JP7189790B2 - Optical fiber lasers and multiplexing optical fiber lasers - Google Patents

Description

The present invention relates to optical fiber lasers and multiplexing optical fiber lasers.

For example, Patent Document 1 and Non-Patent Document 1 disclose a high-power optical fiber laser that outputs high-power laser light. Such high-output optical fiber lasers are applied not only to optical communications, but also to laser devices for laser processing, for example. In recent years, in order to improve the processing speed and processing performance, there has been a demand for a higher output laser device.

U.S. Pat. No. 5,892,615

A.Fujisaki et al., ”Linewidth Controlled 50-W Output Polarization Maintaining Fiber Laser”, Furukawa Review, No.35 March 2009.

As the output power of the optical fiber laser increases, there is a problem that the Raman scattered light generated by the laser light is stimulated and scattered within the optical fiber that constitutes the optical fiber laser, and the power increases rapidly. be. A phenomenon in which Raman scattered light is stimulated and scattered is called SRS (Stimulated Raman Scattering). An increase in the power of the Raman scattered light is not preferable because the energy of the laser light that should be used for the intended purpose is consumed by the increase in the power of the Raman scattered light.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical fiber laser capable of suppressing an increase in the power of Raman scattered light.

In order to solve the above-described problems and achieve the object, an optical fiber laser according to an aspect of the present invention includes an amplification optical fiber and a rear end side disposed at each of both ends of the amplification optical fiber. An optical resonator configured by reflecting means and an output-side reflecting means, a pumping light source for outputting pumping light supplied to the amplification optical fiber, and Raman scattered light generated by laser light oscillated by the optical resonator. and filter means for selectively reducing power, wherein the optical fiber closer to the output side reflection means than the rear end side reflection means has a mode field diameter of 15 μm or less at a laser oscillation wavelength, and the optical resonator The power of the laser light output from the output side reflection means of is 1500 W or more, and the filter means has a power ratio of the laser light to the Raman scattered light immediately after the filter means that is equal to or greater than an allowable value. It is characterized in that it is constructed and arranged to:

An optical fiber laser according to an aspect of the present invention is characterized in that the allowable value is 40 dB or more.

An optical fiber laser according to an aspect of the present invention is characterized in that the power of the Raman scattered light in the output-side reflecting means is 0.15 W or less.

An optical fiber laser according to an aspect of the present invention is characterized in that the filter means reduces the power of the Raman scattered light by -15 dB or less.

An optical fiber laser according to an aspect of the present invention is characterized in that the filter means is arranged between the amplification optical fiber and the output side reflection means.

An optical fiber laser according to an aspect of the present invention is characterized in that the filter means is arranged between the two amplification optical fibers.

An optical fiber laser according to an aspect of the present invention includes an output optical fiber disposed on the opposite side of the amplification optical fiber with the output side reflection means interposed therebetween, and the filter means and the output side reflection means It is characterized in that it is arranged between the output optical fiber.

An optical fiber laser according to an aspect of the present invention is characterized in that it comprises two said output optical fibers, and said filter means is arranged between said two output optical fibers.

A multiplexing type optical fiber laser according to an aspect of the present invention is an optical multiplexer that multiplexes and outputs laser light output from the plurality of optical fiber lasers and the output optical fibers of the plurality of optical fiber lasers. and.

ADVANTAGE OF THE INVENTION According to this invention, it is effective in the ability to suppress the increase in the power of Raman scattered light.

FIG. 1 is a schematic diagram showing the schematic configuration of an optical fiber laser according to Embodiment 1 and the optical power in the optical fiber. FIG. 2 is a diagram showing transmission spectra of Raman light reduction filters. FIG. 3 is a schematic diagram showing the schematic configuration of an optical fiber laser according to Embodiment 2 and the optical power in the optical fiber. FIG. 4 is a diagram showing optical power in an optical fiber in Reference Calculation Example 1. FIG. FIG. 5 is a diagram showing optical power in an optical fiber in Comparative Calculation Example 1. FIG. FIG. 6 is a diagram showing optical power in an optical fiber in Calculation Example 1 and Comparative Calculation Example 1. FIG. FIG. 7 is a schematic diagram showing a schematic configuration of an optical fiber laser and optical power in the optical fiber according to Embodiment 3. FIG. FIG. 8 is a diagram showing optical power in an optical fiber in Reference Calculation Example 2. FIG. FIG. 9 is a diagram showing optical power in an optical fiber in Comparative Calculation Example 2. FIG. FIG. 10 is a diagram showing the optical power in the optical fiber in Calculation Example 2 and Comparative Calculation Example 2; FIG. 11 is a schematic diagram showing the schematic configuration of a multiplexing optical fiber laser according to Embodiment 4 and the optical power in the optical fiber. FIG. 12 is a schematic diagram showing the schematic configuration of an optical fiber laser according to a comparative embodiment and the optical power within the optical fiber.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, this invention is not limited by embodiment described below. In addition, in the description of the drawings, the same or corresponding elements are denoted by the same reference numerals as appropriate, and overlapping descriptions are omitted as appropriate. In addition, the symbol "x" in the figure indicates a fusion splicing portion between major optical fibers.

(Embodiment 1)
FIG. 1 is a schematic diagram showing the schematic configuration of an optical fiber laser according to Embodiment 1 and the optical power in the optical fiber. The optical fiber laser 100 includes a plurality of semiconductor excitation light sources 1, a plurality of optical fibers 2, an optical multiplexer 3, an optical fiber Bragg grating (FBG) 4, an amplification optical fiber 5, and a Raman light reduction filter 6. , an FBG 7 , an optical multiplexer 8 , a plurality of optical fibers 9 , a plurality of semiconductor excitation light sources 10 , and an output optical fiber 11 . Each element is appropriately connected by an optical fiber.

A plurality of semiconductor pumping light sources 1, which are pumping light sources, output pumping light to be supplied to amplification optical fibers 5, respectively. The pumping light has a wavelength capable of optically pumping the amplification optical fiber 5, for example, a wavelength of 915 nm. A plurality of optical fibers 2 each propagate the pumping light output from each semiconductor pumping light source 1 and output it to the optical multiplexer 3 .

The optical multiplexer 3 is composed of a TFB (Tapered Fiber Bundle) in this embodiment. The optical multiplexer 3 multiplexes the pumping light input from each optical fiber 2 with the optical fiber of the signal light port and outputs it to the amplification optical fiber 5 .

The amplification optical fiber 5 is a YDF (Ytterbium Doped Fiber) in which ytterbium (Yb) ions, which are amplification substances, are added to a core portion made of silica-based glass, and an inner clad made of silica-based glass surrounds the core portion. This is a double-clad optical fiber in which a layer and an outer clad layer made of resin or the like are sequentially formed. The core portion of the amplification optical fiber 5 has an NA of, for example, 0.08, and is configured to propagate light emitted from Yb ions, for example, light having a wavelength of 1070 nm, in a single mode. The absorption coefficient of the core portion of the amplification optical fiber 5 is, for example, 200 dB/m at a wavelength of 915 nm. Also, the power conversion efficiency from the excitation light input to the core portion to the laser oscillation light is, for example, 70%.

The FBG 4 , which is the rear-end reflecting means, is connected between the optical fiber of the signal light port of the optical multiplexer 3 and the amplification optical fiber 5 . The FBG 4 has a central wavelength of 1070 nm, for example, and has a reflectance of about 100% in a wavelength band of about 2 nm width around the central wavelength and around it, and almost transmits light with a wavelength of 915 nm. Further, the FBG 7, which is the output-side reflecting means, is connected between the optical fiber of the signal light port of the optical multiplexer 8 and the amplification optical fiber 5. FIG. The FBG7 has a central wavelength, for example, 1070 nm, which is substantially the same as that of the FBG4. To Penetrate.

The FBGs 4 and 7 are arranged at both ends of the amplification optical fiber 5, respectively, and form optical fiber resonators for light with a wavelength of 1070 nm.

A plurality of semiconductor pumping light sources 10, which are pumping light sources, output pumping light to be supplied to the amplification optical fiber 5, respectively. The pumping light has a wavelength capable of optically pumping the amplification optical fiber 5, for example, a wavelength of 915 nm. A plurality of optical fibers 9 propagate the pumping light output from each semiconductor pumping light source 10 and output it to the optical multiplexer 8 .

Like the optical multiplexer 3, the optical multiplexer 8 is composed of a TFB in this embodiment. The optical multiplexer 8 multiplexes the pumping light input from each optical fiber 9 to the optical fiber of the signal light port, and outputs the result to the amplification optical fiber 5 .

In the amplification optical fiber 5, Yb ions in the core are photoexcited by the pumping light, and emit light in a band including a wavelength of 1070 nm. The emitted light with a wavelength of 1070 nm causes laser oscillation due to the optical amplification action of the amplification optical fiber 5 and the action of the optical resonator constituted by the FBGs 4 and 7 .

The output optical fiber 11 is arranged on the opposite side of the FBG 7 and connected to the optical fiber of the signal light port of the optical multiplexer 8 . The oscillated laser light (laser oscillation light) is output from the output optical fiber 11 . The output optical fiber 11 is connected, for example, to a delivery optical fiber. Laser oscillation light is propagated for a given application by a delivery optical fiber.

A Raman light reduction filter 6 as filter means is arranged between the amplification optical fiber 5 and the FBG 7 . Here, the laser oscillation light generates Raman scattering light in the optical fiber through which the laser oscillation light propagates among the optical fibers constituting the optical fiber laser. The optical fiber through which the laser oscillation light propagates mainly includes the amplification optical fiber 5, the FBG 7, the optical fiber constituting the signal light port of the optical multiplexer 8, the output optical fiber 11, and the optical fiber connecting these. The Raman light reduction filter 6 has a function of selectively reducing the power of the generated Raman scattered light.

In this embodiment, the wavelength of the laser oscillation light is 1070 nm, so the wavelength of the Raman scattering light is about 1120 nm. The Raman light reduction filter 6 has a transmittance of −30 dB or less around 1120 nm, as shown in the transmission spectrum of FIG. Therefore, when the Raman scattered light is input to the Raman light reduction filter 6, its power is selectively and greatly reduced.

The Raman light reduction filter 6 can be composed of, for example, a slant type FBG. The slant-type FBG reduces the power of the Raman scattered light by selectively reflecting the Raman scattered light and causing it to leak out of the core of the FBG. When the Raman light reduction filter 6 is configured by a slant type FBG, the following may be used. For example, when fabricating the FBG 7, the coating of the optical fiber for fabricating the FBG may be removed, and patterned ultraviolet rays may be irradiated thereon to write the grating of the FBG 7 and the grating of the slant type FBG. As a result, the Raman light reduction filter 6 and the FBG 7 can be manufactured as an integral part by performing the coating removal step once. Further, this eliminates the work of fusion splicing that is necessary when the Raman light reduction filter 6 and the FBG 7 are manufactured as separate parts, so that the overall length of the optical fiber is shortened and the work process is simplified. can.

The Raman light reduction filter 6 can also be configured by a bulk-type filter or an optical fiber-type filter containing a substance capable of absorbing Raman scattered light, for example. These light absorption filters reduce the power of the Raman scattered light by selectively absorbing the Raman scattered light. In addition, in the light absorption type filter, the energy of the absorbed light is converted into heat, and heat may be generated. Therefore, it is preferable to provide a heat dissipation mechanism such as a heat sink.

The Raman light reduction filter 6 can also be configured by a photonic bandgap fiber (PBGF) configured so that the wavelength band containing Raman scattered light is a forbidden band.

The Raman light reduction filter 6 preferably has a transmittance of -15 dB or less so as to reduce the power of the Raman scattered light by -15 dB or less. It is even more preferable to have

Returning to FIG. 1, the dashed line L1 represents the relationship between the position in the longitudinal direction of the optical fiber through which the laser oscillation light propagates (hereinafter sometimes simply referred to as the longitudinal direction) and the power of the laser oscillation light at that position. Schematically. As indicated by the dashed line L1, the power of the laser oscillation light increases in the amplification optical fiber 5 from FBG4 to FBG7, and is substantially constant on the output optical fiber 11 side of FBG7.

On the other hand, the solid line L2 schematically shows the relationship between the position in the longitudinal direction and the Raman scattering power at that position. As indicated by the solid line L2, the power of the Raman scattered light increases in the amplification optical fiber 5 from FBG4 to FBG7, but is greatly reduced by the Raman light reduction filter 6. FIG. After that, the power of the Raman scattered light increases over the output optical fiber 11, but since the power has been greatly reduced by the Raman light reduction filter 6, the subsequent increase in power is suppressed.

As a result, the degree to which the energy of the laser oscillation light is consumed to increase the power of the Raman scattered light is also reduced, so that the power reduction of the laser oscillation light can be suppressed, and the energy efficiency of the laser oscillation can be suppressed. Furthermore, the power ratio of laser oscillation light to Raman scattered light can be made relatively high. Here, if the laser oscillation light is used as the signal light, the Raman scattered light can be considered as noise light with respect to the signal light. Therefore, the power ratio of the laser oscillation light to the Raman scattered light is sometimes referred to as OSNR (Optical Signal Noise Ratio). It can be said that the higher the OSNR, the less the energy of the laser oscillation light is consumed to increase the power of the Raman scattered light, and the higher the quality of the light output from the output optical fiber 11 is. Therefore, it is preferable to configure and arrange the Raman light reduction filter 6 so that the OSNR is equal to or higher than the allowable value. The allowable value of OSNR is, for example, 40 dB, although it varies depending on the intended use of the laser oscillation light, the required power, and the like. Moreover, it is preferable that the position where the OSNR is equal to or higher than the allowable value is immediately after the Raman light reduction filter 6 which is the filter means. Here, "immediately after" means "immediately after" in the direction in which the power of the Raman scattered light increases.

In particular, in this embodiment, the optical fiber closer to FBG7 than FBG4 has a mode field diameter of 15 μm or less at the laser oscillation wavelength, and the power of laser light output from FGB7 is 1500 W or more. Therefore, the beam quality when used for laser processing is good, but the power density of the light in the optical fiber on the FBG7 side is higher than that on the FBG4 side. Therefore, although the power of the generated Raman scattered light may be significantly increased, the power is significantly reduced by the Raman light reduction filter 6, so that the OSNR is above the allowable value. The optical fiber on the FBG7 side of the FBG4 is an optical fiber through which the laser oscillation light propagates on the FBG7 side of the FBG4. That is, they are mainly the amplification optical fiber 5, the FBGs 4 and 7, the output optical fiber 11, the optical fiber of the signal light port of the optical multiplexer 8, and the optical fiber through which the laser oscillation light propagates in the Raman light reduction filter 6. FIG.

Next, for comparison, the optical power in the optical fiber in the optical fiber laser according to the comparative embodiment as shown in FIG. 12 will be described. Since the optical fiber laser 1000 according to this comparative example has a configuration in which the Raman light reduction filter 6 is removed from the configuration of the optical fiber laser 100 according to Embodiment 1, detailed description of the configuration of the optical fiber laser 1000 is omitted.

A dashed line L1001 in FIG. 12 schematically shows the relationship between the position in the longitudinal direction and the power of the laser oscillation light at that position. On the other hand, the solid line L1002 schematically shows the relationship between the position in the longitudinal direction and the Raman scattering power at that position. In the case of this optical fiber laser 1000, the power of Raman scattered light continuously increases from the FBG 4 to the output optical fiber 11, so the power in the output optical fiber 11 becomes extremely large. Along with this, as indicated by the dashed line L1001, the energy of the laser oscillation light is consumed to increase the power of the Raman scattered light on the output optical fiber 11 side of the FBG 7, so the power further decreases. Furthermore, OSNR also decreases.

式（１）、（２）において、Ｐｓはラマン散乱光のパワー、Ｐｐはレーザ発振光のパワー、ｇＲはラマン利得係数、Ａｅｆｆは有効コア断面積、αｓはラマン散乱光の波長における伝送損失、αｐはレーザ発振光の波長における伝送損失を示しており、いずれも長手方向における位置ｚの関数である。また、λｓはラマン散乱光の波長、λｐはレーザ発振光の波長である。 Here, propagation equations for the power of laser oscillation light and Raman scattering light are expressed by the following equations (1) and (2).

In equations (1) and (2), Ps is the power of Raman scattered light, Pp is the power of laser oscillation light, gR is the Raman gain coefficient, Aeff is the effective core area, and αs is the transmission loss at the wavelength of the Raman scattered light. αp indicates the transmission loss at the wavelength of the laser oscillation light, both of which are functions of the position z in the longitudinal direction. λs is the wavelength of Raman scattering light, and λp is the wavelength of laser oscillation light.

As shown by the formula (1), when [gR/(λs·Aeff)]·Pp is large, that is, when the power of the laser oscillation light is high, the Raman scattered light increases with high efficiency. Further, as shown by the equation (2), when −[gR/(λp·Aeff)]·Ps is large, that is, when the power Ps of the Raman scattering light is large, the rate of decrease in the power of the laser oscillation light becomes large. Therefore, it is effective from the viewpoint of suppressing an increase in the power of Raman scattered light to lower Ps by the Raman light reduction filter 6 before it increases, as in the first embodiment.

As described above, according to the optical fiber laser 100 according to the first embodiment, it is possible to suppress an increase in the power of Raman scattered light.

(Embodiment 2)
FIG. 3 is a schematic diagram showing the schematic configuration of an optical fiber laser according to Embodiment 2 and the optical power in the optical fiber. This optical fiber laser 100A has the configuration of the optical fiber laser 100 according to the first embodiment, in which the amplification optical fiber 5 is divided into amplification optical fibers 5a and 5b, and Raman light reduction is performed between the amplification optical fibers 5a and 5b. Since it has a configuration in which the filter 6 is moved, a detailed description of the configuration of the optical fiber laser 100A is omitted. Also in this embodiment, the optical fiber on the FBG7 side of the FBG4 has a mode field diameter of 15 μm or less at the laser oscillation wavelength, and the power of the laser light output from the FGB7 is 1500 W or more.

In FIG. 3, the dashed line L1A schematically shows the relationship between the position in the longitudinal direction and the power of the laser oscillation light at that position. As indicated by the dashed line L1A, the power of the laser oscillation light increases in the amplification optical fiber 5 from FBG4 to FBG7, and is substantially constant on the output optical fiber 11 side of FBG7.

A solid line L2A schematically shows the relationship between the position in the longitudinal direction and the power of Raman scattering at that position. As indicated by the solid line L2A, the power of the Raman scattered light increases from FBG4 to FBG7, but is greatly reduced by the Raman light reduction filter 6 placed between the amplification optical fibers 5a and 5b. After that, the power of the Raman scattered light increases over the output optical fiber 11, but since the power has been greatly reduced by the Raman light reduction filter 6, the subsequent increase in power is suppressed.

As a result, in the optical fiber laser 100A, the energy of the laser oscillation light is also reduced to the extent that it is consumed to increase the power of the Raman scattered light. A decrease in efficiency can also be suppressed. Furthermore, the OSNR can also be made higher than the allowable value.

As described above, according to the optical fiber laser 100A according to the second embodiment, it is possible to suppress an increase in the power of Raman scattered light.

Next, the result of examining the optical power distribution in the longitudinal direction of the optical fiber laser by calculation will be described.

First, as Reference Calculation Example 1, calculation was performed for the configuration of the optical fiber laser 1000 shown in FIG. FIG. 4 is a diagram showing optical power in an optical fiber in Reference Calculation Example 1. FIG. In FIG. 4, the horizontal axis indicates the position in the longitudinal direction of the YDF amplification optical fiber, and the vertical axis indicates the power of laser oscillation light (laser light power) and the power of Raman scattering light (Raman light power). there is The length of the amplification optical fiber is 20 m. A dashed line L11 is the laser light power, and a solid line L12 is the Raman light power. Further, the Raman optical power is indicated in units of watts in FIG. 4(a) and in units of dBm in FIG. 4(b). The Raman optical power was calculated on the assumption that natural Raman scattering of one photon occurred at a position of 0 m, and then the photon was Raman amplified over 20 m. In addition, in this reference calculation example 1 and the following calculation examples, the calculation is performed without considering the phenomenon that the energy of the laser oscillation light is consumed to increase the Raman scattering light. The mode field diameter of the optical fiber on the FBG7 side of the FBG4 at the laser oscillation wavelength corresponds to 14 μm.

In this reference calculation example 1, the laser light power on the output side of the amplification optical fiber is 1100 W, ie, approximately 60.4 dBm. Raman optical power, on the other hand, is 0.1 W, or 20 dBm. Therefore, if the allowable amount is 40 dB, the OSNR is approximately 40.4 dB, which is above the allowable amount. That is, if the laser light power is 1100 W, the Raman light power of 0.1 W is acceptable.

Subsequently, as Comparative Calculation Example 1, calculations were performed for a design in which the laser light power was increased in the configuration of the optical fiber laser 1000 shown in FIG. FIG. 5 is a diagram showing optical power in an optical fiber in a comparative calculation example. A dashed line L21 is the laser light power, and a solid line L22 is the Raman light power. The Raman optical power is shown in watt units in FIG. 5(a) and in dBm units in FIG. 5(b).

In this Comparative Calculation Example 1, the laser light power at the output side of the amplification optical fiber is 1500 W, ie, approximately 61.8 dBm. At this time, assuming that the OSNR tolerance is 40 dB, the Raman optical power is 0.15 W or less, that is, approximately 21.8 dBm. However, in this comparative calculation example, the Raman optical power was approximately 42 dBm, and the OSNR was smaller than 20 dB, which was significantly below the allowable level.

Next, as Calculation Example 1, calculations were performed assuming that the configuration of the optical fiber laser 100A shown in FIG. The amplification optical fiber of 20 m was divided into 15 m and 5 m lengths, and Raman light reduction filters having a transmittance of about -18 dB were arranged at the positions from 0 m to 15 m.

FIG. 6 is a diagram showing the optical power in the optical fiber in Calculation Example 1. FIG. The dashed line L21 represents the laser light power in this calculation example, and since it is the same curve as the laser light power in the comparative calculation example shown in FIG. 5, the same reference numerals are used. A solid line L22 is the Raman light power in the comparative calculation example and is shown for comparison. A solid line L23 is the Raman light power in the first calculation example. Moreover, FIG.6(b) expands a part of Fig.6 (a). The Raman optical power is shown in watt units in FIGS. 6(a) and 6(b), and in dBm units in FIG. 6(c).

In this calculation example 1, the laser light power at the output side of the amplification optical fiber is 1500 W, ie, approximately 61.8 dBm. At this time, assuming that the OSNR tolerance is 40 dB, the Raman optical power is 0.15 W or less, that is, approximately 21.8 dBm or less. In this calculation example, since a Raman light reduction filter with an appropriate configuration (transmittance) was placed at an appropriate position, the Raman light power was approximately 21.7 dBm, and the OSNR was approximately 40.1 dB, which was above the allowable level.

(Embodiment 3)
FIG. 7 is a schematic diagram showing a schematic configuration of an optical fiber laser and optical power in the optical fiber according to Embodiment 3. FIG. This optical fiber laser 100B has the configuration of the optical fiber laser 100 according to the first embodiment, in which the output optical fiber 11 is split into two output optical fibers 11a and 11b, and a Raman light reduction filter is provided between the output optical fibers 11a and 11b. Since it has a configuration in which the filter 6 is moved, a detailed description of the configuration of the optical fiber laser 100B is omitted. Also in this embodiment, the optical fiber on the FBG7 side of the FBG4 has a mode field diameter of 15 μm or less at the laser oscillation wavelength, and the power of the laser light output from the FGB7 is 1500 W or more.

In FIG. 7, the dashed line L1B schematically shows the relationship between the position in the longitudinal direction and the power of the laser oscillation light at that position. As indicated by the dashed line L1B, the power of the laser oscillation light increases in the amplification optical fiber 5 from FBG4 to FBG7, and is substantially constant on the output optical fiber 11 side of FBG7.

A solid line L2B schematically shows the relationship between the position in the longitudinal direction and the power of Raman scattering at that position. As indicated by the solid line L2B, the power of Raman scattered light increases from the FBG 4 to the output optical fiber 11a, but is greatly reduced by the Raman light reduction filter 6. FIG. After that, the power of the Raman scattered light increases in the output optical fiber 11b, but since the power has been greatly reduced by the Raman light reduction filter 6, the subsequent increase in power is suppressed.

As a result, in the optical fiber laser 100B, the energy of the laser oscillation light is also reduced to the extent that it is consumed to increase the power of the Raman scattered light, so that a decrease in the power of the laser oscillation light can be suppressed. Furthermore, the OSNR can also be made higher than the allowable value.

As described above, according to the optical fiber laser 100B according to the third embodiment, it is possible to suppress an increase in the power of Raman scattered light.

In the optical fiber laser 100B, the Raman light reduction filter 6 is arranged between the output optical fibers 11a and 11b. A configuration in which the light reduction filter 6 is arranged between the FBG 7 and the output optical fiber 11 may be employed.

Next, the result of examining the optical power distribution in the longitudinal direction of the optical fiber laser by calculation will be described.

First, as Reference Calculation Example 2, calculation was performed for the configuration of the optical fiber laser 1000 shown in FIG. FIG. 8 is a diagram showing optical power in an optical fiber in Reference Calculation Example 2. FIG. In FIG. 8, the horizontal axis indicates the position in the longitudinal direction of the output optical fiber, and the vertical axis indicates the laser light power and the Raman light power. The length of the output optical fiber is 10m. A dashed line L31 is the laser light power, and a solid line L32 is the Raman light power. Further, the Raman optical power is indicated in units of watts in FIG. 8(a) and in units of dBm in FIG. 8(b). The mode field diameter of the optical fiber on the FBG7 side of the FBG4 at the laser oscillation wavelength corresponds to 14 μm.

In this reference calculation example 2, the laser light power at the position 0 m that is output from the optical fiber resonator and input to the output optical fiber is 1000 W, ie, 60 dBm. On the other hand, Raman optical power is about 0.1W. Also, the laser light power at the position 10m of the output optical fiber drops to about 999.1W, and the Raman light power increases to about 1W. Therefore, the OSNR is about 30 dB, which is less than 40 dB but acceptable for general use.

Subsequently, as Comparative Calculation Example 2, calculations were performed for the configuration of the optical fiber laser 1000 shown in FIG. 12 with a higher laser light power design. FIG. 9 is a diagram showing optical power in an optical fiber in Comparative Calculation Example 2. FIG. A dashed line L41 is the laser light power, and a solid line L42 is the Raman light power. Further, the Raman optical power is indicated in units of watts in FIG. 9(a) and in units of dBm in FIG. 9(b).

In Comparative Calculation Example 2, the laser light power at the position 0 m that is output from the optical fiber resonator and is input to the output optical fiber is 1500 W, ie, approximately 61.8 dBm. At this time, assuming that the OSNR tolerance is 40 dB, the Raman optical power is 0.15 W or less, that is, approximately 21.8 dBm. However, in this Comparative Calculation Example 2, the Raman optical power at the output optical fiber position of 10 m greatly exceeded 21.8 dBm, and the OSNR was about 20 dB, which was significantly below the allowable level.

Next, as calculation example 2, in the configuration of the optical fiber laser 100B shown in FIG. Calculations were performed assuming a design with increased optical power. The transmittance of the Raman light reduction filter was set to approximately -20 dB.

FIG. 10 is a diagram showing the optical power in the optical fiber in Calculation Example 2. FIG. A dashed line L51 is the laser light power in the second calculation example. A dashed line L41 is the laser light power in Comparative Calculation Example 2 and is shown for comparison. A solid line L42 is the Raman light power in Comparative Calculation Example 2 and is shown for comparison. A solid line L52 is the Raman light power in the second calculation example. The Raman optical power is shown in watts in FIG. 10(a) and in dBm units in FIG. 10(b).

In this calculation example 2, the laser light power at the position 10 m of the output optical fiber is 1500 W, ie, about 61.8 dBm. At this time, assuming that the OSNR tolerance is 40 dB, the Raman optical power is 0.15 W or less, that is, about 21.8 dBm. In this calculation example 2, since the Raman light reduction filter with the proper configuration (transmittance) is placed at the proper position, the Raman light power at the position 10 m of the output optical fiber is about 21.7 dBm, and the OSNR is about 40.0 dBm. It was 1 dB, which was more than the allowable amount.

(Embodiment 4)
FIG. 11 is a schematic diagram showing the schematic configuration of a multiplexing optical fiber laser according to Embodiment 4 and the optical power in the optical fiber. This multiplexing optical fiber laser 200 includes the four optical fiber lasers 100B shown in FIG. 7, an optical multiplexer 21, an output optical fiber 22, and a delivery optical fiber 23.

In FIG. 11, the configuration of each optical fiber laser 100B other than the output optical fiber 11 and the Raman light reduction filter 6 is illustrated as a block. Each output optical fiber 11 propagates the laser oscillation light output from each optical fiber laser 100B and outputs it to the optical multiplexer 21 .

The optical multiplexer 21 is configured to multiplex the laser oscillation light input from each output optical fiber 11 and output to the output optical fiber 22 . The output optical fiber 22 is a multimode fiber with a core diameter of 50 to 100 μm, for example, and the mode field diameter is much larger than 15 μm. Therefore, the power of the generated Raman scattered light is small and is not considered.

The output optical fiber 22 propagates the combined laser oscillation light to the delivery optical fiber 23 . The delivery optical fiber 23 is a multimode fiber with a core diameter of 50 to 100 μm, for example, and the mode field diameter is much larger than 15 μm. Therefore, the power of the generated Raman scattered light is small and is not considered.

In FIG. 11, the solid line L schematically shows the relationship between the position in the longitudinal direction and the power of Raman scattered light at that position. As indicated by the solid line L, in this multiplexing optical fiber laser 200, the light is generated in the output optical fiber 11 extending to the inside of the optical multiplexer 21 and propagates from the output optical fiber 11 side to each optical fiber laser 100B. The power of Raman scattered light is greatly reduced by each Raman light reduction filter 6 . After that, the power of the Raman scattered light increases in each optical fiber laser 100B, but since the power has been significantly reduced by the Raman light reduction filter 6, the subsequent increase in power is suppressed.

In addition, in the multiplexing optical fiber laser 200, the power of the Raman scattered light increases from the FBG 4 to the output optical fiber 11a as in the case of the optical fiber laser 100B shown in FIG. to After that, the power of the Raman scattered light increases in the output optical fiber 11b, but since the power has been greatly reduced by the Raman light reduction filter 6, the subsequent increase in power is suppressed.

As a result, in the multiplexing type optical fiber laser 200, the energy of the laser oscillation light is also reduced to increase the power of the Raman scattered light, so that the power reduction of the laser oscillation light can be suppressed. Furthermore, the OSNR can also be made higher than the allowable value.

As described above, according to the multiplexing optical fiber laser 200 according to the fourth embodiment, it is possible to suppress an increase in the power of Raman scattered light.

In addition, the present invention is not limited by the above-described embodiment. The present invention also includes a configuration obtained by appropriately combining the constituent elements of the above-described embodiments. Further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible.

1, 10 semiconductor excitation light sources 2, 9 optical fibers 3, 8, 21 optical multiplexers 5, 5a, 5b amplification optical fibers 6 Raman light reduction filters 11, 11a, 11b, 22 output optical fibers 23 delivery optical fibers 100, 100A , 100B Optical fiber laser 200 Multiplexing optical fiber laser L, L12, L2, L22, L23, L2A, L2B Solid lines L1, L11, L1A, L1B, L21 Broken lines

Claims (5)

an amplifying optical fiber;
an optical resonator composed of a rear-end reflecting means and an output-side reflecting means arranged at both ends of the amplification optical fiber;
a pumping light source that outputs pumping light to be supplied to the amplification optical fiber;
filter means for selectively reducing the power of Raman scattered light generated by the laser light oscillated by the optical resonator;
an output optical fiber disposed on the opposite side of the amplification optical fiber across the output side reflection means;
with
The optical fiber closer to the output-side reflecting means than the rear-end reflecting means has a mode field diameter of 15 μm or less at a laser oscillation wavelength,
power of the laser light output from the output-side reflecting means of the optical resonator is 1500 W or more;
The filter means is arranged so that the power ratio of the laser light to the Raman scattered light immediately after the filter means is 40 dB or more, and is arranged between the output side reflection means and the output optical fiber. ,
The filter means suppresses an increase in the power of the Raman scattered light in the output optical fiber downstream from the filter means by once reducing the power of the Raman scattered light.
An optical fiber laser characterized by: 2. The optical fiber laser according to claim 1, wherein the power of said Raman scattered light in said output side reflecting means is 0.15 W or less. 3. The optical fiber laser according to claim 1, wherein said filter means reduces the power of said Raman scattered light by -15 dB or less. An optical fiber laser according to any one of claims 1 to 3 , comprising two said output optical fibers, wherein said filter means is arranged between said two output optical fibers. . a plurality of optical fiber lasers according to any one of claims 1 to 4 ;
an optical multiplexer for multiplexing and outputting the laser beams output from the output optical fibers of the plurality of optical fiber lasers;
A multiplexing optical fiber laser comprising:

Images