JPH04229839A - Light wavelength changing module - Google Patents

Abstract

PURPOSE:To secure a second harmonics through high wavelength conversion efficiency by utilizing a high nonlinear optical constant inherent in ethyl cyanophenyl imidazole or a core for a fiber type wavelength changing element. CONSTITUTION:This module is provided with a fiber type wavelength changing element 1 consisting of a core 11 and a clad 12, a laser beam source 3 for emitting a fundamental wave, and a condensing optical system 2 for condensing the fundamental wave and simultaneously radiating the fundamental wave to a core end of the fiber type wavelength changing element 1. In this connection, the core 11 consists of a single crystal of 2-ethyl-1-(4-cyanophenyl)imidazole(ECI), and a (b) axis of this single crystal extends in parallel with a core axis, while an (a) axis or (c) axis extends orthogonally to the (b) axis, and the condensing optical system polarizes the fundamental wave in the (a) axis or (c) axis direction.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion module that converts a fundamental wave emitted from a laser light source into a second harmonic using an optical wavelength conversion element.

0002

2. Description of the Related Art Conventionally, nonlinear optical materials have been used to convert the wavelength of laser light into its second harmonic or the like. As a wavelength conversion element that performs the above-mentioned wavelength conversion, for example, a bulk crystal type is known. However, in bulk crystal type optical wavelength conversion elements, the birefringence of the crystal of the nonlinear optical material is used to achieve phase matching between the fundamental wave and the second harmonic, so even if the nonlinearity is large, the birefringence is No nonlinear optical materials cannot be used.

On the other hand, in recent years, nonlinear optical materials with large nonlinearity and low birefringence or no birefringence can be used, and in addition, it is possible to use nonlinear optical materials that have large nonlinearity and small birefringence or no birefringence. A fiber-type wavelength conversion element is used because it is easy to convert. This fiber type wavelength conversion element has a core made of a single crystal of a nonlinear optical material formed within a cladding made of glass.

[0004] As the nonlinear optical material used for the core of the fiber-type wavelength conversion element, it is preferable to use one having a large nonlinear optical constant in order to increase wavelength conversion efficiency. Examples of these nonlinear optical materials include 2-methyl-4-nitroaniline (hereinafter abbreviated as MNA), p-nitro-(2-hydroxymethyl-pyrrolinyl)phenylene (hereinafter abbreviated as NPP), 3
, 5-dimethyl-1-(4-nitrophenyl)pyrazole (hereinafter abbreviated as PRA) and the like are known.

On the other hand, in order to efficiently extract the obtained second harmonic with the fiber-type wavelength conversion element, it is necessary that the organic nonlinear optical material used in the element has low light absorption in the optical wavelength region in which it is used. is necessary. More specifically, the nonlinear optical material of the optical wavelength conversion element used in the short wavelength technology of semiconductor lasers has a wavelength of 800 nm.
The wavelength is approximately 4, which is the second harmonic region of the semiconductor laser near m.
It is preferable to use a nonlinear optical material that has low light absorption in the blue light wavelength range of 0.00 nm.

[0006] On the other hand, the inventors had previously
In Publication No. 099, 2-ethyl-1(4-cyanophenyl)imidazole (hereinafter abbreviated as ECI) has a wavelength of 40
It is disclosed that almost no light in the vicinity of 0 nm is absorbed.

[0007]

[Problems to be Solved by the Invention] However, the conventionally known MNA, NPP and PRA each have a
Since it exhibits large light absorption in wavelength regions around 80 nm, 500 nm, and 450 nm, there was a problem in that it was unable to efficiently generate second harmonics in the blue region. In addition, in a fiber-type wavelength conversion element using ECI as the core, the crystal orientation of the single crystal of ECI as the core and the polarization direction of the fundamental wave incident on the core are not appropriate.
There was a problem that the second harmonic could not be obtained with high wavelength conversion efficiency.

The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide an optical wavelength conversion module that can also efficiently generate second harmonics in the blue region.

[0009]

[Means for Solving the Problems] As a result of extensive research on ECI, the inventors have found that a single crystal of ECI is shown in FIG.
We obtained a new finding that it is an orthorhombic crystal structure with a crystal structure similar to the following, and its space group is Pca21. Note that FIG. 3a shows the ac plane of the ECI single crystal, and FIG. 3b shows the ECI single crystal.
The ab plane of the single crystal, Figure 3c shows the bc plane of the ECI single crystal.

[0010] The inventors also discovered that E
A new finding was also obtained that the a-axis of a CI single crystal coincides with the X-axis of its dielectric principal axis, the b-axis with the Y-axis of its dielectric principal axis, and the c-axis with the Z-axis of its dielectric principal axis. Ta. Furthermore, the second-order nonlinear optical tensor d of the ECI single crystal is
We have obtained the knowledge that it is expressed by the following number 1.

[0011]

[Math 1]

[0012] Here, considering the dielectric principal axes X, Y, and Z determined with respect to the crystal axes a, b, and c, d31 is light polarized in the X direction (hereinafter referred to as "X polarized light" and "Y polarized light"). The same applies to "Z-polarized light." , when extracting the second harmonic of Z polarized light, d33
is when the fundamental wave of Z polarization is input and the second harmonic of Z polarization is extracted, d24 is when the fundamental wave of Y and Z polarization is input and the second harmonic of Y polarization is extracted, d15 is X These are nonlinear optical constants that contribute when the fundamental wave of Y polarized light and Y polarized light are input and the second harmonic of X polarized light is extracted.

The inventors determined each of the above nonlinear optical constants by X-ray structural diffraction. The values are shown in Table 1.

[0014]

[Table 1]

From Table 1, among the above nonlinear optical constants, d
It can be seen that 31, d33 and d15 are large values.

Therefore, as shown in FIG. 2, when forming a fiber type wavelength conversion element 1 having a core 11 made of ECI, the b-axis (X-axis direction in the dielectric principal axis) of the ECI single crystal is aligned with the core. Orient it so that it extends parallel to the axis. By injecting a fundamental wave polarized in the b-axis direction (Y-axis direction on the dielectric main axis) or c-axis direction (Z-axis direction on the dielectric main axis) of the single crystal of the fiber-type wavelength conversion element 1, large nonlinear optical d31 or d which is a constant
33 can be used to obtain the second harmonic in a single mode.

[0017] In order to utilize the nonlinear optical constant d15, it is necessary to input X-polarized and Z-polarized fundamental waves, but due to the refractive index anisotropy of ECI, the second harmonic is cannot occur. The optical wavelength conversion module according to the present invention for solving the above problems has been made based on the above new findings, and has a core made of an organic nonlinear optical crystal and a cladding made of glass. A fiber-type wavelength conversion element for wavelength converting the fundamental wave incident on the end face, a laser light source that emits laser light as the fundamental wave, and a fiber-type wavelength conversion element for condensing the laser light and irradiating it onto the core end face of the fiber-type wavelength conversion element. An optical wavelength conversion module comprising a condensing optical system, wherein the core has the following formula (1):

[0018]

[Case 2]

The b-axis of this single crystal extends parallel to the core axis, and the a-axis or c-axis extends perpendicularly to the b-axis, and The optical system is characterized in that the optical system polarizes the fundamental wave in the a-axis or c-axis direction.

[0020]

[Operation] In the optical wavelength conversion module according to the present invention, a laser light, which is a fundamental wave, is emitted from a laser light source. The laser beam is focused by a focusing optical system, linearly polarized in the a-axis or c-axis direction of the single crystal that is the core of the fiber-type wavelength conversion element, and irradiated onto the core end face of the fiber-type wavelength conversion element. Ru.

Therefore, the fundamental wave of laser light is E
Using the above-mentioned d31 or d33 among the nonlinear optical constants of the CI single crystal, it is efficiently converted into a second harmonic. In addition, in Figure 4, ECI is 4 × 10-4 m in ethanol.
The absorption spectrum of a solution containing a concentration of ol/l is shown. As shown in FIG. 4, ECI has a wavelength of 400 nm.
Since almost no nearby light is absorbed, in this optical wavelength conversion module, the second harmonic in the blue region is not absorbed by the core.

[0022]

[Examples] The present invention will be explained in more detail below based on Examples. FIG. 1 is an explanatory diagram showing an optical wavelength conversion module A according to the present invention. This optical wavelength conversion module A
consists of a fiber type wavelength conversion element 1, a laser light source 3, and a condensing optical system 2.

[0023] The fiber type wavelength conversion element 1 is an EC
It has a core 11 made of a single crystal of I and a cladding 12 made of glass, and the b axis of the single crystal of ECI is
As shown in Figure 2, it extends parallel to the core axis. Also,
The a- and c-axes of the ECI single crystal extend perpendicularly to the core axis. The condensing optical system 2 is arranged between the laser light source 3 and the fiber-type wavelength conversion element 1, and includes a collimating lens 21, an anamorphic prism pair 22, 23, a λ/2 plate 24, and a condensing lens 25.
are arranged in this order from the laser light source 3 side toward the fiber type wavelength conversion element 1 side.

Further, as the laser light source 3, a laser light source such as a semiconductor laser, which is normally used for generating a fundamental wave of the second harmonic, can be used. According to this optical wavelength conversion module A, the laser beam 31a emitted as a fundamental wave from the laser light source 3 is transmitted through the collimating lens 2.
1, the beam is made into a parallel beam. Next, the light passes through the anamorphic prism pair 22, 23 and the λ/2 plate 24, is focused by the condenser lens 25, and is then irradiated onto the incident end surface 11a of the core 11 of the fiber type wavelength conversion element 1. .

At this time, the λ/2 plate 24 of the condensing optical system 2 is rotated to change the polarization of the laser beam 31a irradiated onto the incident end face 11a of the core 11 into the a of the ECI single crystal forming the core 11. axis or c-axis direction. Therefore, the above E
The entrance end surface 11a of the core 11 made of a CI single crystal has
Since laser light is focused and polarized in the a-axis or b-axis direction, this optical wavelength conversion module A utilizes the large nonlinear optical constant of the ECI single crystal to
The laser beam 31a, which is the fundamental wave, is converted into a second wave with high wavelength conversion efficiency.
The second harmonic 31 is converted into a harmonic and is generated in a ring shape from the output end face 12b of the cladding 12 of the fiber type wavelength conversion element 1.
b can be generated.

Note that the laser light 31a generated from the laser light source 3 is not particularly limited, but when obtaining a second harmonic having a wavelength in the blue light region, the laser light 31a has a wavelength of 80.
The laser beam is around 0 nm. In this case, the ECI single crystal, which is the core 11 of the fiber-type wavelength conversion element 1, hardly absorbs light with a wavelength of around 400 nm, so it is possible to efficiently obtain the second harmonic 31b having a wavelength in the blue region.

Note that the above-mentioned fiber type wavelength conversion element 1 can be manufactured by various methods. For example, ECI
(melting point: 117°C) is maintained in a molten state in a furnace heated to a temperature slightly higher than the melting point (for example, 125°C). Then, one end of a glass capillary tube serving as the cladding of the fiber type wavelength conversion element is immersed in the ECI melt layer, and the ECI melt is filled into the capillary tube by capillary action. Then,
The glass capillary is rapidly cooled to solidify the ECI melt within the capillary and form a fiber.

[0028] The above fiber is placed in a furnace heated above the melting point of ECI to melt the ECI again. Then outside the furnace maintained at a temperature lower than the melting point of ECI,
As shown in FIG. 4, the fiber 10 is melted by drawing it out from one end of the furnace 4 while applying an electrostatic field with a strength of 1 kV/cm or more by the electrode 41 in the direction perpendicular to the core axis of the fiber 10. The ECI 11b in a liquid state is drawn out of the furnace, and a single crystal 11c of ECI is formed from the above-mentioned one end side.

Then, the portion of the fiber 10 where the ECI single crystal 11c is formed is cut with a fiber cutter or the like to obtain the fiber type wavelength conversion element 1. According to the above method, an E of 50 mm or more in length is placed in a glass capillary.
Since a single crystal of CI can be formed, a fiber type wavelength conversion element having a sufficient length can be obtained. Further, the crystal orientation of the obtained single crystal is such that the a-axis and the c-axis extend perpendicularly to the b-axis, as shown in FIG.

The glass capillary used in the above method is made of heat-resistant glass such as Pyrex glass (registered trademark of Corning Corporation), and has an outer diameter of about 1 mm and an inner diameter of 3 mm.
A diameter of approximately μm is used. Further, as the furnace used to melt the ECI again, it is preferable to use a Bridgman furnace capable of applying an electric field as shown in FIG. 4. Further, when the fiber is drawn out of the furnace to form an ECI single crystal, the fiber drawing speed is preferably about 1 mm/hour in order to obtain a good single crystal.

Using the fiber type wavelength conversion element formed by the above method, an optical wavelength conversion module A having the structure shown in FIG. It was fired as a fundamental wave. Then, when the semiconductor laser polarized and focused in the a-axis direction using the method described above is irradiated onto the incident end face 11a of the core 11 of the fiber type wavelength conversion element 1, the wavelength from the output end face 13 of the fiber type wavelength conversion element 1 is 43
Generation of a second harmonic of 0 nm was confirmed.

[0032]

As described above, according to the optical wavelength conversion module according to the present invention, it is possible to utilize the high nonlinear optical constant of ECI, which is the core of the fiber-type wavelength conversion element, so that high wavelength conversion can be achieved. With efficiency, the second harmonic can be obtained. Further, since ECI does not have a light absorption region around a wavelength of 400 nm, the second harmonic in the blue region can be efficiently obtained using a laser beam with a wavelength of about 800 nm as a fundamental wave.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing one embodiment of an optical wavelength conversion module according to the present invention.

FIG. 2 is an explanatory diagram showing the crystal structure of a single crystal of ECI.

FIG. 3 is a three-sided view showing the crystal structure of a single crystal of ECI, which is the core of the fiber-type wavelength conversion element used in the optical wavelength conversion module of the present invention.

FIG. 4 is a graph showing the absorbance of a methanol solution of ECI.

FIG. 5 is an explanatory diagram showing one step of a method for manufacturing a fiber type wavelength conversion element used in the optical wavelength conversion module of the present invention.

[Explanation of symbols]

1 Fiber type wavelength conversion element 2. Condensing optical system 3 Laser light source 11 Core 12 Clad A. Optical wavelength conversion module

Claims (1)

[Claims] 1. A fiber-type wavelength conversion element having a core made of an organic nonlinear optical crystal and a cladding made of glass, for converting the wavelength of a fundamental wave incident on an end face of the core;
An optical wavelength conversion module comprising a laser light source that emits a laser beam as a fundamental wave and a condensing optical system that condenses the laser beam and irradiates the core end face of a fiber type wavelength conversion element, the optical wavelength conversion module comprising: is composed of a single crystal of 2-ethyl-1-(4-cyanophenyl)imidazole represented by the following formula (1): [Chemical 1], and the b-axis of this single crystal extends parallel to the core axis. , the a-axis or the c-axis extends perpendicularly to the b-axis, and the condensing optical system polarizes the fundamental wave in the a-axis or c-axis direction.