JPH05323401A - Wavelength conversion element - Google Patents

Abstract

PURPOSE:To facilitate the production of the wavelength conversion element which can relieve phase matching conditions add has the effect similar to the effect of an element which is gradually changed in inversion period. CONSTITUTION:Plural polarization inversion layer 3 which are periodically inverted in polarization are formed on the main surface of an element substrate 1 consisting of LiNbO3 to constitute the main surface as a periodic polarization inversion structure. A waveguide 4 extending in intersection with the polarization inversion layers 3 is formed. The waveguide direction of the waveguide 4 is curved at a displacement angle theta in the longitudinal direction of the element substrate 1. The theta satisfies equation costheta=(1+rz)/(1-0.5L.r) (where r is a constant).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to quasi phase matching (QPM:
The second harmonic of the incident wave (Sec
The present invention relates to a wavelength conversion element that generates ond Harmonic Generation).

[0002]

2. Description of the Related Art Light incident on a wavelength conversion element (wavelength: λ
₀ , frequency: ω) and its second harmonic (wavelength: ω)
lambda _0/2, Frequency: To 2 [omega) occurs, it is necessary to wavelength conversion element meets phase matching conditions. Therefore, Quasi Phase Matchi (QPM)
It is widely practiced to satisfy the phase matching condition in the wavelength conversion element by using ng). In this QPM element, the phase matching condition is satisfied by forming a polarization inversion layer in which the polarization direction is periodically inverted for each coherence length along the waveguide direction of the waveguide to adopt a polarization inversion structure. .

This coherence length lc is the refractive index of the incident wave and its second harmonic inside the crystal, n (ω),
If n (2ω), it is expressed by the following equation.

[0004]

[Number 1] _{lc = λ 0/4 (n} (2ω) -n (ω)) However, as can be seen from Equation 1, the interior of the refractive index temperature QPM element is changed n (ω), n (2ω ) Or the wavelength λ _{0 of the} incident wave changes, the coherence length lc changes and the phase matching condition is not satisfied. Therefore, in general, the tolerance of the QPM element with respect to temperature and wavelength is small, and for example, the wavelength fluctuation tolerance of the incident wave is as small as 0.2 nm. Further, the same order is required for the processing accuracy of the domain inversion layer.

Therefore, for example, in order to widen the wavelength fluctuation allowable width, a structure has been proposed in which the inversion period of the polarization inversion structure is gradually changed along the waveguide direction of the waveguide, as shown in FIG. In this structure, for example, when the waveguide is formed so that the difference in inversion period between the vicinity of the entrance end and the vicinity of the exit end of the waveguide is about 0.05 μm, the allowable wavelength variation width is 4 nm.
And the phase matching condition is relaxed. However, the difference between the inversion periods near the entrance end and the exit end is 0.05
It is very difficult to manufacture a domain-inverted structure in which the inversion period is gradually changed so as to be about μm, because high precision is required for processing.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a wavelength conversion element having a structure which is easy to manufacture while relaxing the phase matching condition, in view of the above problems.

[0007]

A wavelength conversion element of the present invention comprises a periodic polarization inversion composed of a plurality of polarization inversion layers whose polarization directions are periodically inverted on the main surface of an element substrate made of a nonlinear optical crystal. A wavelength conversion element having a structure and a waveguide extending in a direction intersecting with the polarization inversion layer, wherein the waveguide has a curved waveguide direction.

[0008]

In the wavelength conversion element of the present invention, the light incident on the waveguide travels while curving along the curved waveguide direction of the waveguide, so that the optical path length in each polarization inversion layer is equal to the waveguide of light. It changes gradually with it. Therefore, the phase matching condition is satisfied at at least one place in the waveguide, and the second harmonic is output.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the wavelength conversion element to which the present invention is applied will be described with reference to the accompanying drawings. In FIG. 2, reference numeral 1 denotes an element substrate made of lithium niobate (Li Nb O ₃ ) which is a non-linear optical crystal having a length L in the longitudinal direction and a c-plane of the crystal formed on the main surface 2 and spontaneous polarization of the crystal. Are aligned in the c-axis direction, that is, in the thickness direction of the element substrate 1. This main surface 2
In order to alternately invert the spontaneous polarization in a predetermined cycle, a plurality of polarization inversion layers 3 in which the spontaneous polarization is inverted are formed at regular intervals in the longitudinal direction of the element substrate 1 to form a periodic polarization inversion structure. Has been taken.

The optical waveguide 4 is formed, for example, by a proton exchange method so as to extend in a direction intersecting with each polarization inversion layer 3 and the waveguide direction is curved with respect to the longitudinal direction of the element substrate 1. One end face of the waveguide 4 is a light incident end face 4a, and the other end face is an emitting end face 4b. The degree of bending of the waveguide 4 is determined as follows.

For example, as shown in FIG. 2, in order to relax the phase matching condition, the waveguide direction of the waveguide 4 is formed linearly and the inversion period of the periodic domain inversion structure is incident on the waveguide 4. In the case of the element substrate 1 formed by gradually widening from the end surface 4a toward the emission end surface 4b, when the light guiding direction is set to the z-axis direction and the coordinates of the element substrate 1 are set as shown in the figure, the polarization is inverted. The period Λ is

[0012]

## EQU00002 ## Λ (z) = Λ ₀ / (1 + rz) Λ ₀ : Inversion cycle at z = 0 r <0: Change rate of inversion cycle.

On the other hand, when the waveguide direction of the waveguide 4 is bent from the longitudinal direction of the element substrate 1, the displacement angle of the waveguide 4 in the waveguide direction with respect to the longitudinal direction of the element substrate 1 is θ, as shown in FIG.
Then, the actual inversion period for the guided wave of light is equivalently

[0014]

## EQU3 ## Λ / cos θ.

Therefore, in the wavelength conversion element of FIG. 1, the optical path length of the light in each polarization inversion layer 3 which is equivalent to FIG.

[0016]

## EQU00004 ## Λ (-L / 2) / cos θ = Λ ₀ / (1 + rz). Therefore, the displacement angle θ of the waveguide 4 is

[0017]

## EQU00005 ## The angle satisfies cos .theta. = (1 + rz) / (1-0.5L.r), which is a function of z. That is, the displacement angle θ of the waveguide 4 faces the longitudinal direction of the element substrate 1 on the incident end face, and the displacement angle θ increases with the light guide and gradually deviates from the longitudinal direction of the element substrate 1. For example, the inversion cycle is 3.5 μm and the width direction of the element substrate 1 is ± 0.5 μm.
In the case of causing the deviation of, the displacement angle θ becomes 9.6 °.

Next, the operation of the above embodiment will be described.
For example, light emitted from a semiconductor laser (not shown) (wavelength:
λ ₀ , frequency: ω) is incident on the incident end face 4a of the waveguide 4 as an incident wave, the second harmonic is excited as this light is guided through the waveguide 4, and the second harmonic is emitted from the emitting end face 4
It is output from b.

Since the waveguide 4 for guiding the incident light is curved with respect to the longitudinal direction of the element substrate 1 and its displacement angle θ gradually increases, the actual polarization of the light on the optical path is guided. The inversion period gradually becomes longer from the incident end face 4a toward the emitting end face 4b. That is, by absorbing fluctuations such as wavelength fluctuations of incident light and fluctuations of the refractive index of the element substrate 1 and always satisfying the phase matching condition at least at one place on the waveguide 4, the second harmonic wave is stabilized. Can be output.

Further, since the inversion period of the periodic domain inversion structure is formed to be constant, the processing accuracy of the domain inversion layer 3 can be relaxed,
The wavelength conversion element of this embodiment can be easily manufactured as compared with the element in which the inversion period is gradually changed. The element substrate 1 is not limited to Li Nb O ₃ and can be made of an appropriate nonlinear optical crystal that generates the second harmonic by QPM such as lithium tantalate (Li Ta O ₃ ) and has the same effect.

[0021]

According to the present invention, since the waveguide direction of the waveguide is curved and the actual inversion period in the waveguide gradually changes along the light guide direction, the incident wave Since it is possible to absorb fluctuations such as fluctuations in wavelength and refractive index, the phase matching condition can be relaxed.

[Brief description of drawings]

FIG. 1 is a top view of a conventional wavelength conversion element in which the inversion period of a periodic polarization inversion structure is gradually changed.

FIG. 2 is a perspective view of a wavelength conversion element showing an embodiment of the present invention.

FIG. 3 is a top view of the same.

[Explanation of symbols for main parts]

 1 element substrate 2 principal surface 3 polarization inversion layer 4 waveguide

Claims (1)

[Claims] 1. A periodic domain-inverted structure composed of a plurality of domain-inverted layers whose polarization directions are periodically inverted on a main surface of an element substrate made of a non-linear optical crystal, and in a direction intersecting the domain-inverted layer. A wavelength conversion element having an extending waveguide, wherein the waveguide has a curved waveguide direction.