JP4954737B2 - Optical amplification system, optical fiber laser and optical fiber amplifier using the same - Google Patents

Description

  The present invention relates to an optical amplification system using a rare earth-doped optical fiber such as an optical fiber laser or an optical fiber amplifier, and a residual light removal for removing residual light other than the amplified light emitted from the emission end of the rare earth doped optical fiber. The present invention relates to a structure, and an optical fiber laser and an optical fiber amplifier using the removal structure.

Laser light has come to be used in various fields such as processing machines, medical equipment, and measuring instruments. In the field of processing machines, lasers have excellent light condensing performance, and a small beam spot with high power density can be obtained, so that precision processing is possible, non-contact processing, and hardness that can absorb laser light. Applications are expanding rapidly because it can be processed into materials.
In particular, an optical fiber laser using a rare earth-doped optical fiber as a laser medium has features such as higher efficiency than a conventional solid-state laser, a compact device, and a combination of a laser oscillation medium and a propagation medium. ing. In recent years, particularly high output of fiber lasers has been rapidly progressing, and high output fiber lasers exceeding 1 kW have been reported.

  In order to realize such a very high output with an optical fiber laser, a lot of pumping light must be injected into the rare earth-doped optical fiber. Usually, in a fiber laser using a rare earth-doped optical fiber, the conversion efficiency from pumping light to laser output is at most about 50%, so pumping light more than double the laser output must be input. That is, in the above-described kW class fiber laser, the input pump light is also in the kW class. However, in the case of performing core pumping in which pumping light is injected into the core like a normal rare earth-doped optical fiber laser, it is difficult to input much pumping light because the core diameter is as small as several μm to 10 μm. . Therefore, when constructing an optical fiber laser, a clad pumping method using an optical fiber having a double clad structure is often used. An optical fiber having a double clad structure has a two-layer structure of a first clad and a second clad. The first clad not only works as a clad for the core, but also the first clad itself uses the second clad as a clad. It also works as a core. Since the first cladding usually has a diameter of 100 μm or more, a large amount of excitation light can be injected relatively easily.

The means for injecting the pumping light into the first cladding is roughly divided into a method in which the pumping light is mainly incident from the end face of the rare earth-doped double-clad fiber and a method in which the pumping light is incident from the side surface of the rare-earth-doped double-clad fiber. .
As a method for making the excitation light incident from the end face of the rare earth-doped double clad fiber, there is a method using an optical coupler as disclosed in Patent Document 1. An example of the configuration of an optical fiber laser using the optical coupler disclosed herein is shown in FIG.
In this optical fiber laser 100, the optical coupler 103 has six pump ports 102 made of a multimode optical fiber and one signal port 112 made of a single mode fiber, and these are integrated by melting and stretching. It has one output port 104 formed by the following. An excitation light source 101 is connected to each excitation port 102, and excitation light emitted from the excitation light source 101 is emitted from the emission port 104. The signal light source 111 is connected to the signal port 112, and the signal light emitted from the signal light source 111 is emitted from the emission port 104.
The diameter of the exit port 104 of the optical coupler 103 is equal to or less than the diameter of the first cladding of the rare earth-doped double clad fiber 105, and the NA (numerical aperture) of the exit port is the rare earth doped double clad fiber 105. If it is smaller than the NA of the first cladding, it is possible to enter the pumping light emitted from the exit port 104 via the incident side fusion splicing portion 106 into the first cladding of the rare earth-doped double clad fiber 105 with low loss. It becomes.

In this way, the excitation light input to the first cladding of the rare earth-doped double clad fiber is absorbed by the rare earth ions added to the core when crossing the core while propagating through the first cladding. The rare earth ions in the excited state by absorbing the excitation light cause stimulated emission by the signal light incident from the signal light source, and amplify the signal light. The amplified signal light is incident on the large-diameter single mode optical fiber 108 via the outgoing-side fusion splicing portion 107 and is output as laser light.
US Pat. No. 5,864,644

However, not all of the excitation light input to the first cladding is absorbed by the rare earth ions added to the core, but about 10% of the input excitation light leaks from the emission end without being absorbed. .
The leaked excitation light is incident on the clad of the large-diameter single mode optical fiber. Further, ASE light that has not contributed to amplification of signal light propagating through the core also leaks from the first clad of the rare earth-doped double clad optical fiber and similarly enters the clad of the large-diameter single-mode optical fiber. Furthermore, a splice loss occurs in the core of the exit side fusion splice, and this loss is not coupled from the core of the rare earth-doped double-clad optical fiber to the core of the large-diameter single mode fiber. Incident on the fiber cladding. In particular, in a high-power fiber laser, a considerable amount of laser light leaks into the clad of a large-diameter single mode fiber even with a slight connection loss.

Here, FIG. 2 shows a general structure of the connection portion of the rare earth-doped double clad fiber 105 and the large-diameter single mode fiber 108, that is, the emission-side fusion splicing portion, and the state of light passing through the fusing portion.
In the rare earth-doped double clad fiber, the clad portion has a two-layer structure as described above. In a general rare earth-doped double clad fiber, the first clad 1052 is made of pure quartz, and the second clad 1053 is formed by applying a polymer resin on the outer side thereof. When a rare earth-doped double clad fiber and another optical fiber are fusion-spliced, first, the protective coating 1054 and the second clad 1053 are removed to expose the first clad 1052, and the optical fiber cleaver is used for the core. The end faces are aligned so that the cleave surface is perpendicular to the axial direction. Similarly, the large diameter single mode fiber 108 is cleaved with the protective coating 1083 removed and the clad 1082 exposed. Thereafter, the fiber is heated by discharge heating or irradiation with a carbon dioxide laser with a fusion splicer, and when the end surfaces of both fibers are melted, the fiber end surfaces are connected to each other and fusion-connected. Since the fusion splicing portion 1071 is easily broken when the quartz glass is exposed, the fusion splicing portion 1071 is fixed to the fusing portion protective tube 1073 using an adhesive 1072.

When such a connection is made, residual light such as residual pump light and ASE light propagating to the first cladding of the rare earth-doped optical fiber 105 propagates along a path indicated by reference numeral 1111 in the figure. To do. The residual light that has been confined and propagated in the first clad 1052 by the second clad of the rare earth-doped optical fiber does not leak out to the outside without being leaked to the outside even if the second clad 1053 is removed. Propagates as it is. Further, even if it passes through the fusion splicing portion 1071 and enters the large-diameter single mode fiber, the portion where the protective coating 1083 is removed propagates without leaking outside the cladding 1082, and the protective coating 1083 When propagating up to a certain portion, the refractive index of the protective coating 1083 is higher than the refractive index of the cladding 1082, and therefore leaks from the first cladding 1082 to the protective coating 1083.
On the other hand, the laser light (amplified light) that has been confined and propagated in the core 1051 of the rare earth-doped optical fiber 105 propagates along the path indicated by reference numeral 1112 in the figure and passes through the fusion splicing portion 1071. Most are coupled to the core 1081 of the large-diameter single-mode fiber 108 and propagate in the core 1081 as 1112a, but an amount of laser light corresponding to the connection loss propagates in the clad 1082 as 1112b. The laser beam 1112b leaking to the clad propagates without leaking outside in the clad 1082 in the portion where the protective coating 1083 is removed, and propagates to a portion where the protective coating 1083 is present. Since the refractive index of the protective coating 1083 is high, the protective coating 1083 leaks from the first cladding 1082 to the protective coating 1083. When the light 1111, 1112 b leaked to the protective coating reaches the adhesive 1072, a part of it leaks to the adhesive 1072 (1111 a, 1112 c), and is absorbed and changed to heat. The remainder leaking out to the adhesive 1072 propagates as it is in the protective coating 1083 as residual light (1111b, 1111d).
Such a phenomenon occurs regardless of the diameter of the fiber to be fused with the rare earth-doped double clad fiber. The same phenomenon occurs when a multimode fiber capable of propagating several higher order modes is used instead of a single mode fiber.

In the case of recent high-power fiber lasers, the pumping light power input to the rare earth-doped double clad fiber may range from several tens of watts to kilowatts, and the residual pumping light power may range from several watts to several hundreds of watts. On the other hand, since the laser output may range from several tens of watts to kilowatts, laser light of several tens of watts may leak even with a slight connection loss. When such high-intensity light is incident on the protective coating 1083 of the large-diameter single mode fiber 108, the protective coating may burn out. In particular, the boundary between the protective coating 1083 and the protective coating removing portion is likely to catch fire in many cases where dust adheres or the protective coating residue remains when the protective coating is removed. Accordingly, measures may be taken so that the protective coating does not burn in the fusion splicing portion 107, but there is no sufficient countermeasure method. Therefore, it is very difficult to completely eliminate the residual light leaking to the emission side of the fusion splicing portion 107 and propagating through the protective coating, and residual light reaching several W or more may propagate through the protective coating.
In such high-intensity light propagating parts, the protective coating burns out, or there is a problem that the fire breaks out due to adhesion of dust to the protective coating or contact with other components in the optical fiber laser. .

  The present invention has been made in view of the above circumstances, and a residual light removal structure for removing residual light other than amplified light emitted from the exit end of a rare earth-doped optical fiber in an optical amplification system such as an optical fiber laser or an optical fiber amplifier The purpose is to provide.

The present invention provides an optical amplification system using a rare earth-doped optical fiber, the outgoing end side of the rare earth-doped optical fiber in the optical amplification system, remove the remaining light other than amplifying light emitted from said exit end structure is provided for, the remaining light of the rare earth doped optical fiber, a light absorption wavelength band or fluorescence wavelength band, the structure for removing the residual light, the rare earth doped optical around the protective coating around the protective coating of optical fibers connected to the exit end of the fiber, from said exit end side of the rare earth-doped optical fiber, a first residual light off structure and the second residual light off structure is arranged sequentially, wherein the first residual light off structure, high heat-resistant resin is provided as the residual light transmission material around the protective coating, which absorbs light of the absorption wavelength band or the fluorescence wavelength band on the outer periphery of the high heat-resistant resin Tomeko absorbing material is provided, and the heat radiating portion in thermal contact is provided in the residual light absorbing material, wherein the second residual light off structure, wherein the absorption wavelength band or the fluorescence around the protective coating A residual light transmitting material having a refractive index in the wavelength band higher than that of the protective coating is provided, and a residual light absorbing material that absorbs light in the absorption wavelength band or the fluorescence wavelength band is provided on an outer periphery of the residual light transmitting material. A heat-dissipating part in thermal contact with the residual light absorbing material is provided , and the refractive index of the high heat resistant resin of the first residual light removing structure is protected in the absorption wavelength band or the fluorescence wavelength band. Provided is an optical amplification system characterized by being lower than the refractive index of the coating .

The present invention also provides an optical fiber laser and an optical fiber amplifier using the above-described optical amplification system according to the present invention.

  According to the residual light removal structure of the present invention, residual light other than the amplified light emitted from the exit end of the rare earth-doped optical fiber can be reliably removed in an optical amplification system such as an optical fiber laser or an optical fiber amplifier, Problems such as burning of the protective coating due to residual light can be prevented, and the safety of the optical amplification system can be improved.

  Normally, in a state where the fusion splicing portion is housed in the optical fiber laser, the large-diameter single mode fiber is floating in the air, and only air exists around the protective coating. In general, the refractive index of the protective coating is approximately 1.5, whereas the refractive index in air is 1, so that residual light propagates while repeating total reflection at the interface between the protective coating and air. become. Large-diameter single-mode fiber will eventually come in contact with the housing, dust, and other parts in the fiber laser, but for the first time, residual light will leak out of the protective coating, and the contact will absorb the residual light and generate heat. As a result, it cannot burn the heat and burns out.

In order to eliminate this, it is most effective to eliminate the residual light itself. However, as described above, it is difficult to eliminate the residual light itself that leaks to the outside of the fusion splicing portion.
The present inventor covers the periphery of the protective coating with a residual light transmitting material having a higher refractive index than that of the protective coating and a high transmittance at the residual light wavelength at a location of several centimeters from the fusion splicing portion to the emission side, and Developed a residual light removal structure constructed by providing a residual light absorbing material that absorbs residual light and has good heat resistance on the outside, and a heat dissipation part that dissipates the heat generated by the residual light absorbing material. This proved that the residual light can be sufficiently removed, thereby completing the present invention.

  The light that has propagated through the protective coating leaks to the higher refractive index side if there is a substance having a higher refractive index than the protective coating around the protective coating. Furthermore, since this substance has a high transmittance at the residual light wavelength, the leaked residual light propagates with almost no absorption. When the transmittance is low, residual light is absorbed and heat is generated. Therefore, if the heat resistance of the residual light transmitting material is low, it may be altered or burned out. The residual light transmitting material that is in contact with the protective coating hardly absorbs residual light and therefore does not generate heat that causes burnout.

  Thereafter, the residual light is converted into heat when it reaches the residual light absorbing material. At this time, since the surface area of the residual light absorbing material is larger than the surface area of the original protective coating, it is easy to dissipate the generated heat. The generated heat can be dissipated into the air by the heat dissipating part provided so as to be in thermal contact with the residual light absorbing material.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, the following Examples are only illustrations of this invention and this invention is not limited only to description of the following Examples.

Example 1 is shown in FIG. The residual light removal structure 300 of this embodiment includes an optical resin 301, a metal housing 302, and a heat sink 303.
The metal casing 302 is made of aluminum, has a size of 15 mm × 15 mm × 40 mm, and has a cylindrical structure with a hole of 5 mm × 5 mm in the longitudinal direction, and is fixed on the heat sink 303. The surface is black anodized so as to absorb residual light.
A large-diameter single mode fiber is passed through this hole, and the inside of the hole is filled with optical resin 301.

  The optical resin 301 has a refractive index higher than that of the protective coating in order to bring sufficient residual light from the protective coating, has high transparency so as to absorb residual light and not generate heat, and the absorbing material has residual light. It is desirable to use a material with good heat resistance so that it can withstand the heat generated by absorbing water. In this example, a heat-resistant UV curing type epoxy resin that is easy to adjust the refractive index was used. This resin has a refractive index after curing of 1.55.

  When the length of the metal casing is short, the residual light may not be sufficiently removed, so that the length is preferably 10 mm or more.

Further, when the residual light cannot be completely removed by one residual light removal structure, the residual light may be removed by using a plurality of residual light removal structures.
Further, although the metal casing and the heat sink are separate components, the metal casing itself may have a heat dissipation structure without using the heat sink in order to improve heat dissipation.

In this embodiment, as the rare earth-doped double-clad fiber, because of the use of Yb-doped double clad fiber doped with Yb in core, the coating resin having a large diameter single mode fiber, wavelength light of 900nm~110 0 nm Propagating. The transmittance of the used optical resin in this wavelength region is about 90 % .

  The portion where the residual light removing structure is installed may be directly formed around the cladding by removing the protective coating.

  After the residual light removal structure 300 is configured in this way, an optical fiber laser having the configuration shown in FIG. 1 is manufactured, and this residual light removal structure is located 50 mm from the fusion splicing 107 of the large-diameter single mode fiber 108. 300 was installed, the laser output was adjusted to 30 W, and the surface temperature of the metal casing 302 was measured. The residual pumping light at this time is 10 W, the leakage light from the core calculated from the connection loss is 3 W, and a total of 13 W of light is incident on the clad of the large diameter single mode fiber.

  At the same time as the laser beam was emitted, the temperature of the metal casing rose, but the temperature stabilized in about 30 minutes from the start of emission, and the temperature was about 40 ° C.

Next, the residual light removal structure was removed, and a black body paint was applied to the surface of the removed large-diameter single-mode fiber protective coating. The same measurement was performed. The fire broke out from the part where it was applied.
That is, it was confirmed that by providing the residual light removing structure, it was possible to prevent the large-diameter single mode fiber from being burned out.

In the optical fiber laser of Example 1, when the laser output was 100 W, the metal casing temperature was 80 ° C. In general, the epoxy resin has a glass transition temperature of about 50 ° C. to 150 ° C. With the configuration of Example 1, long-term reliability cannot be ensured with a higher-intensity laser output.
Therefore, as shown in FIG. 4, two residual light removal structures are installed in the longitudinal direction of the large-diameter single mode fiber.

The two residual light removing structures 300 and 400 have the same basic structure, but different residual light transmitting materials are used.
The first residual light removing structure 300 is made of a high heat resistant resin so as to withstand heat generation due to leakage of high intensity residual light. In general, silicone-based and fluorine-based resins are considered to have high heat resistance. In this example, a silicone resin was used. In order to suppress the heat generation of the first residual light removal structure 300, the one having a refractive index comparable to or slightly lower than that of the protective coating is used so that all residual light does not leak from the silicone resin. By using a resin having a slightly lower refractive index, only the component having a large NA is removed by the first residual light removing structure 300 from the residual light propagating through the protective coating, and the component having a small NA is the first. 1 passes through the residual light removal structure 300. That is, by the refractive index of the high heat resistant resin, it is possible to control how much of the total residual light is removed by the first residual light removal structure 300.

  The residual light that has not been removed by the first residual light removal structure 300 is incident on the second residual light removal structure 400. The residual light transmitting material used for the second residual light removing structure 400 may be an epoxy or acrylate resin that is a heat-resistant resin that is less heat resistant than silicone or fluororesin and that allows easy adjustment of the refractive index. By configuring the second residual light removal structure 400 in this way, residual light is removed in the same manner as in the first embodiment.

When these two residual light removal structures 300 and 400 were provided and the laser output was again set to 100 W, the metal housing temperature of the first residual light removal structure 300 was 70 ° C., and the metal housing of the second residual light removal structure 300 The body temperature was 30 ° C. Since the metal housing of the first residual light removal structure 300 is lowered by 10 ° C., it can be seen that the thermal burden is reduced and the two residual light removal structures 300 and 400 are shared. Furthermore, the heat resistance of the residual light transmitting material used is not a problem at any metal casing temperature.
That is, it has been found that by providing two residual light removal structures, it is possible to remove the residual light of a higher-power fiber laser.

It is a block diagram which illustrates the structure of an optical fiber laser. It is a schematic longitudinal cross-sectional view for demonstrating the general structure of the output side fusion splicing part in an optical fiber laser, and the mode of the light which passes a fusion | melting part. It is a perspective view of the residual light removal structure produced in Example 1 which concerns on this invention. It is a schematic block diagram of the residual light removal structure produced in Example 2 which concerns on this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 100 ... Optical fiber laser, 101 ... Excitation light source, 102 ... Excitation port, 103 ... Optical coupler, 104 ... Outlet port, 105 ... Rare earth addition double clad fiber, 106 ... Incident side fusion splicing part, 107 ... Outlet side fusion Connection part, 108 ... large-diameter single mode optical fiber, 1051 ... core, 1052 ... first clad, 1053 ... second clad, 1054 ... protective coating, 1071 ... fusion splicing part, 1072 ... adhesive, 1073 ... fusion part Protective tube, 1081 ... core, 1082 ... clad, 1083 ... protective coating, 1111, 1111a, 1111b, 1112b, 1112c, 1112d ... light (residual light), 1112, 1112a ... light (amplified light), 300 ... residual light removal structure (First residual light removing structure), 301... Optical resin (residual light transmitting substance), 302. Absorbing material), 303 ... heat sink (heat radiating portion), 400 ... second residual light off structure.

Claims (6)

An optical amplification system using a rare earth-doped optical fiber ,
The outgoing end side of the rare earth-doped optical fiber in the optical amplification system, structure to remove residual light other than amplifying light emitted from said exit end is provided,
The residual light is light in the absorption wavelength band or fluorescence wavelength band of the rare earth-doped optical fiber,
Wherein the structure to remove residual light, around the protective coating of optical fibers connected to the exit end of the rare earth doped optical fiber, the exit end side of the rare earth doped optical fiber, a first residual light off structure and The second residual light removal structures are arranged in order;
In the first residual light removing structure, a high heat-resistant resin is provided as a residual light transmitting material around the protective coating, and a residue that absorbs light in the absorption wavelength band or the fluorescence wavelength band on the outer periphery of the high heat-resistant resin. A light absorbing material is provided, and a heat dissipating part in thermal contact with the residual light absorbing material is provided ,
In the second residual light removing structure, a residual light transmitting material having a refractive index in the absorption wavelength band or the fluorescence wavelength band higher than the refractive index of the protective coating is provided around the protective coating. A residual light-absorbing material that absorbs light in the absorption wavelength band or the fluorescence wavelength band is provided on the outer periphery, and a heat dissipation portion that is in thermal contact with the residual light-absorbing material is provided ,
An optical amplification system , wherein a refractive index of the high heat-resistant resin having the first residual light removing structure is lower than a refractive index of a protective coating in the absorption wavelength band or the fluorescence wavelength band . The optical amplification system according to claim 1 , wherein a part of the protective coating whose periphery is covered with the residual light transmitting material is removed. The residual light-absorbing material, wherein provided on the inner wall surface of the exit end side of the connection point and the metal housing which is provided to surround the vicinity of the rare earth-doped optical fiber, the residual light transmission material is the metal housing 3. The optical amplification system according to claim 1, wherein the surface area of the metal casing filled inside is larger than the area of the protective coating covered with the residual light transmitting material. The optical amplification system according to any one of claims 1 to 3, wherein the wavelength of the residual light is 900 to 1100 nm. Optical fiber laser using the optical amplification system according to any of claims 1-4. Optical fiber amplifier using an optical amplification system according to any of claims 1-4.

Images