DE102020127432A1 - Pulsed or continuous wave fiber laser or amplifier with specially doped active fiber - Google Patents

Abstract

A pulsed or continuous wave fiber laser or fiber amplifier has an active fiber with a fiber core and a fiber cladding as well as a doped region with an active doping, which serves to amplify a laser pulse propagating through the fiber. The transversal distribution of the doping concentration of this doping is chosen so that a geometric overlap between the doped area and an intensity profile of the laser radiation to be amplified is Γ < 0.8 and a radial center of gravity Rρ of the transversal distribution of the doping concentration of the active doping is in an area of intensity I of the laser radiation with I(Rp) < 0.8. With this active fiber, for example, a higher pulse energy can be extracted with laser pulses than with active fibers with a completely uniform doping of the core.

Description

Technical field of application

The present invention relates to a pulsed or continuous wave fiber laser or amplifier with an active (optical) fiber that has a fiber core and a fiber cladding and one or more doped regions with an active doping that enables amplification of a laser radiation propagating through the fiber.

Optical beam sources of good beam quality are required for applications in the field of optical data transmission via optical fiber networks, for example in coherent data communication with lasers, in the field of optical countermeasures and in the field of laser weapons and also for material processing with lasers with high pulse energies in the range from a few 100 µJ to a few mJ . Fiber lasers are particularly suitable for generating laser radiation with pulses of high repetition rate. However, the maximum pulse energy that can be generated is limited by parasitic lasing or ASE (Amplified Spontaneous Emission). It is possible to use other types of solid-state lasers for the above applications, but only with a relatively low repetition rate and average power.

For applications in the field of laser weapons and also for material processing with lasers, high laser powers in the range from a few kW to a few 10 kW of good beam quality are required. It is possible to use other types of solid-state lasers for the above applications, but only with a relatively complex architecture and significant effort in waste heat management. Fiber lasers offer great advantages here due to the wave conduction and the large heat exchange jacket surface with a small cooling distance to be bridged between the active core and the cooled jacket.

The object of the present invention is to increase the pulse energy that can be generated with a pulsed fiber laser or fiber amplifier or the power that can be achieved in a continuously operated fiber laser or fiber amplifier.

Presentation of the invention

This object is achieved with the pulsed fiber laser or amplifier according to claim 1 and the continuous fiber laser or amplifier according to claim 2. Advantageous configurations of these fiber lasers or fiber amplifiers are the subject matter of the dependent patent claims or can be found in the following description and the exemplary embodiments.

The proposed fiber lasers or fiber amplifiers have an active fiber with a fiber core and a fiber cladding as well as one or more doped regions with an active doping, which serves to amplify a laser radiation propagating through the fiber. In the case of the continuous (cw) fiber laser or amplifier, the fiber has a hollow fiber core in which the highest intensity I of the laser radiation occurs, based on the transverse intensity profile of the laser radiation guided in the fiber. The active fiber is characterized by a particular transversal distribution of the doping concentration of the active doping. This distribution is chosen such that a geometric overlap between the one or more doped regions and an intensity profile of the laser radiation to be amplified is Γ<0.8 and a radial center of gravity R _ρ of the transverse distribution of the doping concentration of the active doping in a region of one intensity I of the laser radiation with I(R _ρ ) <0.8. The geometric overlap is preferably Γ<0.5, particularly preferably Γ<0.2, and the radial center of gravity R _ρ is I(R _p )<0.5, particularly preferably I(R _p )<0. 25 In the case of a fiber with a circular cross-section and a Gaussian intensity profile of the laser radiation, the amplification-relevant doping is therefore present in a region of the fiber in which the transverse intensity profile of the laser radiation has a laser intensity of <80%, preferably ≤50%, particularly preferably ≤20% (and of course >0%) of the maximum laser intensity of the intensity profile. Outside this region of the fiber there is preferably no amplifying doping. The term amplification-relevant doping relates here to the transversal distribution of the doping concentration of the doping serving to amplify the laser radiation. The doping can be homogeneous or inhomogeneous in the doped area, for example by introducing doped nanoparticles that do not completely dissolve during fiber production. The geometric overlap Γ and the radial center of gravity R _ρ are defined in more detail below.

The solution to the above object is thus achieved by using an active fiber having a specific transverse doping distribution (curve or distribution of doping concentration) and either in a cw fiber laser or amplifier or in a pulsed fiber laser, for example a Q-switched or a Q-switched and mode-locked fiber lasers, or a pulsed fiber amplifier, in particular as a high-power amplifier in a pulsed fiber amplifier chain. For this purpose, the active fiber is doped with laser-active ions in such a way that the selected transversal distribution of the doping concentration means that the laser-active ions responsible for the laser amplification of the laser or signal photons are concentrated or have their highest concentration in an area in which the laser intensity of the passing through Laser radiation or the laser pulse passing through is only low or moderate compared to the maximum intensity in the center of the fiber core. Because of its location outside the center of the fiber core, this area is also referred to below as the outer area. In contrast, in the area of higher laser intensity, i.e. in and near the center of the fiber core, there is no or a significantly lower doping concentration.

The outer region can be completely and uniformly doped, i.e. contain a homogeneous doping distribution, a doping distribution that changes gradually or abruptly in the radial and/or azimuthal direction, a random distribution or one or more doped regions that are not connected to one another, outside of which there are then no the reinforcement serving doping is present. The respective regions can in turn have a homogeneous doping concentration, one that changes gradually or abruptly, or else a randomly distributed doping concentration. Parts of the core and/or cladding of the fiber can also consist of hollow structures which remain empty during operation or which can absorb gases or liquids. This cavity or these gases or liquids can partially serve to conduct light. These gases or liquids can also represent the laser-active medium.

The proposed fiber laser or fiber amplifier differs from known fiber lasers or fiber amplifiers of the prior art only in the specially doped fiber. It can therefore otherwise be constructed identically to fiber lasers or fiber amplifiers known from the prior art, in particular realized both in an end-pumped configuration and in a configuration pumped via the fiber cladding.

In a pulsed fiber laser, the stored energy, and hence the extractable laser pulse energy and laser pulse duration, is limited by the occurrence of parasitic lasing or ASE once the gain exceeds the corresponding gain threshold for parasitic oscillations or ASE. Using a normal state-of-the-art fiber, in which the entire region of the fiber core is uniformly doped, this gain threshold is reached at a relatively low inversion, since the overlap (in the transverse direction) of the doped region inverted by pump laser radiation with the laser intensity profile of the laser pulse to be amplified is high. Due to the proposed transversal distribution of the doping or doping concentration, the actively doped area (or the actively doped areas) overlaps with the laser intensity profile only in a region of the fiber in which the laser intensity is reduced. As a result, the amplification of a laser pulse generated by a given inversion over the length of the fiber is smaller than in the case of the same inversion in the intensity maximum or center of the fiber core. The fiber can thereby be operated at a higher inversion until it produces the same gain.

The initial gain g _i required to generate a specific pulse duration of a laser pulse in Q-switching is described by the main equation of the Q-switching process. The pulse duration is $$t_p\approx \frac{\mathrm{right}n\left(\mathrm{right}\right)}{\mathrm{right}-1-ln\left(\mathrm{right}\right)}{\tau }_c$$

das Verhältnis der logarithmischen Verstärkung vor dem Puls $$g_i=\frac{1}{2}\mathrm{\Gamma }\left({\sigma }_a+{\sigma }_e\right){\langle \Delta N\rangle }_i^{\text{'}}=\frac{1}{2}\mathrm{\Gamma }\left[\left({\sigma }_a+{\sigma }_e\right){\langle \Delta N\rangle }_i-\left({\sigma }_a-{\sigma }_e\right)\langle N\rangle \right]$$

zur logarithmischen Schwellwertverstärkung g_th eines entsprechenden CW-Lasers bei Güteschaltung, z.B. $$g_{th}=\frac{1}{2}\mathrm{\Gamma }\left({\sigma }_a+{\sigma }_e\right){\langle \Delta N\rangle }_{th}^{\text{'}}=\frac{1}{2}\mathrm{\Gamma }\left[\left({\sigma }_a+{\sigma }_e\right){\langle \Delta N\rangle }_{th}-\left({\sigma }_a-{\sigma }_e\right)\langle N\rangle \right]$$

dar. σ_a und σ_e sind die spektroskopischen Wirkungsquerschnitte für Absorption und Emission, ${\langle \Delta N\rangle }_{th}^{\text{'}}$

die Schwell-Inversionsdichte bei freigegebenem Resonator, ${\langle \Delta N\rangle }_i^{\text{'}}$

die Inversionsdichte beim Freigeben des Resonators und (N) die Dichte der laseraktiven Ionen. $$\mathrm{\Gamma }=\frac{{\displaystyle \iint {}_{dotierterBereich\langle N\rangle >0}I\left(r,\phi \right)dA}}{{\displaystyle \iint {}_{Intensit\ddot{a}tsprofil}I\left(r,\phi \right)dA}}$$

ist hierbei der geometrische Überlappungsfaktor, der den Überlapp des dotierten Bereiches mit einer transversalen Intensität I(r,φ) der Lasermode beschreibt. Daher muss in einer Faser mit einem kleineren dotierten Bereich die anfängliche Inversionsdichte ${\langle \Delta N\rangle }_i^{\text{'}}$

entsprechend höher sein, um die gleiche Verstärkung und damit die gleiche Pulsdauer zu erhalten.Therein τ _c represents the photon lifetime in the resonator, η(r) the extraction efficiency and $$\mathrm{right}=\frac{{〈\Delta N〉}_i^{\text{'}}}{{〈\Delta N〉}_{tH}^{\text{'}}}=\frac{G_i}{G_{tH}}$$

the ratio of the logarithmic gain before the pulse $$G_i=\frac{1}{2}\mathrm{\Gamma }\left({\sigma }_a+{\sigma }_e\right){〈\Delta N〉}_i^{\text{'}}=\frac{1}{2}\mathrm{\Gamma }\left[\left({\sigma }_a+{\sigma }_e\right){〈\Delta N〉}_i-\left({\sigma }_a-{\sigma }_e\right)〈N〉\right]$$

for logarithmic threshold gain g _th of a corresponding CW laser with Q-switching, e.g $$G_{tH}=\frac{1}{2}\mathrm{\Gamma }\left({\sigma }_a+{\sigma }_e\right){〈\Delta N〉}_{tH}^{\text{'}}=\frac{1}{2}\mathrm{\Gamma }\left[\left({\sigma }_a+{\sigma }_e\right){〈\Delta N〉}_{tH}-\left({\sigma }_a-{\sigma }_e\right)〈N〉\right]$$

σ _a and σ _e are the spectroscopic cross sections for absorption and emission, ${〈\Delta N〉}_{tH}^{\text{'}}$

the threshold inversion density with the resonator released, ${〈\Delta N〉}_i^{\text{'}}$

the inversion density upon resonator release; and (N) the laser active ion density. $$\mathrm{\Gamma }=\frac{{\displaystyle \iint {}_{\mathrm{i.e}Otie\mathrm{right}te\mathrm{right}Be\mathrm{right}eicH〈N〉>0}I\left(\mathrm{right},\phi \right)\mathrm{i.e}A}}{{\displaystyle \iint {}_{Intensit\ddot{a}tsp\mathrm{right}Ofil}I\left(\mathrm{right},\phi \right)\mathrm{i.e}A}}$$

is the geometric overlap factor that describes the overlap of the doped region with a transversal intensity I(r,φ) of the laser mode. Therefore, in a fiber with a smaller doped region, the initial inversion density must ${〈\Delta N〉}_i^{\text{'}}$

be correspondingly higher in order to obtain the same amplification and thus the same pulse duration.

worin V das dotierte Volumen und ${\langle \Delta N\rangle }_f^{\text{'}}$

die finale Inversionsdichte nach Durchlaufen des Pulses darstellen. Somit erhält man $$E_{out}\propto V{\langle \Delta N\rangle }_i^{\text{'}}\propto g_i\frac{V}{\mathrm{\Gamma }}$$

The energy extracted per pulse is $$E_{O\mathrm{and}t}=\frac{1}{2}HvV\left[{〈\Delta N〉}_i^{\text{'}}-{〈\Delta N〉}_f^{\text{'}}\right]=\frac{1}{2}HvVn\left(\mathrm{right}\right){〈\Delta N〉}_i^{\text{'}}$$

where V is the doped volume and ${〈\Delta N〉}_f^{\text{'}}$

represent the final inversion density after passing through the pulse. So you get $$E_{O\mathrm{and}t}\propto V{〈\Delta N〉}_i^{\text{'}}\propto G_i\frac{V}{\mathrm{\Gamma }}$$

zu $$R_{\rho }=\frac{{\displaystyle {\int }_0^{\infty }r\overline{\rho }\left(r\right)dr}}{{\displaystyle {\int }_0^{\infty }\overline{\rho }\left(r\right)dr}}$$

Es ist nun im Sinne der Erfindung vorteilhaft, wenn dieser Schwerpunkt der Verteilung in einem Bereich der Intensität mit I(R_p) < 0,8, vorzugsweise I(R_p) < 0,5, besonders bevorzugt I(R_ρ) < 0,25 zu liegen kommt. Dies wird später in einem Ausführungsbeispiel anhand einer im Querschnitt ringförmigen Dotierung nochmals beispielhaft gezeigt.This shows that the reduced doping volume counteracts the increased possible inversion density due to the lower overlap factor. However, since the intensity-weighted overlap decreases faster with increasing radial distance from the center of the fiber core or from the maximum of the mode intensity or the transverse laser intensity profile than a ring-shaped surface element 2πrdr available at this radial distance and thus the associated volume 2πrLdr, the pulse energy can be compared be increased with a fully doped core. A measure of this area (and thus the volume) available for a given doping distribution is the radial center of gravity R _ρ of the laser-active doping distribution N(r,φ). This is obtained from the mean radial doping distribution $$\overline{\rho }\left(\mathrm{right}\right)=\frac{1}{2\pi }{\displaystyle {\int }_0^{2\pi }N\left(\mathrm{right},\phi \right)\mathrm{i.e}\phi }$$

to $$R_{\rho }=\frac{{\displaystyle {\int }_0^{\infty }\mathrm{right}\overline{\rho }\left(\mathrm{right}\right)\mathrm{i.e}\mathrm{right}}}{{\displaystyle {\int }_0^{\infty }\overline{\rho }\left(\mathrm{right}\right)\mathrm{i.e}\mathrm{right}}}$$

It is now advantageous within the meaning of the invention if this focus of the distribution is in an intensity range with I(R _p )<0.8, preferably I(R _p )<0.5, particularly preferably I(R _p )<0 ,25 comes to rest. This will be shown again later by way of example in an exemplary embodiment using a doping that is annular in cross section.

The transverse distribution of the doping concentration in the active fiber selected according to the present invention enables a higher inversion density and thus an increase in the stored and extractable energy at a given gain. As a result, with pulsed fiber lasers or pulsed fiber laser amplifiers that use such an active fiber, the pulse energies can be increased compared to the use of fibers with a complete and uniform doping of the core as before. The maximum achievable pulse energy is therefore increased by the chosen transversal doping distribution in comparison to such fibers with a complete and uniform doping of the core. The laser power can also be increased in the case of continuous fiber lasers or fiber amplifiers. For example, guiding the high laser power in a hollow region can increase the thresholds of nonlinear effects.

The proposed active fiber can advantageously be used as an active medium in a fiber laser or also as an amplification medium in a fiber amplifier of a laser, in particular a fiber laser. As before, the active fiber can be pumped both via the fiber cladding and in an end-pumped arrangement by coupling one or both ends of the fiber into the fiber core.

Such fiber lasers or fiber amplifiers can be used, for example, for laser material processing and in particular for optronic countermeasures and laser weapons or also for optical data transmission. This is of course not an exhaustive list.

In a preferred single-mode operation of the active fiber, the fiber diameter and the effective numerical aperture (NA) of the fiber are chosen such that the first higher-order mode of a laser pulse propagating through the fiber or of the laser radiation propagating through the fiber has a significant loss and/or or sufficiently large mode diameters so that the overlap with the lasing ions is small enough to limit significant parasitic gain of this higher mode. For this purpose, the fiber can also be in the form of a fiber microstructured for single-mode operation (endlessly single-mode microstructured fiber).

Both the fiber core and the fiber cladding can additionally be doped with other active ions that serve purposes other than signal amplification. This applies in particular to the function of refractive index matching between core and fiber cladding. Furthermore, the present invention is also not limited to step-index fibers, but can also be used for other fiber types or other types of waveguides that allow correspondingly structured doping, in order to achieve the same advantages. For example, the actual fiber core can be hollow (such as in a hollow-core photonic crystal fiber, photonic bandgap fiber, Kagome fiber or negative-curvature fiber) and the laser-active doping can be introduced into the edge structures of the cavity. This has the particular advantage that the high intensities are carried out in a vacuum or in a gas and thus non-linear effects can be drastically reduced compared to a solid core and the radiation-induced damage threshold can be increased. This applies to both pulsed and continuous modes of operation.

character list

• 1 schematische Darstellungen von drei Beispielen für eine transversale Verteilung der Dotierungskonzentration bei der vorgeschlagenen aktiven Faser;
• 2 ein Beispiel für das Verhältnis der pro Puls extrahierten Energie zwischen einer Ausgestaltung der vorgeschlagenen aktiven Faser und einer aktiven Faser mit gleichmäßiger Dotierung des gesamten Kerns in Abhängigkeit von Dotierungsparametern der vorgeschlagenen aktiven Faser;
• 3 schematische Darstellungen von drei weiteren Beispielen für eine transversale Verteilung der Dotierungskonzentration bei der vorgeschlagenen aktiven Faser;
• 4 eine schematische Darstellung eines weiteren Beispiels für eine transversale Verteilung der Dotierungskonzentration gemäß der vorliegenden Erfindung bei einer photonic crystal Faser mit festem oder hohlem Kern;
• 5 eine schematische Darstellung eines weiteren Beispiels für eine transversale Verteilung der Dotierungskonzentration gemäß der vorliegenden Erfindung bei einer photonic crystal Faser mit festem oder hohlem Kern;
• 6 eine schematische Darstellung eines weiteren Beispiels für eine transversale Verteilung der Dotierungskonzentration gemäß der vorliegenden Erfindung bei einer photonic bandgap Faser mit festem oder hohlem Kern;
• 7 eine schematische Darstellung eines weiteren Beispiels für eine transversale Verteilung der Dotierungskonzentration gemäß der vorliegenden Erfindung bei einer Kagome-Faser mit hohlem Kern; und
• 8 eine schematische Darstellung eines weiteren Beispiels für eine transversale Verteilung der Dotierungskonzentration gemäß der vorliegenden Erfindung bei einer negative-curvature Faser mit hohlem Kern.

The proposed active fiber is explained in more detail below using exemplary embodiments in conjunction with the drawings. Here show:

• 1 schematic representations of three examples of a transverse distribution of the doping concentration in the proposed active fiber;
• 2 an example of the ratio of energy extracted per pulse between an embodiment of the proposed active fiber and an active fiber with uniform doping of the entire core depending on doping parameters of the proposed active fiber;
• 3 schematic representations of three further examples for a transverse distribution of the doping concentration in the proposed active fiber;
• 4 a schematic representation of a further example of a transverse distribution of the doping concentration according to the present invention in a photonic crystal fiber with a solid or hollow core;
• 5 a schematic representation of a further example of a transverse distribution of the doping concentration according to the present invention in a photonic crystal fiber with a solid or hollow core;
• 6 a schematic representation of another example of a transverse distribution of the doping concentration according to the present invention in a photonic bandgap fiber with solid or hollow core;
• 7 Fig. 12 is a schematic representation of another example of a transverse dopant concentration distribution according to the present invention in a hollow-core Kagome fiber; and
• 8th 12 is a schematic representation of another example of a transverse doping concentration distribution according to the present invention in a negative-curvature hollow-core fiber.

Ways to carry out the invention

In order to increase the pulse energy extractable from an active fiber of a pulsed fiber laser or pulsed fiber amplifier or the extractable laser power of a continuous fiber laser or fiber amplifier, it is proposed to generate the amplification doping of the active fiber with a specific transverse distribution of the doping concentration. Here, the transversal distribution of the doping concentration of the active doping is selected such that the geometric overlap between the doped area(s) and the intensity profile of the laser radiation to be amplified is Γ<0.8, preferably Γ<0.5, particularly preferably Γ< 0.2, and the radial center of gravity R _ρ of the transverse distribution of the doping concentration in the region of an intensity I of the laser radiation with I(R _p ) <0.8, preferably I(R _p ) <0.5, particularly preferably I(R _p ) < 0.25. 1 shows three examples, for which the upper part of the figure shows a cross section through the active fiber with fiber core 1 and fiber cladding 2 and the lower part of the figure shows the transversal intensity profile of a laser pulse or laser radiation in the fundamental mode, which is or are the fiber propagates. In all three examples, the doped regions 3 are each in the outer region of the fiber core 1. The fiber core 1 is referred to here as the part of the fiber in which the laser radiation is mainly guided within the fiber. For example, in a simple step-index fiber, this is the innermost part of the fiber with the highest index of refraction. The doped regions 3 are each in a region of the fiber core 1 in which the laser intensity has fallen to a relatively small value compared to the maximum intensity present in the center of the core, as is shown in sub-figures a) to c) of the 1 is evident.

In sub-figure a), several doped regions 3 are used, which in this example are arranged next to one another on a circular path at a constant distance. In relation to the entire fiber, these doped regions 3 represent rod-like volumes that extend along the fiber. The rods can be twisted around the fiber axis. For this purpose, appropriately doped rods are integrated into the core structure later forming the fiber core during the production of the fiber, which are then drawn together with the structure for the fiber cladding to form the fiber. In the embodiment of part b), a complete ring is doped around the center of the fiber core 1, which extends to the edge of the core. Partial image c) again shows a doped ring-shaped area, which is, however, at a distance from the edge of the fiber core 1 .

während bei einem vollständig dotierten Kern mit dem Radius a und identischer Verstärkung g_i die extrahierte Pulsenergie $$E_{out}^{full}\propto g_i\frac{V^{kern}}{{\mathrm{\Gamma }}_{full}}=\frac{\pi a^{2}l}{{\mathrm{\Gamma }}_{full}}=\frac{\pi a^{2}l}{1-e^{-2a^2}}$$

beträgt. Hierbei wurde eine Gauß'sche Intensitätsverteilung mit einem Einheitsradius angenommen.As already mentioned above, the opposing effects of the reduced doping volume (compared to a fully doped fiber core) and the higher possible inversion density lead to an overall increase in the pulse energy. This is based on the design of the 1b) demonstrated again. The doped annular region 3 of 1b) has an inner radius of a ₁ and an outer radius of a, which corresponds to the radius of the fiber core 1. The energy extracted by a laser pulse at gain g _i is proportional to $$E_{O\mathrm{and}t}^{\mathrm{right}inG}\propto G_i\frac{V^{\mathrm{right}inG}}{{\mathrm{\Gamma }}_{\mathrm{right}inG}}=\frac{\pi \left(a^2-a_1^2\right)l}{{\mathrm{\Gamma }}_{\mathrm{right}inG}}=\frac{\pi \left(a^2-a_1^2\right)l}{e^{-2a_1^2}-e^{-2a^2}}$$

while for a fully doped core of radius a and identical gain g _i the extracted pulse energy $$E_{O\mathrm{and}t}^{f\mathrm{and}ll}\propto G_i\frac{V^{ke\mathrm{right}n}}{{\mathrm{\Gamma }}_{f\mathrm{and}ll}}=\frac{\pi a^{2}l}{{\mathrm{\Gamma }}_{f\mathrm{and}ll}}=\frac{\pi a^2\mathrm{<figure-callout\; id="1"\; label="l"\; filenames="DE102020127432A1\_0018.png,DE102020127432A1\_0019.png"\; state="\{\{state\}\}">l</figure-callout>}}{1-e^{-2a^2}}$$

amounts to. Here, a Gaussian intensity distribution with a unit radius was assumed.

In the case of ring-shaped doping 1b) is the ratio between these two energies $$\frac{E_{O\mathrm{and}t}^{\mathrm{right}inG}}{E_{O\mathrm{and}t}^{f\mathrm{and}ll}}=\frac{a^2-a_1^2}{a^2}\frac{1-e^{-2a^2}}{e^{-2a_1^2}-e^{-2a^2}}$$

annähert. Daher kann die extrahierbare Pulsenergie mit einer derartigen Dotierung bzw. Dotierungsverteilung gegenüber einer Faser mit vollständig dotiertem Kern deutlich erhöht werden.This ratio is in 2 for a fiber with a normalized frequency (V parameter) V = 2.405. This corresponds to the maximum V parameter for single-mode light guidance known from fiber theory. From this plot it can be seen that as the ring thickness is reduced to an outer narrow ring (a1 →a), the ratio increases and eventually approaches the limit $$\underset{a_1\to a}{\text{limited}}\frac{E_{O\mathrm{and}t}^{\mathrm{right}inG}}{E_{O\mathrm{and}t}^{f\mathrm{and}ll}}=\frac{e^{2a^2}-1}{2a^2}$$

approaching. The extractable pulse energy can therefore be significantly increased with such a doping or doping distribution compared to a fiber with a fully doped core.

3 finally shows three further examples of a possible doping distribution in the proposed active fiber. In these examples, the fiber is not doped in the fiber core but is doped adjacent to the fiber core or in the fiber cladding sufficiently close to the fiber core. In the lower part of 3 are each as with 1 to recognize the intensity distributions of the laser pulse propagating in the fiber (in the fundamental mode).

In 3a) an embodiment is again shown in which rod-shaped, doped regions 3 are present in an outer region of the fiber, this time adjacent to the fiber core 1 in the fiber cladding 2 . These can be created by placing appropriately doped rods on the outside or close to the core structure before drawing the fiber. The doped rods can, for example, be integrated into the structure for the fiber cladding in a stack and draw process, for example in a photonic crystal fiber. 3b) shows again an annular doping 3, which borders on the edge of the fiber core 1, 3c ) An annular doping 3 at a small distance from the fiber core 1. The amplification effect is also achieved with these configurations, since the laser mode guided in the fiber extends into the fiber cladding 2, as is the case in the lower part of the 3 is indicated.

When using several doped areas 3 as in 1a) or 3a) these need not be formed from the number of regions 3 shown. Rather, only two or also significantly more doped areas 3 can be arranged around the core 1 or the center of the core 1 correspondingly—preferably symmetrically in the case of a small number of <=12 areas. Also, these areas do not necessarily have to have circular cross sections or run in the form of rods over the entire fiber length. Here, too, the course in the longitudinal direction around the fiber axis can be twisted or interrupted.

the 4-8 finally show further examples of a possible doping distribution in the proposed active fiber. In these examples, the fiber has a nearly or completely empty fiber core. This can be filled with a gas or a liquid subsequently or during fiber drawing.

4 shows a photonic crystal fiber with a circular hollow fiber core 1, which is surrounded by solid material of the cladding 2. The longitudinally rod-shaped holes 4 within the shell 2 produce the desired light-guiding effect in the core. The doped region 3 is a circular doping of the innermost cladding material bordering on the core region. This can be within the first row of holes or contain one or more rows of holes. Due to the unavoidable low penetration of the light into this cladding structure, which is ultimately necessary for guiding the light, a small overlap is produced between the doped region 3 and the fiber mode.

5 Fig. 1 shows two other photonic crystal fibers with a circular hollow fiber core 1 surrounded by solid cladding 2 material. The longitudinally rod-shaped holes 4 within the jacket 2 produce, for example, a honeycomb structure for the desired light-guiding effect in the core 1. The doped region 3 here is a doping of the innermost honeycomb material bordering the core region. This can be the material area of the first hole circle row that is in direct contact with core 1 ( 5a ) or contain one or more rows of holes ( 5b ). through the Due to the unavoidable but ultimately necessary low penetration of the light into this cladding structure for guiding the light, a small overlap is produced between the doped region 3 and the fiber mode.

6 shows a photonic bandgap fiber with a circular resonant structure around a hollow or solid fiber core 1 which is surrounded by alternating layers 5 of cladding 2 with, for example, changing refractive indices. Resonance effects between these layers 5 of the cladding 2 produce the desired light guiding effect in the core 1. The doped area 3 is here in the area of one or more of these layers 5. Due to the unavoidable but ultimately necessary for the light guidance small penetration of the light into this layer structure low overlap between doped region 3 and fiber mode generated.

7 shows a kagome fiber with a kagome structure 6 around a hollow fiber core 1. Resonance effects in this structure of the cladding 2 produce the desired light-guiding effect in the core. The doped region 3 here consists of at least part of the webs forming the kagome structure 6 . Due to the unavoidable but ultimately necessary low penetration of the light into this structure for the light guidance, a small overlap between the doped area and the fiber mode is generated.

8th shows a negative-curvature fiber with a cladding 2 consisting of locally circular structures of negative curvature around a hollow fiber core 1. Resonance effects in this structure of the cladding 2 produce the desired light-guiding effect in the core 1. The doped region 3 here consists of one or more of these structures with negative curvature. Due to the unavoidable but ultimately necessary low penetration of the light into this structure for the light guidance, a small overlap between the doped area and the fiber mode is generated.

Reference List

1

Fiber core (= area with significant light intensity)

2

Fiber cladding (= area with greatly reduced to no light intensity)

3

doped area

4

rod-shaped holes

5

layers of the fiber sheath

6

Kagome structure

Claims (13)

Pulsed fiber laser or fiber amplifier with at least one active fiber, - which has a fiber core (1) and a fiber cladding (2) as well as one or more doped regions (3) with an active doping, which enables a laser radiation propagating through the fiber to be amplified, and - In which a transversal distribution of a doping concentration of the active doping is selected such that a geometric overlap between the one or more doped regions (3) and an intensity profile of the laser radiation to be amplified is T<0.8 and a radial center of gravity R _ρ of the transversal distribution of the doping concentration of the active doping in a range of intensity I of the laser radiation with I(R _p ) <0.8. Continuous fiber laser or fiber amplifier with at least one active fiber, - which has a hollow fiber core (1) and a fiber cladding (2) as well as one or more doped regions (3) with an active doping, which enables a laser radiation propagating through the fiber to be amplified, and - in which a transversal distribution of a doping concentration of the active doping is selected such that a geometric overlap between the one or more doped regions (3) and an intensity profile of the laser radiation to be amplified is Γ<0.8 and a radial center of gravity R _ρ the transversal distribution of the doping concentration of the active doping in a region of intensity I of the laser radiation with laser radiation in the hollow fiber core (1). fiber laser or fiber amplifier claim 1 or 2 , characterized in that the geometric overlap between the one or more doped regions and the intensity profile of the laser radiation to be amplified is Γ ≤ 0.5 and the radial center of gravity R _ρ of the transversal Distribution of the doping concentration of the active doping is in a range of intensity I of the laser radiation with I(R _p ) <0.5. fiber laser or fiber amplifier claim 1 or 2 , characterized in that the geometric overlap between the one or more doped regions and the intensity profile of the laser radiation to be amplified is Γ ≤ 0.2 and the radial center of gravity R _ρ of the transverse distribution of the doping concentration of the active doping in a region of intensity I of laser radiation with I(R _p ) < 0.25. Fiber lasers or fiber amplifiers according to any one of Claims 1 until 4 , characterized in that the active doping is present in a plurality of non-contiguous regions (3). fiber laser or fiber amplifier claim 5 , characterized in that the plurality of regions (3) are each rod-shaped and extend along the fiber. Fiber lasers or fiber amplifiers according to any one of Claims 1 until 4 , characterized in that the active doping is present in a continuous region (3) which is ring-shaped in transverse cross section and is located within the fiber core (1) either at the edge of the fiber core (1) or at a distance from the edge of the fiber core (1). Fiber lasers or fiber amplifiers according to any one of Claims 1 until 4 , characterized in that the active doping is present in a coherent region (3) which is ring-shaped in transverse cross section and is located within the fiber cladding (2) either at the edge of the fiber core (1) or at a distance from the edge of the fiber core (1). Fiber lasers or fiber amplifiers according to any one of Claims 1 until 4 , characterized in that the active doping is present in a continuous region (3) which is ring-shaped in transverse cross section and extends on both sides around the edge of the fiber core (1). fiber laser or fiber amplifier claim 5 or 6 , characterized in that the plurality of regions (3) lie within the fiber core (1). fiber laser or fiber amplifier claim 5 or 6 , characterized in that the plurality of areas (3) are within the fiber cladding (2) at the edge of the fiber core (1). fiber laser or fiber amplifier claim 5 or 6 , characterized in that the plurality of regions (3) are partly within the fiber cladding (2) and partly within the fiber core (1) at the edge of the fiber core (1). Fiber lasers or fiber amplifiers according to any one of Claims 1 until 12 , characterized in that the fiber is designed in such a way that laser radiation propagating through the fiber is only amplified in a fundamental mode.

Images