JPH05333391A - Wavelength conversion element - Google Patents

Abstract

PURPOSE:To relieve phase matching conditions by forming an additive layer of the film thickness changing in a waveguide direction on a waveguide and changing the waveguide parameter of the waveguide in a waveguide direction. CONSTITUTION:The waveguide 3 for light is formed across a main surface 2 on an element substrate 1 consisting of a nonlinear optical material. Polarization inversion layers 4 in which spontaneous polarization is inverted along the waveguide direction are formed on the waveguide 3. The additive layer is formed in the upper part of the waveguide 3. The additive layer is formed to the thickness which is smallest at the incident end face 3a of the waveguide 3, increases along the waveguide 3 and is largest at the exit end face 3b. Then, the effective refractive index of the waveguide 3 changes toward the exit end face 3b along the waveguide direction from the incident end face 3a. Namely, the wavelength permissible width at which the second harmonic wave is excited is set wider than the conventional permissible width of the wavelength and, therefore, the phase matching conditions by pseudo transfer matching are satisfied in any region within the waveguide 3 even if the wavelength of the input waves fluctuates more than heretofore.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to quasi phase matching (QP).
M: Quasi Phase Matching)
The present invention relates to a wavelength conversion element that generates Harmonic Generation).

[0002]

2. Description of the Related Art In order to generate a second harmonic by being excited by laser light (wavelength: λ ₀ , frequency: ω) incident on a wavelength conversion element, the wavelength conversion element must satisfy a phase matching condition. Is required. Therefore, pseudo transfer matching (QP
It is widely practiced to establish a phase matching condition in a wavelength conversion element by M (Quasi Phase Matching). This Q
In the PM element, a polarization inversion layer in which polarization is inverted for each coherence length is formed along the waveguide direction of the waveguide, and the phase matching condition is satisfied.

The coherence length lc is the refractive index and the propagation constant of the incident wave and its second harmonic wave inside the crystal, respectively.
If (ω) and n (2ω) are given, they are expressed by the following equation. _{lc = λ 0/4 (n} (2ω) -n (ω)) ............... (1) However, as can be seen from equation (1), the refractive index inside of the temperature changes in the QPM element n ( When ω), n (2ω) changes or the wavelength λ _{0 of the} incident wave changes, the coherence length lc changes and phase matching with the element cannot be achieved.
The tolerance for temperature and wavelength of the device is small. For example,
Since this coherence length lc is in the order of μm and (n (2ω) −n (ω)) is in the order of 10 ⁻² , the wavelength fluctuation allowable width 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.

[0004]

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

[0005]

A wavelength conversion element of the present invention is a wavelength conversion element having a waveguide having a polarization inversion structure in which polarization is periodically inverted in the waveguide direction on an element substrate made of a non-linear optical material. In the device, an additional layer whose film thickness changes in the waveguide direction is formed in the waveguide, and a waveguide parameter of the waveguide is changed in the waveguide direction.

[0006]

In the wavelength conversion element of the present invention, since the effective refractive index of the waveguide gradually changes along the waveguide direction, even if the wavelength of the incident wave fluctuates, the phase is changed in any region of the waveguide. Matching is established.

[0007]

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. 1, reference numeral 1 denotes an element substrate made of lithium niobate (LiNbO ₃ ) which is a nonlinear optical crystal, in which the c-plane of the crystal is formed on the main surface 2, and the spontaneous polarization of the crystal is in the c-axis direction, that is, the element substrate 1 Are aligned in the thickness direction. A light waveguide 3 is formed on the main surface 2 by, for example, a proton exchange method so as to cross the main surface 2. One end surface of this waveguide 3 becomes a light incident end surface 3a, and the other end surface becomes an output end surface 3b. Is becoming A polarization inversion layer 4 in which the spontaneous polarization is periodically inverted in the waveguide direction of light is formed in the waveguide 3, and a polarization inversion structure is adopted. The polarization period of this domain-inverted structure is the coherence length lc of equation (1).
And is constant in the present embodiment, for example.

Further, as shown in FIG. 2, an additional layer 5 made of, for example, zinc sulfide (ZnS) is formed in contact with the upper portion of the waveguide 3. The additional layer 5 has the thinnest film thickness on the incident end face 3a of the waveguide 3 and the thinnest film on the outgoing end face 3b as the film thickness increases along the waveguide direction. The additional layer 5 is formed by vapor deposition of ZnS on the main surface 2 after the waveguide 3 having the domain-inverted structure is formed on the element substrate 1. Then, at the time of vapor deposition, for example, a shielding plate is used so that the film thickness is changed in the waveguide direction.

Next, the operation of the above embodiment will be described.
For example, when light (wavelength: λ ₀ , frequency: ω) emitted from a semiconductor laser (not shown) is incident on the incident end face 3a of the waveguide 3 as an incident wave, this light is guided by the waveguide 3 as The second harmonic is excited and the second harmonic is output from the emission end face 3b.

In the waveguide 3, an additional layer 5 is formed on the waveguide 3 so that the film thickness gradually increases from the incident end face 3a toward the emitting end face 3b along the waveguide direction. , The effective refractive index n _eff of this waveguide 3 is determined by the incident end face 3a.
Changes toward the emission end surface 3b along the waveguide direction.
This can be explained as follows. For example, when the element substrate 1 is compared to the model shown in FIG.
The x-axis and the z-axis are the thickness direction and the light guiding direction of the light. The thicknesses of the waveguide 3 and the additional layer 5 are t and h (z), respectively.
And the refractive indices of the air layer, the additional layer, the waveguide, and the substrate are n _a , n _o , _ng , and n _s . Time of light (wavelength: λ, wave number k = 2π / λ) guided in the waveguide 3 and the waveguide direction z
, The propagation constant β in the waveguiding direction is determined as the root of the following mode equation (3), where exp [i (β · z−ωt)] …………………… (2) Be done.

[0011]

[Equation 1]

However,

[0013]

[Equation 2]

Further, the effective refractive index n _eff is calculated from the above propagation constant by the following equation (14): n _eff = β / k ……………………………… (14) Be done. Therefore, based on the above equations (3) to (14), when the refractive index n ₀ of the additional layer is larger than the refractive index _{ng of the} waveguide (n _o > n _g ), the effective refractive index n _eff is It can be seen that as the thickness of the additional layer 5 increases, it increases as shown in FIG. That is, in the waveguide 3, the effective refractive index n _eff increases along the light guiding direction. Therefore, the refractive indices n (ω) and n (2
It can be seen that ω) similarly increases along the waveguide direction.

In this way, the propagation constant of the waveguide 3 is set in advance along the waveguide direction in consideration of variations in the refractive index of the incident wave and the second harmonic and the wavelength of the fundamental wave due to the temperature change of the element substrate 1. Are gradually changing. Therefore, in the wavelength conversion element of the present embodiment, the allowable wavelength range for exciting the second harmonic is set wider than the conventional allowable wavelength fluctuation range.
Even if the wavelength variation of the incident wave exceeds the conventional wavelength variation allowable range, the phase matching condition by QPM can be satisfied in any region in the waveguide 3. That is, the phase matching condition can be relaxed.

The element substrate 1 is not limited to LiNbO ₃ but may be formed of a second QPM such as lithium tantalate (LiTaO ₃ ).
Can be made with an appropriate nonlinear optical crystal that generates harmonics,
It has the same effect. Further, in the above embodiment, the inversion period of the domain-inverted structure is constant, but the same effect can be obtained even if the inversion period gradually changes from the incident end face 3a to the emitting end face 3b.

Further, the film thickness of the additional layer is not limited to that in the above-mentioned embodiment, and the output end face is formed thinner than the incident end face, or the film thickness along the waveguiding direction changes periodically. If it is appropriately changed in the waveguiding direction, the same effect as that of the above embodiment can be expected.

[0018]

According to the present invention, since an additional layer whose film thickness changes in the waveguide direction of light is formed in the waveguide to change the waveguide parameter of the waveguide in the waveguide direction, Since the allowable wavelength range for exciting the second harmonic is set wider than the conventional allowable wavelength fluctuation range, even if a wavelength fluctuation exceeding the conventional allowable wavelength fluctuation range occurs, it will be The phase matching condition by QPM can be satisfied in the region, and the phase matching condition can be relaxed.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of a wavelength conversion element of the present invention.

FIG. 2 is a vertical cross-sectional view taken along the same waveguide direction as above.

FIG. 3 is a model diagram of a cross section of the same waveguide.

FIG. 4 is a graph illustrating the relationship between the thickness of the additional layer and the effective refractive index of the waveguide.

[Explanation of symbols for main parts]

 1 element substrate 3 waveguide 4 pole inversion layer 5 additional layer

Claims (1)

[Claims] 1. A wavelength conversion element having, on an element substrate made of a non-linear optical material, a waveguide having a polarization inversion structure in which polarization is periodically inverted along the waveguide direction. A wavelength conversion element, characterized in that an additional layer of which the wavelength changes is formed in the waveguide to change the waveguide parameter of the waveguide in the waveguide direction.