JP2005084386A - Double clad fiber and optical device provided with the same, and optical amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a double clad fiber in which thermal damage by light leaking from a pumping light source is suppressed. <P>SOLUTION: The double clad fiber is provided with a solid core 1, a 1st solid clad 2 arranged around the core 1, and a 2nd solid clad 3 arranged around the 1st clad 2, and all of the solid core 1, the 1st clad 2, and the 2nd clad 3 are formed of quartz. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a double clad fiber, an optical apparatus including the same, and an optical amplifier.

  A double clad fiber, which is a kind of optical fiber, is characterized by having optical amplification characteristics, and is widely used in optical devices such as fiber lasers and optical amplifiers.

  The double-clad fiber is provided from the center of the fiber with a core doped with a rare earth element as an optical amplification component, a first clad provided around the core and having a lower refractive index than the core, and around the first clad. And a second clad having a lower refractive index than the first clad.

  In this double clad fiber, the excitation light incident on the first clad propagates through the region surrounded by the second clad while being repeatedly reflected at the interface between the first clad and the second clad, and the excitation light passes through the core. The rare earth element doped in the core is made to be in an inversion distribution state excited by outermost electrons when passing through the substrate, and the light propagating through the core is amplified by the stimulated emission.

  Non-Patent Document 1 describes an LD module pigtail in which pumping light emitted from a pumping light source is coupled to a multimode fiber, that is, a multimode fiber formed of a core in which a diffraction grating is written and a low refractive index polymer. A configuration of an optical amplifier to which a double clad fiber having a second clad formed is connected is disclosed.

In this optical amplifier, a diffraction grating (Fiber Bragg Grating, hereinafter referred to as “FBG”) written in the core of a double clad fiber functions as a dichroic mirror for pump light and signal light reflection, and propagates through the core. It is described as amplifying light.
Proceedings of the 1996 IEICE General Conference C-332 (amplification characteristics of double-clad fiber amplifier)

  However, in order to reduce the size of the LD module, when a double-clad fiber in which the second clad having the above-described configuration is formed of a polymer is used as the pigtail of the LD module, the second light leaks from the excitation light source. There is a possibility that the polymer forming the clad may be burned, and there is a possibility that the optical element of the LD module including the double clad fiber may be thermally damaged.

  This invention is made | formed in view of this point, The place made into the objective is to provide the double clad fiber by which the thermal damage by the leakage light of an excitation light source is suppressed.

  The double clad fiber of the present invention includes a solid core, a first solid clad provided around the solid core, and a second solid clad provided around the first solid clad. The double-clad fiber provided is characterized in that both the solid core and the first and second solid clads are formed of quartz.

  According to the above configuration, since the solid core of the double clad fiber and the first and second solid clads are both made of quartz, the double clad fiber of the present invention is used as a harsh one such as a pigtail of an LD module. Unlike the conventional case where the second solid clad is formed of a polymer, the second solid clad has high heat resistance even when used in a rough environment. There is no risk of burning the actual cladding. Thereby, the double clad fiber of this invention suppresses the thermal damage by the leakage light of an excitation light source.

  In the double clad fiber of the present invention, the solid core may have photosensitivity.

  According to the above configuration, since the solid core of the double clad fiber has photosensitivity, for example, by irradiating the solid core from the side of the fiber of the double clad fiber via the phase mask. The diffraction grating can be written on the solid core.

  In the double clad fiber of the present invention, a diffraction grating may be written in the solid core.

  According to the above configuration, since the diffraction grating, that is, FBG is written in the solid core of the double-clad fiber, light in a specific wavelength range can be reflected or transmitted. Can function as an optical filter.

  In the double clad fiber of the present invention, the outside of the second solid clad may be coated with a metal thin film.

  According to the above configuration, since the second solid cladding is formed of quartz and heat resistance is improved, the outside of the second solid cladding is coated with the metal thin film, and the metal thin film is applied to the body of the LD module. The double-clad fiber can be soldered through.

  An optical device according to the present invention includes the double clad fiber according to the present invention.

  According to the above configuration, the double-clad fiber of the present invention is used as a pigtail of an LD module because thermal damage due to leakage light of the excitation light source is suppressed, and an optical amplifier and a fiber laser incorporating the LD module In addition, an optical device such as an ASE (Amplified Spontaneous Emission) light source also suppresses thermal damage due to leakage light of the excitation light source. Furthermore, since FBG is written in the solid core of the double clad fiber, it is possible to provide an optical filter in the pigtail of the LD module.

  The optical amplifier of the present invention includes a double-clad fiber of the present invention, a pumping light source connected to one end of the double-clad fiber, a core connected to the other end of the double-clad fiber and doped with a rare earth element, A rare earth element-doped fiber having a first clad provided around the core and a second clad provided around the first clad, an incident terminal, an outgoing terminal, and an incoming / outgoing terminal, and the rare earth element doped fiber And an optical circulator having the input / output terminal connected to the opposite side of the double clad fiber.

  According to the above configuration, both the solid core and the first and second solid clads are formed of quartz, and the FBG is written on the solid core. An excitation light source is connected to form an LD module, a rare earth element-doped fiber is connected to the pigtail (the other end of the double clad fiber) of the LD module, and an optical circulator is connected to the end of the rare earth element-doped fiber.

  Therefore, the signal light incident from the incident terminal of the optical circulator propagates in the core of the rare earth element doped fiber via the input / output terminal, is reflected by the FBG in the pigtail, and again, the core of the rare earth element doped fiber. It propagates through the inside and exits from its output terminal via the input / output terminal of the optical circulator. At the same time, the excitation light from the excitation light source enters the first cladding of the rare earth element doped fiber via the double clad fiber, and the excitation light passes between the first cladding and the second cladding of the rare earth element doped fiber. Propagating the region surrounded by the second clad while repeating reflection at the interface, and when the excitation light passes through the core, the rare earth element doped in the core is in an inverted distribution state excited by the outermost electrons, Stimulated release occurs. As a result, the signal light incident from the incident terminal of the optical circulator is amplified in the rare earth element-doped fiber, and the amplified signal light is emitted from the output terminal of the optical circulator.

  Furthermore, the oscillation wavelength from the excitation light source can be locked by partially reflecting the excitation light from the excitation light source by the FBG in the pigtail and returning the excitation light to the excitation light source, thereby stabilizing the oscillation wavelength of the LD module. Can increase the sex.

  As described above, the optical amplifier of the present invention uses the double-clad fiber in which the solid core and the first and second solid clads are both made of quartz as the pigtail of the LD module. Thermal damage due to light is suppressed, and the FBG is written on the solid core of the pigtail (double clad fiber) of the LD module, so that it not only serves as a resonator mirror for amplifying the light, but also as a pump light source It also functions as a mirror for locking the oscillation wavelength of the signal light, and can stabilize the amplification factor of the signal light.

  As described above, the double-clad fiber of the present invention has high heat resistance because both the solid core and the first and second claddings are made of quartz, and is thermally affected by the leakage light of the excitation light source. Damage can be suppressed.

  Hereinafter, embodiments of a double clad fiber, an optical apparatus including the same, and an optical amplifier according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a double clad fiber 10 of the present invention. 1A is a schematic diagram of a fiber longitudinal section of the double clad fiber 10, and FIG. 2B is a schematic diagram of a fiber cross section of the double clad fiber 10.

  This double clad fiber 10 includes a core 1 that is the center of the fiber, a first clad 2 provided around the core 1, a second clad 3 provided around the first clad 2, and a second clad 3. And a protective coating 4 provided to cover.

The core 1 is made of quartz and doped with about 10 mol% germanium (Ge), about 15000 ppm tin (Sn), and the like. In general, since tin (Sn) is an element having a sensitizing action, the core 1 has photosensitivity. Therefore, FBG can be written in the core 1 by irradiating ultraviolet rays from the side of the fiber. The refractive index n ₁ of the core 1 is higher than the refractive index n ₂ of the first cladding 2, for example, the relative refractive index difference _{(Δ = (n 1 -n 2} ) / n 2) is from about 1 %. In addition, the magnitude | size of the diameter of the core 1 is 5-10 micrometers.

  The core 1 can also be doped with rare earth elements such as ytterbium (Yb) and erbium (Er). Thereby, the optical amplification characteristic similar to the rare earth element doped fiber 20 mentioned later can be given to the double clad fiber 10.

The first cladding 2 is made of quartz, and its refractive index n ₂ is almost the refractive index of quartz alone. The outer diameter of the first cladding 2 is 100 to 200 μm. In this embodiment, the shape of the fiber cross section of the first cladding 2 is circular. However, like the shape of the fiber cross section of the first cladding 2a of the rare earth element-doped fiber 20 described later, it is noncircular. It may be.

The second cladding 3 is made of quartz and is doped with fluorine (F) or the like. The refractive index n ₃ of the second cladding ₃ is lower than the refractive index n ₂ of the second cladding. For example, the relative refractive index difference (Δ = (n ₃ −n ₂ ) / n ₂ ) is −1. 3 to -1.4%. The outer diameter of the second cladding 3 is 125 to 250 μm.

  The protective coating 4 is, for example, an ultraviolet curable acrylic resin.

  The double clad fiber 10 of the present invention can be manufactured by heating and drawing a preform and drawing it into a fiber shape. Specifically, an example will be described below.

  FIG. 2 is a schematic cross-sectional view of the preform 15.

  First, a quartz core-forming rod 11 doped with a predetermined amount of germanium (Ge) and boron (B), a first cladding-forming tube 12 made of quartz that is not doped with anything, and a predetermined amount of fluorine. A quartz-made second cladding forming tube 13 doped with (F) is prepared.

  Next, a preform 15 is formed by disposing a first cladding forming tube 12 around the core forming rod 11 and a second cladding forming tube 13 around the first cladding forming tube 12, respectively. To do. The order of disposing the core forming rod 11, the first cladding forming tube 12, and the second cladding forming tube 13 may be changed as appropriate.

  The preform 15 may be formed by a known method such as an MCVD (Modified Chemical Vapor Deposition) method, an OVD (Outside Vapor-phase Deposition) method, or a VAD (Vapor-phase Axial Deposition) method.

  Next, the preform 15 is heated and stretched in a drawing furnace to form a fiber. As a result, the core forming rod 11 of the preform 15 forms the core 1 of the double clad fiber 10, the first cladding forming tube 12 of the preform 15 forms the first clad 2 of the double clad fiber 10, and the preform 15. The second clad forming tube 13 forms the second clad 3 of the double clad fiber 10.

  Further, an ultraviolet curable acrylic resin is applied to the surface of the fiber formed by heating and stretching, and then the protective coating 4 is formed by irradiating ultraviolet rays.

  As described above, the double clad fiber 10 of the present invention is manufactured.

  The double clad fiber 10 can write FBG in the core 1 by irradiating ultraviolet rays through a phase mask.

  Specifically, the fiber surface of the double clad fiber 10 and the phase mask are arranged so as to be in close contact with each other, and, for example, ultraviolet laser light having a wavelength of 248 nm is irradiated to the core 1 through the phase mask. Here, the phase mask is a kind of diffraction grating, and an interference fringe pattern by + 1st order and −1st order diffracted light is formed immediately below the phase mask, and this interference fringe pattern is transferred to the core 1 to become an FBG.

  Next, an optical device including a double clad fiber in which the FBG of the present invention is written will be described.

  First, an optical amplifier will be described as an example of an optical device.

  FIG. 3 is a schematic configuration diagram of the optical amplifying apparatus 40a of the present invention. In the drawing, the protective coating 4 of the double clad fiber 10a is omitted.

  The optical amplifying device 40a includes an excitation light source 32, a double clad fiber 10a in which FBGs 5a and 5b are written, a rare earth element doped fiber 20, an optical circulator 31, and single mode fibers 30a, 30b, and 30c. Yes.

  As the excitation light source 32, various laser light sources can be used, and in this embodiment, an LD (laser diode) is used.

  At one end of the double clad fiber 10a, the protective coating 4 on its surface is removed, and the exposed surface of the second clad 3 is coated with a metal thin film of nickel (Ni) and gold (Au). Furthermore, the double clad fiber 10a is connected to the main body of the excitation light source 32 through a metal thin film at one end thereof, thereby constituting an LD module 35a. Thereby, the inside of the main body of the excitation light source 32 is sealed, and water vapor that adversely affects the LD is prevented from flowing into the main body.

  Further, since both the core 1 of the double clad fiber 10a, the first clad 2 and the second clad 3 are made of quartz, heat resistance is high, and there is no fear of burning due to leakage light from the excitation light source 32. Therefore, the double clad fiber 10a is prevented from being thermally damaged by the leakage light of the excitation light source.

  The rare earth element doped fiber 20 includes a core 1a, a first cladding 2a provided around the core 1a, and a second cladding 3a provided around the first cladding 2a.

  The core 1a is made of quartz and doped with a rare earth element such as ytterbium (Yb) as a light amplification component.

  The first cladding 2a is made of quartz and has a refractive index lower than that of the core 1a. Further, the shape of the fiber cross section of the first cladding 2a may be circular, but is preferably noncircular. For example, by using a non-circular shape such as a hexagonal shape, a triangular shape, or a D-shape, the skew component of the excitation light can be reduced and the excitation light rate can be increased.

  The second clad 3a only needs to have a lower refractive index than the first clad 2a, and may be composed of a polymer having a low refractive index or may be composed of quartz having a plurality of pores.

  The rare earth element doped fiber 20 is connected to the pigtail (double clad fiber 10a) of the LD module 35a.

  In addition, it is preferable that the diameter of the core 1a of the rare earth element doped fiber 20 and the diameter of the core 1 of the double clad fiber 10a are set to be substantially the same from the viewpoint of reducing connection loss.

  The outer diameter of the first cladding 2a of the rare earth element doped fiber 20 is preferably the same as or larger than the outer diameter of the first cladding 2 of the double cladding fiber 10a. Thereby, the excitation light propagating through the first clad 2 of the double clad fiber 10a can be efficiently incident on the first clad 2a of the rare earth element doped fiber 20.

  The FBG 5a written on the excitation light source 32 side of the pigtail of the LD module 35a partially reflects the excitation light from the excitation light source 32 and returns the excitation light to the excitation light source 32. With this FBG 5a, the oscillation wavelength from the excitation light source 32 can be locked, and the stability of the oscillation wavelength of the LD module 35a can be enhanced.

  4A shows the light output characteristics of the 977 nm excitation light source when the LD temperature is 25 ° C., and FIG. 4B shows the light output when the LD temperature is 50 ° C. Output characteristics are shown.

  As can be seen by comparing FIGS. 4A and 4B, when the temperature of the LD rises from 25 ° C. to 50 ° C., the oscillation wavelength shifts toward the longer wavelength.

  FIG. 4C shows the absorption characteristics of the excitation light of the rare earth element-doped fiber 20 doped with ytterbium (Yb).

  The absorption characteristic of the rare earth element-doped fiber 20 has a maximum at a wavelength of 977 nm and is highly wavelength-dependent. Therefore, when the oscillation wavelength of the LD, that is, the wavelength of the excitation light, changes, the amount of absorption of the excitation light changes greatly, and the amplification factor of the signal light propagating in the core changes.

  Thus, by providing the FBG 5a in the core 1 of the double clad fiber 10a, the oscillation wavelength of the LD can be locked at, for example, 977 nm, and the optical amplification factor of the optical amplifier 40a can be stabilized.

  On the other hand, the FBG 5b written on the connection side of the pigtail of the LD module 35a with the rare earth doped fiber 20 totally reflects the signal light incident on the core 1 of the double clad fiber 10a from the core 1a of the rare earth element doped fiber 20. Thus, the rare earth element-doped fiber 20 is returned.

  Each of the single mode fibers 30a, 30b, and 30c has a core 1 ′ and a clad 2 ′ that is provided around the core 1 ′ and has a lower refractive index than the core 1 ′, and an incident signal in the core 1 ′. Light is propagated through the core 1 'while being repeatedly reflected at the interface between the core 1' and the clad 2 '.

  The single mode fiber 30 c is connected to the incident terminal of the optical circulator 31, the single mode fiber 30 a is connected to the input / output terminal of the optical circulator 31, and the single mode fiber 30 b is connected to the output terminal of the optical circulator 31.

  The single mode fiber 30a is connected to the rare earth element doped fiber 20 on the side opposite to the optical circulator 31 side. It is preferable that the diameter of the core 1 ′ of the single mode fiber 30 a and the diameter of the core 1 a of the rare earth element doped fiber 20 are set to be substantially the same from the viewpoint of reducing connection loss.

  In this optical amplifying device 40a, the signal light incident on the incident terminal of the optical circulator 31 through the single mode fiber 30c propagates in the core 1a of the rare earth element doped fiber 20 through the incident / exit terminal, and the pigtail The light is reflected by the FBG 5b in the (double clad fiber 10a), propagates again in the core 1a of the rare earth element doped fiber 20, and is emitted from the output terminal via the input / output terminal of the optical circulator 31.

  At the same time, the excitation light from the excitation light source 32 enters the first cladding 2a of the rare earth element doped fiber 20 via the double clad fiber 10a, and the excitation light is the first cladding 2a of the rare earth element doped fiber 20. Propagating in the region surrounded by the second cladding 3a while repeating reflection at the interface between the first cladding 2a and the second cladding 3a, and when the excitation light passes through the core 1a, the rare earth element doped in the core 1a is used as the outermost shell Stimulated emission occurs in an inverted population state in which electrons are excited.

  As a result, the signal light incident from the incident terminal of the optical circulator 31 is amplified in the rare earth element doped fiber 20, and the amplified signal light is emitted from the output terminal of the optical circulator 31.

  Further, the FBG 5a in the pigtail (double clad fiber 10a) partially reflects the excitation light from the excitation light source 32 and returns the excitation light to the excitation light source 32, thereby locking the oscillation wavelength from the excitation light source 32. . Therefore, the stability of the oscillation wavelength of the LD module 35a can be improved, and stable excitation light can be supplied to the rare earth element-doped fiber 20. As a result, the optical amplifier 40a of the present invention suppresses a change in the amplification factor of the signal light.

  The optical device of the present invention can be applied not only to an optical amplifier but also to a fiber laser and an ASE light source as follows.

  FIG. 5 is a schematic configuration diagram of the fiber laser 40b of the present invention. In the drawing, the protective coating 4 of the double clad fiber 10a is omitted.

  The fiber laser 40b includes an LD module 35a, a rare earth element-doped fiber 20, and a single mode fiber 30.

  The LD module 35a and the rare earth element-doped fiber 20 have the same configuration as the optical amplifier 40a, and a description thereof will be omitted.

  The single mode fiber 30 has an FBG 5 c and an isolator 33 on the connection side with the rare earth element doped fiber 20.

  In this fiber laser 40b, the pumping light from the pumping light source 32 enters the first cladding 2a of the rare earth element doped fiber 20 via the double clad fiber 10a, and activates the rare earth element to generate spontaneous emission light. . The spontaneous emission light generated in the core 1a of the rare earth element doped fiber 20 is incident on the core 1 'of the single mode fiber 30 and reaches the FBG 5c. Of the spontaneous emission light, the light having a wavelength included in the reflection wavelength band of the FBG 5 c is reflected and is incident on the core 1 a in the rare earth element-doped fiber 20 again.

  Here, a resonator is formed between the FBG 5c and the FBG 5b in the double clad fiber 10a of the LD module 35a, and stimulated emission is also caused by the reflected spontaneous emission light. That is, in the fiber laser 40b, resonance occurs between the FBG 5b and the FBG 5c, and laser oscillation is generated by repeated stimulated emission, and high-power laser light is extracted.

  FIG. 6 is a schematic configuration diagram of the ASE light source 40c of the present invention. In the drawing, the protective coating 4 of the double clad fiber 10b is omitted.

  The ASE light source 40c includes an LD module 35b, a rare earth element doped fiber 20, and a single mode fiber 30 '.

  An FBG 5a is formed in the double clad fiber 10b of the LD module 35b, and the oscillation wavelength of the excitation light from the excitation light source 32 is locked.

  The single mode fiber 30 ′ has an optical isolator 33.

  The rare earth element-doped fiber 20 has the same configuration as that of the optical amplifier 40a and the fiber laser 40b, and a description thereof will be omitted.

  In the ASE light source 40c, the excitation light from the excitation light source 32 enters the first cladding 2a of the rare earth element doped fiber 20 via the double clad fiber 10b, and activates the rare earth element to generate spontaneous emission light. . Then, the spontaneous emission light generated in the rare earth element doped fiber 20 is emitted from the fiber end of the single mode fiber 30 ′ via the optical isolator 33.

  As described above, the double-clad fiber of the present invention is used as a pigtail of an LD module because thermal damage due to leakage light of an excitation light source is suppressed, and an optical amplifier, a fiber laser, An optical device such as an ASE light source also suppresses thermal damage due to leakage light from the excitation light source. Further, since the FBG is written in the core of the double clad fiber, an optical filter can be provided in the pigtail of the LD module, and the oscillation wavelength can be locked or it can function as a mirror of the resonator.

  The present invention is not limited to this embodiment, and may have other configurations.

  As described above, the double clad fiber of the present invention is useful as a pigtail of an LD module of an optical device such as an optical amplifier, a fiber laser, or an ASE light source.

It is a mimetic diagram showing double clad fiber 10 concerning an embodiment of the present invention. It is a schematic diagram of the cross section of the preform 15 at the time of manufacturing the double clad fiber 10 which concerns on embodiment of this invention. It is a schematic diagram which shows the optical amplifier 40a which concerns on embodiment of this invention. (A) shows the light output characteristics of the 977 nm excitation light source when the LD temperature is 25 ° C., and (b) the light output characteristics of the 977 nm excitation light source when the LD temperature is 50 ° C. (C) shows the absorption characteristic of the excitation light of the rare earth element doped fiber 20 doped with ytterbium (Yb). It is a schematic diagram which shows the fiber laser 40b which concerns on embodiment of this invention. It is a schematic diagram which shows the ASE light source 40c which concerns on embodiment of this invention.

Explanation of symbols

1, 1 ', 1a Core 2, 2a First clad 2' Clad 3, 3a Second clad 4 Protective coating 5a, 5b, 5c FBG
DESCRIPTION OF SYMBOLS 10 Double clad fiber 11 Core rod 12 1st clad formation part 13 2nd clad formation part 15 Preform 20 Rare earth element doped fiber 30,30a, 30b, 30c, 30 'Single mode fiber 31 Optical circulator 32 Excitation light source 33 Optical isolator 35a, 35b LD module 40a Optical amplifier 40b Fiber laser 40c ASE light source

Claims (6)

A double-clad fiber comprising a solid core, a first solid clad provided around the solid core, and a second solid clad provided around the first solid clad. ,
A double-clad fiber, wherein the solid core and the first and second solid clads are both made of quartz. The double clad fiber according to claim 1, wherein
The double-clad fiber, wherein the solid core has photosensitivity. The double clad fiber according to claim 2,
A double-clad fiber, wherein a diffraction grating is written in the solid core. The double clad fiber according to claim 1, wherein
A double clad fiber, wherein the second solid clad is coated on the outside with a metal thin film.   An optical apparatus comprising the double-clad fiber according to claim 3. A double clad fiber according to claim 3;
An excitation light source connected to one end of the double clad fiber;
A core doped with a rare earth element, connected to the other end of the double clad fiber, a first clad provided to cover the core, and a second clad provided to cover the first clad A rare earth element doped fiber;
An optical circulator having an input terminal, an output terminal, and an input / output terminal, the input / output terminal being connected to the opposite side of the double-clad fiber of the rare earth element-doped fiber;
An optical amplifier comprising:

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