JPH02189528A - Waveguide type wavelength converting element - Google Patents

Abstract

PURPOSE:To eliminate the spherical aberration of SHG output light by providing a side wall, a channel optical waveguide, and a plane optical waveguide on a crystal thin layer which is formed of bulk type crystal and have nonlinear optical phase matching condition of type 1 and applying the basic wave of ordinary light to the channel optical waveguide. CONSTITUTION:The crystal thin layer 1 which is formed of the bulk type crystal and has the nonlinear optical phase matching condition of type I is provided with the side wall 2 which deviates slightly from an angle satisfying the phase matching condition and extends at an angle where the extraordinary light refrac tive index to second harmonic generating (SHG) light becomes a little bit larger than the ordinary light refractive index to fundamental light, and the channel optical waveguide 3 and plane optical waveguide 4 are provided on the crystal thin layer 1 along the side wall 2. Therefore, the basic wave of the ordinary light is entered into the channel optical waveguide 3 and then the secondary higher harmonic of the basic wave which is the extraordinary light is obtained from the plane optical waveguide 4 to lead out SHG output light 6. Consequent ly, the wave front aberration is small because the SHG output light 6 is waveguide light, and the influence of variance is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a waveguide type wavelength conversion element, and more particularly 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: 5e
cond Harmonic Generatton)
The device is extremely important industrially as a device that can 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.

In other words, semiconductor lasers are generally 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 semiconductor lasers is 0.78μm～1.55
It is a wavelength in the near-infrared region of μm.

In order to apply such semiconductor lasers to a wider range of devices such as displays and medical equipment, there is a need for semiconductor lasers that can emit light of shorter wavelengths such as red, green, and blue, but current technology does not allow for this. It is difficult to suddenly realize this type of semiconductor laser, that is, one that can produce short wavelength light as the semiconductor laser itself.

Therefore, if a wavelength conversion element, especially a wavelength conversion element that can efficiently convert wavelength even at the output level 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 SHG element from such a point of view, an optical waveguide is formed in a lithium niobate (LiNbO:+) crystal, near-infrared light is transmitted through this optical waveguide, and from there it is transmitted into the crystal substrate. An SHG element technology has been proposed that obtains the second harmonics radiated by (Cherenkov radiation).
4222, JP-A-61-94031).

[Problem to be solved by the invention]

However, in the SHO 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, S
Since the HG output is substrate radiation, light with a unique wavefront, severe aberrations, and an intensity distribution resembling "thin eyebrows" 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 this aberration.

The purpose of the present invention is to be able to eliminate the drawbacks of the waveguide-type SHG element as described above, to have less wavefront aberration in the SHG output light, and to reduce optical variations in materials and manufacturing errors. An object of the present invention is to provide a waveguide type wavelength conversion element that can absorb light.

[Means to solve the problem]

The waveguide type wavelength conversion element of the present invention is a bulk crystal and has a type I nonlinear optical phase matching condition. A channel optical waveguide is formed on the thin crystal layer along the side wall, and a channel optical waveguide is formed on the thin crystal layer along the side wall. a planar optical waveguide formed on the same crystal thin layer that formed the optical waveguide, the fundamental wave of an ordinary ray is injected into the channel optical waveguide, and the fundamental wave, which is an extraordinary ray, is injected from the planar optical waveguide. It is characterized by obtaining the second harmonic of the wave.

[Effect]

In the present invention, when the fundamental wave of the ordinary ray is injected into the channel optical waveguide, the second harmonic of the fundamental wave, which is the extraordinary ray, is obtained from the planar optical waveguide, and the SHG output light can be extracted. Since the SHG and output light obtained thereby are guided light, there is little wavefront aberration. Furthermore, if the channel optical waveguide is formed along the sidewall and its direction is selected to absorb optical variations in the material and manufacturing variations, it is possible to eliminate the effects of these variations. It is.

〔Example〕

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

FIG. 1 is a diagram for explaining the structure of a waveguide type wavelength conversion element according to an embodiment of the present invention.

In FIG. 1, reference numeral 1 indicates a nonlinear optical effect with optically uniaxial anisotropy, such as barium oxalate crystal (β-888gO4) (hereinafter abbreviated as β-BBO crystal). The orientation of the crystal layer is the X plate (that is, the normal line to the crystal layer is the X axis). On this crystal layer surface, a side wall 2 is provided which extends in a direction (2,) which is tilted by θ degrees (approximately 22 degrees) from the Z axis in the y axis direction, and along this side wall 2, a channel optical waveguide is formed on the β-BBO crystal layer. 3 is formed. A planar optical waveguide 4 is also provided on the same crystal layer in which the channel optical waveguide 3 is formed.

In FIG. 1, the β-BBO crystal N1 constitutes a thin crystal layer having type I nonlinear optical phase matching condition in a bulk crystal, and the side wall 2 is formed from an angle that satisfies the above phase matching condition. It forms a side wall that is slightly shifted and proceeds in the direction of an angle such that the extraordinary refractive index for the 5IIG ahead is slightly larger than the ordinary refractive index for the fundamental light.

A fundamental wave 5 (frequency ω) of an ordinary ray is injected into the channel optical waveguide 3 from a light source (not shown). Further, in the illustrated example, a cylindrical lens 7 is arranged on the crystal end face side opposite to the light source side.

In the SHG element shown in FIG. 1, the fundamental wave 5 is injected into the channel optical waveguide 3, and the second harmonic of the fundamental wave 5, which is an extraordinary ray, ie, the SHG light 6, is extracted from the planar optical waveguide 4 as guided light. , this is supplied to the cylindrical lens 7 as site output light.

When obtaining the SHG output light, that is, the second harmonic with a wavelength of 1/2, since the SHG output light is guided light,
It is not necessary to use a high-grade lens to correct aberrations as in the past, and a stable waveguide type wavelength conversion element can be constructed in which the SHG output light has no wavefront aberration.

As mentioned above, with the conventional SHG output, light with a unique wavefront and severe aberrations is emitted, 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 SHG output light is also spatially guided in at least one direction by a waveguide having a waveguide structure7, such inconvenience will not occur.

The planar optical waveguide 4 described above is provided as a waveguide that holds the SHG light as a TE planar waveguide light, and therefore, as in this embodiment (using the cylindrical lens 7), it is easy to use a beam that is easy to use, that is, circular. Since it can be converted into a collimated beam and does not require a high-grade lens, it is possible to eliminate the drawbacks of the conventional waveguide type SHG element mentioned above, and the S tl G output light is waveguided light. Therefore, a waveguide type wavelength conversion element with less wavefront aberration can be obtained.

In addition, in the configuration shown in FIG. Even if there are physical variations or variations in manufacturing errors during device fabrication, it is possible to obtain a waveguide type wavelength conversion element with a structure that can absorb these optical variations in materials and variations in manufacturing errors.

As described above, the waveguide type wavelength conversion element according to the present invention has a thin crystal layer having a nonlinear optical phase matching condition of type (III) in a bulk crystal, with an angle slightly shifted from the angle that satisfies the phase matching condition, and SHG A side wall 2 is provided extending in the direction of an angle such that the extraordinary refractive index for light is slightly larger than the ordinary refractive index for the fundamental light, and a channel optical waveguide 3 formed on the thin crystal layer along the side wall 2 and a channel optical waveguide 3 formed on the thin crystal layer are provided. It consists of a planar optical waveguide 4 formed on the same crystal '41111 that formed the waveguide 3. A fundamental wave 5, which is an ordinary ray, is injected into the channel optical waveguide 3, and the fundamental wave, which is an extraordinary ray, is injected from the planar optical waveguide 4. 5 second harmonic is obtained.

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

In FIG. 1, this wavelength conversion element constitutes a coherent short-wavelength compact light source together with a semiconductor laser as a light source described below, and the two optical waveguides have a refractive index on the surface of, for example, a bulk β-BBO crystal. A .beta.-BBO crystal layer doped with ions to increase the amount of ions can be uniformly epitaxially grown by flux property or the like, and can be formed using ordinary lithographic techniques and dry or wet etching techniques.

For the channel optical waveguide 3 that holds the fundamental wave 5 of the TM wave which is incident light and has an electric field component parallel to the X-axis of the crystal N1, the thickness thereof is, for example, a wavelength of 0.8 as the fundamental wave.
It is set to be a single mode waveguide when a 38 m semiconductor laser beam is used, and waveguide 4 is a single mode waveguide for light with a wavelength of 0.415 μm, and the fundamental wave For 0°83 μm light, the thickness is set so as to provide a rough cutoff.

SHG light 6, which is extraordinary light, is emitted from the fundamental wave 5, which is ordinary light, transmitted through the channel optical waveguide 3 via a nonlinear optical constant. The traveling direction of the wave front is the same direction as the fundamental wave 5, but the energy propagation direction is shifted by several degrees from it (this effect is called the walk-off effect).
. The SHG light 6 whose energy propagation direction is shifted is TE
The above-mentioned waveguide 4 is provided as a waveguide for holding planar waveguide light. The planar waveguide SHG light 6 is emitted from the crystal end face with a one-dimensional spread angle in the direction perpendicular to the waveguide, and is converted into a circular collimated beam by the cylindrical lens 7 as described above.

The above conditions would exist in both +y1-y directions across the z-axis if there were no side wall 2 and a -like planar structure, and the conversion efficiency to SHG light 6 would be halved, but in this implementation As in the example, since the side wall 2 is provided, conversion in one direction is efficiently achieved.

In addition, the phase matching between the fundamental wave 5 and the SHG light 6 is satisfied as shown below, and the SHG wave can be excited efficiently (while also absorbing the variations as described above).

That is, first, the direction in which the side wall 2 moves is the bulk β-BB
Conditions for phase matching based on the crystal angle achieved in O crystals (
direction in which the refractive index of the two light waves is equal), i.e. type I
(Fundamental wave is ordinary light, SHG light is extraordinary light) Direction (2,)
It takes a value close to .

However, as is generally well known, the transmission refractive index of a waveguide is slightly different from the bulk refractive index due to shape effects. For this reason, even if the direction of the channel optical waveguide 3 is selected in accordance with the matching angle (2,) realized by the bulk β-BBO crystal, it is difficult to obtain correct matching conditions. Furthermore, since the thickness and width of the waveguide vary during manufacturing, it is difficult to set matching conditions correctly.

Therefore, it is necessary to have a structure that can absorb the above-mentioned variations.

Therefore, in the above embodiment, the channel optical waveguide 3 is formed along the side wall 2, and the direction of the channel optical waveguide 3 is slightly shifted (α) from the bulk matching angle.
The angle (2,) is selected so that the extraordinary refractive index for the SHG light is slightly larger than the ordinary refractive index for the fundamental light.

By doing this, the fundamental wave (its transmission refractive index is Nf) traveling through the channel optical waveguide 3 is converted into an SHG wave (its transmission refractive index is N5) having a wavefront propagation direction in the same direction.
na), the SHG wave has a wavefront propagation direction in a direction tilted by a slight angle β (its transmission refractive index is Ns□
cos β), and SHG waves are efficiently excited in that direction.

(Thus, it is possible to obtain a structure in which the SHO output light does not have spherical aberration, and at the same time, it is possible to obtain a structure that can absorb optical variations in materials or manufacturing errors.

Furthermore, in order to increase redundancy, the following may be done.

That is, the width of the channel optical waveguide 3 may be formed so as to continuously and gradually change in the light transmission direction.

Further, in the above embodiments, the case of a -axial crystal was described, but the same effect is also effective in the case of a bicyclic crystal. Furthermore, although we have described the case of a so-called homoepitaxial layer in which a crystal layer with almost the same composition as the substrate is grown on the substrate,
The same can be achieved even if the substrate and the growth layer are heteroepitaxial layers having different compositions.

〔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 the SHG output light has little wavefront aberration. Further, it is also possible to realize a waveguide type wavelength conversion element having a structure that can absorb optical variations in materials, variations in manufacturing errors, etc.

[Brief explanation of the drawing]

FIG. 1 is a perspective view for explaining the structure of a waveguide type wavelength conversion element according to an embodiment of the present invention. 1... Crystal layer 2... Side wall 3... Channel optical waveguide 4... Planar optical waveguide 5... Fundamental wave 6... SHG light 7・・・・・・Cylindrical lens

Claims (1)

[Claims] (1) A thin crystal layer that is a bulk crystal and has Type I nonlinear optical phase matching conditions is slightly deviated from the angle that satisfies the phase matching conditions, so that the extraordinary refractive index for SHG light is different from that for fundamental light. A side wall extending in the direction of an angle slightly larger than the ordinary refractive index is provided, a channel optical waveguide is formed on the thin crystal layer along the side wall, and a channel optical waveguide is formed on the same thin crystal layer on which the channel optical waveguide is formed. a planar optical waveguide formed in the channel optical waveguide, a fundamental wave of an ordinary ray is injected into the channel optical waveguide, and a second harmonic of the fundamental wave, which is an extraordinary ray, is obtained from the planar optical waveguide. Features of waveguide type wavelength conversion element.