JPH095808A - Optical second higher harmonic generator - Google Patents

Abstract

PURPOSE: To make SHG operation possible with high conversion efficiency by forming the refractive index distribution structure of an optical waveguide in such a manner that the refractive index on the periphery of the optical waveguide is equal to the refractive index of a substrate crystal and is made gentle toward the central part of the optical waveguide. CONSTITUTION: The refractive index distribution structure of the optical waveguide 12 of an optical second higher harmonic generating(SHG) element having the optical waveguide 12 is formed so as to be equal to the refractive index of the substrate 11 crystal on the periphery 12A of the optical waveguide 12 and to be made gentle toward the central part of the optical waveguide 12 or to be made higher in the refractive index finely stepwise. As a result, a cut-off condition is less severe and the higher mode is correspondingly cut off even if the width of the optical waveguide 12 is set wide. Namely, the propagation mode of the basic wave and the propagation mode of the optical second higher harmonic wave(SH wave) are more easily regulated to the same lower order mode even if the width of the optical waveguide 12 is set wide in order to enhance the coupling efficiency with a semiconductor laser.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical second high frequency generation (SH) using a semiconductor laser (LD) as a light source of a fundamental wave.
G) The present invention relates to an element.

[0002]

2. Description of the Related Art Conventionally, an element using an LD as a fundamental wave light source is
SHG devices based on quasi phase matching (QPM) are the mainstream. FIG. 4 is a block diagram of such a conventional SHG element. As shown in this figure, an optical waveguide 2 is formed on a substrate 1, and a periodically domain-inverted region 3 is formed so as to intersect the optical waveguide 2. Here, as the substrate 1, LiNbO ₃ , L
iTaO ₃ or KTiOPO ₄ (KTP) is used.

In such an element, a fundamental wave (angular frequency ω) is made incident on the optical waveguide 2, and while propagating through the optical waveguide 2, part of the energy of the fundamental wave is the second harmonic of the optical wavelength 2ω. Wave (SH wave). Phase matching is realized by a periodically poled structure. When the energy of the fundamental wave is Pω and the energy of the SH wave is P ₂ ω, normalized by the square L ² of the length L of the SHG element η _NOR = Pω / (P ₂ ω · L ² ) (1 ) Is called the absolute conversion efficiency.

In the development of SHG devices, one of the objectives is to improve the absolute conversion efficiency. It is also known from theoretical calculation that this absolute conversion efficiency is given by the following equation. [For example, Applied Physics
Letters Vol. 64 (1994), pp. 3
No. 208-3209. ] Η _NOR = C · | J | ² (2) Here, C is a constant determined by the fundamental wave, SH wave, nonlinear optical constant, dielectric constant, wavelength of SH wave, etc., and J is an overlap given by the following equation. It is an integral.

[0005]

[Equation 1]

Here, E ₂ ω (x, z) and E ω (x,
z) is a function that gives the amplitude distributions of the standardized fundamental wave and SH wave in the optical waveguide, respectively. From the above description, in order to increase the absolute conversion efficiency, the point is to increase the value of the overlap integral given by the above equation (3).
For that purpose, E ₂ ω (x, z) and Eω (x, z) are required to be as similar as possible. That is, it is required that the propagation modes of the fundamental wave and the SH wave are the same.

The propagation mode of light is mainly determined by the size (shape) of the optical waveguide and the wavelength of the guided light. In general, the wider the optical waveguide width and the shorter the wavelength, the higher-order propagation mode is selected. On the other hand, it is desirable that the width of the optical waveguide be wide so that the fundamental wave is incident on the optical waveguide as much as possible (in order to increase the coupling efficiency). However, since the wavelength of the fundamental wave and the wavelength of the SH wave are exactly doubled, even if the fundamental wave is the fundamental mode or a higher-order propagation mode higher than the first order, the SH wave is higher The mode may be selected. That is, the fundamental wave and SH
It is difficult to make the wave propagation modes the same.

[0008]

In the conventional optical waveguide structure of the SHG element, no measures have been taken against the above problems. That is, the width of the optical waveguide is wide and the efficiency of incidence of the fundamental wave on the optical waveguide is high, and no positive measures have been taken to make the propagation modes of the fundamental wave and the SH wave the same. If this point is improved, the conversion efficiency can be further improved.

Therefore, the present invention eliminates the above problems,
An object of the present invention is to provide an optical second high frequency generating element that includes an optical waveguide having a structure capable of making the propagation modes of the fundamental wave and the SH wave the same, and that can improve conversion efficiency.

[0010]

In order to achieve the above object, the present invention provides an optical second high frequency generating element comprising a nonlinear optical crystal substrate on which a periodic domain inversion structure and an optical waveguide are formed. The periphery of the waveguide is equal to the refractive index of the substrate crystal, and the refractive index is gradually or finely stepped toward the center of the optical waveguide.

[0011]

According to the present invention, as described above, in the SHG element having the optical waveguide, the refractive index distribution structure of the optical waveguide is such that the periphery (12A) of the optical waveguide (12) is equal to the refractive index of the substrate crystal. Since the refractive index is gradually or finely increased toward the center of the optical waveguide (12), the cut-off condition becomes weaker, and the width of the optical waveguide (12) is reduced. Higher mode is cut off even if is set wide.

That is, even if the width of the optical waveguide (12) is set wide for the purpose of increasing the coupling efficiency with the semiconductor laser, the propagation mode of the fundamental wave and the propagation mode of the SH wave become the same low-order mode. It becomes easier to specify.

[0013]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows an embodiment of the present invention S
FIG. 2 is a configuration diagram of the HG element, and FIG. 2 is a diagram showing the refractive index of the optical waveguide of the SHG element. In this figure, an optical waveguide 12 is formed on a LiNbO ₃ substrate 11 which is a nonlinear optical crystal substrate, and a periodic domain inversion region 13 is formed so as to intersect with the optical waveguide 12.

In such an element, a fundamental wave (angular frequency ω) is made incident on the optical waveguide 12, and while propagating through the optical waveguide 12, a part of the energy of the fundamental wave becomes an angular frequency 2ω.
Is converted into the second harmonic (SH wave) of light. Phase matching is realized by a periodically poled structure. Therefore, in the embodiment of the present invention, the periphery 12A of the optical waveguide 12 is equal to the refractive index of the substrate crystal, and the refractive index is gradually or gradually increased toward the central portion of the optical waveguide 12. Form.

That is, the optical waveguides 12 and 12A are formed so that the refractive index distribution is gentle as shown in FIG. 2 or the refractive index is increased in fine steps. Hereinafter, a specific SHG element of the present invention will be described. Incidentally, in the conventional case, the effective refractive index of the optical waveguide having the stepwise refractive index distribution as shown in FIG. 5 is as shown in FIG.

That is, in FIG. 6, the propagation mode is set to N.
, N = 0 is the basic mode, N = 1 is the primary mode, and so on. The b on the vertical axis represents the normalized effective refractive index, and the point where the effective refractive index curve given for each mode intersects the horizontal axis is the cutoff condition. The horizontal axis V is a value corresponding to half the standardized optical waveguide width value given as V = k ₀ n ₁ a (2Δ) ^1/2 . Here, Δ≈ (n ₁ −n ₂ ) / n ₁ , and n ₁ , n ₂ , and k ₀ are the refractive index of the optical waveguide, the refractive index of the substrate, and the wave number of propagating light in vacuum, respectively. .

In order to form an optical waveguide having a gentle refractive index distribution as shown in FIG. 2 of this embodiment, the mask having a narrow width may be formed by either the ion exchange method or the Ti thermal diffusion method. After the optical waveguide is formed using, the optical waveguide is formed again by using a mask with a slightly wider width than the previous time, so that the periphery of the optical waveguide is equal to the refractive index of the substrate crystal, The refractive index can be increased in fine steps toward the central portion. By further annealing this, it is possible to form a fine step-like shape in which the refractive index changes gently.

In any case, the cut-off condition can be similarly eased for an optical waveguide having a fine stepwise refractive index and an optical waveguide having a gently changing shape. However, the effect of softening the cut-off condition (the cut-off occurs in a wider waveguide) is greater when the annealing treatment is performed so that the refractive index changes gently.

The operation of the optical second high frequency generating element according to the embodiment of the present invention will be described below. FUNDAMENA
TLS OF MICROOPTICS (ACADEM
IC PRESS, 1984) pp. According to 40 to 69, when the width of the optical waveguide is 2a, the condition V of the cutoff of the first mode (half of the standardized optical waveguide width) is V = k ₀ n ₁ a (2Δ) ^{1 / 2} = π / 2 It is given by (4).

Here, Δ≈ (n ₁ −n ₂ ) / n ₁ . Further, n ₁ , n ₂ , and k ₀ are the refractive index of the optical waveguide, the refractive index of the substrate, and the wave number of propagating light in vacuum, respectively.
On the other hand, if the refractive index distribution of the optical waveguide is formed so as not to have a step function but to have a gentle function represented by (x / a) ^a as described above, the cutoff condition V is It changes to Vc which is approximately given by the equation (5).

Vc = {(1/6) + [1 / (α + 2)]-[3 / (α + 3)] + (4/3) [1 / (α + 4)]} ^-1/4 (5) The relationship is shown in FIG. The case where the refractive index distribution structure of the optical waveguide is stepwise corresponds to that α is infinite. In this case, V = 1.57 from the above equations (4) and (5). Further, when α is 2 parabolicly, Vc increases to 2.28.

Here, the width 2a of the optical waveguide required to cut off the first-order mode will be considered. Since a = V / [n ₁ k ₀ (2Δ) ^1/2 ] from the above formula (4), the optical waveguide width is 2a when the refractive index distribution is stepwise, and 2ac when it is parabolic. Then, (2a
c) / (2a) = Vc / V = 2.28 / 1.57 = 1.
Since it is 45, it means that the width of the optical waveguide can be approximately 1.5 times as large as that in the case of being parabolic in the case of the step function.

As described above, in the SHG element having the optical waveguides 12 and 12A as shown in FIG. 1, the cutoff condition is that the refractive index distribution structure of the optical waveguide is as shown in FIG. It can be made sweet by the configuration, and even if the width of the optical waveguide is set wider, the higher-order mode is cut off. That is, even if the width of the optical waveguide is set wide for the purpose of increasing the coupling efficiency with the LD, it becomes easy to define the propagation mode of the fundamental wave and the propagation mode of the SH wave in the same low-order mode.

As described above, the present invention can be applied to an SHG element having an optical waveguide as a constituent element. That is,
As shown in FIG. 1, S using LiTaO ₃ or KTP
By applying it to the HG element, an SHG element having high conversion efficiency can be realized as well. As a substrate, LiNbO
₃ other than LiTaO ₃ or KTiOPO ₄ (KT
P) or the like may be used.

It should be noted that the present invention is not limited to the above-described embodiment, and various modifications are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0026]

As described in detail above, according to the present invention, the following effects can be achieved. In an SHG element having an optical waveguide, the refractive index distribution structure of the optical waveguide is such that the periphery of the optical waveguide is equal to the refractive index of the substrate crystal and the refractive index is gradually or finely stepped toward the center of the optical waveguide. Since it is formed so as to be high, the cut-off condition becomes weak, and even if the width of the optical waveguide is set wider, the higher-order mode is cut off.

That is, even if the width of the optical waveguide is set wide for the purpose of increasing the coupling efficiency with the semiconductor laser,
It becomes easy to define the propagation mode of the fundamental wave and the propagation mode of the SH wave in the same low-order mode. Therefore, the fundamental wave and SH
Waves can be guided in the same mode, which enables SHG operation with high conversion efficiency.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an SHG element showing an embodiment of the present invention.

FIG. 2 is a diagram showing a refractive index of an optical waveguide of an SHG element showing an example of the present invention.

FIG. 3 is a diagram showing a single mode condition of a distributed index planar waveguide of an SHG element showing an example of the present invention.

FIG. 4 is a configuration diagram of a conventional SHG element.

FIG. 5 is a diagram showing the refractive index of a conventional optical waveguide having a stepwise refractive index distribution.

FIG. 6 is a diagram showing an effective refractive index of a conventional optical waveguide having a stepwise refractive index distribution.

[Explanation of symbols]

11 LiNbO ₃ substrate 12 Optical waveguide 12A Periphery of optical waveguide 13 Polarization inversion region

Claims (1)

[Claims] 1. An optical second high-frequency generating device comprising a periodic polarization inversion structure and an optical waveguide formed on a nonlinear optical crystal substrate, wherein the periphery of the optical waveguide is equal to the refractive index of the substrate crystal. Gently toward the center,
Alternatively, the optical second high frequency generating element is characterized in that it is formed so that the refractive index is increased in fine steps.