JPH01257922A - Waveguide type wavelength converting element - Google Patents

Abstract

PURPOSE:To eliminate the wave front aberration of a secondary higher harmonic by providing a phase matching grating which almost satisfies a specific relation between the phase constant of a 1st channel optical waveguide to a fundamental wave and the phase constant of a 2nd channel optical waveguide to the 2nd higher harmonic of the fundamental wave. CONSTITUTION:When the phase constant of a Ti diffusion optical waveguide 3 to the fundamental wave is denoted as beta<(omega)> and the phase constant of an H<+> exchange optical waveguide 4 to the secondary higher harmonic is denoted as beta<(2omega)>, efficient wavelength conversion is performed on condition that beta<(2omega)>-2beta<(omega)>=2pi/LAMBDA, i.e. n<(2omega)>-n<(omega)>=0.415/LAMBDA. The phase matching grating which is a dielectric provided on two optical waveguides 3 and 4 formed at the same position on a crystal substrate 1 serves for said function. At the time of said equivalent refractive index, this period LAMBDA is about 2mum and the phase matching grating 5 can be formed by using a normal SiO2 film forming method and lithography technique.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a wavelength conversion element for a semiconductor laser, which makes it possible to realize a coherent short-wavelength compact light source.

[Conventional technology]

Wavelength conversion elements, particularly 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 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-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. difficult. Therefore, if a wavelength conversion element capable of efficiently converting wavelength even with 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 constructing an optical waveguide-type SHO element, it is possible to avoid a decrease in energy density due to light diffraction, and it is possible to realize a wavelength conversion element with relatively high conversion efficiency even with a light intensity comparable to that of a semiconductor laser. . An example of such a method is to form an optical waveguide in a lithium niobate crystal, transmit near-infrared light through the optical waveguide, and obtain second harmonics that are radiated into the crystal substrate (Cherenkov radiation). There is an invention of an SHG element (Japanese Patent Laid-Open No. 14222/1983).

JP-A-61-94031).

[Problem to be solved by the invention]

The above-mentioned SHG element has the advantage that it does not require precise temperature control because the phase matching conditions between the fundamental wave and the second harmonic are automatically established, but the second harmonic output is Because it is substrate radiation, the wavefront is unique and has severe aberrations.
Light with an intensity distribution similar to that of "thin eyebrows" comes out 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. However, if the second harmonic output light is also channel guided light like the light emitted from the semiconductor laser, such inconvenience will not occur.

An object of the present invention is to eliminate the drawbacks of the conventional waveguide type SHG element described above and to provide a waveguide type wavelength conversion element having a structure in which the second harmonic output light becomes channel guided light.

[Means to solve the problem]

The waveguide type wavelength conversion element of the present invention proceeds in a direction substantially perpendicular to the Z-axis on the X-plate or Y-plate lithium niobate crystal plane, and has two parallel side walls, and an identical space between these two side walls. two channel optical waveguides, first and second, formed at a location and overlapping each other; a phase constant β (ω) for the fundamental wave (frequency ω) of the first channel optical waveguide; The first and second channel optical waveguides are arranged so that the phase constant β(2ω) for the second harmonic of the fundamental wave approximately satisfies the relationship β(2ω)−2β(ω)=2π/Δ. It has a phase matching grating that provides a period A of change in refractive index in the light transmission direction.

〔Example〕

The present invention will be described in detail below based on embodiments and 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 sectional view thereof. I is a LiNbos crystal substrate, and the substrate orientation is the X plate (that is, the normal line to the substrate is the X axis). On this crystal plane, side walls 2a+2b extending in a direction substantially orthogonal to the Z axis are provided, and between these side walls, along these side walls, a first channel optical waveguide 3 is formed on the lithium niobate crystal plane.
is formed. A second channel optical waveguide 4 is also provided at the same position on the same crystal plane on which the first channel optical waveguide 3 is formed. The first channel optical waveguide 3 has T on the lithium niobate crystal plane.
The second channel optical waveguide 4 is a Ti diffusion channel optical waveguide formed by diffusing i, and the second channel optical waveguide 4 is an H+ exchange channel optical waveguide formed by an ion exchange method.

The output light of the semiconductor laser 6 is coupled to the first channel optical waveguide 3 as an incident fundamental wave (frequency ω) 7. The second channel optical waveguide 4 guides the second harmonic 8 converted from the fundamental wave 7 . The waveguide second harmonic 8 emitted from the waveguide end face is converted into circular collimated light 10 by a circular lens 9 .

The two optical waveguides 3 and 4 and the phase matching grating 5 have the following structure. The equivalent refractive index n of the Ti diffused optical waveguide 3 for propagating the fundamental wave 7, which is the incident light from the semiconductor laser 6 and is a TE wave having an electric field component parallel to the Z direction of the crystal substrate 1, with respect to the fundamental wave with a wavelength of 0.83 μm.
(ω) is 2.177, and H
The equivalent refractive index n + zω) of the °-exchanged optical waveguide 4 for the second harmonic having a wavelength of 0.47.5 μm is 2.387, and there is a difference in the phase constant (β) of the two waves. Therefore, no conversion from the fundamental wave to the second harmonic will occur if the current state remains as it is.

The phase constant for the fundamental wave of the Ti diffused optical waveguide 3 is β1ω
), and the phase constant for the second harmonic of the exchanged optical waveguide 4 is β32ω), now β12ω〉−2β1
If there is a period of change in the refractive index with a period Δμm that satisfies the relationship ω)-2π/8, that is, ntzω)-n(ω' = 0.415/A, efficient wavelength conversion can be performed.Crystal Substrate 1 The phase matching grating 5, which is a dielectric material provided on the waveguides of the two optical waveguides 3.4 formed at the same location above, plays this role.In the case of the above equivalent refractive index value, this period Δ is 2 μm
Therefore, the phase matching grating 5 can be formed using a normal Sin film formation method or lithography technique.

The side walls 2a and 2b in the embodiment of FIG. 1 are provided for the following reasons. Due to the optical anisotropy characteristic of the lithium niobate crystal, when the wavefront propagation direction of the volume plane wave is rotated from the Y direction to the Z direction, the refractive index increases. 16 from Y axis
When the deflection is approximately 17 degrees, the refractive index of the volume plane wave is n! 1L
ILI[(2ω) satisfies the same relationship as the above equation. That is, n IIULK (2ω)−n(ω
'=0.415/Δ holds true. This means that the second harmonic is also radiated in a direction deviated by about 16 to 17 degrees from the Y axis. If it were a planar structure without two side walls, this condition would be +2. -2 Exists in both directions, and the second harmonic is converted not only into a wave that propagates through the waveguide 4 but also into a volume wave, and the second harmonic is aimed at the waveguide 8.
The conversion efficiency to is significantly reduced. According to the present invention, since the side walls 2a and 'lb are provided, the waveguide second harmonic 8
The conversion to is efficiently achieved.

On the other hand, if there are fluctuations or temperature changes in the waveguide thickness or crystal refractive index, the equivalent refractive index of the waveguide will change, the above equation will no longer be satisfied, and wavelength conversion will become extremely unstable. To avoid this, by gradually changing the period of the dielectric provided on the waveguide in the direction of light transmission, it is possible to absorb fluctuations in the equivalent refractive index and temperature changes and achieve stable wavelength conversion. I can do it. In the figure, the maximum period is shown as Δl, and the minimum period is shown as Δ.

According to the waveguide type wavelength conversion element having the above configuration, the fundamental wave 7 incident from the semiconductor laser 6 is converted into a second wave having no wavefront aberration.
It is converted into harmonics and output, and converted into circular collimated light 10 by a circular lens 9.

The channel optical waveguide 4 is formed by a single process such as an ion exchange method, and the intensity distribution of the second harmonic emitted from the crystal end face in the vertical direction of the waveguide becomes asymmetrical, which is converted by the circular lens 9. When the collimated light 10 is separated from a Gaussian circular beam in shape, it is also possible to make the emitted light intensity distribution symmetrical by making the channel optical waveguide 4 into a buried structure. This is achieved by using a known technique of additionally diffusing atoms that lower the refractive index, such as magnesium, by thermal diffusion or the like after the above process.

〔Effect of the invention〕

As described above, according to the present invention, a stable waveguide type wavelength conversion element without wavefront aberration in the second harmonic can be obtained.

Further, according to the present invention, since an XFi or Y plate is used as the crystal substrate and no Z plate is used, the semiconductor laser can be directly connected to the end face of the waveguide, and unlike the case of the Z plate, the semiconductor laser can be connected directly to the end face of the waveguide. There is no need to use a plate or tilt the semiconductor laser chip by 90 degrees, which is extremely convenient for mounting.

[Brief explanation of the drawing]

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 FIG. 2 is a sectional view thereof. 1...LiNb0+crystal substrate 2a, 'lb...Side wall 3...First channel optical waveguide 4...Second channel optical waveguide 5...Phase matching grating 6...Semiconductor laser 7...・Fundamental wave 8...Second harmonic 9...Circular lens 10...Circular collimated light Agent Patent attorney Yoshiyuki Iwasa - (to)

Claims (1)

[Claims] (1) Two parallel side walls running in a direction almost perpendicular to the Z axis on the X-plate or Y-plate lithium niobate crystal plane, and a second side wall formed at the same location between these two side walls and overlapping each other. two channel optical waveguides, a first channel optical waveguide and a second channel optical waveguide, a phase constant β^(^ω^) for the fundamental wave (frequency ω) of the first channel optical waveguide, and a second Phase constant β^(^2
Between β^(^2^ω^)-2β^(^ω
A waveguide-type wavelength conversion element having a phase matching grating that provides a period of refractive index change Λ in the light transmission direction of the first and second channel optical waveguides so as to substantially satisfy the relationship: ^) = 2π/Λ.