JPH0310229A - Optical wavelength converting method - Google Patents

Abstract

PURPOSE:To obtain high wavelength conversion efficiency by adequately setting the polarization direction of the basic waves made incident on the PRA crystal expressed by specific molecular formula. CONSTITUTION:The single crystal of the nonlinear optical material 3, 5- dimethyl-1-(4-nitrophenyl) pyrazole: PRA expressed by formula I is used and the linearly polarized basic waves of the respective wavelengths existing between 450 and 4000nm are made incident on this bulk single crystal and the angle phase matching of a type I is attained, by which the sum frequency thereof is obtd. Namely, the phase matching conditions of the type I are nW<1>, 2/lambda1+nW<2>, 2/lambda2=nW<3>, 1/lambda3 when the refractive indices of the wavelengths lambda1, lambda2 when made incident at an arbitrary angle to respective basic waves are designated as nW<1>, nW<2> and the refractive indices of the wavelength lambda3 to the sum frequency is designated as nW<3>. The high conversion efficiency is obtd. if such conditions are satisfied. Since the PRA does not absorb much light near 400nm wavelength and, therefore, the sum frequency of the blue region is efficiently generated. The high wavelength conversion efficiency is obtd. in this way and the sum frequency of the blue region is easily obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical wavelength conversion method for converting two fundamental waves generated from a light source into a sum frequency using an optical wavelength conversion element.

(Prior art) Conventionally, two types of fundamental waves with different wavelengths λ1 and λ2 are incident on a nonlinear optical material, and the wavelength λ3 (1/λ3-1
/λ1+]/λ2) Various attempts have been made to extract the sum frequency of /λ1+]/λ2). Specifically, a bulk crystal type optical wavelength conversion element that performs wavelength conversion in this manner is well known. In addition, J,
Appl, phyS, Vol, 55p
65 (1984), YaO et al. describe in detail the phase matching method during second harmonic generation using KTP, which is a biaxial crystal.

Conventionally, LiNb has been used as an inorganic material as a nonlinear optical material for forming the above-mentioned bulk crystal type optical wavelength conversion element.
O3, KTP, organic interest rate, for example, JP-A-60-250
MNA (2-methyl-4-
NPP (N-(4-nitrophenyl)) as described in J. Opt. Soc. Am.
-L-prolinol), NPAN (N-(4-nitrophenyl)-N-methylaminoacetonitrile) Compared to nonlinear optical materials, it has advantages such as high wavelength conversion efficiency, high dielectric breakdown threshold, and low optical damage due to its large nonlinear optical constant.

(Issue 20 to be solved by the invention) However, since the absorption edge of these organic nonlinear optical materials exists around 450 nm for MNA and 480 nm for NPP, it is difficult to generate a sum frequency in the blue region. It has become. Furthermore, KTP,L which is an inorganic material
The absorption edge of iNb03 and the like is 400 nm or less, and the disadvantages are that it is not possible to obtain a sum frequency in the blue region, and that the figure of merit of wavelength conversion is at least one order of magnitude lower than that of organic materials.

Similarly, when obtaining the sum frequency of long wavelength regions such as green and red, the wavelength conversion efficiency is also low because the figure of merit is small.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical wavelength conversion method that can obtain high wavelength conversion efficiency and also easily obtain a sum frequency in the blue region. The target is

(Means for Solving the Problems) The optical wavelength conversion method of the present invention provides a nonlinear optical wavelength conversion element (35-dimethyl-1( 4-
Using a single crystal of (nitrophenyl) pyrazole (hereinafter referred to as PRA), each wavelength is 450 to 4000.
It is characterized in that a linearly polarized fundamental wave between nanometers is incident on this single crystal, and a sum frequency thereof is obtained by performing angular phase matching of type (2).

(Yi Tsuki J) 1. 0 4
3 It is disclosed in Publication No. 2, and it is also shown in the publication that it has a nonlinear optical effect, but when actually forming and using a bulk crystal type optical wavelength conversion element, there are It was unclear whether good phase matching could be achieved between the fundamental wave and the sum frequency by doing as follows.

The points where Type I angular phase matching can be achieved in the method of the present invention will be explained below. The crystal structure of the PRA is shown in FIGS. 2A, 2B and 20.

Further, FIG. 3 shows its bulk crystal structure. This PR
The crystal of A has an orthorhombic system, and the school of fish is mm2. Therefore, when considering the tensor of nonlinear optical constants, when light linearly polarized in the X direction (hereinafter referred to as It is a nonlinear optical constant, and similarly 63
2 is a nonlinear optical constant when the fundamental wave of Y polarization is input and the sum frequency of X polarization is extracted, d33 is the nonlinear optical constant when the fundamental wave of X polarization is input and the sum frequency of X polarization is extracted, and d24 is The nonlinear optical constant, d15, is the X
This is a nonlinear optical constant when the fundamental wave of X polarization is input and the sum frequency of X polarization is extracted. The magnitude of each nonlinear optical constant is shown in the table below.

becomes. Here, C]31 is the crystal 1 as shown in FIG.
'+l Optical axis x determined for a, b, c,
In the above table, y and z are values obtained by X-ray crystal structure analysis, and z are values measured by the Marker Fringe method, and both units are [x1O-9e su].

Based on this value, when comparing the effective nonlinear constant dert of KTP and the figure of merit, d32 of PRA is approximately 100 times larger.

Also, since PRA is an orthorhombic and biaxial crystal,
The refractive index nX when the polarization plane of light is in the optical axis X-axis (crystal axis c-axis) direction, and the refractive index when the polarization plane of light is in the y-axis (crystal axis b-axis) direction perpendicular to this x-axis. nYs and X, 7
It has a refractive index n2 in the Z-axis (crystal axis a-axis) direction perpendicular to both axes. These refractive indices nx % nY %
The wavelength dispersion of nZ is shown in FIG.

When the wavelength of the fundamental wave exceeds 4000 nm, it can be expected that it will reach the vibrational level of the PRA molecule and that the fundamental wave will be absorbed by the PRA molecule. Therefore, in the present invention, the upper limit of the wavelength of the fundamental wave is set to 4000 nm. On the other hand, as will be explained later, since PRA absorbs more light with wavelengths lower than 400 nm, the wavelength λ3 of the obtained sum frequency is 40 nm.
It must be 0 nm or more. Therefore, considering safety, if the lower limit of the sum frequency wavelength λ3 is set to 405 nm, then the wavelength λ1 of one fundamental wave is at its maximum and the above upper limit = 4000.
nm, in which case the wavelength λ2 of the other fundamental wave is approximately 45 nm.
0 nm (rounded down to the nearest whole number). Wavelength of sum frequency λ3
In order to make the above lower limit value -405 nm or more, if the fundamental wave wavelength λ2 is lower than 450 nm, one of the fundamental wave wavelengths λ1 must be set higher than 4000 nm. Therefore, in the present invention, the lower limit wavelength of the fundamental wave is set to 450 nm.

Regarding the phase matching method when the second harmonic is generated by a diosseous crystal, there is a detailed description in the paper by Yao et al. This will be expanded and explained. That is, as shown in FIG. 5, φ is the angle between the traveling direction of light in the crystal and the optical axis Z of the crystal, and θ is the angle
The angle from the axis in the above direction of travel. Here, the refractive index for each fundamental wave when incident at an arbitrary angle is n v
ln W2, the refractive index for the sum frequency is n'''3,
The refractive index for the fundamental wave and sum frequency of each axis is n
X5nYs Next, let `` cos θ · sin θ nX S ny S nZ % nma3, npu3.

When kx = sjn φ kY = sin φ kz = cos φ -2□ and h3x, the solutions of the above equations (L-1), (1-2), and (1-3) become the phase matching condition. .

th = kx2 (bl+cl)ky2 (at
+c1) kx2 (al +bl) CI = kX2bl C1+lcy” al c1
+, 42a1 bl Bz = kx2 (bz 1 Cz) kYz (az
102) kz 2 (aZ +b2) C2= kx2b2 C2+kY 2a2 C20k
z 2a2 bz B3-kx ” (b3+C3) ky 2 (a3 +c3 ) kz 2 (a3 +b3 ) C3= kx2b3 C3+kY” a3 C3+ 22a3b3 al = (nmo')-2az=(n吏2)-2b1-
(n;')-2bz = (n':Y')-2Cl-
(n bu')-2Cz=(n pu 2)-2a3- b3- c3=The solution to the equation is (n pu')-2 (n'': Y')-2 (n pu 3)- 2 (1-1), (1-2), and (1-3) (decoding is + when i=1 and - when i=2).

The phase matching condition for type ■ is 0°1・2/λz+n"2.2/λ2 = n"3.
+/λ3...(1-4).

Therefore, in the case of the present invention, type I phase matching can be achieved when angles φ and θ that satisfy equation (L-4) exist. Therefore, by using the refractive index of the PRA shown in FIG. 4, it is possible to determine the phase angles θ and φ for the two fundamental waves and their sum frequency that satisfy the equation (1, -4).

In addition, the transmission spectrum of a 200 μm thin film of PRA is shown in Figure 6, and as shown in the figure, this PRA has a wavelength of 4.
It does not absorb much light around 00 nm. Therefore, according to the optical wavelength conversion element using this PRA bulk crystal, it is possible to efficiently generate a sum frequency in the blue region.

(Example) Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

First Embodiment FIG. 1 shows an apparatus for converting two fundamental waves into a sum frequency by the method of the present invention. First, a method for producing the bulk single-crystal type optical wavelength conversion element 10 will be described. Element 10 can be made by the usual Bridgman method. First, PRA in a molten state is poured into a suitable mold and then rapidly cooled.] 2 Then, this PRA is polycrystallized. After that, this PRA,
Temperatures higher than its melting point of 102°C (e.g. 105°C)
By gradually drawing the molten PRA from the inside of the furnace, which is maintained at a temperature of 100,000 yen, to the outside of the furnace, which is maintained at a temperature lower than the melting point, the molten PRA is single-crystalized from the portion drawn out of the furnace. As a result, 5
A PRA that is in a single crystal state over a long range of 0 mm or more and has a uniform crystal orientation is formed, and the optical wavelength conversion element 10 can be made sufficiently long. As is well known, the wavelength conversion efficiency of this type of optical wavelength conversion element is 2 times the length of the element.
Since the wavelength conversion element is proportional to the power, the longer the optical wavelength conversion element is, the higher the practical value becomes.

The PRA single crystal formed as described above is cut along the Y-2 plane that includes the optical axis Y and two axes (the b-axis and the a-axis in the crystal axis), and in the A bulk single-crystal type optical wavelength conversion element 10 was formed by cutting out a piece with a thickness of 2 mm.

A fundamental wave 15 is incident on this optical wavelength conversion element 10 as shown in FIG. In this example, a semiconductor laser 11 and a YAG laser are used as the light sources that generate the fundamental wave.
2I are used, and the wavelength λ emitted from each of them is
Laser light (fundamental wave) with λ2 = 830 nm, λ2 = lOG4nm, i5A and 15B are beam splitter 2
After being multiplexed at 2, the light is input to the optical wavelength conversion element 10.

At this time, the side base water waves 15A and 15B are incident with the incident direction being inclined by 0.6° (=θ) from the X axis to the Y axis side inside the PRA crystal. In this embodiment, the angle is φ-90°.

From the wavelength dispersion of the refractive index of PRA shown in Figure 4, the wavelength 83
For light with a wavelength of 0 nm, n
5077 n , t = 1.7806 n z '= 1. It is fi890. And when θ=0.6° and φ=90°,
The fundamental wave 15A with wavelength λ1 = 830 nm is ny and ny
We feel the intermediate refractive index of n '' = 1.7978. The fundamental wave 15B with wavelength λ2 = 1084 nm also has n
) (and ny, we feel that the refractive index is intermediate, and n゛'' = 1.7805.

The fundamental waves 1.5A and 15B thus incident on the optical wavelength conversion element 10 made of PRA have a wavelength of 466 nm.
The sum frequency of m is converted to 15°. Therefore, from the optical wavelength conversion element]0, this sum frequency 15' and the fundamental wave 15A,
A beam mixed with 15B is emitted. This output beam transmits the sum frequency 15' with a wavelength of 46 Gnm well, while transmitting the fundamental wave 15A with a wavelength of 830 nm and the wavelength 10 [i4
The fundamental wave 15B of nm is passed through the absorbing bandpass filter 13, and only the sum frequency 15'' is taken out. This sum frequency 15' has been confirmed to be Z-polarized light. Therefore, in this example, the above-mentioned (1-4) is satisfied,
Phase matching is achieved between the fundamental waves 15A, 15B and the sum frequency 15'. As described above, the high value d32 is used as the nonlinear optical constant.

From the wavelength dispersion of the refractive index of PRA shown in Figure 4, the wavelength 40
For 6 nm light, nX = 1.5553 ny = 1.9475 nz -1,7902, and in this case, the sum frequency 15'' is Z polarized light, so
For a sum frequency of 15'' with wavelength λ3 = 46B nm, n''=nz=], 7902.

λ1-830 nm, λ2 = ], 064n shown above
m.

λ3 = 466 nm% and n"' = 1
．． 7978, n ′2 = 1°7805, n ′3 =
The value of 1.7902 satisfies the phase matching conditional expression (1.-4) described above.

The above was explained for the case of φ-90°, but when φ=90
In addition to the case of .degree., there are other angles .theta. and .phi. where phase matching can be achieved.

As a comparative example, a bulk single crystal of KTP with a thickness of 2 mm was formed, and fundamental waves], 5A, and 15B were incident on this bulk single crystal using the same device as shown in Fig. 1 to generate a sum frequency. . Sum frequency 15' in this comparative example and the above example
The light intensity of the sample (passed through the bandpass filter 13) was measured, and the sum frequency light intensity in the above example was more than an order of magnitude higher than that in the comparative example.

Second Example> A bulk single crystal of PRA was produced in the same manner as in the first example. He-N as a light source that generates the fundamental wave
Using an e-laser and a semiconductor laser, the wavelengths λ1 = 633 nm and λ2 = I550n emitted from them, respectively.
A laser beam (fundamental wave) of m was made incident on the optical wavelength conversion element 10. At this time, for the side base water wave, θ-2,28',
It is φ-90°. For light with a wavelength of 633 nm, n>, =1. ．． 523B ny = 1.8320 n z = 1.7193, and for light with a wavelength of 550 nm, n x =
1.5012 ny = 1.71i04 nz = 1.6773. And as above, θ-2, 28°, φ-9
When the angle is 0°, the fundamental wave with wavelength λ1=633 nm is n
We feel a refractive index that is intermediate between becomes.

In this way, the fundamental wave incident on the optical wavelength conversion element 10 made of PRA is converted into a sum frequency wave having a wavelength λ3 = 449 nm. For light with a wavelength of 449 nm, n x =1.5632 n Y =1.9792 n z ""1.8107. In this case, the sum frequency becomes Z-polarized light, so for a sum frequency of wavelength λ3 = 449 nm, n ””
” n z =1.8107.

λ1, λ2, λ3 and n"1, n"2n"3 mentioned above
The value of is also determined by changing the phase matching conditional expression (1-4) explained earlier to i
”Add iYj.

Third Example> A bulk single crystal of PRA was produced in the same manner as in the first example. A dye laser and an infrared laser were used as light sources for generating a fundamental wave, and laser beams (fundamental waves) of wavelengths λ1=450 nm and λ22-4000 nm emitted from them were made incident on the optical wavelength conversion element 10. At this time, for the side base water wave, θ = 10.5G°, φ = 90
°. For light with a wavelength of 450 nm, n x = 1.5G29 n v = 1.9780 n z -1,8099, and for light with a wavelength of 4000 nm, n x =
1.4826 n Y '= 1.6401 n z = 1.6070. And as above, θ= 10.56°, φ=
When the angle is 90°, the fundamental wave with wavelength λ1 ``450 nm feels an intermediate refractive index between n) (and ny, and n ''=
It becomes 1.9584. The fundamental wave of wavelength λZ = 4000 nm also has an intermediate refractive index between nx and 09, and n″2 = 1.6330.

In this way, the fundamental wave incident on the optical wavelength conversion element 0 consisting of a PRA is converted into a sum frequency having a wavelength λ3 = 400 nm. For light with a wavelength of 400 nm, n x =1.6001 ny -2,1479 n z =1.9256. In this case, the sum frequency becomes Z-polarized, so for a sum frequency of wavelength λ3 = 400 nm, n″′3
= nz = 1.9256.

λ1, λ2, λ3 and n°゛1, n“2n” mentioned above
The value of 3 also satisfies the phase matching conditional expression (1-4) described above.

Fourth Example> A bulk single crystal of PRA was produced in the same manner as in the first example. Semiconductor lasers were used as two light sources for generating fundamental waves, and laser beams (fundamental waves) with wavelengths λ1 = 830 nm and λ2-1500 nm emitted from them were made incident on the optical wavelength conversion element 9 and the element 10. At this time, for the side base water wave, θ=
19.8°, φ-90°. For light with a wavelength of 830 nm, n)(=1.5132 ny = 1.7979 nz =1.6991, and for light with a wavelength of 1500 nm, nX=1.
5018 n Y = ], 7G22 n 2 = 1. [It is 1783. And as above, θ=19.8°, φ=9
At 0°, the fundamental wave with a wavelength λ1 = 830 nm has a refractive index intermediate between nx and ny, and n'' = 1.7568.The fundamental wave with a wavelength λZ = 1500 nm also has a refractive index intermediate between nX and ny. The refractive index is n''=1.7253.

In this way, the fundamental wave incident on the optical wavelength conversion element 10 made of PRA is converted into a sum frequency of wavelength λ3-534 nm. 0 for light with a wavelength of 534 nm is n x = ], 5362 ny -1,8758 nz = 1.745B. In this case, the sum frequency is Z-polarized light, and the wavelength λ3
For a sum frequency of = 534 nm, n '' = n z
= 1. '745G.

The above-mentioned λ1, λ2, λ3 and n °l, n"2n
The value of u3 also satisfies the phase matching conditional expression (1.-4) described above.

<Fifth Example> A bulk single crystal of PRA was produced in the same manner as in the first example. Semiconductor lasers were used as two light sources for generating fundamental waves, and laser beams (fundamental waves) with wavelengths λ1=900 nm and λ2−L300 nm emitted from them were made incident on the optical wavelength conversion element IO. At this time, for the side base water wave, θ = 18.88°, φ = 90
°. For light with a wavelength of 900 nm, nx = 1.511.1 n y = 1. ．． 7914 n 2 = 1.6954, and for light with a wavelength of 1300 nm, n x -
1,5042 n Y = 1.7 [198 n 2 = 1.6827. And as above, θ-18, 88°, φ-9
At 0°, the fundamental wave with wavelength λ1 = 900 nm feels an intermediate refractive index between ny and ny, and becomes n'・'' = 1.7545. The fundamental wave with wavelength λz = 1300 nm also has a refractive index of nX n
Feeling the intermediate refractive index of y, n ''2 = 1.7352.

In this way, the fundamental wave incident on the optical wavelength conversion element IO made of PRA is converted into a sum frequency having a wavelength λ3 = 532 nm. For light with a wavelength of 532 nm, n x −1,5367 n Y = 1.877 [in z = 1.74137. In this case, the sum frequency is Z-polarized light, and the wavelength λ3
For a sum frequency of = 532 nm, n """' n
z = 1.74B7.

The above-mentioned λ engineering, λ2, λ3 and n.”1, n..2n
The value of w3 also satisfies the phase matching conditional expression (1-4) described above.

Sixth Example> A bulk single crystal of PRA was produced in the same manner as in the first example. Semiconductor lasers were used as two light sources for generating fundamental waves, and laser beams (fundamental waves) with wavelengths λ1=1200 nm and λ22-1300 nm emitted from them were made incident on the optical wavelength conversion element 10. At this time, for both fundamental waves, the angles are θ-18, 88°, and φ-90°. For light with a wavelength of 1200 nm, n x = 1.505 G ny = 1.7740 n z = 1. [1852, and for light with a wavelength of 1300 nm, n x =
1. ．． 5042 3 4 ny = 1.7 [i9g nz = 1.6827. And as above, θ=1-8.88°, φ-
When the angle is 90°, the fundamental wave with a wavelength λ1 = 1200 nm feels an intermediate refractive index between nX and ny, and n'' = 1.7545.The fundamental wave with a wavelength λ2 =] 300 nm also has the refractive index between nX and ny, We feel an intermediate refractive index of n ''2 = 1.7352.

In this way, the fundamental wave incident on the optical wavelength conversion element 10 made of PRA is converted into a sum frequency of wavelength λ3=624 nm. For light with a wavelength of 624 nm, n x ``'1.5244 ny = 1..8347 nz = 1.7209. In this case, the sum frequency of wavelength λ3 = 624 nm is Z polarized light, and n '' = It becomes 1.7209.

” double series λ1, λ2, λ3 and n h″l, n
The value of '2n u 3 also satisfies the phase matching conditional expression (1-4) described above.

Seventh Example> A bulk single crystal of PRA was produced in the same manner as in the first example. Semiconductor lasers were used as two light sources for generating fundamental waves, and laser beams (fundamental waves) with wavelengths λ1=]200 nm and λ2=1500 nm were made incident on the optical wavelength conversion element 10. At this time, for both fundamental waves, θ=18.88° and φ−90°. For light with a wavelength of 1200 nm, nX = 1.5056 n Y = 1.7740 nz = 1.8852, and for light with a wavelength of 1500 nm, nx -
1,5018 nY=1.7622 nz=1.6783. And as above, θ= 18.88°, φ=
When the angle is 90°, the fundamental wave with wavelength λ1 = 1200 nm feels an intermediate refractive index between n) (and ny, and n″-1,
It becomes 7545. The fundamental wave of wavelength λ2 = 1500 nm also has an intermediate refractive index between nx and nY, and n''' = 1.7352.

In this way, the fundamental wave incident on the optical wavelength conversion element 10 made of PRA is converted into a sum frequency wave having a wavelength λ3 = 867 nm. For light with a wavelength of 667 nm, nx = 1.5209 n Y -1,8232 nz = -t, 7t4°. In this case, the sum frequency of wavelength λ3 = 8F37 nm is Z-polarized light, and n ''3 = 1.71.40.

λ1, λ2, λ3 and n w I, n″2n described above
The value of u3 also satisfies the phase matching conditional expression (1.-4) described above.

Hereinafter, two embodiments have been described, and the fundamental wave wavelength λ
The combination of λ1 and λ2 is not limited to the one in the embodiment 1, and can be set in various ways. For example, type I angular phase matching can be achieved by setting as follows. Note that the sum frequency wavelength λ3 in each case is also shown.

(Effects of the Invention) As explained in detail above, in the optical wavelength conversion method of the present invention, by appropriately setting the polarization direction of the fundamental wave incident on the PRA crystal, angular phase matching between the fundamental wave and the sum frequency is achieved. According to this method, the high nonlinear optical constant of PRA can actually be used in a bulk crystal type nonlinear optical material, and extremely high wavelength conversion efficiency can be achieved. Furthermore, since PRA has an absorption edge near 400 nm, the method of the present invention using a bulk single crystal of PRA makes it possible to efficiently extract the sum frequency in the blue region.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an example of an apparatus for implementing the method of the present invention; FIGS. 2A, 2B, and 20 are crystal structure diagrams of PRA used in the present invention in the C-axis, a-axis, and b-axis directions, respectively; FIG. 3 is a diagram of the bulk crystal structure of the PRA, FIG. 4 is a graph showing the wavelength dispersion of the refractive index of the PRA, and FIG. 5 is a graph showing the fundamental wave traveling direction and the optical axis Z inside the crystal according to the present invention. FIG. 6 is a graph showing the transmission spectrum of the PRA. 10... Optical wavelength conversion element 11... Semiconductor laser 15A, 15B... Fundamental wave 15'... Sum frequency 21... YAG racer Rika Toru ← g 4 Osai

Claims (1)

[Claims] Light that makes two linearly polarized fundamental waves incident on a bulk single crystal of a nonlinear optical material represented by the following molecular formula ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ and converts them into a sum frequency. A wavelength conversion method in which the wavelength is 450 to 4000
Injecting a fundamental wave between nm into the nonlinear optical material,
An optical wavelength conversion method characterized by obtaining a sum frequency by obtaining Type I angular phase matching.