WO2023066986A1

WO2023066986A1 - Laser device and method for shaping a laser pulse

Info

Publication number: WO2023066986A1
Application number: PCT/EP2022/079075
Authority: WO
Inventors: Florian Graf; Aleksander BUDNICKI
Original assignee: Trumpf Laser Gmbh
Priority date: 2021-10-21
Filing date: 2022-10-19
Publication date: 2023-04-27
Also published as: DE102021127336A1

Abstract

The invention relates to a laser device (2), comprising – a seed laser (14), which is designed as a passively mode-locked fibre oscillator (1), wherein: the passively mode-locked fibre oscillator (1) has a bidirectional loop (3) and a unidirectional loop (5); the bidirectional loop (3) and the unidirectional loop (5) are coupled together by a 3x3 coupler (7); the bidirectional loop (3) has a first amplifying fibre (9); and at least one operating parameter of the seed laser (14) is adjusted so that a laser pulse exiting from a decoupling port (10) of the 3x3 coupler (7) has a pulse shape with a depression at a central wavelength (c), and comprising - an amplifier device (12), which is designed to amplify the laser pulse generated by the seed laser (14), wherein the at least one operating parameter of the seed laser (14) is matched to the amplifier device (12) so that a gain narrowing in the amplifier device (12) is at least partially compensated for by the pulse shape of the laser pulse.

Description

Laser device and method for shaping a laser pulse

DESCRIPTION

The invention relates to a laser device and a method for shaping a laser pulse.

In a laser device that uses the amplification scheme of so-called "Chirped Pulse Amplification" (CPA), the spectrum of an excitation laser (seed laser) typically narrows successively in the subsequent amplification stages due to the finite amplification bandwidth of the amplifier medium. This effect, referred to below as gain narrowing, is also referred to as gain narrowing. Due to the narrowing of the spectrum, the bandwidth-limited temporal pulse duration of the output pulses of the laser device is lengthened.

It is known from US Pat. No. 9,401,580 B1 to avoid this effect by arranging a passive pulse shaping filter behind the excitation laser and in front of the optical amplifier chain in order to shape the input pulse coming from the excitation laser in such a way that the gain narrowing effect occurring in the subsequent amplifier chain is precompensated becomes. The use of the passive pulse-shaping filter requires an additional optical component and thus a complex construction of the laser device, as well as power losses.

The invention is therefore based on the object of creating a laser device and a method for shaping a laser pulse, the disadvantages mentioned being reduced and preferably not occurring.

The object is achieved by providing the present technical teaching, in particular the teaching of the independent claims and the preferred embodiments disclosed in the dependent claims and the description.

The object is achieved in particular by creating a laser device which has an excitation laser which is designed as a passively mode-locked fiber oscillator. The A passively mode-locked fiber oscillator has a bidirectional loop and a unidirectional loop, the bidirectional loop and the unidirectional loop being coupled together by a 3x3 coupler. The bi-directional loop includes a first gain fiber. At least one operating parameter of the excitation laser is set in such a way that a laser pulse emerging from an outcoupling port of the 3×3 coupler has a pulse shape that has a depression at a central wavelength of the pulse shape. In this way, the input pulse generated by the excitation laser and preferably intended for coupling into an amplifier chain is advantageously shaped in the excitation laser itself, in particular in such a way that a gain narrowing effect occurring in the subsequent amplifier chain is precompensated. The laser device according to the invention can thus be used without an additional optical component such as a pulse-shaping filter, also in connection with amplification, in particular according to the CPA principle, so that the construction of the laser device is simple, with no losses caused by such an additional optical component. The pre-compensation of the input pulse also advantageously avoids the bandwidth-limited pulse duration of the output pulses of an amplifier chain used with the laser device being lengthened. As a result, a CPA system with a shorter pulse duration is made possible.

In particular, the at least one operating parameter of the excitation laser is set in such a way that the laser pulse emerging from the coupling-out port has a spectral pulse shape that has a depression at the central wavelength.

In the context of the present technical teaching, a central wavelength is understood to mean, in particular, a mean value of the pulse shape plotted over the wavelength scale if the pulse shape is symmetrical, or - in particular in the case of a symmetrical pulse shape - a median of the spectrum and/or the spectral power density of the pulse shape, or another suitable measure for determining a mean or central wavelength of the pulse shape, in particular a centroid of the pulse shape on the wavelength scale.

In the context of the present technical teaching, the fact that the pulse shape has a depression at the central wavelength is understood in particular to mean that a spectral power density of the pulse shape in the region of the central wavelength is lower than in pulse flanks of the pulse shape above and below the region of the central wavelength. The pulse shape therefore has a dent, particularly in the area of the central wavelength. In particular, a spectral power density of the pulse shape plotted against the wavelength scale at the location of the Center wavelength less than at least one other location on the wavelength scale below the center wavelength and at least one other location on the wavelength scale above the center wavelength. In particular, a maximum of the spectral power density of the pulse shape does not lie at the location of the central wavelength. In particular, the spectral power density preferably has two maxima, namely a first maximum below the central wavelength and a second maximum above the central wavelength.

A fiber oscillator is understood to mean, in particular, a laser oscillator which has at least one optical component, in particular for guiding and/or influencing light, which has a fiber or consists of a fiber. In a preferred embodiment it is possible for all optical components of the fiber oscillator to be fiber components, ie components which in particular have a fiber or consist of a fiber, in particular fiber-based components or fiber-coupled components.

A loop is understood to mean an optical part of the fiber oscillator which has a first end and a second end, both the first end and the second end being coupled to the same connection component of the fiber oscillator, here in particular to the 3x3 coupler. This means, in particular, that light pulses that run through the loop starting from the connection component return along the loop to the connection component. Such a loop can be designed as a ring as a whole; in particular, in this case the loop consists of a ring part. However, it is also possible for such a loop to have at least one ring part and at least one linear branch light-conductingly connected to the ring part, in particular precisely one ring part and precisely one linear branch.

A bidirectional loop is understood to mean, in particular, a loop in which light pulses can propagate both from the first end to the second end and from the second end to the first end—ie in both directions. In particular, the bidirectional loop with the first amplification fiber has the function of a saturable absorber.

A unidirectional loop is understood to mean, in particular, a loop in which light pulses can propagate along the loop only in one distinct direction, either from the first end to the second end or from the second end to the first end. An isolator device is preferred in the unidirectional loop, in particular a Isolator, arranged, wherein the isolator device is set up to allow light pulses to pass through only in one direction, but to block them in the other direction, for example by utilizing the Faraday effect, or in another suitable manner. The isolator means is preferably arranged in a ring part of the unidirectional loop.

The bi-directional loop is preferably a first fiber loop.

A fiber loop is understood to be a loop that has a fiber at least in some areas or consists of a fiber. In one embodiment, the fiber loop consists entirely of one fiber or is composed of a plurality of interconnected fibers.

The unidirectional loop is preferably a second fiber loop. In one embodiment, the unidirectional loop is formed as a unidirectional ring.

In an embodiment of the laser device, the bidirectional loop has an asymmetry. In particular, in one embodiment it is provided that the bidirectional loop is configured asymmetrically for two light pulses passing through the bidirectional loop in opposite directions.

In particular, the bidirectional loop has an asymmetry element, in particular an asymmetrically arranged first amplification element for asymmetrical amplification, and/or an asymmetrically arranged attenuation element for asymmetrical attenuation of the light pulses propagating in opposite directions along the bidirectional loop. The asymmetry element is generally set up and/or arranged to generate a difference in the respective self-phase modulation between a light pulse propagating in a certain first direction along the bidirectional loop and a light pulse propagating in the other, second direction along the bidirectional loop. In one embodiment, however, the asymmetry element can also be in the form of a non-reciprocal phase shifter.

The asymmetrically arranged first reinforcement element can preferably be variably adjusted with regard to the reinforcement. In particular, if the first amplification fiber is designed as the asymmetrically arranged first amplification element, that is to say asymmetrical element, variable amplification can be implemented by varying the pump power. Alternatively or additionally, the asymmetrically arranged attenuation element can preferably be variably adjusted with regard to the attenuation.

In general, a variable, non-linear phase shift between the two counter-propagating light pulses in the bidirectional loop can be realized via a variable setting of the asymmetry element; In particular, the phase shift can be adjusted by variable activation of the asymmetry element.

In particular, according to one embodiment, the first reinforcement fiber can be arranged asymmetrically in the bidirectional loop. This means in particular that the first reinforcement fiber is arranged closer to the first end of the bidirectional loop than to the second end, or vice versa. Alternatively, according to another embodiment, it can be provided that an asymmetrically arranged attenuating element, in particular an asymmetrically arranged decoupling element, for example a tap coupler, or a filter, a polarization attenuator or the like is arranged in the bidirectional loop. The embodiments mentioned can also be combined with one another.

In particular, the bidirectional loop is preferably designed as a nonlinear, amplifying loop mirror (NALM). In this case, the bidirectional loop has an asymmetry, so that different light pulses, which pass through the bidirectional loop in different directions, pass through a longer part of the bidirectional loop with different intensity levels, depending on their direction of circulation, because - in relation to the running distance of the bidirectional loop - be strengthened and/or weakened sooner or later. Because of the self-phase modulation in the bidirectional loop, this leads to a non-linear phase shift between two light pulses that pass through the bidirectional loop in opposite directions, this phase shift itself in turn being intensity-dependent. The phase shift between the two light pulses in turn influences their coupling behavior at the 3x3 coupler. In this way, light pulses of higher intensity are preferably fed in the appropriate propagation direction via the 3×3 coupler from the bidirectional loop into the unidirectional loop, as a result of which the bidirectional loop designed as NALM in particular can fulfill the function of a saturable absorber. The loop arrangement made up of the bidirectional loop and the unidirectional loop, which are coupled to one another via the 3×3 coupler, and thus also the fiber oscillator overall, preferably has a so-called figure 8 configuration.

According to one development of the invention, it is provided that the laser device has an amplifier device that is set up to amplify the laser pulse generated by the excitation laser. The at least one operating parameter of the excitation laser is matched to the amplifier device in such a way that the pulse shape of the laser pulse, which has the depression at the central wavelength, at least partially, preferably completely, compensates for a gain narrowing that occurs in the amplifier device. In this respect, the advantages already described above are realized in a special way if the laser device itself has the amplifier device. In particular, the amplifier device has a plurality of amplifiers or amplifier stages. In particular, the amplifier device is designed as an amplifier chain. In particular, the amplifier device is designed as a CPA system, ie the amplification in the amplifier device takes place according to the principle of "chirped pulse amplification" (CPA).

According to a development of the invention, it is provided that the at least one operating parameter of the excitation laser is selected from a group consisting of: a fiber length of the bidirectional loop, a fiber length of the first amplifying fiber, a nonlinear phase shift in the bidirectional loop, in particular the arrangement or configuration of a non-reciprocal phase shifter in the bidirectional loop and/or a pump power coupled into the first gain fiber, a power split in the 3x3 coupler, and a phase shift in the 3x3 coupler.

In the context of the present technical teaching, an operating parameter is understood to be a parameter that influences or determines the operation or the mode of operation of the excitation laser. This can be a parameter that is constant over time, one that is determined in particular by the design or the choice of material for the excitation laser; however, it can also be a parameter that is variable over time, in particular an adjustable or predeterminable parameter, which can be changed in particular during operation of the excitation laser. In particular, an operating parameter of the excitation laser is such a parameter that influences or determines the nonlinear phase shift between the two laser pulses traversing the bidirectional loop in opposite directions. By suitably selecting such an operating parameter and thereby suitably adapting the non-linear Phase shift, the pulse shape for the laser pulse, in particular the deepening at the central wavelength, can be selected in such a way that an amplification narrowing occurring in a subsequent amplifier device is at least partially, preferably completely, compensated.

In one embodiment of the laser device, the at least one operating parameter is an operating parameter of the asymmetry element. In particular, the at least one operating parameter is preferably selected or specified by setting the asymmetry element.

In particular, the 3×3 coupler has a plurality of ports, in particular six ports. In one embodiment of the laser device, the 3x3 coupler is designed symmetrically, which means in particular that light pulses are distributed in equal proportions to the different ports of the 3x3 coupler. In particular, the 3×3 coupler thus has a symmetrical power distribution. A port is understood to be a connection of the 3x3 coupler that acts as an input or as an output, and can be connected in particular to a fiber in a light-conducting manner.

According to a development of the invention, it is provided that the 3x3 coupler has a first coupler side and a second coupler side, each with - per coupler side - three ports, the unidirectional loop being connected to the first coupler side and the bidirectional loop to the second coupler side, and the decoupling port being a free port on the first coupler side. In this way in particular, the desired pulse shape with the indentation at the central wavelength can be advantageously achieved. In the context of the present technical teaching, a free port is understood to mean, in particular, a port that is not connected to the unidirectional loop. The decoupling port as a free port is therefore in particular a port not occupied by the unidirectional loop. This does not preclude the coupling-out port from being connected to at least one other optical element and being occupied in this sense.

According to a development of the invention, it is provided that the 3x3 coupler has a first port, a second port and a third port on the first coupler side of two coupler sides of the 3x3 coupler, with the 3x3 coupler on the second coupler side of the two coupler sides having one fourth port, a fifth port and a sixth port. The first port is optically connected directly to the fourth port via a fiber section, with the second port being optically connected directly to the fifth port via a fiber section is, wherein the third port is optically connected directly via a fiber section with the sixth port. In particular, light pulses that propagate between two ports that are directly connected to one another experience no phase jump. In particular, the 3x3 coupler is set up in such a way that light pulses can crosstalk between the direct connections of the ports, experiencing a phase shift which is preferably 2TC/3, regardless of between which two connections a light pulse crosstalks.

In one embodiment of the fiber oscillator, the 3x3 coupler is generally arranged to impart a phase shift of 2TC/3 ZU to light pulses that crosstalk between different proximate connections of the ports of the 3x3 coupler. Another embodiment is also possible, in which the 3×3 coupler is designed asymmetrically, other phase shifts then resulting for the light pulses crosstalking between different direct optical connections of the ports. This effect can be used in a targeted manner in order to tune the spectral pulse shape, in particular to compensate for a subsequent reduction in amplification. In particular, this effect can be used in a targeted manner in combination with a suitable tuning of the pump power for amplification in the bidirectional loop for tuning the spectral pulse shape.

In a first embodiment of the laser device, a first end of the bidirectional loop is optically connected to the fourth port, a second end of the bidirectional loop is optically connected to the fifth port, and the output coupling port is the second port of the 3x3 coupler. The sixth port can preferably be used to decouple additional light pulses from the fiber oscillator, either as additional useful light or for monitoring.

In another, second embodiment of the laser device, the first end of the bidirectional loop is optically connected to the fifth port, the second end of the bidirectional loop is optically connected to the sixth port, and the output coupling port is the second port of the 3x3 coupler. The fourth port can preferably be used to decouple additional light pulses from the fiber oscillator, be it as additional useful light or for monitoring.

A first end of the unidirectional loop is optically connected to the third port, and a second end of the unidirectional loop is optically connected to the first port. The unidirectional loop is preferably set up in such a way - in particular by the Isolator means that a light pulse along the unidirectional loop can only get from the third port to the first port - or vice versa.

In the present specification, specific embodiments of the 3x3 coupler are described in particular with regard to specific possible arrangements and connections of ports of the 3x3 coupler. The person skilled in the art will readily recognize that other embodiments exist which are equivalent, nearly equivalent or at least functionally identical to the arrangements described, but in any case fulfill the same purpose.

Without wishing to be bound to the theory and the specific embodiment, the function of the fiber oscillator in connection with the aforementioned second embodiment is explained in more detail below: A light pulse entering the 3x3 coupler from the unidirectional loop via the first port is three light pulses with the same pulse energy divided between the fourth port, the fifth port and the sixth port. The light pulses at the fifth port and at the sixth port each experience a phase shift of 2/3 compared to the light pulse entering at the first port. The light pulse at the fifth port is referred to below as the first light pulse, and the light pulse at the sixth port as the second light pulse. The first light pulse now runs through the bidirectional loop starting from the first end to the second end - namely from the fifth port to the sixth port, with the second light pulse running through the bidirectional loop in the opposite direction - namely from the sixth port to the fifth port.

Because of the asymmetrical configuration of the bidirectional loop, the first light pulse and the second light pulse now experience different phase shifts or B integrals during their propagation along the bidirectional loop. The difference in the B integrals or the phase shift between the first light pulse and the second light pulse depends in particular on the original intensity of the light pulses - before passing through the bidirectional loop - and the amplification and/or attenuation in the asymmetry element, in particular the amplification in the first amplification fiber, ie in particular from a pumping level of the first amplification fiber. The attenuation can also be made variable, if necessary, in order to influence the phase shift.

Arriving at the fifth port, the second light pulse now partially talks over into the direct optical connection between the sixth port and the third port and again experiences a phase shift of 2TC/3. The first arriving at the sixth port Light pulse is partially forwarded directly to the third port without experiencing a phase shift. An output pulse at the third port resulting from superposition—in particular constructive interference—of the first light pulse and the second light pulse thus depends in particular on the B integrals that the light pulses experience during their propagation along the bidirectional loop.

The 3x3 coupler is set up in such a way that even with a vanishing non-linear phase shift between the first light pulse and the second light pulse, there is a finite transmission of preferably approximately 10% of the input pulse energy and a non-vanishing slope of the phase-dependent transmission curve, which results in a laser pulse structure from the noise out significantly simplified. In particular, this facilitates the start, in particular a self-start, of the mode-locked operation. With increasing phase shift, the transmission increases. Thus, the bidirectional loop favors light pulses with higher peak power and can thus fulfill the function of a saturable absorber.

The non-linear phase shift between the first light pulse and the second light pulse can be set variably by varying the pump power for the first amplification fiber in the bidirectional loop.

At the same time, that portion of the second light pulse arriving at the fifth port that does not cross over into the direct optical connection between the sixth port and the third port is partially transferred directly to the second port, with a portion of the first light pulse arriving at the sixth port going into the direct optical connection Connection between the fifth port and the second port crosstalks and in turn experiences a phase shift of 2TC/3. From the corresponding superposition - in particular destructive interference - of the corresponding components of the first light pulse and the second light pulse, the spectral pulse shape with the desired depression results at the second port, in particular as a function of the non-linear phase shift between the first light pulse and the second light pulse in the bidirectional loop. The spectral pulse shape at the second port can therefore be set in particular by setting or specifying the non-linear phase shift.

The mode of operation of the aforementioned first embodiment is analogous. According to a development of the invention, it is provided that the fiber oscillator has normal dispersion overall. The fact that the fiber oscillator has a normal dispersion overall, or that - in other words, but with the same meaning - a total dispersion of the fiber oscillator is in the normal dispersion range, means in particular that a light pulse passing through the fiber oscillator after a pass through the fiber oscillator - i.e. every component of the fiber oscillator - was once happened - has experienced normal dispersion. This in turn means that, compared to a temporal shape of the light pulse before passing through the fiber oscillator, in the temporal shape of the light pulse after passing through the fiber oscillator, higher frequencies lag while lower frequencies lead. Higher frequencies rush through the fiber oscillator more slowly than lower frequencies. This does not necessarily mean that every optical component of the fiber oscillator has normal dispersion; rather the effect results at least for the sum of the optical components. Thus, while in a preferred embodiment it is possible for all optical components of the fiber oscillator to have normal dispersion, in another preferred embodiment it is also possible for at least a first optical component of the fiber oscillator to have anomalous dispersion, with the fiber oscillator having at least one other , second optical component having a normal dispersion that overcompensates for the anomalous dispersion of the first optical component such that the overall dispersion of the fiber oscillator is normal.

If the wavelength of the fiber oscillator is in the normal dispersion range, for example when using ytterbium or neodymium as a doping element, no further additional measures are required, in particular no dispersion compensation elements, to keep the overall dispersion of the fiber oscillator in the normal range. However, according to one embodiment, the fiber oscillator can also have at least one dispersion compensation element in such a case in order to shift the dispersion to a desired range within the normal dispersion range, in particular for fine-tuning the dispersion. In particular, the total dispersion can be reduced in terms of absolute value with the aid of a dispersion compensation element. According to one embodiment, the dispersion compensation element can be embodied in particular as a chirped grating, in particular as a chirped fiber Bragg grating, or as a dispersion compensation fiber, in particular as Photonic Crystal Fiber (PCF), in particular with a suitably adapted, preferably tailor-made dispersion. On the other hand, if the wavelength of the fiber oscillator is in the anomalous dispersion range, for example when using erbium, thulium or holmium as a doping element, the fiber oscillator preferably has at least one dispersion compensation element in order to bring the total dispersion into the normal range. The at least one dispersion compensation element is preferably designed as a dispersion compensation fiber or as a chirped grating, in particular as a chirped fiber Bragg grating. A dispersion compensation fiber is also referred to as a dispersion-compensated fiber or a dispersion-adapted fiber. Such a dispersion compensation fiber can, for example, have a fiber core that includes rings with different refractive indices.

Alternatively, it is provided that the fiber oscillator has an anomalous dispersion overall. The fact that the fiber oscillator has an anomalous, i.e. negative, dispersion overall, or that - in other words, but with the same meaning - a total dispersion of the fiber oscillator is in the anomalous dispersion range, means in particular that a light pulse passing through the fiber oscillator after one pass through the fiber oscillator - i.e. each Fiber oscillator component passed once - experienced anomalous dispersion. This in turn means that in comparison to a temporal form of the light pulse before passage through the fiber oscillator, in the temporal form of the light pulse after passage through the fiber oscillator, higher frequencies lead due to dispersion, while lower frequencies lag. Higher frequencies rush through the fiber oscillator faster than lower frequencies. However, this effect can be at least partially compensated for by other effects, in particular non-linearities, in particular by non-linear self-phase modulation. That the fiber oscillator as a whole has anomalous dispersion does not necessarily mean that every optical component of the fiber oscillator has anomalous dispersion; rather the effect results at least for the sum of the optical components. Thus, while in a preferred embodiment it is possible for all optical components of the fiber oscillator to have anomalous dispersion, in another preferred embodiment it is also possible for at least a first optical component of the fiber oscillator to have normal - positive - dispersion, with the fiber oscillator has at least one other, second optical component that has an anomalous dispersion that overcompensates for the normal dispersion of the first optical component such that the overall dispersion of the fiber oscillator is anomalous. If the wavelength of the fiber oscillator is in the anomalous dispersion range, for example when using erbium, thulium or holmium as doping elements, no further additional measures are required, in particular no dispersion compensation elements, to keep the overall dispersion of the fiber oscillator in the anomalous range. According to one embodiment, however, the fiber oscillator can also have at least one dispersion compensation element in such a case in order to reduce the total dispersion in terms of amount, in particular to bring it close to the normal dispersion range, and thus to obtain pulses that are spectrally broader and therefore shorter in time. According to one embodiment, a dispersion compensation element used for this purpose can be designed in particular as a dispersion compensation fiber or as a grating, in particular as a fiber Bragg grating. Such a dispersion compensation fiber is also referred to as a dispersion-compensated fiber or a dispersion-adapted fiber. Such a dispersion compensation fiber can, for example, have a fiber core that includes rings with different refractive indices.

If the total dispersion of the fiber oscillator is in the anomalous dispersion range, solitons preferably form in the fiber oscillator or, in the case of the dispersion-compensated fiber oscillator, dispersion-managed solitons. The anomalous dispersion on the one hand and the non-linear self-phase modulation have different signs, so that the phase differences, which result from dispersion on the one hand and self-phase modulation on the other hand, at least largely, preferably completely, cancel each other out.

If the wavelength of the fiber oscillator is in the normal dispersion range, for example when using ytterbium or neodymium as doping elements, in one embodiment the fiber oscillator has at least one dispersion compensation element in order to bring the total dispersion into the anomalous range. The at least one dispersion compensation element is preferably designed as a chirped grating, in particular as a chirped fiber Bragg grating.

According to a development of the invention, it is provided that the first amplifying fiber is doped with at least one element selected from a group consisting of ytterbium, neodymium, erbium, thulium and holmium, or with a combination of at least two of these elements, in particular He/Yb or Tm/Ho. In particular, in one embodiment, the first amplifying fiber is doped with exactly one of the elements mentioned. In another embodiment, the first reinforcement fiber is a combination of at least two of the elements mentioned, doped in particular with a combination of exactly two of the elements mentioned. In another embodiment, the first gain fiber is doped with erbium and ytterbium (Er/Yb). In another embodiment, the first gain fiber is doped with thulium and holmium (Tm/Ho). The doping element or, if appropriate, the combination of doping elements determines in particular an optical wavelength for the fiber oscillator: if the first amplifying fiber includes ytterbium or neodymium as a doping element, the wavelength is approximately 900 nm to 1100 nm. If the first amplifying fiber includes erbium as a doping element, the wavelength is approximately at 1500 nm. If the first amplifying fiber comprises thulium or holmium as a doping element, the wavelength is approximately at 1900 nm to 2100 nm.

According to a development of the invention, it is provided that the excitation laser has at least one dispersion compensation element.

In an embodiment of the laser device, the dispersion compensation element is selected from a group consisting of: a chirped fiber Bragg grating and a dispersion compensating fiber.

According to a development of the invention, the 3×3 coupler is designed as a symmetrical coupler, with the bidirectional loop being set up to impart a nonlinear phase shift of 1 rad to 3 rad—between the opposing light pulses. Particularly in this way, the desired pulse shape with the dip at the center wavelength can be obtained. Alternatively--as already explained above--it is also possible for the 3×3 coupler to be designed asymmetrically.

In one embodiment of the laser device it is provided that the unidirectional loop has no amplification element. In this case, the first amplification fiber as the first amplification element is advantageously the only amplification element of the fiber oscillator, in particular the only amplification fiber. The fiber oscillator can thus have a very simple and inexpensive structure.

In an alternative preferred embodiment, it is provided that the unidirectional loop has an - additional - second reinforcement element, in particular a second reinforcement fiber, with an isolator element - in a preferred embodiment the isolator device of the unidirectional loop provided anyway - in the direction of propagation of a light pulse in of the unidirectional loop is located between the second reinforcement member and the first reinforcement fiber. Additionally or alternatively, an isolator element is preferably arranged in the propagation direction of the light pulse between the first amplification fiber and the second amplification element. In this way, losses in particular can advantageously be compensated for, in that light pulses in the fiber oscillator are amplified not only in the first amplification fiber but also in the second amplification element. At the same time, this allows greater freedom in the choice of amplification for the first amplification fiber and thus freer adjustment of the phase shift between the first light pulse and the second light pulse, since a change in the overall amplification of the fiber oscillator when the amplification in the first amplification fiber is varied can be adjusted accordingly by means of the second Reinforcement element can be compensated. In a preferred configuration, the insulator element can be designed as an insulator or as a circulator.

The second gain fiber is preferably doped with the same element as the first gain fiber.

The bidirectional loop preferably has an in-coupling device which is set up to in-couple pump light into the first amplifying fiber. The in-coupling device arranged in the bidirectional loop can at the same time also be used for in-coupling pump light into the additional second amplifying element, in particular into the second amplifying fiber. In addition, the preferably asymmetrically arranged in-coupling device can serve as an asymmetric element, in particular as an asymmetrically arranged attenuating element.

Alternatively, it is preferably possible for a coupling device to be arranged in the unidirectional loop, which is set up to couple pump light into the additional second amplification element, in particular the second amplification fiber. The in-coupling device is preferably used at the same time for in-coupling pumped light into the first amplifying fiber.

Alternatively, it is preferably also possible for the bidirectional loop to have a first coupling device for coupling pumped light into the first gain fiber, with the unidirectional loop having a second coupling device set up to couple pumped light into the second gain element. The coupling device, be it the first coupling device or the second coupling device or a single coupling device, is preferably designed as a wavelength division multiplex coupler (Wavelength Division Multiplexer—WDM).

In one embodiment of the laser device it is provided that the unidirectional loop has a reflecting arm, with a reflecting element being arranged in the reflecting arm. According to one embodiment, additional optical functions can also be implemented via the reflector element, in particular the function of a bandwidth limitation element and/or a dispersion compensation element. The reflective arm offers advantages in particular with regard to the arrangement and insulation on both sides of an additional second reinforcement element in the reflective arm.

The reflective arm preferably has at least one fiber or preferably consists of at least one fiber.

The reflector element is preferably arranged at a reflective end of the reflective arm. The reflective arm is preferably formed as a linear branch of the unidirectional loop which is optically connected to a ring portion of the unidirectional loop. The reflecting arm, in particular the linear branch, has the reflector element at the reflection end and is light-conductingly connected to the ring part at a connection end opposite the reflection end. A light pulse traversing the unidirectional loop traverses the reflective arm twice, once from the terminal end to the reflective end, and then back from the reflective end to the terminal end.

The reflector element is preferably designed to be partially transparent—or, to put it the other way around, partially reflective—so that a predetermined proportion of light is coupled out of the fiber oscillator via the reflector element.

In one embodiment of the laser device, it is provided that the reflector element is in the form of a wavelength-fixing element, that is to say in particular as an element that is set up to fix the central wavelength for the fiber oscillator. Thus, the reflector element advantageously enables the central wavelength with which the fiber oscillator is operated to be clearly defined. This offers the great advantage of high reproducibility with increased variability at the same time in order to obtain a specific, desired wavelength as the central wavelength. This can in particular in subsequent processes whose Efficiency depends on the wavelength can be crucial, for example in material processing processes, in an amplification chain, and/or in frequency conversion.

In one embodiment of the laser device, it is provided that the reflector element is in the form of a fiber Bragg grating. The fiber Bragg grating can preferably function as a dispersion compensation element, as a wavelength fixing element, and/or as a bandwidth limitation element. In order to be able to function as a dispersion compensation element, the fiber B ragg grating is preferably in the form of a chirped fiber B ragg grating. The fiber B-ragg grating can also act as a wavelength-fixing element or as a bandwidth-limiting element if it is designed as an unchirped fiber B-ragg grating.

In one embodiment of the laser device it is provided that the fiber oscillator has a dispersion compensation element. The dispersion compensation element is preferably formed by the reflector element, in that the reflector element is designed as a chirped fiber Bragg grating. Alternatively or additionally, the dispersion compensation element is a dispersion-compensating fiber. Alternatively or additionally, the first amplification fiber is designed to be dispersion-compensated.

In one embodiment of the laser device, it is provided that the reflecting arm is connected to the ring part of the unidirectional loop in a light-conducting manner via a circulator element. The circulator element preferably serves at the same time as an isolator device for the unidirectional loop. The ring part has a first ring branch, which is optically connected at a first ring branch end to the 3×3 coupler—in particular to the third port—and at a second ring branch end to the reflecting arm, in particular to the circulator element. The ring part also has a second ring branch which is optically connected at a first ring branch end to the reflecting arm, in particular to the circulator element, and at a second ring branch end to the 3x3 coupler - in particular to the first port. In particular, with regard to the aforementioned second embodiment - without wanting to be bound to the theory and the specific embodiment - the following mode of operation results: A light pulse entering the first ring branch via the third port of the 3x3 coupler passes through this to the circulator element, is this is coupled into the connection end of the reflecting arm, runs through the reflecting arm to the reflector element arranged at the reflection end, is at least partially reflected there, runs along the reflecting arm back to the connection end, is coupled there by the circulator element into the second ring branch, and runs through this up to the first port of the 3x3 coupler. Thus, the first ring branch and the second ring branch are each traversed once by the light pulse, while the reflecting arm is traversed twice—back and forth.

In one embodiment of the laser device, provision is made for a second amplification fiber to be arranged in the unidirectional loop, in particular as the additional second amplification element already mentioned above. Preferably, the second amplifying fiber is located in the reflective arm. This proves to be particularly advantageous since the second amplification fiber is traversed twice by a light pulse propagating in the unidirectional loop, so that the light pulse is amplified twice. Furthermore, the second reinforcement fiber is advantageously separated from the first reinforcement fiber by the circulator element—particularly in both directions—so that the two reinforcement fibers do not adversely affect one another.

The fiber oscillator preferably has a coupling device for coupling pump light into the fiber oscillator, in particular into the unidirectional loop, outside the unidirectional loop, in particular outside the loop arrangement—in the propagation direction of a light pulse coupled out by the reflector element—behind the first reflector element. In this way, pumped light can advantageously be coupled into the unidirectional loop via the reflector element. However, the coupling device can also be arranged within the unidirectional loop, in particular in the reflecting arm.

In one embodiment of the laser device it is provided that the fiber oscillator has a bandwidth limitation element. In particular, the bandwidth limitation element is arranged in the unidirectional loop. With the bandwidth limitation element, it is particularly advantageously possible to define the central wavelength for the fiber oscillator. In particular, the bandwidth limitation element has a bandwidth of at least 1 μm to at most 20 nm, preferably from at least 10 μm to at most 15 nm.

In one embodiment of the laser device, the reflector element, in particular the fiber Bragg grating, is designed as the bandwidth limitation element. In particular, the fiber Bragg grating can also act as a bandwidth limitation element when it is in the form of an unchirped grating. Alternatively or additionally, the fiber oscillator has a bandpass filter as the bandwidth limitation element.

In one embodiment of the laser device, it is preferably provided that the bandwidth limitation element, in particular the reflector element or the bandpass filter, is designed to be adjustable with respect to its central wavelength. This enables a particularly high level of flexibility in the choice of the central wavelength of the fiber oscillator. In one embodiment, the bandwidth limiting element is a temperature dependent grating, or a grating sensitive to stretching or compression at its center wavelength.

In one embodiment of the laser device, provision is made for an additional amplification fiber to be arranged in the unidirectional loop, which is referred to here as the third amplification fiber for the purpose of linguistic differentiation, regardless of whether a second amplification fiber is also present. This configuration is particularly preferred in an embodiment of the fiber oscillator in which the unidirectional loop consists of a ring part, the unidirectional loop in particular having no linear branch, in particular no reflective arm. Thus, the third reinforcement fiber is arranged in particular in the ring part of the unidirectional loop. Losses in the fiber oscillator can advantageously be compensated for with the third amplifying fiber

According to another embodiment, however, it is provided that the third reinforcement fiber is provided in addition to a second reinforcement fiber provided in the reflecting arm, in which case the third reinforcement fiber is also preferably arranged in the annular part of the unidirectional loop.

In particular, the third amplifying fiber is doped with the same element as the first amplifying fiber - and in particular as the second amplifying fiber.

In particular, the fiber oscillator in the unidirectional loop has a decoupling device for decoupling light pulses. In this way it is possible to decouple light pulses - whether as useful light or to check the fiber oscillator - not only via the second port or the fourth port of the 3x3 coupler, but additionally or alternatively via the decoupling device. The decoupling device is preferably designed as a tap coupler.

In one embodiment of the laser device it is provided that all optical components of the fiber oscillator are polarization-maintaining. This proves to be a particularly advantageous embodiment for the fiber oscillator.

In one embodiment of the laser device, it is provided that all optical components of the fiber oscillator are formed by fibers or consist of fibers, being in particular fiber-based components or fiber-coupled components. In particular, the fiber oscillator preferably has no free-ray components. In this case, there is no adjustment effort in connection with the fiber oscillator.

In another embodiment, however, it is also possible for the fiber oscillator to have at least one optical component which is designed as a free-beam component.

In one embodiment, the fiber oscillator has a pulse repetition rate of 1 MHz to 150 MHz.

In one embodiment, the laser device has a pump light source. The pumped light source and the fiber oscillator are connected to one another in a light-conducting manner, so that pumped light from the pumped light source can be coupled into the fiber oscillator.

In particular, the pumped light source is optically connected to the first amplifying fiber, so that pumped light from the pumped light source can be used to pump the first amplifying fiber.

In one embodiment, the laser device has a control device.

The control device is in particular operatively connected to a variably controllable asymmetry element of the bidirectional loop in order to set the variable asymmetry element, in particular to set the non-linear phase shift between the light pulses running through the bidirectional loop in the opposite direction, in particular such that the phase shift is 1 to 3 rad.

In particular, the control device is set up to set the at least one operating parameter of the excitation laser, in particular an operating parameter of the variable asymmetry element, to be selected in such a way that the laser pulse emerging from the outcoupling port of the 3×3 coupler has the predetermined pulse shape with the depression at the central wavelength. In particular, the control device is set up to carry out the method according to the invention described below or one or more of the specific embodiments of the method described below.

In particular, the control device is preferably operatively connected to a variably controllable first reinforcement element in order to set the variably controllable first reinforcement element in terms of its gain.

In one embodiment, the control device is operatively connected to the pumped light source and set up to set a pulse duration of the fiber oscillator by selecting the pump power of the pumped light source. The control device is set up in particular to select the pump power of the pumped light source such that the non-linear phase shift between the light pulses running through the bidirectional loop in the opposite direction is 1 to 3 rad, in particular such that the predetermined pulse shape is achieved.

Alternatively or additionally, the control device is operatively connected to a variably controllable attenuation element in order to adjust the variably controllable attenuation element with regard to its attenuation, in particular such that the non-linear phase shift between the light pulses running through the bidirectional loop in the reverse direction is 1 to 3 rad, in particular such that the predetermined pulse shape is reached.

Alternatively or additionally, the control device is operatively connected to the bandwidth limitation element, which can be adjusted with respect to its central wavelength, and is set up to set the central wavelength of the bandwidth limitation element. In a particularly advantageous manner, this allows flexible selection and, in particular, also variation of the central wavelength of the fiber oscillator.

In one embodiment, the fiber oscillator has another filter element in addition to the adjustable bandwidth limitation element, wherein an overlapping area between the bandwidth limitation element and the filter element can be adjusted by adjusting the bandwidth or central wavelength of the adjustable bandwidth limitation element. In this way, an effective bandwidth of the Combination of the bandwidth limitation element and the filter element and thus in turn the central wavelength can be set with high precision.

The bandwidth limitation element can in particular be thermally or mechanically adjustable, for example by heating or cooling, or by stretching or compressing.

Adjustable bandwidth limitation can also be effected with a Fabry-Perot filter in which a distance between two surfaces responsible for the Fabry-Perot property is changed.

In one embodiment, the control device is set up to generate a first, higher asymmetry in the bidirectional loop in a start operating mode by controlling the variably controllable asymmetry element, in order to promote a rapid start of the laser activity in the fiber oscillator, the control device being set up to to control the variably controllable asymmetry element in a continuous operating mode in order to generate a second, smaller asymmetry in the bidirectional loop in order to ensure stable continuous operation of the fiber oscillator. In particular, the control device is set up to correspondingly control a variably controllable attenuation element, in particular to set a first, higher attenuation in the start operating mode and to set a second, lower attenuation in the continuous operating mode.

The object is also achieved in that a method for shaping a laser pulse is created, a predetermined pulse shape being provided for the laser pulse, in particular being specified. The laser pulse is generated by an excitation laser, which is designed as a passively mode-locked fiber oscillator. The passively mode-locked fiber oscillator has a bidirectional loop and a unidirectional loop, the bidirectional loop and the unidirectional loop being coupled together by a 3x3 coupler, the bidirectional loop having a first gain fiber. At least one operating parameter of the excitation laser is matched to the predetermined pulse shape in such a way that the laser pulse emerging from an outcoupling port of the 3×3 coupler has the predetermined pulse shape. In particular, within the scope of the method, the laser device according to the invention or a laser device according to one or more of the embodiments described above is used in order to generate and shape the laser pulse. In particular, the advantages that have already been explained in connection with the laser device result in connection with the method. According to a development of the invention, it is provided that the laser pulse is amplified in an amplifier device, the predetermined pulse shape being determined or specified in such a way that an amplification narrowing occurring in the amplifier device is at least partially compensated.

The invention is explained in more detail below with reference to the drawing. show:

FIG. 1 shows a schematic representation of a first exemplary embodiment of a laser device with an excitation laser designed as a passively mode-locked fiber oscillator;

Figure 2 is a schematic representation of a second embodiment of a

laser device;

Figure 3 is a schematic representation of a third embodiment of a

laser device;

Figure 4 is a diagrammatic illustration of the operation of a bandwidth limiting element in a fiber oscillator;

Figure 5 is a schematic representation of a fourth embodiment of a laser device, and

FIG. 6 shows a schematic representation of the mode of operation of the laser device.

Fig. 1 shows a schematic representation of a first exemplary embodiment of a laser device 2 with an excitation laser 14 designed as a passively mode-locked fiber oscillator 1. The fiber oscillator 1 has a bidirectional loop 3 and a unidirectional loop 5, with the bidirectional loop 3 and the unidirectional loop 5 passing through a 3×3 coupler 7 coupled to one another, in particular connected in a light-conducting manner. A first amplification fiber 9 is arranged in the bidirectional loop 3 . At least one operating parameter of the excitation laser 14 is set in such a way that a laser pulse emerging from an outcoupling port 10 of the 3×3 coupler 7 has a—spectral—pulse shape that has a depression at a central wavelength. In this way, it is advantageously possible to use the correspondingly suitable amplifier device 12, which may occur in a downstream amplifier device 12—shown only schematically here To compensate pulse deformation in the excitation laser 14 itself at least partially, preferably completely. Advantageously, no additional optical components are then required for this compensation, so that the laser device 2 can have a simple structure with low optical losses at the same time.

The laser device 2 preferably has the amplifier device 12 , the amplifier device 12 being set up to amplify the laser pulse generated by the excitation laser 14 . The at least one operating parameter of the excitation laser 14 is matched to the amplifier device in such a way that the pulse shape of the laser pulse at least partially, preferably completely, compensates for the narrowing of the gain occurring in the amplifier device 12 .

The at least one operating parameter of the excitation laser 14 is preferably selected from a group consisting of: a fiber length of the bidirectional loop 3, a non-linear phase shift in the bidirectional loop 3, in particular the optional arrangement or configuration of a non-reciprocal phase shifter - only shown schematically here 16 in the bidirectional loop and/or a pump power coupled into the amplifying fiber 9, a power split in the 3x3 coupler 7, and a phase shift in the 3x3 coupler 7.

The bidirectional loop 3 advantageously assumes the function of a saturable absorber, so that the fiber oscillator 1 can in particular dispense with a semiconductor-based saturable absorber mirror (Semiconductor Saturable Absorber Mirror), SESAM for short. In this way, the otherwise existing problem of degradation and misalignment in connection with a SESAM is also completely avoided. In particular, no problems with degradation and/or misalignment occur in connection with the bidirectional loop 3 . A suitable choice of the first amplification fiber 9, in particular an element with which the first amplification fiber 9 is doped, can provide suitable wavelengths for the fiber oscillator 1, in particular in the range from approximately 900 nm to 1100 nm (ytterbium, neodymium) over 1500 nm ( erbium) to about 1900 to 2100 nm (thulium, holmium). By specifically tuning the overall dispersion of the fiber oscillator 1 in the anomalous or normal range, well-defined dispersion characteristics are advantageously provided. In a preferred embodiment, the amplification fiber 9 is designed as a bandwidth limitation element 59 and/or as a dispersion compensation element 60 . In particular, the amplification fiber 9 can assume the function of bandwidth limitation due to its gain bandwidth. In a preferred embodiment, the fiber oscillator 1 has an anomalous dispersion overall. In another preferred embodiment, the fiber oscillator 1 has normal dispersion overall.

The first amplifying fiber 9 is preferably doped with at least one element selected from a group consisting of: ytterbium, neodymium, erbium, holmium, and thulium. The first amplifying fiber 9 can also be doped with a combination of at least two of the elements mentioned, in particular with a combination of exactly two of these elements, in particular Er/Yb or Tm/Ho.

The bidirectional loop 3 preferably has an asymmetry for two light pulses which pass through the bidirectional loop 3 in opposite directions, in particular in the form of an asymmetry element 4. This asymmetry can be achieved in particular by an asymmetrically arranged first amplifying element 6 and/or an asymmetrically arranged attenuating element 8 in of the bidirectional loop 3 can be effected. In the exemplary embodiment illustrated here, the first reinforcement fiber 9 is arranged asymmetrically in the bidirectional loop 3 as the first reinforcement element 6 . In particular, the bidirectional loop 3 is designed as a non-linear, amplifying loop mirror (NALM).

A coupling device 11 for coupling pump light is preferably arranged in the bidirectional loop 3 . The coupling device 11 is preferably designed as a wavelength division multiplex coupler (WDM). The coupling device 11 can also act as an attenuating element 8 here. A tap coupler, for example, can also be arranged in the bidirectional loop 3 as the attenuating element 8 .

An isolator device 13 , in particular an isolator 15 , is preferably arranged in the unidirectional loop 5 .

The 3x3 coupler 7 is preferably set up to impart a phase shift of 2TC/3 ZU to light pulses which crosstalk between different direct connections of a plurality of ports 17 of the 3x3 coupler 7 . In particular, the 3×3 coupler 7 has a first coupler side 18 and a second coupler side 20, each with three ports 17. The unidirectional loop 5 is preferably connected to the first coupler side 18, with the bidirectional loop 3 being connected to the second coupler side 20. The decoupling port 10 is preferably a free port 17, i.e. a port 17 not occupied by the unidirectional loop 5, of the first coupler side 18.

The 3×3 coupler 7 is preferably designed as a symmetrical coupler, and the bidirectional loop 3 is preferably set up to impart a phase shift of 1 rad to 3 rad between the opposing light pulses. Alternatively, however, it is also possible for the 3×3 coupler 7 to be designed asymmetrically.

A specific embodiment of the 3×3 coupler 7 is described below with reference to FIG. Numerous other embodiments are possible which are equivalent, nearly equivalent or at least functionally identical to the arrangement described, but in any case fulfill the same purpose.

The 3x3 coupler 7 has according to the embodiment shown here on the first coupler side 18 in particular a first port 17.1, a second port 17.2 and a third port 17.3, and on the second coupler side 20 a fourth port 17.4, a fifth port 17.5 and a sixth port 17.6 on. The first port 17.1 is directly across a first fiber section

22.1 is optically connected to the fourth port 17.4, the second port 17.2 is optically connected directly to the fifth port 17.5 via a second fiber section 22.2, and the third port 17.3 is optically connected directly to the sixth port 17.6 via a third fiber section 22.3. A first end 19 of the unidirectional loop 5 is optically connected to the third port 17.3. A second end 21 of the unidirectional loop 5 is optically connected to the first port 17.1. Due to the configuration and arrangement of the isolator device 13, light pulses can only propagate along the unidirectional loop 5 from the third port 17.3 to the first port 17.1 in the exemplary embodiment shown here. A first end 23 of the bidirectional loop 3 is optically connected to the fifth port 17.5. A second end 25 of the bidirectional loop 3 is optically connected to the sixth port 17.6. The second port not connected to the unidirectional loop 5, i.e. free

17.2 is the decoupling port 10. The fourth port 17.4 can preferably be used in order to decouple additional light pulses from the fiber oscillator 1, either as additional useful light or for monitoring.

A light pulse entering the 3x3 coupler 7 from the unidirectional loop 5 via the first port 17.1 is converted by the 3x3 coupler 7 into three light pulses with the same pulse energy on the fourth Port 17.4, the fifth port 17.5 and the sixth port 17.6 divided. The light pulses at the fifth port 17.5 and at the sixth port 17.6 each experience a phase shift of 2TC/3 compared to the light pulse entering at the first port 17.1. The light pulse at the fifth port 17.5 is referred to below as the first light pulse, and the light pulse at the sixth port 17.6 as the second light pulse. The first light pulse now runs through the bidirectional loop 3 starting from its first end 23 to its second end 25, with the second light pulse running through the bidirectional loop 3 in the opposite direction.

Due to the asymmetrically arranged first amplification fiber 9 in the bidirectional loop 3, the first light pulse and the second light pulse now experience different phase shifts or B integrals during their propagation along the bidirectional loop 3. The difference in the B integrals or the phase shift between the first light pulse and the second light pulse depends in particular on the original intensity of the light pulses - before passing through the bidirectional loop 3 - and the amplification in the first amplification fiber 9, i.e. in particular on a pump level of the first amplification fiber 9. In particular, the non-linear phase shift is preferably 1 rad to 3 wheel.

Arriving at the fifth port 17.5, the second light pulse now speaks partially over into the direct optical connection between the sixth port 17.6 and the third port 17.3 and again experiences a phase shift of 2TC/3. The first light pulse arriving at the sixth port 17.6 is partially forwarded directly to the third port 17.3 without experiencing a phase shift. An output pulse at the third port 17.3 resulting from superposition—in particular constructive interference—of the first light pulse and the second light pulse thus depends in particular on the B integrals that the light pulses experience during their propagation along the bidirectional loop 3.

Light components that get back into the first port 17.1 are eliminated by the isolator device 13. Only light pulses that enter the unidirectional loop 5 via the third port 17.3 are let through into the unidirectional loop 5. The bidirectional loop 3 acts as a saturable absorber.

A proportion of the second light pulse arriving at the fifth port 17.5, which does not crosstalk into the direct optical connection between the sixth port 17.6 and the third port 17.3, is in turn partially forwarded directly to the second port 17.2. In addition, part of the first arriving at the sixth port 17.6 speaks Light pulse in the immediate optical connection between the fifth port 17.5 and the second port 17.2 and experiences a phase shift of 2TC / 3. The pulse shape of the light pulse exiting at the decoupling port 10, ie the second port 17.2, results from the superposition--particularly destructive interference--of the corresponding portions of the first light pulse and the second light pulse at the second port 17.2.

In another exemplary embodiment of the laser device 2, the first end 23 of the bidirectional loop 3 is connected to the fourth port 17.4 in a light-conducting manner. The second end 25 of the bidirectional loop 3 is optically connected to the fifth port 17.5. The mode of operation of this other exemplary embodiment is analogous to the mode of operation explained above.

In the first embodiment of the fiber oscillator 1, the unidirectional loop 5 has no gain medium. In particular, the first amplification fiber 9 is the only amplification medium here, in particular the only amplification fiber of the fiber oscillator 1.

Laser device 2 also preferably has a pumped light source 29, pumped light source 29 being optically connected to fiber oscillator 1, in particular to coupling device 11, so that pumped light from pumped light source 29 can be coupled into fiber oscillator 1.

Fig. 2 shows a schematic representation of a second exemplary embodiment of the laser device 2.

Elements that are the same and have the same function are provided with the same reference symbols in all figures, so that reference is made to the previous description in each case.

In this exemplary embodiment, the unidirectional loop 5 has a reflecting arm 31 in which, in the second exemplary embodiment shown here, a reflector element 35 designed as a fiber Bragg grating 33 is arranged. The reflecting arm 31 is connected to a ring part 39 of the unidirectional loop 5 via a circulator element 37 in a light-conducting manner. In particular, the ring part 39 has a first ring branch 41 which is connected to the third port 17.3 of the 3x3 coupler 7 with a first ring branch end 43 and is connected to the circulator element 37 with a second ring branch end 45 . The ring part 39 also has a second ring branch 47 which is connected to the circulator element 37 with a first ring branch end 49 and having a second ring end 51 connected to the first port 17.1 of the 3x3 coupler 7. The circulator element 37 acts here as an isolator device 13. A light pulse passing through the unidirectional loop 5 starting from the third port 17.3 to the first port 17.1 passes through the ring branches 41, 47 once, but passes through the reflecting arm 31 twice, namely once towards the reflector element 35 , and once from the reflector element 35 back.

A second amplification fiber 53 is arranged in the reflecting arm 31 as an additional second amplification element 52, which is preferably doped with the same element with which the first amplification fiber 9 is also doped. However, the second amplification element 52, in particular the second amplification fiber 53, can also be arranged at a different point in the fiber oscillator 1.

The reflector element 35 is preferably designed to be partially transmitting or partially reflecting, with a predetermined proportion of light being coupled out of the fiber oscillator 1 via the reflector element 35, and pumping light for the second amplification fiber 53 being coupled into the unidirectional loop 5 via the reflector element 35.

The circulator element 37 acts in particular as an isolator element 57 in the unidirectional loop 5.

The reflector element 35 is preferably designed as a bandwidth limitation element 59; In particular, the fiber Bragg grating 33--which is unchirped according to one embodiment--is preferably designed as a bandwidth-limiting element 59. In a preferred embodiment, it is possible for the bandwidth limiting element 59 to be adjustable—in particular thermally or mechanically—with respect to its bandwidth.

The bandwidth limitation element 59 preferably has a bandwidth of at least 1 μm to at most 20 nm, preferably from at least 10 μm to at most 15 nm.

In particular, if the fiber Bragg grating 33 is designed as a chirped fiber Bragg grating 33, it can additionally or alternatively function as a dispersion compensation element 60.

As an alternative or in addition, it is optionally possible for a dispersion-compensating fiber 71 to be arranged as a dispersion-compensation element 60 in the bidirectional loop 3 . In a preferred embodiment, the laser device 2 has a control device 61, the control device 61 being operatively connected to the pumped light source 29 and set up, in particular to set the non-linear phase shift in the bidirectional loop 3 and thus in particular at the same time the predetermined pulse shape by selecting the pump power of the pumped light source 29 .

The control device 61 is alternatively or additionally operatively connected to the bandwidth limiting element 59, which is preferably adjustable in terms of its bandwidth - in particular thermally or mechanically - and is set up to set a bandwidth of the bandwidth limiting element 59, in particular to be able to preferably cover a larger pulse duration range than - possibly only by choosing the pump power.

Fig. 3 shows a schematic representation of a third exemplary embodiment of the laser device 2.

In this third exemplary embodiment, the unidirectional loop 5 consists of the ring part 39 - it accordingly has no reflective arm 31 - and has the additional second reinforcement element 52, here a third reinforcement fiber 63, in the ring part 39, with the third reinforcement fiber 63 preferably having is doped with the same element as the first amplification fiber 9. The isolator device 13 is arranged as the isolator element 57 behind the third amplification fiber 63 in the propagation direction.

The isolator device 13 is here at the same time as a second coupling device 65--in addition to the coupling device 11, which is a first coupling device--for coupling pump light for the third amplification fiber 63, in particular as a wavelength-multiplier coupler.

In addition, a—preferably adjustable—bandpass filter 67 is optionally arranged as the bandwidth limitation element 59 in the unidirectional loop 5 in the propagation direction before the third amplifying fiber 63 .

Furthermore, a decoupling device 69 is optionally arranged in the unidirectional loop 5, which is preferably designed as a tap coupler. In particular, useful light or light for monitoring the fiber oscillator 1 can be selectively coupled out via the coupling-out device 69 . 4 shows a diagrammatic explanation of the mode of operation of the bandwidth limitation element 59. A plot of a spectral power density of a light pulse against the wavelength is shown at a). at b) a power density of the laser pulse over time is plotted against time. A first, dashed curve Kl shows in the plots according to a) and b) a spectral or temporal shape of the light pulse before it passes through the bandwidth limitation element 59, and a second, solid curve K2 shows the corresponding shape of the light pulse after passing through the bandwidth limitation element 59

If the fiber oscillator 1 has normal dispersion overall, the interaction of normal dispersion and self-phase modulation generates strongly chirped light pulses in the fiber oscillator 1, which spread spectrally and temporally during their propagation. The bandwidth limitation element 59 advantageously cuts off parts on both sides of the spectrum and thus shortens the light pulses not only spectrally, but also temporally, due to the strong chirp. In this way in particular, the boundary condition of the periodicity for a light pulse circulating in the fiber oscillator 1 can be met.

Depending on the—preferably in particular thermally or mechanically adjustable—bandwidth of the bandwidth limiting element 59, spectrally broader or narrower light pulses are obtained in particular. Depending on this, light pulses that are shorter or longer in time can be generated by means of the laser device 2 .

5 shows a schematic representation of a fourth exemplary embodiment of the laser device 2. In this fourth exemplary embodiment, a dispersion-compensating fiber 71 is arranged in the unidirectional loop 5 as a dispersion-compensating element 60. FIG.

In particular, with the aid of the dispersion compensation element 60, regardless of its configuration—in particular according to FIG. 2 or FIG. 5—it is possible to adjust the—in particular normal—total dispersion of the fiber oscillator 1 such that it is reduced, in particular close to zero.

A bandwidth limiting element 59 is preferably also provided in this exemplary embodiment. In particular, the first amplifying fiber 9 can be embodied as a bandwidth limiting element 59 . As an alternative or in addition, a bandpass filter, for example, can also be provided as the bandwidth limitation element 59 . Irrespective of the specific design of the fiber oscillator 1--in particular according to one of the exemplary embodiments described above, all optical components of the fiber oscillator 1 are preferably designed to maintain polarization.

All optical components of the fiber oscillator 1 are preferably fiber components, or fiber-based components, or fiber-coupled components. In particular, the fiber oscillator 1 preferably has no free beam component.

Fig. 6 shows a schematic representation of the mode of operation of the laser device 2. A) shows a first diagram in which the normalized spectral power density S(X) is plotted in arbitrary units on the ordinate and the wavelength X is plotted on the abscissa, with the first diagram shows laser pulses as input pulses for the amplifier device 12 of a CPA system. In b) a second diagram is shown, in which the normalized spectral power density S(X) in arbitrary units is also on the ordinate and the wavelength /- - on the same scale as in a) - is plotted on the abscissa, with the second Diagram laser pulses are shown as the output pulses of the amplifier device 12 respectively assigned to the input pulses.

The amplifier device 12 has a finite amplification bandwidth, which results in a narrowing of the gain for laser pulses passing through the amplifier device 12 .

In the first diagram at a), K3 designates a third dashed curve, which represents an uncompensated, in particular Gaussian, laser pulse. In the second diagram at b), this input pulse is associated with a fifth dashed curve labeled K5, which shows the corresponding output pulse of the amplifier device 12 that results when the input pulse passes through the amplifier device 12 according to the third curve K3. The effect of the narrowing of the spectral gain can be clearly seen, with the fifth curve K5 having a significantly smaller spectral bandwidth than the third curve K3. As a result, in particular the bandwidth-limited temporal pulse duration of the compressed laser pulse of the CPA system is disadvantageously lengthened.

In the first diagram at a), K4 again designates a fourth, solid curve, which represents a compensated laser pulse that is matched to the gain narrowing occurring in the amplifier device 12 and that has a depression 24 in particular at a central wavelength U. This pulse shape is advantageous for narrowing the gain Amplifier device 12 is compensated, so that the corresponding output pulse of the amplifier device 12 shown in the second diagram at b) with a sixth solid curve K6 has no or only a slight spectral narrowing. This advantageously results in a significantly shorter bandwidth-limited temporal pulse duration of the compressed laser pulse of the CPA system, in particular in comparison to the laser pulse represented by the fifth curve K5.

As part of a method for shaping a laser pulse, a predetermined pulse shape is preferably provided for a laser pulse, in particular the predetermined pulse shape is determined in such a way that an amplification narrowing occurring in the amplifier device 12 is at least partially, preferably completely, compensated. The laser pulse is then generated by means of the excitation laser 14 designed as a passively mode-locked fiber oscillator 1, with at least one operating parameter of the excitation laser 14 being matched to the predetermined pulse shape in such a way that the laser pulse emerging from the decoupling port 10 of the 3×3 coupler 7 has the predetermined pulse shape.

Claims

EXPECTATIONS

1. Laser device (2), with an excitation laser (14), which is designed as a passively mode-locked fiber oscillator (1), wherein the passively mode-locked fiber oscillator (1) has a bidirectional loop (3) and a unidirectional loop (5), wherein the bidirectional loop (3) and the unidirectional loop (5) are coupled together by a 3x3 coupler (7), the bidirectional loop (3) having a first amplification fiber (9), and wherein

- at least one operating parameter of the excitation laser (14) is set in such a way that a laser pulse emerging from an outcoupling port (10) of the 3x3 coupler (7) has a pulse shape which has a trough at a central wavelength (X _c ), and with an amplifier device (12), which is set up to amplify the laser pulse generated by the excitation laser (14), the at least one operating parameter of the excitation laser (14) being matched to the amplifier device (12) in such a way that the pulse shape of the laser pulse produces a Amplifier device (12) occurring gain narrowing is at least partially compensated.

2. Laser device (2) according to claim 1, wherein the at least one operating parameter of the excitation laser (14) is selected from a group consisting of: a fiber length of the bidirectional loop (3), a fiber length of the first gain fiber (9), a non-linear phase shift in the bidirectional loop (3), in particular the arrangement or configuration of a non-reciprocal phase shifter (16) in the bidirectional loop (3) and/or a pump power coupled into the first amplification fiber (9), a power distribution in the 3x3 coupler ( 7), and a phase shift in the 3x3 coupler (7).

3. Laser device (2) according to one of the preceding claims, wherein the 3x3 coupler (7) has a first coupler side (18) and a second coupler side (20) each with three ports (17), the unidirectional loop (5) having the first coupler side (18) and the bidirectional loop (3) is connected to the second coupler side (20), and wherein the decoupling port (10) is a free port (17) of the first coupler side (18).

4. Laser device (2) according to any one of the preceding claims, wherein the 3x3 coupler (7) on a first coupler side (18) of two Kopplerseiten (18,20) of the 3x3 coupler (7) has a first port (17.1), a second port (17.2) and a third port (17.3), the 3x3 coupler (7) on a second coupler side (20) of the two coupler sides (18,20) having a fourth port (17.4), a fifth port (17.5) and a sixth port (17.6), wherein the first port (17.1) is optically connected to the fourth port (17.4) directly via a first fiber section (22.1), the second port (17.2) directly via a second fiber section (22.2) is optically connected to the fifth port (17.5), the third port (17.3) being optically connected directly to the sixth port (17.6) via a third fiber section (22.3), a first end (19) of the unidirectional loop (5) is optically connected to the third port (17.3), a second end (21) of the unidirectional loop (5) being optically connected to the first port (17.1), a first end (23) of the bidirectional loop (3) being optically connected to the fourth port (17.4) is optically connected, wherein a second end (25) of the bidirectional loop (3) is optically connected to the fifth port (17.5), and wherein the decoupling port (10) is the second port of the 3x3 coupler (7) or wherein the first end (23) of the bidirectional loop (3) is optically connected to the fifth port (17.5), wherein the second end (25) of the bidirectional loop (3) is optically connected to the sixth port (17.6). , and wherein the decoupling port (10) is the second port (17.2) of the 3x3 coupler (7).

The laser device (2) according to any one of the preceding claims, wherein the fiber oscillator (1) has normal dispersion as a whole, or wherein the fiber oscillator (1) has anomalous dispersion as a whole.

6. Laser device (2) according to one of the preceding claims, wherein the first amplifying fiber (9) is doped with at least one element selected from a group consisting of ytterbium, neodymium, erbium, thulium, and holmium, or with a combination from at least two of these elements, in particular Er/Yb or Tm/Ho.

7. Laser device (2) according to one of the preceding claims, wherein the excitation laser (14) comprises at least one dispersion compensation element (60), preferably selected from a group consisting of: a chirped fiber Bragg grating (33) and a dispersion compensating fiber (71).

8. Laser device (2) according to one of the preceding claims, wherein the 3x3 coupler (7) is constructed as a symmetrical coupler, wherein the bidirectional loop (3) is arranged to impart a non-linear phase shift of 1 rad to 3 rad.

9. A method for shaping a laser pulse, wherein - a predetermined pulse shape is provided for the laser pulse, the laser pulse being generated by means of an excitation laser (14) which is designed as a passively mode-locked fiber oscillator (1), the passively mode-locked fiber oscillator (1) a bidirectional loop (3) and a unidirectional loop (5), the bidirectional loop (3) and the unidirectional loop (5) being coupled together by a 3x3 coupler (7), the bidirectional loop (3) having a first Having reinforcing fiber (9), wherein

- at least one operating parameter of the excitation laser (14) is matched to the predetermined pulse shape in such a way that the laser pulse emerging from a coupling-out port of the 3x3 coupler (7) has the predetermined pulse shape and the laser pulse is amplified in an amplifier device (12), the predetermined pulse shape is determined such that in the amplifier device (12) occurring amplification narrowing is at least partially compensated.