Passively mode-locked fiber oscillator, laser device, and non-linear cpa-amplification system comprising such a fiber oscillator

Abstract

The invention relates to a passively mode-locked fiber oscillator (1) for generating seed pulses, said fiber oscillator being suitable for non-linear CPA amplification systems (73), comprising ˗ a bidirectional loop (3) and a unidirectional loop (5), wherein ˗ the bidirectional loop (3) and the unidirectional loop (5) are coupled together by a loop coupler (7), ˗ the bidirectional loop (3) has a first amplification fiber (9), ˗ the fiber oscillator (1) has a total fiber length with which the repetition rate of the fiber oscillator (1) equals maximally 10 MHz, ˗ the fiber oscillator (1) has at least one dispersion compensation element (60), and ˗ the at least one dispersion compensation element (60) is designed such that the fiber oscillator (1) has a total dispersion of 0.04 ps² to 0.1 ps².

Description

[0001] DESCRIPTION

[0002] Passively mode-locked fiber oscillator, laser device and nonlinear CPA amplification system with such a fiber oscillator

[0003] The invention relates to a passively mode-locked fiber oscillator, a laser device with such a fiber oscillator and a non-linear CPA amplification system with such a fiber oscillator or with such a laser device.

[0004] There are two main classes of optical pulse lasers: mode-locked and Q-switched. Mode-locked lasers can generate ultrashort optical pulses with high repetition rates, while Q-switched lasers are generally used for generating high-energy pulses at relatively low repetition rates. If sub-ps pulses are to be generated, a mode-locked oscillator system is typically chosen. Typically, the repetition rate is then set to the desired value after the oscillator using a pulse selection device, i.e., a pulse picker, e.g., an acousto-optic modulator (AOM) or an electro-optic modulator (EOM).

[0005] Passively mode-locked fiber oscillators typically have repetition rates of more than 10 MHz. Achieving lower repetition rates is particularly difficult for fiber oscillators constructed exclusively from fiber components and without free-space sections. In particular, there is no simple way to adjust the repetition rate if the fiber oscillator consists exclusively of fiber components and no free-space section. This is all the more true when high quality requirements are placed on the spectrum of the generated laser pulses, such as when the laser pulses are used as seed pulses for a nonlinear CPA amplification system. Spectra of such laser pulses should exhibit the lowest possible residual ripple, in particular, no ripples or modulations.

[0006] The invention is based on the object of creating a passively mode-locked fiber oscillator, a laser device with such a fiber oscillator, and a nonlinear CPA amplification system with such a fiber oscillator or with such a laser device, wherein the aforementioned disadvantages are at least reduced, preferably not occurring at all. This object is achieved by providing the present technical teaching, in particular the teaching of the independent claims and the embodiments disclosed in the dependent claims and the description.

[0007] The object is achieved in particular by providing a passively mode-locked fiber oscillator for generating seed pulses, wherein the seed pulses generated by the fiber oscillator are suitable for nonlinear CPA amplification systems. The fiber oscillator has a bidirectional loop and a unidirectional loop, wherein the bidirectional loop and the unidirectional loop are coupled to one another by a loop coupler, and wherein the bidirectional loop has a first amplification fiber. The fiber oscillator has a total fiber length at which a repetition rate of the fiber oscillator is at most 10 MHz. In addition, the fiber oscillator has at least one dispersion compensation element, wherein the at least one dispersion compensation element is designed such that the fiber oscillator has a total dispersion in a predetermined range of 0.04 ps. 2 up to 0.1 ps 2By appropriately selecting the total fiber length, a repetition rate of 10 MHz or less, in particular less than 10 MHz, can be provided, particularly without the need for a free-space path. At the same time, by adjusting the overall normal total dispersion to the predetermined range of 0.04 ps 2 up to 0.1 ps 2ensures that the spectrum of the generated seed pulses is suitable for use as input pulses for amplification in a nonlinear CPA amplification system. In particular, the seed pulses generated with the fiber oscillator according to the invention have a spectrum with low residual ripple, in particular with low ripples. Double pulses are also advantageously avoided. Setting the total dispersion within the predetermined range is important for the spectrum of the generated laser pulses and the functioning of the fiber oscillator: If the dispersion is too low, reproducibly stable operation of the fiber oscillator cannot be guaranteed; if the dispersion is too high, a spectrally narrow filter is required for stable operation. Short pulses and thus wider spectra can therefore only be generated through the influence of significant nonlinear broadening per oscillator round trip, which creates spectral ripples / modulations.Particularly in the range of total dispersion proposed here, smooth spectra are obtained that are sufficiently broad, especially with regard to subsequent amplification in a CPA amplification system. By appropriately selecting the fiber length, the repetition rate of the fiber oscillator can be tailored to an industrial application. In particular, no pulse selection device is required. This is particularly advantageous because it reduces the complexity of the overall system and also avoids degradation effects, thus improving the quality of the generated laser pulses, at least in the long term.

[0008] In particular, the spectrum of the generated seed pulses has less than 20% ripples.

[0009] The fact that the seed pulses generated by the fiber oscillator are suitable for non-linear CPA amplification systems means in particular that the fiber oscillator is suitable for generating such seed pulses which are suitable for use in non-linear CPA amplification systems, that is to say which are particularly suitable for being amplified in non-linear CPA amplification systems.

[0010] In particular, the fiber oscillator is configured to generate laser pulses with a bandwidth-limited pulse duration of less than 1 ps.

[0011] In particular, the dispersion compensation element is designed as a fiber component.

[0012] A fiber oscillator is understood, in particular, to be a laser oscillator that has at least one optical component, in particular for light guidance and / or light influencing, that has a fiber or consists of a fiber. In a preferred embodiment, all optical components of the fiber oscillator are fiber components, i.e., components that in particular have a fiber or consist of a fiber, in particular fiber-based components or fiber-coupled components.

[0013] A loop is understood to be an optical part of the fiber oscillator which has a first end and a second end, wherein both the first end and the second end are coupled to the same connection component of the fiber oscillator, here in particular to the loop coupler. This means in particular that light pulses which pass through the loop from the connection component return along the loop to the connection component. Such a loop can be designed as a ring overall; in particular, the loop in this case 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 connected to the ring part in a light-conducting manner, in particular exactly one ring part and exactly one linear branch.A bidirectional loop is understood to be 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 - i.e. in both directions.

[0014] A unidirectional loop is understood, in particular, to be a loop in which light pulses can only propagate along the loop in a specific direction, either from the first end to the second end or from the second end to the first end. Preferably, an isolator device, in particular an isolator, is arranged in the unidirectional loop, wherein the isolator device is configured to allow light pulses to pass 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 device is preferably arranged in a ring part of the unidirectional loop.

[0015] The bidirectional loop is preferably a first fiber loop.

[0016] A fiber loop is understood to be a loop that has at least one fiber in some areas or consists of one fiber. In a preferred embodiment, the fiber loop consists entirely of one fiber or is composed of a plurality of interconnected fibers.

[0017] The unidirectional loop is preferably a second fiber loop. In particular, the unidirectional loop is preferably formed as a unidirectional ring.

[0018] According to a further development of the invention, the total fiber length is designed such that the fiber oscillator has a repetition rate of at most 5 MHz.

[0019] The repetition rate f rep of the fiber oscillator depends on the total fiber length l according to the following equation: with the optical path length nl, which is the product of the refractive index n of the fiber components of the fiber oscillator and the total fiber length l. Equation (1) can be used in particular if the fiber components all have the same refractive index, or if at least the refractive indices of the different fiber components do not differ too much from each other, in which case, in particular, an average refractive index n can be used. If the refractive index of individual

[0020] Fiber components are strong, however, the following consideration can be made: where to determine the total optical path length over the partial optical path lengths n t l t of the individual fiber components z is summed up.

[0021] To determine the total fiber length, the fiber length of the unidirectional loop and the bidirectional loop are added together. If the fiber oscillator also has a reflective arm—particularly as part of the unidirectional loop—the fiber length of the reflective arm is multiplied by a factor of 2 and added to the sum of the fiber length of the bidirectional loop and the fiber length of the unidirectional loop, or to the fiber length of the part of the unidirectional loop not included in the reflective arm.

[0022] According to a further development of the invention, the total fiber length is at least 20 m. In this way, a repetition rate of at most 10 MHz, in particular at most 5 MHz, can be achieved.

[0023] In one embodiment, the fiber oscillator is self-starting.

[0024] According to a further development of the invention, the loop coupler is designed as a 3x3 coupler. In particular, this configuration enables self-starting of the fiber oscillator in a simple manner.

[0025] The 3x3 coupler preferably has a plurality of ports, in particular six ports. In one embodiment of the laser device, the 3x3 coupler is symmetrical, which means, in particular, that light pulses are distributed equally among the various ports of the 3x3 coupler. In particular, the 3x3 coupler thus has a symmetrical power distribution. A port is understood to be a connection of the 3x3 coupler that can act as an input or output, in particular, can be connected to a fiber in a light-conducting manner.

[0026] In one embodiment, the 3x3 coupler has a first coupler side and a second coupler side, each with three ports per coupler side, wherein the unidirectional loop is connected to the first coupler side and the bidirectional loop is connected to the second coupler side.

[0027] In one embodiment, the 3x3 coupler has a first port, a second port, and a third port on the first of two coupler sides of the 3x3 coupler, and the 3x3 coupler has a fourth port, a fifth port, and a sixth port on the second of the two coupler sides. The first port is directly connected to the fourth port in a light-conducting manner via a fiber section, the second port is directly connected to the fifth port in a light-conducting manner via a fiber section, and the third port is directly connected to the sixth port in a light-conducting manner via a fiber section. Light pulses propagating between two directly connected ports, in particular, do not experience any phase shift.The 3x3 coupler is particularly designed so that light pulses can crosstalk between the immediate connections of the ports, experiencing a phase shift that is preferably 2TC / 3, regardless of the two connections between which a light pulse crosstalks.

[0028] In one embodiment of the fiber oscillator, the 3x3 coupler is generally configured to impart a phase shift of 2TC / 3 to light pulses that crosstalk between different direct connections of the ports of the 3x3 coupler. Another embodiment is also possible in which the 3x3 coupler is configured asymmetrically, resulting in different phase shifts for the light pulses that crosstalk between different direct optical connections of the ports.

[0029] In a first embodiment of the laser device, a first end of the bidirectional loop is optically connected to the fourth port, and a second end of the bidirectional loop is optically connected to the fifth port. The second port and / or the sixth port can preferably be used to additionally couple light pulses out of the fiber oscillator, either as useful light or for monitoring.

[0030] In another, second embodiment of the laser device, the first end of the bidirectional loop is optically connected to the fifth port, wherein the second end of the bidirectional loop is optically connected to the sixth port. The second port and / or fourth port can preferably be used to couple light pulses out of the fiber oscillator, either as useful light or for monitoring. A first end of the unidirectional loop is optically connected to the third port, wherein a second end of the unidirectional loop is optically connected to the first port. The unidirectional loop is preferably configured—in particular by the isolator device—such that a light pulse along the unidirectional loop can only travel from the third port to the first port—or vice versa.

[0031] In particular, this description describes specific embodiments of the 3x3 coupler, taking into account certain possible arrangements and connections of ports of the 3x3 coupler. Those skilled in the art will readily recognize that other embodiments exist that are equivalent, nearly equivalent, or at least functionally identical to the described arrangements, but in any case fulfill the same purpose.

[0032] Without wishing to be bound by theory or the specific embodiment, the function of the fiber oscillator is explained in more detail below in connection with the aforementioned second embodiment: A light pulse entering the 3x3 coupler from the unidirectional loop via the first port is split into three light pulses with equal pulse energy, distributed to the fourth port, the fifth port, and the sixth port. The light pulses at the fifth and sixth ports each experience a phase shift of 2TC / 3 compared to the light pulse entering 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 passes through the bidirectional loop starting from the first end to the second end - namely from the fifth port to the sixth port, while the second light pulse passes through the bidirectional loop in the opposite direction - namely from the sixth port to the fifth port.

[0033] Particularly in an asymmetrical configuration of the bidirectional loop, the first light pulse and the second light pulse 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 an asymmetry element, in particular the amplification in the first amplification fiber, thus in particular on a pump level of the first amplification fiber. The attenuation can also be configured variably if necessary to influence the phase shift.Having arrived at the fifth port, the second light pulse partially transmits into the direct optical connection between the sixth port and the third port, again experiencing a phase shift of 2TC / 3. The first light pulse arriving at the sixth port is partially transmitted 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 experienced by the light pulses during their propagation along the bidirectional loop.

[0034] The 3x3 coupler is configured in such a way that, even with a vanishing nonlinear phase shift between the first light pulse and the second light pulse, a finite transmission of preferably approximately 10% of the input pulse energy and a non-vanishing slope of the phase-dependent transmission curve are achieved. This significantly simplifies laser pulse generation from noise. In particular, this facilitates the start, especially self-start, of 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.

[0035] By varying the pump power for the first gain fiber in the bidirectional loop, the nonlinear phase shift between the first light pulse and the second light pulse can be variably adjusted.

[0036] At the same time, the portion of the second light pulse arriving at the fifth port that does not crosstalk into the direct optical connection between the sixth port and the third port is partially transmitted directly to the second port, with a portion of the first light pulse arriving at the sixth port crosstalking into the direct optical connection between the fifth port and the second port and in turn experiencing a phase shift of 2TC / 3.

[0037] The functionality of the aforementioned first embodiment is analogous.

[0038] As an alternative to the loop coupler being designed as a 3x3 coupler, it is also possible for the loop coupler to be designed as a 2x2 coupler. To make the fiber oscillator self-starting, a phase shifter is preferably additionally provided in this case. According to a further development of the invention, the first amplification fiber is doped with ytterbium. This enables the overall normal total dispersion of the fiber oscillator to be tuned to the predetermined range in a particularly simple manner.

[0039] Alternatively or additionally, the first reinforcement fiber is doped with at least one element selected from a group consisting of neodymium, erbium, thulium, and holmium.

[0040] In one embodiment, the first amplification fiber is doped with exactly one of the elements selected from the group consisting of ytterbium, neodymium, erbium, thulium, and holmium. In another embodiment, the first amplification fiber is doped with a combination of at least two of the mentioned elements, in particular with a combination of exactly two of the mentioned elements. In one embodiment, the first amplification fiber is doped with erbium and ytterbium (Er / Yb). In another embodiment, the first amplification fiber is doped with thulium and holmium (Tm / Ho).

[0041] The doping element or, where appropriate, the combination of doping elements determines in particular an optical wavelength for the fiber oscillator: If the first amplification fiber comprises ytterbium or neodymium as a doping element, the wavelength is approximately 900 nm to 1100 nm. If the first amplification fiber comprises erbium as a doping element, the wavelength is approximately 1500 nm. If the first amplification fiber comprises thulium or holmium as a doping element, the wavelength is approximately 1900 nm to 2100 nm.

[0042] The fact that the fiber oscillator exhibits normal total dispersion, or—in other words, but equivalently—that the total dispersion of the fiber oscillator lies within the normal dispersion range, means in particular that a light pulse passing through the fiber oscillator has experienced normal dispersion after one pass through the fiber oscillator—that is, each component of the fiber oscillator has passed through once. This, in turn, means that, compared to the temporal shape of the light pulse before passing through the fiber oscillator, higher frequencies lag behind, while lower frequencies lead, in the temporal shape of the light pulse after passing through the fiber oscillator. Higher frequencies therefore pass 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 occurs at least for the sum of the optical components. While it is therefore possible in a preferred embodiment 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, wherein the fiber oscillator has at least one other, second optical component that has normal dispersion, which overcompensates for the anomalous dispersion of the first optical component, so that the dispersion of the fiber oscillator as a whole is normal.

[0043] If the wavelength of the fiber oscillator lies within the normal dispersion range, for example, when using ytterbium or neodymium as the doping element, the dispersion compensation element is provided and configured to keep the total dispersion of the fiber oscillator within the predetermined range for the fiber oscillator. However, if the wavelength of the fiber oscillator lies within the anomalous dispersion range, for example, when using erbium, thulium, or holmium as the doping element, the dispersion compensation element is additionally provided and configured to bring the total dispersion into the normal range and thereby into the predetermined range.

[0044] In one embodiment, the fiber oscillator comprises a plurality of dispersion compensation elements as the dispersion compensation element. It is then particularly important that the overall effect of the various dispersion compensation elements is such that the total dispersion of the fiber oscillator lies within the predetermined range.

[0045] In one embodiment, the dispersion compensation element is arranged in the unidirectional loop.

[0046] According to a further development of the invention, the at least one dispersion compensation element is selected from a group consisting of a chirped fiber Bragg grating and a dispersion-compensating fiber. Alternatively or additionally, the first amplifying fiber is configured to compensate for dispersion. The dispersion-compensating fiber or the dispersion-compensating first amplifying fiber is also referred to as a dispersion-compensating fiber or dispersion-matched fiber. Such a dispersion-compensating fiber can, for example, have a fiber core comprising rings with different refractive indices.

[0047] In one embodiment, the dispersion-compensating fiber exhibits anomalous dispersion, in particular if the fiber oscillator otherwise exhibits, at least substantially, in particular overall—in particular with the exception of the dispersion-compensating fiber—a normal dispersion that is greater than the predetermined total dispersion. The anomalous dispersion of the dispersion-compensating fiber is matched to the normal dispersion of the remaining components of the fiber oscillator such that the total dispersion lies within the predetermined range.

[0048] In another embodiment, the dispersion compensation element formed as a chirped fiber Bragg grating is formed as a reflector element of a reflecting arm of the unidirectional loop.

[0049] In one embodiment, the bidirectional loop has an asymmetry. In particular, one embodiment provides for the bidirectional loop to be asymmetrically configured for two light pulses passing through the bidirectional loop in opposite directions.

[0050] According to a further development of the invention, the bidirectional loop has an asymmetry element.

[0051] In one embodiment, the asymmetry element is an amplification element arranged asymmetrically in the bidirectional loop - in particular variably adjustable - for an asymmetric amplification of the light pulses propagating in opposite directions along the bidirectional loop.

[0052] Alternatively or additionally, the asymmetry element is an attenuation element arranged asymmetrically in the bidirectional loop - in particular variably adjustable - for an asymmetric attenuation of the light pulses propagating in opposite directions along the bidirectional loop.

[0053] The asymmetry element is generally configured 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.

[0054] The asymmetrically arranged amplification element is preferably variably adjustable in terms of amplification. In particular, if the first amplification fiber is configured as the amplification element, variable amplification can be achieved by varying the pump power. Alternatively or additionally, the asymmetrically arranged attenuation element is preferably variably adjustable in terms of attenuation.

[0055] In general, a variable phase shift between the two counter-propagating light pulses in the bidirectional loop can be achieved by variable adjustment of the asymmetry element; in particular, the phase shift can be adjusted by variable control of the asymmetry element.

[0056] In particular, according to one embodiment, the first amplification fiber can be arranged asymmetrically in the bidirectional loop. This means, in particular, that the first amplification 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, an asymmetrically arranged attenuation element, in particular an asymmetrically arranged output coupling element, for example a tap coupler, or a filter, a polarization attenuator, or the like, can be arranged in the bidirectional loop. The aforementioned embodiments can also be combined with one another.

[0057] In particular, the bidirectional loop is preferably designed as a nonlinear amplifying loop mirror (NALM). In this case, the bidirectional loop exhibits an asymmetry such that different light pulses traversing the bidirectional loop in different directions pass through a longer portion of the bidirectional loop with different intensity levels, depending on their direction of travel, because they are amplified and / or attenuated sooner or later, relative to the path of the bidirectional loop. Due to the self-phase modulation in the bidirectional loop, this leads to a phase shift between two light pulses traversing the bidirectional loop in opposite directions. This phase shift itself is intensity-dependent. The phase shift between the two light pulses, in turn, influences their coupling behavior at the loop coupler.In this way, light pulses above a certain intensity threshold are effectively fed from the bidirectional loop into the unidirectional loop via the loop coupler in the appropriate propagation direction, allowing the bidirectional loop, designed as a NALM, to function as a saturable absorber. In particular, the NALM fiber length is adjusted so that the phase difference in the NALM leads to constructive interference at the fiber output. The loop arrangement consisting of the bidirectional loop and the unidirectional loop, which are coupled to each other via the loop coupler, and thus also the fiber oscillator as a whole, preferably has a so-called figure-8 configuration.

[0058] In one embodiment, the unidirectional loop has no gain medium. In this case, the first gain fiber is advantageously the only gain medium of the fiber oscillator, in particular the only gain fiber. The fiber oscillator can thus have a very simple and cost-effective design.

[0059] In an alternative preferred embodiment, the unidirectional loop comprises an additional amplification medium, in particular a second amplification fiber, wherein an isolator element—in a preferred embodiment, the isolator device of the unidirectional loop already provided—is arranged in the propagation direction of a light pulse—preferably in the unidirectional loop—between the amplification element and the first amplification 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 amplification element. The amplification element can advantageously be used to compensate for losses, in particular, by amplifying light pulses in the fiber oscillator not only in the first amplification fiber, but also in the additional amplification medium.At the same time, this allows for greater freedom in selecting the gain for the first amplification fiber and thus a more flexible adjustment of the phase shift between the first light pulse and the second light pulse, since a change in the overall gain of the fiber oscillator when varying the gain in the first amplification fiber can be compensated accordingly using the additional gain medium. In particular, this design enables inefficient operation of the bidirectional loop, explained in more detail below, which is beneficial for the quality of the spectra of the generated seed pulses.

[0060] In a preferred embodiment, the isolator element can be designed as an isolator or as a circulator.

[0061] The second amplification fiber is preferably doped with the same element as the first amplification fiber. The bidirectional loop preferably has a coupling device configured to couple pump light into the first amplification fiber. The coupling device arranged in the bidirectional loop can also simultaneously serve to couple pump light into the additional amplification medium, in particular into the second amplification fiber. Furthermore, the preferably asymmetrically arranged coupling device can serve as an asymmetry element, in particular as an asymmetrically arranged attenuation element.

[0062] Alternatively, it is preferably possible for a coupling device to be arranged in the unidirectional loop, which is configured to couple pump light into the additional amplification medium, in particular the second amplification fiber. Preferably, the coupling device also serves to simultaneously couple pump light into the first amplification fiber.

[0063] Alternatively, it is preferably also possible for the bidirectional loop to have a first coupling device for coupling pump light into the first amplification fiber, wherein the unidirectional loop has a second coupling device which is configured to couple pump light into the additional amplification medium.

[0064] 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 multiplexer (WDM).

[0065] According to a further development of the invention, the unidirectional loop comprises a reflective arm, with a reflector element arranged in the reflective arm. According to one embodiment, additional optical functions can also be realized via the reflector element, in particular the function of a bandwidth limiting element and / or a dispersion compensation element. The reflective arm offers advantages, particularly with regard to the arrangement and bilateral insulation of an additional gain medium in the reflective arm.

[0066] The reflective arm preferably has at least one fiber or preferably consists of at least one fiber.

[0067] The reflector element is preferably arranged at a reflection end of the reflective arm. The reflective arm is preferably designed as a linear branch of the unidirectional loop, which is light-conductingly connected to a ring part of the unidirectional loop. The reflective 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 passing through the unidirectional loop passes through the reflective arm twice: once from the connection end to the reflection end, and then back from the reflection end to the connection end.

[0068] The reflector element is preferably partially transparent - or, conversely, partially reflective - so that a predetermined proportion of light is coupled out of the fiber oscillator via the reflector element.

[0069] In one embodiment of the fiber oscillator, the reflector element is designed as a wavelength-fixing element, i.e., in particular, as an element configured to establish a central wavelength for the fiber oscillator. Thus, the reflector element advantageously enables a clear definition of the central wavelength at which the fiber oscillator is operated. This offers the significant advantage of high reproducibility while simultaneously increasing variability in order to obtain a specific, desired wavelength as the central wavelength. This can be particularly crucial in downstream processes whose efficiency depends on the wavelength, for example, in material processing, in an amplification chain, and / or in frequency conversion.

[0070] In one embodiment, the reflector element is designed as a fiber Bragg grating. The fiber Bragg grating can preferably function as the dispersion compensation element, as a wavelength fixing element, and / or as a bandwidth limiting element. In order to function as the dispersion compensation element, the fiber Bragg grating is preferably designed as a chirped fiber Bragg grating. The fiber Bragg grating can also function as a wavelength fixing element or as a bandwidth limiting element when designed as an unchirped fiber Bragg grating.

[0071] In one embodiment, the dispersion compensation element is formed by the reflector element in that the reflector element is designed as a chirped fiber Bragg grating.

[0072] In particular, the reflector element is the dispersion compensation element. In one embodiment, the reflective arm is optically connected to a ring part of the unidirectional loop via a circulator element. The circulator element preferably also serves as an isolator device for the unidirectional loop. The ring part has a first ring branch, which is optically connected to the loop coupler at a first ring branch end—in particular to the third port of the 3x3 coupler—and to the reflective arm at a second ring branch end. The ring part also has a second ring branch, which is optically connected to the reflective arm at a first ring branch end and to the loop coupler at a second ring branch end—in particular to the first port of the 3x3 coupler.A light pulse entering the first ring branch via the loop coupler travels through it to the circulator element, is coupled by it into the connecting end of the reflective arm, travels through the reflective arm to the reflector element arranged at the reflective end, is at least partially reflected there, travels along the reflective arm back to the connecting end, is coupled there by the circulator element into the second ring branch, and travels through the second ring branch again to the loop coupler. Thus, the light pulse each travels through the first ring branch and the second ring branch once, while the reflective arm travels twice—back and forth.

[0073] In particular, a second amplification fiber is arranged in the unidirectional loop, in particular in the reflective arm, in particular as the additional amplification medium already mentioned above. The second amplification fiber is preferably arranged in the reflective arm. This proves to be particularly advantageous because a light pulse propagating in the unidirectional loop passes through the second amplification fiber twice, so that the light pulse is amplified twice. Furthermore, the second amplification fiber is advantageously separated from the first amplification fiber by the circulator element—in particular in both directions—so that the two amplification fibers do not adversely influence each other.

[0074] The second amplifying fiber is preferably doped with the same element as the first amplifying fiber.

[0075] The fiber oscillator preferably has a coupling device for coupling pump light into the fiber oscillator, in particular into the unidirectional loop, located outside the unidirectional loop, in particular outside the loop arrangement, behind the first reflector element in the propagation direction of a light pulse coupled out by the reflector element. In this way, pump 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 reflective arm.

[0076] In one embodiment, the fiber oscillator has a bandwidth-limiting element. The bandwidth-limiting element is preferably arranged in the unidirectional loop. The interplay of normal dispersion and self-phase modulation generates strongly chirped light pulses in the fiber oscillator, which broaden spectrally and temporally as they propagate. The bandwidth-limiting element advantageously cuts off components on both sides of the spectrum and thus—due to the strong chirp—shortens the light pulses not only spectrally but also temporally. In this way, in particular, the boundary condition of periodicity for a light pulse circulating in the fiber oscillator can be met.

[0077] The bandwidth limiting element preferably has a bandwidth of at least 4 nm to at most 100 nm, in particular to at most 30 nm, in particular from at least 5 nm to at most 20 nm.

[0078] In one embodiment, the bandwidth-limiting element is designed as a bandpass filter. This represents a suitable configuration of the bandwidth-limiting element.

[0079] Alternatively or additionally, it is preferably provided that the reflector element, in particular the fiber Bragg grating, is designed as a bandwidth-limiting element. This is particularly advantageous because it then eliminates the need for an additional component for bandwidth limitation. In this case, the fiber Bragg grating can be designed as an unchirped fiber Bragg grating or as a chirped fiber Bragg grating.

[0080] The fiber Bragg grating may additionally or alternatively function as the dispersion compensation element, in particular if it is designed as a chirped fiber Bragg grating.

[0081] Alternatively or additionally, a dispersion compensating fiber is preferably arranged in the unidirectional loop as the dispersion compensation element.

[0082] In one embodiment, an additional amplification fiber is arranged in the unidirectional loop, which is referred to here as a third amplification fiber for the purpose of linguistic differentiation, regardless of whether a second amplification fiber is additionally present. This configuration is particularly preferred in an embodiment of the fiber oscillator in which the unidirectional loop consists of a ring part, wherein the unidirectional loop in particular has no linear branch, in particular no reflective arm. Thus, the third amplification fiber is arranged in particular in the ring part of the unidirectional loop. The third amplification fiber can advantageously compensate for losses in the fiber oscillator and / or deliberately inefficient operation of the bidirectional loop.

[0083] However, according to another embodiment, it is provided that the third amplifying fiber is provided in addition to a second amplifying fiber provided in the reflecting arm, wherein in this case too, the third amplifying fiber is preferably arranged in the ring part of the unidirectional loop.

[0084] The third amplifying fiber is preferably doped with the same element as the first amplifying fiber - and preferably as the second amplifying fiber.

[0085] The fiber oscillator in the unidirectional loop preferably has an output coupling device for outputting light pulses. In this way, it is possible to output light pulses—whether as useful light or for testing the fiber oscillator—not only via the loop coupler, in particular the second port or the fourth or sixth port of the 3x3 coupler, but additionally or alternatively via the output coupling device. Due to the interplay of dispersion on the one hand and self-phase modulation on the other along the fiber oscillator, the output light pulses have different temporal widths depending on their location of output. Light pulses with different temporal widths can therefore be output, in particular, from the loop coupler, in particular from the second port of the 3x3 coupler, the fourth port of the 3x3 coupler, and via the output coupling device.

[0086] The decoupling device is preferably designed as a tap coupler.

[0087] The bandwidth-limiting element, in particular the reflector element or the bandpass filter, is preferably designed to be adjustable in terms of its bandwidth, preferably as a temperature-dependent grating or as a grating whose bandwidth is sensitive to expansion or compression. In one embodiment, all optical components of the fiber oscillator are designed to maintain polarization. This proves to be a particularly advantageous configuration for the fiber oscillator.

[0088] In one embodiment, all optical components of the fiber oscillator are formed by fibers or consist of fibers, in particular fiber-based components or fiber-coupled components. In particular, the fiber oscillator preferably has no free-space components. In this case, no adjustment effort is required in connection with the fiber oscillator.

[0089] However, in another embodiment it is also possible for the fiber oscillator to have at least one optical component which is designed as a free-beam component.

[0090] The object is also achieved by providing a laser device comprising a pump light source and a fiber oscillator according to the invention or a fiber oscillator according to one or more of the previously described embodiments, wherein the pump light source and the fiber oscillator are connected to one another in a light-conducting manner, so that pump light from the pump light source can be coupled into the fiber oscillator. In connection with the laser device, the advantages already described in connection with the fiber oscillator are particularly evident.

[0091] According to a further development of the invention, it is provided that the laser device has a control device.

[0092] The control device is preferably operatively connected to a variably controllable asymmetry element of the bidirectional loop in order to adjust the variable asymmetry element, in particular to adjust the nonlinear phase shift between the light pulses passing through the bidirectional loop in the opposite direction, in particular such that a difference in the B-integrals of two laser pulses passing through the bidirectional loop in opposite directions is approximately 1 rad. Advantageously - contrary to intuition - the fiber oscillator and in particular the bidirectional loop, i.e., the NALM, is deliberately operated inefficiently in order to limit the entire nonlinear phase per oscillator round trip despite the low repetition rate.In particular in this embodiment, the unidirectional loop preferably has an additional amplification medium, in particular a second and / or third amplification fiber, which can advantageously compensate for the inefficient operation of the NALM.

[0093] In particular, the control device is configured to adjust the variable asymmetry element such that the difference in the B-integrals of two laser pulses passing through the bidirectional loop in opposite directions is 1 rad.

[0094] Alternatively, the control device is configured to adjust the variable asymmetry element such that the nonlinear phase shift between the light pulses passing through the bidirectional loop in the opposite direction is at most 2 / 3, preferably 2 / 3. This preferably allows a larger pulse duration range to be covered than—possibly only—by selecting the pump power.

[0095] In particular, the control device is preferably operatively connected to a variably controllable amplification element in order to adjust the variably controllable amplification element with regard to its amplification.

[0096] In one embodiment, the control device is operatively connected to the pump light source and configured to adjust a pump power of the pump light source such that a difference in the B-integrals of two laser pulses passing through the bidirectional loop in opposite directions is approximately 1 rad.

[0097] In particular, the control device is configured to adjust the pump power of the pump light source such that the difference in the B-integrals of two laser pulses passing through the bidirectional loop in opposite directions is 1 rad

[0098] Alternatively, the control device is configured to adjust the pump power of the pump light source such that the non-linear phase shift between the light pulses passing through the bidirectional loop in the reverse direction is at most 2 / 3, preferably 2 / 3.

[0099] Alternatively or additionally, the control device is preferably operatively connected to a variably controllable attenuation element in order to adjust the attenuation of the variably controllable attenuation element, in particular to achieve one of the aforementioned effects with regard to the B-integrals or the phase shift. Alternatively or additionally, it is provided that the control device is operatively connected to the bandwidth-limiting element, in particular the reflector element or the bandpass filter, which is adjustable with regard to its bandwidth, optionally in interaction with a further optical element, in particular a further bandwidth-limiting element, and is configured to adjust a bandwidth of the bandwidth-limiting element, in particular to at least 4 nm to a maximum of 100 nm, in particular to a maximum of 30 nm, in particular from at least 5 nm to a maximum of 20 nm.

[0100] Preferably, the fiber oscillator comprises a further filter element in addition to the adjustable bandwidth limiting element, wherein an overlap range between the bandwidth limiting element and the filter element can be adjusted by adjusting the bandwidth of the adjustable bandwidth limiting element. In this way, an effective bandwidth of the combination of the bandwidth limiting element and the filter element can be adjusted very efficiently, in particular to at least 4 nm to at most 100 nm, in particular to at most 30 nm, in particular from at least 5 nm to at most 20 nm.

[0101] The strip width limiting element can in particular be thermally or mechanically adjustable, for example by heating or cooling, or by stretching or compressing.

[0102] An adjustable bandwidth limitation can also be achieved with a Fabry-Perot filter, in which a distance between two surfaces responsible for the Fabry-Perot property is changed.

[0103] The control device is preferably configured to generate a first, higher asymmetry in the bidirectional loop in a start-up 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 is configured to control the variably controllable asymmetry element in a continuous mode of operation in order to generate a second, lower asymmetry in the bidirectional loop in order to ensure stable continuous operation of the fiber oscillator. In particular, the control device is configured to control a variably controllable attenuation element accordingly, in particular to set a first, higher attenuation in the start-up mode and to set a second, lower attenuation in the continuous mode of operation.The object is also achieved by creating a nonlinear CPA amplification system comprising a fiber oscillator according to the invention or a fiber oscillator of one or more of the previously described embodiments for generating seed pulses. Alternatively, the CPA amplification system comprises a laser device according to the invention or a laser device according to one or more of the previously described embodiments for generating seed pulses. Optionally, the amplification system comprises a pulse selection device for selecting or discarding seed pulses. This can be provided, in particular, although this is not necessary due to the described design of the fiber oscillator, in particular in order to be able to additionally change the repetition rate if necessary.The amplification system comprises a pulse stretcher for temporally stretching the seed pulses, an amplifier arrangement for amplifying the temporally stretched seed pulses, and a compression device for temporally compressing the amplified, temporally stretched seed pulses. In connection with the amplification system, the advantages already described in connection with the fiber oscillator or the laser device arise in particular.

[0104] In the context of the present technical teaching, the fact that the amplification system is designed as a CPA amplification system means, in particular, that the amplification in the amplification system is carried out according to the principle of "chirped pulse amplification" (CPA). In the context of the present technical teaching, a nonlinear CPA system is understood, in particular, to be a CPA system that allows a specific nonlinear phase during amplification or propagation in the fiber.

[0105] Optionally, the amplification system comprises a pulse selection device for selecting or discarding time-stretched seed pulses, amplified time-stretched seed pulses, and / or compressed amplified seed pulses.

[0106] In one embodiment, the amplifier arrangement comprises at least one preamplifier and at least one main amplifier. In particular, in one embodiment, the amplifier arrangement comprises a plurality of preamplifiers and one main amplifier.

[0107] According to a further development of the invention, it is provided that a total non-linear phase of the amplification system is at least % rad, preferably more than % rad. This means in particular that the amplification system is configured such that its total non-linear phase is at least % rad, preferably more than % rad. This has the advantage that, for a given stretch factor, the maximum extractable pulse energy from the amplification system can be increased. Although the compressibility of the amplified pulses suffers from the non-linear phase, it can be significantly influenced, in particular, by a suitable choice of the input spectrum. In order to achieve the highest possible pulse energy with simultaneous good compressibility of the pulses, the requirements for the shape of the input spectra are therefore very high, and the fiber oscillator proposed here is advantageously suited to meeting these high requirements.

[0108] The invention is explained in more detail below with reference to the drawings, which show:

[0109] Figure 1 is a schematic representation of a first embodiment of a passively mode-locked fiber oscillator;

[0110] Figure 2 is a schematic representation of a second embodiment of a passively mode-locked fiber oscillator;

[0111] Figure 3 is a schematic representation of a third embodiment of a passively mode-locked fiber oscillator;

[0112] Figure 4 is a schematic representation of a fourth embodiment of a passively mode-locked fiber oscillator, and

[0113] Figure 5 is a schematic representation of an embodiment of a nonlinear CPA amplification system with a passively mode-locked fiber oscillator.

[0114] Fig. 1 shows a schematic representation of a first embodiment of a passively mode-locked fiber oscillator E. The 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 to one another, in particular optically connected, by a loop coupler 7, here a 3x3 coupler. A first amplification fiber 9 is arranged in the bidirectional loop 3. The fiber oscillator 1 has a total fiber length at which a repetition rate of the fiber oscillator is at most 10 MHz, and a total normal dispersion of 0.04 ps. 2 up to 0.1 ps 2Advantageously, the bidirectional loop 3 can assume the function of a saturable absorber. By specifically tuning the total dispersion of the fiber oscillator 1 within the predetermined range, well-defined dispersion properties are advantageously provided. In a preferred embodiment, the amplification fiber 9 is designed as a dispersion compensation element 60—and optionally additionally as a bandwidth limiting element 59. In particular, the amplification fiber 9 can assume the function of bandwidth limiting due to its gain bandwidth.

[0115] The first reinforcement fiber 9 is preferably doped with at least one element selected from a group consisting of: ytterbium, neodymium, erbium, holmium, and thulium. The first reinforcement fiber 9 can also be doped with a combination of at least two of the aforementioned elements, in particular with a combination of exactly two of these elements.

[0116] Preferably, the bidirectional loop 3 has an asymmetry for two light pulses that 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 gain element 6 and / or an asymmetrically arranged attenuation element 8 in the bidirectional loop 3. In the exemplary embodiment illustrated here, the first gain fiber 9 is arranged asymmetrically in the bidirectional loop 3 as a gain element 6. In particular, the bidirectional loop 3 is designed as a nonlinear amplifying loop mirror (NALM).

[0117] Preferably, a coupling device 11 for coupling pump light is arranged in the bidirectional loop 3. The coupling device 11 is preferably designed as a wavelength division multiplex (WDM) coupler. The coupling device 11 can also act as an attenuator 8. A tap coupler, for example, can also be arranged as the attenuator 8 in the bidirectional loop 3.

[0118] An isolator device 13, in particular an isolator 15, is preferably arranged in the unidirectional loop 5.

[0119] The loop coupler 7, designed as a 3x3 coupler, is preferably configured to impart a phase shift of 2TC / 3 ZU to light pulses that crosstalk between various direct connections of a plurality of ports 17 of the 3x3 coupler. Opposing light pulses in the NALM are then imparted a corresponding phase shift.

[0120] In the following, a specific embodiment of the 3x3 coupler is described with reference to Figure 1, taking into account a specific possible arrangement and connection of the ports 17 of the 3x3 coupler. Numerous other embodiments are possible that are equivalent, nearly equivalent, or at least functionally identical to the described arrangement, but in any case fulfill the same purpose.

[0121] According to the exemplary embodiment illustrated here, the 3x3 coupler has, in particular, a first port 17.1, a second port 17.2, a third port 17.3, a fourth port 17.4, a fifth port 17.5, and a sixth port 17.6. 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 design and arrangement of the isolator device 13, light pulses can propagate along the unidirectional loop 5 only from the third port 17.3 to the first port 17.1. 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 17.2 and the fourth port 17.4 are preferably used to couple light pulses out of the fiber oscillator 1, either as useful light or for monitoring.

[0122] A light pulse entering the 3x3 coupler from the unidirectional loop 5 via the first port 17.1 is split by the 3x3 coupler into three light pulses with equal pulse energy, which are sent to the fourth port 17.4, the fifth port 17.5, and the sixth port 17.6. The light pulses at the fifth port 17.5 and the sixth port 17.6 each experience a phase shift of 2TC / 3 compared to the light pulse entering 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 passes through the bidirectional loop 3, starting from its first end 23 and moving to its second end 25, while the second light pulse passes through the bidirectional loop 3 in the opposite direction.

[0123] 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, in particular on a pump level of the first amplification fiber 9. Having arrived at the fifth port 17.5, the second light pulse now partially communicates into a direct optical connection between the sixth port 17.6 and the third port 17.3, again experiencing a phase shift of 2TC / 3. The first light pulse arriving at the sixth port 17.6 is directly routed to the third port 17.3 without experiencing a phase shift. An output pulse at the third port 17.3 resulting from the superposition of the first light pulse and the second light pulse thus depends in particular on the B-integrals experienced by the light pulses during their propagation along the bidirectional loop 3.

[0124] Light components that return to 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 allowed to pass. The bidirectional loop 3 functions as a saturable absorber.

[0125] In the first embodiment of the fiber oscillator 1, the unidirectional loop 5 has no gain medium. In particular, the first gain fiber 9 is the only gain medium, in particular the only gain fiber of the fiber oscillator 1.

[0126] Figure 1 also shows an embodiment of a laser device 27 which has a pump light source 29 and the fiber oscillator 1, wherein the pump light source 29 is light-conductingly connected to the fiber oscillator 1, in particular to the coupling device 11, so that pump light from the pump light source 29 can be coupled into the fiber oscillator 1.

[0127] Fig. 2 shows a schematic representation of a second embodiment of the fiber oscillator 1.

[0128] Identical and functionally identical elements are provided with the same reference numerals in all figures, so that reference is made to the preceding description in each case.

[0129] In this embodiment, the unidirectional loop 5 has a reflective arm 31, in which, in the second embodiment shown here, a reflector element 35 designed as a fiber Bragg grating 33 is arranged. The reflective arm 31 is connected in a light-conducting manner to a ring part 39 of the unidirectional loop 5 via a circulator element 37. In particular, the ring part 39 has a first ring branch 41, which is connected by a first ring branch end 43 to the third port 17.3 of the 3x3 coupler, and by a second ring branch end 45 to the circulator element 37. The ring part 39 also has a second ring branch 47, which is connected by a first ring branch end 49 to the circulator element 37 and by a second ring branch end 51 to the first port 17.1 of the 3x3 coupler. The circulator element 37 acts here as an isolator device 13. A unidirectional loop 5 starting from the third port 17.3 light pulse passing to the first port 17.1 passes through the ring branches 41, 47 once each, but through the reflecting arm 31 twice, namely once towards the reflector element 35 and once back from the reflector element 35.

[0130] A second amplification fiber 53 is arranged in the reflecting arm 31 as an additional amplification medium 52, which is preferably doped with the same element with which the first amplification fiber 9 is doped. However, the amplification medium 52, in particular the second amplification fiber 53, can also be arranged elsewhere in the fiber oscillator 1.

[0131] The reflector element 35 is preferably designed to be partially transmissive or partially reflective, wherein on the one hand a predetermined proportion of light is coupled out of the fiber oscillator 1 via the reflector element 35, and on the other hand pump light for the second amplifying fiber 53 is preferably coupled into the unidirectional loop 5 via the reflector element 35.

[0132] The circulator element 37 acts in particular as an isolator element 57 in the unidirectional loop 5.

[0133] The reflector element 35 is preferably designed as a bandwidth limiting element 59; in particular, the fiber Bragg grating 33—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 with respect to its bandwidth—in particular thermally or mechanically.

[0134] The bandwidth limiting element 59 preferably has a bandwidth of at least 4 nm to at most 100 nm, in particular to at most 30 nm, in particular from at least 5 nm to at most 20 nm.

[0135] In particular, if the fiber Bragg grating 33 is designed as a chirped fiber Bragg grating 33, it can additionally or alternatively function as the dispersion compensation element 60. Figure 2 also shows a second embodiment of the laser device 27, which in a preferred embodiment has a control device 61, wherein the control device 61 is operatively connected to the pump light source 29 and is preferably configured to adjust a pulse duration of the fiber oscillator 1 by selecting the pump power of the pump light source 29.

[0136] The control device 61 is alternatively or additionally configured to adjust the nonlinear phase shift between the light pulses passing through the bidirectional loop in the opposite direction by appropriately controlling the pump light source 29, in particular such that a difference in the B-integrals of two laser pulses passing through the bidirectional loop in opposite directions is approximately 1 rad. Advantageously—contrary to intuition—the fiber oscillator 1 and in particular the NALM are deliberately operated inefficiently in order to obtain smooth spectra at a low repetition rate. This inefficient operation is advantageously compensated for by the additional amplification medium 52, in particular the second amplification fiber 53.

[0137] The control device 61 is alternatively or additionally operatively connected to the bandwidth limiting element 59, which is preferably designed to be adjustable with respect to its bandwidth - in particular thermally or mechanically - and is configured to set a bandwidth of the bandwidth limiting element 59, in particular in order to preferably be able to cover a larger pulse duration range than - possibly only - by selecting the pump power.

[0138] Fig. 3 shows a schematic representation of a third embodiment of the fiber oscillator 1.

[0139] In this third embodiment, the unidirectional loop 5 consists of the ring part 39 - it accordingly has no reflective arm 31 - and has the additional gain medium 52, here a third gain fiber 63, in the ring part 39, wherein the third gain fiber 63 is preferably doped with the same element as the first gain fiber 9. The isolator device 13 is arranged as the isolator element 57 behind the third gain fiber 63 in the propagation direction.

[0140] The isolator device 13 is also designed here 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 division multiplex coupler. Furthermore, a—preferably adjustable—bandpass filter 67 is optionally arranged in the unidirectional loop 5 as the bandwidth limiting element 59, upstream of the third amplification fiber 63 in the propagation direction.

[0141] Furthermore, an output coupling device 69, which is preferably designed as a tap coupler, is optionally arranged in the unidirectional loop 5. Via the output coupling device 69, in particular, useful light or light for monitoring the fiber oscillator 1 can be output. In this third embodiment, the first amplification fiber 9 can also function as the dispersion compensation element 60.

[0142] Fig. 4 shows a schematic representation of a fourth embodiment of the fiber oscillator 1. In this fourth embodiment, a dispersion-compensating fiber 71 is arranged in the unidirectional loop 5 as a dispersion compensation element 60.

[0143] By means of the dispersion compensation element 60, regardless of its design - in particular according to Figure 2 or Figure 4 - it is possible to adjust the - normal - total dispersion of the fiber oscillator 1 to the predetermined range.

[0144] Preferably, a bandwidth-limiting element 59 is also provided in this exemplary embodiment. In particular, the first amplifying fiber 9 can be designed as a bandwidth-limiting element 59. Alternatively or additionally, a bandpass filter, for example, can also be provided as the bandwidth-limiting element 59.

[0145] Regardless of the specific design of the fiber oscillator 1 - in particular according to one of the previously described embodiments - all optical components of the fiber oscillator 1 are preferably designed to maintain polarization.

[0146] Preferably, all optical components of the fiber oscillator 1 are fiber components, or fiber-based components, or fiber-coupled components. In particular, the fiber oscillator 1 preferably has no free-space component.

[0147] Fig. 5 shows a schematic representation of an embodiment of a non-linear CPA amplification system 73 with a passively mode-locked fiber oscillator 1.

[0148] The nonlinear CPA amplification system 73 comprises the fiber oscillator 1, in particular the laser device 27, for generating seed pulses. Optionally, the CPA amplification system 73 comprises a pulse stretcher 75 for selecting or discarding seed pulses, as well as a pulse selection device 77 for temporally stretching the seed pulses, an amplifier arrangement 79 for amplifying the temporally stretched seed pulses, in particular with at least one preamplifier 81 and at least one main amplifier 83, and a compression device 85 for temporally compressing the amplified, temporally stretched seed pulses. A total nonlinear phase of the CPA amplification system 73 is preferably at least 1 / 2 rad.

Claims

REQUIREMENTS 1. A passively mode-locked fiber oscillator (1) for generating seed pulses, suitable for nonlinear CPA amplification systems (73), comprising a bidirectional loop (3) and a unidirectional loop (5), wherein the bidirectional loop (3) and the unidirectional loop (5) are coupled to each other by a loop coupler (7), wherein the bidirectional loop (3) has a first amplification fiber (9), wherein the fiber oscillator (1) has a total fiber length at which a repetition rate of the fiber oscillator (1) is at most 10 MHz, wherein the fiber oscillator (1) has at least one dispersion compensation element (60), and wherein the at least one dispersion compensation element (60) is configured such that the fiber oscillator (1) has a total dispersion of 0.04 ps 2 up to 0.1 hp 2 exhibits.

2. Fiber oscillator (1) according to claim 1, wherein the total fiber length is configured such that the fiber oscillator (1) has a repetition rate of at most 5 MHz.

3. Fiber oscillator (1) according to any one of the preceding claims, wherein the total fiber length is at least 20 m.

4. Fiber oscillator (1) according to one of the preceding claims, wherein the loop coupler (7) is configured as a 3x3 coupler.

5. Fiber oscillator (1) according to one of the preceding claims, wherein the first reinforcing fiber (9) is doped with ytterbium.

6. Fiber oscillator (1) according to one of the preceding claims, wherein the at least one dispersion compensation element (60) is selected from a group consisting of a chirped fiber Bragg grid (33) and a dispersion compensating fiber (71).

7. Fiber oscillator (1) according to one of the preceding claims, wherein the bidirectional loop (3) is an asymmetry element, in particular an asymmetrically arranged - preferably It features a variably adjustable reinforcement element and / or an asymmetrically arranged attenuating element.

8. Fiber oscillator (1) according to one of the preceding claims, wherein the unidirectional loop (5) has a reflective arm (31), wherein a reflector element (35) is arranged in the reflective arm, and wherein preferably the reflective arm (31) is connected via a circulator element (37) to a ring part (39) of the unidirectional loop (5) in a light-conducting manner, wherein preferably a second reinforcing fiber (53) is arranged in the unidirectional loop (5), in particular in the reflective arm (31).

9. Laser device (27), comprising a pump light source (29) and a fiber oscillator (1) according to any one of claims 1 to 8, wherein the pump light source (29) and the fiber oscillator (1) are connected to each other in a light-conducting manner, so that pump light from the pump light source (29) can be coupled into the fiber oscillator (1).

10. Laser device according to claim 9, comprising a control device (61), wherein the control device (61) is operatively connected to the pump light source (29) and is configured to adjust a pump power of the pump light source (29) such that a difference in the B integrals of two laser pulses traversing the bidirectional loop (3) in opposite directions is approximately 1 rad, preferably 1 rad.

11. Nonlinear CPA amplification system (73), comprising a fiber oscillator (1) according to any one of claims 1 to 8 or a laser device (27) according to any one of claims 9 or 10 for generating seed pulses, optionally a pulse selection device (77) for selecting or discarding seed pulses, a pulse stretcher (75) for stretching the seed pulses in time, an amplifier arrangement (79) for amplifying the time-stretched seed pulses, and a compression device (85) for time-compressing the amplified, time-stretched seed pulses.

12. Amplification system (73) according to claim 11, wherein an entire nonlinear phase of the amplification system (73) is at least % rad.

Images