JP3049077B2 - Functional optical fiber - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a functional optical fiber having a core doped with an active substance, and for example, to a functional optical fiber used for applications such as light emission and amplification. .

[Conventional technology]

In order to perform optical amplification, laser emission, and the like using an optical fiber, a functional optical fiber in which various rare earth elements are added to a core as an active material has been reported. In particular, functional optical fibers doped with Nd, Er, Ho, etc. are expected to be applied in fields such as communication as fiber lasers, fiber amplifiers, fiber sensors, and the like.

[Problems to be solved by the invention]

However, in a functional optical fiber in which a rare earth element is added to a core, an emission wavelength or an optical amplification wavelength cannot be arbitrarily set due to its inherent energy level. For this reason, there have been cases where functional optical fibers cannot be put to practical use. For example, Nd The transition is expected to be applied to the 1.31 μm wavelength optical communication field. Nevertheless, the maximum optical gain can be obtained with the Nd-doped optical fiber at the wavelength 1.33 μm on the longer wavelength side than the wavelength 1.31 μm, and the optical amplification gain is not selected at the wavelength 1.31 μm (OFC '90 Post Deadline Paper).

In view of the above circumstances, the present invention is directed to a functional optical fiber having a core to which an active substance such as a rare earth element is added, which is used for optical amplification that causes a desired shift in its emission wavelength or optical amplification wavelength. The purpose of the present invention is to provide a functional optical fiber.

[Means and Actions for Solving the Problems]

To achieve the above object, a functional optical fiber according to the present invention includes a core doped with a rare earth element or a transition metal as an active material, and a clad surrounding the core and having a lower refractive index than the core, A stress applying structure for applying stress to the core is provided, and a desired shift is generated in the emission wavelength of the active substance or the optical amplification wavelength of the active substance in accordance with the stress applied to the core.

In the above-mentioned functional optical fiber, since the stress is applied to the core by the stress applying structure, an energy shift occurs in the energy level of the rare earth element or the transition metal added as the active substance. As a result, a desired shift can be generated in the emission wavelength of the active substance or the light amplification wavelength of the active substance in accordance with the stress applied to the core. Alternatively, the wavelength-dependent characteristic distribution of light emission or optical amplification can be changed.

For example, as a preferred embodiment of the above-mentioned functional optical fiber, a coating layer having a linear expansion coefficient different from that of the cladding may be provided outside the cladding, and this may be a stress applying structure.

If the coefficient of linear expansion of the coating layer is large, when the functional optical fiber is cooled below the temperature at which the coating layer and the cladding are formed, the coating layer exerts stress on the cladding, and this pressure further increases. It extends to the core. in this case,
Depending on the pressure applied to the core, it is possible to cause a desired shift in the emission wavelength of the active substance or the light amplification wavelength of the active substance.

As another preferred embodiment of the functional optical fiber,
A mechanical stress may be applied to the core and the clad to form a stress applying structure.

As a method of applying the mechanical stress, for example, the functional optical fiber may be compressed, pulled, or twisted in a predetermined direction. Thereby, various stresses can be applied to the core. As described above, a desired shift or the like can be generated in the emission wavelength of the active substance or the light amplification wavelength of the active substance in accordance with the pressure applied to the core.

As still another preferred embodiment of the functional optical fiber, a difference may be provided between the coefficient of linear expansion of the clad and the coefficient of linear expansion of the core, and this may be a stress applying structure.

If the coefficient of linear expansion of the cladding is large, the cladding exerts pressure on the core when the functional optical fiber is cooler than the temperature at which the core and the cladding are formed. In this case, depending on the pressure applied to the core,
A desired shift or the like can be generated in the emission wavelength of the active substance or the light amplification wavelength of the active substance.

As still another preferred embodiment of the functional optical fiber, a stress applying structure may be provided by providing a stress applying portion having a coefficient of linear expansion different from that of the clad inside the clad.

Similarly to the above, the stress applying section acts on the clad to deform it and generate stress in the core.

As still another preferred embodiment of the functional optical fiber, a stress applying structure may be formed by rapidly cooling the functional optical fiber at the time of drawing.

In this case, the surface of the functional optical fiber is rapidly cooled, so that a compressive stress acts on the clad and a tensile stress acts on the core.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to FIG. 1 to FIG. Note that the present invention relates to a functional optical fiber for optical amplification used for applications such as light emission and optical amplification. Hereinafter, this is simply referred to as a functional optical fiber.

FIG. 1 is a view showing a first embodiment of a functional optical fiber according to the present invention. The functional optical fiber of the present embodiment is
A core 12 made of phosphate glass to which Nd ³⁺ is added as an active material and a clad 13 made of phosphate glass to which Nd ³⁺ is not added are provided. Furthermore, around the cladding 13,
A stress applying jacket 11, which is a coating layer, is provided.
As a material of the stress applying jacket 11, glass having a linear expansion coefficient larger than that of the cladding 13 is used.

Hereinafter, the structure of the functional optical fiber of the embodiment will be briefly described.

The above-mentioned core 12, clad 13 and stress applying jacket 11
Is prepared by a melting method. From these glasses, a core rod, a cladding pipe, and a stress applying jacket pipe are produced. These rods and pipes are fiberized by a rod-in-tube method to form a functional optical fiber. In this functional optical fiber, since the stress applying jacket 11 has a larger linear expansion coefficient than the cladding 13, the core 12 and the cladding 13 are given compressive stress.

The first embodiment can be modified in various ways. For example, silicate glass,
Use of fluoride glass or the like is possible. It is needless to say that a rare earth element such as Er can be used as the active substance, but other transition metals can also be used. Further, as the stress applying jacket, it is desirable to use a glass that does not thermally affect the core and the clad and has good weather resistance in order to enable drawing. However, if the stress applying jacket is not drawn simultaneously, Al, In-Sn, Sn
It is also possible to use a metal material such as. In this case, it is desirable to use a material having a higher hardness than glass.
Further, the coefficient of linear expansion of the stressed jacket may be less than the coefficient of linear expansion of the cladding and core. In this case, it is considered that tension acts on the core.

FIG. 2 is a diagram schematically showing the arrangement of ions in the phosphate glass forming the core. The functional optical fiber of the first embodiment causes a desired shift in the emission wavelength or the optical amplification wavelength. It considers the reason why it can be done. The black dot in the center indicates Nd ³⁺ which is an active ion. Others show the constituent ions of the glass by the glass forming component and the glass modifying component.

The energy level of Nd ³⁺ is split and shifted by a ligand field created by surrounding anions and cations. In this case, the ligand field received by the active ion mainly depends on the distance between the active ion and each anion and cation. For example, consider the case where a uniform compressive stress is applied to the phosphate glass forming the core. By applying such compressive stress, the distance between active ions and surrounding ions is uniformly reduced. As a result, it is considered that the degree of splitting and shifting of the energy level of Nd ³⁺ greatly varies. That is, it is considered that the emission wavelength or the optical amplification wavelength by Nd ³⁺ in the functional optical fiber can be shifted by the presence of an appropriate compressive stress.

By using such a functional optical fiber, for example, a fiber laser, a fiber amplifier and the like can be manufactured.
In the case of a fiber laser, the functional optical fiber containing a predetermined active substance, an excitation light source that generates excitation light, an optical unit that causes the excitation light to enter the functional optical fiber from the excitation light source, and a functional optical fiber And an optical resonator that feeds back emitted light caused by the active material to the functional optical fiber. The fiber amplifier is basically the same as the fiber laser. In this case, however, an optical resonator is not required, and conversely, an optical means for introducing a signal light to be amplified is required.

Hereinafter, the first embodiment will be described more specifically.

To prepare the core rod, finish composition 15 nA ₂ O-
So that _{15Al 2 O 3 -70P 2 O 5} , were formulated raw materials. Further, an oxide of Nd was added as an active substance to the prepared raw material so that the concentration of Nd ³⁺ was 1000 ppm with ^respect to the above-mentioned finished composition. Thereafter, the prepared raw material was melted by heating, vitrified by rapid cooling, and formed into a core rod by machining.

A clad pipe and a pipe for a stress applying jacket were prepared in the same manner. However, the composition of the glass of the clad pipe was _{3PbO-15Na 2 O-15Al 2} O 3 -67P 2 O 5. The composition of the stress-applying jacket pipe is 45Na ₂ O-15Al ₂ O
₃ −40P ₂ O ₅

Next, by a known rod-in-tube method, the core rod, the clad pipe and the pipe for the stress applying jacket were welded and drawn to prepare a functional optical fiber. For comparison, an optical fiber was prepared by welding and drawing only the core rod and the clad pipe.

Next, the fluorescence peak of the obtained functional optical fiber was measured. Specifically, an excitation light source (here, Ti
-A sapphire laser was used. 0.78μm from)
Was introduced into one end of the above functional optical fiber, and the fluorescence output from the other end was measured with an optical spectrum analyzer. In a sample consisting of only the core and the clad, it was observed that the fluorescence peak was present at a wavelength of 1.324 μm. On the other hand, in the sample provided with the stress applying jacket, it was observed that the fluorescence peak was present at a wavelength of 1.312 μm.

As is clear from the above, the fluorescence peak wavelength can be shifted to the shorter wavelength side by using the stress applying jacket. Therefore, it is considered that the peak wavelength of the optical amplification gain can also be shifted to the shorter wavelength side. In the case of the embodiment, the glass used for the stress applying jacket has a high expansion coefficient and a low melting point relative to the core and the clad. For this reason, it is considered that as the thickness of the stress applying jacket increases, compressive stress acts on the core glass.

FIG. 3 shows a functional optical fiber according to a second embodiment of the present invention.

As shown in the figure, a functional optical fiber 31 is wound around a winding core material 32. Furthermore, functional optical fibers
In order to fix 31, a resin mold 33 is formed around it. The functional optical fiber has Nd ³⁺
And a clad made of phosphate glass to which Nd ³⁺ is not added.
The manufacturing method is the same as that of the first embodiment.

The feature of this embodiment is that the functional optical fiber 31 is fixed by the resin mold 33 while being wound with high tension. In this case, the functional optical fiber is pulled with a constant stress in the optical axis direction. As a result, it is possible to shift the emission wavelength or the optical amplification wavelength due to Nd ^{3+ in} the core of the functional optical fiber.

In the embodiment, the functional optical fiber 31 is wound by applying a tension. However, the functional optical fiber 31 may be wound while being twisted. In this case, a torsional stress acts on the functional optical fiber 31.

FIG. 4 shows a functional optical fiber according to a third embodiment of the present invention.

The functional optical fiber of FIG. 4 (a) is used as an active substance.
A core 41 consisting of phosphate glass doped with Nd ^3+, and a cladding 42 made of phosphate glass without the addition of Nd ^3+. In the layer of the clad 42, stress applying portions 42a and 42b extending in the same direction as the optical axis of the core 41 are provided so as to sandwich the core 41. This functional optical fiber was produced by fusing and drawing a glass rod for a core and a stress applying portion and a cladding pipe provided with three holes for accommodating the glass rods.

In the embodiment, the stress applying portions 42a and 42b are larger than the linear expansion coefficient of the clad 42 and have a lower melting point than the clad 42. Therefore, it is considered that a tensile stress acts on the clad 42 due to the residual stress at the time of producing the functional optical fiber, and a tensile stress also acts on the core 41 adjacent thereto.
Therefore, it is possible to shift the emission wavelength or the optical amplification wavelength by Nd ^{3+ in} the core of the functional optical fiber.

FIG. 4 (b) shows a modification of the functional optical fiber of FIG. 4 (a). The difference is that four stress applying portions 42a, 42b, 42c, 42d are provided in the cladding 42. In this case, a more uniform and larger stress can be applied to the core 41 than in the case of the functional optical fiber of FIG. 4 (a).

As a modification of the functional optical fiber of FIGS. 4A and 4A, the entire cladding may be used as a means for applying stress. In other words, as the cladding material,
If a glass or the like having a higher linear expansion coefficient and a lower melting point than that of the material U is used, a compressive stress can be applied to the core due to residual stress when a functional optical fiber is manufactured.

FIG. 5 shows a functional optical fiber according to a fourth embodiment of the present invention.

In the fourth embodiment, stress is generated in the core depending on the conditions of the manufacturing process of the functional optical fiber. Glass corresponding to the composition of the core and the clad is prepared by a melting method. Note that an active substance is added to the core.
Next, a core rod and a cladding pipe are produced from the prepared core and cladding glass, respectively. These rods and pipes are welded to form a preform for drawing. Up to this point, there is no difference from the conventional manufacturing process.

Next, the preform is converted into a fiber by the drawing apparatus shown in FIG. The preform 52 is fixed to the feeding device 22 and descends gradually. The preform 52 is heated by the heater, softened, and drawing starts. The drawn optical fiber 51 is wound around a winding drum 56 via a capstan 55. At this time, if the softening temperature is set to a low temperature, or if the optical fiber 51 after drawing is used as an example, a large stress will remain in the core and the clad.

〔The invention's effect〕

As described above, according to the present invention, since the stress is applied to the core by the stress applying structure, the energy level of the rare earth element or the transition metal, which is the active substance, is shifted or split. As a result, a shift occurs in the emission wavelength or the optical amplification wavelength corresponding to the interval between the energy levels of the active substances, or the emission characteristics change. That is, if the magnitude and type of the stress applied to the core are appropriately adjusted, a desired shift can be generated in the emission wavelength or the optical amplification wavelength due to relaxation of the electrons in the excited active material, Alternatively, the light emission characteristics in a predetermined wavelength band can be changed.

[Brief description of the drawings]

FIG. 1 is a partial sectional view of a first embodiment of a functional optical fiber according to the present invention, FIG. 2 is a diagram schematically showing an arrangement of ions in a core glass, and FIG. 3 is a function of the second embodiment. FIG. 4 is a diagram illustrating a functional optical fiber according to a third embodiment, and FIG. 5 is a diagram illustrating a method of manufacturing the functional optical fiber according to the fourth embodiment. 11 ... Coating layer, 12 ... Core, 13 ... Clad, 31 ... Functional optical fiber, 41 ... Core, 42 ... Clad, 42a, 4
2b, 42c, 42d: Stress applying section.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshiki Chikusa 1, Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Norihisa Keito 1, Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric (72) Inventor Minoru Watanabe Minami 1-chome, Taya-cho, Sakae-ku, Yokohama, Kanagawa Prefecture Sumitomo Electric Industries, Ltd. (72) Inventor Yutaka Katsuyama 1-1-6 Uchisaiwai-cho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-62-111208 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/35

Claims (6)

(57) [Claims] 1. A functional optical fiber for optical amplification, comprising: a core to which a rare earth element or a transition metal is added as an active substance; and a cladding surrounding the core and having a lower refractive index than the core. A light amplification structure, wherein a stress applying structure for applying stress to a core is provided, and a desired shift is generated in an emission wavelength of the active substance or a light amplification wavelength of the active substance in accordance with the stress applied to the core. For functional optical fiber. 2. The functionality according to claim 1, wherein the stress applying structure is formed by providing a coating layer having a different coefficient of linear expansion from the cladding outside the cladding. Optical fiber. 3. The functional optical fiber according to claim 1, wherein the stress applying structure applies a mechanical stress to the core and the clad. 4. The structure according to claim 1, wherein the stress applying structure is formed by providing a difference between a coefficient of linear expansion of the clad and a coefficient of linear expansion of the core. Functional optical fiber. 5. The functional element according to claim 1, wherein the stress applying structure is formed by providing a stress applying section having a coefficient of linear expansion different from that of the clad inside the clad. Optical fiber. 6. The functional optical fiber according to claim 1, wherein said stress applying structure is formed by quenching when drawing the functional optical fiber.