JPH0361929A - Wavelength converting optical element - Google Patents

Abstract

PURPOSE:To allow the collimating or condensing of a light second harmonic wave beam by changing the equiv. grating pitch of the diffraction grating with respect to a radial direction according to the spreading angle of the second harmonic wave beam. CONSTITUTION:A basic wave 14 is made incident into a waveguide part 11 from a light incident end face and the Cherenkov reflected light propagated in a substrate 10, i.e., the light second harmonic wave 15 is emitted from a light exit end face 13. Further, this radiated light 15 is converted by a wave front converting element 12 to collimated beams 16 of light. The wave front converting element 12 is formed of the diffraction grating consisting of a part of elliptic patterns. This diffraction grating has A1, A2 grating spaces respectively in two orthogonal directions. The grating pitches of the diffraction grating, therefore, vary in different radial directions and the setting of the grating pitches corresponding to the spread angles of the second harmonic wave beams respectively in the different radial directions is possible. The collimating or condensing of the light second harmonic wave having no axisymmetry is possible in this way.

Description

Detailed Description of the Invention [Objective 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 meter illumination, etc. The present invention relates to a wavelength conversion optical element that combines a wave path and a wavefront conversion element.

(Prior Art) In recent years, short-wavelength coherent light sources have been developed for application to high-density optical disk systems, all-in-one systems, display systems, and the like. In an optical disk system, the spot diameter of the light focused on the disk surface is proportional to the wavelength of the light source, so a short wavelength light source is essential to achieve high density.

As a short-wavelength light source, semiconductor lasers have the advantages of being small, lightweight, and low power consumption, so the development of short-wavelength lasers using new materials is progressing, and it has already reached 0.6 μm.
InGaAIP semiconductor lasers with an oscillation wavelength in the red band have reached the level of practical use. However, although research has been carried out on green or blue lasers with shorter wavelengths, a laser that continuously oscillates at room temperature has not been obtained, and there is still no prospect of practical use.

On the other hand, as another means of realizing a short wavelength light source, there is optical second harmonic generation (SHG) using a nonlinear optical crystal.
More research has been conducted than ever before. In order to realize compact size and low power consumption, attempts have been made to use a semiconductor laser as a fundamental wave light source and to turn a nonlinear optical crystal into a waveguide. For example, proton exchange L i as shown in Fig.
A blue light source has been obtained as the optical second harmonic of a semiconductor laser using an N b Oi waveguide (T, Tanuc
hl at al,: 0pLo-electronl
cs, Vol, 2. No, 1. pp, 5g-58
(1987), ).

In addition, 100 in the figure is a L i N b O3 substrate, 101
104 indicates a waveguide, 104 indicates a fundamental wave, and 10B indicates an output light (optical second harmonic). This method emits optical second harmonics into the waveguide substrate n=t by Cerenkov radiation, and has the advantage that phase matching by angle control and temperature control is not required compared to the conventional SHG method.

However, since Cerenkov synchrotron radiation has a complex wavefront emitted from a light source distributed on the axis, a special optical system is required to collimate or focus the second harmonic of the emitted light. , it is difficult to focus the beam to a diffraction-limited spot diameter. Light emitted from a light emitting source distributed on the axis in an isotropic medium is called a conical wave, and such a conical wave is generated by an optical element called an axicon (J, H, McLeod: J, Opi, Soc). , ^
m,, Vo1.44, No, 8゜pp, 592-597
(1954)) allows for collimation. A typical example of an axicon is a conical prism.

However, the Cherenkov radiation in L i N b 03 described above does not form a perfect conical wave. This is because the refractive index of a nonlinear optical crystal generally varies depending on the propagation direction and polarization direction of light. That is, LiNb0. The second harmonic radiated into the substrate also has an emission angle depending on the direction and is not axially symmetrical.

Therefore, such a beam cannot be collimated using an axisymmetric axicon.

(Problem to be Solved by the Invention) Conventionally, in SHG that uses Cerenkov radiation that does not require phase matching, the emitted beam is not axially symmetrical due to the anisotropy of the substrate, so the emitted beam must be collimated or focused. It was difficult to do so.

The present invention has been made in consideration of the above circumstances, and its purpose is to collimate or condense a second harmonic beam having no axis symmetry that is emitted from an optical waveguide due to Cerenkov radiation. It is an object of the present invention to provide a wavelength conversion optical element that can contribute to the practical use of compact, short-wavelength light sources.

[9! [Means for Solving the Problems] 1' of the present invention is based on the second
In order to enable collimation or condensation of the harmonic beam, the wavefront conversion element is formed by a diffraction grating with an elliptical or concentric pattern, and the grating pitch with equal polarization of the diffraction grating in the radial direction is adjusted to the spread angle of the second harmonic beam. The goal is to change it accordingly.

That is, in the present invention, at least one of the waveguide layer and the substrate is formed of a nonlinear optical material, and the refractive index of the waveguide layer and the substrate is such that the fundamental wave becomes the waveguide mode and the second harmonic becomes Cerenkov radiation. A wavelength conversion optical element comprising a set optical waveguide and a wavefront conversion element that is provided in contact with or opposite to a light output end face of the optical waveguide and converts the wavefront of light emitted from the end face, The wavefront conversion element is formed by a diffraction grating consisting of a part of an elliptical pattern, and preferably, when the refractive index of the substrate for ordinary light and extraordinary light at the wavelength of the second harmonic of light is n , the ratio of the long sleeve to the short axis of the elliptical pattern is noon,
It is set so that

Further, in the present invention, at least one of the waveguide layer and the substrate is formed of a nonlinear optical material, and the refractive index of the waveguide layer and the substrate is set so that the fundamental wave is a waveguide mode and the second harmonic is Cherenkov radiation. A wavelength converting optical element comprising: an optical waveguide having a light output from the optical waveguide; and a wavefront conversion element that is provided in contact with or opposite to a light output end face of the optical waveguide and converts a wavefront of light emitted from the end face. The conversion element is formed of a diffraction grating consisting of a part of a concentric pattern, and at least one of the light output end face and the wavefront conversion element is arranged at an angle from a plane perpendicular to the waveguide layer, Preferably, the inclination angles of the light emitting end face and the wavefront conversion element are set to be the same.

(Function) According to the present invention, by forming the diffraction grating forming the wavefront conversion element into an elliptical pattern, the grating pitch of the diffraction grating becomes nested in different radial directions, and the second harmonic is generated in different radial directions. The grating pitch can be set according to the beam spread angle. Therefore, in the leading wave type SHG using the Cerenkov radiation method, it is possible to collimate or condense optical second harmonics that do not have axial symmetry.

Furthermore, even if the diffraction grating is arranged in a concentric circular pattern, at least one of the light emitting end face and the wavefront conversion element is tilted from the plane perpendicular to the waveguide layer. The grating pitch can be set according to the spread angle of the second harmonic beam, and it becomes possible to collimate or condense the optical second harmonic without axial symmetry.

(Example) Hereinafter, details of the present invention will be explained with reference to illustrated embodiments.

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, 10 is Li
The substrate 10 is a NbO3 substrate, and a waveguide section 11 is formed on this substrate 10 to constitute an optical waveguide. A wavefront conversion element 12 made of an elliptical pattern diffraction grating is arranged opposite to or in contact with the light output end face 13 of this optical waveguide. and,
The fundamental wave 14 is input into the waveguide 11 from the light incidence end face,
Cherenkov radiation light (optical second harmonic) 15 propagated into the substrate 10 is emitted from the light emitting end face 13, and this radiation light 15 is further converted into parallel light 16 by the wavefront conversion element 12.

Here, if the refractive index of the substrate IO with respect to the wavelength λ1 of the fundamental wave 14 and the wavelength λ2 (-λ, /2) of the optical second harmonic 15 is nl+n2, then n, <nEPF <n
2 +++ The material of the substrate 10 is selected so as to satisfy (1).

However, n EPF is the effective refractive index of the optical waveguide with respect to the fundamental wave. L i N b Oi is the nonlinear optical constant d3
Since N has a large value, the substrate orientation, fundamental wave propagation direction, and polarization direction are selected so that this d33 can be utilized. That is, when a Z plate is used as the substrate, the waveguide mode of the fundamental wave is the y propagation (or X propagation) TM mode, and when an X plate (or yN) is used, the waveguide mode of the fundamental wave is This is the TE mode of y propagation (or X propagation).

When the fundamental wave 14 with wavelength λ1 is incident on the end face of this optical waveguide, the nonlinear optical effect of LiNbO3 causes this light to
It is converted into a 1/2 optical second harmonic, and θ
Substrate 1 as Cerenkov radiation 15 with angle C
Propagates through 0. θ.

Ha n　I! It has the following relationship with pp and n2.

n 2 Cogθc −n gpp −
(2) This Cherenkov radiation is refracted at the substrate end face 13 and radiated into the air at an angle θ. Here, θ.

and θC have the following relationship.

n2sfnθ(-sinθo ・” (
3) The second harmonic radiated into the air is transmitted to the wavefront conversion element 12
is converted into parallel light 16 by . This parallel light 16 is
It is possible to focus down to the diffraction limit subo-soto diameter using a normal lens. An example of the structure of the wavefront conversion element 12 is shown in FIG. As shown in the figure, the wavefront conversion element 12 is formed of a diffraction grating that is a part of an elliptical pattern. This diffraction grating has grating spacings of AI and A2 in two orthogonal directions, respectively.

Next, the principle of the above wavefront conversion element will be explained. First, in order to find the refractive index of the substrate for the second harmonic, consider a refractive index ellipsoid as shown in FIG. Here, the propagation direction of the fundamental waveguide mode is set to X, and a vector n is set in the same direction as the polarization direction. Let S be the Poynting vector representing the traveling direction of the second harmonic wave, and let nz be a vector that matches the polarization direction of the second harmonic and whose absolute value is the magnitude of the refractive index for the second harmonic: J3 wave. . S and x
S is expressed by the following equation, where θ is the angle with To, and ψ is the angle between the projection of S onto the yz plane and the two axes.

T'-(cosθ, sinθs1nφ+ 5ln
fl cosφ) ・(4) Also, nl+ 02 as n-(○, O, n,) ...(5) n
z − (x, V + z) ... (6)
Then, each vector must satisfy the following relationship.

(x/no)2+(y/no)2” (z/n-)2
−1 − (7)s*n2 =0
... (8) s ・ (nl x
Q2) -0-(9) Here, no1n* represents the refractive index for ordinary light and extraordinary light at the wavelength of the second harmonic.

From equations (4) to (9), the refractive index n2 for the second harmonic
is given by the following equation.

n2s" nz 2 ......(10) θ in the above equation corresponds to the Cherenkov radiation angle θ in the configuration shown in Figure 1. Since 02 in equation (10) needs to satisfy equation (2), , (2), and (10), θC is given by the following equation.

That is, in the configuration shown in FIG. 1, the Cerenkov radiation angle θC generally varies depending on the angle ψ. ψ is the X plate, the angle from the waveguide plane when using TE polarization, and the Z plate, 7M
When polarized light is used, it corresponds to its complementary angle. Due to the ψ dependence of equation (11), the angle θ in the air of the second harmonic radiated from the end face.

also changes depending on ψ. In the plane containing the Z axis (ψ-0), and 2
Let θ be the Cherenkov radiation angle in air in the plane perpendicular to the axis (ψ−π/2). 1 and θ. 2, then from equations (3) and (11), sin θo+−n
o(1-n EFP2/ n *') ”” −(1
2) sln θ02-n! (1-n F!pp2/
n *') ”” −(13) LiNbo, in the case of
n o and n e are the wavelength λ [am] and the temperature T [K
] as a function of (tl, V, tlo
bden et at,: Phys, Lett,,
Vol, 23°No, 3. pp, 243-244 (
196B),).

n.

4.9130 0.00278 λ n , 2- 4.5567 - 0.00224 λ 2 + 2.005X 10-'T 2 The wavelength characteristics of the refractive index of Li NbO at room temperature (25°C) are shown in Fig. show. When the wavelengths of the fundamental wave and the second high-order wave are 0.84 μm and 0.42 μm, respectively,
nl + nz r n in formulas (1) to (3)
The relationship of EFP is shown in the figure. FIG. 5 shows the relationship between the radiation angle of the second harmonic in the substrate and in the air and ΔN calculated using the above formula. Here, ΔN is the difference between the effective refractive index for the guided mode and the substrate refractive index, that is, ΔNs*ngpp
The refractive index difference Δn−n between the waveguide and the substrate is given by nl (ct>)−(ie).

−nl (n, fil'l, (ω)) has the following relationship.

0 Ku Δ N Ku Δ n
(17) In the example shown in FIG. 5, there are two cases in which ψ in equation (11) is O and π/2.

When θC is given by equation (11), the grating spacing of the 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 A also changes depending on the angle ψ, corresponding to the ψ dependence of the Cerenkov radiation angle. ψ−0
Let A be A when , and At-A2 be At-A2 when ψ-π/2, then A, , A2 can be expressed as follows, respectively.

A1-mλ2/no/(1-ngpp2/n*')"
"・" (19) A2-mλ2/IT@/ 0-
navF2/na ') '' - (20) 0 x π/2, A takes an intermediate value between equations (19) and (20), and as a result, the equiphase line of the wavefront conversion element grating is divided into long sleeve and short sleeve. It becomes an ellipse with the axis ratio n.:n.

FIG. 6 shows the relationship between the lattice spacing A I + A 2 and ΔN when first-order diffraction is used (m-1). From the perspective of grating processing, it is better to have a larger value of 8, but from the figure it can be seen that for this purpose it is necessary to increase ΔN. From Figure 6, when ΔN -0,135, %nEPF (
ω)-n, (2ω), and θ.

−〇. -〇, and at this time, it becomes A-■.

It is known that waveguide formation by proton exchange can provide a large refractive index difference for extraordinary light; for example, Δn
A value of -0.13 has been reported (Taniuchi et al.: Applied Physics, Vol. 56. No. 12. pp. 1637
-1614 (19117)). Therefore, ΔN is (1, 1
It is possible to set it to a certain degree. At this time, from FIG. 6, the lattice spacing A is approximately 1 μm. A grating of this level can be created by, for example, electron beam lithography (G, IIatakoshl et al.: ＾pp
1. opt,, Vol.

24, No, 24. pp, 4307-4311 (1
985),).

The grating shown in FIG. 2 has a blazed shape with a sawtooth cross section in order to increase diffraction efficiency, but such a grating can also be formed by controlling the dose during electron beam lithography. . In addition, the first
The illustrated example shows an example of a wavefront conversion element for collimating the emitted light, but by changing the grating pattern, it is possible to create a wavefront conversion element that converts the emitted light into a spherical wave that converges or diverges. In this case, the grating pattern also has an elliptical shape. However, the gratings are not equally spaced in each direction as shown in FIG. 2, but are unevenly spaced.

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

FIG. 7 shows a second embodiment of the present invention, in which a concentric pattern grating is used as the wavefront conversion element. In the figure, 20 is a L i N b O3 substrate, 21 is a waveguide, and 22 is a wavefront conversion element consisting of a diffraction grating with a concentric pattern. As shown in this figure, it is possible to correct the anisotropy of the Cerenkov radiation angle by using a concentric pattern of wavefront conversion elements or tilting the end face of the waveguide substrate. Here, the inclination angles of the concentric grating wavefront conversion element and the end face of the waveguide substrate are φ and Φ, respectively.
shall be. Assuming that there are two waveguides and the waveguide mode is 7M mode with y propagation, in order to collimate the emitted light with this arrangement, the grating spacing A is given by the formula (20), using A2, Further, the inclination angle may be set so that φ and Φ satisfy the following equation.

sln[51n-' ln2+5ln(θC1-Φ))
+Φ−φ]+slnφ= sinθ02 ・”<
21) Using the relationships such as (2) and (3), the above equation can be transformed as follows.

1-(SIn(Io+eO8φ-t++:pps1n4
1) 2sln(I-φ)+(Slflθo+con-
nEppsinΦ)cos(Φ-φ)+slnφ-si
n θ02 ”' (22) Here, θ.1 and θo2 are (12), (1, 3)
It is the same as the expression. When using the TE mode of y-propagation in the X-plate waveguide, A1 given by equation (19) may be used as the grating spacing A, and θ.1 and θ.2 may be exchanged in the above equation.

The example in FIG. 7 shows a case where both the wavefront conversion element and the end face of the waveguide substrate are tilted, but it is also possible to correct the anisotropy of the radiation angle even if only one side is tilted. In the figure, 20 is a L i N b O3 substrate, 21 is a waveguide, 24 is a fundamental wave, and 25 is Cherenkov radiation propagated into the substrate 10 (
26 indicates collimated parallel light.

FIG. 8 shows a third embodiment of the present invention, in which the 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 formula. That is, the inclination angle φ of the concentric grating wavefront conversion element is given by the following formula (X plate, y
propagation, for 7M mode).

However, A −(1−cos7to+)2+5ln2θat−
8lr12θ. It is 2. Similarly, in the case of the X plate and TE mode, the above equation can be replaced by the following equation.

However, B −(1-cosθo2)2+5ln2θo2-3i
n2θo1. The sign of φ is different between the case of the X plate and the case of the X plate. That is, in equation (23), φ<0. In equation (24), φ>O. Figure 9 shows Δ defined by equation (16).
The relationship between N and φ is shown.

FIG. 10 shows a fourth embodiment of the present invention, in which only the end face of the waveguide is tilted. In this case, by setting φ−〇 in equation (22), the end face inclination angle Φ can be found as shown in the following equation (in case of 2 plates, 7M mode)
.

tanΦ= (Slnf)o+-9lnθ02)/(
ngpp-cosθ02) (25) Similarly, in the case of xli, TE mode, tanΦ
= (sinθo2−9inllo+)/(nEpp−
cos θ01) (26) Similarly to the case where the wavefront conversion element is tilted and used, the X plate and the X plate have the same sign of Φ. FIG. 11 shows the relationship between ΔN and Φ. As can be seen by comparing FIG. 9 and FIG. 11, the effect of anisotropic correction of the Cherenkov radiation angle is greater for Φ than for φ. That is, it is better to incline the end face of the substrate than to incline the wavefront conversion element with a slight inclination. This becomes more noticeable as ΔN becomes larger. When ΔN −0,135, that is,
When n EFF(ω) - n, (2ω), it seems contradictory that correction by φ is required even though the correction by Φ is 0. niθ. , −θ. 2-0, the lattice spacing becomes infinite, and φ becomes indeterminate.

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 tilted in the same direction. In this arrangement, the wavefront conversion element can be bonded to the end surface of the substrate, so alignment is easy. The tilt angle at this time can be expressed as Φ-φ in equation (22). That is, in the case of the 2[,7M mode, the end face inclination angle Φ is given by the following equation.

However, C” (napp-1)2+5ln2θ,,-5in2
θo2. Similarly, in the case of x tentative, 7M mode, the above equation is replaced by the following equation.

However, C= (nI:pp-1)2+5ln2θ, 2-sin
2θo1. Furthermore, in Fig. 13, (27) and (28
The relationship between ΔN and Φ, which can be obtained by the above formula, is shown.

As described above, according to the first embodiment of the present invention, a wavefront conversion element consisting of an elliptical pattern diffraction grating as shown in FIG. It becomes possible to collimate or condense the second harmonic beam emitted from the wave path. In addition, as in the second to fifth embodiments, by adopting an arrangement in which at least one of the wavefront conversion element and the end face of the optical waveguide mounting plate is tilted, a concentric pattern diffraction that can be easily created as a wavefront conversion element is achieved. A grating can be used, again making it possible to obtain a collimated second harmonic beam. Therefore, entering 4. By using a semiconductor laser as a light source, it is possible to realize a short wavelength green or blue light source that cannot be obtained with conventional semiconductor lasers.

Note that the present invention is not limited to the embodiments described above. For example, as a substrate for an optical waveguide, L i N
b LiTa0i instead of O3.

Inorganic nonlinear materials such as KNbO3, LiIO3, and KTP, and organic nonlinear materials such as MNA and DAN can be used. Furthermore, the configuration of the optical waveguide is not limited to one in which a 7XIJ film-like waveguide layer is formed on a substrate;
As shown in the figure, a waveguide portion may be formed by impurity diffusion on the surface of a substrate 90 made of a nonlinear material such as L i N b 03. Furthermore, both the substrate and the waveguide layer are not necessarily nonlinear. It is not necessary that the material is an optical material, and at least one of these materials may be a nonlinear optical material.In addition, various modifications can be made without departing from the gist of the present invention.

[Effects of the Invention] As detailed above, according to the present invention, the wavefront conversion element is formed of a diffraction grating having an elliptical or concentric pattern, and the equi-dramatic grating pitch of the diffraction grating in the radial direction is adjusted to the second harmonic beam. Since it is changed according to the spread angle of
It is possible to collimate or condense the optical second harmonic beam having no axial symmetry emitted from the optical waveguide by Cerenkov radiation, and it becomes possible to easily realize a compact and short wavelength light source.

[Brief explanation of drawings]

FIG. 1 is a 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 diagram showing an example of a wavefront conversion element used in the above embodiment, and FIG. LiNbO
A diagram for explaining the refractive index of 3, Figure 4 is L i
A characteristic diagram showing the relationship between the refractive index and wavelength of N b 03,
FIG. 5 is a characteristic diagram showing the relationship between Cerenkov radiation angle and ΔN, FIG. 6 is a characteristic diagram showing the relationship between lattice spacing of the wavefront conversion element and ΔN, and FIGS. Figures 7, 8, 10 and 12 are cross-sectional views showing the element configuration, and Figures 9, 11 and 13 are for explaining the inclination angle φ. FIG. 14 is a perspective view showing a modification of the present invention, and FIG. 15 is a perspective view showing an example of a conventional wavelength conversion optical element. 10.20, 30.40.50...L i N b
03 substrate, 11, 21.31, 41.51... waveguide section, 12.22.32.42.52... wavefront conversion element,
13... Waveguide substrate end surface, 14.24... Fundamental wave,
5... Cherenkov synchrotron radiation, 16 degrees 6... Outgoing light.

Claims (4)

[Claims] (1) At least one of the waveguide layer and the substrate is formed of a nonlinear optical material, and the refractive index of the waveguide layer and the substrate is set so that the fundamental wave is a waveguide mode and the second harmonic is Cherenkov radiation. A wavelength conversion optical element comprising an optical waveguide and a wavefront conversion element that is provided in contact with or opposite to a light output end face of the optical waveguide and converts a wavefront of light emitted from the end face, wherein the wavefront conversion element 1. A wavelength conversion optical element characterized in that the wavelength conversion optical element is formed of a diffraction grating consisting of a part of an elliptical pattern. (2) When the refractive index of the substrate for ordinary light and extraordinary light at the wavelength of the second harmonic of light is respectively n_0 and n_■, the ratio of the long axis and short axis of the elliptical pattern is n_0:
The wavelength conversion optical element according to claim 1, characterized in that n_■. (3) At least one of the waveguide layer and the substrate is formed of a nonlinear optical material, and the refractive index of the waveguide layer and the substrate is set so that the fundamental wave is a waveguide mode and the second harmonic is Cherenkov radiation. A wavelength conversion optical element comprising an optical waveguide and a wavefront conversion element that is provided in contact with or opposite to a light output end face of the optical waveguide and converts a wavefront of light emitted from the end face, wherein the wavefront conversion element is formed of a diffraction grating consisting of a part of a concentric pattern, and at least one of the light output end face and the wavefront conversion element is inclined from a plane perpendicular to the waveguide layer. (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.