JPH06194703A - Wavelength converting element - Google Patents

Abstract

PURPOSE:To stably convert the output beam of a semiconductor laser to secondary higher harmonics at a high efficiency even in the case where the wavelength is fluctuated and the ambient temperature is changed. CONSTITUTION:In a surface of a non-linear optical crystal 1, which is formed of linear grid spontaneous polarization in the depth direction of a substrate so as to be reversed periodically, a wave-guide 3 is formed, of which equivalent refractive index is gradually changed in the beam transmission direction. Even in the case where the wavelength of the basic wave is fluctuated and the refractive index of the crystal is changed by a change of the ambient temperature, the Cerenkov angle, which satisfies the phase aligning condition of the basic wave and the secondary higher harmonics, always coincides with the main direction of the diffraction of the secondary polarization wave to be excited by the basic wave. A wavelength converting element having a stabilized high efficiency is thereby obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention 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.

[0002]

2. Description of the Related Art A wavelength conversion element, especially a second harmonic generation (SHG) element, is extremely important industrially as a device for obtaining coherent short-wavelength light which is difficult to obtain with an excimer laser or the like.

Semiconductor lasers are used in various optical communication equipments and optical information equipments as a light source that oscillates a compact and high-power coherent light. Currently, the wavelength of light obtained from this semiconductor laser is in the red to near infrared region of 0.63 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 green and blue is required, but it is difficult to realize this kind of semiconductor laser with the current technology. If a wavelength conversion element that can efficiently perform wavelength conversion even at the output of a semiconductor laser can be realized, its effect will be great.

With the recent development of semiconductor laser manufacturing technology,
It has become possible to obtain higher output characteristics than ever before. Therefore, if an optical waveguide type SHG element is constructed,
There is a possibility that the reduction of the energy density due to the diffraction of light can be avoided, and the wavelength conversion element can be realized with a relatively high conversion efficiency even with the light intensity of a semiconductor laser. As an example of such a method, an optical waveguide is formed in a lithium niobate crystal, near-infrared light is transmitted through this optical waveguide, and a second harmonic wave that is radiated into the crystal substrate (Cherenkov radiation) is obtained. There is an invention of the SHG element (JP-A-60-14222, JP-A-61-94031 and JP-A-2-93).
No. 624). The SHG element of this system has a characteristic that a precise temperature control is not necessary because the phase matching condition between the fundamental wave and the SHG wave is automatically obtained. However, on the other hand, there are the following two major drawbacks. The first is the fundamental wave, which is guided light, and S, which is emitted light.
Since the electromagnetic field distribution is significantly different from that of HG light, the conversion efficiency from the fundamental wave to SHG light is low, and the output level of the semiconductor laser (up to about 100 mW) is about 0.2%, which is not practical.

That is, the factors contributing to the efficiency of conversion from the guided fundamental wave to the radiated second harmonic are, of course, the magnitude of the nonlinear optical constant and the phase matching. Matching the electromagnetic field distribution between the wave and the SHG radiation is another important factor. When this is analyzed in detail, the fundamental wave propagating through the waveguide generates a polarized wave having a frequency twice that of the distribution corresponding to the field distribution of the fundamental wave via the nonlinear optical constant. When the direction of the main peak of the diffraction pattern of this polarized wave and the direction matching the phase velocity (refractive index) of the fundamental wave are equal, the polarized wave is cumulatively accumulated in that direction,
Efficient SHG light is emitted.

In the above-mentioned conventional example, the Cherenkov radiation angle is 16.2 when the wavelength of the fundamental wave is 0.84 μm.
It is reported that the degree. On the other hand, the mode distribution of the fundamental wave guided through the waveguide is approximated to a Gaussian distribution with a diameter of about 2 μm, and the distribution of the generated secondary polarization wave is also equal to this.
The diffraction pattern of this polarized wave is approximated by an Airy diffraction pattern, although it is roughly approximated to the maximum in the waveguide traveling direction, and the intensity sharply decreases at an angle in the oblique direction of the substrate depth. The angle at which the intensity of the polarized wave having a diameter of 2 μm is 0 at the beginning of the Airy diffraction image is about 7 degrees, and the Cherenkov radiation angle 16.
The intensity of the polarization wave is extremely small because it is outside of 2 degrees. The inefficiency of the conventional invention described above can be roughly interpreted in this way.

If the direction of the main peak of the polarized wave diffraction pattern and the Cherenkov radiation angle can be made equal, a highly efficient waveguide type wavelength conversion element can be constructed.

The second difficulty of the invention of the conventional Cherenkov radiation type wavelength conversion element is that the variation of the emission angle of the SHG light and the efficiency associated therewith are caused by the ambient temperature and the wavelength variation of the oscillation light of the semiconductor laser which is the fundamental wave. It is a big change. That is, most optical crystals have an absorption edge near the wavelength of blue light. Therefore, the wavelength variation of the refractive index and the temperature variation in this vicinity are large. As is well known, the Cherenkov radiation angle in the above configuration is determined by the ratio cos of the equivalent refractive index of the fundamental wave guided light and the extraordinary refractive index at the SHG light wavelength. The extraordinary refractive index at the SHG light wavelength changes greatly with temperature and wavelength, which changes the emission angle of the SHG, resulting in SHG.
The output fluctuates greatly, which is inconvenient for practical use. In particular, this effect is to try to make the radiation angle shallower than the substrate in order to increase the efficiency, devise the structure of the waveguide, and make the equivalent refractive index for the fundamental wave close to the extraordinary optical refractive index of the substrate for SHG light. If you do, the effect is particularly strong.

Due to the above-mentioned problems, the conventional type wavelength conversion element has not been put to practical use.

[0010]

SUMMARY OF THE INVENTION The present invention has a low conversion efficiency of the above-described conventional waveguide SHG element, and the conversion efficiency is caused by a change in the refractive index caused by a fundamental wavelength variation, ambient temperature change, or the like. It is intended to provide a wavelength conversion element having a feature that the manufacturing conditions are relaxed while eliminating the difficulty that the value fluctuates greatly.

[0011]

Means for Solving the Problems On the surface of a non-linear optical crystal in which spontaneous polarization is periodically inverted in a linear lattice shape, the transmission direction of the fundamental wave light is the lattice vector created by the spontaneous polarization inversion period. Non-linear optics that are almost orthogonal to each other and form an optical waveguide whose equivalent refractive index gradually changes in the light transmission direction, or that spontaneous polarization is periodically inverted in a linear lattice pattern on the crystal surface. An optical waveguide is provided on the surface of the crystal in which the transmission direction of the fundamental wave light is slightly deviated from the orthogonal direction with respect to the lattice vector formed by the spontaneous polarization inversion period, and the equivalent refractive index of which gradually changes in the light transmission direction. An optical waveguide having a constant equivalent refractive index is formed on the surface of a nonlinear optical crystal formed by periodically inverting the spontaneous polarization in a linear lattice shape on the surface of the crystal. Periodic Whether the path of the optical waveguide is curved so that the angle formed by the direction of the lattice vector and the light transmitting direction of the optical waveguide gradually changes along the light transmitting direction of the optical waveguide. , In addition, periodic inversion of spontaneous polarization is formed in a curved lattice on the surface of the nonlinear optical crystal,
By providing a linear optical waveguide having a constant equivalent refractive index on the surface, or by any one of the above-mentioned three means, the guided wavelength conversion with high efficiency and stable efficiency against temperature change, wavelength change, etc. The device is obtained.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments with reference to the drawings.

(Embodiment 1) FIG. 1 is a diagram showing the structure of a waveguide type wavelength conversion element according to a first embodiment of the present invention.
1 is a lithium niobate (LiNbO ₃ ) crystal which is an effective second-order nonlinear optical material, and the substrate orientation is z. In this crystal, the spontaneous polarization inversion period 2 is formed in layers in the depth direction of the substrate by a known method (method of forming the spontaneous domain inversion period in the crystal pulling direction during crystal growth). .

A waveguide for guiding the fundamental wave light is formed on the surface of the crystal substrate 1. In this example, a material (for example, titanium oxide (TiO ₂ )) whose refractive index at the fundamental wavelength is larger than the extraordinary light refractive index (n _e (ω)) at the fundamental wavelength of the substrate LiNbO ₃ crystal is used as the substrate surface. Then, the channel waveguide 3 is thinly loaded in a ridge shape.
A fundamental wave (for example, a wavelength of 0.83 μm) is introduced into the channel waveguide 3.
m), the oscillation light of the semiconductor laser 10 is injected, and the TM fundamental wave guided mode is injected. This fundamental wave light excites a secondary polarization wave having a frequency doubled via the nonlinear optical constant d _{3 3} of the crystal. As the fundamental wave light propagates, the secondary polarized waves radiated one after another interfere with each other in the direction in which the phases of the fundamental wave and the second harmonic wave are matched to each other and propagate in the form of a strengthening beam. As described in the problems of the prior art, if the radiation pattern of the secondary polarized wave is pseudo, but the propagation direction of the secondary higher harmonic whose phase is matched with the fundamental wave, that is, the Cherenkov angle is the same, The conversion efficiency from the fundamental wave to the SHG light 6 is high.

In this embodiment, the crystal substrate has a periodic layer in which the spontaneous polarization is inverted in advance in the depth direction. As shown in FIG. 2, the phase of the secondary polarization wave is inverted by 180 degrees between the layers in which the spontaneous polarization is inverted. The traveling direction θ _B of the composite wave front of the polarization wave (5 in FIG. 2) emitted from each layer becomes the Bragg angle in the periodic field, and Sin θ _B = λ (2ω) / (Λ _a · n (2
ω)) is given. Where λ (2ω) and n (2
ω) is the wavelength of the second harmonic in air and the refractive index there, and Λ _a is the spontaneous polarization inversion period. On the other hand, Cherenkov angle theta _C is given by _{Cosθ C = N (ω) /} n (2ω). Here, N (ω) is the equivalent refractive index of the fundamental wave guided mode (4 in FIG. 2). Spontaneous polarization inversion period Λ _a and fundamental wave guided mode equivalent refractive index N such that θ _B = θ _C.
By setting (ω), high conversion efficiency from the fundamental wave to SHG light is realized.

For example, when the spontaneous polarization inversion period Λ _a confirmed in the literature is set to 3 μm, the Bragg angle θ _B becomes about 3.43 degrees with respect to the above wavelength. The condition for giving the Cherenkov angle θ _C equal to this angle is N (ω) = 0.
998n (2ω) and Ti forming a ridge waveguide
The height of the ridge portion of O ₂ should be set to about 300 nm. These can be easily realized.

Further, as described in the above-mentioned subject, the oscillation wavelength of the semiconductor laser differs between lots, and the wavelength varies by about 3 angstrom / degree depending on the temperature. Unless the wavelength conversion element has the effect of absorbing these wavelength differences and temperature fluctuations, the cost cannot be industrially established. In this embodiment, such effects are embodied as follows.

The shipping use regarding the variation of the oscillation wavelength of a commercially available semiconductor laser is regulated to about ± 10 nm.
When the fundamental wave wavelength varies from 0.83 μm to 0.01 μm, the Bragg angle also varies. In order for this angle to become the Cherenkov angle again, the equivalent refractive index N (ω) of the fundamental wave is 2.
It may be set so as to cover the range of 317 to 2.297.

That is, as shown in FIG. 1, the width of the ridge portion of the ridge type waveguide for guiding the fundamental wave is not uniform in the light transmitting direction, but is provided in a wedge shape so that the equivalent refractive index gradually changes. There is. Since the transmission refractive index becomes smaller as the width of the ridge portion becomes narrower, even if the wavelength or the material refractive index changes, the direction (Bragg angle) in which the secondary polarization wave due to the inversion period of the spontaneous polarization interferes and is strengthened, and the phase Wavelength conversion strongly occurs at the waveguide portion of the ridge width that gives an equivalent refractive index that matches the matching condition (Cherenkov angle). The condition that the two angles always match is satisfied. Therefore, stable wavelength conversion is realized.

(Embodiment 2) FIG. 3 is a view showing the structure of a waveguide type wavelength conversion device according to a second embodiment of the present invention.
Is an effective second-order nonlinear optical material, lithium niobate (LiNbO ₃ ) crystal, and the substrate orientation is the y-plate,
The substrate surface is on the zx plane. This crystal is from Example 1.
Similarly to the above, the spontaneous polarization reversal period 2 is formed in layers in the z direction of the principal surface of the substrate by the known method. On the surface of the crystal substrate 1, a waveguide 3 for guiding the fundamental wave light in a direction slightly swung from the x direction.
Has been formed. The method of forming the waveguide is the same as that of the first embodiment. That is, the refractive index at the fundamental wavelength is
A material (for example, titanium oxide (TiO ₂ )) larger than the extraordinary refractive index (n _e (ω)) of the fundamental wavelength of LiNbO ₃ crystal, which is the substrate, is thinly loaded in a ridge shape on the substrate surface to form a channel waveguide. There are three. Oscillation light 11 of a semiconductor laser having a fundamental wave (for example, a wavelength of 0.83 μm) is injected into the channel waveguide 3 to generate a TE fundamental wave guided mode.

The operating principle of this embodiment is substantially similar to that of the first embodiment, although the relationship between the spontaneous polarization inversion layer period and the azimuth of the waveguide forming surface differs by 90 degrees from that of the first embodiment. However, it is not a completely similar structure. That is, when the lattice vector K _a of the spontaneous polarization inversion period and the fundamental wave vector K (ω) are orthogonal to each other as in the first embodiment, the second harmonic vector K (2ω) satisfying the condition is Exists on both sides of the waveguide, S
Since the HG light is dispersed and output on both sides, it is inefficient and difficult to use. In this embodiment, the light transmission direction of the fundamental wave optical waveguide is shifted from the x axis so that the SHG light is emitted to one side of the waveguide while avoiding this. FIG. 4A shows the matching relationship between wave number vectors. The wave number vector of the fundamental wave is incident with an inclination of θ (ω) from the x-axis. This figure shows a case where the fundamental wave equivalent refractive index (N (ω)) is smaller than the refractive index (n _e (2ω)) of the second harmonic. Two fundamental wave vector K (ω) and second harmonic vector K (2ω)
When one and the other form a triangle via the spontaneous polarization inversion period vector K _a , SHG light is efficiently generated. That is, the matching condition is expressed by the following equation.

K (2ω) Sin θ (2ω) = 2K (ω)
Sinθ (ω) + K _a K (2ω) Cosθ (2ω) = 2K (ω) Cosθ
(Ω) Since the wave number vector of the fundamental wave is inclined by θ (ω) with respect to the x-axis, a closed triangle is not formed depending on K _a facing in the opposite direction. That is, the matching condition is not satisfied, and the SHG light is efficiently emitted only in the direction satisfying the condition. Figure 4
As shown in (b), when the vector of the fundamental wave is taken in the x-axis direction without being tilted, the direction of the second-order harmonic vector (2ω) satisfying the matching condition is symmetrical in two directions with the x-axis interposed. The second harmonic is present and the power of the second harmonic is distributed and the efficiency is not increased.

Further, as in the first embodiment, since the waveguide structure is formed so as to disperse the fundamental wave equivalent refractive index in the light transmitting direction, it can withstand the wavelength fluctuation and the refractive index fluctuation. It is as a structure. For example, as a design example, the pitch Λ _a of the spontaneous polarization inversion grating is set to 5 μm, the angle between the fundamental wave waveguide direction and the spontaneous polarization inversion grating vector K _a is set to 60 degrees, and the fundamental wave equivalent refraction is from 2.257 to 2.25. If set so as to cover the range of 277, 0.8 of the fundamental wavelength will be
This device operates in the range of 3 ± 0.01 μm.

In this embodiment, since the SHG light is radiated into the substrate in the first embodiment, and the thickness of the substrate is required to be larger than usual to some extent, the radiation is in the horizontal plane of the substrate.
It has the feature that the thickness of the substrate is not required.

In the above first and second embodiments, the case where the substrate is cut out from the crystal formed at the time of growing the spontaneous polarization inversion period and used is described. A substrate obtained by a method of forming an inversion domain into an arbitrary shape by a process after forming the substrate from the crystallized crystal may be used. Although the y-plate is used in the description of the above embodiment, in this case, the z-plate is used and both the fundamental wave and the SHG light utilize the deflection (TM wave) having an electric field component perpendicular to the substrate. It will be.

(Embodiment 3) FIG. 5 is a view showing the structure of a waveguide type wavelength conversion device according to a third embodiment of the present invention.
Like in the first and second embodiments, is a lithium niobate (LiNbO ₃ ) crystal which is an effective second-order nonlinear optical material, the substrate orientation is the z plate, and the substrate surface is in the zx plane. In this crystal, the inversion period 2 of the spontaneous polarization is formed in a curved stripe shape on the main surface of the substrate by a known method. On the surface of the crystal substrate 1, a straight ridge waveguide for guiding the fundamental wave light is formed. The method of forming the waveguide is the same as in the first and second embodiments.

The oscillation light 10 of the semiconductor laser is injected into this waveguide to excite the TM substrate wave guided mode 4. The fundamental wave is converted into the second harmonic wave 6. In this embodiment, as a structure for providing a margin (redundancy) of the element with respect to wavelength fluctuation and refractive index fluctuation, the magnitude of the equivalent refractive index of the fundamental wave mode (that is, The length of the wave number vector K (ω) is not dispersed in the light transmission direction, but the angle formed between the spontaneous polarization inversion lattice vector K _{a and} the fundamental wave number vector K (ω) is dispersed. That is, the width of the ridge waveguide 3 is constant in the light transmitting direction, remembering angle in the direction of the grating of the spontaneous polarization, on the size constant spontaneous polarization inversion grating vector kappa _a, the wave vector of the fundamental wave kappa (omega ) Angle is dispersed. As a result, even if the wavelength or the refractive index changes, K (ω), K (2ω) and K _a
A highly efficient wavelength conversion is performed in the vicinity of the curved portion of the waveguide forming the triangle whose vector is closed.

For example, the spontaneous polarization reversal grating has a pitch Λ _a =
When it is configured in a concentric circle shape with a radius of about 5 μm and a radius of about 60 mm, this device operates within the range of 0.83 ± 0.01 μm of the fundamental wave wavelength without fluctuation in conversion efficiency.

(Embodiment 4) In Embodiment 3, an angle is formed in the direction of the lattice of spontaneous polarization to provide a margin (redundancy) of the element with respect to wavelength fluctuation and refractive index fluctuation. As a complementary structure, as shown in FIG. 4, the spontaneous polarization inversion period can be formed in a linear shape so that the locus of the fundamental wave waveguide has a curvature. The same effect as that of the third embodiment can be realized so that it can be easily inferred.

In the above embodiments, a lithium niobate crystal was used as the nonlinear optical crystal. However, similar to this crystal, the nonlinear optical effect is large and lithium tantalate is a crystal capable of spatially reversing the spontaneous polarization. Crystals and K
TP (KTiOPO ₄ ) crystals and the like are known, and it is of course possible to use these. It has been known that the organic non-linear material can periodically reverse the orientation of molecules like the LB film. The effects of the present invention can be realized even by using such materials and film forming methods.

As the material for forming the fundamental wave waveguide and the method for forming the same, many methods which have already proved useful in the integrated optical technology can be used.

[Brief description of drawings]

FIG. 1 is a perspective view showing a structure of a first embodiment of the present invention.

FIG. 2 is a sectional view showing the structure of the first embodiment of the present invention.

FIG. 3 is a perspective view showing a structure of a second embodiment of the present invention.

FIG. 4 is a diagram showing matching conditions of wave number vectors.

FIG. 5 is a perspective view showing the structure of a third embodiment of the present invention.

FIG. 6 is a top view showing the structure of the fourth exemplary embodiment of the present invention.

[Explanation of symbols]

1 LiNbO ₃ crystal 2 spontaneous polarization inversion period 3 ridge type optical waveguide 4 fundamental wave mode 5 second order polarization wave 6 SHG light 10 semiconductor laser 11 oscillation light (fundamental wave) 31 ridge type waveguide ridge section

Claims (3)

[Claims] 1. A waveguide in which the equivalent refractive index of fundamental wave light gradually changes in the light transmission direction is formed on the surface of a nonlinear optical crystal in which spontaneous polarization is periodically inverted in the substrate depth direction. A wavelength conversion element characterized by the above. 2. The surface of a nonlinear optical crystal in which spontaneous polarization is periodically inverted along the surface of a substrate, the transmission direction of the fundamental wave light is almost equal to the lattice vector formed by the spontaneous polarization inversion period. A wavelength conversion element characterized by forming a waveguide having a finite angle which is close to a right angle and whose equivalent refractive index is gradually changed in the light transmitting direction. 3. An optical waveguide is formed on the surface of a nonlinear optical crystal in which spontaneous polarization is periodically inverted along the surface of a substrate, and the direction of a lattice vector formed by the spontaneous polarization inversion period and the optical waveguide. The wavelength conversion element is characterized in that the relative angle formed with the light transmission direction of (1) gradually changes along the light transmission direction of the optical waveguide.