JPH0820658B2 - Optical wavelength conversion element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention uses a fiber-type optical wavelength conversion element for converting a fundamental wave into a second harmonic wave having a half wavelength thereof, and the element. More particularly, the present invention relates to an optical wavelength conversion element and an optical wavelength conversion module having a function of converting a wavefront of a wavelength conversion wave from a conical wavefront to a spherical wavefront.

(Prior Art) Conventionally, various attempts have been made to perform wavelength conversion (shortening of wavelength) of laser light into a second harmonic wave or the like using a non-linear 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 uses birefringence of a crystal in order to satisfy a phase matching condition, there is a problem that a material having a large nonlinearity but having no birefringence or a small material cannot be used.

As an optical wavelength conversion element capable of solving the above problems, a so-called fiber type is proposed. This optical wavelength conversion element is an optical fiber with a core made of a nonlinear optical material filled in the clad, and is published by the Society of Applied Physics, Micro Optics Research Group, VOL.3, No.2, p28
To 32 show one example. In this fiber-type optical wavelength conversion element, phase matching can be easily achieved between the guided mode of the fundamental wave in the core part and the radiation mode of the second harmonic wave to the cladding (so-called Cherenkov radiation). Therefore, recently, research on this fiber type optical wavelength conversion element has been actively conducted.

(Problems to be Solved by the Invention) By the way, the wavelength-converted wave obtained as described above is taken out from the end face of the clad and used for various purposes. I often want to use it after focusing on. For example, in the case of using a wavelength converted wave for optical recording, from the viewpoint of improving the recording density,
It is desired to narrow down the wavelength converted wave to a particularly small spot.

However, in the fiber Cherenkov type optical wavelength conversion element described above, there is a problem in that even if the wavelength converted wave taken out of the element is attempted to be narrowed through a general spherical lens, it does not converge to a small spot. Therefore, it is an object of the present invention to provide an optical wavelength conversion element that enables a wavelength converted wave to be narrowed down to a small spot, and an optical wavelength conversion module using the element.

(Means and Actions for Solving the Problems) The present invention is, in the fiber Cherenkov type optical wavelength conversion element as described above, the wavelength conversion wave radiated in the clad (emitted at a predetermined phase matching angle). Then, the light wavelength conversion element of the present invention is used in this optical wavelength conversion element of the fiber Cherenkov type. The clad end face from which the wavelength-converted wave is emitted is formed as an aspherical lens surface that converts the wavefront of the wavelength-converted wave from a conical wavefront to a spherical wavefront.

Further, another optical wavelength conversion element of the present invention is also the fiber Cherenkov type optical wavelength conversion element, in which a clad end face from which the wavelength conversion wave is emitted, a concentric circle for converting the wavefront of the wavelength conversion wave from a conical wavefront to a spherical wavefront. It is characterized in that a grating is formed.

Further, the present invention, in order to solve the above-mentioned problems, the above-mentioned optical wavelength conversion element of the fiber Cherenkov type, the wavelength conversion wave emitted from the cladding end face of this optical wavelength conversion element is arranged at the position of incidence, Provided is a light wavelength conversion module for converting a wavefront of a converted wave from a conical wavefront to a spherical wavefront, the lens having one light passage surface having a conical surface and the other light passage surface having a spherical surface.

Further, the present invention provides an optical wavelength conversion module provided with a concentric circular grating element for converting the wavefront of the wavelength conversion wave from a conical wavefront to a spherical wavefront, instead of the lens as described above.

The lens and the concentric grating element may be arranged apart from the cladding end face from which the wavelength-converted wave is emitted, or may be closely fixed to this end face.

If the wavefront of the wavelength-converted wave can be converted into a convergent spherical wavefront by the action of the lens or concentric circle grating element as described above, the clad end surface that is an aspherical lens surface, and the concentric circle grating formed on the clad end surface, The converted wave can be narrowed down to a small spot, and even if it is converted into a divergent spherical wavefront, it can be narrowed down to a small spot by passing the wavelength converted wave through an ordinary spherical lens.

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

1 and 2 show an optical wavelength conversion device 10 according to the first embodiment of the present invention. This optical wavelength conversion element 10
Is an optical fiber in which a core 11 made of a nonlinear optical material is filled in a hollow portion at the center of a clad 12. As the above-mentioned nonlinear optical material, it is preferable to use an organic nonlinear optical material having high wavelength conversion efficiency as described above. In this example, 3,5-dimethyl-1- (4-nitrophenyl) pyrazole (hereinafter referred to as JP-A-62-210432) (hereinafter,
The core 11 is formed of the PRA).

Here, as an example, the core 11 is the PRA described above,
A method of manufacturing the optical wavelength conversion element 10 will be described for the case of forming SFS3 glass. First, a hollow glass fiber 12 'serving as the clad 12 is prepared. As an example, the glass fiber 12 'has an outer diameter of 3 mm and a hollow portion having a diameter of 2 μm. And as shown in FIG.
The PRA 11 'is kept in a molten state in a furnace or the like, and one end of the glass fiber 12' is infiltrated into the melt. Then, due to the capillary phenomenon, the PRA 11 'in the melted state enters into the hollow portion of the glass fiber 12'. The temperature of the melt is
In order to prevent decomposition of PRA11 ', the temperature should be slightly higher than its melting point (102 ° C). Then glass fiber 12 ′
When cooled rapidly, the PRA11 'that had entered the hollow part was polycrystallized.

Then, this glass fiber 12 'is put into a furnace kept at a temperature higher than the melting point of PRA 11' (for example, 102.5 ° C),
By gradually pulling it out of the furnace maintained at a temperature lower than the melting point, the melted PRA 11 'is single-crystallized from the portion pulled out of the furnace. As a result, the core 11 having an extremely long single crystal state and having a uniform crystal orientation is formed, and the light wavelength conversion element 10 can be made sufficiently long.

After the core 11 is filled as described above, both ends of the glass fiber 12 'are appropriately cut. At this time, the element end face 10a which is the light incident end face is cut perpendicularly to the fiber axis, and the cladding end face 12a on the opposite side is finished to an aspherical lens surface by a known polishing method or the like. As a result, the optical wavelength conversion element as shown in FIG. 1 and FIG.
You get 10. The shape of the clad end surface 12a will be described in detail later.

The light wavelength conversion element 10 is used as shown in FIG. That is, for example, a semiconductor laser (wavelength: 870 nm) 16 is used as a fundamental wave generating means, and a laser beam (fundamental wave) 15 which is a divergent beam emitted from the semiconductor laser 16 is made into a parallel beam by a collimator lens 25, and a beam shaping prism. The cross-sectional shape is made into a perfect circle by 26 and 27, and the light is focused by the objective lens 18 and then irradiated to the element end face 10a (end face of the core 11). As a result, the laser light 15 enters the core 11. This fundamental wave 15 constitutes the core 11.
The PRA converts the second harmonic wave 15 'having a wavelength of 1/2.
This second harmonic 15 'is radiated into the cladding 12 and the element 10
Proceed inside to the end face side. The phase matching is performed between the guided mode in the core part of the fundamental wave 15 and the radiation mode in the cladding part of the second harmonic wave 15 '(so-called Cherenkov radiation).

The second harmonic wave 15 'is emitted from the clad end surface 12a to the outside of the device. The fundamental wave 15 guided in the core 11 is emitted from the end surface 11a of the core 11. The light beam 15 ″ including the second harmonic wave 15 ′ and the fundamental wave 15 is passed through a filter 18 that passes only the second harmonic wave 15 ′, and only the second harmonic wave 15 ′ is extracted. The wave 15 'is narrowed down to a minute spot P by the action of the clad end surface 12a which is an aspherical lens surface.Although Fig. 2 does not show a device using this second harmonic wave 15', In the device of this type, the second harmonic wave 15 'is often used in this way for the reasons described above.

Next, the shape of the cladding end face 12a will be described in detail. In this embodiment, as shown in detail in FIG. 4, the clad 12 is formed sufficiently thick so that all the second harmonics 15 'radiated into the clad 12 at the phase matching angle .theta. The light is directly emitted from the cladding end face 12a to the outside of the device without being totally reflected. To do so, assuming that the diameter of the core 11 is d and the length of the outer peripheral portion of the clad 12 is L (see FIG. 4), the diameter D of the clad 12 is set to D> 2L · tan θ + d (1) do it. If it looks like this, the clad
The wavefronts of all the second harmonic waves 15 'traveling in 12 are oriented in the direction indicated by the arrow W in Fig. 4 within one plane including the core axis, and are therefore conical as a whole. Therefore, if the shape of the clad end surface 12a is set as an aspherical lens surface in which a conical surface J shown by a broken line in FIG. 4 and a spherical surface K shown by a chain line are combined, the second harmonic wave 1 emitted to the outside of the clad 12 is obtained.
The 5'wavefront becomes a converging spherical wavefront. Here, the shape of the cladding end face 12a will be described in detail by using the yz coordinate system shown in FIG. Since this optical system is an axially symmetric system, it is sufficient to consider such a yz plane, and spherical aberration and the like will not be considered below (the same applies hereinafter). The shape formula of the conical surface J is z ₁ = a | y |, while the shape formula of the spherical surface is Therefore, the geometrical expression z (y) of the cladding end surface is Becomes However n _C : Refractive index of cladding θ: Phase matching angle n _A : Refractive index outside the element (1 for air). The clad end face has a shape defined by the above formula (2).
By forming 12a, the second harmonic 1
The wavefront of 5'becomes a converging spherical wavefront, and the second harmonic wave 15 '
Will converge to a small spot P.

Next, a second embodiment of the present invention will be described with reference to FIGS. In FIGS. 5 and 6, the same elements as those in FIGS. 1 to 4 are designated by the same reference numerals, and the description thereof will be omitted unless otherwise necessary (the same applies hereinafter). In the optical wavelength conversion device 20 of the second embodiment, the cladding end face 12a from which the second harmonic wave 15 'is emitted is formed perpendicular to the core axis, and the concentric grating 21 is provided on the surface thereof. There is. Such a grating 21 can be formed by a known photolithography method or the like. Also in this embodiment, the diameter D of the clad 12 is formed to be sufficiently large, for example, about 1 to 3 mm so that the above-mentioned expression (1) is established, and thus the above-mentioned grating 21 is relatively easily formed. be able to. The m-th ring zone radius rm of the grating 21 will be described below.

As shown in FIG. 5, considering the position z in the optical axis direction and the position r in the radial direction of the concentric grating 21, the refractive index of the cladding 12 is n _C , the phase matching angle of the second harmonic wave 15 ′ is θ, and the convergence distance is Be f. Since it is an axially symmetric system, it is considered only on one cross section. The phase φ of the grating 21 is φ = φ _OUT −φ _IN = 2πm + const. (M is an integer), where φ _IN is the phase of the incident wave (conical wave front) and φ _OUT is the phase of the output wave (spherical wave front). is there. And φ _IN = (rn _C sin θ + zn _C cos θ) 2π / λ And z = 0 on the grating surface,
After all Assuming that r = 0 when m = 0 as the standard condition, const. = − F · 2π / λ Therefore, the following quadratic equation (1-n _C ² sin ² θ) r ² −2n _C sin θ (mλ−f ) R + (2f
The solution r _{m obtained} by solving −mλ) mλ = 0 for an integer _m is the ring zone radius of the m-th grating.

It should be noted that the grating 21 as described above can also be used to converge the second harmonic wave 15 'on the reflecting surface of the optical disk. In that case, in consideration of refraction of the second harmonic wave 15' within the optical disk, The band radius should be set.

In this embodiment as well, the wavefront of the second harmonic wave 15 'can be converted from a conical wavefront to a converging spherical wavefront by passing through such a grating 21.
It is possible to narrow down the harmonic wave 15 'to a minute spot P.

Next, a third embodiment of the present invention will be described with reference to FIG. The device of the third embodiment includes a fiber Cherenkov type optical wavelength conversion element 31 similar to that already described,
This is a light wavelength conversion module 40 including a lens 42 arranged so as to face the end face (light emission end face) 12a of the cladding 12 of the light wavelength conversion element 31. In the lens 42, the surface 42a on the light wavelength conversion element 31 side is a conical surface, and the surface 42b on the opposite side is a spherical surface. By using the lens 42 having such a shape, the second light emitted from the light wavelength conversion element 31
When the harmonic wave 15 'passes through the lens surface 42a, the wavefront is converted from a conical wavefront into a plane wavefront. Further, as the second harmonic wave 15' passes through the lens surface 42b, the wavefront converges from the plane wavefront. Converted to a spherical wavefront and thus the second
The harmonic wave 15 'is focused on the minute spot P.

Note that the same operation as described above can be obtained by using a lens having one conical surface and the other surface being a flat surface, and a lens having at least one spherical surface.

Next, a fourth embodiment of the present invention will be described with reference to FIG. The device of the fourth embodiment includes a fiber Cherenkov type optical wavelength conversion element 31 similar to that already described,
This is a light wavelength conversion module 50 including a grating element 51 arranged so as to face the end face (light emission end face) 12a of the cladding 12 of the light wavelength conversion device 31. The grating element 51 includes a concentric circular grating on one surface of the transparent member 52 (the surface opposite to the light wavelength conversion element 31).
53 is formed. In this case also, the wavefront of the second harmonic wave 15 'is converted into a convergent spherical wavefront by passing the second harmonic wave 15' emitted from the cladding end surface 12a of the optical wavelength conversion element 31 through the grating element 51 as described above. be able to.

Hereinafter, the shape of the concentric circle grating 53 will be described in detail. As shown in FIG. 9, the position z in the optical axis direction,
Considering the radial position r of the concentric grating 53, the exit angle of the second harmonic wave 15 ′ from the cladding 12 is θ ₀ , and the convergence distance is f. The phase φ of the grating 53 is φ = φ _OUT −φ _IN = 2πm + const. (M is an integer), where φ _IN is the phase of the incident wave (conical wave front) and φ _OUT is the phase of the output wave (spherical wave front). is there. And φ _IN = (rsin θ ₀ + zcos θ ₀ ) 2π / λ And z = 0 on the grating surface,
After all If r = 0 when m = 0 as a standard condition, const. = − F · 2π / λ Therefore, the following quadratic equation (cos ² θ ₀ ) r ² + 2sin θ ₀ (f−mλ) r + (2f−mλ )
The solution r _{m obtained} by solving mλ = 0 for an integer m becomes the ring zone radius of the m-th grating.

The concentric circle grating 53 as described above can be created by a method such as electron beam drawing or laser direct drawing, and can also be created holographically using a device as shown in FIG. In FIG. 10, reference numeral 60 represents a plane wave, and 61 represents the plane wave 60 in the second wave.
Concave conical lens for converting into a diverging conical surface wave 60 'similar to the harmonic wave 15'.Concave conical surface lens for converting 42, 63 and 63 are half mirrors, 64 and 65 are mirrors, and 66 is a plane wave converging spherical surface. wave
It is a spherical lens that converts to 60 ″, and a photosensitive body 67 carried on the transparent member 52 is irradiated with a diverging conical surface wave 60 ′ and a converging spherical wave 60 ″ that are coaxially aligned with each other to form a grating. To be done.

Next, a fifth embodiment of the present invention will be described with reference to FIG. The device of the fifth embodiment also includes the optical wavelength conversion element 31.
And a concentric grating element 71. In this example, the concentric grating 73 formed on the surface of the transparent member 72 is the second harmonic wave.
It is designed to transform a 15 'cone wavefront into a divergent spherical wavefront. The second harmonic wave 15 ′ thus wavefront-converted is passed through the collimator lens 74 formed of a spherical lens to form a parallel beam, and the beam width can be expanded. Of course, the second harmonic 15 'thus collimated
Can be narrowed down to a minute spot P by passing it through a condenser lens 75 composed of a spherical lens.

In the fourth and fifth embodiments, the grating elements 51 and 71 are arranged apart from the light wavelength conversion element 31, but the grating element is closely fixed to the cladding end face 12a of the light wavelength conversion element 31. I don't mind.

Further, the grating elements 51 and 71 as described above are
The surface on which the concentric gratings 53 and 73 are formed may be arranged so as to face the optical wavelength conversion element 31 side.

Although the embodiment for converting the spherical wave into the second harmonic has been described above, the present invention is not limited to this, and an optical wavelength conversion element for converting the fundamental wave into the third harmonic or a sum of two kinds of fundamental waves. It is also applicable to an optical wavelength conversion element that converts a wavelength into a frequency or a difference frequency.

(Effects of the Invention) According to the present invention as described in detail above, since the wavefront of the wavelength-converted wave emitted from the fiber-type optical wavelength conversion element can be a spherical wavefront, this wavelength-converted wave can be used as it is, or It becomes possible to focus on a minute spot through a collimator lens or a condenser lens which is an ordinary spherical lens. Therefore, according to the present invention, it becomes possible to use the wavelength-converted wave for an optical recording device or the like that needs to narrow the light beam to be used to a minute spot, and the range of application of the wavelength conversion technique is significantly expanded. .

[Brief description of drawings]

FIG. 1 is a schematic perspective view showing a light wavelength conversion element of a first embodiment of the present invention, FIG. 2 is a side view showing a usage state of the light wavelength conversion element of the first embodiment, and FIG. FIG. 4 is an explanatory view for explaining a method of manufacturing a wavelength conversion element, FIG. 4 is a side sectional view showing the above-mentioned optical wavelength conversion element in an enlarged manner, and FIGS. And FIG. 7 is a side sectional view showing a light wavelength conversion module according to the third and fourth embodiments of the present invention, and FIG. 9 is a light wavelength conversion of the fourth embodiment. FIG. 10 is an explanatory view for explaining the shape of a concentric circle grating of the module, FIG. 10 is an explanatory view for explaining a method of manufacturing the optical wavelength conversion module of the above-mentioned fourth embodiment, and FIG. It is a sectional side view which shows a wavelength conversion module. 10, 20, 31 …… Optical wavelength converter, 11 …… Core 12 …… Clad, 12a …… Clad end face 15 …… Basic wave, 15 ′ …… Second harmonic 21, 53,73 …… Concentric grating 40 , 50, 70 …… Optical wavelength conversion module 42 …… Lens 42a, 42b …… Lens surface 51, 71 …… Grating element

Claims (4)

[Claims] 1. A fiber in which a core of a nonlinear optical material is filled in a clad having a refractive index lower than that of the fiber, and an optical wavelength conversion in which a fundamental wave incident on the core is wavelength-converted and radiated into the clad. In the element, the clad end face from which the wavelength-converted wave is emitted is an aspherical lens surface that converts the wavefront of the wavelength-converted wave from a conical wavefront to a spherical wavefront. 2. A fiber in which a core of a nonlinear optical material is filled in a clad having a refractive index lower than that of the fiber, and a fundamental wavelength incident on the core is wavelength-converted to emit light into the clad. In the element, an optical wavelength conversion element characterized in that a concentric circular grating for converting the wavefront of the wavelength-converted wave from the conical wavefront to the spherical wavefront is formed on the end face of the cladding from which the wavelength-converted wave is emitted. 3. A fiber having a core of a non-linear optical material filled in a clad having a refractive index lower than that of the core, wherein the fundamental wave incident on the core is wavelength-converted and emitted into the clad. The element and the wavelength conversion wave emitted from the clad end surface of this optical wavelength conversion element are arranged at the position where they are incident, and the wavefront of this wavelength conversion wave is converted from a conical wavefront to a spherical wavefront. , A light wavelength conversion module including a lens having the other light passing surface formed into a spherical surface. 4. A fiber in which a core of a nonlinear optical material is filled in a clad having a refractive index lower than that of the clad, and the wavelength of the fundamental wave incident on the core is converted to radiate into the clad. An optical wavelength conversion module comprising an element and a concentric circular grating element arranged at a position where the wavelength conversion wave emitted from the clad end surface of the optical wavelength conversion element is incident and converting the wavefront of this wavelength conversion wave from a conical wavefront to a spherical wavefront. .