JP5202820B2 - Optical coupler, fiber laser and optical fiber amplifier - Google Patents

Description

  The present invention relates to a fiber laser and an optical fiber amplifier using a rare earth-doped optical fiber as an oscillation medium, an optical coupler capable of efficiently entering pump light from a number of pump light sources into the rare earth-doped optical fiber, and the optical coupler. The present invention relates to a fiber laser and an optical fiber amplifier provided with a coupler.

  Laser light is used in various fields such as processing machines, medical institutions, and measuring instruments. In the field of processing machines, lasers are excellent in light condensing performance, and a small beam spot with high power density can be obtained, so precision processing is possible, and non-contact processing and hard materials that can absorb laser light. Its application is expanding rapidly due to the advantages such as being capable of being processed. In particular, the use of fiber lasers has expanded due to the feature that a rare earth-doped optical fiber can be used as both a laser oscillation medium and a propagation medium. Recently, a very high power kW class fiber laser has been developed.

  In order to realize such a very high output in a fiber laser, it is important how much pumping light can be injected into the rare earth-doped optical fiber. In the case of a normal rare earth-doped optical fiber, the core is pumped by pumping light into the core. However, since the core diameter is as small as several μm to 10 μm, a lot of pumping light is thrown into the core. Technically difficult. Therefore, when constructing a fiber laser, clad pumping 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 functions 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. In addition, since the first cladding usually has a diameter of 100 μm or more, it becomes possible to inject a large amount of excitation light 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, for example, there is a method using an optical coupler as disclosed in Patent Document 1.

A structural example of a fiber laser using an optical coupler disclosed in Patent Document 1 is shown in FIG. The optical coupler 103 has seven pumping ports 102 made of a multimode optical fiber and one output port 104 formed by melting and stretching them to be integrated. 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 diameter of the exit port 104 of the optical coupler 103 is equal to or less than the diameter of the first clad of the rare earth-doped double clad fiber 105, and the numerical aperture of the exit port (hereinafter referred to as NA) is added to the rare earth. If it is smaller than the NA of the double clad fiber 105, the excitation light emitted from the emission port 104 can be incident on the first clad of the rare earth-doped double clad fiber 105 with low loss.

In this way, the excitation light input to the first cladding of the rare earth-doped double clad fiber crosses the core while propagating through the first cladding, but is absorbed by the rare earth ions added to the core at that time. The The rare earth ions that have absorbed the excitation light emit spontaneous emission light having a wavelength different from that of the excitation light, and light propagating through the core of the spontaneous method emission light is amplified while propagating to ASE (amplified spontaneous emission light). ).
On the other hand, fiber gratings are provided at both ends of the rare earth-doped double clad fiber 105. The fiber grating provided on the laser emission side is the output coupler 107 and has a characteristic of reflecting light having a part of the wavelength of spontaneous emission light emitted from the rare earth-doped double clad fiber 105. The rate is not 100%, and it has an appropriate reflectance so as to obtain a desired output. The fiber grating provided at the other end is a high reflection mirror 106, which has the same reflection wavelength as that of the output coupler 107, but has a reflectance of almost 100%, and has no effect on the excitation light wavelength. Make it transparent.
The highly reflective mirror 106 and the output coupler 107 constitute a resonator, and the ASE emitted from the rare earth-doped double clad fiber 105 is reflected by this mirror and reciprocates in the resonator, so that the rare earth doped double clad fiber The laser beam is increased every time it passes through 105, and eventually laser oscillation occurs. A part of the laser beam passes through the output coupler 107 and is output as laser beam through the isolator.
US Pat. No. 5,864,644

In the fiber laser configured as shown in FIG. 1, in order to increase the output of the fiber laser, it is important to input more pumping light into the first cladding of the rare earth-doped double cladding fiber. When the above-described optical coupler is used, a single-emitter pumping semiconductor laser (pumping LD) is connected to each incident port of the optical coupler, and pumping light is incident on a rare earth-doped double clad fiber. The pumping light power that can be incident on each port is determined by the pumping LD used, but is generally about 5 W per pumping LD. On the other hand, the number of incident ports of the multiport coupler is about several ports to about 20 ports, and the pumping light power that can be input is limited to about 100 W at the maximum. In order to increase the pumping light power that can be input, it is necessary to fabricate an optical coupler having a large number of incident ports. In addition, since a very high excitation light power is input, there is a possibility that the optical coupler may be burned out due to heat generated by the loss even if it is a slight loss.
Therefore, all the ports cannot be used unless they have very low loss optical couplers, and there has been a problem that the manufacturing yield becomes very poor.

  The present invention has been made in view of the above circumstances, and in a fiber laser and an optical fiber amplifier using a rare earth-doped optical fiber as an oscillation medium, pumping light from a large number of pumping light sources can be efficiently incident on the rare earth-doped optical fiber. The purpose is to provide an optical coupler.

In order to achieve the above object, the present invention has at least two or more excitation light incident ports and one exit port formed without melting and integrating the optical fibers of the incident ports, a first optical coupler excitation light incident to the excitation light incident port is emitted from the core of the output port, and at least two or more excitation light incident port, the optical fiber of the entrance port molten integrated with And a second optical coupler having a core diameter and numerical aperture of the outgoing port smaller than the core diameter and numerical aperture of the incident port of the first optical coupler, And an output port of the first optical coupler is connected to each of the exit ports of the plurality of second optical couplers.

The present invention is also an optical coupler for use in an optical fiber amplifier, which pumps the energy level of rare earth ions emitted from a pumping light source and added to the core of an optical fiber that is an amplification medium of the optical fiber amplifier. At least two or more excitation light incident ports into which light is incident, one signal light incident port into which signal light emitted from a signal light source and amplified by an optical fiber amplifier is incident, and an optical fiber of each of these ports to melt integrally includes an exit port formed without stretching, the excitation light incident on the excitation light incident port is emitted from the first cladding of the output port, which is incident from the signal light input port a first optical coupler where the signal light is emitted from the core of the output port, at least two or more, and the excitation light incident port, melting together the optical fiber of the entrance port And a second optical coupling having a core diameter and numerical aperture of the output port smaller than the core diameter and numerical aperture of the incident port of the first optical coupler. and a vessel, each output port of the first optical coupler of the excitation light incident port to the plurality of second optical coupler to provide an optical coupler, characterized in that formed by connecting.

  In the optical coupler of the present invention, it is preferable that the numerical aperture of the output port of the first optical coupler is 0.45 or less and the core diameter is 400 μm or less.

  The present invention also provides an optical coupler according to the present invention, a pumping light source connected so that pumping light can be incident on a pumping light incident port of the second optical coupler, and an output port of the second optical coupler. And a rare earth-doped optical fiber connected to the optical fiber.

  The present invention also provides an optical coupler according to the present invention, a pumping light source connected so that pumping light can be incident on a pumping light incident port of the second optical coupler, and an output port of the second optical coupler. And a rare earth-doped optical fiber connected to the optical fiber amplifier.

In the optical coupler of the present invention, the output port of the second optical coupler is connected in tandem to the incident port of the first optical coupler, and the core diameter of the output port of the second optical coupler is Since each NA is configured to be smaller than the incident port fiber diameter and NA of the first optical coupler, the light emitted from the outgoing port of the second optical coupler is incident on the first optical coupler without loss. The pumping light from a large number of pumping light sources can be efficiently incident on the rare earth-doped optical fiber, and the high-power pumping light can be output from the output port of the first optical coupler.
A fiber laser and an optical fiber amplifier according to the present invention include an optical coupler according to the present invention, a pumping light source connected to a pumping light incident port of the second optical coupler so that pumping light can enter, and a second light. Since it has a rare earth doped optical fiber connected to the output port of the coupler, it becomes possible to efficiently enter the pump light from a large number of pump light sources into the rare earth doped optical fiber, and low loss and high power amplified light. Can be emitted.

  In the optical coupler of the present invention, the output port of the second optical coupler is connected in tandem to the incident port of the first optical coupler, and the core diameter of the output port of the second optical coupler is Each of the NAs is characterized by having a configuration smaller than the incident port fiber diameter and NA of the first optical coupler. As a result, the light emitted from the exit port of the second optical coupler can be incident on the first optical coupler without loss, and the pumping light from many pumping light sources can be efficiently incident on the rare earth-doped optical fiber. Therefore, high-power excitation light can be emitted from the emission port of the first optical coupler.

The relationship between the core diameter (r _in ) and NA (NA _in ) of the optical fiber at the entrance port and the core diameter (r _out ) and NA (NA _out ) of the optical fiber at the exit port is
NA _in × r _in = NA _out × r _out
Therefore, the optical fibers of the entrance port and the exit port of the first and second optical couplers are selected according to this relationship. As a result, an easily manufactured optical coupler having a small number of incident ports can be used, the number of incident ports can be easily increased, and a large amount of excitation light can be input.
In addition, since only the optical coupler with a small number of incident ports is used, all the ports can be manufactured with a high yield even with a low-loss optical coupler.

  FIG. 2 is a block diagram showing Embodiment 1 of the optical coupler according to the present invention. In FIG. 2, reference numeral 200 denotes a second optical coupler, 201 denotes an excitation light source, 202 denotes an incident port of the second optical coupler, 203 denotes an exit port of the second optical coupler, and 210 denotes a first optical coupling. 212, an incident port of the first optical coupler, and 213 an outgoing port of the first optical coupler.

  The first optical coupler 210 has seven incident ports 212 and one output port 213. The output port 213 of the first optical coupler 210 is used by being connected to a rare earth-doped double clad fiber. In general, in the rare earth-doped double clad fiber, the first clad is made of quartz glass and the second clad is made of a low refractive index polymer. In this case, the first clad has an NA of about 0.45 and a diameter of about 400 μm. Therefore, in order to make the excitation light emitted from the emission port 213 enter the first cladding with low loss, it is preferable that the NA of the emission port 213 is 0.45 or less and the core diameter is 400 μm or less.

  A holey fiber may be used instead of the rare earth-doped double clad fiber. In this case, since the NA of the first cladding is about 0.7, the NA of the emission port 213 may be 0.7 or less.

  From the above equation, the relationship between the core diameter of the incident port 212 and the NA so that the NA of the exit port 213 satisfies 0.45 and the core diameter of 400 μm is as shown in FIG.

  In this example, when the first optical coupler 210 was manufactured by using a multimode fiber having a core diameter of 300 μm and NA of 0.22 for the incident port 212, the light having an output port of 213 having a core diameter of 400 μm and NA of 0.43 is used. A combiner could be obtained.

  Similarly, in the second optical coupler 200, the NA of the emission port 203 should be 0.22 or less and the core diameter should be 300 μm or less so that light can be incident on the first optical coupler without loss. Don't be. In this example, when the second optical coupler 200 was manufactured using a multi-mode fiber having a fiber core diameter of 100 μm and NA of 0.22 as the incident port 202, only the seven multi-mode fibers were fused and integrated. The second optical coupler 200 having a core diameter of the exit port 203 of 275 μm and NA of 0.22 could be obtained without stretching.

  As shown in FIG. 2, 49 incident ports 202 were obtained by connecting the exit ports 203 of the second optical coupler 200 to the seven incident ports 212 of the first optical coupler 210 described above. It becomes an optical coupler, and very high excitation light power can be obtained from the incident port 213 of the first optical coupler 210. In addition, since the second optical coupler 200 only needs to be melted and integrated without being stretched, it is very easy to manufacture, and there is an advantage that a plurality of second optical couplers 200 to be used can be manufactured with high yield. Conversely, the first optical coupler 210 may be only fused and integrated, and the second optical coupler 200 may perform stretching. In this case, since the stretching is not performed, the first optical coupler 210 can be reduced in loss. Thus, the ability to reduce loss with the first optical coupler 210 to which very high-intensity light is incident is very effective in avoiding the risk of burning.

  For example, when the pumping light source 201 capable of outputting 5 W of pumping light per unit is connected to all of the incident ports, the second optical coupler 200 has 35 W and the first optical coupler 210 has about 245 W of pumping light. Is incident. If there is a loss in the optical coupler, the lost light is converted into heat, so even if the first optical coupler 210 has a loss of about 0.5 dB, heat of about 25 W is generated. Therefore, the optical coupler must have a very low loss. The loss of the first optical coupler 210 and the second optical coupler 200 used in this example was 0.2 dB (average of seven incident ports), which was a very small loss. Further, the output port 203 of the second optical coupler 200 is connected to each incident port 212 of the first optical coupler 210, and further, 5 W per unit is connected to each incident port 202 of the second optical coupler 200. When the pumping light source 201 that can output the pumping light was actually connected, 224 W of pumping light power could be obtained at the emission port 213 of the first optical coupler 210.

  If this output port 213 is used instead of the optical coupler 103 of FIG. 1, for example, a fiber laser can be configured.

  FIG. 4 is a configuration diagram showing an optical coupler according to Embodiment 2 of the present invention and an optical fiber amplifier (or fiber laser) using the optical coupler. In FIG. 4, the same components as those in the first embodiment are denoted by the same reference numerals. In FIG. 4, reference numeral 214 denotes a signal light incident port, 215 denotes a signal light source, and 216 denotes a Yb-doped double clad fiber as an example of a rare earth-doped fiber.

  The first optical coupler 210 has a structure in which one of the seven excitation light incident ports shown in FIG. 2 is a signal light incident port 214 made of a single mode fiber. The exit port 213 of the first optical coupler 210 has a double clad structure with a mode field diameter of 20 μm, a first clad diameter of 400 μm, and a first clad NA of 0.43.

  Similarly to the first embodiment, the exit port 203 of the second optical coupler 200 is connected to the excitation light incident port 212. Therefore, a total of 42 pumping light sources 201 can be connected to the second optical coupler 200. The excitation light incident from the excitation light incident port 212 of the second optical coupler 210 is output from the first cladding of the emission port 213 of the first optical coupler 210.

  A signal light source 215 is connected to the signal light incident port 214 of the first optical coupler 210. The signal light incident on the core of the signal light incident port 214 of the first optical coupler 210 from the signal light source 215 is output from the core of the output port 213.

  Further, a Yb-doped double clad fiber 216 is connected to the emission port 213 of the first optical coupler 210. In these, the signal light emitted from the core of the emission port 213 of the first optical coupler 210 and the excitation light emitted from the first cladding are incident on the core and the first cladding of the Yb-doped double clad fiber 216, respectively. Connected so that. With such a configuration, the signal light emitted from the signal light source 215 can be amplified by the Yb-doped double clad fiber 216.

  In this embodiment, a semiconductor laser (LD) having a wavelength of 915 nm and a pumping light power of 5 W is used as the pumping light source 201, and a Yb-doped optical fiber ring laser having a wavelength of 1064 nm and a signal power of 100 mW is used as the signal light source 215. By using 216 having a clad NA of 0.46, a clad diameter of 400 μm, a core diameter of 20 μm, and a clad absorption of 0.6 dB / m at a wavelength of 915 nm, an output of 82 W is obtained from the output end of the Yb-doped double clad fiber 216. It was.

It is a block diagram which illustrates the structure of the conventional fiber laser. It is a block diagram which shows Example 1 of the optical coupler which concerns on this invention. 6 is a graph showing the relationship between the core diameter of an incident port and NA in order to satisfy the NA of the exit port of the first optical coupler in Example 1 of 0.45 and the core diameter of 400 μm. It is a block diagram which shows Example 2 of the optical coupler which concerns on this invention, and an optical fiber amplifier (or fiber laser) using the same.

Explanation of symbols

DESCRIPTION OF SYMBOLS 200 ... 2nd optical coupler, 201 ... Excitation light source, 202 ... Incident port, 203 ... Outlet port, 210 ... 1st optical coupler, 212 ... Incident port, 213 ... Outlet port, 214 ... Signal light incident port, 215 ... Signal light source, 216 ... Yb-doped double clad fiber (rare earth-doped fiber).

Claims (5)

The pumping light incident port has at least two or more pumping light incident ports and one exit port formed without melting and integrating the optical fibers of the incident ports. A first optical coupler that exits from the core of the exit port;
At least two or more excitation light incident port consists of a one exit port formed by further stretching the optical fiber of the input port and melting integral, core diameter and numerical aperture of the exit port is first A second optical coupler smaller than the core diameter and numerical aperture of the incident port of the optical coupler;
An optical coupler comprising a plurality of second optical couplers connected to an excitation light incident port of the first optical coupler. An optical coupler used in an optical fiber amplifier,
At least two pumping light incident ports that are incident from the pumping light source, which is excited by the energy level of the rare earth ions added to the core of the optical fiber that is the amplification medium of the optical fiber amplifier, and is emitted from the pumping light source ; There is one signal light incident port on which the signal light emitted from the light source and amplified by the optical fiber amplifier enters, and an output port formed by melting and integrating the optical fibers of each of these ports without stretching. and excitation light incident on the excitation light incident port is emitted from the first cladding of the exit port, a first optical coupler incident signal light is emitted from the core of the output port from the signal light input port When,
At least two or more, and the excitation light incident port consists of a single output port of an optical fiber formed by further stretched by molten integration of the input port, the core diameter and the numerical aperture of the exit port A second optical coupler smaller than the core diameter and numerical aperture of the incident port of the first optical coupler;
An optical coupler comprising: the excitation light incident port of the first optical coupler connected to each of the output ports of the second optical couplers. 3. The optical coupler according to claim 1, wherein the numerical aperture of the output port of the first optical coupler is 0.45 or less and the core diameter is 400 μm or less. The optical coupler according to any one of claims 1 to 3, an excitation light source connected so that excitation light can enter the excitation light incident port of the second optical coupler, and emission of the second optical coupler And a rare earth-doped optical fiber connected to the port. The optical coupler according to any one of claims 1 to 3, an excitation light source connected so that excitation light can enter the excitation light incident port of the second optical coupler, and emission of the second optical coupler An optical fiber amplifier comprising a rare earth-doped optical fiber connected to a port.

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