JPH02189527A - Waveguide type wavelength converting element - Google Patents

Abstract

PURPOSE:To eliminate the wave front aberrations of output light and to stabilize the output light by providing a plane waveguide having periodic gratings on either one of left and right crystal planes between which the channel light guide progressing in the direction nearly orthogonal with the Z-axis on the crystal plane of lithium niobate is inserted. CONSTITUTION:An X-plate is used as the crystal substrate 1 of the lithium niobate (LiNbO3) and the channel light guide 2 progressing in the direction nearly orthogonal with the Z-axis on the crystal plane thereof is formed. The plane waveguide 3 having the periodic gratings 4 of an equivalent refractive index is provided on either one of the crystal planes between which the channel light guide 2 is inserted. The output light subjected to the wavelength conversion to have the wavelength different from the wavelength of a basic wave is, therefore, obtd. as the plane guided light from the plane waveguide 3 having the periodic gratings of the equivalent refractive index by injecting the basic wave into the channel light guide 2. The output light is taken out as the light having no wave front aberrations in this way and the need for using a high-grade lens for correcting the aberrations is eliminated. The direct connection to a semiconductor laser 10 is possible.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a waveguide-type wavelength conversion element, and in particular, a waveguide-type wavelength conversion device suitable for a semiconductor laser that enables the realization of a coherent short-wavelength compact light source. Regarding elements.

[Conventional technology]

Wavelength conversion elements, especially second harmonic generation (S i (G:
5econd Harmonic Generation
on) elements are extremely important industrially as devices that obtain coherent short wavelength light that is difficult to obtain with excimer lasers and the like.

The usefulness of the wavelength conversion element is that when the light source used is a semiconductor laser, it is possible to construct a light source that can extract a desired short wavelength light by wavelength conversion even if a semiconductor laser of an existing wavelength is used.

That is, semiconductor lasers are generally used as small, high-output coherent laser light sources in various optical communication devices and optical information devices, but currently, the wavelength of light obtained from semiconductor lasers is 0.78 μm ~ The wavelength is 1.55 μm in the near-infrared region.

In order to apply such semiconductor lasers to a wider range of devices such as displays, there is a need for semiconductor lasers that can emit light of shorter wavelengths such as red, green, and blue. Semiconductor laser, namely 1. It is difficult to suddenly create a semiconductor laser that can produce short wavelength light.

Therefore, if a wavelength conversion element, especially a wavelength conversion element that can efficiently convert wavelength even with the output of a semiconductor laser, could be realized, the effect would be enormous.

Therefore, research and development of wavelength conversion elements for semiconductor lasers is progressing, and in recent years, with the development of semiconductor laser manufacturing technology, it has become possible to obtain products with higher output characteristics than before. Waveguide type SHG as a conversion element
elements are attracting attention.

That is, by configuring an optical waveguide type SHG 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.

Conventionally, as an example of a waveguide-type wavelength conversion element from this point of view, an optical waveguide is formed in a lithium niobate (LiNb03) crystal, near-infrared light is transmitted through this optical waveguide, and then radiated into the crystal substrate. (Cherenkov radiation) second
Techniques for SHG elements that obtain harmonics have been proposed (Japanese Patent Laid-Open No. 14222-14222, Japanese Patent Laid-open No. 61-94
Publication No. 031).

[Problem to be solved by the invention]

However, in the SHG element using this method, there is still room for improvement in terms of wavefront aberrations as described below.

In other words, in the conventional SHG element described above, the phase matching condition between the fundamental wave and the SHG wave is automatically established.
Although it has the advantage of not requiring precise temperature control, 5R
(Since the G output is substrate radiation, light with a unique wavefront, severe aberrations, and an intensity distribution similar to "thin eyebrows" comes out from the edge of the substrate. Therefore, this light has a Gaussian intensity distribution. converting it into a normal, usable beam requires a high-grade lens that corrects this aberration.

An object of the present invention is to provide a waveguide-type wavelength conversion element that can eliminate the drawbacks of the conventional waveguide-type wavelength conversion element as described above and has no wavefront aberration in output light.

[Means to solve the problem]

The waveguide-type wavelength conversion element of the present invention has a channel optical waveguide propagating in a direction substantially perpendicular to the Z axis on an X temporary or Y plate lithium niobate crystal plane, and a channel optical waveguide propagating in a direction substantially perpendicular to the Z axis on an X temporary or Y plate lithium niobate crystal plane; It is characterized by having a planar waveguide having a periodic grating with an equal refractive index formed on either the left or right crystal plane with the channel optical waveguide in between.

[Effect]

In the present invention, if a fundamental wave is injected into a channel optical waveguide that travels in a direction substantially perpendicular to the Z axis on an X-plate or Y-plate lithium niobate crystal plane, it is possible to Wavelength-converted output light having a different wavelength from the fundamental wave is obtained as planar waveguide light.

This output light is extracted as having no wavefront aberration, so there is no need to use a high-grade lens for aberration correction as in the conventional case. Further, the use of an X plate or a Y plate enables direct connection with a semiconductor laser even when a light source is configured in combination with a semiconductor laser.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a diagram showing the structure of a waveguide type wavelength conversion element which is an embodiment of the present invention.

In this example, lithium niobate (L i N b O
z) An X plate is used as the crystal substrate, and a configuration is adopted in which two optical waveguides, a channel optical waveguide and a planar waveguide having a periodic grating with an equal refractive index, are formed therein.

That is, in FIG. 1, reference numeral 1 indicates LiN b
It is an Ox crystal plate, and the substrate orientation is the X plate (that is, the normal line to the substrate is the X axis). A channel optical waveguide 2 is formed on this crystal plane, which extends in a direction substantially perpendicular to the Z axis (therefore, in the Y direction in this embodiment). Then, a planar waveguide 3 having a periodic grating 4 with an equal refractive index is provided on either one of the crystal planes sandwiching the channel optical waveguide 2 on the same crystal plane on which the channel optical waveguide 2 is formed. There is.

A semiconductor laser 10 as a light source is disposed on one end surface side of the channel optical waveguide 2, and a cylindrical lens 11 in the illustrated example is disposed on the output end surface side of the planar waveguide 3.

In the wavelength conversion element shown in FIG.
The output light from 0 is wavelength-converted by inputting it into the channel optical waveguide 2, and is taken out as planar waveguide light through the planar waveguide 3, which is then given to the cylindrical lens 1 as the SHG output light.

When obtaining the SHG output light, that is, the second harmonic with a wavelength of 1/2, it can be extracted as plane waveguide light, so it is not necessary to use a high-grade lens to reduce aberrations (m) as in the conventional method. Therefore, it is possible to construct a stable waveguide type wavelength conversion element in which the SHG output light has no wavefront aberration.

As mentioned above, with conventional systems, the SHG output produces light with a unique wavefront and severe aberrations, so a high-grade lens for aberration correction is required to convert it into an easy-to-use beam with a Gaussian intensity distribution. On the other hand, if the SHO output light is channel waveguided light like the output light of the semiconductor laser used, or even if not, if it is plane waveguided light that is guided in at least one dimension, A circular lens or a cylindrical lens like this example (cylindrical lens 11) can be used to easily generate parallel beams,
That is, it can be converted into collimated SHG light 12, and the inconveniences of the conventional method do not occur. Therefore, it is possible to eliminate the drawbacks of the conventional waveguide type SHG element mentioned above.
A waveguide type wavelength conversion element having a structure in which SHG output light becomes planar waveguide light can be obtained.

Furthermore, in the configuration shown in FIG. 1, unlike the conventional example described above, as mentioned above, the X plate is used as the crystal plate used, and the Z plate is not used, so the semiconductor laser 10 is directly attached to the end face of the waveguide. Can be connected. Specifically, the output light of the semiconductor laser can be coupled directly or via a lens, via a half-wave plate as in the case of using a Z plate, or by tilting the semiconductor laser chip used by 90 degrees. There's no need.

In this way, the waveguide type wavelength conversion element according to the present invention has x
A channel optical waveguide 2 traveling in a direction almost perpendicular to the Z axis on a plate or Y plate lithium niobate crystal plane, and either the left or right side of the channel optical waveguide 2 on the same crystal plane that formed the channel optical waveguide 2. It consists of a planar waveguide 3 having a periodic grating 4 of equal refractive index formed on one crystal plane, and with this configuration, a semiconductor laser IO is used as a light source and SMC light is obtained by wavelength conversion. In this case, the fundamental wave (frequency ω) is applied to the channel optical waveguide 2.
The SHG output light is A waveguide type wavelength conversion element without wavefront aberration can be obtained.

The waveguide type wavelength conversion element shown in FIG. 1 will be described in more detail below, including numerical examples.

In FIG. 1, the semiconductor laser 10 and the wavelength conversion element are
The semiconductor laser 10 is a semiconductor laser that oscillates at a wavelength of 0.83 μm, for example, and its output light is transmitted through a channel optical waveguide 20.
Connected directly or through a lens from one end surface. As this light travels through the channel optical waveguide 2, SHG light with a wavelength of 0.415 μm is generated via the nonlinear optical constant ariff of the crystal. This SMC light is radiated into the planar waveguide 3 as planar waveguide light. The light exiting the planar waveguide 3 is converted into a circular parallel collimated beam, ie, collimated SHG light 12, by a cylindrical lens 11.

The above two optical waveguides have the following structures in more detail.

The channel optical waveguide 2 that holds the fundamental wave of the TE wave, which is incident light and has an electric field component parallel to the Z axis on the X plane of the crystal substrate 1, is made of metal titanium (Ti), which is a commonly used method.
The thickness is set so as to form a single mode waveguide for semiconductor laser light having a fundamental wave wavelength of 0.83 μm. The waveguide 3 that holds the SHG planar waveguide light, which is a TE wave with the same polarization as the fundamental wave, is formed on the same crystal plane as the waveguide 2.゛) Provided by the exchange method. This waveguide 3 is a single mode waveguide for light with a wavelength of 0.415 μm, which is SMC light, and has a roughly cutoff for light of 0°83 μm, which is the fundamental wave. Its thickness is set as follows. Furthermore, in order to form a period of equal polarized refractive index, here, a grating 4 made of, for example, a film of Sin is provided on the surface of the planar waveguide 3.

Phase matching between the fundamental wave and the SMC light is achieved as follows, and efficient conversion can also be performed as described below.

First, a Ti diffusion channel optical waveguide 2 through which the fundamental wave propagates.
The equipolarized refractive index n-1 for a fundamental wave with a wavelength of 0.83 μm is about 2.17, and the direction is the same as that of the fundamental wave when the planar waveguide 3 does not have a periodic grating with an equipolarized refractive index (i.e., the Y direction).
The equipolarized refractive index (nHd) for a TE wave with a wavelength of 0.415 pm propagating to is about 2.32.

Therefore, since there is a difference in the phase constants of the two waves, conversion from the fundamental wave to SMC light does not occur in the same propagation direction.

However, when the wavefront propagation direction of the SMC light is rotated from the Y direction to the Z direction, the equipolarized refractive index increases.

For example, with a variation of about 20 to 25 degrees from the Y axis and an equipolarized refractive index n(2) for the 0.415 pm TE wave of a planar waveguide.
is equal to the equipolarized refractive index with respect to the fundamental wave propagating in the Y direction in the channel optical waveguide 2, so in principle, the SMC light should be emitted in this direction. However, the fundamental wave is a wave that is close to a plane wave that propagates in the Y direction, and the SMC light is a wave that is close to a plane wave that propagates in the vibration direction by about 20 to 25 degrees from the Y axis. Since the wavefront directions of the fundamental wave and the SMC light differ greatly by 20 to 25 degrees, the conversion efficiency is extremely low.

Therefore, in order to increase conversion efficiency, the fundamental wave and S
It is desirable that the propagation direction differs by a slight angle β from the MC light. Therefore, now β(mal) 2β(W)
2 2π/Δ, that is, n(2t+J) −nllall z 9.415 /
The period of change in refractive index with a period Δμm that satisfies the relationship of Δ is
A small angle (0,5
If they are provided with a difference of about 1 degree, efficient conversion from the fundamental wave of the channel light to the SMC light of the planar waveguide light will be performed.

Specifically, regarding the formation of the periodic grating 4, in the case of the above equipolarized refractive index value, the period Δ is about 2 μm,
This pattern can be formed using normal lithography technology.

Furthermore, if there are fluctuations or temperature changes in the thickness or crystal refractive index of the waveguide 3, the equipolarized refractive index of the waveguide 3 will change, the above equation will no longer be satisfied, and the SHG conversion will become extremely unstable. In order to avoid this, if the period of the dielectric material provided on the waveguide 3, that is, the period of the grating, is gradually changed in the light transmission direction, fluctuations in the equipolarized refractive index and temperature changes can be suppressed. It is also possible to realize stable SHG conversion by absorption.

In the above embodiment, the planar waveguide 3 is formed by a single process such as Ti diffusion method or ion exchange method, and the intensity distribution in the vertical direction of the waveguide at the end of the SMC emitted from the crystal end face is If asymmetry occurs and the collimated SHG light 12 converted by the cylindrical lens 11 is separated from the Gaussian circular beam in shape, the channel optical waveguide 2 may be made into an embedded structure to make the emitted light intensity distribution symmetrical. It is possible.

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.

In addition, in the above embodiment, a case was described in which a dielectric grating was provided on the planar waveguide 3 to form a period with an equal polarized refractive index, but the present invention is not limited to this. It is also possible to periodically change the proton exchange thickness during diffusion.

〔Effect of the invention〕

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

Moreover, unlike the conventional example, an X plate or a Y plate is used, and a Z plate is not used, so when a semiconductor laser is used as a light source, the semiconductor laser can be directly connected to the end face of the waveguide. There is no need to use a half-wave plate or to tilt the semiconductor laser chip by 90 degrees as in the above, and this is extremely convenient for mounting, and is particularly suitable as a wavelength conversion element for semiconductor lasers.

[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. 1...Lithium niobate (LiNb03) crystal plate 2...Ti diffusion channel optical waveguide 3...
Planar waveguide 4... Equal polarized refractive index periodic grating 10... Semiconductor laser 11... Cylindrical lens 12... Collimator S HG optical agent Patent attorney Yoshiyuki Iwasa

Claims (1)

[Claims] (1) A channel optical waveguide that travels in a direction almost perpendicular to the Z axis on an X-plate or Y-plate lithium niobate crystal plane, and the left and right sides of the channel optical waveguide on the same crystal plane that formed this channel optical waveguide. A waveguide-type wavelength conversion element comprising: a planar waveguide having a periodic grating of equivalent refractive index formed on one of the crystal planes.