WO2017012898A1 - Beam modulator with frequency conversion, and corresponding method and laser machining machine - Google Patents

Abstract

The invention relates to a beam modulator (1) for modulating the power of a laser beam (2), comprising an actuatable deflecting device (3) that is arranged in the beam path of the incident laser beam (2) and has two actuation states in which the incident laser beam (2) is deflected to different lengths (α₁, α₂). According to the invention, at least one non-linear crystal (5) is arranged in the beam path of the two laser beams (4a, 4b) deflected to different lengths, said crystal converting the frequency of one of the two laser beams deflected to different lengths with greater efficiency and converting the frequency of the other laser beam with less efficiency. A separator (7) is arranged in the beam path of the laser beam (6a, 6b) which is frequency-converted with greater efficiency and of the laser beam which is frequency-converted with less efficiency, said separator spatially separating the frequency-converted components and the non-frequency converted components of the laser beams (6a, 6b), wherein one of the two spatially separated laser beams (9a, 9b, 9c) forms the power-modulated output laser beam.

Description

 Beam modulator with frequency conversion and associated

The invention relates to a beam modulator for power modulation of a laser beam with a controllable deflection device arranged in the beam path of the incident laser beam, which has two drive states in which the incident laser beam is deflected differently, and an associated laser processing machine and a method for power modulation a laser beam. In other words, the deflection device has a first drive state in which the incident laser beam is deflected at a first angle, and a second drive state in which the incident laser beam is deflected at a different second angle. In the case of lasers, in particular pulsed lasers, highly dynamic modulation of the output power of the laser beam is required for many applications.

Such power modulation can be achieved with a fast deflection unit, which can be formed for example by an acousto-optic modulator (AOM) or an electro-optical deflector. In order for the unmodulated and the modulated laser beam can be separated from each other, however, large deflection or a large space are required, which limits the design of the beam modulator significantly. The present invention is therefore the object of developing a beam modulator of the type mentioned in that deflectors can be used with lower deflection angles.

This object is achieved in that at least one nonlinear crystal is arranged in the beam path of the two differently deflected laser beams, the frequency of one of the two differently deflected laser beams with higher efficiency and the other laser beam with lower efficiency, and that in the beam path of higher Efficiency frequency-converted and the frequency converted with less efficiency La- serstrahls a separator is arranged, which spatially separates the frequency-converted and non-frequency-converted portions of the laser beams, wherein one of the two spatially separated laser beams forms the output power modulated output laser beam. In the case of an electrically controlled deflection device, the two activation states of the deflection device can be realized for example by:

- applying and not applying an electrical drive signal;

 - application of drive signals with different power;

 - application of drive signals of different amplitude;

- applying drive signals with different frequency; or

- Apply control signals with different power and different frequency. According to the invention can be dispensed with large deflection angle in connection with the downstream frequency conversion. The deflection angle of one of the two differently deflected laser beams only has to be sufficiently large to leave the angular acceptance range of the non-linear crystal required for frequency conversion and consequently not be frequency-converted or with significantly reduced efficiency. Ideally, the deflection angle and the crystal length that affect the angular acceptance range of the crystal are matched to one another such that maximum frequency conversion occurs in one of the two deflection states and minimal or no frequency conversion occurs in the other.

The separation of the modulated from the unmodulated radiation or the frequency-converted from the non-frequency-converted radiation then takes place by means of the separator. The invention enables efficient Leistungsmodul- tion and due to the low deflection angle a small space. Materials suitable for frequency conversion include lithium niobate, potassium dihydrogen phosphate, beta barium borate, or lithium triborate.

Preferably, the differently deflected laser beams both impinge on the at least one non-linear crystal, so that the same laser power always strikes the non-linear crystal in both drive states, which consequently experiences the same heating as possible in both activation states. In particularly preferred embodiments, in one of the two drive states of the deflection device, the incident laser beam is deflected by the deflection device at an angle of 0 °, ie, the incident laser beam is deflected only in the other drive state. Particularly preferably, one of the two differently deflected laser beams impinges on the nonlinear crystal within the acceptance angle range of the at least one nonlinear crystal required for frequency conversion, while the other laser beam strikes the nonlinear crystal outside the acceptance angle range. Preferably, in the beam path of the two differently deflected laser beams, a non-linear frequency doubling crystal (SHG (second harmonic generation) crystal) for generating a laser beam with twice the frequency or a combination of at least two non-linear crystals for frequency tripling (THG (third harmonic generation) or yet arranged higher frequency multiplication.

Preferably, the deflection device is embodied as an electrically controlled AOM which, for example, only deflects the incident laser beam in one of its two drive states. Low cost AOMs with low deflection angles can be used. As AOM mostly LiNbO ₃ - or PbMoO ~ crystals, glass or quartz are used. Preferably, the separator is designed as a dichroic mirror, which spatially separates the frequency-converted and the non-frequency-converted portions of the laser beams. Instead of a dichroic mirror, mirrors or other beam splitters or other arrangements may also be used. The output laser beam may be formed by either the non-frequency converted or the frequency converted laser beam, in which case no power modulated output laser beam is generated in the latter case in the event of AOM failure. The power component of the output beam can be adjusted by how much the input power is split between the two deflection states. A control unit supplies the deflection unit with corresponding activation signals. Alternatively, the output power can also be done by modulating the input power. However, in many applications it is advantageous to keep the input power constant and to modulate the power in the arrangement described herein.

The beam modulator preferably has a detector, which detects the power of at least one of the spatially separated laser beams, and a control unit, which controls the deflection unit on the basis of the detected laser power. The invention also relates to an associated laser processing machine for processing workpieces with a laser beam generator for generating the laser beam, with a beam modulator according to one of the preceding claims and with a deflection control of the beam modulator, in particular electrically, controlling machine control.

In another aspect, the invention also relates to a method for power modulating a laser beam, wherein the incident laser beam is selectively deflected at two different angles, wherein the two differently deflected laser beams are frequency-converted with different efficiency, wherein the frequency-converted and the non-frequency-converted Shares of the laser beams are spatially separated from each other and wherein one of the two spatially separated laser beams forms the power modulated output laser beam.

The power of at least one of the spatially separated laser beams is preferably detected and the deflection of the incident laser beam is controlled on the basis of the detected laser power.

Further advantages and advantageous embodiments of the subject invention will become apparent from the description, the claims and the drawings. Likewise, the features mentioned above and the features listed further can be used individually or in combination in any combination. The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention.

Show it:

1 shows a beam modulator according to the invention with an SHG crystal for frequency doubling of the laser radiation;

 Fig. 2 shows a beam modulator according to the invention with two non-linear

Crystals for frequency tripling the laser radiation; 3 shows a beam modulator according to the invention with power control; and

 Fig. 4 shows a laser processing machine with the invention

 Beam modulator.

In the following description of the figures identical reference numerals are used for identical or functionally identical components.

The in Fign. 1 and 2 are used for highly dynamic power modulation of a laser beam 2.

The beam modulators 1 each have a arranged in the beam path of the incident laser beam 2, for example electrically controllable deflection 3, which does not deflect the incident laser beam 2 in an electrically non-driven, first state (non-deflected laser beam 4a with deflection angle Oi = Ö °) and deflected in a second, electrically controlled state (deflected laser beam 4b with deflection angle a ₂ > 0 °). The deflection device 3 is, for example, an electrically controllable AOM. In the beam path of the two laser beams 4a, 4b, a non-linear SHG crystal 5 is arranged. Within the SHG angular acceptance range of the SHG crystal 5, which is required for SHG frequency conversion, the non-deflected laser beam strikes the SHG crystal 5 at right angles in the exemplary embodiment shown and is therefore at least partially converted to twice the frequency (frequency-converted laser beam 6a and non-frequency converted beam 6c). On the other hand, the deflected laser beam 4b impinges on the SHG crystal 5 outside of the SHG angular acceptance range and is therefore not frequency-converted (or less efficiently) (non-frequency-converted laser beam 6b). In the beam path of the frequency-converted and non-frequency-converted laser beams 6a, 6b, 6c, a separator in the form of a wavelength-dependent beam splitter 7, for example, a dichroic mirror, arranged for the wavelength of the frequency-converted laser beam 6a reflective and for the wavelength of the non-frequency-converted Laser beam 6b, 6c is transmissive. The in radiant For example, arranged at an angle of 45 ° beam splitter 7 transmits the non-frequency-converted laser beam 6b, 6c (transmitted laser beams 9b, 9c) and deflects the frequency-converted laser beam 6a by 90 ° (deflected laser beam 9a). One of the laser beams 9a or 9b / 9c is used as a power-modulated output laser beam of the beam modulator 1, whereas the unused other laser beam can be directed into a beam trap (not shown).

At the deflection device 3 10 different electrical control signals are applied in the two drive states of a control unit to divert the incident laser beam 2 different levels. By electrical switching (in this case binary switching on and off of the electrical control signal) between the two drive states of the deflection device 3, the output laser beam can be correspondingly power-modulated or the ratios of these two states can be set. In addition, it can be set by the control unit 10 how the power of the incident laser beam 2 is split between the undeflected and the deflected laser beam 4a, 4b.

Since the differently deflected laser beams 4a, 4b both impinge on the non-linear crystal 5, the same laser power always impinges on the non-linear crystal 5 in both drive states, which consequently experiences the same heating as possible in both drive states.

As an alternative to the wave-dependent beam splitter 7, another separator for spatially separating the two output laser beams 9a, 9b may also be used, e.g. also mirrors or other beam splitters.

From the beam modulator of Fig. 1, the beam modulator 1 shown in Fig. 2 differs only in that here a combination of two nonlinear crystals 5, 5 'is used for frequency tripling. The first crystal 5 is an SHG crystal and at least partially generates the second harmonic, ie a frequency doubling. In the second crystal 5 ', at least part of the fundamental frequency (laser beam 6c) and the second harmonic (laser beam 6a) in a sum frequency process then become the third harmonic, that is to say a freewheeling frequency. triplex, generated. The non-deflected laser beam 4a impinges on both crystals 5, 5 'in each case within the angular acceptance ranges required for both frequency conversions and is therefore at least partially converted to three times the frequency. This laser beam converted to three times the frequency is designated 6d. The deflected laser beam 4b, however, strikes at least one of the two crystals 5, 5 'outside the angular acceptance range, whereby no or significantly less power is frequency-converted. The beam splitter 7 deflects only the laser beam 6d converted to three times the frequency by 90 ° (deflected laser beam 9a) and transmits the other laser beams 6a, 6b, 6c (transmitted laser beams 9b, 9c).

Since the laser beams 6a-6c all impinge on the second crystal 5 'in each case, the same laser power always impinges on the second crystal 5' in both activation states, which consequently experiences the same heating as possible in both activation states.

Instead of the two crystals 5, 5 'arranged one behind the other, it is also possible for three or more several crystals to be arranged one behind the other. However, instead of tripling the wavelength, other conversion schemes are possible, e.g. two successively arranged SHG crystals for a Frequenzvervierfach ung.

The deflection angle of the deflected laser beam 4b may, for example, be matched to the first minimum of the sinc function of the nonlinear crystal 5, 5 '.

In the Fign. 1 and 2, the non-deflected laser beam 4a is frequency-converted, and since the entire laser power of the fundamental wavelength is available, no power loss occurs in the modulation. Instead of as shown in Figs. 1 and 2, to convert the non-deflected laser beam 4a, the nonlinear crystal 5, 5 'may also be arranged such that the deflected laser beam 4b strikes the crystals 5, 5' within the angular acceptance ranges of the crystals 5, 5 'and therefore frequency-converted becomes. The non-deflected laser beam 4a, on the other hand, hits outside the angular acceptance regions of the crystals 5, 5 'to the crystals 5, 5' and is therefore not frequency converted. Also in this case, the frequency-converted and the non-frequency-converted laser beams are spatially separated from each other by means of the beam splitter 7. In this case, therefore, the deflected laser beam 4b is frequency-converted and thus no power is converted in case of failure of the AOM 3.

The beam modulator 1 shown in FIG. 3 differs from the beam modulator of FIG. 1 only in that here a part of the output laser beam 9a is directed via a partially reflective mirror 11 to a detector / sensor 12 and in a control unit (eg machine control) 13 with a nominal power value is compared and the deflection unit 3 and its control unit 10 is controlled by the control unit 13 accordingly. Thus, one can regulate to constant power or to a predetermined power. Alternatively, the one or more unused laser beams 9b, 9c can be measured and from this the power of the output laser beam 9a can be calculated and compared with a desired value. However, changes in the conversion efficiency of the nonlinear crystals, e.g. due to degradation / defects / temperature / misalignment can not be considered or compensated. Fig. 4 shows schematically a laser processing machine 20 with a laser beam generator 21 for generating the laser beam 2, with the beam modulator 1 and with a deflector 3 and their control unit 10 electrically controlling machine controller 22. By means of the output laser beam 9a workpieces 23 with the desired modulation of Laser power are processed.

Claims

claims 1. beam modulator (1) for power modulation of a laser beam (2), with a controllable deflection device (3) arranged in the beam path of the incident laser beam (2) and having two drive states in which the incident laser beam (2) is deflected at different distances (α-ι _, 02),  characterized,  that in the beam path of the two differently deflected  Laser beams (4a, 4b) at least one non-linear crystal (5; 5 ') is arranged, which frequency converts one of the two differently deflected laser beams with higher efficiency and the other laser beam with lower efficiency, and  a separator (7) is arranged in the beam path of the frequency converter of higher efficiency and of lower frequency conversion (6a, 6b), which spatially separates the frequency-converted and the non-frequency-converted components of the laser beams (6a, 6b), one of them the two spatially separated laser beams (9a, 9b, 9c) forms the power-modulated output laser beam. 2. beam modulator according to claim 1, characterized in that in one of the two driving states of the deflection device (3) of the incident laser beam (2) from the deflection device (3) is deflected at an angle (c) of 0 °. 3. beam modulator according to claim 1 or 2, characterized in that the different far deflected laser beams (4a, 4b) both each on the at least one nonltnearen crystal (5, 5 ') impinge. 4. Beam modulator according to one of the preceding claims, characterized in that one (4a) of the two differently deflected laser beams within the required frequency conversion for acceptance angle range of the at least one non-linear crystal (5; 5 ') on the nichtiinearen crystal (5; 5') and the other laser beam (4b) strikes the nonlinear crystal (5; 5 ') outside of the acceptance angle range. 5. beam modulator according to any one of the preceding claims, characterized in that in the beam path of the two differently deflected laser beams (4 a, 4 b) a nonlinear frequency doubling crystal (5) or a combination of at least two nonlinear crystals (5, 5 ') is arranged for frequency multiplication. 6. beam modulator according to any one of the preceding claims, characterized in that the deflection device (3) is designed as an electrically driven AOM. 7. beam modulator according to any one of the preceding claims, characterized in that the separator (7) is designed as a dichroic mirror. 8. beam modulator according to any one of the preceding claims, characterized in that the output laser beam is formed by the frequency-converted laser beam (9a). 9. beam modulator according to one of claims 1 to 7, characterized in that the output laser beam is formed by the non-frequency-converted laser beam (9b, 9c). 10. Beam modulator according to one of the preceding claims, characterized by a deflection device (3), in particular electrically, controlling control unit (10). 11. Beam modulator according to claim 10, characterized in that the control unit (10) controls the deflection device (3) such that the incident laser beam (2) is split in an adjustable power ratio to the deflected and non-deflected beam (4a, 4b). 12. A beam modulator according to any one of the preceding claims, characterized by a detector (12) which detects the power of at least one of the spatially separated laser beams (9a, 9b, 9c), and by a control unit (13) based on the detected laser power, the deflection - unit (3) controls. 13. A laser processing machine (20) for processing workpieces (23), with a laser beam generator (21) for generating the laser beam (2), with a beam modulator (1) according to any one of the preceding claims and with a deflection device (3) of the beam modulator ( 1), in particular electrically, controlling machine control (13, 22). 14. A method of power modulating a laser beam (2), wherein the incident laser beam (2) is selectively deflected at two different angles (eh, a ₂ ), the two being deflected differently far Laser beams (4a, 4b) are frequency-converted with different efficiency, wherein the frequency-converted and non-frequency-converted portions of the laser beams (6a, 6b) are spatially separated from each other and wherein one of the two spatially separated laser beams (9a, 9b, 9c) the power modulated output laser beam forms. 15. Method according to claim 14, characterized in that the power of at least one of the spatially separated laser beams (9a, 9b, 9c) is detected and the deflection of the incident laser beam (2) is controlled on the basis of the detected laser power.