EP3405761A1

EP3405761A1 - Relative phase measurement for the coherent combining of laser beams

Info

Publication number: EP3405761A1
Application number: EP17702012.0A
Authority: EP
Inventors: Matthias Häfner; Thomas Metzger
Original assignee: Trumpf Scientific Lasers & Co KG GmbH
Current assignee: Trumpf Scientific Lasers & Co KG GmbH
Priority date: 2016-01-18
Filing date: 2017-01-16
Publication date: 2018-11-28
Also published as: CN108780010B; CN108780010A; WO2017125345A1; US20190052045A1; US11114814B2; DE102016100721B3

Abstract

The invention relates to a phase control system (5) for controlling the relative phase (φ) of two laser beams (17A, 17B) of a laser system (1) to be coherently combined, in particular permitting the provision of a phase-controlled sum laser beam (19). An optical unit (7) of the phase control system (5) comprises a beam input (33) for receiving a measurement portion (19') of two coherent laser beams (17A, 17B) that are/are to be superimposed in a collinear manner to form a sum laser beam (19), and provides measuring beams (31A, 31B, 31C) or measuring beam regions (131A, 131B, 131C), which are used with associated photodetectors (41A, 41B, 41C) for the output of photodetector signals. In order to determine the relative phase from the photodetector signals, the phase control system (5) has an evaluation unit (9) and a delay device (11) for introducing at least one of the two laser beams (17A, 17B) into the beam path. The optical unit is designed in such a way that the measuring beams (31A, 31B, 31C) or measuring beam regions (131A, 131B, 131C) are assigned different phase offsets.

Description

COMBINE RELATIVE PHASE MEASUREMENT TO COHERENT

OF LASER RADIATION

The present invention relates to the coherent combining of laser beams, in particular of two pulsed laser beams, such as may be generated by (high power amplifier) laser systems for scientific applications. In particular, the present invention relates to a phase control system for controlling the relative phase of two laser beams to be coherently combined and a laser system having such a phase control system. Furthermore, the invention relates to a method for the coherent combination of laser beams.

Coherent combining allows multiple parallel amplifier stages (eg, fiber amplifiers, multipass amplifiers, or regenerative amplifiers) and / or oscillators to be optically combined to form a single output beam. In the coherent combination of two laser beams, the polarization state of the sum beam and in particular the stability of the polarization state are a decisive performance feature.

For fiber-based amplifier systems, a coherent combining of different research groups has been demonstrated, see e.g. B. "Coherent addition of fiber-amplified ultrashort laser pulses", E. Seise, et al., Dec. 20, 2010, Vol. 26, OPTICS EXPRESS 27827 and "Coherently-combined two channel femtosecond fiber CPA system producing 3 mJ pulse energy", A. Klenke et al., 21 Nov. 2011, Vol. 24, OPTICS EXPRESS 24280. In general, it is known to check the state of polarization with measuring methods such as the Hänsch

Couillaud detection (HCD) or Pound-Drewer-Hall detection, see e.g. "Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity", T.W. Hänsch et al., 1980 Opt. Comm. 35 (3). In part, heterodyne measuring methods are used, which are less suitable for short pulse laser systems with low repetition rates, see e.g. "Coherent beam combining and phase noise measurements of ytterbium fiber amplifiers", S.J. Augst, et al, 2004 Opt. Lett. 29 (5).

It is an object of this disclosure to provide a concept for controlling the relative phase between laser beams to be combined. Another Aspect is based on the object to enable a stable coherent combination in a laser system.

At least one of these objects is achieved by a phase control system according to claim 1, by a laser system according to claim 11 or 12 and by a method for coherently combining laser beams according to claim 17 or 20. Further developments are given in the subclaims.

In one aspect of the present invention, a phase control system for controlling the relative phase of two laser beams coherently to be combined of a laser system provided for providing a phase-locked sum laser beam comprises an optical unit. The optical unit comprises a beam input for receiving a measurement component of two collinear laser beams which are collinearly superimposed to form a sum laser beam, in particular with substantially orthogonal polarization states. The optical unit further comprises a beam splitter for generating at least three measuring beams from the coherent laser beams of the measuring component, which propagate on associated measuring beam paths, or a propagation section in which the coherent laser beams of the measuring component propagate spatially superimposed at an angle and form at least three measuring beam areas. In this case, the at least three measuring beam paths of the at least three measuring beams are formed by projection onto adapted polarization directions or the at least three measuring beam ranges by path length differences for generating different phase offsets of the associated measuring beams or measuring beam ranges. Furthermore, the optical unit comprises at least three photodetectors for outputting photodetector signals, wherein the photodetectors are respectively assigned to one of the measurement beam paths or one of the measurement beam areas and the photodetector signals correspond to the measurement beams or measurement beam areas at the different phase offsets. Furthermore, the optical unit comprises an evaluation unit which generates, based on the at least three photodetector signals, a control signal which corresponds to a relative phase between the coherent laser beams based on an evaluation of a polarization state or interference behavior of the measurement component, and a delay device for introduction into the latter

Beam path of at least one of the two collinear laser beams, which has an optical path length which is adjustable in response to the control signal. In one embodiment, a phase control system for controlling the relative phase of two laser beams to be coherently combined of a laser system provided for providing a phase-locked sum laser beam comprises an optical unit. The optical unit comprises a beam input for picking up a measurement component of two collimated laser beams collimated to form a sum laser beam, a beam splitter for generating (at least) three measurement beams from the measurement component, (at least) three measurement beam paths for the (at least) three measurement beams are each formed to project a polarization state of an associated measurement beam onto a polarization direction, and (at least) three photodetectors each associated with one of the measurement beam paths for outputting photodetector signals corresponding to the measurement beams projected onto the polarization directions. Furthermore, the phase control system has an evaluation unit which, based on the (at least) three photodetector signals, generates a control signal which corresponds to a relative phase between the coherent laser beams based on an evaluation of a polarization state of the measured component, and a delay device for introduction into the beam path of at least one of two collinear laser beams having an optical path length which is adjustable in response to the control signal.

In a further embodiment, such a phase control system has an optical unit for generating two measuring beams corresponding to the coherent laser beams, which propagate in a propagation section at a splitting angle and are spatially superimposed in a central area. The optical unit further comprises a polarizing filter, e.g. in the interference region for generating interfering polarization states which lead to an interference fringe pattern in a direction given by the splitting angle. The central area comprises at least three measuring beam areas with defined local different phase offsets. The optical unit furthermore has at least three photodetectors each assigned to one of the measurement beam areas for outputting photodetector signals with respect to the various phase offsets, as well as a previously described evaluation unit and a previously described delay device.

In another aspect, a laser system for providing a coherent combining-based sum laser beam includes a seed laser beam source for providing a first seed laser beam and a second seed laser coherent seed laser beam, a first amplifier arm including a first optical amplifier unit Generating a first amplified laser beam based on the first seed laser beam, and a second amplifier arm having a second optical amplifier unit for generating a second amplified laser beam based on a second seed laser beam component. Furthermore, the laser system comprises a combination unit for the colocal overlay of the beam path of the first amplified laser beam and of the beam path of the second amplified laser beam for generating a sum laser beam. In this case, the first laser beam in a first polarization state and the second laser beam in a second polarization state different from the first polarization state are superimposed. Furthermore, the laser system has a phase control system as sketched above, in which a portion of the sum laser beam is coupled into the beam input of the optical unit as measurement component and the delay device is provided in the first amplifier arm and / or in the second amplifier arm.

In a further aspect, a laser system for providing a coherent combining-based sum laser beam comprises a laser beam source for providing a first laser beam and a second laser beam coherent to the first laser beam, a collimating unit for collinearly superimposing the beam path of the first laser beam and the beam path of the second laser beam for generation the sum laser beam, wherein the first laser beam in a first polarization state and the second laser beam are superimposed in a second polarization state different from the first polarization state, and a phase control system as outlined above. As part of the measurement, a part of the sum laser beam is in the beam input of the optical unit

coupled and the delay device is provided in the beam path of the first laser beam and / or in the beam path of the second laser beam.

In a further aspect, a method for coherently combining laser beams comprises the following steps: superposing two laser beams with different polarization states on a common propagation path to form a sum laser beam, determining in a substantially intensity-independent homodyne measuring method a relative phase between the two laser beams and Adjusting an optical path length difference to stabilize the state of polarization of the sum laser beam. The interferometric measurement principle described herein by way of example in some embodiments herein, which is based on the above aspects, is based on a kind of homodyne phase measurement between two contributing polarization components. Compared to the Hänsch-Couillaud detection (HCD) mentioned above, the concept disclosed herein may have the advantage of intensity independence. Thus, when using HCD, a phase stability in the range of 0.1 rad rms is reported, whereby HCD can not distinguish between intensity and phase fluctuations. The latter can have the consequence that when using HCD in a control loop additional phase disturbances are impressed on the combined sum laser beam by occurring intensity fluctuations in the input beams.

Unlike HCD, the interferometric measurement principle disclosed herein may allow separation of intensity and phase variations. Thus, the interferometric measuring principle disclosed herein may enable better phase stabilization. Moreover, the measurement principle can make it possible to compensate for intensity fluctuations in the output beam by deliberately introducing elliptical polarization and subsequent polarization filtering. From the Längenmessinterferometrie it is known that in length measurements for the phase measurement resolution limits in the range of 0.001 rad rms and better can be achieved, which can be based on the measurement principle disclosed herein on the measurement of a relative phase.

Furthermore, the phase detection disclosed herein is principally applicable to a wide range of laser repetition rates. Thus, at low laser repetition rates, the photodetector signals can be evaluated in isolation for each individual laser pulse (single-shot

Evaluation). As a result, the control bandwidth of the phase stabilization can be kept very far. An example of data processing that allows a single-shot evaluation is a "sample-and-hold" implementation in the photodetectors In some refinements of laser systems, especially at high laser powers, the laser beams can be combined with thin-film polarizers ,

In general, the splitting into the measuring beams can take place with non-polarizing beam splitters, such as diffraction gratings or partially reflecting mirror combinations. Herein, general 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 shows a schematic representation of a laser system with a seed laser, two regenerative amplifier units and a phase control system,

2 is a schematic representation of a first exemplary optical unit of a phase control system,

3 is a schematic representation of a second exemplary optical unit of a phase control system,

Fig. 4 is a schematic representation of a third exemplary optical unit of a phase control system and

5 is a schematic representation of another optical unit of a phase control system based on the evaluation of an interference fringe pattern.

Aspects described herein are based, in part, on the finding that in the coherent combination of two laser beams, an essentially intensity-independent measurement of the polarization state of the sum laser beam allows improved control of the coherent superposition of the sub-beams. Fluctuations in the laser power can be reduced by avoiding or reducing the intensity dependence of the phase measurement in their influence on the coherent combination.

The polarization state can be measured by measuring the relative phases between the two input beams. By way of example, the interferometric measuring principle disclosed herein and a corresponding downstream signal processing, the phase with high linearity can be determined substantially largely independent of the intensity. For example, such an intensity-independent phase value for the phase difference (ie the relative phase) can be provided in the form of a quadrature signal. In particular, the measurement of the state of polarization may be indirect over the phase between the two orthogonal polarizations based on the evaluation of an interference fringe pattern In addition to the intensity independence, the embodiments disclosed herein may, among other things, facilitate easy adjustment (particularly due to the disclosed common path phase measurement configurations) and be made insensitive to vibration and drift events. This is a difference to HCD, which is based on a inherently stable reference cavity. Moreover, the embodiments disclosed herein may, inter alia, provide a unique phase signal even at large phase jumps. Again, this is a difference to HCD, where drifting out of the resonance range can lead to loss of uniqueness. The goal of coherent combining is to combine a plurality of laser beams generated, for example, by parallel amplifier stages and / or oscillators into a single output laser beam. This requires coherence of the laser beams, so that, for example, all amplifier stages are fed with coherent laser light, for example coherent laser pulses. In the following, the concept is used by way of example with reference to a laser system with a common seed laser for two regenerative amplifier stages as the source, so that coherent amplified laser beams can be combined.

1 schematically shows an exemplary laser system 1 with a seed laser 2 as a seed laser beam source, two regenerative amplifier units 3A, 3B and a phase control system 5. Phase control system 5 comprises, for example, an optical unit 7, an evaluation unit 9 and a delay device 11. A primary Laser beam 13 of seed laser 2 is split by a beam splitter 15A into two (coherent) sub-beams, which are identified in FIG. 1 as first seed laser beam 13A and second seed laser beam 13B. Each sub-beam is supplied to the associated amplifier unit 3A, 3B for generating a first amplified laser beam 17A based on the first seed laser beam portion 13A and a second amplified laser beam 17B based on the second seed laser beam 13B, respectively. With the aid of another beam splitter 15B, the amplified laser beams 17A, 17B are collinearly superimposed to form a sum laser beam 19.

The interferometric measuring principle disclosed herein can be used accordingly in the coherent superimposition of non-amplified and / or amplified laser beams. Thus, in FIG. 1, similar configurations can be superposed with none or only one of the amplifier units 3A, 3B coherent non-amplified and / or amplified laser beams, wherein at least in one of the two superposed beam paths, the delay device 11 is provided.

The conglomerate of the two regenerative amplifier units 3A, 3B with the upstream and downstream beam splitters 15A, 15B constitutes a Mach-Zehnder interferometer. The beam splitters 15A, 15B can be implemented, for example, as a beam splitter cube and / or as thin-film polarizers. At the output of this Mach-Zehnder interferometer, which is formed by the two regenerative amplifier units 3A, 3B, one obtains the superimposed sum laser beam 19 with a polarization state of whatever kind.

In FIG. 1, deflecting mirrors 21 and lambda half wave plates 23 for changing the polarization states of the various laser beams are also shown schematically. It can be seen that the primary laser beam 13 has a polarization state 25 superimposed on two polarization states with respect to the function and orientation of the beam splitter 15A, the two polarization states 25A, 25B (for example P and S polarization) after the polarization states

Beam splitting the first seed laser beam 13A and the second seed laser beam 13B and further be maintained in the amplifier units 3A, 3B.

In the exemplary embodiment of FIG. 1, for the superimposition in the second beam splitter 15B, the polarizations of the amplified laser beams 17A, 17B are aligned correspondingly with the lambda half wave plates 23. For example, the output polarization of each amplifier unit 3A, 3B can be adjusted so that the amplified laser beam 17A is reflected in a polarizing beam splitter cube and the amplified laser beam 17B is transmitted. At the output of the beam splitter 15B, the superimposition of both beams is obtained so that the sum laser beam 19 is characterized by a polarization state 27 which is based on the superposition of the polarization states 27A, 27B of the amplified laser beams 17A, 17B. In alternative embodiments, for example, the beam splitter 15B may be interchanged with the deflecting mirror 21, so that waveplates are not necessarily needed for superposition.

The use of lambda half-wave plates 23 shown in FIG. 1 furthermore makes it possible to be able to set the intensity ratios correctly and thus to adapt the contributions of the amplified laser beams and according to their polarization states to the sum laser beam 19. In the exemplary embodiment shown in FIG. 1, the combined sum laser beam 19 may be interpreted as a superposition of two orthogonal linear variable phase-shift polarization states. The phase shift is based inter alia on different optical path lengths in the amplifier units 3A, 3B. The optical

Path length in the amplifier unit can be adapted, for example, with the delay device 11. Thus, in the case of delay elements based on piezo elements, for example, path length changes of a few 100 μm and in the case of the combination of piezo element and linear displacement table, path length changes of several centimeters, for example, may be made possible. In the case of pulsed systems, the delay device 11 comprises, for example, a retroreflector arranged on a translation unit for superimposing the pulse envelope ends and a piezo arrangement for high-resolution phase matching. Furthermore, acousto-optic delay units can be used. In particular, the high-resolution phase matching is part of the Phasenregelungssys- system 5.

As shown in FIG. 1, the phase control system 5 in the output beam path can, for example, be the light transmitted by a deflection mirror 22 (also referred to herein as measurement portion 19 ') or the beam emerging at a second output of the combining beam splitter 15B (dotted beam 29 in FIG Fig. 1) use.

It is the task of the phase control system 5 to measure the polarization state of the sum laser beam in order to stabilize it with the delay device 11, whereby a coherent combination with quasi-constant relative phase can be performed. For example, coherently combined laser beams provide a basis for constant and reproducible experimental conditions.

FIG. 2 shows a first exemplary construction of an optical unit and also clarifies the signal acquisition and evaluation.

In the concept of the phase analysis and subsequent control explained below, collinearly superimposed partial beams in the form of a measurement component 19 'of the sum laser beam 19 are used for phase measurement. The concept is based on the different polarization states 27A, 27B of the partial beams 17A, 17B to be coherently combined. Of the Measurement portion 19 'has the polarization state 27 of the sum laser beam 19, which results inter alia from the phase between the two coherently combined partial beams 17A, 17B. The polarization state 27 is measured using three measuring beams 31A, 31B, 3 IC and correspondingly stabilized by optical delay of one of the two partial beams 17A, 17B. The evaluation does not take place via a fringe pattern, which would be a sign for an angle between the sub-beams, but via intensity signals of special polarization components, which are generated by the three measuring beams 31A, 31B, 31C. As exemplified in connection with FIGS. 2 to 4, in some embodiments, therefore, only three signals are to be evaluated with simple algorithms in order to obtain an intensity-independent phase signal. As a result, measurement rates up to the MHz range are possible.

The optical unit 7A shown in FIG. 2 represents a common-path interferometer, the core property of which is temporal phase fluctuations between the two superimposed and mutually orthogonally polarized at the output of the previously explained Mach-Zehnder configuration of the amplifier units 3A, 3B Rays 17A, 17B transform into a variable-direction linear phase vector characterizing the sum laser beam 19. Such optical systems are sometimes referred to as geometric-phase interferometer. Common path interferometers are known, for example, as a length-measuring interferometer and as a phase detector for interference lithography, see e.g. "Laser linear and angular displacement interferometer", V.P. Kiryanov, et al., Optoelectronics, Instrumentation and Data Processing, no. 4, 1994 Avtometriya.

According to the concepts disclosed herein, an additional interferometer is used for phase detection, whereby substantially pure phase information can be obtained.

Further, unlike the common-path interferometer in a classical Mach-Zehnder interferometer, the orthogonal polarizations would be projected onto a common plane to get interference. However, the possibility of holding an intensity-independent quadrature signal can be lost. The common path property renders the structure disclosed herein particularly robust with respect to adjustment errors, vibrations, etc. According to the concept disclosed herein, the measurement of the relative phase is carried out by means of a purely phase-sensitive interferometer.

In the optical unit 7A, the measuring portion 19 'is received at the beam input 33 and impressed with the aid of a lambda-quarter plate 35 is a phase-dependent polarization vector. Referring to the polarization states 27A, 27B of the amplified laser beams 17A, 17B, the quarter-wave plate 35 transduces the two linear polarizations underlying polarization state 27 into two counter-circular polarizations 27 '. As well as the superimposition of the linear polarizations, the superposition of these two circular polarizations 27 'leads to a linear polarization whose orientation depends on the relative phase of the circular polarizations or the orthogonal input polarizations. The relative phase is thus converted into the orientation of the resulting polarization vector. In this case, a complete rotation of the polarization vector corresponds to a relative phase shift of one wavelength.

A splitting grating 37 splits into the three measuring beams 31 A, 31 B, 3 IC, to which corresponding measuring beam paths 32 A, 32 B, 32 C are assigned. In each measuring beam path 32A, 32B, 32C is in each case a linear polarizer 39A, 39B, 39C. The intensity behind the polarizers 39A, 39B, 39C is converted in each case with a photodiode 41A, 41B, 41C into electrical signals, which are then fed to a control electronics of the evaluation unit 9, for example. In the case of large-diameter measuring beams, the measuring beam paths 32A, 32B, 32C may further each have a lens 43 for focusing the measuring beams onto the photodiodes 41A, 41B, 41C. If the polarization vector of each measuring beam 31A, 31B, 3 IC rotating in dependence on the relative phase passes through the associated polarizer 39A, 39B, 39C, the intensity behind the polarizer 39A, 39B, 39C is proportional to the projection of the polarization vector onto the transmission plane of the polarizer 39A, 39B, 39C. The intensity is therefore a sine function with the period lambda / 2 (lambda is usually the mean wavelength of the laser radiation here). The forward direction of the polarizers 39A, 39B, 39C thus determines the phase of a sinusoidal intensity signal detected with the photodiodes 41A, 41B, 41C. Exemplary intensity profiles 43 A, 43 B, 43 C are shown schematically in FIG. 2. However, each one of the intensity profiles 43A, 43B, 43C is also dependent on the intensities of the amplified laser beams 17A, 17B. The division into the three measuring beams 31A, 31B, 3 IC now allows generation of an intensity-independent phase vector 45 by subtraction of two pairs of photodetector signals. For this purpose, as schematically indicated in FIG. 2, the polarizers 39A, 39B, 39C form projections in three spatial directions, so that three sinusoidal signals having the relative phases "+ 90 °", "0 °" and "-90 °" are obtained. hereinafter referred to as I (+ 90 °), 1 (0 °) and I (-90 °).

Forming differences of the sine signals of the photodiodes 41A, 41B, 41C according to I (+ 90 °) -I (0 °) and I (-90 °) - 1 (0 °), two signals I (+ 45 °) and I are obtained (-45 °), which together represent a quadrature signal 47 with X and Y values. Intensity fluctuations of the amplified laser beams 17A, 17B result in each case only an amplitude change, which is identical for both signals I (+ 45 °) and I (-45 °). The relative phase φ can now be determined according to cp = arctan (I (+ 45 °) / 1 (-45 °)), with the identical amplitude changes shortening out. The phase signal thus generated, the relative phase between the amplified laser beams 17A, 17B, in particular the phase vector 45, can be used as an error signal of a phase stabilization control loop, which can be used in particular for driving the delay device 11.

In the following, the intensity independence of the phase measurement is explained by way of example. It is assumed that two input beams are orthogonally polarized in the x and y directions and the y polarized beam has a phase shift φ to the x polarized beam. For simplicity, it is further assumed that the waveplate is lossless and the E-field can be decomposed into two orthogonal components (corresponding to the axes of the waveplate) of the same amplitude.

For the x-polarized beam and the y-polarized beam, the E fields can be written as follows: E-field of the first input beam, M-polarized λ / 4-plate, ¥ ektorie! Le decomposition in fast and slow axis here Ideal case: equal amplitudes in both axes E-field of the second input beam, y-polarized φ: phase shift

After the superposition of the two beams results for the sum laser beam:

The amplitudes and phases were in the fast or slow

Axis direction of the wave plate summarized:

If one determines the intensities in the two axis directions, these diesel show

ben dependencies of

The respective offset A and the respective amplitude B of the photodiode signal are thus independent of the orientation of the polarizer / polarizer.

The evaluated phase information is therefore also essentially independent of the intensities of the two amplified laser beams 17A, 17B in the herein disclosed concept for relative phase measurement and in particular in the schematically sketched in the figures 1 to 3 optical units with corresponding signal processing. FIGS. 3 and 4 schematically show alternative embodiments of optical units which also allow measuring methods for an intensity-independent phase signal. In the following explanations, the numerals are retained as much as possible for substantially similar features.

In contrast to the construction shown in FIG. 2, in the embodiment of the optical unit 7B according to FIG. 3 at the beam input 33 there is no conversion of the partial beams forming the measuring beam into circular polarization, but only two of the three measuring beams, here by way of example the measuring beams 31A, 3 IC, are phase shifted with wavelength plates. Thus, the measuring beam paths 32A and 32C 'each have a quarter-wave plate 51A, 51B, wherein the orientation of the fast / slow axes is rotated by 90 ° to each other. The latter is indicated in FIG. 3 by vectors in the drawing plane (lambda quarter plate 51A) or perpendicular to the drawing plane (lambda quarter plate 51B). Furthermore, all three measuring beam paths 32A ', 32B', 32C 'have identically oriented polarizers 49, that is, the projection directions are identical for all three measuring beam paths 32A', 32B ', 32C'.

In the following, alternative embodiments are described in which, in contrast to the optical unit 7A, no circular polarization is generated at the beam input 33 and no rotating polarization vector is generated. In order to interfere with the orthogonal polarization contributions in the measurement component, e.g. a polarizer can be placed at 45 ° in the beam path. Thus, for example, at the middle photodiode a phase-dependent sine modulation. With the aid of retardation plates, one could set a fixed offset phase in each additional measuring beam path. For example, you can use two quarter-wave plates, with the fast axis of one plate is horizontal and the other vertically aligned. In this way one would obtain a phase offset of +/- 90 ° in the photodiode signal. Alternatively, other phase offsets can be made, for example, about 90 ° and 180 °. In principle, any further offsets can be set in such constructions, wherein at least three measuring beams are to be provided for a quadrature signal processing.

The optical unit 7C shown in FIG. 4 represents a corresponding modification of the optical unit 7B shown in FIG. 3. In essence, the structures are the same, so that for convenience the corresponding reference numerals have been omitted. Of the Difference relates to the measuring beam path 32C "in which a lambda-half wave plate 53 effects a phase modification instead of the quarter-wave plate The wave plates 51A, 53 are aligned in their orientation with respect to the polarization in the respective measuring beam path 32A ', 32C" in that the fast axes are parallel to one another, and in particular (in the optimal case) at the same time parallel to one of the polarization directions of the measuring beams. Accordingly, in the respective measuring beam paths 32A ', 32C ", there are phase-shifted linear polarizations, ie there is no rotating vector in this embodiment, but a change between linear and circular polarization The configurations of the optical units 7B, 7C in turn allow for example Generation of a quadrature signal 47, with which the coherent combination can be regulated on the basis of an intensity-independent phase vector 45 rotating in dependence on the relative phase The evaluation unit 9 comprises electronic components and / or a computer system for analog and / or digital evaluation, wherein the evaluation steps outlined in FIG For example, the electrical signals of the photodiodes 41A, 41B, 41C first pass through one

Transimpedance amplifier, which outputs a voltage proportional to the light intensity at the output. With the aid of differential amplifiers, the sine or cosine component of the quadrature signal is formed from two voltage signals in each case. Additional suitable functional groups can be used to adjust the amplitude and offset of the electrical signals, resulting in an ideal quadrature signal that can be sampled with an analog-to-digital converter. Further processing is usually done digitally using a computer system. Alternatively, the output signals of the transimpedance amplifiers may be sampled directly, the adaptation and generation of the quadrature signal then being implemented in the computer system. In particular, the pure phase measurement on the photodetectors allows an adjustment of the off-set by adjusting the individual photodetector signals and an adjustment of the amplitude of the phase vector 45. Thus, for the individual measurement beam paths, a substantially identical signal intensity can be readjusted electronically. In general, digital computer systems and / or analog signal processing can be used to calculate the phase position and / or the control signals for the delay device. In the exemplary embodiments, projection directions rotated by π / 2 relative to one another were addressed, since these can lead to a good contrast. Since the transmission of the polarizers for rotations by π, for example at 0 ° and 180 °, is the same, results for the period of the quadrature signal lambda / 2. This means that filter angles of 45 °, 0 ° and + 45 ° each correspond to a phase of 90 °, 0 °, + 90 ° in the transmitted intensity signal. The angle data in the figures thus relate to the photodiode signals. In general, other non-identical polarization directions and projection directions can also be used in the optical units, for example when more than three measuring beams are available, with possibly an adaptation of the evaluation, in particular without forming a quadrature signal.

In summary, the relative phase measurement concept disclosed herein in conjunction with FIGS. 1-4 allows a method for coherently combining laser beams in which two laser beams having different polarization states are superimposed on a common propagation path to form a sum laser beam. In this case, in a substantially intensity-independent homodyne measuring method, the relative phase between the two laser beams is determined and an optical path length difference for stabilizing the polarization state of the sum laser beam is adjusted. In the method, the substantially intensity independent homodyne measurement method may include one or more of the following steps: splitting off a measurement portion of the sum laser beam, splitting the measurement portion into three measurement beams, forming three polarization states, and projecting the three polarization states onto a common projection direction or projecting the three measurement beams in three projection directions. Further embodiments can be seen from the description of the exemplary embodiments of the laser system and of the optical units.

Hereinafter, further embodiments of a homodyne measurement will be described, in which intensity measurements at corresponding phase offset positions of one over a Beam cross-section extending interference pattern are made to provide photodetector signals with different phase offsets for the phase control. For the measurement, coherent laser beams extend in particular at an angle, so that overlapping areas of the laser beams have traveled through different path lengths and adjacent phase offset positions have different path length differences, which lead to an interference fringe pattern.

The angle between the directions of propagation of the laser beams can already be present at the beam input of the phase control system or can be specially generated on the basis of collinearly superimposed laser beams. For example, similar to the superimposition in Fig. 1 with two beam splitters with a corresponding adjustment of the mirror 21 at the output of the beam splitter 15 A, a corresponding angle between the laser beams are present.

FIG. 5 shows an exemplary approach for generating a splitting angle starting from a collinearly superposed measuring portion 119 '. The measurement portion 119 'is based on two e.g. respectively, but and orthogonal polarized (possibly amplified) laser beams. In Fig. 5, the linear polarizations are indicated schematically by arrows 127. A birefringent prism 137 causes an angle α between coherent laser beams 117A, 117B due to the polarization-dependent refraction. Due to the orthogonal polarizations of the laser beams 117A, 117B, a polarizing filter 139 is introduced downstream of the birefringent prism 137 into the two largely overlapping beam paths to cause interference. The polarization filter 139 is aligned in such a way that downstream of the beam, the laser beams 117A, 117B have at least partially interfering polarization states. For example, 5, a transmission direction 165 at 45 ° to the orthogonal linear polarization directions is indicated in FIG.

The generated (split) angle is selected such that the laser beams 117A, 117B in a propagation section 118A only diverge slightly, so that the laser beams 117A, 117B overlap in a central area 118B. This results in an interference pattern 161 over the beam cross section in the direction of the angle a. In Fig. 5, an interference pattern is schematically indicated in the drawing plane from bottom to top.

Generally, the angles are in the range of 0.01 ° to 0.02 °, so that at beam diameters, for example, from 3 mm to 20 mm adjacent phase offset differences of eg ± 90 ° at a distance of eg 1 mm and are measurable with corresponding photodetectors.

For clarification, three measuring beam regions 131A, 131B, 13 IC are shown in FIG. Between two adjacent ones of the three measuring beam areas 131A, 131B, 13C1 there is in each case a phase offset difference of 90 ° in the interference pattern 161. With three photodetectors 141A, 141B, 141C (for example photodiodes) arranged in a measurement plane 163, photodetector signals corresponding to the phase offsets can be supplied to an evaluation unit for evaluation and generation of a control signal. The control signal corresponds to the relative phase between the coherent laser beams and can be used in accordance with the control of a delay device. The generation of the control signal can be effected, for example, as in the signal processing described in connection with FIGS. 1 to 4. In some embodiments based on the evaluation of an interference fringe pattern, in addition to the use of individual detectors, for example, a line or pixel sensor can be used, which is positioned in the measurement plane.

Furthermore, tilting of the measuring plane used (in the plane spanned by the angle) causes a projection of the fringe pattern onto this plane, whereby the fringe spacing in the measuring plane can be adjusted since it increases with increasing angle. Tilting of the measuring plane used may e.g. to adjust the phase offset between the individual detectors. Tilting is possible, for example. when a detector unit with predetermined spacing is to be used for a smaller fringe period. The tilt angle is thereby increased (starting from the orientation orthogonal to the laser beams) until the fringe period and detector spacing match one another.

In summary, the concept of measuring the relative phase using interference fringes, which is disclosed inter alia in connection with FIG. 5, also permits a method for coherently combining laser beams, in which two laser beams with different polarization states are superposed on a common propagation path to form a sum laser beam. In the method, the substantially intensity-independent homodyne measuring method can comprise the following steps: superposing two coherent laser beams with interfering polarization states such that at least three measuring beam areas 131A, 131B, 13 IC are assigned in the beam cross section of the superimposed coherent laser beams different Phasenoffsets; Determining a relative phase between the two laser beams based on the at least three measuring beam regions 131A, 131B, 13 IC, in particular detecting intensity values for each of the at least three measuring beam regions 131A, 131B, 13CIC and providing them as input variables for signal processing, in particular for quadrature signal processing ; and adjusting an optical path length difference between the two laser beams before superimposing them to stabilize the interference pattern based on the determined relative phase.

Examples of amplifier units are fiber amplifier units, e.g. a crystal fiber amplifier unit, titanium: sapphire amplifier units, bar amplifier units, plate amplifier units, disc amplifier units, optical parametric amplifier units and semiconductor amplifier units, and in particular regenerative, single-pass and / or multi-pass amplifier units.

Thus, (pulsed or cw) amplifier systems, for example Ti: sapphire based or fiber laser based amplifier systems, eg in multipass configuration or slab based, can be coherently combined using the homodyne and substantially amplitude independent phase measurement described herein. For example, repetition rates of pulsed laser systems can range from one or a few Hz up to 100 kHz, or even one or more MHz. Exemplary pulse durations are in the ns, ps, and fs range. Exemplary regenerative laser systems and regenerative laser stages that can be coherently combined with the concepts disclosed herein are disclosed, for example, in "High-repetition-rate picosecond pump laser based on a Yb: YAG disk amplifier for optical parametric amplification", T. Metzger et al , Opt. Lett., 34, 2123-2125 (2009).) An exemplary fiber amplifier based laser system is described in the aforementioned paper by A. Klenke et al .. Furthermore, the concept described herein can also be applied to scalable optical amplifier arrangements. as disclosed, for example, in DE 10 2010 052 950 A1 In particular, after two or more pairs of amplifier systems have each been coherently combined, the superimposed output beams can also be coherently superimposed again, in this way correspondingly scaled output powers can be achieved. 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 disclose every possible intermediate value or subgroup of units for the purpose of the original disclosure as well as for the purpose of restricting the claimed invention, in particular also as the limit of a range indication.

Claims

claims

A phase control system (5) for controlling the relative phase (φ) of two laser beams (17A, 17B) to be coherently combined of a laser system (1) provided for providing a phase-locked sum laser beam (19), comprising:

an optical unit (7, 107) comprising

a beam input (33) for receiving a measurement portion (19 ') of two collinear laser beams (17A, 17B) to collide superimposed to form a sum laser beam (19), in particular having substantially orthogonal polarization states, a beam splitter (37) for generating at least three measurement beams (31 A, 31 B, 3 IC) from the coherent laser beams (17A, 17B) of the measuring component (19 ') propagating on associated measuring beam paths (32A, 32B, 32C), or

a propagation section in which the coherent laser beams (17A, 17B) of the measurement component (19 ') propagate spatially superimposed at an angle and form at least three measurement beam regions (131A, 131B, 13Ic),

wherein the at least three measuring beam paths (32A, 32B, 32C) of the at least three measuring beams (31A, 31B, 3 IC) are projected onto matched polarization directions or

the at least three measuring beam regions (131A, 131B, 13 IC) by path length differences

for generating different phase offsets of the associated measuring beams (31A, 31B, 3 IC) or measuring beam areas (131A, 131B, 13CIC), and at least three photodetectors (141A, 141B, 141C) for outputting photodetector signals, the photodetectors (141A , 141B, 141C) are each assigned to one of the measuring beam paths (32A, 32B, 32C) or each one of the measuring beam regions (131A, 131B, 13C) and the photodetector signals are assigned to the measuring beams (31A, 31B, 3C) or measuring beam regions ( 131A, 131B, 131C) at the different phase offsets,

an evaluation unit (9) which generates a control signal (45) based on the at least three photodetector signals, the relative phase (φ) between the coherent laser beams (17A, 17B) based on an evaluation of a polarization state of the measurement component (19 ') or an interference behavior the proportion of measurement (19 ') corresponds, and a delay device (11) for introducing into the beam path of at least one of the two collinear laser beams (17A, 17B), which has an optical path length which is adjustable in dependence on the control signal (45).

Second phase control system (5) according to claim 1 for controlling the relative phase (φ) of two laser beams to be combined coherently (17A, 17B) of a laser system (1), which is provided for providing a phase-controlled sum laser beam (19)

the beam input (33) of the optical unit (7) is designed to receive a measuring component (19 ') of two coherent laser beams (17 A, 17 B) superposed collimated to form a sum laser beam (19) and

the at least three measurement beam paths (32A, 32B, 32C) of the measurement component (19 ') are each designed to project a polarization state of an associated measurement beam onto a polarization direction.

3. phase control system (5) according to claim 1 or 2, wherein the beam splitter (37) as a non-polarizing beam splitter, in particular as a diffraction grating or as a partially reflecting mirror combination for distributing the measuring component (19 ') to the three measuring beam paths (32A, 32B, 32C) is, and / or

wherein at least one of the at least three measuring beam paths (32A, 32B, 32C) has a polarizer (39A, 39B, 39C, 49) for projecting a polarization state onto a polarization direction and / or a focusing lens (37) for increasing the signal.

4. Phase control system (5) according to claim 2 or 3, wherein the optical unit (7A) has a quarter wave plate (35) which is downstream of the beam input (33) and upstream of the beam splitter (37) is arranged and in particular for conversion of orthogonal linear polarizations (27) of the coherent laser beams (17A, 17B) is provided in left and right rotating circular polarizations (27 '), and

wherein the three polarization directions associated with the projections of the measuring beams (31A, 31B, 3CC) are rotated by 45 ° and 90 °, respectively, and the associated three measuring beam paths (31A, 31B, 31C) in particular each have a linear polarizer which is such are oriented so that the transmitted polarization directions are rotated by 45 ° or 90 ° to each other.

5. phase control system (5) according to claim 2 or 3, wherein the at least three measuring beam paths (32A, 32B, 32C) are each formed to project a measuring beam path specific polarization state to a common polarization direction and in particular each have a linear polarizing filter (49) whose transmitted polarization - are oriented essentially identically, and

wherein for generating the measuring beam path-specific polarization states two of the at least three measuring beam paths (32A, 32B, 32C) phase-shifting optical elements (51A, 51B, 53), which cause a defined relative phase delay of one of the polarization directions, so that phase-shifted photodetector signal waveforms (43 A, 43B , 43C).

6. phase control system (5) according to claim 5, wherein for generating the measuring beam path-specific polarization states of the at least three measuring beam paths each have a lambda quarter wave plate (51A, 51B), wherein the lambda-quarter wave plates (51 A, 51 B) in their Orientation with respect to the polarization in the respective measuring beam path (32A, 32C) rotated by 90 ° to each other and in particular the orientation of the fast and slow axes interchanged and aligned to the polarization directions of the measuring beams, or wherein for generating the Meßstrahlengangspezifischen polarization states one of the at least three measuring beam paths one Lambda quarter wave plate (51A) and one of the at least three measuring beam paths (32A, 32B ', 32C ") have a lambda half wave plate (53), wherein the wave plates (51A, 53) in their orientation with respect to the polarization in respective measurement beam path (32A, 32C ") are aligned such that the fast axis Parallel to each other, and in particular at the same time parallel to one of the polarization directions of the measuring beams are, so that set in the respective measuring beam paths (32Α ', 32C ") to mutually phase-shifted linear polarizations.

7. phase control system (5) according to claim 1 for controlling the relative phase (φ) of two laser beams to be combined coherently (17A, 17B) of a laser system (1), which is provided for providing a phase-controlled sum laser beam (19)

in the propagation section of the optical unit (107), the coherent laser beams (17A, 17B) propagate at an angle and are spatially superimposed in a central area, and the optical unit (107) further comprising a polarizing filter, in particular disposed in the propagating section, for generating interfering polarization states of the coherent laser beams (17A, 17B), the states of polarization in an angular direction resulting in an interference pattern exhibiting the least interference. at least three measuring beam areas (131A, 131B, 13 IC) assign the different phase offsets.

8. Phase control system (5) according to claim 6, wherein

the beam input (33) is designed for recording the measuring component (19 ') of collinear superimposed coherent laser beams (17 A, 17 B) and

wherein the optical unit (107) further comprises a polarization state dependent beam splitter (137), in particular a birefringent prism, for generating the angle between the two coherent laser beams (17A, 17B) such that the coherent laser beams (17A, 17B) in the central region are at least form three measuring beam areas (131A, 131B, 131C), or

wherein the optical unit (107) further comprises

a first polarization state dependent beam splitter to separate the polarized beam portions of the measurement portion,

a second, in particular polarization state-dependent or non-polarization state-dependent, beam splitter,

optical elements for deflecting the separated beam portions on the second beam splitter, wherein at least one of said optical elements is arranged such that the superimposed at the output of the second beam splitter beam portions enter at an angle in the propagation section.

9. phase control system (5) according to any one of the preceding claims, wherein the evaluation unit (9) is further adapted to temporal variations of the relative phase (φ) between the coherent laser beams (17A, 17B) in a change in direction of a phase vector (45).

10. phase control system (5) according to any one of the preceding claims, wherein the evaluation unit (9) is designed to subtract from two pairs of photodetector signals and in particular comprises a difference based quadrature signal processing (47) and / or wherein the delay device (11) is designed for the coarse adjustment of the optical path length and for the fine adjustment of the optical path length, and in particular has a motor-driven, for example piezo-mechanical displacement unit of a folding mirror or retroreflector, and / or

wherein, in particular, an acousto-optical delay unit is used to stabilize and / or specifically change the phase position of the input beams, and / or

wherein the calculation of the phase position and / or the control signals for the delay device by means of a digital computer system and / or an analog signal processing takes place.

11. A laser system (1) for providing a coherent combining based sum laser beam (19) with:

a seed laser beam source for providing a first seed laser beam (13A) and a second seed laser beam (13B) coherent to the first seed laser beam (13A), a first amplifier arm including a first optical amplifier unit (3A) for generating a first seed laser beam amplified laser beam (17A) based on the first seed laser beam (13A),

a second amplifier arm having a second optical amplifier unit (3B) for generating a second amplified laser beam (17B) based on the second seed laser beam (13B),

a combination unit (15B) for collinearly superposing the beam path of the first amplified laser beam (17A) and the beam path of the second amplified laser beam (17B) to produce the sum laser beam (19), the first laser beam (17A) in a first polarization state and the second laser beam (17B) are superimposed in a second polarization state different from the first polarization state, and a phase control system (5) according to one of the preceding claims, wherein as part of measurement (19 ') a part of the sum laser beam (19) or parts of the first propagating at an angle amplified laser beam (17A) and the second amplified laser beam (17B) are coupled into the beam input (33) of the optical unit (7) and the delay device (11) in the first amplifier arm (3A) and / or in the second amplifier arm (3B ) is provided.

12. A laser system (1) for providing a coherent combining based sum laser beam (19) with: a laser beam source for providing a first laser beam (13A) and a second laser beam (13B) coherent with the first laser beam (13A),

a combination unit (15B) for collinear superimposition of the beam path of the first laser beam (13A) and the beam path of the second laser beam (13B) to produce the sum laser beam (19), the first laser beam (13A) in a first polarization state and the second laser beam (13B ) are superimposed in a second polarization state different from the first polarization state, and

a phase control system (5) according to one of the preceding claims, wherein as part of the measurement (19 ') part of the sum laser beam (19) or angularly propagating parts of the first laser beam (13A) and the second laser beam (13B) in the beam input (19 33) of the optical unit (7) is or are coupled and the delay device (11) in the beam path of the first laser beam (13A) and / or in the beam path of the second laser beam (13B) is provided.

The laser system (1) according to claim 11 or claim 12, wherein the evaluation unit (9) of the phase control system (5) determines a relative phase (φ) between the first (amplified) laser beam (17A) and the second (amplified) laser beam (17B) which characterizes the polarization state of the sum laser beam (19) and the determination substantially independent of intensity fluctuations in the first (amplified) laser beam (17A) and / or in the second (amplified) laser beam (17B), in particular in the first optical amplifier unit (17A) and / or the second optical amplifier unit (17B), and / or wherein the delay device (11) for adjusting a relative optical path difference between the first (amplified) laser beam (17A) and the second (amplified) laser beam (17B), in particular the first amplifier arm (17A) and the second amplifier arm (17B) in response to the control signal (45) for stabilizing the polarization State of the sum laser beam (19) is formed.

14. The laser system (1) according to claim 11 or 13, wherein the seed laser beam source comprises a primary seed laser (2) and a beam splitter (15A) for splitting a primary seed laser beam (13) of the primary seed laser (2) the first seed laser beam (13A) and the second seed laser beam (13B), or wherein the seed laser beam source comprises two coherently coupled seed laser systems and / or

wherein the first optical amplifier unit (3A) and / or the second optical amplifier unit (3B) each comprise a fiber amplifier unit, a crystal fiber amplifier unit, a Ti tan: sapphire amplifier unit, a rod amplifier unit, a plate amplifier unit, a disc amplifier unit, an optical parametric amplifier unit, or a semiconductor amplifier unit and in particular a regenerative and / or single-pass amplifier unit and / or multi-pass amplifier unit.

15. A laser system (1) according to any one of claims 11 to 14, wherein

the first polarization state and the second polarization state are substantially orthogonal linear polarizations,

the optical unit (7A) has a lambda quarter wave plate (35) for converting the line polarizations into left-handed or right-handed circular polarizations, and in particular a diffraction grating for distributing the measured portion (19 ') onto the three measuring beam paths (32A, 32B, 32C). and

the three measuring beam paths (32A, 32B, 32C) are each designed to project the polarization state to three directions of polarization rotated by 45 ° or 90 ° relative to one another, and in particular each have a polarizer (39A, 39B, 39C) as input variables for quadrature signal processing in particular; whose projection directions are rotated by 45 ° or 90 ° so that optical path length changes of the first (amplified) laser beam (17A) and the second (amplified) laser beam (17B), in particular in the first amplifier arm and / or in the second amplifier arm, in particular phase-shifted Pho - todetektorsignaländerungen result.

16. A laser system (1) according to any one of claims 11 to 14, wherein

the optical unit (7B, 7C) has in particular a diffraction grating (37) for distributing the measuring component onto the three measuring beam paths (32A, 32B, 32C), and

as input variables for quadrature signal processing, the three measuring beam paths (32A, 32B, 32C) are each designed to project a measuring beam path specific polarization state to a common polarization direction, and in particular each have a NEN polarizer (49) whose projection directions are substantially identical, and wherein the Generation of the Meßstrahlengangspezifischen polarization states two of the three measuring beam paths (32A, 32B, 32C) each have a quarter-wave plate (51A, 51B), wherein the lambda quarter-wave plates (51A, 51B) are rotated by 90 ° to each other, or two of the three measuring beam paths (32A, 32B, 32C) have a lambda-quarter wave plate (51A) and a half-wave wave plate (53), respectively, so that optical path length changes of the first (amplified) laser beam (17A) and the second (amplified) laser beam (17B), in particular in first amplifier arm and / or result in the second amplifier arm, phase-shifted photodetector signal changes.

17. A method for coherently combining laser beams with:

Superposition of two coherent laser beams with different polarization states on a common propagation path to form a sum laser beam,

In a substantially intensity-independent homodyne measuring method, determine a relative phase between the two laser beams and

Adjusting an optical path length difference between the two laser beams before forming the sum laser beam to stabilize the polarization state of the sum laser beam based on the determined relative phase.

18. The method of claim 17, wherein the substantially intensity-independent homodyne measuring method comprises the steps of:

Providing a measurement portion of the sum laser beam,

Splitting the measuring component into at least three measuring beams,

Forming at least three polarization states,

Projecting the at least three polarization states in a common direction of projection and

Detecting intensity values for each of the at least three measurement beams for providing the intensity values as input variables for signal processing, in particular for quadrature signal processing.

19. The method of claim 17, wherein the substantially intensity-independent homodyne measuring method comprises the steps of:

Providing a measurement portion of the sum laser beam,

Splitting the measuring component into at least three measuring beams, and

Projecting the at least three measuring beams in at least three projection directions and detecting intensity values for each of the at least three measuring beams to provide the intensity values as input variables for signal processing, in particular for quadrature signal processing.

20. A method for coherently combining laser beams with:

Superposition of two coherent laser beams with interfering polarization states such that at least three measurement beam areas (131A, 131B, 13C1) in the beam cross section of the superimposed coherent laser beams are assigned different phase offsets,

Determining a relative phase between the two laser beams based on the at least three measuring beam regions (131A, 131B, 131C), in particular detecting intensity values for each of the at least three measuring beam regions (131A, 131B, 131C) and as input variables for a signal processing, in particular for a Quadrature signal processing, be provided, and

Adjusting an optical path length difference between the two laser beams before superimposing them to stabilize the interference pattern based on the determined relative phase.