WO2024201650A1 - Optical fiber and optical amplifier - Google Patents

Abstract

This optical fiber is provided with: a core region to which rare earth ions are added; a plurality of holes that each have a center axis parallel to the center axis of the core region, the plurality of holes being disposed so as to surround the core region; and a clad region that is disposed so as to include the core region and the plurality of holes in the interior thereof, the clad region having a refractive index lower than that of the core region.

Description

Optical Fibers and Optical Amplifiers

This disclosure relates to optical fibers and optical amplifiers.

Non-Patent Document 1 discloses a rare-earth doped fiber in which a rare-earth element is added to the core region in order to amplify the signal light propagating through the core region. In a cladding-pumped rare-earth doped fiber, pumping light is introduced into the cladding region and excites rare-earth ions in all core regions contained in the rare-earth doped fiber. Therefore, multiple signal lights can be amplified simultaneously with a single pumping light source.

M. Wada et al., “High Density Few-mode Multicore Fiber Amplifier for Energy Efficient SDM Transmission,” ECOC2019 W.2.C.5 (2019)

In the optical fiber disclosed in Non-Patent Document 1, the pumping light has a uniform light intensity distribution in the cladding region, and there is pumping light that does not overlap with the core region. Since the pumping light that does not overlap with the core region does not contribute to the amplification of the signal light propagating through the core region, there is a problem in that the pumping efficiency decreases.

The present disclosure has been made in consideration of the above problems. Its purpose is to provide an optical fiber and an optical amplifier that can reduce the pumping light that does not overlap the core region and improve the pumping efficiency so that the pumping light contributes to the amplification of the signal light propagating through the core region.

In order to solve the above-mentioned problems, an optical fiber according to one embodiment of the present disclosure comprises a core region doped with rare earth ions, a plurality of holes having a central axis parallel to the central axis of the core region and arranged to surround the core region, and a cladding region arranged to contain the core region and the plurality of holes therein and having a refractive index lower than that of the core region.

An optical amplifier according to one aspect of the present disclosure includes the optical fiber described above, an excitation light source that emits excitation light that excites rare earth ions, an optical coupler that introduces the signal light and excitation light into one end of the optical fiber, and an optical splitter that splits the signal light from the light output from the other end of the optical fiber.

According to the present disclosure, it is possible to reduce the amount of pump light that does not overlap the core region, thereby improving pump efficiency, so that the pump light contributes to the amplification of the signal light propagating through the core region.

FIG. 1 is a cross-sectional view illustrating the structure of an optical fiber according to an embodiment of the present disclosure. FIG. 2 is a cross-sectional view showing the structure of an optical fiber according to a modified example of the present disclosure. FIG. 3 is a schematic diagram showing the intensity distribution of pump light in an optical fiber that does not include holes. FIG. 4 is a schematic diagram illustrating the intensity distribution of excitation light in an optical fiber according to an embodiment of the present disclosure. FIG. 5 is a schematic diagram showing the intensity distribution of excitation light when the contribution of the photonic crystal structure is taken into account in an optical fiber according to an embodiment of the present disclosure. FIG. 6 is a schematic diagram illustrating a configuration of an optical amplifier according to an embodiment of the present disclosure.

Next, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the description, the same parts will be given the same reference numerals and duplicate explanations will be omitted.

[Optical fiber configuration]
Fig. 1 is a cross-sectional view showing the structure of an optical fiber according to an embodiment of the present disclosure. Fig. 2 is a cross-sectional view showing the structure of an optical fiber according to a modified example of the present disclosure. Figs. 1 and 2 show cross-sectional views of a cross section perpendicular to the extending direction (direction of the central axis) of the optical fiber FB. The optical fiber FB includes a core region 10, holes 20, and a cladding region 30.

The core region 10 is doped with rare earth ions. Examples of rare earth ions that may be doped into the core region 10 of the optical fiber FB include, but are not limited to, Er ³⁺ , Pr ³⁺ , Tm ³⁺ , Yb ³⁺ , Nd ³⁺ , and Dy ³⁺ . The rare earth ions are selected according to the band of the signal light to be amplified. Multiple types of rare earth ions may be doped.

The number and arrangement of the core regions 10 in the optical fiber FB are not limited to the examples shown in Figs. 1 and 2. For example, the optical fiber FB may have any number of core regions 10. In addition, in the optical fiber FB, the multiple core regions 10 may be arranged in a circular ring shape, or in a square or hexagonal close-packed arrangement.

The air holes 20 have a central axis parallel to the central axis of the core region 10 and are arranged to surround the core region 10. A plurality of air holes 20 are arranged in the optical fiber FB. A photonic crystal structure can be obtained by regularly arranging the air holes 20 in the cladding region 30.

In addition, FIG. 1 shows a 54-hole structure as the photonic crystal structure, in which seven holes near the center of a row of hexagonally close-packed holes 20 are missing. Also, FIG. 2 shows a 54-hole structure as the photonic crystal structure, in which one hole near the center of a row of hexagonally close-packed holes 20 is missing. The photonic crystal structure is not limited to the examples given here, and may be a hexagonally close-packed or square arrangement.

By arranging the core region 10 so as to surround the core region 10, an excitation light guide region RA is formed near the core region 10. The excitation light guide region RA is a part of the cladding region 30, and is a region surrounded by the air holes 20.

The photonic crystal structure limits the mode of the excitation light propagating in the excitation light guide region RA. In particular, since the excitation light propagates in a limited mode in the excitation light guide region RA, rare earth ions can be excited in all of the core regions 10 arranged in the excitation light guide region RA.

Furthermore, by appropriately modifying the photonic crystal structure during the design stage of the optical fiber FB, it is possible to select the electric field intensity distribution within the optical fiber FB. In other words, the pumping light can be concentrated in the pumping light guide region RA so that the pumping light contributes to the amplification of the signal light propagating through the core region 10. As a result, the pumping light that does not overlap the core region 10 can be reduced, and the pumping efficiency by the optical fiber FB can be improved.

The diameter of each air hole 20 may be 2 μm or more. This is because if the diameter of the air hole 20 were shorter than the wavelength of the light passing through the optical fiber FB, the air hole 20 itself would scatter the light passing through the optical fiber FB, increasing the scattering loss. Furthermore, the diameter of the air hole 20 is set to 2 μm or more in consideration of the controllability of the air hole 20 when forming the optical fiber FB by performing the drawing process (drawing) in which the base material is heated and stretched.

The refractive index of the holes 20 may be lower than the refractive index of the cladding region 30. This prevents a decrease in the coupling efficiency (optical amplification efficiency) between the excitation light and the signal light.

The cladding region 30 is arranged so as to include the core region 10 and the multiple holes 20 therein, and has a lower refractive index than the core region 10. The core region 10 and the above-mentioned excitation light guide region RA are arranged near the center of the cladding region 30.

[Excitation light distribution]
Next, a description will be given of changes in the distribution of pump light caused by the presence or absence of holes 20 in the cladding region 30. Fig. 3 is a schematic diagram showing the intensity distribution of pump light in an optical fiber without holes. Fig. 4 is a schematic diagram showing the intensity distribution of pump light in an optical fiber according to an embodiment of the present disclosure.

In both Figures 3 and 4, the intensity distribution of the excitation light EL is shown on a straight line along the radial direction of the optical fiber FB. For reference, the intensity distribution of the signal light SL is also shown.

As shown in FIG. 3, in the cladding region 30 where no air holes 20 are arranged, the pumping light EL spreads throughout the cladding region 30, resulting in pumping light EL that does not overlap with the core region. The pumping light EL that does not overlap with the core region does not contribute to the amplification of the signal light SL propagating through the core region, resulting in reduced pumping efficiency.

On the other hand, as shown in FIG. 4, in the cladding region 30 where the air holes 20 are arranged, the pumping light EL is concentrated in the pumping light guide region RA. Therefore, the pumping light EL that does not overlap with the core region 10 is reduced. As a result, the pumping light EL contributes to the amplification of the signal light SL propagating through the core region 10, and the pumping efficiency by the optical fiber FB is improved.

FIG. 5 is a schematic diagram showing the intensity distribution of the excitation light EL when the contribution of the photonic crystal structure is taken into account in an optical fiber according to an embodiment of the present disclosure. The photonic crystal structure limits the number of modes of the excitation light EL, making it possible to increase the intensity of the excitation light EL in the core region.

In FIG. 5, an example is shown in which the pump light EL propagates in the fundamental mode, but the number of modes of the pump light EL may be two or more, i.e., a multimode state. By setting the distance between the core regions 10 to 25 μm or less, the signal light SL is coupled between the cores. As a result, the gain deviation between the cores caused by the non-uniform intensity distribution of the pump light EL can be averaged.

[Configuration of optical amplifier]
Next, an optical amplifier AP using the optical fiber FB described in the above embodiment will be described. Fig. 6 is a schematic diagram showing the configuration of an optical amplifier according to an embodiment of the present disclosure. The optical amplifier AP includes a pumping light source LD, an isolator TS1, an optical coupler PC, the optical fiber FB described in the above embodiment, and an isolator TS2.

The excitation light source LD emits excitation light. The excitation light is light that excites the rare earth ions added to the optical fiber FB.

The isolator TS1 introduces the excitation light from the excitation light source LD to the optical coupler PC. The isolator TS1 blocks the light traveling from the optical coupler PC to the excitation light source LD.

The optical coupler PC introduces the signal light and the pumping light into one end of the optical fiber FB. The optical coupler PC introduces the signal light into the core region 10 of the optical fiber FB, and introduces the pumping light into the pumping light guide region RA of the cladding region 30, which is surrounded by holes 20.

The optical coupler PC may be provided with a position adjustment mechanism (not shown) that adjusts the position at which the excitation light is introduced into the excitation light guide region RA. The position adjustment mechanism may control the position in the cross section of the excitation light guide region RA where the excitation light is coupled, and excite a predetermined number of modes of the excitation light.

The signal light input to one end of the optical fiber FB together with the pump light is amplified while traveling through the optical fiber FB and is output from the other end of the optical fiber FB.

The isolator TS2 (optical splitter) splits the signal light from the light output from the other end of the optical fiber FB. The split signal light is introduced into the output optical fiber B2. The isolator TS2 blocks the pump light traveling from the optical fiber FB to the output optical fiber B2.

[Effects of the embodiment]
As described above in detail, the optical fiber according to this embodiment comprises a core region doped with rare earth ions, a plurality of holes having a central axis parallel to the central axis of the core region and arranged so as to surround the core region, and a cladding region arranged so as to contain the core region and the plurality of holes therein, and having a refractive index lower than that of the core region.

This reduces the amount of pump light that does not overlap the core region, improving pump efficiency so that the pump light contributes to the amplification of the signal light propagating through the core region.

The optical fiber according to this embodiment may have a photonic crystal structure in which a plurality of holes are regularly arranged in a cross section perpendicular to the central axis of the core region. The photonic crystal structure limits the mode of the excitation light propagating in the excitation light waveguide region. In particular, since the excitation light propagates in a limited mode in the excitation light waveguide region, rare earth ions can be excited in all of the core regions arranged in the excitation light waveguide region.

Furthermore, by appropriately modifying the photonic crystal structure during the optical fiber design stage, it is possible to select the electric field intensity distribution within the optical fiber. In other words, the pump light can be concentrated in the pump light guide region so that the pump light contributes to the amplification of the signal light propagating through the core region. As a result, the amount of pump light that does not overlap the core region can be reduced, improving the pumping efficiency of the optical fiber.

Furthermore, the optical fiber according to this embodiment may have a plurality of holes arranged in a square or hexagonal close-packed arrangement in a cross section perpendicular to the central axis of the core region. This makes it possible to effectively concentrate the excitation light in the excitation light guide region. As a result, the excitation light that does not overlap the core region can be reduced, and the excitation efficiency of the optical fiber can be improved.

Furthermore, in the optical fiber according to this embodiment, the diameter of each air hole may be 2 μm or more. This suppresses the air holes themselves from scattering the light passing through the optical fiber, and prevents the installation of air holes from increasing scattering loss. Furthermore, the controllability of the air holes can be maintained during the elongation process in which the base material is heated and elongated.

Furthermore, in the optical fiber according to this embodiment, the refractive index of the air holes may be lower than the refractive index of the cladding region. This prevents a decrease in the coupling efficiency (optical amplification efficiency) with the signal light in the optical fiber having air holes.

The optical amplifier according to this embodiment includes the optical fiber described above, an excitation light source that emits excitation light that excites rare earth ions, an optical coupler that introduces signal light and excitation light into one end of the optical fiber, and an optical splitter that splits the signal light from the light output from the other end of the optical fiber. This makes it possible to amplify with high efficiency the signal light input to one end of the optical fiber together with the excitation light.

Furthermore, in the optical amplifier according to this embodiment, the optical coupler may introduce the signal light into the core region and introduce the pump light into a pump light guide region in the cladding region that is surrounded by holes. This makes it possible to increase the intensity of the pump light in the core region. Furthermore, the position in the cross section of the pump light guide region where the pump light is coupled can be controlled, making it possible to excite a predetermined number of modes of the pump light. Furthermore, it is possible to average out the gain deviation between cores caused by the non-uniform intensity distribution of the pump light.

The contents of the present disclosure have been described above in accordance with the embodiments, but it will be apparent to those skilled in the art that the present disclosure is not limited to these descriptions, and that various modifications and improvements are possible. The descriptions and drawings that form part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operational techniques will be apparent to those skilled in the art from this disclosure.

This disclosure naturally includes various embodiments not described here. Therefore, the technical scope of this disclosure is determined only by the invention-specific matters related to the scope of the claims that are appropriate from the above explanation.

REFERENCE SIGNS LIST 10 core region 20 air hole 30 cladding region RA pumping light guide region AP optical amplifier B1 input optical fiber B2 output optical fiber FB optical fiber LD pumping light source PC optical coupler TS1, TS2 isolator (optical splitter)

Claims (7)

a core region doped with rare earth ions;
a plurality of holes having central axes parallel to a central axis of the core region and arranged to surround the core region;
a cladding region arranged to contain the core region and the plurality of holes therein, the cladding region having a refractive index lower than that of the core region;
An optical fiber comprising: The optical fiber according to claim 1, wherein the plurality of holes are regularly arranged in a cross section perpendicular to the central axis of the core region to form a photonic crystal structure. The optical fiber according to claim 2, wherein the plurality of holes are arranged in a square or hexagonal close-packed manner in the cross section. The optical fiber according to claim 1, wherein the diameter of each of the holes is 2 μm or more. The optical fiber of claim 1, wherein the refractive index of the holes is lower than the refractive index of the cladding region. An optical fiber according to any one of claims 1 to 5;
an excitation light source that emits excitation light for exciting the rare earth ions;
an optical coupler that introduces the signal light and the pumping light into one end of the optical fiber;
an optical demultiplexer that demultiplexes the signal light from the light output from the other end of the optical fiber;
An optical amplifier comprising: The optical coupler includes:
Introducing the signal light into the core region;
7. The optical amplifier according to claim 6, wherein the pumping light is introduced into a pumping light guide region of the cladding region surrounded by the holes.

Images