DE102012012982A1 - Laser array, has fiber amplifier comprising laser source that exhibits specific beam parameter product, where laser radiation emitted by laser source couples into core of multi-mode fiber of fiber amplifier - Google Patents

Abstract

The array has a fiber amplifier comprising a laser source i.e. multi-mode diode laser stack (1), and a multi-mode fiber (4) i.e. core-pumped single or double shell fiber, where a laser radiation emitted by the laser source couples into a core of the multi-mode fiber of the fiber amplifier. The laser source exhibits a beam parameter product that amounts to 2.4-40 mm milliradian. The laser source is formed by fiber or solid state lasers. The multimode fiber of the fiber amplifier comprises a multi-shed cladding structure with a signal cladding and a pumping cladding.

Description

Technical application

The present invention relates to a laser arrangement comprising a laser source and a fiber amplifier with a multi-mode fiber, wherein laser radiation emitted by the laser source is coupled into the multimode fiber of the fiber amplifier and amplified there.

State of the art

Many applications based on the use of laser radiation require the guidance of the laser radiation via fibers, in particular glass fibers. With typical solid-state lasers, such as, for example, bar lasers, multimode laser radiation can be generated at average powers of more than 1 kW and pulse peak powers over 1 MW. However, the coupling of the laser radiation of these solid-state lasers into a transport fiber causes difficulties. In addition, the beam quality of such lasers is not sufficient for some applications.

For an improved coupling of laser radiation into transport fibers, fiber-based arrangements can be used, with which pulsed laser radiation with average powers of 100 watts and pulse peak powers up to 5 MW can be generated. In this case, diode lasers which emit fundamental-mode laser radiation are used as the laser source. For example, show Lombard et al., "Diode-pumped Yb doped large-core fiber amplifier with a multimode-to-singlemode photorefractive converter," in Photorefractive Effects, Materials, and Devices, P. Delaye, C. Denz, L. Mager , and G. Montemezzani, eds., Vol. 87 of OSA Trends in Optics and Photonics (Optical Society of America, 2003), paper 547 , a laser array comprising a fundamental mode diode laser and a fiber amplifier having a multi-mode fiber for generating pulsed fundamental mode laser radiation.

However, the radiation generated in this way often can not be transmitted with glass fibers over longer distances, since in the transmission non-linear effects occur, in particular stimulated Raman scattering. Furthermore, the amplification of the fundamental mode laser radiation to produce high output power requires a large number of amplifier stages, which cause a complex structure of the device.

The object of the present invention is to provide a laser arrangement with fiber amplifier, which in contrast has a less complex structure and allows the generation of pulsed and continuous laser radiation, which can be transmitted over longer distances in a transport fiber.

Presentation of the invention

The object is achieved with the laser arrangement according to claim 1. Advantageous embodiments of the laser arrangement are the subject of the dependent claims or can be found in the following description and the embodiments.

The proposed laser arrangement comprises a pulsed or continuously emitting laser source of one or more diode lasers or one or more fiber or solid state lasers and a fiber amplifier with a multimode fiber. Laser source and fiber amplifier are arranged such that the laser radiation emitted by the laser source is coupled into the core of the multimode fiber of the fiber amplifier, which is partially doped with laser-active atoms or ions, and amplified there. The multimode fiber may be pumped, for example, by one or more diode lasers or by one or more fiber lasers. In the case of a double-jacket or multi-layer fiber, the pumping light can be coupled in via a pump jacket provided for this purpose or else directly through the fiber ends into the core of the multimode fiber. Here, a multimode fiber is a fiber whose core has a combination of radius a and numerical aperture NA which satisfy the following equation at the wavelength λ of the laser radiation used: V = 2π · a · NA / λ >> 2,405.

Corresponding to a 7.5-fold diffraction-limited beam with a beam parameter product of SPP = 2.4 mm mrad at 1 μm, values of V ≥ 15 are to be understood here.

The proposed laser arrangement is characterized in that a multimode laser source is used as the laser source. A multimode laser source is to be understood as meaning a laser source which emits laser radiation with a multiplicity of independent modes or has a beam parameter product of SPP ≥ 2.4 mm mrad. This corresponds to a diffraction factor of M ² ≥ 7.5 at a wavelength of 1 μm. The diffraction factor M ² is defined by: M ² = ωφ / λ / π, where λ corresponds to the wavelength of the laser radiation, ω corresponds to the radius of the beam waist of the laser radiation and φ corresponds to half the aperture angle of the laser radiation in the far field.

The laser source can in this case be formed, for example, by a fiber laser, solid-state laser or a multimode diode laser, for example in the form of an edge emitter with an asymmetrical emission profile or as a VCSEL (vertical cavity surface-emitting laser). In a further embodiment, the laser radiation is superimposed by a plurality of emitters or diode lasers, which can be arranged, for example, in a bar or stack (multiemitter). These may then also be emitters which emit fundamental mode laser radiation. Likewise, the laser source can be realized by superposition of several fundamental mode fiber or solid state lasers.

The arrangement is particularly advantageous if the signal generated by the laser source in the form of one or more diode, fiber or solid-state lasers with a pulse duration in the range from 1 ps to 100 .mu.s is modulated, for example by direct modulation of the driver current of a diode. The pulsed laser radiation generated by the laser source is then coupled into the core of the multimode fiber of the fiber amplifier having at least one fiber section in which the core is doped with laser active atoms or ions and optically pumped via one or more pump lasers. The light of the laser source can either be coupled directly into the multimode fiber, the core of which is doped with such laser-active atoms or ions, or first in a non-laser-active multimode fiber or a corresponding fiber section, to which or then with the laser-active atoms or ions doped multimode fiber is directly or indirectly connected or coupled.

In an advantageous embodiment, a homogenizing fiber section for mixing the modes, which may also be formed as a separate multimode fiber, is used in front of the section of the multimode fiber doped with laser-active atoms or ions. With the homogenization in this fiber section and a top hat (rectangular shape) of the beam profile in the spatial and / or angular distribution can be achieved. The homogenization and support of a top-hat distribution can, for example, by using a non-circularly symmetric core of the fiber, z. B. D-shaped, quadrangular, hexagonal or octagonal, can be achieved.

By the here proposed combination of a multimode fiber in the fiber amplifier with a multimode laser source, hereinafter also referred to as a signal source, for example. From one or more diode lasers, higher average power and higher pulse power in the fiber amplifier can be produced with a smaller number of amplifier stages. Furthermore, this combination allows the generation of amplified multimode laser radiation transmission in transport fibers without the risk of non-linear effects, since in this case large core diameter can be used in the range of 50 to 400 microns over which distributes the power. This allows a less complex structure of the laser arrangement and the laser radiation can be transmitted over long distances in transport fibers. The use of diode lasers over Q-switched solid-state lasers has the further advantage that no pulse duration repetition rate coupling occurs in diode lasers. The pulse-pause ratio can be varied by the modulation of the drive current in a large range and is preferably at least 1: 100, wherein the pulse duration is preferably shorter than 100 microseconds. With such an arrangement, pulse peak powers of more than 1 kW can be generated with an average power of> 10 W.

The multimode fiber of the fiber amplifier can be designed as a shell-pumped double-cladding fiber, for example with a D-shaped or polygonal cladding, or as a core-pumped single or double cladding fiber. Of course, the reinforcement can be both single-stage and multi-stage, depending on the desired performance.

In addition, techniques for controlling the modes capable of propagation in the fiber can be used in the fiber amplifier. For example, the fiber may be underfilled and suitably wound to limit the number of modes that can be propagated. For example, for a numerical aperture of NA = 0.1 sufficient for the coupling and guiding of the laser radiation, a multimode fiber of the fiber amplifier with a core diameter of 100 μm and a numerical aperture of NA = 0.2 can be used for the underfilled operation to reach. By means of suitable radii of curvature when winding this fiber, it is then achieved that certain modes are not transmitted via the fiber.

In a further embodiment of the laser arrangement, a pump coupler is used, which couples the pump radiation into a pump casing of the multimode fiber against the signal direction, ie against the direction of the laser radiation to be amplified. In this case, a sufficient isolation of the pumping pump pump arms for the protection of the pump laser, in particular in the case of diode lasers as a pump laser, must be achieved. This is achieved either by underfilling the signal core, ie the core of the multimode fiber, or by a multi-clad structure of the multimode fiber of the fiber amplifier. By underfilling the signal core, in which the fiber has a larger numerical aperture than the coupled laser radiation, the proportion of the amplified laser radiation is reduced, which decouples from the signal core and thus can run back into the pumping arms. In a multiple cladding structure, the signal core is additionally enclosed by a signal jacket with a larger numerical aperture, which lies between the pump cladding and the signal core. This also ensures that a smaller proportion of the amplified laser radiation can enter the pumping arms.

As laser-active ions or atoms for the doping of the core of the multimode fiber, for example, elements such as ytterbium, neodymium, erbium, thulium or holmium can be used. Of course, this is not an exhaustive list. When ytterbium is used, a pump wavelength in the range of 900 to 1050 nm is required. When using shell-pumped double cladding fibers with ytterbium-doped core diameters of 100-400 μm, the large overlap of pump light in the cladding and the doped region of the fiber makes it possible to achieve a sufficiently high absorption even at pump wavelengths of ≥ 1000 nm, ie> 1 dB / m. Advantageous for this are ytterbium doping concentrations n _ges in the range of 5 · 10 ¹² to 12 · 10 ¹² m ^-3 in combination with doped core diameters of 100-400 microns and pumping sheath diameters of 400-600 microns, so that fiber lengths of 3-10 m with Pump wavelengths of 1000-1040 nm can be used. The advantage of these pump wavelengths is on the one hand in a reduced heat input into the active fiber by a smaller Stokes shift. On the other hand, when using these pump wavelengths, the inversion in the ytterbium-doped fiber is limited upwards by pumping transparency to z. B. 23.5% at a pump wavelength of 1000 nm or 8.4% at a pump wavelength of 1025 nm. Thus, the tendency of the fiber amplifier to photodarking ie the turbidity of the fiber is reduced by induced color centers, as well as for self-pulsing. Due to a limited inversion to approx. <25%, the occurrence of increased spontaneous emission (ASE) at a low duty cycle is also reduced.

Preferably, the laser source is selected to have a beam parameter product of at least 2.4 mm · mmrad in a 50 μm diameter core diameter multimode fiber having a numerical aperture of NA = 0.1 and at most 40 mm · mmrad at a core diameter of Multimode fiber of 400 μm and a numerical aperture of NA = 0.2.

With such a laser arrangement, laser radiation of high laser power can be generated, which can be transmitted through a glass fiber over more than 10 m, without non-linear effects occurring. All components of the fiber amplifier can be spliced together. The only remaining free jet route relates to the coupling of the signal laser, which, however, takes place at low average powers and is therefore not critical. The generated multimode radiation can be much easier to couple into transport fibers, so that the requirements for insulators, beam switches and fiber connectors decrease. Direct generation of a multi-mode, low-coherence signal can achieve higher signal homogeneity than is the case with concepts with a fundamental-mode signal source where the beam must be subsequently homogenized.

Brief description of the drawings

The present laser arrangement will be briefly explained again by means of exemplary embodiments in conjunction with the drawings. Hereby show:

1 an example of a laser arrangement according to the present invention;

2 Examples of the construction of the multimode fiber of the fiber amplifier;

3 further examples of the construction of the multimode fiber of the fiber amplifier; and

4 an example of the design of the multimode fiber of the fiber amplifier for the isolation of Pumparme.

Ways to carry out the invention

1 schematically shows an example of a possible embodiment of the present laser arrangement. In this example, a laser diode stack is used as the laser source 1 used from several laser diodes whose laser radiation is superimposed. The superimposition takes place by means of a corresponding beam-shaping element 2 that the radiation of the laser diodes of the laser diode stack 1 into a homogenization fiber 3 couples. In this homogenization fiber 3 The superimposed laser radiation is homogenized before it is removed from the homogenization fiber 3 in the reinforcing fiber 4 arrives. This reinforcing fiber 4 is a multimode fiber doped with laser active atoms or ions. The fiber is optically pumped via suitable pump laser (not shown in the figure) whose radiation is coupled via multimode pump coupler into the pump casing of the multimode fiber.

2 shows three examples of the construction of the multimode fiber in cross section. The multimode fiber or reinforcing fiber 4 consists of a signal core 5 made of quartz glass, by a pumping jacket 6 of quartz glass one opposite the signal core 5 surrounded by a lower refractive index. The pump coat 6 is in turn of a protective coat 7 surrounded by plastic. The three Examples show two polygonal and one D-shaped configuration of the pumping mantle 6 , through which a more even coupling of the pump light in the signal core 5 is reached.

3 shows an example of an embodiment of the multimode or amplification fiber 4 which achieves an additional mix of fashions and support the formation of a top-hat distribution in location and / or angle. Such a shaped core can also be used in the homogenization fiber 3 of the 1 be used. Again, the multimode fiber consists of the signal core 5 , the pump coat 6 and the protective mantle 7 , The signal core 5 However, quartz glass here does not have a circular-symmetrical cross-section, but is square in the left-hand example, and octagonal in the right-hand example. This deviation from the circular symmetry achieves the mixture of modes and the support of the formation of the top hat distribution.

4 finally shows a section of the multimode or amplification fiber 4 in the case of a coupled against the signal direction via a pump coupler pump light. In such a case, sufficient insulation of the pumping arms 8th of the pump coupler can be achieved in order to protect the pump diodes used as a pump laser, for example, before the coupling of amplified laser radiation. This can be achieved as in the present case by a multi-clad structure of the multimode fiber, which is a signal core 5 a signal coat 9 as well as the pump coat 6 having. As can be seen from the figure, the pump light 10 against the direction of the laser signal to be amplified 11 in the pump coat 6 coupled. Through the signal coat 9 is achieved that stray light 12 the amplified laser signal in the signal coat 9 is guided and not in the Pumparme 9 can couple the pump coupler. For this purpose, the signal coat 9 a larger numerical aperture than the signal core 5 exhibit. Thus, for example, with a signal core of 50 μm and a numerical aperture of NA = 0.1, a signal coat of 100 μm and a numerical aperture of NA = 0.2 and a pump coat of 400 μm with a numerical aperture of NA = 0.4 be used to achieve a suitable isolation.

LIST OF REFERENCE NUMBERS

1

laser diode stack

2

Beam shaping element

3

Homogenisierungsfaser

4

reinforcing fiber

5

signal core

6

pump casing

7

mantle

8th

pumping arm

9

signal coat

10

pump light

11

laser signal

12

scattered light

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Lombard et al., "Diode-pumped Yb doped large-core fiber amplifier with a multimode-to-singlemode photorefractive converter," in Photorefractive Effects, Materials, and Devices, P. Delaye, C. Denz, L. Mager , and G. Montemezzani, eds., Vol. 87 of OSA Trends in Optics and Photonics (Optical Society of America, 2003), paper 547 [0003]

Claims (14)

Laser arrangement with fiber amplifier, comprising a laser source ( 1 ) and a fiber amplifier with a multimode fiber ( 4 ), wherein from the laser source ( 1 ) emitted laser radiation in a core ( 5 ) of the multimode fiber ( 4 ) of the fiber amplifier, characterized in that the laser source ( 1 ) is a multimode laser source having a beam parameter product of SPP ≥ 2.4 mm mrad. Laser arrangement according to claim 1, characterized in that the laser source ( 1 ) is formed by a multi-mode diode laser. Laser arrangement according to claim 1, characterized in that the laser source ( 1 ) is formed by a plurality of diode lasers whose laser radiation is superimposed. Laser arrangement according to claim 1, characterized in that the laser source ( 1 ) is formed by a fiber or solid state laser or by a plurality of fiber or solid state laser whose laser radiation is superimposed. Laser arrangement according to one of claims 1 to 4, characterized in that the beam parameter product of the laser source is between 2.4 and 40 mm · mrad. Laser arrangement according to one of claims 1 to 5, characterized in that the laser source is a temporally modulated operated laser source. Laser arrangement according to one of claims 1 to 6, characterized in that the multimode fiber ( 1 ) of the fiber amplifier at least one in the core ( 5 ) with laser-active atoms or ions-doped fiber section, in front of which a laser beam which homogenises the coupled-in laser radiation (US Pat. 3 ) is arranged. Laser arrangement according to claim 7, characterized in that the laser radiation homogenizing fiber portion ( 3 ) has a non-circular cross-section core ( 5 ) having. Laser arrangement according to one of Claims 1 to 8, characterized in that the multimode fiber ( 4 ) of the fiber amplifier, a shell-pumped double or multi-layer fiber with D-shaped or polygonal pump jacket ( 6 ). Laser arrangement according to one of claims 1 to 9, characterized in that the multimode fiber ( 4 ) of the fiber amplifier a Mehrfachmantelstruktur with a signal coat ( 9 ) and a pumping jacket ( 6 ), in which the signal jacket ( 9 ) one opposite the core ( 5 ) of the multimode fiber ( 4 ) has increased numerical aperture. Laser arrangement according to one of claims 1 to 9, characterized in that the multimode fiber ( 4 ) of the fiber amplifier is connected to a pump coupler, via the pump light counter to a propagation direction of the laser source ( 1 ) in the core ( 5 ) coupled laser radiation is coupled, wherein the core ( 5 ) of the multimode fiber ( 4 ) has a higher numerical aperture at least in the region of the coupling of the pumping light than it has for the coupling and guiding of the laser radiation of the laser source ( 1 ) is required. Laser arrangement according to one of Claims 1 to 8, characterized in that the multimode fiber ( 4 ) of the fiber amplifier is a core-pumped single or double cladding fiber. Laser arrangement according to one of claims 1 to 12, characterized in that the core ( 5 ) of the multimode fiber ( 4 ) of the fiber amplifier has a higher numerical aperture than that for the coupling and guiding of the laser radiation of the laser source ( 1 ) and wound to limit the number of propagatable modes in the fiber. Laser arrangement according to one of Claims 1 to 13, characterized in that the multimode fiber ( 4 ) of the fiber amplifier is a cladding-pumped double cladding fiber having an ytterbium-doped core whose core diameter is between 100 and 400 μm, the laser arrangement comprising a pump laser which pumps the fiber amplifier at a pump wavelength of 1000 nm.

Images