Single crystal optical fiber and manufacture therefor

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to a single crystal optical fiber applicable to laser oscillation, optical amplification and various nonlinear optical effects, and a method for manufacturing the same.

【0002】

[Problems to be solved by conventional techniques and inventions]

Conventionally, single crystal optical fibers have been described in the literature (MM Fejer et al .: Rev. Sci. Instrum., 55 (11), 1984, pp1791-1796) and the literature (YS Luh et al .: J. Crystal Growth, 78,1986, pp. As shown in 135-143), it is a needle-like crystal and does not have a core or clad structure that utilizes the difference in refractive index. Therefore, light cannot be efficiently confined in the fiber, resulting in light loss. There was a problem that it was big.

【0003】

Therefore, when an element to which the nonlinear optical effect is applied is manufactured from a single crystal fiber, there is a problem that it is difficult to obtain the light intensity required to cause the nonlinear optical effect.

【0004】

To solve this problem, as shown in the literature (MJF Digonnet et al .: J. Lightwave Technol. LT-5,1987, pp.642), the refractive index is needle-shaped on the side surface of the needle-shaped single crystal. A structure in which a glass material having a refractive index lower than that of a single crystal is attached and a needle-shaped single crystal is used as a core portion and a glass material is used as a clad portion has been studied.

【0005】

Conventionally, in order to form such a structure, a method has been used in which a needle-shaped single crystal is placed in a molten glass material, the glass material is attached to the needle-shaped single crystal, and the glass material is cooled and solidified. However, in this method, since the glass material is heated to the melting point or higher and then cooled, microbending occurs in the needle-shaped single crystal due to the difference in expansion coefficient between the needle-shaped single crystal and the glass material, and light propagation occurs. There was a problem that the loss increased.

【0006】

Further, since the difference in refractive index from the acicular single crystal is too large for an organic material such as an optical adhesive, there has been a problem that the efficiency of the nonlinear optical effect is lowered due to an increase in the number of propagation modes of the waveguide.

【0007】

In addition, as shown in the literature (S. Sudo et al .: Opt. Lett., 12,1987, pp.938), an additive that lowers the refractive index of the needle-shaped single crystal is diffused from the outside of the needle-shaped single crystal. The internal diffusion method is being studied. However, in this method, it is necessary to diffuse the additive to a considerable depth of the acicular single crystal in order to obtain a sufficient clad layer thickness, which causes a crystal structure irregularity and an increase in light propagation loss. There was a point.

【0008】

An object of the present invention is to solve such a conventional problem and to provide a single crystal optical fiber having a small light loss and capable of causing a non-linear effect with a small light power, and a method for manufacturing the same.

【0009】

[Means for solving problems]

The single crystal optical fiber according to the present invention that achieves the above object is characterized in that a polymer film having a refractive index lower than that of the needle-shaped single crystal is formed on the surface of the needle-shaped single crystal.

【0010】

On the other hand, the method for producing a single crystal optical fiber includes a step of covering the needle-shaped single crystal with a precursor made of a polymer material having a refractive index lower than that of the needle-shaped single crystal, and a step of polymerizing the precursor of the polymer material. It is characterized in that a polymer film is formed on the surface of the needle-shaped single crystal by the step of forming the needle-shaped single crystal.

【0011】

[Action]

According to the present invention, first, the needle-shaped single crystal is substantially covered with a precursor of a polymer material, then the precursor is polymerized by heating or irradiation with ultraviolet rays, and a polymer film is formed on the surface of the needle-shaped single crystal. Form. As a result, it becomes possible to obtain a single crystal optical fiber having a core and a clad structure without microbending and irregular crystal structure.

【0012】

[Example]

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

【0013】

FIG. 1 is a schematic view of a needle-shaped single crystal optical fiber according to this embodiment. As shown in the figure, the single crystal optical fiber 10 is formed of a needle-shaped single crystal 11 and a polymer film 12 formed on the surface of the needle-shaped single crystal 11, and the polymer film 12 The refractive index is lower than that of the needle-shaped single crystal 11.

【0014】

As a step of manufacturing the single crystal optical fiber 10, as shown in FIGS. 1 (A) and 1 (B), a polymer material (needle-shaped single crystal) that forms a polymer film 12 on the outer peripheral surface of the needle-shaped single crystal 11 is formed. The precursor 13 (which has a refractive index lower than the refractive index of 11) is attached in the state of a solution or a melt, and the polymerization reaction shown in "Crystal 1" below is caused by heating or irradiation with ultraviolet rays, for example. , A polymer film 12 made of a polymer material (PPV (polyphenylene vinylene) in "Chemical Formula 1") is formed on the surface of the needle-shaped single crystal 11.

【0015】

[Chemical 1]

<img file="JPH0634829A_D0001.tif" />【0016】

Example 1 In this example, a lithium niobate single crystal fiber produced by a laser melting method was used as the needle-shaped single crystal 11, and polyphenylene vinylene (PPV) having a refractive index of 2.1 was used as the polymer material. The PPV precursor (I) polymerizes as shown in "Chemical formula 1" to become PPV (II). The single crystal fiber manufacturing process of this example will be described with reference to FIG. 1. First, as shown in FIG. 1 (A), the needle-shaped single crystal 11 has a PPV precursor 11 as a precursor 11 of a polymer material ( The alcohol solution of I) adheres evenly. This is heated to 100 ° C for 1 hour in an inert gas such as helium gas. By this heating, the PPV precursor (I) is polymerized to PPV (II), and as a result, as shown in FIG. 1 (B), the acicular single crystal 11 is covered with the polymer film 12 made of PPV (II). In addition, a single crystal optical fiber 10 can be obtained.

【0017】

The cross-sectional structure and refractive index distribution of the single crystal optical fiber 10 produced by this example are shown in FIGS. 2 (A) and 2 (B), respectively. The single crystal optical fiber 20 according to this embodiment is formed of a core 21 made of lithium niobate needle-like single crystal and a clad 22 made of PPV around the core in the central portion. The outer diameter of the core 21 is 50 μm, and the outer diameter of the clad 22 is 100 μm.

【0018】

The refractive index distribution is stepped as shown in FIG. 2 (B), and the refractive index of the core 21 is 2.23 and the refractive index of the clad 22 is 2.10, indicating that a good waveguide structure is formed.

【0019】

In the conventional case where the polymer film made of PPV was not produced, the transmission loss of the single crystal optical fiber at a wavelength of 1.06 μm was 1.2 db / cm, whereas the transmission loss of the single crystal optical fiber according to this example. There was a big improvement of 0.2db / cm.

【0020】

Moreover, when examining the generation of the second harmonic from 1.06 μm light, the conversion efficiency of the conventional fiber without forming the PPV film is 1% / W / cm.²In contrast, the fiber produced in this example was 5% / W / cm.²The conversion efficiency of was obtained, and the improvement of the conversion efficiency was confirmed.

【0021】

Example 2 In this embodiment, the needle-shaped single crystal 11 is internally diffused by using magnesium as an additive for lowering the refractive index of the lithium niobate single crystal optical fiber produced by the laser melting method. Crystals are used, and PPV is used as the polymer material. A PPV film is formed on the surface of the needle-shaped single crystal in the same manner as in Example 1.

【0022】

The cross-sectional structure and refractive index distribution of the single crystal optical fiber produced by this example are shown in FIGS. 3 (A) and 3 (B), respectively. The single crystal optical fiber 30 according to this embodiment is formed of a lithium niobate needle-shaped single crystal 31 and a polymer film 32 made of a surrounding PPV in the central portion. The outer diameter of the needle-shaped single crystal 31 is 50 μm, and the outer diameter of the polymer film 32 made of PPV is 100 μm.

【0023】

As shown in Fig. 3 (B), a graded refractive index distribution is formed inside the needle-shaped single crystal by magnesium internal diffusion, and step-like refraction is formed at the boundary between the needle-shaped single crystal and the polymer film 32 composed of PPV. There is a rate distribution. From the refractive index distribution, the effective core diameter inside the needle-shaped single crystal is estimated to be 30 μm.

【0024】

In the conventional case where the polymer film made of PPV was not formed, the transmission loss of the single crystal optical fiber at a wavelength of 1.06 μm was 1.7db / cm, whereas the transmission loss of the single crystal optical fiber according to this embodiment was 1.7db / cm. There was a big improvement of 0.7db / cm.

【0025】

Moreover, when the second harmonic generation from 1.06 μm light is examined, the conversion efficiency is 2% / W / cm in the fiber in which the polymer film made of PPV is not formed.²However, in the fiber produced in this example, it was 15% / W / cm.²The conversion efficiency of was obtained, and the improvement of the conversion efficiency was confirmed. This result shows that it is necessary to obtain a sufficient clad layer thickness even when the refractive index is distributed inside the needle-shaped single crystal by magnesium diffusion, which shows the advantage of the present invention. ..

【0026】

In this embodiment, the lithium niobate single crystal and PPV as the polymer material have been described as an example, but the production method of the present invention is not limited to lithium niobate and PPV, and for example, neodymium is added. YAG (Y₃Al₅O₁₂) Of course, it can be applied to a combination of a polymer material and an acicular single crystal which are polymerized from a precursor such as acicular crystal and polyimide and whose refractive index is lower than the refractive index of the target acicular single crystal. Is.

【0027】

[Effect of the invention]

As described above, the present invention has an advantage that a low-loss core and clad structure can be produced in a single crystal optical fiber. Taking a core and clad structure with low loss in the fiber structure is effective for improving the efficiency of the nonlinear optical effect, and can generate various nonlinear optical effects such as optical amplification, laser oscillation, harmonic generation, and optical parametric. It is possible to provide crystalline optical fibers.

[Simple explanation of drawings]

[Figure 1]

It is a figure which shows the basic structure of the manufacturing process of the single crystal fiber which concerns on this Example.

[Figure 2]

(A) is a cross-sectional view showing the structure of the single crystal fiber produced in Example 1 of the present invention. (B) is a figure which shows the refractive index distribution of the single crystal fiber produced in Example 1 of this invention.

[Fig. 3]

(A) is a cross-sectional view showing the structure of the single crystal fiber produced in Example 2 of the present invention. (B) is a figure which shows the refractive index distribution of the single crystal fiber produced in Example 2 of this invention.

[Explanation of symbols]

10,20,30 Single crystal optical fiber 11 Needle-shaped single crystal 12 Polymer film 13 Precursor of polymer material