JP2012238781A - FIBER LASER OSCILLATOR AND FIBER LASER AMPLIFIER USING Yb ADDITION GLASS FIBER - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly efficient fiber laser oscillator and fiber laser amplifier using Yb addition glass, capable of applying excitation with wavelength of around 976 nm in a stable state even when fluctuations in an excitation light wavelength occur.SOLUTION: The fiber laser oscillator includes: a first semiconductor laser that radiates first excitation light with a wavelength of 973-979 nm; a second semiconductor laser that radiates second excitation light with a wavelength of 880-970 nm; Yb addition glass gain fiber having total reflection type fiber Bragg grating on excitation light ingoing side and partial reflection type fiber Bragg grating on excitation light outgoing side; a fiber combiner provided on the excitation light ingoing side of optical fiber; a first optical fiber for connecting the first semiconductor laser with the fiber combiner; and a second optical fiber for connecting the second semiconductor laser with the fiber combiner.

Description

The present invention relates to a fiber laser, and more particularly to a fiber laser oscillator and a fiber laser amplifier provided with a Yb (ytterbium) -doped glass fiber as a gain medium.

A fiber laser is a kind of solid-state laser and uses an optical fiber as a medium. A general fiber laser uses a rare earth doped fiber as an amplifier, and the optical path is entirely composed of an optical fiber. The rare earth doped optical fiber is an optical fiber in which a minute amount of rare earth element is added to the core portion. Rare earth elements have the property of generating fluorescence in the wavelength band from visible to near infrared. When appropriate excitation light is incident on the rare earth-doped optical fiber, fluorescence is generated by increasing the energy state of the rare earth element. When the excitation light intensity is high, stimulated emission is performed, so that it can be used as a laser oscillator or an optical fiber amplifier that amplifies weak signal light.

A fiber laser that _{uses an} Er (erbium) _-doped glass fiber as a gain medium and oscillates laser light having a wavelength of 2.8 μm by transition of the Er ion from the laser upper level ⁴ I _11/2 to the laser lower level ⁴ I _13/2 Is known that the wavelength of emitted oscillation light shifts to the longer wavelength side when excitation light having a wavelength that causes ground level absorption is incident at a high intensity.

This phenomenon is explained by the energy level diagram as follows. In general, the energy of laser oscillation light is often explained by a simple transition process between two upper and lower levels. Actually, one energy level is separated into a plurality of Stark levels having a narrow energy interval, and the Stark level is occupied from the low energy side. In the laser lower level ⁴ I _13/2 , the higher the excitation intensity, the higher the Stark level contributes to the oscillation, the energy interval with the upper level ⁴ I _11/2 is narrowed, and the oscillation wavelength is longer. Shift to the side.

In the fiber laser oscillator using the Er-doped glass fiber according to Patent Document 1, the first pumping light (960 nm to 1020 nm) in which the ground level absorption occurs and the laser lower level ⁴ with respect to the problem of wavelength shift due to the pumping intensity. By simultaneously using the second pumping light that causes excitation level absorption from I _13/2, it is possible to stably and efficiently obtain a laser beam having a constant wavelength regardless of the ground level pumping intensity. ing.

JP 2005-150229 A

  Unlike fiber lasers using general-purpose Er-doped glass, Yb-doped glass fibers with small quantum defects are mainly used in the field of industrial high-average output fiber lasers. The Yb-doped glass has negligible excitation level absorption like that of the Er-doped glass, so that there is no problem of wavelength shift of oscillation light that is desired to be explained in the background art.

  Two types of configurations are known as typical fiber laser configurations using a Yb-doped glass fiber as a gain medium. In the first configuration, a plurality of excitation light sources are bundled into one fiber by a multimode fiber combiner and coupled from the end face of the gain fiber without a space. In the second configuration, excitation light is spatially coupled from the end face of the gain fiber. As the excitation light source, a semiconductor laser that emits a wavelength close to the absorption peak of the gain medium is used.

  Next, an absorption spectrum of a double clad fiber (DCF) using Yb-doped glass will be described. As the DCF, NMA LMA-YDF-20 / 400 (core diameter 20 μm / NA 0.06, excitation cladding diameter 400 μm / NA 0.46) is assumed. DCF using Yb-added glass has a gradual maximum near a wavelength of 915 nm and a sharp peak having an absorption coefficient about four times that near a wavelength of 976 nm. In order to suppress quantum defects and increase oscillation efficiency in a fiber laser, it is generally preferred to select a wavelength near 976 nm as an excitation wavelength, which is closer to the oscillation wavelength (usually 1030 nm to 1100 nm). However, the peak near the wavelength of 976 nm is as narrow as about 8 nm in full width at half maximum, and a change in absorption coefficient of 10% or more occurs with a wavelength variation of 1 nm.

  Consider a case where a typical fiber laser configuration is excited using a semiconductor laser having a center wavelength of 976 nm and a spectral width of 2 nm (FWHM). In calculation, about 13.5 dB (about 95.5%) of excitation light is absorbed by the Yb-doped glass core in the gain fiber 7 m. If the gain fiber is set longer than this and a higher absorption rate is secured, a non-pumped portion with weak pumping light absorption is formed in the core of the gain fiber, which may cause reabsorption of oscillation light.

In actual operation, it is conceivable that the pumping light wavelength deviates from the design value due to variations in the semiconductor lasers to be mounted, the stability of the cooling system, the deterioration of the semiconductor lasers, and the like. In the gain fiber 7m, the change in the pumping light absorption efficiency into the gain fiber core when the center wavelength of the semiconductor laser is around 976 nm was calculated (see FIG. 4). Since the absorption peak at the center wavelength (976 nm) is narrow, the absorption efficiency varies greatly from 85% to 96% with changes in the center wavelength ± 3 nm.

  As described above, in a fiber laser using Yb-doped glass, when a semiconductor laser having a wavelength of about 976 nm is applied, the fiber laser has a response to changes in the pumping light wavelength spectrum due to semiconductor laser driving conditions, temperature changes, inherent variations, and the like. The oscillation output fluctuates sensitively. If the gain fiber is lengthened to suppress the fluctuation, a non-excitation portion is formed in the ytterbium-doped fiber, reabsorption of oscillation light occurs, and oscillation becomes unstable.

  The present invention has been made to solve the above-described problems. A Yb-doped glass capable of applying excitation with a wavelength near 976 nm that is stable and highly efficient even when there is a fluctuation in the wavelength of the excitation light. The object is to obtain the fiber laser oscillator and the fiber laser amplifier used.

  A fiber laser oscillator according to the present invention includes a first semiconductor laser that emits first excitation light having a wavelength of 973 nm to 979 nm, a second semiconductor laser that emits second excitation light having a wavelength of 880 nm to 970 nm, and incident excitation light. A gain fiber made of Yb-doped glass having a total reflection type fiber Bragg grating on the side and a partial reflection type fiber Bragg grating on the pumping light exit side, a fiber combiner provided on the pumping light incident side of the optical fiber, A first optical fiber that connects the semiconductor laser and the fiber combiner, and a second optical fiber that connects the second semiconductor laser and the fiber combiner.

  According to the present invention, the first pumping light in the vicinity of the wavelength of 976 nm and the second pumping light having a wavelength lower than that of the pumping light are simultaneously incident on the gain medium. It becomes possible to manufacture a Yb-doped glass fiber laser oscillator which is stable.

1 is a configuration diagram showing a fiber laser oscillator according to Embodiment 1. FIG. The absorption spectrum of the double clad fiber using Yb addition glass is shown. It is the graph which showed the absorptivity of 1st excitation light and 2nd excitation light with respect to the longitudinal direction position of the gain fiber core using Yb addition glass. It is a graph which shows the change of the pumping light absorption factor by the pumping light center wavelength shift in the fiber laser oscillator using Yb addition glass. The absorption curve at the time of the conventional 976 nm excitation is also shown collectively. 6 is a configuration diagram illustrating a fiber laser oscillator according to a second embodiment. FIG. FIG. 6 is a configuration diagram showing a fiber laser oscillator according to a third embodiment. It is a block diagram which shows the example of the internal structure of a fiber coupling semiconductor laser package. FIG. 10 is a configuration diagram showing a fiber laser amplifier according to a fourth embodiment. FIG. 10 is a configuration diagram showing another form of the fiber laser amplifier according to the fourth embodiment.

Embodiment 1 FIG.
FIG. 1 is a configuration diagram of a fiber laser oscillator using a Yb-doped glass fiber according to Embodiment 1 of the present invention. The fiber laser oscillator 100 includes a collimating lens 2c, a fiber Bragg grating (FBG) 5, a gain fiber 6, a pumping optical fiber combiner 7, semiconductor lasers 9, 10, and an optical fiber 24. The semiconductor lasers 9a and 9b emit a first wavelength. The semiconductor laser 10 emits a second wavelength. The semiconductor lasers 9a, 9b, and 10 are coupled to the optical fiber 24.

For the gain fiber 6 having a Yb-doped core, for example, a double clad fiber such as LMA-YDF-20 / 400 manufactured by Nufern is used. Fiber Bragg gratings 5 a and 5 b are fused to both ends of the gain fiber 6. With respect to the fiber laser oscillation light 4, a total reflection type FBG 5a and a partial reflection type FBG 5b constitute a laser resonator. In the gain fiber 6, the total reflection type FBG 5a is provided on the excitation light incident side, and the partial reflection side FBG 5a is provided on the excitation light emission side.

Fibers in which FBGs 5 a and 5 b are written are fused to both ends of the gain fiber 6. A pumping optical fiber combiner 7 is fused to the fiber in which the FBG 5a is written. Semiconductor lasers 9 and 10 for emitting lasers having different wavelengths are coupled to the excitation light incident side of the excitation optical fiber combiner 7. A fiber Bragg grating is a fiber-type device in which a diffraction grating is formed in the core of an optical fiber to provide a function as an optical filter. The FBG can reflect only light satisfying the Bragg reflection condition, that is, light having an arbitrary wavelength.

The first excitation light and the second excitation light are incident on the gain fiber 6. The first excitation light has a first wavelength with a center wavelength of 976 nm and a spectral width of 2 nm (FWHM). The reason why the two semiconductor lasers 9a and 9b are arranged is that it is assumed that the output cannot be secured with one semiconductor laser. The second excitation light has a second wavelength near the wavelength of 915 nm. On the fiber laser emission side, the fiber laser oscillation light 4 transmitted through the FBG 5b is emitted from the fiber end, and is emitted as a collimated beam by the collimator lens 2c. The length of the gain fiber 6 is set to such a length that 10 dB absorption is obtained with respect to the second wavelength. That is, the length of the gain fiber 6 is assumed to be 7 m (± 0.5 m).

  According to such a configuration, about 10% of the component of the pump light having the second wavelength (near 915 nm) remains until the end of the gain fiber 6, and this component becomes a loss. However, the second wavelength provides a gain region having no non-excitation portion over the entire length of the gain fiber 6, and the reabsorption loss of the fiber laser oscillation light 4 can be suppressed.

As shown in FIG. 2, at the excitation wavelength of the gain fiber 6, the absorption coefficient near the wavelength of 976 nm has a peak about four times the absorption coefficient near the wavelength of 915 nm. Assuming that the center wavelength is 976 nm and FWHM = 2 nm as the first wavelength, the relationship between the longitudinal position of the gain fiber 6 and the absorptance of the first excitation light and the second excitation light is as shown in FIG. FIG. 3 shows the calculation result for the double clad fiber having the absorption spectrum shown in FIG. 2 with the spectral width of the excitation light being 2 nm (FWHM).

In the case of the present embodiment having 10 dB (90.0%) absorption up to the end of the gain fiber 6 for the second wavelength, the absorption of the first wavelength up to the end of the gain fiber 6 is about 30 dB ( 99.9%). On the other hand, by applying the excitation fiber combiner 7 to the coupling of the first excitation light and the second excitation light as in the present configuration, the configuration can be easily saved in space, and a rigid and highly reliable configuration can be realized.

  It is assumed that the power of the semiconductor laser 10 emitting the second wavelength occupies 20% (± 5%) of the total excitation power. Since the absorption of the gain fiber 6 with respect to the second wavelength is 10 dB, 2% of the total pumping power is a transmission loss of the second pumping light having the second wavelength. On the other hand, the absorption of pumping light from the semiconductor laser 9 emitting the first wavelength that accounts for 80% of the total pumping power is 30 dB, and almost all components contribute to pumping. That is, the design is such that 98% of the total pumping power contributes to the pumping of the gain fiber 6.

Here, it is assumed that the center wavelength of the semiconductor laser 9 emitting the first wavelength is shifted from 976 nm. As the cause of the wavelength shift, a plurality of factors such as individual differences of semiconductor lasers, fluctuations in the cooling system, current values, and deterioration can be considered. FIG. 4 shows changes in excitation efficiency when the center wavelength of the excitation light having the first wavelength is changed. Here, the spectral width of 2 nm is assumed to be constant.

This graph shows a comparison of changes in excitation efficiency during conventional 976 nm excitation. As shown in FIG. 4, in the configuration of this example, the excitation light absorptance with respect to the shift of the center wavelength is stable even though excitation is performed in the vicinity of 976 nm where the spectrum width is narrow. Even when the shift of the center wavelength of the first excitation light is assumed to be 976 ± 3 nm, the absorptance is 96.6 to 97.9%, showing a very high absorptivity and can be kept within a minute fluctuation of about 1%. It becomes possible.

  Here, the absorption of the second wavelength (915 nm) is 10 dB. However, if there is a gain fiber length or pumping light power that makes the entire region of the gain fiber 6 a gain region, it is possible to have absorption of 10 dB or more. . Although the second wavelength was set to 915 nm, the second wavelength has lower absorption than the vicinity of the first wavelength of 976 nm, and the absorption to the Yb-doped glass is 0.1 dB / m or more in FIG. Yes, any wavelength that can ensure an allowable gain is applicable. That is, any wavelength in the range of 880 to 970 nm may be applied to the second wavelength, but a wavelength in the range of 900 to 930 nm is particularly preferable.

  Furthermore, although the pumping power of the second wavelength is 20% of the total pumping power, the ratio is arbitrary as long as there is sufficient pumping power to make the entire area of the gain fiber 6 a gain region. However, since the pump light near the first wavelength of 976 nm has a smaller quantum defect than the second pump light and contributes to the improvement of oscillation efficiency, the power of the first pump light is equal to the power of the second pump light. The above is desirable from the viewpoint of oscillation efficiency.

Further, there is no problem even if the second wavelength includes the third wavelength and the fourth wavelength within the range applicable to the second wavelength. If the entire region of the gain fiber 6 can be a gain region as the entire pumping light component of the second and subsequent wavelengths, it is included in the first embodiment. In the configuration of FIG. 1, the 3 × 1 pumping optical fiber combiner 7 is used for introducing pumping light. However, the number of pumping light input fibers may be plural, and the semiconductor laser emitting the first and second wavelengths may be used. There is no problem if the number of fibers to be coupled is one or more.

Embodiment 2. FIG.
In the second embodiment, when the excitation light is introduced into the gain fiber 6, it is made incident from the end face of the fiber spatially using the lenses 2a, 2b and 2d as shown in FIG. At that time, the first excitation light and the second excitation light are coaxially coupled by an optical element such as the dichroic mirror 13 and coupled to the fiber resonator.

According to such a configuration, it is not necessary to previously couple the pumping light source to the entrance of the fiber combiner, cost reduction can be expected, and the degree of freedom in designing the pumping light source and the like that can be applied increases.

Embodiment 3 FIG.
In the second embodiment, the semiconductor laser 9 that emits the excitation light having the first wavelength and the semiconductor laser 10 that emits the second wavelength are operated as separate modules. In the third embodiment, as shown in FIG. 6, one or more semiconductor laser modules 17 that emit a mixture of the first wavelength and the second wavelength are applied. Inside the semiconductor laser module 17, for example, a semiconductor laser 19 that emits a first wavelength and a semiconductor laser 20 that emits a second wavelength are mounted in the package as shown in FIG. The first excitation light 21 and the second excitation light 22 emitted from each form a combined beam 23 via the dichroic mirror 18 and are coupled to the optical fiber 24. The optical fiber 24 for transmitting the pumping light is connected to the pumping optical fiber combiner 7.

  According to such a configuration, the combined high-brightness semiconductor laser light source can be coupled to the pumping port of the pumping optical fiber combiner 7, so that the pumping power per port can be improved. Output becomes easy. Moreover, since the same fiber laser output can be realized with a small number of pumping ports, the pumping optical fiber combiner 7 with a small number of pumping ports can be applied.

In general, when the number of excitation ports increases, the excitation light transmittance of each port decreases, the amount of heat generated in the combiner increases, and the reliability decreases. This configuration has an advantage that a highly reliable pump fiber combiner with a small number of pump ports can be applied. Furthermore, fiber combiners with a large number of pump ports are generally difficult to manufacture and expensive. This configuration also has an advantage that a low-cost fiber laser oscillator can be configured.

Embodiment 4 FIG.
The fourth embodiment relates to a fiber laser amplifier. Unlike the first to third embodiments, the fiber laser amplifier 200 does not have the FBGs 5a and 5b. As shown in FIG. 8, the signal light enters the core of the gain fiber 6 from the seed light source 14 through the optical isolator 16 and the signal feedthrough excitation combiner 15. A semiconductor laser 9 having a first wavelength and a semiconductor laser 10 having a second wavelength are coupled to the pumping port fiber of the signal feedthrough pumping combiner 15. Alternatively, as shown in FIG. 9, a semiconductor laser module 17 that emits a mixture of the first wavelength and the second wavelength is coupled.

  According to such a configuration, signal light emitted from a stable but weak seed light source can be amplified efficiently and stably.

  DESCRIPTION OF SYMBOLS 1 Semiconductor laser, 2a-2d lens, 4 Fiber laser oscillation light, 5a Total reflection type FBG, 5b Partial reflection type FBG, 6 Gain fiber, 7 Excitation optical fiber combiner, 9a-9b Fiber which emits 1st wavelength Coupled semiconductor laser, 10 Fiber-coupled semiconductor laser emitting the second wavelength, 11 Semiconductor laser emitting the first wavelength, 12 Semiconductor laser emitting the second wavelength, 13 Dichroic mirror, 14 Seed light source, 15 Signal feed Through-pump fiber combiner, 16 optical isolators, 17a to 17c, fiber-coupled semiconductor laser that emits mixed first and second wavelengths, 18 dichroic mirror, 19 semiconductor laser that emits first wavelength, 20 second A semiconductor laser that emits a wavelength of 21; the first excitation light; 2 Second excitation light, 23 the first wavelength and the excitation light combining a second wavelength, 24 pump light transmission optical fiber

Claims (7)

A first semiconductor laser that emits first excitation light having a wavelength of 973 nm to 979 nm;
A second semiconductor laser that emits second excitation light having a wavelength of 880 nm to 970 nm;
A gain fiber made of Yb-doped glass having a total reflection type fiber Bragg grating on the excitation light incident side and a partial reflection type fiber Bragg grating on the excitation light emission side;
A fiber combiner provided on the excitation light incident side of the gain fiber;
A first optical fiber connecting the first semiconductor laser and the fiber combiner;
A fiber laser oscillator comprising: the second semiconductor laser; and a second optical fiber connecting the fiber combiner. A first semiconductor laser that emits first excitation light having a wavelength of 973 nm to 979 nm;
A second semiconductor laser that emits second excitation light having a wavelength of 880 nm to 970 nm;
A gain fiber made of Yb-doped glass having a total reflection type fiber Bragg grating on the excitation light incident side and a partial reflection type fiber Bragg grating on the excitation light emission side;
A fiber laser oscillator comprising: an optical element that guides the first pumping light and the second pumping light to the gain fiber. A first semiconductor laser that emits first excitation light having a wavelength of 973 nm to 979 nm;
A second semiconductor laser that emits second excitation light having a wavelength of 880 nm to 970 nm;
A gain fiber made of Yb-doped glass having a total reflection type fiber Bragg grating on the excitation light incident side and a partial reflection type fiber Bragg grating on the excitation light emission side;
A fiber combiner provided on the excitation light incident side of the gain fiber;
A first optical fiber connecting the first semiconductor laser and the fiber combiner;
A second optical fiber connecting the second semiconductor laser and the fiber combiner;
An optical element for guiding the first pumping light and the second pumping light to the gain fiber;
A fiber laser oscillator, wherein the first semiconductor laser, the second semiconductor laser, and the optical element are packaged. The fiber laser oscillator according to any one of claims 1 to 3, wherein the gain fiber is set to a length that allows absorption of 10 dB with respect to the second pumping light. 5. The fiber laser oscillator according to claim 4, wherein the power of the second pumping light is 20% of the power of the total pumping light. A seed light source that emits seed light;
A first semiconductor laser that emits first excitation light having a wavelength of 973 nm to 979 nm;
A second semiconductor laser that emits second excitation light having a wavelength of 880 nm to 970 nm;
A gain fiber made of Yb-doped glass;
A fiber combiner provided on the excitation light incident side of the gain fiber;
A first optical fiber connecting the first semiconductor laser and the fiber combiner;
A second optical fiber connecting the first semiconductor laser and the fiber combiner;
A fiber laser amplifier comprising: the seed light source; and a third optical fiber connecting the fiber combiner. A seed light source that emits seed light;
A first semiconductor laser that emits first excitation light having a wavelength of 973 nm to 979 nm;
A second semiconductor laser that emits second excitation light having a wavelength of 880 nm to 970 nm;
A gain fiber made of Yb-doped glass;
A fiber combiner provided on the excitation light incident side of the gain fiber;
A first optical fiber connecting the first semiconductor laser and the fiber combiner;
A second optical fiber connecting the seed light source and the fiber combiner;
An optical element for guiding the first pumping light and the second pumping light to the gain fiber;
A fiber laser amplifier, wherein the first semiconductor laser, the second semiconductor laser, and the optical element are packaged.

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