DE102012106063A1 - Laser device comprises an uncoupling window for photons, which is arranged at an end of a multi-mode fiber that is coupled to single-mode fiber, for preventing reflection of photons of lasing wavelength back into the multi-mode fiber - Google Patents

Abstract

The laser device has a multi-mode fiber (1) with an optically active material, having an end coupled to a single-mode fiber (2). The diameter of laser core (4) is 1.5 times as large as that of the core (6) of single-mode fiber. The core (6) is fitted bluntly on laser core such that the diameter of laser core is selected for making a multi-mode beam to emerge from uncoupling window (9) for photons, with the pumping of active material. The window is arranged at the other end of multi-mode fiber for preventing reflection of photons of lasing wavelength back into the multi-mode fiber.

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a laser device comprising a multimode fiber with a laser core having optically active material, a singlemode fiber optically coupled to one end of the multimode optical fiber, a fiber Bragg grating inscribed in a core of the singlemode fiber as a mirror for photons of at least a certain laser wavelength and having a coupling window for laser light at the other end of the multimode fiber, wherein a diameter of the laser core of the multimode fiber is at least 1.5 times as large as a diameter of the core of the singlemode fiber. 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 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 laser wavelength is off of the 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 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.

A laser device of the type described above, ie with the features of the preamble of independent claim 1, is known from US 2010/0322575 A1 known. Here, the multimode fiber and the portion of the singlemode fiber are located between the multimode fiber and the fiber Bragg grating in a laser cavity bounded by the fiber Bragg grating and a dichroic mirror at the other end of the singlemode fiber becomes. This dichroic mirror also forms the coupling-out window for the laser light from the multimode fiber. In addition, the active material in the laser core of the multimode fiber is through this pumped dichroic mirror through. The beam path of the pumping light and the laser light are separated by means of another dichroic mirror outside the laser resonator. In this known laser device, the singlemode fiber serves as a mode filter for the laser light generated in the laser resonator. That is, the known laser device emits monofrequent laser light exclusively in the fundamental mode defined by the singlemode fiber. The efficiency of the known laser device is limited by the fact that the fundamental mode forming in the laser resonator does not cover larger parts of the laser core of the multimode fiber and accordingly can not exploit all the portions of the inversion of the active material caused by the pump light.

OBJECT OF THE INVENTION

The invention has for its object to show a simply constructed narrow-band emitting laser device with an active material in their laser core having multimode fiber and very 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

In a laser device according to the invention comprising a multimode fiber with a laser core having optically active material, with a singlemode fiber optically coupled to one end of the multimode optical fiber, with a fiber Bragg grating inscribed in a core of the singlemode fiber A mirror for photons at least a certain laser wavelength and with a coupling window for laser light at the other end of the multimode fiber, wherein a diameter of the laser core of the multimode fiber is at least 1.5 times as large as a diameter of the core of the single-mode fiber and wherein Multimode fiber with a jump in the diameter of its laser core to the diameter of the core of the singlemode fiber is attached to the singlemode fiber, so that the core of the singlemode fiber only a proportion of the cross-sectional ratio of the two cores corresponding proportion of the laser core of multimode Fiber to the singlemode fiber receives running light, the laser light enters after once through the multimode fiber from the coupling-out window.

In the laser device according to the invention, there is no laser resonator. That is, even at least at one, preferably at both ends of the laser device, even light having the wavelength of the generated laser light is substantially not reflected. Rather, the laser light is produced in a single pass through the multimode fiber from light reflected at the fiber Bragg grating of the singlemode fiber and correspondingly having a wavelength defined by the fiber Bragg grating. The laser light forming on this basis in a single pass through the multimode fiber increasingly also adapts to higher modes whose propagation in the multimode fiber is possible during its passage, and thereby also effectively enables the inversion of the active material achieved by the pump light exploit in such parts of the laser core, the light in the fundamental mode are not accessible.

The avoidance of a laser resonator at the end of the laser device with the singlemode fiber is achieved in that only a small part of the guided in the laser core of the multimode fiber and emerging in the direction of the single-mode fiber enters the core of the singlemode fiber and is reflected therein at the appropriate wavelength by the fiber Bragg grating. However, the vast majority of this light does not enter the core of the singlemode fiber, but passes it.

At the end of the multimode fiber opposite the singlemode fiber, the avoidance of a resonator mirror is achieved in that the decoupling window provided there for the laser light is partially mirrored, but at least not mirrored, that is to say the resonator mirror. H. at least let the majority of the laser light coming from the multimode fiber exit.

Preferably, the portion of the laser light reflected from the resonator mirror is only small and is in a range of 2% to 10%, preferably 4 to 6%, d. H. about 5%. A reflection of about 4% is often achieved by polishing a free end of the multimode fiber without special coating. Although the laser light reflected into the multimode fiber is amplified in the multimode fiber, it then emerges at least substantially unused at the other end of the multimode fiber and does not run back again to the coupling-out window.

At this other end of the multimode fiber, the singlemode fiber with the inscribed fiber Bragg grating causes only photons of the desired laser wavelength to be reflected back at that end and back into the multimode core in the transverse fundamental mode. Such photons occur when pumping the optical active material of the multimode fiber as part of the spontaneous emission spectrum on its own. This fraction is positively selected over the other wavelengths and modes by the singlemode fiber with the inscribed fiber Bragg grating from the spontaneous emission spectrum and reflected 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. On average, the photons of the laser light have traversed significantly less distance than their length in the multimode fiber before they emerge from the coupling-out window.

That a device according to the invention is able to output a continuous wave laser beam not only without a resonator but also without active seeding of the pumped multimode fiber is extremely surprising. In particular, it has not been expected that the spontaneous emission of fluorescent light remaining after the inversion of the pumped laser core has been removed by the amplification of the first photons traveling from the fiber Bragg grating to the outcoupling window will still sufficiently provide photons which will yield their selection by the singlemode fiber and their reflection at the fiber Bragg grating ensure the emission of the laser light from the laser core of the multimode fiber with its wavelength. It should be noted that the singlemode fiber and the fiber Bragg grating do not act as mode and wavelength filters for laser light often reciprocating in a laser resonator.

Even if no laser resonator is maintained with the partial reflection of the laser light at the coupling-out window in the laser device according to the invention, a certain degree of this partial reflection, in particular in the above-mentioned range of 2 to 10%, makes sense, in order to avoid that the inversion of the active material in the laser core of the multimode fiber near the end where the singlemode fiber is disposed. Namely, the degradation of substantial parts of this inversion by the low intensities of the light with the wavelength of the laser light reflected back from the fiber Bragg grating in the singlemode fiber into the laser core is omitted here. Only farther away from the singlemode fiber is the intensity of the laser light generated from this light so great that the inversion of the active material is substantially cleared, in which case also the increasing mode locking, which is used in the laser device according to the invention, is of importance is. A high static level of inversion of the active material in the laser core at the end of the multimode fiber near the singlemode fiber is associated with the risk that laser light randomly reflected back into the outcoupler window from the outside passes back through the multimode fiber to this area and the inversion uncontrolled clearing away. In this case, very high laser energies can be released in a pulsed manner, which can have consequences until the local evaporation of the material of the multimode fiber and / or the single-mode fiber, in particular at its interface. However, if at the output window, a small portion of the laser light is regularly reflected back into the multimode fiber, this eliminates the inversion at the opposite end of the multimode fiber again and again. Although the associated energy is not used because the amplified by the inversion laser light emerges at the opposite end of the coupling window end of the multimode fiber. However, this is accepted in the laser device according to the invention, in order to avert the described danger. In principle, only a small reflection of photons at the coupling-out window of the laser device according to the invention back into the multimode fiber serves to purposefully exert no influence on the laser light generated from this other end. This deliberately avoids that photons of any wavelength and any mode, which would be reflected in principle at this coupling window, get back into the multimode fiber and amplified there, because this would at least result in an unnecessary consumption of pumping energy.

The laser device according to the invention does not only come out without a mirror at the other end of the multimode fiber; but a not very limited reflection of photons back into the multimode fiber even affects 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 is not more than 10% at all relevant wavelengths of the spontaneous emission spectrum of the optically active material of the multimode fiber. This can be achieved concretely in that an exit surface of the coupling-out window is partially mirrored for the relevant wavelengths. The same goal is achieved when a surface of the outcoupling window having residual reflection is tilted or curved with respect to the axis of the multimode fiber to confine that 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 a partially reflecting coating, but in the form of an additional optical element which is connected to the multimode fiber with a reflection-free transition. the Those skilled in the art are known techniques to which a so-called splicing counts.

Likewise, the singlemode fiber is preferably connected to the multimode fiber with a reflectionless transition. In this case, the significant jump according to the invention occurs at the diameter of the cores between the multimode fiber and the singlemode fiber. In particular, the laser device according to the invention has no 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. At least 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 pumping 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 due to the only one-time passage of the laser light through the multimode fiber cheaper, since this is pumped in quasi 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 especially the case when the multimode fiber has only small losses for the higher modes and corresponding mode locking and the Mode crosstalk occurs strongly. However, the special feature here 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, ie 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 have only one wavelength. 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 efficiently converts the excitation energy of the optically active region into a laser beam 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 adiabatic with the aid of a funnel or a gradual 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, ie singlemode 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 will of course be at small core diameters achieved much easier than 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 proportion of the waves running in the direction of the grid went past it. 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.

Performed experiments clearly show that the amplifier only forward radiation at a Wavelength generated. 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

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Cited patent literature

• US 6625182 B1 [0002]
• US 2010/0322575 A1 [0004]

Claims (15)

Laser device comprising: - a multimode fiber ( 1 ) with a laser core ( 4 ) comprising optically active material, - a singlemode fiber optically coupled to one end of the multimode optical fiber ( 2 ), - one into a core ( 6 ) of the singlemode fiber ( 2 ) registered fiber Bragg gratings ( 8th ) as a mirror for photons of at least one specific laser wavelength and - a coupling window ( 9 ) for laser light at the other end of the multimode fiber ( 1 ), - wherein a diameter of the laser core ( 4 ) of the multimode fiber ( 1 ) is at least 1.5 times as large as a diameter of the core ( 6 ) of the singlemode fiber ( 2 ), characterized in that - the multimode fiber ( 1 ) with a jump in the diameter of its laser core ( 4 ) to the diameter of the core ( 6 ) of the singlemode fiber ( 2 ) to the singlemode fiber ( 2 ) is added so that the core ( 6 ) of the singlemode fiber ( 2 ) only one of the cross-sectional ratio of the two cores corresponding proportion of in the laser core ( 4 ) of the multimode fiber ( 1 ) to the singlemode fiber ( 2 ) receives light, and - that the laser light after a single pass through the multimode fiber ( 1 ) emerges from the coupling-out window. Laser device according to claim 1, characterized in that a reflection of the decoupling window ( 9 ) for the laser light back into the multimode fiber ( 1 ) Is 2% to 10%, preferably 4 to 6%. Laser device according to claim 1 or 2, characterized in that an exit surface of the decoupling window ( 9 ) is partially reflected. 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 at the ends facing away from the coupling-out window of the multimode fiber ( 1 ) and the singlemode fiber ( 2 ) is provided an absorbing device for escaping light. Laser device according to one of the preceding claims, characterized in that the diameter of the laser core ( 4 ) of the multimode fiber ( 1 ) is at least 2 times, preferably at least 3 times as large as the diameter of the 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 a pump light source ( 15 ), the pump light at the singlemode fiber ( 2 ) opposite the other end into the multimode fiber ( 1 ) coupled. Laser device according to claim 10, characterized in that the pump light source ( 15 ) has at least one laser diode. Laser device according to one of claims 10 or 11, 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 12, 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 claims 1 to 13, 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 nucleus ( 4 ) of the singlemode fiber ( 2 ) is inscribed.

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