Arrangement for pulse control in oscillator-amplifier systems using oscillators of different wavelengths

Abstract

Arrangement for energy control of rapidly recurring pulses of a laser system - a main oscillator (11) emitting a beam (111) with a wavelength λ1, - a shaping optic (131) for the main oscillator, which forms a focus for the beam (111) from the main oscillator to a beam (161) with a size and position according to a specification, - an auxiliary oscillator (21) that emits a beam (211) with a different wavelength λ2, - a shaping optic (231) for the auxiliary oscillator, which forms a focus for the beam (211) from the auxiliary oscillator to a beam (261) with a size and a position according to a specification, - an acousto-optic modulator (63) to which a high-frequency driver (66) is associated, wherein the driver generates a high-frequency power at a frequency f1 and a high-frequency power at a frequency f2 and couples it into the modulator, wherein the high-frequency power A1(f1, t) at frequency f1 is used for diffraction and modulation of the beam of the main oscillator and the high-frequency power A2(f2, t) at frequency f2 is used for diffraction and modulation of the beam of the auxiliary oscillator, resulting in a beam (171) from the beam (161) and another beam (271) from the beam (261), - a shaping optic (82) that separates the diffracted and modulated beams (171, 271) of the two oscillators from other diffraction orders and couples them superimposed into a downstream amplifier arrangement, - an amplifier (86) by which the power and pulse energy of the superimposed beams is upscaled, - a spectral filter (68) with which the pulses or the power of an amplified beam of the main oscillator of wavelength λ1 are separated from the beam of an amplified beam of the auxiliary oscillator of wavelength λ2. wherein the (161, 261) shaping optics (131, 231) are designed and arranged such that the two beams meet at a defined point, wherein the foci of the two beams are of equal size and are located at the interface, wherein the angles of incidence of the beams (161, 261) into the modulator (63) and the frequencies f1 and f2 are matched such that the diffracted and modulated beams (171, 271) superimpose coaxially, wherein the wavelengths λ1 and λ2 are selected such that the amplifier (86) has a comparable gain for them.

Description

[0001] Oscillator power amplifiers are increasingly being used to increase the power or energy of lasers, including particularly powerful short and ultrashort pulse lasers, since the high required power / energy cannot be achieved with an oscillator alone. The oscillator is referred to as the seeder in the following descriptions. The most commonly used seeders are mode-locked or Q-switched fiber lasers, solid-state lasers or diode lasers. Such lasers are used in materials processing, among other things. In addition to properties such as pulse length and pulse energy, the controllability and flexibility of the pulses are of key importance. For example, in material processing systems with variable speeds, the pulses must be synchronizable with the system, and their pulse repetition rate and pulse energy must be adjustable. Laser parameters such as pulse length and beam quality must remain constant.

[0002] The pulse energy and beam parameters depend on the pulse repetition rate, as a high pulse repetition rate leads to low pulse energy and increased extraction / saturation of the energy stored in the gain medium. Furthermore, varying extraction of the stored energy leads to variations in the thermal lensing and thus in beam propagation parameters such as beam size and beam divergence.

[0003] It is essential to keep the extraction / saturation in the gain medium, especially in the amplifier, constant over time. For a nominal pulse repetition rate, the relevant time constant for averaging is approximately equal to the inverse of the nominal pulse repetition rate.

[0004] For a ps laser with very high pulse energy and / or high average power, an amplifier array consisting of multiple amplifiers is used. The amplifiers are used in series to achieve the required high gain.

[0005] The laser array behaves temporally like a master, runs at a pulse repetition rate, and pulses cannot be triggered externally. This contradicts many applications where the pulses must be triggered by process conditions / events and the pulse energy must be varied. In addition, laser parameters such as pulse energy, pulse width, and beam propagation parameters must remain constant. Beam propagation parameters include propagation direction, divergence, and beam waist position.

[0006] Document D1 DE10 2020 000 999 A1 specifies arrangements for controlling the energy of rapidly recurring pulses. They consist of a main oscillator and an associated modulation arrangement, an auxiliary oscillator and an associated modulation arrangement, with the main oscillator emitting a pulsed beam whose peak pulse power is at least a factor of 10 higher than that of the auxiliary oscillator, and an optical arrangement for coaxially superimposing the beams from the main oscillator and the auxiliary oscillator. Furthermore, an amplifier is used to increase the power and energy of the superimposed beam. Furthermore, a temporal filter is used. The temporal filter consists of at least one nonlinear element. The nonlinear element influences the beam through nonlinear effects depending on the instantaneous intensity.This imparts different properties to the main oscillator beam and the auxiliary oscillator beam. Thus, the amplified beam is split into two beams with different temporal sequences based on their instantaneous intensity.

[0007] An arrangement known from document D1 for generating coaxial superimposed beams from a main oscillator of wavelength λ1 and from an auxiliary oscillator of wavelength λ2 shows Fig. 4. A modulator (631) including a driver is assigned to the main oscillator (11), and a modulator (632) including a driver is assigned to the auxiliary oscillator (21). The shaped and modulated beams (161) and (261) are coaxially superimposed using a deflection mirror (701) and a dichroic mirror (702). Electro-optical modulators can also be used in this alternative embodiment.

[0008] As is known from document D1, the coaxial superposition of beams (161) and (261) can also be achieved by a deflection mirror (701) and a polarization radiator (707). For gain media with a polarization dependence, a retardation plate (703) and a polarization (706) can be used. This linearizes the polarizations of the coaxially superimposed beams, as shown in Fig. 5 is shown.

[0009] Such arrangements only work in conjunction with a downstream nonlinear process such as frequency conversion. They cannot be used in cases where a nonlinear process is not required or permissible.

[0010] The object of the present invention is to provide laser arrangements that can achieve constant laser parameters even with any temporal sequence of pulses. The applicability should be independent of nonlinear processes. This allows for the realization of a laser arrangement with an external free-trigger option and PSO function. Furthermore, the pulse energy should be adjustable according to external specifications.

[0011] In general, a modulator consists of a first element that modifies the beam properties such as propagation direction or polarization, a second element that acts as a driver, and a third element that groups, sorts, or splits the beams according to their properties. In the following, the first element is referred to as the modifying element and the third element as the filtering element. In the case of acousto-optical modulators, the first element is an acousto-optical grating cell, the second element is a radio-frequency unit, and the third element is a spatial filter / beam dump. In the case of electro-optical modulators, the first element is a Pockels cell, the second element is a high-voltage unit, and the third element is a polarization beam splitter.

[0012] The solution to the problem is an optical amplifier arrangement consisting of - a main oscillator (11) emitting a beam (111) with a wavelength λ1, - a shaping optic (131) for the main oscillator, which forms a focus for the beam (111) from the main oscillator into a beam (161) with a size and a position according to a specification, - an auxiliary oscillator (21) emitting a beam (211) with a different wavelength λ2, - a shaping optic (231) for the auxiliary oscillator, which forms a focus for the beam (211) from the auxiliary oscillator to a beam (261) with a size and a position according to a specification, - an acousto-optical modulator (63) to which a high-frequency driver (66) is assigned, the driver generating a high-frequency power at a frequency f1 and a high-frequency power at a frequency f2 and coupling it into the modulator, the high-frequency power A1(f1, t) at the frequency f1 being used to diffract and modulate the beam of the main oscillator and the high-frequency power A2(f2, t) at the frequency f2 being used to diffract and modulate the beam of the auxiliary oscillator, thereby creating a beam (171) from the beam (161) and a further beam (271) from the beam (261), - a shaping optics (82) which separates the diffracted and modulated beams (171) and (271) of the two oscillators from other diffraction orders and couples them superimposed into a downstream amplifier arrangement, - an amplifier that upscales the power and pulse energy of the superimposed beams, - a spectral filter used to separate the pulses or the power of an amplified beam of the main oscillator at wavelength λ1 from the beam of an amplified beam of the auxiliary oscillator at wavelength λ2. wherein the shaping optics (131) and (231) are designed and arranged such that the two beams (161) and (261) meet at a defined point, the foci of the two beams being of equal size and located at the interface, the angles of incidence of the beams (161) and (261) into the modulator (63) and the frequencies f1 and f2 being coordinated with one another such that the diffracted and modulated beams (171) and (271) superimpose coaxially.

[0013] An amplifier is used whose gain medium amplifies both the beam with wavelength λ1 and the beam with wavelength λ2. The wavelengths λ1 and λ2 are chosen so that the small-signal gains for the two wavelengths do not differ significantly. This allows the beam from the main oscillator and the beam from the auxiliary oscillator to be amplified comparably. Furthermore, a spectral filter is placed behind the amplifier to separate the beams of different wavelengths.

[0014] A dichroic mirror or a birefringent spectral filter, for example, can be used as a spectral filter. The spectral filter splits the amplified beam from the main oscillator and the amplified beam from the auxiliary oscillator. This creates an amplified beam with the properties of the main oscillator, such as wavelength and pulse length, and with a specified temporal sequence.

[0015] For a ns pulse duration, a Q-switched oscillator can be used as the main oscillator. In an ultrashort pulse laser system, a ps oscillator or an fs oscillator is used as the main oscillator. A pulsed oscillator with a pulse length considerably longer than that of the main oscillator can be used as the auxiliary oscillator (601). Preferably, a cw oscillator is used as the auxiliary oscillator. The cw oscillator should have a continuous and stable output. An example of cw oscillators with constant and continuous output are cw oscillators with a single longitudinal mode.

[0016] The pulses of the main oscillator are selected according to external inputs using the modulator. The beam power of the auxiliary oscillator is determined according to external inputs and adjusted using the modulator.

[0017] In the following, the laser arrangement is explained using the example of a ps oscillator-amplifier system, which consists of a main oscillator with ps pulse duration and an auxiliary oscillator that emits constant and continuous radiation.

[0018] The optical arrangement for coaxial superposition can be formed by a partially transparent mirror. For amplifiers with polarization-independent amplification, the main and auxiliary oscillators are coaxially superposed, a polarizer is used. In this case, a main oscillator with a linearly polarized beam is used. The beam from an auxiliary oscillator is also linearly polarized. The polarizations of the two beams are perpendicular to each other. To separate the amplified partial beam from the main oscillator and the amplified partial beam from the auxiliary oscillator, a polarizer is placed behind the amplifier.

[0019] In the following, the laser arrangement is explained using the example of a ps oscillator-amplifier system, which consists of a main oscillator with ps pulse duration and an auxiliary oscillator that emits constant and continuous radiation.

[0020] Fig. 1 shows an optical amplifier arrangement with a main oscillator (11) and an auxiliary oscillator (21), each emitting a beam (111) and (211). The beam (111) emitted by the oscillator (11) has a wavelength λ1 and is transformed by shaping optics (131) into a convergent beam (161) having a focus. The beam (211) emitted by the auxiliary oscillator (21) has a wavelength λ2 and is transformed by shaping optics (231) into a convergent beam (261) having a focus. The shaping optics (131) and (231) are designed and arranged such that the foci of the convergent beams (161) and (261) are of equal size and lie at an intersection of the two beams. An acousto-optic modulator (63) is arranged at the interface. The acousto-optic modulator is driven by a driver (66).The driver (66) generates time-varying high-frequency powers A1(f1, t) and A2(f2, t) at two different frequencies f1 and f2 according to a specified value. The angles of incidence of the beams (161) and (261) and the frequencies f1 and f2 are selected such that the beam (171) diffracted by the beam (161) and the beam (271) diffracted by the beam (261) coaxially overlap. The coaxially superimposed beams are coupled into an amplifier (86) by means of a shaping optics (82), where their power / energy is increased by the amplifier. In practice, the amplifier can be formed by one or more amplification stages to achieve the necessary gain.

[0021] In order to achieve a high diffraction efficiency, the angle of incidence of at least one of the beams (161) and (261) and one of the frequencies f1 and f2 are chosen so that the condition for diffraction at the blaze angle is fulfilled.

[0022] In Fig. Figure 2 shows an example of the optical amplifier arrangement according to this present application. A shaping optic (181) is used for the main oscillator (11) and a shaping optic (281) is used for the auxiliary oscillator (21). The shaping optics are designed and arranged such that the beam (111) is transformed into a beam (186) and the beam (211) into a beam (286), wherein the beams (186) and (286) are collimated with the same beam waist position and are parallel to each other. Furthermore, the distance between the beams is adjusted according to a specification. A lens (811) is used. With this lens, the beams (186) and (286) are focused into the beams (161) and (261). The two beams have foci of the same size. The two foci are located at the interface where the acousto-optic modulator is positioned.

[0023] Advantageously, the driver is configured so that at least one of the frequencies f1 and f2 can be changed within a small range so that the diffraction angle of at least one of the beams can be changed so that the coaxial overlap of the beams can be improved in an electronic manner.

[0024] To increase the power and pulse energy, the coaxially superimposed beams are fed into an amplifier (86). The beams are coupled into the amplifier via an optical system (82). An amplified beam (911) with higher power / pulse energy is generated behind the amplifier. The beam (911) contains two sub-beams. One is the amplified beam (99) from the main oscillator (11) at wavelength λ1, and the other is the amplified beam (99) from the auxiliary oscillator (21) at wavelength λ2. A spectral filter (68) is used to split the two beams.

[0025] The spectral filter can be, for example, a dichroic mirror or a birefringent spectral filter.

[0026] As it is in Fig. As shown in Figure 3, a modulator (76) is used for further temporal shaping and / or modification of the pulses. The modulator can be an electro-optical modulator or an acousto-optical modulator. It can also be used to modulate the power and pulse energy. The modulator transforms the amplified beam into an output beam (99) with a temporal profile according to a specified setting.

[0027] It is assumed that the oscillator-amplifier arrangement is pumped constantly and continuously. The operation of the oscillator-amplifier arrangement can be explained as follows using exemplary designs and an ultrashort pulse laser.

[0028] For simplified explanation, Fig. 6a to 6j show the triggering and temporal shaping of the beams and the driver powers. Fig. Figure 6a shows the periodic pulses of the master oscillator at wavelength λ1. The typical pulse frequency is a few tens of MHz. Fig. Figure 6b shows the constant and continuous power of the cw auxiliary oscillator with wavelength λ2. The ps laser system should have a nominal pulse repetition rate f nom The time span to the nominal pulse repetition rate is τnom=1fnom. Fig. Figure 6c shows the triggering of T1, T2, ...T6 according to an external setting. The time intervals between the adjacent triggers are not constant. There are three situations: T4 - T3 > τ nom , T4 - T3 = τ nom and T4 - T3 < τ nom . At the trigger pulse T n For example, the driver of the main modulator performs the following actions (see Fig. 2c, Fig. 2d and Fig. 2e): 1. At the rising edges of the trigger pulses, the driver switches the high-frequency power of frequency f1 for the main oscillator to a zero state, preferably at times T1, T2, T3, T4, ...Tn, so that no pulses from the main oscillator with wavelength λ1 pass through. At the same time, the driver switches the high-frequency power of frequency f2 for the auxiliary oscillator to a zero state, so that no power from the auxiliary oscillator with wavelength λ2 passes through. In summary, no beam passes through the amplifier, and the inversion / amplification in the amplifier medium builds up. 2. After a build-up time of Tn2-Tn1, the driver switches the high-frequency power of frequency f1 for the main oscillator to the conducting state for a short period of time (Tn3-Tn2), so that a useful pulse p n or a useful pulse burst of wavelength λ1 (cf. Fig. 6f) with the highest possible amplitude for amplification. The high-frequency power of frequency f2 for the auxiliary oscillator remains at zero, so that the pulse energy of the useful pulse P n The number of pulses of a useful pulse burst of wavelength λ1 can be determined by the time period Tn3-Tn2 in relation to Ts = 1 / f s (f s is the pulse repetition rate of the seeder) can be controlled according to a specification. 3. In the event that T n -T(n-1) > 1 / f nom holds, the high-frequency power of frequency f1 for the main oscillator remains in the zero state (cf. Fig. 6d), while the high-frequency power of frequency f2 for the auxiliary oscillator changes to a defined conduction state after a short time (Tn4-T(n-1)3) and remains there until the next trigger pulse arrives (cf. Fig. 6e). This allows the modulator to produce a conditioning beam power b nthe wavelength λ2 with a predefined and low amplitude (cf. Fig. 6g) to the inversion (81 in Fig. 7b) and thus adjust the gain of the laser medium for constant laser parameters. 4. If T n -T(n-1) = 1 / f nom , the high-frequency powers f1 and f2 for the main oscillator and the auxiliary oscillator remain at the ground state, so that an amplification is built up for the n-th useful pulse or useful pulse burst. 5. If the following trigger pulse comes faster than the nominal pulse repetition rate, namely T n -T(n-1) <= 1 / f nom, the high-frequency power of frequency f1 for the main modulator briefly switches to the on-state on the rising edge. At the same time, the high-frequency power of frequency f2 for the auxiliary oscillator remains in the zero state to release a useful pulse or useful pulse burst of wavelength λ1. In this case, the subsequent pulse energy is lower due to the shorter inversion build-up time.

[0029] After receiving the next trigger pulse ((n+1)-th pulse), the process starts again.

[0030] According to the explanations above, the Fig. 6h the temporal form of the coaxially superimposed rays (171) and (271). Fig. Figure 6i shows the temporal shape of the amplified beams (911) after the amplifier. The amplified beam (911) contains the amplified pulses P1, P2, ...Pn of wavelength λ1 from the main oscillator and the power B1, B2, ...Bn of wavelength λ2 from the auxiliary oscillator. After the spectral filter, the amplified beam of the auxiliary oscillator is filtered out at wavelength λ2. Fig. Figure 6j shows the pulse train of the useful pulses from the main oscillator with a wavelength λ1.

[0031] The structure and procedure described above ensure that the energies of the useful pulses or useful pulse bursts of wavelength λ1, P1, P2, ...Pn are approximately equal, and that the power stored in the laser medium is extracted at a constant average. This ensures that the thermal state in the laser medium remains constant, and the beam propagation parameter is stable over time. This solves the problem of this invention to a first approximation.

[0032] A frequency-conversion laser uses a nonlinear medium. The frequency conversion process is highly intensity-dependent and highly wavelength-selective. In this case, the auxiliary pulses can be filtered out using the nonlinear process instead of a spectral filter.

[0033] According to the amplifier arrangement, the reinforced conditioning outputs (B1, B2... in Fig. 6i and Fig. 93 in Fig. 7c) and the amplified useful pulses (P1, P2... in Fig. 6e and Fig. 91 in Fig. ), which have been scaled in power or pulse energy. Although the peak power of the conditioning powers B1, B2, ...Bn is considerably lower than the pulse energy of the desired pulses P1, P2, ...Pn, the conditioning powers are disruptive for many applications and must therefore be filtered out.

[0034] Fig. Figure 7b shows the temporal progression of the inversion or gain (81) in the amplifier medium. It is evident that, due to the control of the diode laser discussed above, the amplifier medium exhibits the same output inversion or gain for each pulse. As in Fig. As shown in Figure 7c, amplified pulses (99) with identical laser parameters as the pulse energy (171) can be generated.

[0035] As with ps laser systems, the conditioning power (93) after the amplifier can be cut off using a spectral filter and the useful pulses (99) can be passed through for use.

[0036] For many applications, the energy of the useful pulses must be adjustable. The energy of the useful pulses can be adjusted by adjusting the conditioning power of the auxiliary oscillator. In this case, the amplifier is constantly pumped. To keep the thermo-optical properties of the amplifier constant, the total extracted power is kept constant by the useful pulses and the conditioning power. If the useful pulses are to contain less power, more power must be extracted from the amplifier by the conditioning power. Furthermore, the extractable power from the amplifier is limited. Conversely, the power and energy of the useful pulses can be adjusted or modulated by adjusting or modulating the conditioning power.

[0037] Fig. Figure 8b shows a trigger sequence. A useful pulse is selected for each trigger. To adjust the pulse energy downstream of the amplifier, a bias level is set for the driver of the auxiliary modulator (see Figure 8b). Fig. 8c). The bias level generates a continuous conditioning power b0 (cf. Fig. 8d). The continuous conditioning performance b0 is amplified to the conditioning performance B0 (cf. Fig. 8e) and leads to a reduction of the inversion in the amplifier. This increases the achievable energy of the useful pulses (P1, P2, ...P5 in Fig. 8ei). This allows the energy of the useful pulses to be adjusted. Furthermore, the bias level can be modulated according to a specified value, thus adjusting the energy of the useful pulses.

[0038] In this application the following applies: - Amplified beam of the main oscillator of wavelength λ1 = useful pulses of wavelength λ1 - Amplified beam of the auxiliary oscillator of wavelength λ2 = conditioning pulses (conditioning power) of wavelength λ2 = auxiliary pulses of wavelength λ2

Claims

[1] Arrangement for energy control of rapidly recurring pulses of a laser system from - a main oscillator (11) emitting a beam (111) with a wavelength λ1, - a shaping optic (131) for the main oscillator, which forms a focus for the beam (111) from the main oscillator into a beam (161) with a size and a position according to a specification, - an auxiliary oscillator (21) emitting a beam (211) with a different wavelength λ2, - a shaping optic (231) for the auxiliary oscillator, which forms a focus for the beam (211) from the auxiliary oscillator to a beam (261) with a size and a position according to a specification, - an acousto-optical modulator (63) to which a high-frequency driver (66) is assigned, the driver generating a high-frequency power at a frequency f1 and a high-frequency power at a frequency f2 and coupling them into the modulator, the high-frequency power A1(f1, t) at the frequency f1 serving to diffract and modulate the beam of the main oscillator and the high-frequency power A2(f2, t) at the frequency f2 serving to diffract and modulate the beam of the auxiliary oscillator, whereby a beam (171) is created from the beam (161) and a further beam (271) is created from the beam (261), - a shaping optics (82) which separates the diffracted and modulated beams (171, 271) of the two oscillators from other diffraction orders and couples them superimposed into a downstream amplifier arrangement, - an amplifier (86) by which the power and pulse energy of the superimposed beams are scaled up, - a spectral filter (68) with which the pulses or the power of an amplified beam of the main oscillator with wavelength λ1 are separated from the beam of an amplified beam of the auxiliary oscillator with wavelength λ2. The shaping optics (131, 231) (161, 261) are designed and arranged such that the two beams meet at a defined point, the foci of the two beams being of equal size and located at the interface, the angles of incidence of the beams (161, 261) into the modulator (63) and the frequencies f1 and f2 being coordinated with one another such that the diffracted and modulated beams (171, 271) superimpose coaxially, the wavelengths λ1 and λ2 being selected such that the amplifier (86) has a comparable gain for them. [2] Arrangement for energy control of rapidly recurring pulses of a laser system according to claim 1, characterized bythat in the amplifier (86) a gain medium is used which amplifies both the beam with the wavelength λ1 and the beam with the wavelength λ2, wherein the small-signal gains for the two wavelengths differ only insignificantly. [3] Arrangement for energy control of rapidly recurring pulses of a laser system according to claim 1 or 2, characterized by in that the shaping optics (181,281) are designed and arranged such that the beams are transformed such that they are collimated with an equal beam waist position and are parallel to one another, the distance between the beams (186, 286) being set according to a specification, a lens (811) being used, the lens (811) focusing the beams (186, 286) into beams (161, 261), the two beams having foci of the same size and the two foci being located at the interface and in the modulator (63). [4] Arrangement for energy control of rapidly recurring pulses of a laser system according to one of claims 1 to 3, characterized by that an electro-optical or acousto-optical modulator (76) is used for further temporal shaping and / or modification of the pulses. [5] Arrangement for energy control of rapidly recurring pulses of a laser system according to one of claims 1 to 4, characterized by that a spectral filter (68) is used to separate the useful pulses of wavelength λ1 from the auxiliary pulses of wavelength λ2. [6] Arrangement for energy control of rapidly recurring pulses of a laser system according to one of claims 1 to 5, characterized by , the main oscillator is a short pulse oscillator, a ps or an fs oscillator and the auxiliary oscillator is a constant and continuous power oscillator.

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