JPH07104538B2 - Optical wavelength conversion module - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical wavelength conversion module for converting a fundamental wave generated from a light source into a second harmonic having a half wavelength by an optical wavelength conversion element. Is.

(Prior Art) Conventionally, various attempts have been made to convert the wavelength of laser light (shorten the wavelength) by utilizing the second harmonic generation by a nonlinear optical material. Specific examples of the optical wavelength conversion element for performing wavelength conversion in this manner are shown in "Basics of Optoelectronics" by A. YARIV, Kunio Tada, Takeshi Kamiya (Maruzen Co., Ltd.), p200-204. The bulk crystal type is well known. However, since this optical wavelength conversion element utilizes the birefringence of the crystal in order to satisfy the phase matching condition, there is a problem that a material having no birefringence or a small material cannot be used even if the nonlinearity is large.

As an optical wavelength conversion element that can solve the above problems,
A so-called fiber type has been proposed. This optical wavelength conversion element is an optical fiber with a core made of a nonlinear optical material filled in the clad, and the journal of the Japan Society of Applied Physics, Micro Optics Research Group VOL.3, No.2, p28-32
Shows an example of this. Since this fiber-type optical wavelength conversion element can easily achieve phase matching between the fundamental wave and the second harmonic, research on this fiber-type optical wavelength conversion element has recently been actively conducted. .

In order to improve the wavelength conversion efficiency of the fiber type optical wavelength conversion element as described above, it is desirable to use an optical material having a high nonlinear optical constant as the core. As an optical material having a high non-linear optical constant, for example, Japanese Patent Laid-Open No. 60-25033 has been used.
No. 4, MNA (2-methyl-4-nitroaniline), NPP (N- (4-nitrophenyl)) described in J.Opt.Soc.Am.B Vol.4 p977 (1987). -L-prolinol), NPAN (N- (4-nitrophenyl) -N-
Methylaminoacetonitrile) and the like are known.

(Problems to be Solved by the Invention) However, when a fiber-type optical wavelength conversion element is formed using the above-mentioned nonlinear optical material, the crystal is oriented in a direction in which the maximum nonlinear optical constant of each material can be used. Therefore, the wavelength conversion efficiency of the light wavelength conversion element is not so high after all.

Further, the wavelength conversion efficiency of the light wavelength conversion element increases as the element length increases, but it is difficult to obtain a uniform single crystal with the above-mentioned materials, and thus it is unsuitable for making a long light wavelength conversion element. There are also problems.

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 module having high wavelength conversion efficiency and capable of easily obtaining the second harmonic in the blue region. It is a thing.

(Means for Solving the Problems) The optical wavelength conversion module of the present invention is composed of the fiber type optical wavelength conversion element and the light source device as described above. The core has the following molecular formula The nonlinear optical material (3,5-dimethyl-1- (4
-Nitrophenyl) -1,2,4-triazole: hereinafter referred to as TRI) is used in a single crystal state, and the crystal orientation direction of TRI that constitutes this core is such that the b-axis is substantially the major axis direction of the core. The light source device is set so as to extend, and the light source device is configured so that a fundamental wave linearly polarized in the direction of the a-axis or the c-axis of the crystal orthogonal to the b-axis is incident on the optical wavelength conversion element. Is.

(Operation) The TRI is disclosed in the specification of Japanese Patent Application No. 61-53884 by the applicant of the present application, and it is also shown in the specification that it has a nonlinear optical effect. How to set the crystal orientation and how to set the polarization direction of the fundamental wave incident on the crystal orientation in forming the optical wavelength conversion element of It was unknown.

Hereinafter, the point that a high wavelength conversion efficiency is obtained by setting the crystal orientation of the nonlinear optical material and the linear polarization direction of the fundamental wave as described above will be described.

The crystal structure of the TRI is shown in Figures 2A, 2B and 2C. The bulk crystal structure is shown in FIG. The TRI crystals are orthorhombic and the point cloud is mm2. So the tensor of the nonlinear optical constant is Becomes Here, d ₃₁ is the crystal axis a, as shown in FIG.
Considering the optical axes X, Y, and Z defined for b and c,
Light linearly polarized in the X direction (hereinafter referred to as X polarized light. Y, Z
Is also the same. ) Is a non-linear optical constant when the Z-polarized second harmonic is extracted by inputting) as a fundamental wave, and similarly, d ₃₂ is a case where the Z-polarized second harmonic is extracted by injecting the Y-polarized fundamental wave. Non-linear optical constant, d ₃₃ is a non-linear optical constant when the Z-polarized fundamental wave is incident to extract the second harmonic of Z-polarized light, and d ₂₄ is the Y-polarized second harmonic when the Y and Z-polarized fundamental waves are incident. A non-linear optical constant for extracting a higher harmonic wave, d ₁₅ is a non-linear optical constant for extracting a second higher harmonic wave of X-polarized light by injecting the fundamental waves of X- and Z-polarized light. The magnitude of each nonlinear optical constant will be described below.

Since the refractive index of TRI has not been clarified yet, the value of b _IJK for which the nonlinear optical constant d _IJK can be derived by the following equation d _IJK = N · f ₁ (2ω) f _J (ω) f _K (ω) b _1JK Show. Note that N is the number of molecules per unit volume, f (ω), f (2ω)
Are the local electric field correction factors for the fundamental wave and the second harmonic, respectively.

Note that these b _IJK values are values by X-ray crystal structure analysis, and the unit is [× 10 ⁻³⁰ esu].

From this table, it can be seen that d ₃₂ , d ₃₃ , d ₂₄ and d ₁₅ can take large values. Therefore, as shown in FIG. 4, in forming the fiber type optical wavelength conversion element 10 by filling the clad 12 with the core 11 made of TRI, the crystal of TRI is b-axis (X-axis in the optical axis). Is oriented so as to extend in the axial direction of the core (this can be realized by the method described below),
This optical wavelength conversion element 10 has a crystal c-axis (optical axis is Z-axis)
Alternatively, if a linearly polarized fundamental wave is made incident in the direction of the a-axis (Y-axis in the optical axis), the above-mentioned large nonlinear optical constant d
₃₂ and d ₃₃ will be available.

In order to use the nonlinear optical constant d ₂₄ , both Y and Z polarized light are used, and in order to use the nonlinear optical constant d ₁₅ ,
Since it is necessary to input both X and Z polarized light into the fiber type optical wavelength conversion element, it is difficult to achieve single mode and high efficiency due to the refractive index anisotropy of TRI. On the other hand, when Y-polarized light or Z-polarized light is input to the fiber-type optical wavelength conversion element, the single mode can be sufficiently realized and the efficiency can be improved.

Fig. 5 shows the transmission spectrum of TRI in a solvent (concentration 4 × 10 ^-4 mol / l, solvent ethanol). As shown in the figure, this TRI absorbs much light near the wavelength of 400 nm. There is no. Therefore, the optical wavelength conversion element using this TRI as a core can efficiently generate the second harmonic in the blue region.

(Embodiment) <First Embodiment> FIG. 1 shows an optical wavelength conversion module according to a first embodiment of the present invention. This optical wavelength conversion module is composed of a fiber type optical wavelength conversion element 10 and a light source device 20 for inputting a fundamental wave to the optical wavelength conversion element 10.

Here, a method for producing the light wavelength conversion element 10 will be described. First, a hollow glass fiber 1 that becomes the clad 12.
2'is prepared. The glass fiber 12 'is made of SFS3 glass, for example, and has an outer diameter of about 100 μm and a hollow portion diameter of 6 μm. Then, as shown in FIG. 6, the TRI was kept in a molten state in the furnace or the like, and the melt 1
Immerse one end of the glass fiber 12 'into the 1'.
Then, due to the capillary phenomenon, the TRI in the melt state enters into the hollow portion of the glass fiber 12 '. The temperature of the melt 11 'is set to a temperature slightly higher than its melting point (152 ° C) in order to prevent decomposition of TRI. Then glass fiber 1
When 2'is rapidly cooled, the TRI entering the hollow part becomes polycrystallized.

Then, the optical fiber 12 'is gradually pulled out from the inside of the furnace kept at a temperature higher than the melting point of TRI (for example, 153 ° C) to the outside of the furnace kept at a temperature lower than the melting point, so that the molten TRI is heated in the furnace. Single crystal is formed from the part that is pulled out. As a result, the core 11 is formed in a single crystal state over a long range of 50 mm or more, and the crystal orientation is also uniform, and the light 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 proportional to the length of the element, so the longer the optical wavelength conversion element, the higher the practical value.

When the TRI is filled in the glass fiber 12 'in a single crystal state as described above, the crystal orientation state is such that the b axis (the optical axis is the X axis) extends in the core axis direction as shown in FIG. Becomes

In order to single crystallize TRI as described above,
For example, a method using a Bridgman furnace as disclosed in Japanese Patent Application No. 61-075078 by the applicant of the present application can be used. Further, the drawing speed of the glass fiber 12 'may be, for example, about 5 mm / h.

After the core 11 was filled as described above, both ends of the glass fiber 12 'were cut with a fiber cutter to form a light wavelength conversion element 10 having a length of 10 mm. As shown in FIG. 1, the light wavelength conversion element 10 is combined with the light source device 20 to form a light wavelength conversion module. In this embodiment, the semiconductor laser 21 is used as a light source for generating the fundamental wave, and the wavelength 820 nm emitted from the semiconductor laser 21 is used.
Laser beam (fundamental wave) 15 of is converted into a parallel beam by a collimator lens 22, then passed through an anamorphic prism pair 23 and a λ / 2 plate 25, focused by a condenser lens 26 into a small beam spot, and The incident end face 10a of the wavelength conversion element 10 is irradiated. As a result, this fundamental wave 15
Enters the light wavelength conversion element 10. As mentioned above, the core
The TRI that constitutes 11 has a crystal orientation state in which the X axis extends in the core axis direction. On the other hand, in this example, λ / 2 of the light source device 20 is used.
By rotating the plate 25, the fundamental wave 15 in the Y polarization state is input to the optical wavelength conversion element 10.

The fundamental wave 15 that has entered the optical wavelength conversion element 10 is the second harmonic wave 1 with a wavelength of 1/2 (= 410 nm) due to the TRI that constitutes the core 11.
Converted to 5 '. The second harmonic wave 15 'repeats total reflection between the outer surfaces of the clad 12 and travels in the element 10, and the guided mode in the core part of the fundamental wave 15 and the clad part of the second harmonic wave 15'. The phase is matched with the radiation mode to (so-called Cherenkov radiation).

A beam 15 ″, which is a mixture of the second harmonic wave 15 ′ and the fundamental wave 15, is emitted from the emission end face 10 b of the light wavelength conversion element 10.
This outgoing beam 15 ″ is passed through a condenser lens 27 and collected, and then the second harmonic wave 15 ′ at 410 nm is satisfactorily transmitted, while the fundamental wave 15 at 820 nm is absorbed by a bandpass filter 28. And the second harmonic 15 'is extracted.
It was confirmed that the second harmonic wave 15 'was Z-polarized light using a polarizing plate or the like. That is, in this example, the above-mentioned TRI nonlinear optical constant d ₃₂ is used. This second harmonic 1
The light intensity of 5'was measured with an optical power meter 29, and the wavelength conversion efficiency was determined to be about 1% in terms of 1 W.

<Second Example> An optical wavelength conversion element having a TRI core was prepared under the same conditions as in the first example except that SF8 glass was used as the glass fiber constituting the clad. Also in this embodiment, the TRI crystal is oriented so that the b axis extends in the core axis direction.

When a Z-polarized YAG laser light with a wavelength of 1064 nm was input as a fundamental wave to this optical wavelength conversion element, the Z-polarized second
The generation of harmonics was confirmed. That is, in this case, the nonlinear optical constant d _{33 of} TRI is used. The wavelength conversion efficiency at this time was about 1% in terms of 1 W, as in the first embodiment.

<Third Example> An optical wavelength conversion element having TRI as a core was prepared in the same manner as in the first example except that SF10 glass was used as the glass fiber constituting the clad. Also in this embodiment, the TRI crystal is oriented so that the b axis extends in the core axis direction.

When Y-polarized YAG laser light with a wavelength of 1064 nm was input as a fundamental wave to this optical wavelength conversion element, the second polarized Z-polarized light was obtained.
The generation of harmonics was confirmed. That is, in this case, the nonlinear optical constant d _{32 of} TRI is used. The wavelength conversion efficiency at this time was about 1% in terms of 1 W, as in the first embodiment.

(Effects of the Invention) As described in detail above, according to the optical wavelength conversion module of the present invention, the high nonlinear optical constant of TRI can be actually used in the fiber type nonlinear optical material, and the optical wavelength conversion element is sufficient. Since it can be formed for a long time, extremely high wavelength conversion efficiency can be realized. In addition, since TRI has an absorption edge near 400 nm, this optical wavelength conversion module can also efficiently extract the second harmonic in the blue region by using laser light of approximately 800 nm as the fundamental wave. Becomes

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the present invention, and FIGS. 2A, 2B and 2C are TRs used in the present invention.
Crystal structure diagrams of I in the b-axis, c-axis, and a-axis directions, FIG. 3 is a bulk crystal structure diagram of the above TRI, and FIG. 4 is a schematic diagram showing the crystal orientation of the core in the optical wavelength conversion device according to the present invention. FIG. 5 is a graph showing the transmission spectrum of the above-mentioned TRI, and FIG. 6 is an explanatory diagram for explaining a method for producing the optical wavelength conversion element according to the present invention. 10 ... Optical wavelength conversion element, 11 ... Core 11 '... TRI melt, 12 ... Clad 12' ... Glass fiber, 15 ... Basic wave 15 '... Second harmonic, 20 ... Light source device 21 …… Semiconductor laser, 22 …… Collimating lens 23 …… Anamorphic prism pair 25 …… λ / 2 plate, 26,27 …… Condensing lens

Claims (1)

[Claims] 1. The following molecular formula in the clad: And a light wavelength conversion element in which a single crystal nonlinear optical material represented by is filled as a core, and a crystal of the optical material is oriented so that its b axis extends substantially in the core axis direction; An optical wavelength conversion module comprising: a light source device for injecting a linearly polarized fundamental wave in a direction of an a-axis or a c-axis of a crystal orthogonal to the b-axis.