JP2676743B2 - Waveguide type wavelength conversion element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a wavelength conversion element for a semiconductor laser, which enables realization of a coherent short-wavelength small-sized light source. (Conventional technology and its problems) A wavelength conversion element, especially a second harmonic generation (SHG) element, is extremely important industrially as a device for obtaining coherent short-wavelength light that is difficult to obtain with an excimer laser or the like. 2. Description of the Related Art Semiconductor lasers are used in various optical communication devices and optical information devices as light sources that oscillate small, high-output coherent light. Currently, the wavelength of light obtained from this semiconductor laser is a wavelength 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 of shorter wavelengths such as red, green, and blue is required, but with the current technology, this kind of semiconductor laser is suddenly realized. difficult. Therefore, if a wavelength conversion element that can efficiently perform wavelength conversion even with an output of a semiconductor laser can be realized, its effect will be great. In recent years, semiconductor laser manufacturing techniques have been developed, and higher output characteristics have been obtained than ever before. Therefore, by constructing an optical waveguide type SHG element, it is possible to avoid a decrease in energy density due to diffraction of light, and it is possible to realize a wavelength conversion element having a relatively high conversion efficiency even with the light intensity of a semiconductor laser. As such an example, a method of forming an optical waveguide in a lithium niobate crystal, transmitting near-infrared light through the optical waveguide, and obtaining a second harmonic emitted from the crystal substrate to the crystal substrate (Chierenkov radiation) There are SHG elements (JP-A-60-14222, JP-A-61-94031). This type of SHG element has the characteristic that precise temperature adjustment is not required because the phase matching condition between the fundamental wave and the SHG wave is automatically taken, while the SHG output is substrate radiated light, so the wavefront is Light with a peculiar, tight aberration, and an intensity distribution like "thin eyebrows" emerges from the end face of the substrate. Therefore, converting this light into a normal, easy-to-use beam with a Gaussian intensity distribution requires a sophisticated lens to correct this aberration. Such inconvenience does not occur if the SHG output light is channel guided light as is the case with the light emitted from the semiconductor laser. Therefore, an object of the present invention is to eliminate the drawbacks of the above-described conventional waveguide type SHG element, and to provide a waveguide type wavelength conversion element having a structure in which SHG output light becomes channel waveguide light. (Means for Solving the Problems) According to the present invention, the first and second two-channel optical waveguides are formed on the C plate lithium niobate crystal plane so as to overlap each other. The channel waveguide is a single-mode optical waveguide for the fundamental wave light, the second channel waveguide is a single-mode optical waveguide for the second harmonic light, and cut for the fundamental wave light. And a frequency of the fundamental wave of the first channel optical waveguide is ω, a wave number of the first channel optical waveguide with respect to the fundamental wave is β (ω), and a second harmonic wave number of the fundamental wave. Is β (2ω), β (2ω) -2β (ω)
= 2π / Λ, the refractive index change period Λ is provided in the light transmission direction of the first and second channel optical waveguides so that the SHG output light is a waveguide type without wavefront aberration. A wavelength conversion element is obtained. (Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective view showing the structure of a waveguide type wavelength conversion element according to an embodiment of the present invention, and FIG. 2 is a longitudinal sectional view of the embodiment shown in FIG. 1 is a LiNbO ₃ crystal substrate, and the substrate orientation is a z-plate (that is, the normal line standing on the substrate is the z-axis). In the present embodiment, two optical waveguides 2 and 3 formed at the same place on the crystal substrate 1 and a dielectric 4 provided on these waveguides and giving a periodic change of the guided light equivalent refractive index are used. It is configured. The optical waveguides 2 and 3 have the following structure. TM which is incident light and has an electric field component perpendicular to the C-plane of the crystal substrate 1
The optical waveguide 2 that holds the fundamental wave 5, which is a wave, is formed by thermally diffusing metallic titanium (Ti), which is a commonly used method, and its thickness is, for example, 0.83 μm as a fundamental wave.
When the m semiconductor laser light is used, it is provided to have a thickness of about 3 μm and serves as a single mode waveguide. The waveguide 3 which is the SHG light of the fundamental wave, holds the guided light TM wave of the same knitting wave as the fundamental wave, and emits it as the emitted light 6 is formed by being overlapped with the waveguide 2. The thickness is set to about 0.43 μm by the exchange of protons (H ⁺ ) with phosphoric acid or the like. This waveguide is a single-mode waveguide for 0.45 μm wavelength light, which is SHG light for 0.83 μm fundamental wave, and is in a cut-off state for 0.83 μm fundamental wave light. is there. The equivalent refractive index n (ω) for the fundamental wave propagating in the Ti diffusion waveguide 2 having a wavelength of 0.83 μm is 2.177, and the equivalent refractive index for the H ⁺ exchange waveguide 3 wavelength of 0.415 μm SHG light in which the SHG light should propagate. n (2ω) is 2.387. Therefore, 2
Since there is a difference in the phase constant (β) of the two waves, the conversion from the fundamental wave to SHG light does not occur in this state. Now, β (2ω) −2β (ω) = 2π / Λ, that is, if there is a period of change of the refractive index of the period Λμm that satisfies the relationship of n (2ω) −n (ω) = 0.415 / Λ, it is efficient. SHG conversion is performed. Dielectric 4 provided on the waveguides of two optical waveguides 2 and 3 formed at the same place on the crystal substrate
Plays this role. For the equivalent refractive index values above,
This period Λ is about 2 μm, and can be easily formed by using a normal Linguelie technique. FIG. 3 is a vertical sectional view showing a modification of the embodiment shown in FIG. If the waveguide thickness or crystal refractive index fluctuates or changes with temperature, the equivalent refractive index of the waveguide changes, the above equation is not satisfied, and the SHG conversion becomes extremely unstable. In order to avoid this, as shown in FIG. 3, by gradually changing the period of the dielectric provided on the waveguide in the light transmitting direction, fluctuations in the transmission refractive index and temperature changes are absorbed. It is possible to realize stable SHG conversion. (Effect of the Invention) As described above, according to the present invention, it is possible to obtain a stable waveguide type wavelength conversion element in which SHG output light has no wavefront aberration.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing the structure of a waveguide type wavelength conversion element according to an embodiment of the present invention, FIG. 2 is a longitudinal sectional view of the embodiment shown in FIG. 1, and FIG. FIG. 1 is a vertical sectional view showing a modified example of the embodiment of FIG. 1.

Claims (1)

(57) [Claims] The C-plate is composed of two first and second channel optical waveguides formed on the crystal plane of lithium niobate so as to overlap each other, and the first channel waveguide is a single-mode optical waveguide for the fundamental wave light. The second channel waveguide is a single-mode optical waveguide for the second harmonic light and has a structure that is cut off for the fundamental wave light. The frequency of the fundamental wave is ω, the wave number of the first channel optical waveguide with respect to the fundamental wave is β (ω), and the wave number of the second harmonic of the fundamental wave is β (2
ω), the period Λ of the refractive index change is provided in the light transmission directions of the first and second channel optical waveguides so that the relationship of β (2ω) -2β (ω) = 2π / Λ is substantially satisfied. A waveguide type wavelength conversion element characterized by the above. 2. The first channel waveguide is an optical waveguide formed by thermal diffusion of metallic titanium, and the second channel waveguide is an optical waveguide formed by proton exchange. The waveguide type wavelength conversion element according to item 1.