WO2019002378A1

WO2019002378A1 - Dynamic seeding of laser amplification systems

Info

Publication number: WO2019002378A1
Application number: PCT/EP2018/067268
Authority: WO
Inventors: Dominik Bauer; Aleksander BUDNICKI; Rainer Flaig; Florian Jansen
Original assignee: Trumpf Laser Gmbh
Priority date: 2017-06-28
Filing date: 2018-06-27
Publication date: 2019-01-03
Also published as: DE102017114399A1; CN110915079A; CN110915079B

Abstract

The invention relates to a laser amplification system (1) comprising a seed laser pulse source unit (3) for providing at least two seed pulse sequences for subsequent amplification, wherein seed pulses (3') of the at least two seed pulse sequence have a seed pulse duration (T, TA, TB) and a seed amplitude (A) varying within a range during the seed pulse duration (T, TA, TB) and adjustable in the profile of the seed pulses. The laser amplification system (1) also has an amplifier stage and in particular an amplification chain (5) made of (for example fiber) amplifier stages (5A, 5B, 5C), which output an output pulse sequence (5'). The laser amplification system (1) is formed in such a way that an amplitude profile of an output pulse (5') of the output pulse sequence can be traced back to contributions from at least two seed pulses (3').

Description

DYNAMIC SEEDING OF LASER AMPLIFIER SYSTEMS

The present invention relates to laser amplifier systems, in particular for generating high power laser pulses. Furthermore, the invention relates to a method for dynamically seeding a laser amplifier system, such as a fiber laser amplifier system.

When amplifying pulses in laser amplifier systems, it is possible to achieve outputs which can lead to a pulse shape change due to saturation of the amplification. For example, the pulse shape of a nanosecond pulse may change during amplification in a high gain fiber laser amplifier system when the fiber laser amplifier system is operated sufficiently near or above saturation. Pulse shape changes may make it difficult to achieve a pulse shape desired by the user, or in extreme cases may also result in the destruction of optical components such as a fiber due to excessive pulse peak power. It is known that via a compensation in the seed pulse form, a defined pulse shape can be achieved at the output of a laser amplifier system, in particular at high output powers. The compensation in the seed pulse form can be made by controlling a seed laser. For example, For example, the energization of a diode laser acting as a seed laser (also referred to herein as a seed diode) can be varied. The achievable dynamics when driving a seed diode is limited, for example, by reaching the laser threshold. At the top, a maximum amplitude is e.g. is limited by a damage threshold of the semiconductor structure and / or by thermal effects in the semiconductor structure of the underlying laser diode structure. Furthermore, pulse shaping, e.g. a pulse pruning, with a control intervention e.g. between two stages of an amplifier chain by means of e.g. acousto-optic modulators (AOM, AOD, AOTF, ...), electro-optic modulators (EOM, Pockels cells, ...) or mechanical switches. The trimming of the pulses often takes place late in an amplifier chain, which can result in high power losses.

One aspect of this disclosure is based on the object of providing pulse shaping of a seed laser pulse which enables efficient amplification.

At least one of these objects is achieved by a laser amplifier system according to claim 1, by a laser amplifier system according to claim 5, by a method for grounding a gain chain according to claim 16 and by a method for selecting a gain chain according to claim 18. Further developments are given in the subclaims.

In one aspect, a laser amplifier system comprises a seed laser pulse source unit for providing at least two seed pulse trains for subsequent amplification, seed pulses of the at least two seed pulse sequences have a respective seed pulse duration and a seed amplitude which varies within a range during the respective seed pulse duration and can be adjusted in its course. Furthermore, a laser amplifier system comprises at least one amplifier stage for amplifying the at least two seed pulse trains and for outputting an output pulse train with output pulses having an output pulse duration. The at least two seed pulse trains are coupled into the amplifier stage in such a way that an amplitude characteristic of an output pulse of the output pulse train is based on contributions from at least two seed pulses, which are each assigned to one of the at least two seed pulse sequences. In a further aspect, a laser amplifier system comprises a seed laser pulse source unit for providing at least two seed pulse trains for subsequent amplification, seed pulses of the at least two seed pulse trains having a respective seed pulse duration and one within a range during the respective seed pulse duration have varying and adjustable in their course seed amplitude. The laser amplifier system further comprises a gain chain comprising a sequence of at least two gain stages coupled in series, the gain in the gain stages being sequential to form intermediate pulses associated with the gain stages, and the gain chain outputting an output pulse sequence having output pulse duration output pulses an amplitude course of an output pulse of the output pulse train to contributions of at least two seed pulses, each associated with one of the at least two seed pulse sequences, goes back.

In another aspect, a method for generating a train of amplified output pulses comprises the steps of providing at least two seed pulse trains for subsequent amplification, wherein seed pulses of the at least two seed pulse trains each have a seed pulse duration and one within one Range varying during the seed pulse duration and in their

Have adjustable seed amplitude, and amplifying the seed pulses in an amplifier stage, so that there is an output pulse train having an output pulse duration having output pulses, wherein the at least two seed pulse trains are coupled into the amplifier stage such that an amplitude characteristic of an output pulse of the output pulse train to contributions from at least two seed pulses, each associated with one of the at least two seed pulse sequences, goes back.

In a further aspect, a method of generating a train of amplified output pulses comprises the steps of providing at least two seed pulse trains for subsequent amplification, seed pulses of the at least two seed pulse trains each having a seed pulse duration and a seed pulse duration, respectively Range during the seed pulse duration having varying and progressively adjustable seed amplitude, and amplifying the seed pulses in sequentially arranged amplifier stages to form intermediate pulses associated with the amplifier stages, such that an output pulse sequence results in output pulses having an output pulse duration, an amplitude characteristic of an output pulse of the output pulse sequence being based on contributions from at least two seed pulses which are each assigned to one of the at least two seed pulse sequences.

In some embodiments, the seed laser pulse source unit may be configured to provide at least two sub-seed pulse trains to be amplified, the sub-pulses of which have a partial pulse duration. The seed laser pulse source unit comprises e.g. at least one laser pulse source, e.g. an adjustable energizable laser diode. At least one of the partial pulse durations may be shorter than an output pulse duration of the output pulses. In each case one of the at least two partial seed pulse sequences to be amplified can deliver one of the at least two contributions from at least two seed pulses and the at least two contributions can be jointly supplied to one of the amplifier stages for the formation of one of the intermediate pulses. In some embodiments, the laser amplifier system, and in particular the control of the seed laser pulse source unit, may be configured such that the at least two seed pulses, which contribute to the amplitude characteristic of an output pulse, are temporally spaced, temporally adjoined, or overlapping in time. In some embodiments, the laser amplifier system, and in particular the control of the seed laser pulse source unit, may be configured such that the at least two seed pulses that contribute to the amplitude response of an output pulse are spaced apart in time, temporally adjoin one another or overlap in time. Thus, seed pulses may e.g. with a time offset smaller than the seed pulse duration, spaced apart. Furthermore, the laser amplifier system, and in particular the control of the seed laser pulse source unit, can be configured such that at least one seed pulse, which contributes to the amplitude characteristic of an output pulse, is temporally spaced with respect to an intermediate pulse, adjoined in time thereto or in time therewith overlaps. Thus, e.g. the at least one seed pulse having a time offset smaller than the seed pulse duration (T) is spaced from the intermediate pulse.

Furthermore, a temporal segment of the output pulses may be due to at least one seed laser pulse source whose seed pulses have passed through all amplifier stages of the amplification chain. Alternatively or additionally, the seed pulses contributing to the amplitude progression of an output pulse of one of the at least two seed pulse sequences may represent part-pulses of a sub-seed pulse sequence to be amplified whose partial pulse duration is shorter than the output pulse duration of the output pulses. In some embodiments, the laser amplifier system may include an optical delay unit for generating a laser pulse time offset between pulses that contribute to the amplitude characteristic of an output pulse, in particular seed pulses or subpulses, a split unit for splitting a laser pulse into two or more subpulses, a combiner unit for combining the Partial pulses, such as an X: (100-X) (fiber) combiner, and / or an attenuation unit for reducing the amplitude of laser pulses, partial pulses and / or intermediate pulses. The optical delay unit may, for example, generate a laser pulse time offset between a seed pulse of one of the at least two seed pulse sequences and an intermediate pulse or between seed pulses of different ones of the at least two seed pulse sequences

In some developments, the laser amplifier system may have at least one pulse-shaping device for changing the amplitude of one of the intermediate pulses. The pulse-shaping device can in particular be designed as an amplitude-changing unit arranged between two adjacent amplifier stages, for example as acousto-optical modulator or electro-optical modulator. Alternatively or additionally, at least one pulse-shaping device may be provided following the amplification for changing the amplitude of the output pulses of the output pulse train.

In some embodiments, the seed laser pulse source unit may include at least one laser pulse source in the form of an adjustable energizable laser diode, and optionally, for each of the at least two seed pulse trains, a separate laser pulse source.

In some embodiments, the laser amplifier system may further comprise a drive unit, which is designed for setting the seed pulse shape, in particular for driving a seed source, a combination unit and / or a pulse-shaping device. For example, the drive unit may be designed to derive the seed pulse profile from a target pulse profile, in particular a target amplitude characteristic of one of the output pulses, and the seed pulse form, in particular the contributions from the at least two seed pulses and / or the circumcision of the amplitude at least one of the associated intermediate pulses to adjust accordingly. Furthermore, the drive unit may be designed to control the intensity of the pulse-shaping device during material processing, in particular during the beam guidance around a curved path, in order to reduce the output pulses.

Further, in some embodiments of the methods, an amplitude characteristic of one of the output pulses may be due to a truncation of the amplitude of at least one seed pulse / intermediate pulse. Furthermore, the amplitude profiles of the seed pulses and optionally the truncation of the amplitude of at least one associated intermediate pulse can be adapted to one another in such a way that a dyad Namely, the gain after the amplification is greater than the dynamic range of a single seed pulse.

In some embodiments of the method, at least one seed pulse sequence can be coupled into the amplifier stage in such a way and, optionally, at least one seed pulse sequence can be coupled into a subsequent amplifier stage in such a way that an intermediate pulse to be amplified is formed, whose amplitude profile depends on contributions from at least two seed pulses. Pulses, each associated with one of the at least two seed pulse sequences, goes back.

The pulsed multi-site interventions proposed herein in a laser amplifier system provide a high dynamic range that can enable achievement of a target pulse shape at the output of the laser amplifier system, particularly at the end of a high gain amplifier chain.

In contrast to a purely temporal pulse shaping between two stages, which basically requires that a significant power component of the pulse is attenuated and thus lost, the herein disclosed highly dynamic Seeden of (eg fiber) lasers can be in principle at least partially lossless and significantly more efficient and allow an increased dynamic range. Thus, a combination of the concepts disclosed herein with known technologies may enable pulse control to an unprecedented extent.

This is usually exemplified by (laser) pulses. In this case, a pulse may inter alia include a burst pulse sequence as a "burst pulse." Accordingly, a pulse below its pulse envelope comprises the burst pulse sequence, Furthermore, a seed pulse sequence generally comprises one or more pulses.

Herein, concepts are disclosed that allow to at least partially improve aspects of the prior art. In particular, further features and their expediencies emerge from the following description of embodiments with reference to the figures. From the figures show:

1 is a schematic representation of a laser system with multiple amplifier stages,

Fig. 2 is a schematic representation of a desired for a high gain

Seed pulse shape with high amplitude dynamics,

3 shows a schematic representation of a pulse shape which can be achieved, for example, with a laser diode and diode current modulation,

Fig. 4A and Fig. 4B are schematic representations of achievable by means of partial pulse combination

Pulse shapes, 5 is a schematic representation of a cascaded partial pulse-based seed

Multi-interventional design in a laser amplifier system,

6 and FIG. 7 are schematic representations of the partial pulse combination for seed-pulse

Generation,

8 is a schematic representation of the partial pulse generation and partial pulse

Combination based on a single laser pulse source,

9 is a schematic representation for achieving the desired target pulse shape by means of amplitude truncation at a plurality of intervention sites in a laser amplifier system and

10 is a schematic representation to illustrate an amplitude truncation.

Aspects disclosed herein are based, in part, on the recognition that pulse circumcision at the end of an amplification process may be inefficient because, depending on the pulse shape, a high power component may be lost through pulse shaping. Furthermore, a decoupled high (loss) power is safe, and thus consuming to dissipate. Furthermore, modulation of laser pulses at very high powers can be technically difficult to implement.

The aspects disclosed herein are further based, in part, on the recognition that large (exponential) amplification may require controlling the pulse shape (a single pulse or a burst pulse train) in a high dynamic gain process. For pulse shaping with high dynamics, pulse-forming interventions can be made which adapt the shape of a pulse underlying the amplification to a stepped amplification process. In this case, the shaping interventions can be made at at least two different locations within an amplifier chain and / or by means of two or more different approaches. The starting point is in each case a pulse formation of a seed pulse of a seed laser pulse source. This first shaping engagement provides seed pulses in a seed pulse train having a seed pulse duration and a seed amplitude varying within a range during the seed pulse duration and adjustable in course. This can e.g. be done by an adjustable, for example, increasing, energization of a (seed) laser diode. In general, the seed pulse can be designed as a single pulse or as a burst pulse train.

Since the dynamics of the seed laser pulse source, in the above example, on the area in which the

Current supply is limited, is limited due to the already mentioned laser threshold and maximum amplitude of a seed diode, it is proposed herein, with a second shaping engagement (generally with other shaping interventions) the limit in the control of the dynamic range of the pulse amplitude of a seed pulse to expand. In each case, the formative interventions via analytical models such as the Frantz-Nodvik equation or other numerical approaches simulated and adapted the shaping interventions accordingly. Additionally or alternatively, the shaping interventions may be performed by measuring pulse parameters and corresponding iterative enhancements of the interventions.

In some embodiments, a single output pulse of a laser amplifier system may be due to multiple (directly contiguous and overlapping) sub-pulses (as a single pulse or as a burst pulse train). Moreover, in order to further increase the dynamics, the partial pulses may also be applied at different locations of the amplifier chain, e.g. between adjacent amplifier stages. Alternatively or additionally, an amplitude adjustment can also be made between different amplifier stages. As a result, the total power losses can be minimized or at least reduced.

In some embodiments described below, e.g. a pulse is preformed by modulating the current of a laser diode. Before the subsequent amplifier stage and / or between two successive amplifier stages, the pulse (as a single pulse or as a burst pulse sequence) can also be shaped by an optical modulator. As a result, pulse deformations that occur during the amplification process, for example due to the saturation of the gain, are precompensated or corrected in between. Since such interventions occur in a low power range of the amplification process, they are accompanied by lower losses.

The above-mentioned analytical models, based for example on the Frantz-Nodvik equation, may allow seed pulse shapes, possibly a specially required combination of several partial pulses and / or trimming, with regard to the desired target pulse shape or knowledge of parameter changes made in the laser system to determine and adjust the amplitude. Such control concepts can also be carried out by means of a controlling unit, for example, the involved seed diodes performing control unit during operation of a laser system, in particular in real time or when setting an operating mode. Many applications of short-pulse lasers, for example in the nano- and picosecond range, can benefit from such control over the pulse shape at the output of a laser system or an amplifier stage. In the following, various measures for pulse shaping will be explained in more detail by way of example in connection with the figures. 1 shows a multi-stage laser amplifier system 1 with a seed laser pulse source unit 3, a gain chain 5 and an (optional) final amplifier stage 7. The various units and their components can be implemented, for example, via fiber couplers / combiners (as an example of a fiber optic unit). specific (pulse) combiner unit 9A), fiber splice portions 9B, transport fiber portions (exemplified between units / components indicated by lines 9C), and / or free jet portions 9D. Although the gain chain 5 described below is primarily based on fiber laser amplifier stages, laser amplifier stages may generally include, but are not limited to, (rod-type) fiber, rod, slab, and / or disk laser amplifier stages.

The seed laser pulse source unit 3 comprises one or more seed lasers 3A, 3B, 3C (seed sources) for providing at least one seed pulse train with seed pulses 3 'for subsequent amplification. By way of example, three seed lasers are shown in FIG. Examples of seed lasers include DFB, DBR or Fabry-Perot laser diodes, as well as mode-locked ultrashort pulse (UKP) lasers or microchipliers. Furthermore, the seed laser pulse source unit 3 comprises a (diode) driving unit 1 1 for setting the seed pulse shape (s). The drive unit 11 is connected via dotted control connections to, for example, three seed lasers 3A, 3B, 3C. For example, the controller may adjust the modulation of the current (i.e., the energization) of the seed diode (s) to the subsequent gain. Furthermore, the seed power of one or more seed lasers 3A, 3B, 3C can be adjusted with a attenuation unit. An example is one

Abschwächeinheit 4 in the beam path of the seed laser 3B schematically drawn and connected to the drive unit 1 1. Die Abwächeinheit 4 ist mit dem Seed-Laser 3B verbunden. Further, the drive unit 11 may generally adapt parameters of the seed laser pulse source unit 3 (generally, the laser amplifier system 1) to a specific application of the generated laser beam.

The reinforcing chain 5 may be e.g. a sequence of fiber amplifier stages 5A, 5B, which are embodied, for example, as cocurrently or countercurrently pumped, core- or shell-pumped fiber amplifiers. By way of example, a pumping diode 6 is shown in FIG. 1, which is divided in its power and fed to the amplifiers 5A and 5B in a reverse-propagating manner (in opposite directions). In the fiber amplifier stages 5A, 5B, the coupled seed pulse 3 'is sequentially amplified to form intermediate pulses 3 "assigned to the fiber amplifier stages 5A, 5B or preamplified for the final amplifier stage 7 To clarify that other amplifier systems can also be used as amplifier stages, an amplifier stage 5C, for example, is not designed as a fiber amplifier stage.

The amplification chain 5 can furthermore have modulators 13, 15 (eg amplitude modulators such as acousto-optical modulators or electro-optical modulators or spectral shaping units such as spatial light modulators (SLM)) for additional temporal or spectral pulse shaping or power stabilization. One or more monitoring units 17 also serve to stabilize the power and / or monitor the pulse shape. For example, the modulators 13, 15 and the monitoring units 17 are also controlled by the control unit. tion unit 1 1 driven or give data to this. Through the power monitoring 17, for example, the seed lasers 3A, 3B, 3C (seed power), the pump diode 6 (pump power) and the modulators 13, 15 (amplitude modulation / spectral shaping) with the aim of stabilization or adaptation of the drive unit. 1 1 are controlled.

Resulting output pulses 5 'of the amplification chain 5 can be used directly, for example for material processing. Alternatively, the resulting output pulses 5 'of the gain chain 5 may be further supplied as seed pulses to the final amplifier stage 7 for generating power amplifier pulses 7. The final amplifier stage 7 is, for example, an amplifier designed as a main amplifier and schematically indicated in FIG. 1, with a disk-shaped laser-active medium.

The thus amplified output amplifier pulses 7 'or even the output coil 5' can e.g. in machine tools for laser cutting, laser welding and material processing, such as e.g. for micromaterialbearbeitung, or used for frequency conversion. They can also be used for scientific purposes, such as pumping OPCPA, as well as in spectroscopy.

The amplification-related pulse deformations mentioned at the outset can arise in the fibers used in the amplification chain 5 with correspondingly high amplification (and corresponding inversion). Accordingly, during amplification in the (fiber) amplifier stages 5A, 5B, 5C, the original seed pulse shape may change due to saturation effects. For example, the front part of the seed pulse or the first pulses of a burst pulse train can experience a significantly increased gain compared to the rear part, as schematically indicated in FIG. 1 with respect to the seed pulse 3 'and the intermediate pulse 3 " in the exemplary representation of FIG. 1 (as well as in the exemplary representations of FIGS. 5 and 8), the pulse shape is schematically indicated over the propagation direction, in contrast to the time representation in FIGS. 2 to 4B.

The drive unit 11 may be designed according to the concepts disclosed herein for setting seed pulse shape (s), in particular for driving a seed source, a combining unit and / or pulse-forming device (15, 60). Furthermore, the drive unit 11 may optionally be designed to derive a seed pulse course from a target pulse course. In particular, a target amplitude characteristic of one of the output pulses can be derived. Accordingly, the drive unit 1 1 can set the seed pulse shape (s), in particular the contributions from the at least two seed pulses and / or the truncation of the amplitude of at least one of the associated intermediate pulses. The derivative may be based, for example, on the already mentioned algorithms and based on measured power values. Of course, a drive unit can also be constructed from a plurality of individual drive units. It is noted that in some amplifier configurations, the effect of the amplification process on the pulse shape may be substantially negligible, for example, if the aforementioned saturation is not achieved. This may, for example, be the case in a correspondingly designed disk amplifier (wherein in general a pulse shape change may also occur in a disk amplifier). In a case with negligible pulse shape change, the target pulse shape is essentially already the output pulse shape of the gain chain 5 and not the pulse shape after the final amplifier stage 7. The pulse shape after the final amplifier stage 7 may be almost identical in shape to the output pulse shape of the gain chain 5, albeit intensity-enhanced , For example, it can only have changes that are negligible for the subsequent station, for example, a material processing device. This is to be taken into account when determining the target pulse shape and the corresponding control of the seed diodes and / or modulator (s) of the drive unit 1 1. For example, a rectangular pulse with pulse durations in the nanosecond or picosecond range, as indicated by way of example in FIG. 1, may be required as the target pulse shape for a laser processing process. Other examples include a plateau pulse with or without rising and / or falling edge, generally trapezoidal or any pulse shapes. If, for example, a rectangular pulse of about 250 ns pulse duration is to be present at the end of the amplifier chain 5, a seed pulse shape is needed which precompensates the overproportional gain of the front region, for example by an exponential increase in amplitude.

2, an exponential increase of the amplitude A with increasing time t is shown schematically for an "ideal" seed pulse (for example for a rectangular output pulse) with a pulse duration T. For such an amplitude increase, the seed laser diode would be, for example, exponentially increasing Power to operate.

However, the laser threshold and maximum amplitude mentioned at the outset limit the dynamic range of a seed laser diode between minimum and maximum current (or between minimum and maximum seed pulse amplitude).

In FIG. 3, a realizable increase in the amplitude A of a seed pulse with time t is shown schematically for a laser diode. It can be seen that the laser threshold of the laser diode can form a plateau P with low amplitude (just above the laser threshold) in the front pulse part. Furthermore, a usable maximum amplitude Amax of the seed pulse is e.g. by a

Damage threshold of the semiconductor structure and / or limited by thermal effects in the semiconductor structure of the laser diode. Consequently, in view of the realizable increase in amplitude, the pulse shaping encounters limits of the dynamic range of the possible activation of the laser diode. For the aforementioned modulation of the energization of laser diodes may not be sufficient to compensate in advance for a deformation of the seed pulses during the amplification process in such a way that the target pulse shape adjusts itself after the amplification. This can occur in addition to the already mentioned case in which the amplifier system has a gain that is too high in terms of deformation, even if the target pulse form itself should have a high (not precompensatable) dynamic. Pulse-shaping devices such as an AOM may also be limited in their dynamic range.

The concepts described herein may allow for a desired target pulse shape for a e.g. subsequent laser processing operation also in such cases, e.g. at high laser powers after the amplification process. The proposed pulse shaping with high dynamics takes place by shaping interventions at at least two different points of an amplification process. In addition to shaping the seed pulse - e.g. can be a pulse through modulation of the

Diode current to be preformed - at least one further forming intervention is made.

The further shaping intervention may further comprise the concept that a seed pulse to be amplified is built up from a plurality of partial pulses. For example, a plurality of partial pulses may be used with different dynamic ranges (reduced, for example, via the attenuation unit 4 in FIG. 1). As a further shaping intervention, in addition or as an alternative, partial pulses of partial seed pulse sequences can be introduced into the amplification process at different points of the amplifier chain 5. For example, as shown in Fig. 1, the introduction of the laser pulses of the seed laser 3C take place before the fiber amplifier stage 5B. Alternatively or additionally, laser pulses may be applied to a fiber amplifier stage, e.g. after the fiber amplifier stage 5B, are introduced.

A basic idea here is that a cascaded arrangement of partial pulse seed sources is responsible for temporally successive segments of a seed pulse, the pulse durations of the segments usually being shorter than the pulse duration of the output pulses. For example, in the above example, partial pulses / intermediate pulses with pulse durations in the range of 10% to 90% of, for example, 250 ns can be combined. In general, shorter or longer pulse durations of, for example, 50 ns or 10 μ8 can also be combined from partial pulses / intermediate pulses. In this case, partial pulse seed sources can each be embodied as a single seed diode (as implemented for simplifying the description in FIGS. 5 to 7) or else go back to a common (seed) seed diode (see FIG. 8). In general, an amplified output pulse is then based on the amplification of at least two seed pulses of different seed pulse sequences. Figures 4A and 4B illustrate the partial pulse approach in the generation of pulse shapes (a single pulse or a pulse envelope of a burst pulse train) with high dynamics. FIG. 4A schematically shows a superimposition of contributions from two partial pulses 21A, 21B (with partial pulse durations TA, TB of, for example, approximately half the pulse duration T), each having an amplitude profile as shown in FIG. 3, the associated amplitude profiles being different Lower limits (plateau amplitudes) and upper limits (maximum amplitudes) have. The partial pulse 21 A in the lower amplitude range essentially forms a time-advanced segment of the seed pulse and the partial pulse in the higher amplitude range essentially forms a temporally subsequent segment of the seed pulse.

Combine more and more partial pulses - e.g. 4B shows a combination of four partial pulses with partial pulse durations Ti - one can approach the "ideal" amplitude characteristic shown in Fig. 2. In the example shown, a first partial pulse seed source controls the amplitude of a first partial pulse / segment and an N- The partial pulse seed source controls the amplitude of an Nth partial pulse / segment, whereby the duration of the N segments, ie the partial pulse duration of the partial pulses, can be (substantially) the same or at least partially different.

Furthermore, contributions from subpulse seed sources may overlap in time. For example, a first partial pulse seed source can radiate from beginning to end, and a second partial pulse seed source is switched on from an adjustable point in time.

In some embodiments, as already shown in connection with FIG. 1, the sub-pulse seed sources are located at different positions of the amplifier chain 5. Thus, the sequential amplification itself can be used to increase the dynamics of the pulse amplitudes of the sub-pulse seed sources ,

The following embodiments are explained by way of example for seed diodes as seed lasers and fiber amplifier stages as amplifier stage. However, other types of seed lasers and amplifier stages, such as those already mentioned, may be used depending on the amplifier system.

FIG. 5 shows, as a further exemplary embodiment, an amplifier chain 20 repeatedly repeated by the amplification process for a plurality of (eg fiber) amplifier stages. By way of example, a first seed diode 23A controls the amplitude of the (temporally) first partial pulse of a partial seed pulse sequence 23A '. a seed diode 23B that of the second sub-pulse of a sub-seed pulse train 23B ', ... and a seed diode 23N that of the Nth sub-pulse. In general, a seed laser can also control one or more subsequent partial pulses. After the first seed diode 23A is a first amplifier stage 25A (for example a fiber amplifier stage) in which the first partial pulse of the partial seed pulse train 23A 'is coupled in for amplification. The amplified first partial pulse / intermediate pulse is via a combiner

(Combining unit 9A) (for example in the free-jet or fiber-coupled) is combined with the second (not yet amplified) partial pulse of the partial seed pulse sequence 23B 'so that the resulting pulse is e.g. a longer pulse duration (for example, the sum of the pulse durations of the first and second partial pulses). In general, the subpulses / intermediate pulses can be temporally spaced apart from one another, in particular with a time offset which is smaller than the seed pulse duration, can be combined, join in time with one another or overlap in time (merge into one another).

Each subpulse can be assigned a time range (for example, a segment) of the output pulse ultimately leaving the amplifier chain 5 '. The high dynamics of such an assignment goes back to the use of the sequence of amplifier stages 25 A, 25B, ... 25N. It should be noted here that the time position of a time range (segment) does not have to correspond to the position of the associated seed diode in the sequential structure. Rather, they can differ from each other. Depending on the target pulse shape, a partial pulse for a corresponding time range is introduced into the amplification process. Usually, partial pulses are introduced into the cascade earlier for more intensified time ranges, as partial pulses for less strongly amplified time ranges of the output laser pulse. In Fig. 4B, for example, four partial pulses are shown, which are amplified corresponding to four segments of the generated output pulse. That is, the amplified output pulse generally comprises electromagnetic radiation due to the four seed pulses, thus based on the amplification of four seed pulses of different seed pulse sequences.

In the embodiment according to FIG. 6, the high dynamics are generated via a combination unit 9A. If you go for example of two (or more) equally strong seed diodes 33A, 33B (as an example of a

Seed laser), one channel of the combining unit 9A may for example transmit 90% of the partial pulse of the seed diode 33A, the other channel of the combining unit 31 transmitting the partial pulse of the seed diode 33B only to 10%. A temporal offset and / or overlap of the partial pulses can be effected by the control of the seed diodes 33A, 33B. A correspondingly resulting combined laser pulse is fed to an amplifier stage 35. Analogously, more than two seed diodes can be combined with corresponding selectable ratios, offsets and / or overlaps, in particular spaced from each other in time, adjoin one another in terms of time or overlapping in time. For example, at least two of the seed pulses may be spaced apart with a time offset less than the seed pulse duration. The embodiment according to FIG. 6 (as well as the embodiments of the following figures) can for example be integrated into the cascade of FIG. 5 in one or more of the seed diodes 23A,... In the embodiment according to FIG. 7, a unit 41 is used in the combination of two (or more) partial pulses of two seed lasers 43 A, 43 B. The unit 41 acts, for example, uniformly over the pulse duration on the amplitude profile of one of the partial pulses. For example, an optical arm assigned to the seed laser 43 A has an amplifier or an attenuation unit as a unit 41, so that the partial pulses combined in a combiner unit 9A, for example, cover different, possibly adjustable, amplitude ranges. Thus, an increased amplitude dynamics is generated, which can be used in the subsequent amplification in an amplifier stage 45 for precompensation. The timing of the seed lasers 43A, 43B can in turn be performed according to the segment concerned (at a distance, overlapping, merging, etc.). In further embodiments, multiple sub-pulses may be generated with only one seed laser. As shown by way of example in FIG. 8, a seed pulse of a seed diode 53 can first be optically split with a splitter 51 in two subpulses propagating along associated optical arms 53A, 53B. Based on thus formed partial seed pulse sequences 53A ', 53B', the desired amplitude dynamics are generated so that an output pulse (its electromagnetic radiation) is due to at least two seed pulses of different (sub) seed pulse sequences.

For example, in Fig. 8, in the optical arm 53A, a partial pulse (e.g., as in Fig. 7 with the unit 41) is attenuated or amplified. The other optical arm 53B has a delay, for example over a fiber link 57 (or even a free beam propagation). Subsequently, both partial pulses are again combined according to the affected segments with distance, overlapping, merging into one another, etc. in a combination unit 9A (balanced or weighted) and fed to an amplifier stage 55.

In addition, one or more pulse-shaping devices (modulators) can be used for temporal pulse shaping between amplifier stages.

Similar to FIG. 5, FIG. 9 shows an amplifier chain 20 'which has a cascade of amplifier stages 61. The coupled seed pulses of a laser, for example a seed laser diode 63, are amplified sequentially in the amplifier stages 61 to form intermediate pulses assigned to the amplifier stages 61. Amplifier chain 20A outputs correspondingly amplified output pulses, each based on a coupled seed pulse amplified and amplitude modulated as discussed below. Between successive amplifier stages 61, pulse shaping devices 60 for changing the amplitude of an associated intermediate pulse may be provided. For example, the pulse-shaping device 60 is a unit for changing the amplitude, ie the amplitude curve during the pulse duration of the intermediate pulse. It is designed, for example, as an optical modulator, for example as an acousto-optical modulator or electro-optical modulator, which is designed to decouple energy during the pulse duration of the intermediate pulse, so that the pulse shape changes, in particular the amplitude in the corresponding temporal range / Segment reduced. FIG. 10 illustrates the concept of truncating the amplitude of intermediate pulses having an amplitude characteristic 71. For example, an acousto-optic modulator is controlled in such a way that the front side of the intermediate pulse (small t values) experiences losses which decrease with increasing t values. In this way, the amplification can be formed in such a way that the amplification takes place, as it were, with an amplitude characteristic which approximates the "ideal" amplitude characteristic 73 (similar to FIG. 2).

The intermediate pulses may further be supplemented (as explained above) with further partial pulses before or after the pulse-shaping device 60 in order to achieve the desired dynamic range. Correspondingly, amplified output pulses, generally their electromagnetic radiation, are each attributed to several coupled seed pulses, and thus each of the output pulses is based on the amplification of several seed pulses. Moreover, the pulse shaping device 61 may be e.g. used for ASE suppression.

In this way, pulse deformations may be corrected, for example, due to the saturation of the gain before or between the fiber amplifier stages 61.

Looking back to FIG. 1, it can be seen that the structure illustrated therein schematically shows the weighted combining of (sub) pulses of the seed sources 3A, 3B. Furthermore, the structure comprises a coupling in of further partial pulses of the seed source 3C in front of the amplifier stage 5B as well as an example of an amplitude adaptation before the amplifier stages 5B and 5C.

In view of the many concepts disclosed herein, for example, a fiber-seed laser may be used whose (possibly stretched) seed pulses are split and given at least one side to a delay unit which delays the split seed pulses. The divided and possibly delayed seed pulses may optionally additionally or alternatively be modulated with a modulator in the respective branch in their energy and subsequently recombined. In addition, a modulator for merging may be provided to to cut down the composite pulse. Thereafter, a number of other amplifiers, which may be fiber, rod, slab, disc-based, for example, form an amplifier chain. Between these amplifiers a supplementary pulse shaping as shown in the examples of fiber amplifier chains can be made.

With regard to the initially mentioned seed source of a UKP laser, the seed pulse sequence can have an envelope whose amplitude profile can be adjusted in accordance with the concepts disclosed herein. Further, UKP seed pulses may be closer together than the seed clock dictates. This must be taken into account in the determination of the target pulse shape and the corresponding control of the seed diodes and / or modulator (s) of the drive unit 1 1, for example for a reduction of the repetition rate.

The concepts disclosed herein further allow the output pulse (e.g., with powers in the W range) coming from the (e.g., fiber-based) amplifier chain to be used as an input pulse for e.g. Slice laser mass amplifier system with one or more (disk laser) amplifier stages to use. The high power laser beam (e.g., in the kW range) ultimately provided by the laser assembly is then applied to the particular application, for example, a laser processing application. In some applications, it may be advantageous to modulate the laser beam with high power, for example on a workpiece to be machined, quickly in the performance, in particular to interrupt the irradiation. For this purpose, external, the laser beam generating laser assembly downstream, modulators, etc. can be used. These allow the laser arrangement to be operated at a fixed (the "optimal") operating point and thus at constant and known beam parameters, however, such modulators are cost-intensive and in some cases complex to implement, since they and possibly subsequent structures are sometimes very handle (dissipate) high decoupled power.

In some embodiments, an external modulator may be provided prior to laser processing to adjust the output power to processing histories. Thus, for example, according to a (eg crystal-based) power amplifier, the output pulse can be cropped by an external modulator without significantly changing the pulse shape (a single pulse or a pulse envelope in burst pulse sequences). This can be desired, for example, when driving around a bend in material processing, for example, cutting a curve to keep the introduced energy constant, for example. Using the concepts disclosed herein, alternatively or in addition to an external modulator, the power of the input pulses may be varied for a main amplifier following a gain chain, eg for the disk multiplex amplifier. That is, for example, the output pulses of the amplifier chain 5 in FIG. 1, for example the peak pulse power extracted from the amplification chain or the repetition rate, are adapted. The procedure using highly dynamic seedling will be explained below as an example for the disk multiplex amplifier as the main amplifier. However, the procedure can be transferred to a wide variety of main amplifier systems and amplifier chains. As noted, other active medium amplifier systems such as a slab or a rod-type fiber can also be used with respect to the concepts disclosed herein.

After a change in the coupled-in output pulses of the amplifier chain 5, the power extraction from the laser disk of the disk multiplex amplifier also changes. This can result in a change in gain in the main amplifier, a change in the gain from the laser disk, and a change (typically in the millisecond range) in thermal stresses, e.g., after a short time (typically in the range of microseconds). lead the laser disc.

To compensate for these two aspects, the pump power for the disk multiplex amplifier can be changed in a modulation of the supplied pulse power such that the dissipated power and thus the thermal lens, which arises in the laser disk, remains constant to a first approximation.

Further, the change in the gain of the disk multiplex amplifier may be compensated via a dynamic change in the input pulse power, or the change may be limited by an effect limiting the maximum gain (e.g., cross-ASE, auxiliary resonator or controlled pump power in the main amplifier).

The change in the pump power in the main amplifier can be done either controlled or regulated, for example. For the latter case, a measurement of the disk power or another, the thermal lens descriptive parameter (eg disk temperature) can be done. For example, the beam parameters of the laser beam or of an auxiliary laser beam, which propagates collinearly to the laser beam, can be measured and used as a control signal. A suitable model can be used, because, for example, depending on the laser configuration, heat generation can increase or decrease with reduced input pulse power and constant pump power. A disadvantage of the procedure described above is that the laser gain, in particular the laser gain over the pulse duration, can not be kept constant, so that when the input pulse power changes, corresponding pulse peaks can occur. Such pulse Elevations may be suppressed, for example, by suitably adjusting the input pulse energy, and in particular by adjusting the amplitude response of the injected output pulses using the concepts of dynamic seed disclosed herein.

An example flow may summarize the following steps:

(1) The output power of the laser array is modulated down by the input pulse generation of the disk multipass amplifier, for example, to obtain half the output power.

(2) The saturation of the disk multiplex amplifier changes and the disk is e.g. warmer as the inversion level increases. At the same time, the gain of the disk multiplex amplifier increases.

(3) The pump power is reduced so that the gain decreases.

(4) The input pulse power must be raised again slightly to maintain a constant output power after the disc multiplex amplifier.

(5) There is better saturation and the laser disc gets colder. Accordingly, the pump power for the disk multiplex amplifier is increased, the input power is to be reduced again and so forth.

Whenever pulse peaks are to be expected within the scope of a step taken, the amplitude characteristic of the output pulses of the amplification chain coupled into the main amplifier can be adapted accordingly. In addition, the beam caustics may change due to the modulation, so that the output beam size may also have to be adapted, whereby the changes may be based on different time scales. It will be appreciated that the amplitude adjustment to the respective target amplitude curve is generally a complex control process, e.g. can be parameterized via a suitable model.

It is explicitly pointed out that all features disclosed in the description and / or the claims are considered separate and independent of each other for the purpose of original disclosure as well as for the purpose of limiting the claimed invention independently of the feature combinations in the embodiments and / or the claims should. It is explicitly stated that all range indications or indications of groups of units of every possible intermediate value or subgroup of units for the purpose of the original disclosure as well for the purpose of limiting the claimed invention, in particular also as a limit of a range indication.

Claims

claims

1. laser amplifier system (1) with

a seed laser pulse source unit (3) for providing at least two seed pulse sequences for subsequent amplification, wherein seed pulses (3 ') of the at least two seed pulse trains have a respective seed pulse duration (T, TA, TB) and one within one Range during the respective seed pulse duration (T, TA, TB) varying and adjustable in their seed amplitude (A), and

at least one amplifier stage (5A, 5B, 5C) for amplifying the at least two seed pulse sequences and for outputting an output pulse train with output pulses (5 ') having an output pulse duration,

wherein the at least two seed pulse trains are coupled into the amplifier stage (5A, 5B, 5C) such that an amplitude characteristic of an output pulse (5 ') of the output pulse train is based on contributions from at least two seed pulses (3'), each one of the at least two seed pulse sequences are assigned, goes back.

2. laser amplifier system (1) according to claim 1, wherein

the laser amplifier system (1), and in particular the control of the seed laser pulse source unit (3), is designed such that the at least two seed pulses (3 '), which contribute to the amplitude characteristic of an output pulse (5'), are temporally spaced from each other , temporally adjoin or temporally overlap, and

Optionally, the at least two seed pulses (3 ') are spaced apart with a time offset less than the seed pulse duration (T).

3. laser amplifier system (1) according to claim 1 or 2, further comprising

an optical delay unit (57) for generating a laser pulse time offset between the at least two seed pulses (3 ') which contribute to the amplitude characteristic of an output pulse (5').

4. laser amplifier system (1) according to one of the preceding claims, wherein

the amplifier stage (5A, 5B, 5C) is a fiber, rod, slab, and / or disk laser amplifier stage.

5. laser amplifier system (1) with

a seed laser pulse source unit (3) for providing at least two seed pulse sequences for subsequent amplification, wherein seed pulses (3 ') of the at least two seed pulse trains have a respective seed pulse duration (T, TA, TB) and one within one Area during the have respective seed pulse duration (T, TA, TB) varying and adjustable in their course seed amplitude (A), and

a gain chain (5) comprising a sequence of at least two gain stages (5A, 5B, 5C) coupled in series, the gain in the gain stages (5A, 5B, 5C) being sequentially formed by the gain stages (5A, 5B, 5C) and the amplification chain (5) outputs an output pulse train with output pulses (5 ') having an output pulse duration,

wherein an amplitude curve of an output pulse (5 ') of the output pulse train is based on contributions from at least two seed pulses (3'), which are each assigned to one of the at least two seed pulse sequences.

6. laser amplifier system (1) according to claim 5, further comprising

an optical delay unit for generating a laser pulse time offset between a seed pulse of one of the at least two seed pulse sequences and an intermediate pulse (3 ") and / or an optical delay unit (57) for generating a laser pulse time offset between seed pulses of different ones of the at least two seed pulse sequences.

7. laser amplifier system (1) according to claim 5 or 6, wherein

the laser amplifier system (1), and in particular the control of the seed laser pulse source unit (3), is designed such that the at least two seed pulses (3 '), which contribute to the amplitude characteristic of an output pulse (5'), are temporally spaced from each other , temporally adjoin or temporally overlapping, and wherein optional seed pulses having a time offset smaller than the seed pulse duration (T) are spaced apart, and / or

wherein the laser amplifier system (1), and in particular the control of the seed laser pulse source unit (3), is designed such that at least one seed pulse (3 '), which contributes to the amplitude characteristic of an output pulse (5'), with respect to an intermediate pulse in time is spaced, temporally adjacent thereto, or temporally overlapping therewith, and optionally wherein the at least one seed pulse having a time offset smaller than the seed pulse duration (T) is spaced from the intermediate pulse.

8. laser amplifier system (1) according to one of claims 5 to 7, wherein

a temporal segment of the output pulses (5 ') originates from at least one seed laser pulse source whose seed pulses pass through all amplifier stages (5A, 5B, 5C) of the amplification chain (5), and / or

wherein the amplification chain (5) comprises a fiber, rod, slab, and / or disk laser amplifier stage.

9. laser amplifier system (1) according to any one of the preceding claims, wherein the to the amplitude characteristic of an output pulse (5 ') contributing seed pulses one of the at least two seed pulse sequences to be amplified sub-pulses of a sub-seed pulse train (23Α', 23B '; 53A ', 53B') whose partial pulse duration (TA, TB) is shorter than the output pulse duration of the output pulses (5 ').

10. laser amplifier system (1) according to any one of the preceding claims, further comprising

at least one distribution unit (51) for dividing a seed laser pulse, a laser pulse source in two or more pulses and / or

at least one attenuation unit (4) for reducing the amplitude of laser pulses, partial pulses and / or intermediate pulses and / or

at least one pulse-shaping device (15, 60) for changing the amplitude (A) of one of the seed pulses, wherein the pulse-shaping device (15, 60) as acousto-optical modulator or electro-optical modulator for a truncation of the amplitude (A) at least one of the seed pulses is formed in at least one temporal subarea, and / or

at least one combiner unit (9A), such as an X: (100-X) fiber combiner, for combining

seed pulses of at least two laser pulse sources with each other or

of seed pulses of at least one laser pulse source with intermediate pulses (5 ') or

of intermediate pulses (5 ') with intermediate pulses (5').

1 1. Laser amplifier system (1) according to one of the preceding claims, wherein

the seed laser pulse source unit (3) has at least one laser pulse source in the form of an adjustable energizable laser diode (23A), and optionally for each of the at least two seed pulse sequences its own laser pulse source.

12. laser amplifier system (1) according to one of the preceding claims, wherein

the seed laser pulse source unit (3) comprises two laser pulse sources, wherein the pulses of the first laser pulse source, in particular via an attenuation unit or an amplifier, generate an intermediate pulse train with the pulses of the second laser pulse source in a combination unit (9A).

13. laser amplifier system (1) according to claim 12, further comprising

at least one pulse-shaping device (15, 60) for changing the amplitude (A) of one of the intermediate pulses (3 "), wherein

the pulse-shaping device (15, 60) as an acousto-optical modulator or electro-optical modulator for a truncation of the amplitude (A) of at least one of the associated intermediate pulses (3 ") is formed in at least a temporal portion and optionally between two adjacent amplifier stages (5A, 5B, 5C) is arranged.

14. Laser amplifier system (1) according to one of the preceding claims, further with

a control unit (1 1), which is designed for setting the seed pulse shape, in particular for driving a seed source, a combination unit (9A) and / or a pulse-shaping device (15, 60), wherein

the control unit (1 1) is optionally designed to derive the seed pulse profile from a target pulse profile, in particular a target amplitude characteristic of one of the output pulses (5 '), and the seed pulse form, in particular the contributions from the at least two seed pulses ( 3 ') and / or the circumcision of the amplitude (A) of at least one of the associated intermediate pulses (3 "), adjust accordingly.

15. Laser amplifier system (1) according to one of the preceding claims, further with

at least one pulse-shaping device following the amplification for changing the amplitude of the output pulses (5 ') of the output pulse train, wherein

the pulse-shaping device is designed as an acousto-optical modulator or electro-optical modulator for a truncation of the amplitude of the output pulses (5 ') of the output pulse train and in particular after a crystal-based power amplifier to reduce the intensity of the pulse is formed without significant change in the pulse shape, said

the control unit (1 1) is optionally designed to control the intensity of the pulse-shaping device during material processing, in particular in the case of beam guidance around a curved path, in order to reduce the output pulses.

16. A method for generating a sequence of amplified output pulses (5 ') with

Providing at least two seed pulse sequences (23Α ', 23Β', 53A ', 53B') for a subsequent amplification, wherein seed pulses (3 ') of the at least two seed pulse trains (23Α', 23B ', 53A', 53B ') each have a seed pulse duration (T, TA, TB) and within a range during the seed pulse duration (T, TA, TB) varying and adjustable in their course seed amplitude (A), and

Amplifying the seed pulses (3 ') in an amplifier stage (5A, 5B, 5C) such that an output pulse train results with output pulses (5') having an output pulse duration,

17. The method of claim 16, wherein an amplitude characteristic of one of the output pulses (5 ') also goes back to a truncation of the amplitude (A) of at least one seed pulse (3') and / or

wherein the amplitude curves of the seed pulses (3 ') are adapted to each other such that a dynamic range after amplification is available which is greater than the dynamic range of a single seed pulse (3').

18. A method for generating a sequence of amplified output pulses (5 ') with

Amplifying the seed pulses (3 ') in sequentially arranged amplifier stages (5A, 5B, 5C) to form intermediate pulses (3 ") associated with the amplifier stages (5A, 5B, 5C), such that an output pulse train having output pulses ( 5 '),

wherein an amplitude characteristic of an output pulse (5 ') of the output pulse train is based on contributions from at least two seed pulses (3'), which are each assigned to one of the at least two seed pulse sequences.

19. The method according to any one of claims 16 to 18, wherein

at least one seed pulse sequence is coupled into the amplifier stage (5A, 5B, 5C) in such a way and optionally wherein at least one seed pulse sequence is coupled into a subsequent amplifier stage (5A, 5B, 5C) in such a way,

in that an intermediate pulse to be amplified is formed whose amplitude profile goes back to contributions from at least two seed pulses (3 '), which are each assigned to one of the at least two seed pulse sequences.

20. The method of claim 18 or 19, wherein

an amplitude characteristic of one of the output pulses (5 ') also goes back to a truncation of the amplitude (A) of at least one intermediate pulse and / or

wherein the amplitude curves of the seed pulses (3 ') and optionally the truncation of the amplitude (A) of at least one associated intermediate pulse (3 ") are adapted to one another such that a dynamic range after amplification is available which is greater than the dynamic range of an individual Seed pulse (3 ').