2...Channel waveguide, 3.-3i02 film, 4.-ZnO3l.

Claims (1)

[Claims] Constant d_ involved in the second-order nonlinear optical effect between the same polarized light
In the z-plane of the crystal material with z_z (z=1, 2, 3),
A straight channel-shaped waveguide is formed, a dielectric film having a refractive index lower than that of the waveguide is provided on the channel-shaped waveguide, and further on the dielectric film, The invention is characterized in that a film that behaves as a dielectric material that exhibits no electrical conductivity for fundamental waves and exhibits electrical conductivity for secondary harmonics is provided with a width that covers the channel waveguide changing in the light transmission direction. Waveguide type wavelength conversion element.