JP2835087B2 - Wavelength conversion optical element - Google Patents

Description

Description: Object of the Invention (Industrial application field) The present invention relates to a wavelength conversion optical element for obtaining a short wavelength light source used for optical information processing, optical measurement, and the like, and particularly to an optical waveguide. And a wavefront conversion element.

(Prior Art) In recent years, a short-wavelength coherent light source has been developed for the purpose of application to a high-density optical disk system, a measurement and display system, and the like. In optical disk systems,
Since the spot diameter of the light focused on the disk surface is proportional to the wavelength of the light source, a short-wavelength light source is indispensable for realizing high density.

As a short-wavelength light source, semiconductor lasers have the advantages of small size, light weight, and low power consumption. Therefore, development of short-wavelength lasers using new materials has been promoted, and the oscillation wavelength has already been increased to the 0.6 μm band (red). InGaAlP-based semiconductor lasers have reached the level of practical use. However, although studies have been conducted on shorter wavelength green or blue lasers, lasers that continually oscillate at room temperature have not been obtained, and there is no prospect of practical application.

On the other hand, as another means for realizing a short wavelength light source, there is optical second harmonic generation (SHG) using a nonlinear optical crystal.
More research has been done than before. In order to realize compactness and low power consumption, attempts have been made to use a semiconductor laser as a fundamental wave light source and to make a nonlinear optical crystal into a waveguide. For example, a proton-exchanged LiNbO ₃ waveguide as shown in FIG. A blue light source has been obtained as an optical second harmonic of a semiconductor laser (T. Taniuchi et al .: Opto-electro
nics, Vol.2, No.1, pp53-58 (1987.). In the figure, 100 is L
An iNbO ₃ substrate, 101 indicates a waveguide, 104 indicates a fundamental wave, and 106 indicates emitted light (light second harmonic). This method emits the second harmonic of the light into the waveguide substrate by Cherenkov radiation, and has an advantage that phase matching by angle control and temperature control is not required as compared with the conventional SHG method.

However, since Cherenkov radiation has a complicated wavefront radiated from a light source distributed on the axis, a special optical system is required to collimate or collect the second harmonic of the output light. It is difficult to narrow the beam to a diffraction-limited spot diameter. Light emitted from a light source that is distributed on an axis in an isotropic medium is called a cone wave,
Such a conical wave is generated, for example, by an optical element called an axicon (JHMcLeod: J. Opt. Soc. Am., Vol. 44, No. 8, pp. 592-59).
7 (1954)), it is possible to collimate. A typical example of an axicon is a conical prism.

However, the aforementioned Cherenkov radiation in LiNbO ₃ does not become a perfect conical wave. This is due to the fact that the refractive index of the nonlinear optical crystal generally differs depending on the direction of light propagation and the direction of deflection. That is, the second radiation radiated into the LiNbO ₃ substrate
The output angles of the harmonics also differ depending on the direction, and are not axially symmetric. Therefore, such a beam cannot be collimated using an axis-symmetric axicon.

(Problems to be Solved by the Invention) As described above, conventionally, in an SHG using Cherenkov radiation that does not require phase matching, the output beam is not axially symmetric due to the anisotropy of the substrate. It was difficult to do.

The present invention has been made in view of the above circumstances, and an object thereof is to collimate or condense a second harmonic beam having no axial symmetry emitted from an optical waveguide by Cherenkov radiation. An object of the present invention is to provide a wavelength conversion optical element which can be made compact and can contribute to practical use of a short wavelength light source.

[Structure of the Invention] (Means for Solving the Problems) The gist of the present invention is to provide a wavefront conversion element with an ellipse or an elliptical shape in order to enable collimation or focusing of a second harmonic beam emitted from an end face of an optical waveguide. An object of the present invention is to change the equivalent grating pitch of the diffraction grating in the radial direction according to the divergence angle of the second harmonic beam.

That is, in the present invention, at least one of the waveguide layer and the substrate is formed of a non-linear optical material, and the refractive index of the waveguide layer and the substrate is adjusted so that the fundamental wave becomes a waveguide mode and the second harmonic becomes Cherenkov radiation. In a wavelength conversion optical element comprising a set optical waveguide and a wavefront conversion element that is provided in contact with or facing the light emitting end face of the optical waveguide and converts a wavefront of light emitted from the end face, are those of the wavefront converting element so as to form a diffraction grating consisting of a portion of an ellipse pattern, preferably n ₀ the refractive index of the substrate with respect to ordinary light and extraordinary light in the wavelength of the light second harmonic, respectively, n _e Where the ratio of the major axis to the minor axis of the elliptical pattern is n ₀ : _ne
It is set so that

Further, in the present invention, at least one of the waveguide layer and the substrate is formed of a non-linear optical material, and the refractive indices of the waveguide layer and the substrate are set so that the fundamental wave becomes a waveguide mode and the second harmonic becomes Cherenkov radiation. Wavelength conversion optics, comprising: a guided optical waveguide; and a wavefront conversion element provided in contact with or facing a light emitting end face perpendicular to the optical axis of the optical waveguide, and converting a wavefront of light emitted from the end face. In the device, the wavefront conversion element is formed by a diffraction grating composed of a part of concentric circular patterns at equal intervals, and at least one of the light emitting end face and the wavefront conversion element is arranged to be inclined from a plane perpendicular to the waveguide layer. Preferably, the light emitting end face and the wavefront conversion element are set to have the same inclination angle.

(Operation) According to the present invention, by making the diffraction grating forming the wavefront conversion element an elliptical pattern, the grating pitch of the diffraction grating is different in different radial directions, and the second harmonic beam is different in different radial directions. Grid pitch can be set according to the spread angle of. Therefore, in the optical waveguide type SHG based on the Cherenkov radiation method, it is possible to collimate or condense the optical second harmonic having no axial symmetry.
Further, even when the diffraction grating is a concentric pattern, at least one of the light emitting end face and the wavefront conversion element is disposed at an angle from a plane perpendicular to the waveguide layer, so that each of the diffraction gratings is formed in a different radial direction as in the case of the elliptical pattern. The grating pitch can be set according to the divergence angle of the second harmonic beam, and collimation or condensing of the optical second harmonic having no axial symmetry can be performed.

(Examples) Hereinafter, details of the present invention will be described with reference to the illustrated examples.

FIG. 1 is a perspective view showing a schematic configuration of a wavelength conversion optical element according to a first embodiment of the present invention. In the figure, reference numeral 10 denotes a LiNbO ₃ substrate, on which a waveguide 11 is formed to form an optical waveguide. A wavefront conversion element 12 composed of a diffraction grating having an elliptical pattern is arranged facing or in contact with the light emitting end face 13 of the optical waveguide. Then, the fundamental wave 14 enters the waveguide 11 from the light incident end face, and the Cherenkov radiated light (light second harmonic) 15 propagated in the substrate 10 is emitted from the light emitting end face 13, and the radiated light 15 is further emitted. Are converted into parallel light 16 by the wavefront conversion element 12.

Here, each refractive index of the substrate 10 with respect to the wavelength lambda ₂ wavelength lambda ₁ and the light second harmonic 15 of the fundamental wave 14 ₍₌ λ _1/2)
Assuming that n ₁ and n ₂ , the material of the substrate 10 is selected so as to satisfy n ₁ <n _EFF <n ₂ (1). Here, n _EFF is the effective refractive index for the fundamental wave of the optical waveguide. LiNbO ₃ Because nonlinear optical constant d ₃₃ has a large value, the substrate orientation to take advantage of this d _33, the propagation direction and the polarization direction of the fundamental wave is selected. That is, when the z-plate is used as the substrate, the guided mode of the fundamental wave is a TM mode of y-propagation (or x-propagation), and when the x-plate (or y-plate) is used, the guided mode of the fundamental wave is This is the TE mode of y propagation (or x propagation).

When the fundamental wave 14 having the wavelength λ ₁ is incident on the end face of the optical waveguide, the light is converted to λ _1/2 by the nonlinear optical effect of LiNbO _3.
And propagates through the substrate 10 as Cerenkov radiation 15 having an angle of θ _C with respect to the waveguide 11. θ _C has the following relationship with n _EFF and n ₂ .

n ₂ cos θ _C = n _EFF (2) The Cherenkov radiation is refracted at the substrate end face 13 and emitted into the air at an angle θ. Here, θ ₀ and θ _C have the following relationship.

n ₂ sin θ _C = sin θ ₀ (3) The second harmonic radiated into the air is converted by the wavefront conversion element 12 into parallel light 16. This parallel light 16 can be narrowed down to a diffraction-limited spot diameter by a normal lens. FIG. 2 shows an example of the structure of the wavefront conversion element 12. As shown in the figure, the wavefront conversion element 12 is formed by a diffraction grating composed of a part of an elliptical pattern. This diffraction grating has grating intervals of _{１ 1} and Λ ₂ in two orthogonal directions, respectively.

Next, the principle of the above wavefront conversion element will be described.
First, to determine the refractive index of the substrate for the second harmonic,
Consider an index ellipsoid as shown in FIG. here,
The propagation direction of the fundamental wave guided mode is x, and a vector ▼ is taken in the same direction as the polarization direction. A pointing vector that indicates the traveling direction of the second harmonic matches the polarization direction of the second harmonic, and the vector whose absolute value is the magnitude of the refractive index for the second harmonic is denoted by ▼.
Is defined as θ, and the angle between the projection of s on the yz plane and the z axis is defined as ψ.

= (Cosθ, sinθ sinφ, sinθ cosφ)… (4) Then, each vector needs to satisfy the following relationship.

(x / n ₀ ) ² + (y / n ₀ ) ² + (z / _ne ) ² = 1 (7) Here, n _0, n _e represents the refractive index for ordinary light and extraordinary light in the wavelength of the second harmonic. (4) than to (9) below, the refractive index n ₂ with respect to the second harmonic is given by the following equation.

Θ in the above equation corresponds to the Cherenkov radiation angle θ _C in the configuration of FIG. Since n _{2 in} equation (10) needs to satisfy equation (2), θ _C is given by the following equation from equations (2) and (10).

That is, in the configuration as shown in FIG. 1, the Cherenkov radiation angle θ _C generally differs depending on the angle ψ. ψ corresponds to the angle from the waveguide surface when using x-plate and TE-polarized light, and the complementary angle when using z-plate and TM-polarized light. Equation (11)
Due to the dependence, the angle θ _{0 in} the air of the second harmonic emitted from the end face also changes depending on ψ. In the plane including the z-axis (ψ =
0) and the Cherenkov radiation angles in air in the plane perpendicular to the z-axis (ψ = π / 2) are θ ₀₁ and θ ₀₂ , respectively, and from equations (3) and (11), sin θ ₀₁ = for ^{_{n 0 (1-n EFF 2}} / n e 2) 1/2 ... (12) sinθ 02 = n E (1-n EFF 2 / n e 2) 1/2 ... (13) LiNbO 3, n 0 , _ne are the wavelength λ [μm] and the temperature T
The function of [K] is given by the following equation (HVHobden e
t al .: Phys, Lett., Vol. 23, No. 3, pp. 243-244 (196
6). ).

FIG. 4 shows the wavelength characteristics of the refractive index of LiNbO ₃ at room temperature (25 ° C.). Set the wavelength of the fundamental wave and the second harmonic to 0.
The relationship among n ₁ , n ₂ , and n _{EFF in} the equations (1) to (3) when 84 μm and 0.42 μm are shown in the figure. FIG. 5 shows the relationship between ΔN and the radiation angle of the second harmonic in the substrate and in the air calculated using the above equation. Here, .DELTA.N the difference between the effective refractive index and a substrate refractive index for the guided mode, i.e. _{ΔN = n EFF -n e (ω} ) ... given by (16), the refractive index difference between the waveguide and the substrate [Delta] n = n ₀ −n
₁ (n ₁ = _ne (ω)) has the following relationship.

0 <ΔN <Δn (17) In the example of FIG. 5, ψ in equation (11) is 0 and π
/ 2 are shown.

When θ _C is given by equation (11), the grating interval of a diffraction grating used as a wavefront conversion element for collimating this light is given by the following equation.

As can be seen from the above equation, the lattice spacing 変 わ る also changes with the angle し て, corresponding to the ψ dependence of the Cherenkov radiation angle. ψ =
Assuming that Λ when 0 is Λ ₁ and Λ when ψ = π / 2 is Λ ₂ , Λ ₁ and Λ ₂ are respectively expressed as follows.

_{Λ 1 = mλ 2 / n 0} / (1-n EFF 2 / n e 2) 1/2 ... (19) Λ 2 = mλ 2 / n e / (1-n EFF 2 / n e 2) 1/2 ... (20) When 0 <ψ <π / 2, Λ takes an intermediate value between the expressions (19) and (20), and eventually the equiphase line of the wavefront conversion element grating has a ratio of the major axis to the minor axis of n _0. : an elliptical of n _e.

FIG. 6 shows the relationship between ΔN and the lattice spacings Λ ₁ , Λ ₂ when using first-order diffraction (m = 1). From the viewpoint of the grating processing, it is better that Λ is large, but from the figure, it can be seen that ΔN needs to be increased for that purpose. Sixth
From the figure, when ΔN = 0.135, n _EFF (ω) = n _e (2ω)
And θ _C = θ ₀ = 0, and then Λ = Λ.

It is known that in the formation of a waveguide by proton exchange, a large difference in refractive index with respect to extraordinary light can be obtained.
A value of n = 0.13 has been reported (Taniuchi et al .: Applied Physics, Vol. 56, No. 12, pp. 1637-1614 (1987)). Therefore, Δ
N can be set to about 0.1. At this time,
From FIG. 6, the lattice spacing Λ is about 1 μm. Such a grating can be created by, for example, electron beam lithography (G. Hatakoshi et al .: Appl. Opt., Vol. 24, N.
o.24, pp.4307-4311 (1985). ).

The grating shown in FIG. 2 has a blazed shape with a sawtooth cross section in order to increase the diffraction efficiency, but such a grating can also be formed by controlling the dose during electron beam writing. . Although the example of FIG. 1 shows an example of a wavefront conversion element for collimating the output light, a wavefront conversion element for converting the output light into a convergent or divergent spherical wave by changing the grating pattern is possible. It is. Also in this case, the grating pattern is also elliptical. However, the gratings are not equally spaced in each direction as shown in FIG. 2, but are irregularly spaced gratings.

As described above, the grating pattern shown in FIG. 2 can be created by electron beam lithography or the like, but since it is an elliptical pattern, it is difficult to create it by other methods such as machining. Although a concentric pattern grating can be relatively easily formed by using an NC lathe, the concentric pattern grating cannot be used as a wavefront conversion element in the arrangement shown in FIG. Therefore, a configuration for using a grating having a concentric pattern as a wavefront conversion element will now be described.

FIG. 7 shows a second embodiment of the present invention, in which a grating having a concentric pattern is used as a wavefront conversion element. In the figure, reference numeral 20 denotes a LiNbO ₃ substrate, 21 denotes a waveguide, and 22 denotes a wavefront conversion element composed of a diffraction grating having a concentric pattern. As shown in this figure, it is possible to correct the anisotropy of the Cherenkov radiation angle by using the wavefront conversion element or the waveguide substrate end face in a concentric pattern inclined. Here, the inclination angles of the concentric grating wavefront conversion element and the end face of the waveguide substrate are φ and φ, respectively. The waveguide is z
Plate, the waveguide mode is a TM mode of y propagating, in such an arrangement, in order to collimate the emitted light, the grating spacing lambda of the grating using lambda ₂ given by (20), also φ and The tilt angle may be set so that Φ satisfies the following equation.

sin [sin ^-1 {n ₂₁ sin (θ _C1 −φ)} + φ + φ] + sin φ = sin θ ₀₂ (21) Using the relations of equations (2) and (3), the above equation is transformed as follows: .

Here, θ ₀₁ and θ ₀₂ are the same as the expressions (12) and (13). In the case of using the TE mode y propagation in x Itashirube waveguides, using lambda ₁ given by the grating spacing lambda (19) equation, to be replaced with theta ₀₁ and theta ₀₂ in the above equation.

Although the example of FIG. 7 shows a case where both the wavefront conversion element and the end face of the waveguide substrate are used in an inclined state, it is possible to correct the anisotropy of the radiation angle by inclining only one of them. In addition, 20 in the figure
LiNbO ₃ substrate, 21 is a waveguide, 24 is a fundamental wave, 25 is Cherenkov radiation light (second harmonic light) propagated in the substrate 10, and 26 is collimated parallel light.

FIG. 8 shows a third embodiment of the present invention, in which a radiation angle is corrected by tilting a wavefront conversion element having a concentric pattern. To find the tilt angle in this case (21)
It is sufficient to set Φ = 0 in the equation. That is, the inclination angle φ of the concentric grating wavefront conversion element is given by the following equation (for z-plate, y-propagation, and TM mode).

Here, A = (1−cos θ ₀₁ ) ² + sin ² θ ₀₁ −sin ² θ ₀₂ . Similarly, in the case of x-plate and TE mode, the above equation is replaced with the following equation.

Here, B = (1−cos θ ₀₂ ) ² + sin ² θ ₀₂ −sin ² θ ₀₁ . The sign of φ differs between the case of the z plate and the case of the x plate. That is, φ <0 in Expression (23) and φ> 0 in Expression (24). FIG. 9 shows the relationship between ΔN and φ defined by equation (16).

FIG. 10 shows a fourth embodiment of the present invention, in which only the end face of the waveguide is inclined. In this case, by setting φ = 0 in equation (22), the end surface tilt angle Φ is obtained as in the following equation (in the case of a z-plate, TM mode).

tanΦ = (sin θ ₀₁ −sin θ ₀₂ ) / (n _EFF −cos θ ₀₂ ) (25) Similarly, in the case of x-plate and TE mode, tan Φ = (sin θ ₀₂ −sin θ ₀₁ ) / (n _EFF − cosθ ₀₁ ) (26) As in the case where the wavefront conversion element is used at an angle, the z plate and x
The sign of Φ is different from the plate. FIG. 11 shows the relationship between ΔN and Φ. As can be seen by comparing FIG. 9 and FIG. 11,
The effect of anisotropy correction of the Cherenkov radiation angle is larger for Φ than for φ. In other words, a slight inclination is required when the substrate end surface is inclined rather than when the wavefront conversion element is inclined. This is more remarkable as ΔN increases. When ΔN = 0.135, ie, n
When the _{EFF (ω) = n e (} 2ω), even though the correction by Φ 0, but the required correction by φ seems contradictory, which, in this case θ ₀₁ = θ
_{This is because 02} = 0, the lattice spacing becomes infinite, and φ becomes indefinite.

FIG. 12 shows a fifth embodiment of the present invention, in which both the end face of the waveguide substrate and the wavefront conversion element are inclined in the same direction. In this arrangement, the wavefront conversion element can be bonded to the end face of the substrate, so that the alignment is easy. The inclination angle at this time may be set as Φ = φ in equation (22). That is, in the case of the z plate and the TM mode, the end surface inclination angle Φ is given by the following equation.

Here, C = (n _EFF −1) ² + sin ² θ ₀₁ −sin ² θ ₀₂ . Similarly, in the case of x-plate and TM mode, the above equation is replaced by the following equation.

Here, C = (n _EFF −1) ² + sin ² θ ₀₂ −sin ² θ ₀₁ . FIG. 13 shows the relationship between ΔN and Φ given by equations (27) and (28).

As described above, according to the first embodiment of the present invention, in the wavelength conversion optical element of the Cherenkov radiation method, by providing the wavefront conversion element including the elliptical pattern diffraction grating as shown in FIG. The second emitted from the waveguide
It becomes possible to collimate or condense the harmonic beam. Further, by adopting an arrangement in which at least one of the wavefront conversion element and the end face of the optical waveguide substrate is used in an inclined manner as in the second to fifth embodiments, a diffraction grating having a concentric pattern that can be easily formed as a wavefront conversion element. Can be used, and also in this case, a collimated second harmonic beam can be obtained. Therefore, by using a semiconductor laser as the incident light source, it is possible to realize a green or blue light source with a short wavelength, which cannot be obtained with a conventional semiconductor laser.

Note that the present invention is not limited to the above-described embodiments. For example, instead of LiNbO ₃ , an inorganic nonlinear material such as LiTaO ₃ , KNbO ₃ , LiIO ₃ , KTP, etc.
Organic nonlinear materials such as MNA and DAN can be used. The configuration of the optical waveguide is not limited to those of forming the thin film waveguide layer on a substrate, LiNbO ₃ as shown in FIG. 14
A waveguide portion formed by impurity diffusion may be formed on the surface of the substrate 90 made of a non-linear material such as. Further, both the substrate and the waveguide layer need not necessarily be nonlinear optical materials, and at least one of them may be a nonlinear optical material. In addition, various modifications can be made without departing from the scope of the present invention.

[Effects of the Invention] As described in detail above, according to the present invention, the wavefront conversion element is formed by a diffraction grating having an elliptical or concentric pattern, and the equivalent grating pitch of the diffraction grating in the radial direction is adjusted by the second harmonic beam. Since it is changed according to the divergence angle, it is possible to collimate or condense the optical second harmonic beam having no axial symmetry emitted from the optical waveguide by Cherenkov radiation, and to use a small, short-wavelength light source. It can be easily realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a schematic configuration of a wavelength conversion optical element according to a first embodiment of the present invention, FIG. 2 is a view showing an example of the structure of a wavefront conversion element used in the above-described embodiment, and FIG. view for explaining the refractive index of _3, FIG. 4 is a characteristic diagram showing the relationship between the refractive index and the wavelength of LiNbO _3, Figure 5 is a characteristic diagram showing the relationship between the Cerenkov radiation angle and .DELTA.N, Figure 6 FIG. 7 is a characteristic diagram showing the relationship between the lattice spacing of the wavefront conversion element and ΔN, and FIGS. 7 to 13 are diagrams for explaining another embodiment of the present invention. 10 and 12 are cross-sectional views showing the element configuration, FIGS. 9, 11, and 13 are characteristic diagrams showing the relationship between the inclination angle φ and ΔN, and FIG. 14 is a diagram showing a modification of the present invention. FIG. 15 is a perspective view showing an example of a conventional wavelength conversion optical element. 10, 20, 30, 40, 50 ... LiNbO ₃ substrate, 11, 21, 31, 41, 51 ... waveguide unit, 12, 22, 32, 42, 52 ... wavefront conversion element, 13 ... waveguide Substrate end face, 14, 24… fundamental wave, 15… Cherenkov radiation, 16, 26… outgoing light.

Claims (4)

(57) [Claims] 1. The refractive index of a waveguide layer or a substrate is set so that at least one of a waveguide layer and a substrate is formed of a nonlinear optical material, a fundamental wave is a waveguide mode, and a second harmonic is Cherenkov radiation. A wavelength conversion optical element, comprising: an optical waveguide that is provided, and a wavefront conversion element that is provided in contact with or opposite to a light emitting end face of the optical waveguide and converts a wavefront of light emitted from the end face. A wavelength conversion optical element, wherein the conversion element is formed by a diffraction grating composed of a part of an elliptical pattern. Wherein when the refractive index of the substrate with respect to ordinary light and extraordinary light in the wavelength of the light second harmonic was n _0, n _e, respectively, the ratio of the major axis to the minor axis of the elliptical pattern n _0: optical wavelength conversion element according to claim 1, characterized in that it is a n _e. 3. The refractive index of the waveguide layer and the substrate is set so that at least one of the waveguide layer and the substrate is formed of a nonlinear optical material, the fundamental wave is in a waveguide mode, and the second harmonic is Cherenkov radiation. Wavelength conversion optics, comprising: a guided optical waveguide; and a wavefront conversion element provided in contact with or facing a light emitting end face perpendicular to the optical axis of the optical waveguide, and converting a wavefront of light emitted from the end face. In the device, the wavefront conversion element is formed of a diffraction grating composed of a part of an equidistant concentric pattern, and at least one of the light emitting end face and the wavefront conversion element is inclined from a plane perpendicular to the waveguide layer. A wavelength conversion optical element, characterized in that: 4. The wavelength conversion optical element according to claim 3, wherein the light emitting end face and the wavefront conversion element have the same inclination angle.