WO2016169933A1

WO2016169933A1 - Optical resonator for a high-power fibre laser

Info

Publication number: WO2016169933A1
Application number: PCT/EP2016/058666
Authority: WO
Inventors: Ulrich Hefter
Original assignee: Rofin Sinar Laser Gmbh
Priority date: 2015-04-20
Filing date: 2016-04-19
Publication date: 2016-10-27

Abstract

The invention relates to an optical resonator (1) for a high-power fibre laser having an optically active medium which, according to the invention, comprises a multimode fibre (2) having a fibre core (10) doped at least in sections, wherein the multimode fibre (2) is arranged between a highly reflective first reflection element (3) and an at least partly transmissive second reflection element (4). The first reflection element (3) comprises a dielectric mirror (5). The invention furthermore relates to a fibre laser (100) comprising an optical resonator (1) embodied in this way. The first reflection element (3) can be embodied as a curved dichroic mirror, which firstly simplifies the coupling-in of the pump radiation from a pump module (7) and also reduces the resonator losses. The second reflection element (4) can be embodied as a partly reflective fibre Bragg grating.

Description

Title: Optical resonator for a high-power fiber laser

description

The invention relates to an optical resonator for a high-power fiber laser with an optically active medium comprising a fiber having an at least partially doped fiber core, wherein the fiber between a highly reflective first reflection element and an at least partially transmitting second reflection element is arranged.

Optical resonators for generating laser radiation are well known from the prior art. For this purpose, it is particularly known to define an optical resonance space between two reflection elements, which is optically pumped to generate laser radiation. In the case of fiber lasers, the optically active medium of the optical resonator is formed by an active fiber which has an at least partially doped fiber core. A reflection element arranged on the input side of the optical resonator is generally highly reflective in order to minimize the losses occurring, while a further output-side reflection element is at least partially transmissive in order to enable a coupling out of the laser radiation generated in the resonance space. The second reflection element can therefore also be referred to as an output coupling element (English: Output coupler).

For many laser applications, the generation of ideally one-mode laser radiation with at least approximately Gaussian beam profile is desirable. However, in order to suppress the generation of higher modes, the core diameter of the fiber serving as the optically active medium must be limited to a value only in the range of a few multiples the wavelength of the generated laser radiation is. A division of fibers into monomode or multimode fibers can be made on the basis of the modality. With a modality of less than 2.4, a single-mode fiber is present, with larger values, the fiber is a multimode fiber. The modality of a fiber is defined by the V-number:

Here, λ denotes the wavelength in vacuum, a the diameter of the fiber core, and NA the numerical aperture, which is correspondingly defined by the refractive indices of the fiber core n _cor e and the clad n _c iaddin _g .

Since the V-number or the modality is a function of the wavelength, corresponds to a modality of 2.4 at a wavelength of 1 μΐτι about a core diameter of 13 μΐτι. In general, therefore, the core diameter of single-mode fibers in the range of less than 20 μΐτι. In conjunction with non-linear effects, such as the Raman effect or self-phase modulation, which occur in such monomode fibers, this limits the output power of the laser to about 2 kW to 3 kW.

This limitation is particularly disadvantageous in the field of high-performance applications such as laser welding. To provide a larger output power, among other things, a parallel coupling of several single-mode fiber lasers has been proposed. Other approaches relate to the optical pumping of the primary optical resonator by means of further fiber lasers. In so-called Mopa arrangements (English: master oscillator power amplifier), an optical amplifier of a laser array is connected downstream. However, these concepts are generally relatively complicated, expensive to implement, and in part, are associated with a reduction in efficiency. It is an object of the present invention to avoid the mentioned disadvantages of the devices known in the prior art and to provide an optical resonator for a high-power fiber laser with high output power, which has a particularly simple design.

The object is achieved by an optical resonator of the aforementioned type with the characterizing features of patent claim 1.

Advantageous embodiments of the invention are the subject of the subclaims.

In an optical resonator for a high-power fiber laser, an optically active medium according to the invention comprises a multimode fiber having an at least partially doped fiber core. The multimode fiber is arranged between a highly reflective first reflection element and an at least partially transmitting second reflection element. The first reflection element comprises a dielectric mirror.

The invention is based on the observation that the above-mentioned nonlinear effects increasingly occur in long fibers with comparatively small core diameters. As such, the nonlinear effects can be suppressed by using fibers with larger core diameters. As a result, the output power can be increased, since potential damage caused by the nonlinear effects only occur at correspondingly increased intensities. Moreover, with the increased core diameter (improved core / sheath ratio) compared to the sheath, there is an increased absorption which allows for the use of shorter fibers. These shorter fibers advantageously serve to further reduce the nonlinear effects.

However, such fibers are no longer suitable for the propagation of single-mode laser radiation, but correspond to the core diameter and numerical apertures of the fibers used multimode fibers, including the Favor education of higher fashions. However, optical resonators with multimode fibers produce comparatively broadband laser radiation, so that increased loss occurs at the fiber Bragg gratings suitable for multimode operation. Often, only a reflection of about 50% of such reflection elements is achieved. This is unsatisfactory especially for the training for highly reflective reflection elements.

The laser radiation generated by the optically pumped multimode active fiber according to the invention has an increased spectral bandwidth compared to single-mode laser radiation. This leads to increased losses at the reflection elements delimiting the optical resonance space. In particular, the loss occurring on the input-side reflection element is disadvantageous since this reflection element is preferably highly reflective in order to minimize power losses. However, the fiber Bragg gratings typically provided as reflective elements are highly reflective only for a narrowband spectral range. In addition, the reflection behavior of fiber Bragg gratings is mode-dependent, so that increased losses occur in multimode fibers. The invention thus proposes to support the multi-mode operation, the first reflection element, which is arranged on the input side of the optical resonance chamber, ie at the end from which pump radiation is coupled into the resonance chamber during operation to perform as a dielectric mirror. Dielectric mirrors are highly reflective for a larger spectral range and, in contrast to fiber-optically implemented Bragg gratings, have no pronounced mode dependence of the reflection behavior. As a result, therefore, the losses occurring on the input side of the optical resonator can be minimized if multimode laser radiation is generated in the optical resonator.

Such an optical resonator is suitable for high-power fiber lasers with output powers of more than 2 kW, in particular more than 3 kW. In other words, the resonant space formed between the first and second reflective elements is formed immediately by a mu-mode fiber. In the optical resonator no means, such as etalons or the like, are introduced, which would be suitable for limiting the oscillation behavior of the resonator essentially to a single-mode operation. Thus, contrary to the conventional teaching, the invention proposes to support a true multi-mode operation in which a multiplicity of modes are amplified in the resonator. This direct approach makes it possible to further improve the efficiency of the optical resonator or of the high-power fiber laser comprising this optical resonator. As a result, greater output power can be used.

The optical resonator according to the invention is preferably a primary resonator. An amplification of the laser beam generated in the optical resonator is not absolutely necessary.

The V-number defined at the outset, which characterizes the modality of the mu ltimode fiber of the optical resonator, is preferably 5 or more. Particularly preferably, the V-number of the multimode fiber assumes a value between 5 and 10. It has been found that in this area enough modes flock, so that Stability problems are avoided.

The multimode fiber is preferably designed so that not only a few transversal modes can oscillate but a plurality of transverse modes. The number of oscillating transverse modes is preferably at least 10.

It has been found that sufficient reflectivity can be provided at the outcoupling side of fiber Bragg gratings when the V-number of the ultimode fiber is in the range between 5 and 10. The losses occurring at the outcoupling side then remain below a critical threshold, so that the resonance conditions relevant for the formation of a stable resonator can be satisfied. The at least partially transmitting second reflection element provided for decoupling of laser radiation amplified in the optical resonator is preferably designed as a fiber Bragg grating. A decoupling of laser radiation on the output side is comparatively uncritical, so that reflection elements can be used here whose reflectivity averaged over the bandwidth is, for example, only 10%. The fiber Bragg grating is preferably written inexpensively in the corresponding region of the multimode fiber.

In preferred embodiments, the dielectric mirror is disposed on a quartz support member. Quartz has relatively low absorption for the wavelengths typically employed, and is thus particularly suitable for high power applications where the heating generated by the laser power must be minimized. According to mutually alternative embodiments, the carrier element is block-shaped or tubular. In any case, with the integrated directly on the support element dielectric mirror is given a monolithic structure, which allows a particularly easy and easy to adjust mounting. Such training can be implemented particularly well as a fiber optic overall concept.

In a development of the invention, it is provided to form the dielectric mirror concavely in the direction of a coupling-in side of the multimode fiber. The radius of curvature of the dielectric mirror is selected so that the laser radiation emerging from the fiber core of the multimode fiber is reflected back onto it. With correspondingly thin fiber cores, the dielectric mirror has a substantially constant radius of curvature. The multimode fiber of the optical resonator is preferably at least partially doped with a rare earth metal, in particular erbium, Er (1.5μ), ytterbium, Yb (1μ), holmium, Ho 21μ), thulium and / or neodymium. A core diameter of the fiber core of the multimode fiber is according to preferred embodiments, at least 50 μΐτι, in particular 50 μΐτι to 100 μΐτι. Accordingly, a shell surrounding the fiber core, can be coupled via the pump radiation in the fiber core, a thickness of several hundred μΐτι, in particular 400 μΐτι on.

The invention further relates to a fiber laser with an optical resonator described above. The fiber laser includes a pump module for optically pumping the multimode active fiber of the optical resonator.

In the following, possible embodiments of the invention will be explained in more detail with reference to the drawings. Showing:

1 shows an optical resonator according to a possible embodiment of the invention in a schematic representation,

2 shows a first reflection element limiting the optical resonator on the input side according to a first embodiment, in a schematic sectional illustration,

3 a first reflection element limiting the optical resonator on the input side according to a second embodiment, in a schematic sectional illustration.

Corresponding parts are provided in all figures with the same reference numerals.

FIG. 1 shows a fiber laser 100 having an optical resonator 1 comprising a multi-mode active fiber 2. A fiber core of the multimode fiber 2 is partially doped with neodymium and has a core diameter of 50 μΐτι on.

The resonant space of the optical resonator 1 is limited by two reflection elements 3, 4. On the input side, a first reflection element 3 is provided which comprises a dielectric mirror 5 which is monolithic Construction is integrated on a support element 6 made of quartz. Possible embodiments of the first reflection element 3 are shown in detail in the sectional views of Figures 2 and 3. The second reflection element 4, which is arranged on the output side of the optical resonator 1 and shown in FIG. 1, is designed as a fiber Bragg grating, which in the example shown has a transmission for the multimode laser radiation of about 10% produced during operation. The optical resonator 1 is coupled on the input side to a pump module 7. The pump module 7 comprises, in a manner known per se, means for generating a population inversion in the active multimode fiber 2. For this purpose, it is provided to couple pump radiation into the active multimode fiber 2, which is generated, for example, by laser diodes.

On the output side, a transport fiber 8 connects to the serving as an output coupler second reflection element 4, by means of which the generated multimode radiation can be directed in particular for material processing on a workpiece.

FIG. 2 shows a first exemplary embodiment of the first reflection element 3 in monolithic construction. The dielectric mirror 5 is arranged on the end of the support element 6, which is designed in the example shown as a solid quartz block and concave curved towards a Einkoppelseite 9 of the multimode fiber 2. The radius of curvature of the dielectric mirror 5 in this case corresponds to an optical length which results from the longitudinal length L of the carrier element 6 taking into account the refraction.

The multimode fiber 2 is connected in a form-fitting manner by splicing to the carrier element 6. The core diameter d of the fiber core 10 is 50 μΐτι, the cladding diameter D of the fiber core 10 surrounding shell 11 is 400 μΐτι. Such a dimensioning of the multimode fiber 2 favors the formation of multimode laser radiation in common gene fiber dopings. The ultimode fiber 2 furthermore has a protective coating 12 which forms the jacket 11.

FIG. 3 shows a second embodiment of the first reflection element 3. The dielectric mirror 5 is fixed on the end to the support element 6, which has a tubular shape. Also, the carrier element 6 of the second embodiment is made of quartz. The curvature ius of the dielectric mirror 5 preferably corresponds to the distance of the dielectric mirror 5 from the coupling side 9 of the ultimode fiber 2, which corresponds to the longitudinal length L. The distance of the dielectric mirror 5 from the coupling side 9 can be particularly easily adjusted by an axial displacement of the tu bus-shaped reflection element 3bezügl the multimode fiber 2. The dimensioning of the core diameter d of the fiber core 10 and the cladding diameter D of the shell 11 essentially corresponds to the dimensioning of the embodiment shown in FIG. 2, so that here too the formation of multimode laser radiation is favored. The invention has been described above with reference to preferred embodiments. However, it should be understood that the invention is not limited to the specific embodiment of the exemplary embodiments shown; rather, the person skilled in the art can derive variations from the description without departing from the essential basic idea of the invention. Reference number list

1 optical resonator

2 mu ltimode fiber

3 first reflection element

4 second reflection element

5 dielectric mirror

6 support element

7 Pu mpmodu l

8 transport fiber

9 coupling side

10 fiber core

11 coat

12 protective coating

100 fiber lasers

L length

d Core Diameter

D jacket diameter

Claims

claims

1. An optical resonator (1) for a high-power fiber laser with an optically active medium, comprising an at least partially doped fiber core (10) comprising multimode fiber (2), wherein the multimode fiber (2) between a highly reflective first reflection element (3) and an at least partially transmitting second reflection element (4) is arranged, wherein the first reflection element (3) comprises a dielectric mirror (5).

Optical resonator (1) according to claim 1, characterized in that the V-number characterizing the multimode fiber (2) is 5 or more, preferably between 5 and 10.

Optical resonator (1) according to claim 1 or 2, characterized in that the second reflection element (4) is designed as a fiber Bragg grating.

Optical resonator (1) according to one of the preceding claims, characterized in that the dielectric mirror (5) is arranged on a support element (6) made of quartz.

Optical resonator (1) according to claim 4, characterized in that the carrier element (6) is block-shaped or tubular.

6. An optical resonator (1) according to one of the preceding claims,

characterized in that the dielectric mirror (5) is concavely curved in the direction of a coupling-in side (9) of the multimode fiber (2).

Optical resonator (1) according to one of the preceding claims, characterized in that the multimode fiber (2) is at least partially doped with a rare earth metal, in particular erbium, ytterbium and / or neodymium. Optical resonator (1) according to one of the preceding claims, characterized in that a core diameter (d) of the fiber core (10) of the multimode fiber (2) at least 50 μΐτι, in particular 50 μΐτι to 100 μΐτι amounts.

9. fiber laser (100) comprising a multi-mode fiber (2) having optical resonator (1) according to one of the preceding claims and a pump module 7 for optically pumping the multimode fiber (2).