DE102011055130A1 - Laser device with an optically active material having multimode optical fiber - Google Patents

Abstract

A laser device has a multimode fiber (1) with optically active material in its laser core (4). At one end of the multimode fiber (1), a singlemode fiber (2) is optically coupled. In the singlemode fiber (2), a fiber Bragg grating (8) is inscribed as a mirror for photons of a given laser wavelength. At the other end of the multimode fiber (1), a reflection of photons back into the multimode fiber (1) preventing outcoupling window (9) is provided.

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a laser device with a multimode optical fiber having optically active material with a singlemode fiber optically coupled to one end of the multimode optical fiber and having a fiber Bragg grating inscribed in the singlemode fiber as a mirror for photons of at least one particular one laser wavelength. A single-mode fiber is to be understood as meaning such an optical fiber which only conducts a transverse mode with respect to all relevant wavelengths of the emission spectrum of the optically active material.

STATE OF THE ART

A laser device of the type described above, ie with the features of the preamble of independent claim 1, is known from US Pat. No. 6,625,182 B1 known. Here, the multimode optical fiber is formed as a solid-state laser rod of optically active material, such as Nd: YAG. The optically active material is embedded in a resonator. At one end of this resonator, a singlemode fiber with a written-fiber Bragg grating is provided as a mirror. The opposite end of the solid state laser rod is coated with a mirror through which the optically active material is pumped. Laser light from the resonator is coupled through the fiber Bragg grating into the singlemode fiber continuing behind the Bragg grating. The singlemode fiber with the inscribed fiber Bragg grating provides in the resonator of the known laser device for a selective reflection of photons of the transverse fundamental mode with a certain, predetermined by the fiber Bragg grating laser wavelength back into the optically active medium. The optically active material thus experiences enhanced feedback on the transverse fundamental mode and at the predetermined laser wavelength. The resonator further enhances this effect. However, the laser light only adopts the properties of the selectively reflected photons as far as the active solid-state laser rod allows the amplification of these properties. In addition, in order to obtain a high gain of the selectively reflected photons, the resonator must provide a large number of reflections since the solid-state laser rod is relatively short. That is, both mirrors of the resonator must reflect significantly. The laser light of the known laser device coupled out to a certain extent via the singlemode fiber behind the fiber Bragg grating should for example be used to optically pump an erbium-doped fiber amplifier.

For fiber lasers based on a double-core fiber with active laser core, it is known that the excitation energy spatially distributed in the active laser core can be degraded more efficiently by higher modes than only by a transverse fundamental mode. The higher modes have a larger overlap with the pump volume, especially in the side areas. A fiber laser with multimode laser core is therefore far more efficient in terms of converting the pump energy to laser light than a singlemode laser core. However, the fabrication of fiber Bragg gratings in multimode cores is known to be significantly more difficult than in singlemode cores, so it is difficult to build a multimode resonator with integrated mirrors for a given laser wavelength. In order to operate a fiber laser with a multimode active fiber nevertheless at a certain laser wavelength, it is known to use an oscillator / amplifier arrangement. As an oscillator used z. Example, a single-mode fiber laser whose resonator mirrors are formed by two fiber Bragg gratings. At the partially transmissive grating, laser light enters the active multimode fiber at the laser wavelength given by the singlemode fiber laser and at the fundamental transversal mode. The active multimode fiber then only works as an amplifier, which is triggered by the oscillator, which thus operates virtually as a seed laser, at the laser wavelength in the transverse fundamental mode. Usually, a special optics is added between the singlemode oscillator and the multimode amplifier to adjust the mode relationships. The pumping of the optically active material in the laser core of the multimode fiber can be known to occur over any or both ends of the multimode laser fiber.

OBJECT OF THE INVENTION

The invention has for its object to show a simply constructed narrow-band emitting fiber laser with high overall efficiency.

SOLUTION

The object of the invention is achieved by a laser device having the features of independent patent claim 1. Preferred embodiments of the laser device according to the invention are defined in the dependent claims.

DESCRIPTION OF THE INVENTION

A laser device according to the invention has a multimode fiber with optically active material in its laser core. At one end of the multimode fiber, a singlemode fiber is optical coupled. In the singlemode fiber, a fiber Bragg grating is inscribed as a mirror for photons of a laser wavelength. At the other end of the multimode fiber there is provided a reflection of photons back into the multimode fiber disengaging window.

From the out of the US Pat. No. 6,625,182 B1 Thus, the laser device according to the invention deviates not only from the fact that the multimode optical fiber is not a solid-state rod but a multimode fiber. In addition, there is the difference that laser light generated by the laser device is coupled out at the end of the multimode optical waveguide opposite the singlemode fiber and not via the singlemode fiber. Furthermore, the laser device according to the invention differs in that the coupling window used has as far as possible no reflection back into the multimode fiber. The latter difference is therefore equivalent to the fact that the laser device according to the invention has no laser resonator limited by mirrors at its two ends.

The lack of reflection of photons back into the multimode fiber at the opposite end of the singlemode fiber serves to purposefully exert no influence on the generated laser light from this other end. That is, it is specifically avoided that photons of any wavelength and any mode are reflected into the multimode fiber and amplified there, because this would at least result in unnecessary consumption of pump energy.

At the other end of the multimode fiber, the singlemode fiber with the inscribed fiber Bragg grating as a mirror causes only photons of the desired laser wavelength to be reflected back at this end and back into the multimode core in the transverse fundamental mode. Such photons occur when pumping the optically active material of the multimode fiber as part of the spectrum of spontaneous emission on its own. This portion of the spectrum is positively selected over the other wavelengths and modes by the singlemode fiber with the inscribed fiber Bragg grating and sent back into the multimode fiber. There, the photons undergo induced amplification over the entire length of the multimode fiber in a single pass until they exit the outcoupling window at the other end of the multimode fiber as amplified laser light.

The laser device according to the invention does not only come out without a mirror at the other end of the multimode fiber; but un-suppressed reflection of photons back into the multimode fiber even interferes with the desired emission of the inventive laser device at the desired laser wavelength in the fundamental mode. For this reason, it is preferred if the reflection of the decoupling window into the multimode fiber at all relevant wavelengths of the spontaneous emission spectrum of the optically active material of the multimode fiber is at most 1%, preferably at most 0.5%. This can be achieved concretely in that an exit surface of the coupling-out window is antireflected for the relevant wavelengths. The same goal is achieved when a surface of the coupling-out window, which has a residual reflection, is inclined or curved relative to the axis of the multimode fiber in order to prevent this reflection back into the multimode fiber.

Preferably, in the laser device according to the invention, the coupling-out window is not formed directly at the end of the multimode fiber, for example by an anti-reflective coating, but in the form of an additional optical element which is connected to the multimode fiber with a reflection-free transition. The person skilled in appropriate techniques are known to which a so-called splicing counts.

Likewise, the singlemode fiber is preferably connected to the multimode fiber with a reflectionless transition. It can also occur a significant jump in the diameter of the cores between the multimode fiber and the singlemode fiber. In fact, in the case of the laser device according to the invention, it is preferable to dispense with a so-called adiabatic adaptation between the diameters of the two cores, for example by means of an interposed telescope. Thus, the core of the singlemode fiber absorbs only a proportion of the light passing through the core of the multimode fiber to the singlemode fiber, which proportion corresponds to the cross-sectional ratio of the two cores. Typically, the diameter of the laser core of the multimode fiber is at least 1.5 times the diameter of a core of the singlemode fiber, which on the one hand differentiates the two types of fibers and, on the other hand, in the multimode fiber, the coupling of large amounts of pump energy in the Laser core relieved. In the laser device according to the invention, the ratio of the two core diameters to each other can also be slightly 3 or higher.

The multimode fiber and / or the singlemode fiber of the laser device according to the invention may be, in particular, double-core fibers having a refractive index difference between their inner core, which is also referred to herein as a core, and its outer core or cladding. These are usually so-called quartz-quartz fibers. In principle, however, other multimode fibers and singlemode fibers, For example, with a structured core with gas-filled spaces usable. In the multimode fiber, the inner core of the double-core fiber with the optically active material forms the laser core, while the outer core or cladding guides the pumping light for pumping the optically active material and is therefore also referred to as pumping jacket.

The laser core of the multimode fiber may in particular be doped with a rare earth ion, such as ytterbium, erbium, thulium or holmium. The doping is matched to the desired laser wavelength and the available pump light.

In principle, the core of the singlemode fiber can also be doped. Optionally, the same doping as in the case of the multimode fiber is suitable. However, the expense of doping and pumping the singlemode fiber is often not in favorable relation to the thus additionally achievable improvement of the laser properties.

A pump light source of the laser device according to the invention can couple pumping light through the singlemode fiber into the multimode fiber. In this case, the fiber Bragg grating in the singlemode fiber, since typically is not pumped at the laser wavelength, no obstacle to the pump light. Alternatively or additionally, a pump light source may be provided with the pump light through the coupling window in the multimode Fiber is coupled. This is favorable due to the only one-time passage of the laser light through the multimode fiber, since this is pumped quasi in countercurrent to the ever-increasing laser light. The stronger the laser light already is, the more pump energy it can take over by its further amplification. It is understood that the pumping light is in any case not only passed over the core, but also over the entire adjacent jacket of the respective fiber.

Typically, a pump light source of the laser device according to the invention comprises at least one laser diode. The length of the multimode fiber is typically tuned to the absorption length of the pump light in the multimode fiber, i. H. at least about the same size as this absorption length. However, the length of the multimode fiber can also be selected according to the criteria of low reabsorption of the already generated laser light. This may result in other optimally suitable fiber lengths.

The optically active material of the laser device according to the invention is preferably pumped continuously, and the laser light then emerges correspondingly continuously from the coupling-out window.

By a suitable choice of a diameter of the laser core or the numerical aperture of the multimode fiber of the laser device according to the invention, care can be taken that, when pumping the optically active material in the laser core of the multimode fiber, a singlemode laser beam emerges from the coupling-out window. The diameter of the laser core and the numerical aperture are to be chosen comparatively small for this purpose. A singlemode laser beam can be advantageously used for various applications, for example in material processing, in medicine, in measurement technology or in corresponding scientific applications.

If a larger diameter of the laser core with a higher numerical aperture of, for example, greater than 0.1 is selected, on the other hand, a multimode laser beam with several transverse modes emerges from the coupling-out window. This is particularly the case when the multimode fiber has only small losses for the higher modes and, according to mode locking and mode crosstalk, strongly occurs. What is special here, however, is that the higher transverse modes induced and amplified in a single pass through the multimode fiber have the same wavelength as the transverse fundamental mode, i. H. the desired laser wavelength. Would the higher modes of a fiber Bragg grating, z. From a grating in an attached multimode fiber, they would have a significant frequency separation from the fundamental mode and from each other.

In addition, the laser device according to the invention exhibits a particularly high overall efficiency when a multimode laser beam is output, because through the involvement of the higher modes, the volume of the laser core of the multimode fiber in which the optically active material is pumped is better utilized than merely by the transverse fundamental mode becomes. Thus, in the one-time pass through the multimode fiber by means of the higher modes, much additional pump energy is absorbed into the laser light.

The fiber Bragg grating in the singlemode fiber of the laser device according to the invention can be photochemically inscribed therein. Such firing of the fiber Bragg grating can be done, for example, with the aid of interference patterns of high-energy laser radiation. The person skilled in appropriate techniques are known.

Usually, the fiber Bragg grating will reflect at only a certain laser wavelength, and the laser light emitted by the laser device according to the invention will then only be this one wavelength up. In principle, however, the fiber Bragg grating can also be designed so that it reflects not only at one but at several specific laser wavelengths. The laser light emitted by the laser device according to the invention then has these several laser wavelengths, insofar as they are supported by their optically active material. In this way, the overall efficiency of the laser device according to the invention can be further increased by exploiting an even larger volume of the pumped optically active material, without overloading parts of the multimode fiber by excessive electromagnetic field strengths. An example of fiber Bragg gratings reflecting at several particular laser wavelengths are so-called chirped gratings in which the refractive index variation has a modulated period length.

A typical laser device according to the invention comprises a double-core fiber whose inner core is designed as a multimode optical waveguide and doped with optically active material. The pump casing surrounding the laser core guides the pump radiation. At one end of the multimode optical fiber is a dual-core fiber with passive single-mode core optically coupled. In the singlemode core, a fiber Bragg grating is inscribed as a mirror for photons of a given laser wavelength. The other end of the multimode optical fiber is prepared in such a way that the optical end surface of the multimode core reflects as little radiation back into the laser core as possible. Pump radiation, which is coupled via the end surfaces or the lateral surfaces, is guided by means of the outer jacket. Wavelength- and mode-selective feedback in the singlemode core achieves targeted and selective self-excitation of the multimode core, which converts the excitation energy of the optically active region into a laser beam efficiently and only in one direction and at one wavelength.

Advantageous developments of the invention will become apparent from the claims, the description and the drawings. The advantages of features and of combinations of several features mentioned in the introduction to the description are merely exemplary and can come into effect alternatively or cumulatively, without the advantages having to be achieved by embodiments according to the invention. Without thereby altering the subject matter of the appended claims, as regards the disclosure of the original application documents and the patent, further features can be found in the drawings, in particular the illustrated geometries and the relative dimensions of several components and their relative arrangement and operative connection. The combination of features of different embodiments of the invention or of features of different claims is also possible deviating from the chosen relationships of the claims and is hereby stimulated. This also applies to those features which are shown in separate drawings or are mentioned in their description. These features can also be combined with features of different claims. Likewise, in the claims listed features for further embodiments of the invention can be omitted.

BRIEF DESCRIPTION OF THE FIGURES

In the following the invention will be further explained and described with reference to preferred embodiments shown in the figures.

1 shows a basic structure of the laser device according to the invention.

2 to 4 show possible embodiments of a fiber splice and a decoupling window of the laser device according to 1 ; and

5 to 7 show possibilities of optical pumping of optically active material of the laser device according to 1 ,

DESCRIPTION OF THE FIGURES

The illustrations of the figures are not to scale. In particular, the diameters of the components of the laser device according to the invention are compared to their length too large reproduced.

1 shows a multimode fiber 1 and a singlemode fiber 2 standing at a fiber splice site 3 connected to each other. The multimode fiber 1 and the singlemode fiber 2 are double-core fibers. The multimode fiber 1 has a laser core 4 with optically active material and a pumping jacket 5 on. The singlemode fiber 2 includes a core 6 and a coat 7 , The core 6 The singlemode fiber has such a small diameter that it conducts only the transverse fundamental mode of the emitted light in the relevant emission spectrum of the optically active material. The laser core 4 has a larger diameter and therefore also conducts higher transverse modes. In the singlemode fiber 2 is a fiber Bragg grating 8th enrolled. At the singlemode fiber 2 opposite end of the multimode fiber 1 is a coupling window 9 intended for laser light. The coupling-out window is designed so that a reflection of laser light back into the multimode fiber 1 is suppressed.

In the following 2 to 4 is on the left an embodiment of the fiber splice site 3 and on the right an embodiment of the decoupling window 9 in opposite 1 shown enlarged. The various embodiments of the fiber splice site 3 can be arbitrary with the different embodiments of the coupling-out window 9 be combined.

2 shows a fiber splice site 3 at which the multimode fiber 1 and the singlemode fiber 2 both with cores 4 and 6 different diameter as well as with coats 5 and 7 different diameter to form a step 10 are joined together. The advantage of such an arrangement is that the respective thinner fiber is cheaper to obtain. However, at this fiber splice site 3 Under certain circumstances pump light from the pump jacket 5 decoupled.

In the embodiment of the decoupling window 9 according to 2 is the optical interface 11 at the output of the multimode fiber 1 abpoliert obliquely.

3 also shows a fiber splice site 3 at which the multimode fiber 1 and the singlemode fiber 2 both with cores 4 and 6 different diameter as well as with coats 5 and 7 different diameter to form a step 10 are joined together, in which case the diameter of the jacket 7 the singlemode fiber 2 is larger. There is a risk that pumping light, via the singlemode fiber 2 into the multimode fiber 1 is to be coupled, at the fiber splice site 3 from the coat 7 couples out.

The decoupling window 9 according to 3 has one with the aid of a dielectric coating 12 non-reflective decoupling surface at the end of the multimode fiber 1 on, the reflections of laser light into the multimode fiber 1 better prevented than an obliquely abpolierte interface.

4 shows a fiber splice site 3 at which the multimode fiber 1 and the singlemode fiber 2 despite cores 4 and 6 different diameter without step are joined together by the coats 5 and 7 have the same diameter. So exists at the fiber splice site 3 no risk of pump light leakage, and the optically active material can easily pass from both ends of the multimode fiber 1 be stimulated.

The decoupling window 9 according to 3 has an end cap 13 on that yourself with a layer 14 is anti-reflective. This layer 14 Can additionally on an oblique interface of the end cap 13 be upset. The 5 to 7 show possibilities for optical excitation of the optically active material in the laser core 4 the multimode fiber 1 , The with the siglemode fiber 2 connected multimode fiber 1 can either only from one end with pump light from a pump light source 15 be supplied ( 5 and 6 ) or simultaneously from both ends. In addition, the supply of pumping light over the two ends of the lateral surface of the pumping mantle 5 possible ( 7 ).

To the laser device with the in 1 The following fundamental considerations have been made and experimental investigations carried out. The aim of the experimental investigations was to develop a laser device with very high efficiency. The efficiency of the laser device was compared to other often preferred properties, such. B. the beam quality, in the foreground. Since the excited volume is better utilized by a multimode laser than by a single-mode laser, in general, the efficiency of a laser in multimode operation is also higher than in singlemode operation. The investigations therefore took place on a multimode fiber laser.

The optimization was carried out using the example of a thulium fiber laser, which can emit between 1.9 μm and 2.1 μm. Since the thulium excitation levels are described by a quasi-3-level scheme, the best performance of the laser can only be achieved with good cooling of the active medium. This has been achieved in the thulium fiber laser used in that the geometric surface-to-volume ratio is particularly advantageous.

The thulium fiber laser used works best when the radiation is completely guided in the fiber. An external mirror should therefore be replaced by an internal fiber Bragg grating. In known manner, the fiber Bragg grating was not written in the active fiber with the doped laser core, but in a passive fiber. This allows the active fiber to be better cooled and used in full length. The waveguide properties of the passive core should be adapted to the corresponding properties of the active laser core so that no reflection or scattering losses occur at the junction. In the usual way then have the two cores comparable core diameter, which in turn can then absorb the same transverse modes, measured at its transverse extent.

If the active and passive fibers are designed as singlemode fibers for the transverse fundamental mode, it is relatively easy to build up a stable resonator from a combination of the two fibers with the fiber Bragg grating in the passive fiber. The situation is different with larger core diameters for multi-mode operation.

The standard HR gratings inscribed in the multimode fiber do not fulfill their intended purpose. Part of the radiation that is to be completely reflected on the grid passes it. This is usually 20% -30% of the total output.

The reason for this is to be assumed in the specific interaction of the lattice with the higher modes. In the fiber, there is a coupling between the different transverse modes, such that the higher modes can operate at the same wavelength as given by the grating in the fundamental mode. However, as soon as a higher mode with the same wavelength as the fundamental mode propagates towards the grating, it does not see exactly that grating, since the corresponding field distribution in the waveguide sees the grating only at a shifted wavelength, which is typically a few nanometers away.

From this finding, it follows that a fiber laser in a conventional cavity multimode fiber, which is characterized by two significant counterpropagating waves, can not be effectively operated by a narrow band, conventionally fabricated fiber Bragg grating. Part of the radiation passes this grid.

Nevertheless, in order to be able to profit from a multimode fiber appropriately, an oscillator / amplifier arrangement (MOPA) is used in the prior art. However, this configuration is much more complex because it requires both a resonator and an amplifier. The oscillator works in the fundamental mode, here the Bragg gratings are effective. In addition to the highly reflective grating, a partially reflecting grating is also usually used on the outcoupling side of the resonator. As a result, the operation of the oscillator is more stable.

The oscillator operates in fundamental mode and therefore has a relatively small core diameter. To guarantee a higher power compatibility, the amplifier is designed with a larger core diameter. The mode matching between the oscillator and the amplifier is usually done adiabatically by means of a funnel or a stepwise approximation of the smaller core diameter to the larger core diameter. With sufficient oscillator power, only the fundamental mode is excited and amplified in the amplifier. However, it is also a certain multimode operation is not excluded.

With this arrangement, an external back-reflection at the output of the amplifier can be dangerous, just like any backward wave that can gain power in the amplifier. Because of the adiabatic adaptation, a relatively large amount of power can be fed back into the oscillator even in the opposite direction, as a result of which the power compatibility of the oscillator can be exceeded and the system is destroyed. In order to prevent any kind of feedback to the oscillator, which in addition would negatively affect the power and wavelength stability, an optical diode is often inserted between the oscillator and the amplifier. All this together is ultimately a relatively complex structure.

Regardless of the effects just described, writing a fiber Bragg grating into a multimode fiber is not as easy as in a fundamental mode fiber, i. H. Single-mode fiber. The reason is simply the correspondingly wider expansion of the core in multimode fibers. Fiber Bragg gratings are written directly into the fiber with a standing interference pattern of a high power laser. In the area of the fiber core, the most homogeneous possible field distribution of the writing laser is required. This is of course achieved much easier with small core diameters than with larger core diameters. Therefore, the grids in fundamental mode fibers have always shown significantly better spectroscopic properties in the investigations.

Even relatively moderately excited fibers already have a very significant reinforcement. If one operates an amplifier completely without a feedback (mirror), then such a fiber begins to emit laser light in two directions, starting from a specific pump power, initiated by the spontaneous emission. The process usually starts at the wavelength of the emission spectrum of the optically active material, where the gain is greatest. The simple passage through the amplifier is enough to reduce much of the inversion.

If one had a good multimode lattice in a multimode core and would add (splice) this lattice written into a passive fiber to the active multimode amplifier, one would first create a resonator. The second mirror of this resonator would be formed by the Fresnel reflection on the refractive index jump of the output of the amplifier. The output radiation would be composed of several modes. The fundamental mode would be reflected at the wavelength of the fundamental mode, the higher modes at the corresponding other wavelengths that are in the nanometer range next to the fundamental mode wavelength. This would create a slightly broader emission spectrum.

However, experiments have shown that an ideal multimode grid is not so easy to produce. A considerable share of the direction Lattice running waves went past this. Nevertheless, the goal was pursued to create a simple, essentially two-component laser device, which is also suitable for high output power.

It is known that highly amplifying laser materials (eg semiconductor lasers), which are located in a resonator, by additional external feedback of a coupled fundamental mode fiber with inscribed fiber lattice, partly to the mode profile and to the wavelength predetermined by the lattice to adjust. However, such a structure has a very high reflectivity (> 30%) due to the Fresnel reflection on the facets of the semiconductor crystal, which already predefines a dominant oscillating resonator.

How a multimode amplifier fiber behaves when simply connecting a simple fundamental mode fiber with inscribed grating to one end analogously to the arrangement described above is theoretically predictable. As described above, if the multimode fiber is included in a resonator with significantly reflective mirrors, it should behave in a similar manner. However, a multimode grating as a partially reflecting resonator mirror was again required for an integrated system. This would bring you back to the old problem of the ideal multimode grating.

However, if one restates the above question in a modified form, namely: how does the amplifier behave when there are no mirrors, but when there is only one fundamental mode reflector attached (spliced) to one side, the answer is no longer theoretically predictable , Normally, one would not even consider this arrangement, since it is to be assumed that the singly amplified radiation in turn exits the multimode fiber in both directions due to the spontaneous emission, thereby essentially passing the fundamental mode grating, because the cross-sectional areas can differ by a factor of 4 or higher. At best, only the proportion reflected in the area of the fundamental mode reflector is reflected back.

However, it has been experimentally proven that the low residual reflection of the fundamental mode lattice significantly influences the amplifier. The wavelength-selectively reflected by the coupled reflector wavelength sufficient to seeden the amplifier such that the emission takes place only at the predetermined wavelength. However, as long as reflections take place at the end faces of the amplifier, there is no real preferential direction of the emitted radiation. But if the reflections are minimized at the end surfaces, z. B. by inclined exit surfaces or attached window, then the residual reflection of the coupled fundamental mode grating is so dominant that it drives the entire radiation in the direction of opposite coupling window via a gain process.

An amplifier designed for minimal reflection at the end surfaces can efficiently degrade inversion throughout the fiber core by mode- and wavelength-selective feedback in one pass and produce appropriate laser radiation at the outcoupling side. This process is particularly efficient when the amplifier is excited against the direction of amplification via the outer cladding. The inversion varying over the length thus follows the intensity of the amplified wave. This process still works even if the ratio of fundamental mode core cross section to multimode core cross section is greater than 10.

Experiments performed clearly show that the amplifier produces only forward radiation at one wavelength. The wavelength is given by the grating in the fundamental mode fiber. The higher modes of the output radiation also have this wavelength. Thus, it is clear that this radiation generated in the forward direction can only be induced by mode coupling. Obviously, this process is much more efficient than a common estimate would suggest. If the higher modes were reflected on the grating and only pushed in the forward direction, then they would have a different wavelength from the fundamental mode wavelength, which would be easy to resolve in the spectrometer, but where it is not observed.

LIST OF REFERENCE NUMBERS

1

 Multimode fiber

2

 Single mode fiber

3

 fiber splice

4

 laser core

5

 pump casing

6

 core

7

 coat

8th

 Fiber Bragg Grating

9

 output window

10

 step

11

 interface

12

 layer

13

 endcap

14

 layer

15

 Pump light source

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 patent literature

• US 6625182 B1 [0002, 0007]

Claims (18)

Laser device comprising: - a multimode optical fiber having optically active material, - a singlemode fiber optically coupled to one end of the multimode optical fiber ( 2 ) and - one into the singlemode fiber ( 2 ) registered fiber Bragg gratings ( 8th ) as a mirror for photons of at least one specific laser wavelength, characterized in that - the multimode optical fiber is a multimode fiber ( 1 ) with the optically active material in its laser core ( 4 ) and - that at the other end of the multimode fiber ( 1 ) a reflections of photons back into the multimode fiber preventing outcoupling window ( 9 ) is provided. Laser device according to claim 1, characterized in that the reflection of the decoupling window ( 9 ) into the multimode fiber ( 1 ) is at most 1.0%, preferably at most 0.5%. Laser device according to claim 1 or 2, characterized in that an exit surface of the decoupling window ( 9 ) is antireflective. Laser device according to one of the preceding claims, characterized in that a coupling window ( 9 ) forming optical element ( 13 ) is connected to the multimode fiber with a reflectionless transition. Laser device according to one of the preceding claims, characterized in that the singlemode fiber ( 2 ) with a reflectionless transition with the multimode fiber ( 1 ) connected is. Laser device according to one of the preceding claims, characterized in that a diameter of the laser core ( 4 ) of the multimode fiber ( 1 ) is at least 1.5 times, preferably at least 3 times as large as a diameter of a core ( 6 ) of the singlemode fiber ( 2 ). Laser device according to one of the preceding claims, characterized in that the multimode fiber ( 1 ) and / or the single-mode fiber ( 2 ) is a double-core fiber. Laser device according to one of the preceding claims, characterized in that the laser core of the multimode fiber ( 1 ) is doped with a rare earth ion. Laser device according to one of the preceding claims, characterized in that also the core of the singlemode fiber ( 2 ) is doped. Laser device according to one of the preceding claims, characterized in that a pump light source ( 15 ), the pumping light through the singlemode fiber ( 2 ) into the multimode fiber ( 1 ) coupled. Laser device according to one of the preceding claims, characterized in that a pump light source ( 15 ) is provided, the pumping light through the coupling window ( 9 ) into the multimode fiber ( 1 ) coupled. Laser device according to claim 10 or 11, characterized in that the pump light source ( 15 ) has at least one laser diode. Laser device according to one of claims 10 to 12, characterized in that the pump light source ( 15 ) continuously pumping light into the multimode fiber ( 1 ) coupled. Laser device according to one of Claims 10 to 13, characterized in that the length of the multimode fiber ( 1 ) is the same as the absorption length of the pump light in the multimode fiber ( 1 ). Laser device according to one of the preceding claims, characterized in that a diameter of the laser core ( 4 ) of the multimode fiber ( 1 ) is selected so that when pumping their optically active material, a singlemode laser beam from the coupling window ( 9 ) exit. Laser device according to one of claims 1 to 14, characterized in that a diameter of the laser core ( 4 ) of the multimode fiber ( 1 ) is selected so that when pumping their optically active material, a multimode laser beam from the coupling window ( 9 ) exit. Laser device according to one of the preceding claims, characterized in that the fiber Bragg grating ( 8th ) photochemically into the singlemode fiber ( 2 ) is inscribed. Laser device according to one of the preceding claims, characterized in that the fiber Bragg grating ( 8th ) at several specific laser wavelengths.

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