JP2679407B2 - Optical wavelength conversion element - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical wavelength conversion element used in the fields of optical information processing utilizing coherent light, optical measurement / control fields, printing / platemaking fields, and medical fields.

2. Description of the Related Art In order to efficiently convert the wavelength of light into the second harmonic, the phase matching condition must be satisfied in the wavelength conversion element. As this method, an angle phase matching method, a temperature phase matching method, a method using a waveguide, etc. have been proposed and used conventionally. Recently, a method called quasi-phase matching using a periodic structure has been attracting attention as a phase matching method. For example, Physical Review (Phys. Rev.) Volume 127
1918 1962 JA Armstrong et al. This is because the polarization direction in the crystal is periodically reversed and the fundamental wave
It is intended to compensate for the amount of phase mismatch between ₁ and the harmonic P ₂ .

Here, polarization reversal means reversing the direction (moment) of the crystal axis. A polarization inversion SHG element is an application of this technique to an SHG element.

The polarization-inverted SHG element has a structure in which the domain-inverted regions are formed perpendicularly to the three-dimensional optical waveguide and periodically, and when the light that becomes the fundamental wave is coupled to the optical waveguide, the second harmonic wave with half the wavelength inside the element Is generated and emitted from the optical waveguide.

Since this polarization inversion type SHG element is proportional to the square characteristic, it is possible to increase the output. The characteristics of the squared output of the polarization inversion type will be described. In an ordinary optical waveguide in which the polarization is not inverted, the output draws a sine curve with respect to the element length due to the difference in the propagation constants of the fundamental wave and the harmonics.
If the boundary inverted by polarization inversion is aligned with the peaks and valleys of the sine curve, the output will increase in the region where the output had decreased, and the output will increase. The squared characteristic is shown because the optical output is proportional to the square of the optical electric field in the waveguide.

The period of the domain inversion region is determined by the difference between the propagation constants of the fundamental wave and the harmonic wave. This propagation constant difference is ΔK, and the element length is l
Then It is. However, P ₂ is the harmonic output. The output of this P ₂ is It vibrates with a period Λ represented by.

 An outline of the structure obtained by this method is shown in FIG.

At this time, the period Λ along the light traveling direction in the domain-inverted region
Is Λ = n, where Δk is the propagation constant difference between the fundamental wave and the harmonic wave.
(2π / Δk) (where n is an odd number), and its width W is = Λ / 2.

As shown in FIG. 4, the harmonic output at this time is such that the harmonic output 6 guided in the region where branching and inversion is not performed only draws a sine curve with respect to the element length l. The area where the slope of the sine curve of the wave output 6 is negative (l ₁ ≤l≤l ₂ ,
The output 7 of the higher harmonic wave when the polarization of (l ₃ ≦ l ≦ l ₄ ) is inverted remarkably increases as shown in the graph. This is because the light guided in the crystal without polarization reversal becomes 0 at the photoelectrode at l = l ₂ and l = l ₄ , but by making the polarization reversal region 3 as described above, l = l _Since the optical electric field at 2 can be doubled when l = l ₁ , the harmonic output 7 at l = l ₂ is 4 times that when l = l ₁ .

3 times the optical field when the l = l ₁ in the same manner as l = l _3, l =
The optical field at l ₄ is 4 times that at l = l ₄ , so output 7
Are 9 times and 16 times, respectively, when l = l ₁ .
That is, in order to increase the harmonic output, it can be understood that the portion of the sine curve 6 having a negative slope may be polarization-inverted.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention In order to manufacture a waveguide type and polarization inversion element, it is necessary to form a region 3 in which the polarization is periodically inverted as shown in FIG. At present, the method of forming this region by heat treatment is the mainstream, but according to this method, there is a problem that the depth y of the region depends on the width x, and there is a problem that the region width x is narrow as shown in FIG. There is a problem that the inversion depth required as an element cannot be obtained as shown in FIG. Further, if the order of the cycle is made higher in order to obtain a sufficient depth, the harmonic output will decrease as shown in FIG. In FIG. 6, 6a is a non-polarization inversion type optical wavelength conversion element, 7a is a polarization inversion type optical wavelength conversion element having a primary period,
8a is a polarization inversion type optical wavelength conversion element having a third period,
Reference numerals 6, 7 and 8 indicate harmonic outputs of 6a, 7a and 8a, respectively.
As can be seen from this figure, the conversion element 7a of the first cycle can obtain the highest harmonic output, and the output decreases as the cycle order increases. If the period is of degree n, the output will be reduced to 1 / n ² for samples whose length is kn / 2 times the period (where k is an integer). In order to compensate for this, for example, it is necessary to take a method of making the length n times, but at this time, there is a problem that the tolerance of the fundamental wavelength becomes 1 / n.

In view of the above point, the present invention has an object to provide an optical wavelength conversion element having a domain-inverted region having a depth sufficient to obtain domain inversion and capable of obtaining a large harmonic output.

Means for Solving the Problem In order to obtain a higher harmonic output, the same output as the higher harmonic output 6 shown in FIG. You must use the cycle. In order to make the width x of the inversion region wider than that of the first order, at least the third order cycle is used. In this case, the output is 1/9, which is about one digit lower. Therefore, in order to solve this, a non-polarization reversal region and a polarization reversal region within the period are used by using a second-order period in which an output higher than the third-order period is obtained and a domain-inversion region deeper than the first-order period is obtained. The ratio of the width of is set to 1: 3.

Action Using the above method, the width of the inversion region is widened to three times that of the first order, and the output is 1 for the third order.
Compared to the next time, it will be reduced by 1 time, but it will be suppressed to 1/4.

Therefore, a sufficient depth can be obtained as the inversion region, and the output is about 2.3 times as large as that in the third-order period, and it is possible to manufacture a highly efficient wavelength conversion element.

EXAMPLE FIG. 1 is a perspective view of a light wavelength conversion element according to the present invention, and FIG. 2 is a sectional view thereof. In this embodiment, as the substrate 1
LiNbO ₃ is used. SiO ₂ is deposited on this substrate 1.
The pattern forms a quadratic stripe whose width is 3/4 of this cycle. When this substrate is heat-treated near the Curie temperature, a region where the polarization is inverted is formed immediately below SiO ₂ . After that, SiO ₂ is removed and a three-dimensional optical waveguide is produced in a direction orthogonal to the stripe pattern to complete the optical wavelength conversion element.

The depth of the domain-inverted region depends on the width x of this region. This is as shown in FIG. 7, and in the case of the second order, since the period of the inversion region is about 6 μm for the device having such a structure, the width of the inversion region is about 4.5 μm, and therefore the inversion depth is The height is about 0.85 μm. The waveguide depth is designed to be 0.7 μm in consideration of loss and nonlinearity. Therefore, it is possible to form a polarization inversion deeper than that of the optical waveguide. By the way, when the first-order period is used, the area width x is 1.5 μm
Therefore, the inversion depth is only 0.02 μm.

FIG. 8 compares the theoretical output of this element with that of the third-order period.

In FIG. 8, 8a is a polarization reversal type optical wavelength conversion element having a third period, and 9a is a polarization reversal type light in which the ratio of the width of the non-inversion region 4 to the inversion region is 1: 3 in the second period of the present invention. It is a wavelength conversion element. Reference numerals 8 and 9 indicate harmonic outputs of 8a and 9a, respectively. As is clear from this figure, by increasing the period from the third order to the second order, the output of the harmonics is significantly increased, and at the same time, the depth of the domain inversion region is the width x of the domain inversion region in the first period.
Although it was not possible to make it deeper than the waveguide 2 because it was small, it is possible to obtain a sufficient depth for polarization inversion by increasing the width of the domain inversion region in the period as in the present invention.

When the output of blue light was observed using a semiconductor laser (wavelength 0.83 μm) for this device, the fundamental wave power in the waveguide was 50.
An output of 1 mW was obtained for mW. The output from the element of the first cycle manufactured as a reference was 40 μW, which was an improvement of 25 times.

Although using LiNbO ₃ as the substrate here, LiNbO ₃
By using, it is possible to manufacture a device that is double-digit strong against optical damage and further increase the input, so that the output can also be increased in proportion to the square thereof. Although an example of an optical waveguide type element has been described here, phase matching occurs similarly even if the crystal itself is a bulk shape in which the polarization inversion region and the non-polarization inversion region are used without using the optical waveguide. Can be obtained.

EFFECTS OF THE INVENTION As described above, according to the present invention, the polarization direction of the surface of the substrate having the optical waveguide is inverted in a quadratic period, and the width of the inversion region is set to 3/4 of the period. A domain-inverted region having a sufficient depth with respect to the depth can be produced. Moreover, the theoretical efficiency is reduced to 1/4 of the case of the first cycle, but the actual output is improved 25 times, and it is also possible to make up for the theoretical reduction in output by the element length. , Its substantial effect is great.

[Brief description of the drawings]

FIG. 1 is a perspective view of an optical wavelength conversion device according to an embodiment of the present invention, FIG. 2 is a sectional view thereof, FIG. 3 is a schematic view of a conventional polarization inversion wavelength conversion device, and FIG. 4 is quasi phase matching. Fig. 5 is a characteristic diagram of the harmonic output by Fig. 5, Fig. 5 is a sectional view showing the relationship between the polarization inversion depth and the waveguide thickness in the element, and Fig. 6 is the characteristic of the harmonic output by the difference in the period of the polarization inversion type wavelength conversion element. Figure, 7th
FIG. 8 is a characteristic diagram showing the relationship between the width of the domain inversion region and the depth formed at that time, and FIG. 8 is a characteristic diagram of the output of the device of the present invention. 1 ... LiNbO ₃ substrate, 2 ... Optical waveguide, 3 ... Polarization inversion region, 4 ... Non-polarization inversion region, 6 ... Harmonic output when not polarization inversion, 7 ... Polarization inversion In the case of (1st period), 8 ... Harmonic output in the case of polarization inversion (3rd period), 9 ... Harmonic output in the case of polarization inversion (Secondary period, area ratio) 1: 3), 10 ... polarization direction, P ₁ ... fundamental wave, P ₂ ... harmonic wave.

 ─────────────────────────────────────────────────── --Continued from the front page (56) References JP-A-2-63026 (JP, A) JP-A-2-93624 (JP, A) Electronics Letters, Vol. 25 No. 3 (1989) P. 174-P. 175 Appl. Phys. Lett. , Vol. 56 No. 18 (1990) P. 1725-P. 1727

Claims (2)

(57) [Claims] 1. An optical waveguide on a surface of a LiNbO ₃ crystal, and periodically has a region in which a width of a polarization inversion region and a non-polarization inversion region is 3: 1 with respect to a traveling direction of light of the optical waveguide. Letting ΔK be the propagation constant difference between the fundamental wave P ₁ and the harmonic wave P ₂ , the period is 2π /
An optical wavelength conversion element characterized by ΔK. 2. The optical wavelength conversion device according to claim 1, wherein a LiTaO ₃ crystal is used instead of the LiNbO ₃ crystal.