JPH05341340A - Wavelength converting element - Google Patents

Abstract

PURPOSE:To provide a wavelength converting element having the structure with relaxed phase matching condition by providing a temp. controlling means imparting a temp. gradient in the waveguide direction of a waveguide. CONSTITUTION:A optical waveguide 3 is formed so as to intersect respective polarization inversion layers formed on the main surface 2 of an element substrate 1. An insulated buffer layer 4 is formed of silicon dioxide on the main surface 2 while covering the upper part of the waveguide 3. Thin film heaters 5 as a temp. controlling means are formed in a pair by, for example, vapor- depositing titanium respectively at the upper part in the vicinity of an incident end face 3a and at the upper part in the vicinity of the emitting end face 3b of the waveguide 3 via the buffer layer 4. Since the refractive indexes to an incident wave and to a second higher harmonic are respectively changed along the waveguide direction corresponding to the temp. gradient along the waveguide direction of the waveguide 3, the phase matching condition is satisfied in at least one region of the waveguide 3 even when the wavelength of the incident wave in fluctuated. Consequently, the phase matching condition is relaxed.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Light incident on a wavelength conversion element (wavelength:
lambda _0, Frequency: is excited in omega) its second harmonic (wavelength: lambda _0/2, Frequency: To 2 [omega) occurs, must be phase-matching condition is satisfied in the wavelength conversion element Becomes Therefore, quasi phase matching (QPM: Quasi Phase Ma)
It is widely practiced to establish a phase matching condition in a wavelength conversion element by tching). In this QPM element, the phase inversion condition is established 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 form a polarization inversion structure. There is.

The coherence length lc is the refractive index of the incident wave inside the crystal and the refractive index of its second harmonic, n.
Assuming that (ω) and 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. In addition, generally QP
The tolerance for the temperature and wavelength of the M element is small, and for example, the wavelength fluctuation allowable width of the incident wave is as small as 0.2 nm. Therefore, the same order is required for the processing accuracy of the domain inversion layer.

With this structure, for example, when a waveguide is formed in which the inversion period is gradually changed 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 wavelength fluctuation occurs. The allowable width is expanded to about 4 nm, and the phase matching condition is relaxed. However, fabrication of a polarization inversion structure in which the inversion period is gradually changed so that the difference in inversion period between the vicinity of the incident end and the vicinity of the emission end is about 0.05 μm requires high precision in processing. Being very difficult.

[0006]

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.

[0007]

A wavelength conversion element of the present invention has a waveguide on an element substrate made of a nonlinear optical crystal,
A wavelength conversion element that outputs the second harmonic of an incident wave,
It has a temperature control means for providing a temperature gradient in the waveguide direction of the waveguide.

[0008]

According to the present invention, since the refractive index for the incident wave and the second harmonic changes according to the temperature gradient along the waveguide direction of the waveguide, respectively, the wavelength of the incident wave changes. Even if f varies, the phase matching condition is satisfied at least in any region of the waveguide.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a wavelength conversion device to which the present invention is applied will be described with reference to FIGS. In FIG.
Reference numeral 1 is an element substrate made of a nonlinear optical crystal. This element substrate 1 is made of lithium niobate (Li Nb O ₃ ) and has a c-plane of the crystal as a main surface 2 and a c-axis direction of the crystal as a thickness direction of the element substrate 1. The main surface is cut and formed to have a length l and a width w. The spontaneous polarization of the element substrate 1 is aligned with the c-axis direction. A polarization inversion layer in which the spontaneous polarization is periodically inverted is formed on this main surface to form a periodic polarization inversion structure. The inversion period of this polarization inversion structure is determined based on the coherence length lc of the equation 1, and in this embodiment, it is inverted at a constant period, for example. An optical waveguide 3 is formed so as to intersect with each polarization inversion layer by, for example, a proton exchange method, and one end face of this waveguide 3 becomes an incident end face 3a of light and the other end face becomes an emission end face 3b. ing.

Further, an insulating buffer layer 4 is formed on the main surface while covering the upper portion of the waveguide 3 with silicon dioxide (SiO 2).
₂ ). And this buffer layer 4
The thin film heaters 5 serving as temperature control means are formed in pairs above and below the entrance end face 3a and the exit end face 3b of the waveguide 3 via, for example, by vapor deposition of titanium (Ti).

Next, the operation of this embodiment will be described. With the thin film heater 5, for example, the temperature of the incident end side of the waveguide 3 at the temperature T _in
Then, the emission end side is controlled to the temperature T _o (T _in > T _o ), respectively. If there is no great difference in temperature between the element substrate 1 and the surroundings and heat radiation from the element substrate 1 to the surroundings can be almost ignored,
As shown in FIG. 2, a temperature gradient is generated in the waveguide direction of the waveguide, and for example, the temperature of each region of the length dx in which the waveguide 3 is divided into infinitesimal small portions in the waveguide direction is measured along the waveguide direction. Change. Light (wavelength: λ ₀ , frequency: ω) emitted from, for example, a semiconductor laser (not shown) is incident on the wavelength conversion element having a temperature gradient along the waveguide 3 as an incident wave on the waveguide 3. When incident on the end face 3a, the second harmonic is excited in any of the waveguide regions where the phase matching condition is satisfied, and the second harmonic is output from the emitting end face 3b.

Therefore, in order to satisfy the phase matching condition,
A temperature gradient is given in advance to the waveguide direction of the waveguide 3 to change the temperature of the waveguide in the waveguide direction, and each refractive index n for each region dx partitioned infinitely small in the waveguide direction of the waveguide 3. (Ω), n
If (2ω) is changed according to the temperature, the phase matching condition can be satisfied at least in any region of the waveguide 3 even if the wavelength of the incident wave changes.

Therefore, the phase matching condition can be relaxed and the second harmonic can be output even when the wavelength of the incident wave changes. The temperature change in the guiding direction of the waveguide 3, even if the temperature gradient is controlled to a temperature lower than the temperature T _o of the exit end temperature T _in the incident end side to the opposite contrary to the above embodiment (T _in <T _o ), which has a similar effect.

Next, a second embodiment will be described with reference to FIG. In FIG. 3, similarly to the first embodiment, the element substrate 1 is provided with the periodically poled structure, the waveguide 3, and the buffer layer 4. Then, a thin film heater 5 is formed as a temperature control means for giving a temperature gradient to the waveguide 3 via the buffer layer 4. This thin film heater 5
Is formed by vapor deposition at Ti and changes the temperature of the waveguide 3 along the waveguide direction. For example, the density of the thin film heaters 5 per unit length in the waveguide direction is the highest on the incident end side. The thin film heater 5 is formed so that the density of the thin film heater 5 gradually decreases along the wave direction and becomes the lowest on the emission end side.

Also in this embodiment, as in the case of the first embodiment, there is a temperature gradient in the waveguide direction of the waveguide 3 and the temperature is different for each region dx in the waveguide direction. Since the refractive indices n (ω) and n (2ω) are changed, respectively, the phase matching condition can be relaxed and the second harmonic can be output even when the wavelength of the incident wave changes.
Further, as the temperature control means in the waveguide direction of the waveguide 3,
Not limited to the above-mentioned embodiment, the thin film heater 5 and the buffer layer 4 are provided for each region of the waveguide appropriately partitioned along the waveguide direction.
Formed through, and controlling the temperature of each region,
It is possible to obtain the same effect as that of each of the above embodiments.

In the above embodiment, the heating means is used as the temperature control means, but the same effect can be obtained even if a temperature gradient is given to the waveguide by the cooling means. The element substrate 1 is not limited to Li Nb O ₃ , but lithium tantalate (Li
It can be made of an appropriate nonlinear optical crystal that generates the second harmonic by QPM such as Ta O ₃ ) and has the same effect.

In this embodiment, the inversion period of the domain inversion structure is constant, but the same effect can be obtained by gradually changing the inversion period from the incident end face 3a to the emitting end face 3b.

[0018]

According to the present invention, the temperature control means gives a temperature gradient in the waveguide direction of the waveguide so that the incident wave and the second harmonic wave are generated in each region of the waveguide in the waveguide direction in accordance with the temperature thereof. Since the refractive index of each of the waveguides changes, the phase matching condition is satisfied at least in any region of the waveguide even if the wavelength of the incident wave fluctuates, so that the phase matching condition can be relaxed.

[Brief description of drawings]

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

FIG. 2 is a graph showing the temperature distribution in the waveguide direction of the above waveguide.

FIG. 3 is a perspective view of a wavelength conversion element showing a second embodiment of the present invention.

[Explanation of symbols for main parts]

 1 element substrate 3 waveguide 5 thin film heater as temperature control means

Claims (1)

[Claims] 1. A wavelength conversion element having a waveguide on an element substrate made of a non-linear optical crystal and outputting a second harmonic of an incident wave, the temperature control providing a temperature gradient in the waveguide direction of the waveguide. A wavelength conversion element comprising means.