KR101746921B1 - Laser processing apparatus and pulse laser beam output method - Google Patents

Abstract

Provided is a laser machining apparatus capable of changing pulse energy by adjusting laser power for each laser pulse.
The laser power source applies a high-frequency voltage to the discharge electrode of the laser oscillator in accordance with the duty cycle of the excitation command signal to be inputted. An excitation pattern command signal including duty cycle information is input to the laser control device. The laser control device applies an excitation command signal to the laser power source based on the duty cycle information included in the excitation pattern command signal. The machine control device gives a pulse output timing signal for instructing the output timing of the pulse laser beam and an excitation pattern command signal to the laser control device. The stage holds the object to be processed. The optical system guides the pulse laser beam output from the laser oscillator to the object to be processed held on the stage.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser processing apparatus and a pulsed laser beam output method,

The present application claims priority based on Japanese Patent Application No. 2014-188369 filed on September 17, 2014. The entire contents of which are incorporated herein by reference.

TECHNICAL FIELD The present invention relates to a laser machining apparatus for processing a pulse laser beam incident on an object to be processed, and a pulse laser beam output method applied to the laser machining apparatus.

Patent Documents 1 to 3 disclose a power supply device for controlling a laser output by applying a high-frequency voltage supplied to a discharge electrode of a carbon dioxide gas laser.

In the power supply device disclosed in Patent Document 1, on-off control of the inverter is performed based on the command pulse that is pulse-frequency-modulated in accordance with the processing condition of the workpiece. Pulse-frequency-modulated voltage pulses are output from the inverter and applied to the discharge electrodes. Thus, it is possible to improve the machining quality.

In the power supply device disclosed in Patent Document 2, the pulse width of the output voltage of the inverter is controlled so that the detection value of the discharge current coincides with the set value. The switching frequency of the inverter is increased in accordance with the decrease of the discharge current so that the rise of the output current of the inverter is delayed from the rise of the output voltage of the inverter. Thus, high efficiency and low noise can be achieved for the laser power source.

In the power supply device disclosed in Patent Document 3, pulse width modulation is employed to control the output of the RF power source of the gas discharge laser. The average power of the digital pulse train is selectively changed by incrementally varying the duration of the digital pulse in the digital pulse train given to the RF power source.

Japanese Laid-Open Patent Publication No. 2013-089788 Japanese Patent Publication No. 03496369 Japanese Patent Publication No. 2013-507790

In order to improve the laser processing quality, it is required to control the pulse energy of the pulsed laser beam. The pulse energy is obtained by time-integrating the instantaneous value of the laser power from the rising time point to the falling time point. When it is desired to increase the pulse energy, a method of lengthening the pulse width and a method of increasing the laser power can be considered. Conventionally, when a carbon dioxide gas laser is used as the laser light source, the pulse energy is changed by changing the pulse width of the laser pulse. This is because it was difficult to change the laser power for each laser pulse and it was easy to change the pulse width.

However, even if the pulse energies are the same, it has been found that when the pulse width and the laser power are different, the processing quality is also different. A technique of changing the pulse energy by adjusting the pulse width is insufficient for performing high quality processing and a technique of changing the pulse energy by adjusting the laser power is desired.

An object of the present invention is to provide a laser machining apparatus capable of changing pulse energy by adjusting laser power for each laser pulse. Another object of the present invention is to provide a pulse laser beam output method applied to a laser processing apparatus.

According to one aspect of the present invention,

A laser oscillator including a pair of discharge electrodes and outputting a pulsed laser beam;

A laser power source for applying a high frequency voltage to the discharge electrode in accordance with a duty cycle of an excitation command signal to be inputted,

Wherein when the excitation pattern command signal including the duty cycle information and the pulse output timing signal for commanding the output timing of the pulse laser beam are input to the laser power source based on the duty cycle information included in the excitation pattern command signal, A laser control device for giving an excitation command signal,

A processor control device for applying the pulse output timing signal and the excitation pattern command signal to the laser control device,

A stage for holding an object to be processed,

An optical system for guiding the pulsed laser beam output from the laser oscillator to an object to be processed held on the stage,

Is provided.

According to another aspect of the present invention,

A laser oscillator including a pair of discharge electrodes and outputting a pulsed laser beam;

A laser power source for applying a high frequency voltage to the discharge electrode in accordance with a duty cycle of an excitation command signal to be inputted,

A control device for applying the excitation command signal to the laser power source based on the duty cycle information inputted when the duty cycle information is inputted,

An input device for inputting the duty cycle information to the control device,

A stage for holding an object to be processed,

An optical system for guiding the pulsed laser beam output from the laser oscillator to an object to be processed held on the stage,

Is provided.

According to another aspect of the present invention,

A method of outputting a pulsed laser beam by exciting a plasma by applying a high frequency voltage pulsed to a discharge electrode of a gas laser oscillator through an H bridge circuit,

There is provided a method of outputting a pulsed laser beam which changes the average power within the pulse width of the laser pulse by changing the duty cycle of the conduction time of the H bridge circuit every laser pulse.

The average power within the pulse width of the pulse laser beam can be changed by applying a high frequency voltage to the discharge electrode in accordance with the duty cycle of the excitation command signal. The average power within the pulse width can be changed for each laser pulse by changing the duty cycle of the excitation command signal for each laser pulse.

1 is a schematic diagram of an optical system of a laser machining apparatus according to an embodiment and a block diagram of a control system.
2 is a cross-sectional view of the laser oscillator.
3 is an equivalent circuit diagram of a high frequency power supply.
4 is a timing chart of the excitation pattern command signal, the pulse output timing signal, the excitation command signal, the high frequency voltage, the discharge current, and the pulse laser beam.
5A is a graph showing an example of the waveforms of the excitation pattern command signal, the excitation command signal, and the pulse laser beam. FIG. 5B is a graph showing the waveforms of the excitation pattern command signal, the excitation command signal, Fig.
6 is a graph showing an example of the relationship between the switching frequency and the average power within the pulse width of the pulse laser beam.

Fig. 1 shows a schematic diagram of an optical system of a laser machining apparatus according to an embodiment and a block diagram of a control system. The laser power source 11 applies the high frequency voltage Ve to the discharge electrode 12 of the laser oscillator 10 in a pulsed manner. The laser power source 11 includes a direct current power source 13 and a high frequency power source 14. For the laser oscillator 10, for example, a carbon dioxide gas laser oscillator is used. The laser control device 15 gives the control value Cv to the DC power supply 13 and gives the excitation command signal Ec to the RF power supply 14 based on the instruction from the machine control device 35 do.

The DC power supply 13 controls the DC voltage applied to the RF power supply 14 based on the control value Cv. The high frequency power supply 14 applies the high frequency voltage Ve to the discharge electrode 12 based on the excitation command signal Ec. More specifically, the average output in the pulse width of the pulse laser beam is controlled by the control value Cv. Here, the "average pulse output within the pulse width" means a value obtained by averaging the laser output in a period in which the laser pulse is output, and is equal to a value obtained by dividing the pulse energy by the pulse width. On the other hand, the "average output of the pulse laser" generally used means a value obtained by averaging the laser output in unit time including the time when the laser pulse is not output. The average output of the pulse laser is smaller than the average output in the pulse width.

For example, in the case of performing punching processing on a printed board or the like, the average output within the pulse width, the pulse width, the repetition frequency of the pulse, and the like are controlled to coincide with the target value.

The high-frequency power supply 14 includes a plurality of switching elements 14a, and converts the direct current supplied from the direct-current power supply 13 into an alternating current. The switching element 14a is on-off controlled by the excitation command signal Ec input from the laser control device 15. [ The detailed configuration of the high frequency power supply 14 will be described later with reference to Fig.

As the laser medium (gas) gas of the laser oscillator 10, for example, a mixed gas of a carbonic acid gas and a nitrogen gas is used. When a high frequency voltage Ve is applied from the laser power source 11 to the discharge electrode 12, the plasma is excited between the discharge electrodes, and the pulse laser beam Lp is output from the laser oscillator 10.

The pulse laser beam Lp output from the laser oscillator 10 is split into a transmission beam and a reflection beam by the partial reflecting mirror 21. [ The reflected beam is incident on the photodetector 20. [ When detecting the light, the photodetector 20 sends a detection value Dv corresponding to the light intensity to the laser control device 15. [ The photodetector 20 includes, for example, a mercury cadmium telluride (MCT) photoconductive element having sensitivity to the wavelength range of a carbon dioxide gas laser.

The transmission beam straightly passing through the partial reflecting mirror 21 passes through the light-guiding optical system and is incident on the object 31 to be processed. The light guiding optical system includes a spot position stabilization optical system 22, an aspheric surface lens 23, a collimator lens 24, a mask 25, a field lens 26, a fold mirror 27, a beam syringe 28 ), And an f? Lens 29. The object to be processed 31 is held on the stage 30.

The pulse laser beam Lp transmitted through the spot position stabilization optical system 22 is incident on the aspheric lens 23. [ The spot position stabilizing optical system 22 includes a plurality of convex lenses and is provided at a position where the aspheric lens 23 is disposed even if trembling occurs in the traveling direction of the pulsed laser beam Lp output from the laser oscillator 10 Thereby stabilizing the position of the beam spot. The aspherical lens 23 changes the beam profile of the pulsed laser beam Lp. For example, the beam profile of a Gaussian shape is changed to a beam profile of a top flat shape.

The pulsed laser beam Lp transmitted through the aspherical lens 23 is collimated by the collimator lens 24 and is then incident on the mask 25. The mask 25 includes a transmission window and a shielding portion, and shapes the beam cross-section of the pulsed laser beam Lp. The pulse laser beam Lp transmitted through the transmission window of the mask 25 is incident on the beam syringe 28 via the field lens 26 and the fold mirror 27. [ The beam syringe 28 scans the laser beam in the two-dimensional direction by a command from the machine control device 35. [ As the beam injector 28, for example, a galvano scanner is used.

The pulsed laser beam Lp scanned by the beam syringe 28 is condensed by the f? Lens 29 and is incident on the object 31 to be processed. The field lens 26 and the f? Lens 29 image the transmission window of the mask 25 on the surface of the object to be processed 31. The stage 30 can move the object 31 in a direction parallel to the surface of the object 31 by a command from the machine control device 35. [ At least one of the beam syringe 28 and the stage 30 functions as a moving mechanism for moving the incident position of the pulse laser beam Lp on the surface of the object 31. [

The machine control device 35 gives the pulse output timing signal Pt and the excitation pattern command signal Ep to the laser control device 15. [ The laser control device 15 applies the excitation command signal Ec to the laser oscillator 10 during a period in which the pulse output timing signal Pt is input. The duty cycle of the excitation command signal Ec and the repetition frequency of the pulse are designated by the excitation pattern command signal Ep. The repetition frequency of the pulse of the excitation command signal Ec coincides with the switching frequency of the high frequency power supply 14.

Fig. 2 shows a cross-sectional view of the laser oscillator 10. Fig. A blower 40, a pair of discharge electrodes 12, a heat exchanger 46, and a laser medium gas are accommodated in the laser chamber 50. A discharge space 42 is defined between the pair of discharge electrodes 12. [ As a discharge occurs in the discharge space 42, the laser medium gas is excited. 2, a cross section orthogonal to the longitudinal direction of the discharge electrode 12 is shown. Each of the discharge electrodes 12 includes a conductive member 43 and a ceramic member 44. The ceramic member 44 isolates the conductive member 43 from the discharge space 42.

A circulation path for returning from the blower 40 to the blower 40 via the discharge space 42 and the heat exchanger 46 is formed in the laser chamber 50. The heat exchanger (46) cools the laser medium gas which has become hot by the discharge.

A pair of terminals 51 are mounted on the wall surface of the laser chamber 50. The conductive member 43 of the discharge electrode 12 is connected to the terminal 51 by the in-chamber current path 52, respectively. The terminal 51 is connected to the laser power source 11 by the off-chamber current path 55. [

3 shows an equivalent circuit diagram of the high-frequency power supply 14. As shown in Fig. The high frequency power supply 14 includes an H bridge circuit having two bridge arms 14A and 14B. Each of the bridge arms 14A and 14B includes two switching elements 14a connected in series to each other. The discharge electrode 12 is connected to the intermediate point of the two bridge arms 14A and 14B through the transformer 14C. The DC power supply 13 applies a DC voltage to the H bridge circuit. Off control of the switching element 14a is performed by the excitation command signal Ec from the laser control device 15. [

The switching element 14a on the high potential side of one bridge arm 14A and the switching element 14a on the low potential side of the other bridge arm 14B are turned on in a state in which all the switching elements 14a are off, The switching element 14a on the low potential side of one of the bridge arms 14A and the switching element 14a on the high potential side of the other bridge arm 14B are turned on A high frequency voltage is applied to the discharge electrode 12 by repeating the procedure of returning to the off state.

4 shows an example of a timing chart of the excitation pattern command signal Ep, the pulse output timing signal Pt, the excitation command signal Ec, the high frequency voltage Ve, the discharge current Ie and the pulse laser beam Lp . The excitation pattern command signal Ep includes duty cycle information and switching frequency information. As one example, the excitation pattern command signal Ep is transmitted from the machine control device 35 (Fig. 1) to the laser control device 15 (Fig. 1) in general serial communication or parallel communication. The machine control device 35 raises the pulse output timing signal Pt after transmitting the excitation pattern command signal Ep. When it is desired to change the duty cycle or the switching frequency within one laser pulse, the excitation pattern command signal Ep includes a plurality of duty cycle information and switching frequency information, and information for instructing the switching timing .

The pulse output timing signal Pt commands the start and stop of the output of the excitation command signal Ec by the rise and fall of the pulse output timing signal Pt. When the pulse output timing signal Pt rises at time t1, the laser control device 15 starts outputting the excitation command signal Ec to the high frequency power supply 14. When the pulse output timing signal Pt falls at the time t3, the laser control device 15 stops outputting the excitation command signal Ec to the high frequency power supply 14.

In Fig. 4, when the excitation command signal Ec is in the state Ec0, it indicates that all the switching elements 14a of the high frequency power supply 14 (Fig. 3) are off. When the excitation command signal Ec is in the state Ec1, the switching element 14a on the high potential side of one bridge arm 14A and the switching element 14a on the low potential side of the other bridge arm 14B ON state, and when the excitation command signal Ec is in the state Ec2, the switching element 14a on the low potential side of one bridge arm 14A and the switching element 14a on the high potential side of the other bridge arm 14B Indicating that the switching element 14a is in the ON state.

When the excitation command signal Ec is applied to the high frequency power source 14, the high frequency power source 14 applies the high frequency voltage Ve to the discharge electrode 12 in accordance with the duty cycle and the switching frequency of the excitation command signal Ec . When the high-frequency voltage Ve is applied to the discharge electrode 12, the plasma is excited between the discharge electrodes 12, and the discharge current Ie flows. Is slightly delayed from the rising time t1 of the pulse output timing signal Pt and the output of the pulse laser beam Lp starts at time t2.

At the time t3, when the excitation command signal Ec stops, the application of the high-frequency voltage Ve to the discharge electrode 12 also stops, and the discharge current Ie begins to decrease. As a result, the output power of the pulse laser beam Lp also begins to decrease.

Next, the relationship between the duty cycle information of the excitation pattern command signal Ep, the excitation command signal Ec, and the pulse laser beam Lp will be described with reference to Figs. 5A and 5B.

5A shows an example of the waveforms of the excitation command signal Ec and the pulse laser beam Lp. The states (Ec1, Ec0, Ec2, Ec0) appear in this order within one cycle of the excitation command signal Ec. The respective times T1 of the state Ec1 and the state Ec2 and the period T2 of the excitation command signal Ec are instructed by the excitation pattern command signal Ep (Fig. 4). The switching frequency fs of the high frequency power supply 14 is 1 / T2. The duty cycle Dcc of the excitation command signal Ec, that is, the duty cycle of the conduction time of the H bridge circuit constituting the high frequency power supply 14 (Fig. 3), is represented by 2 x T1 / T2. As can be seen from Fig. 5, when the duty cycle Dcc is changed, the ratio of the conduction time to the non-conduction time is changed.

When the duty cycle Dcc and the switching frequency fs of the excitation command signal Ec are the conditions shown in Fig. 5A, the average power in the pulse width of the pulse laser beam Lp is Wa1. By multiplying the average power Wa1 in the pulse width by the pulse width, the pulse energy is obtained.

Fig. 5B shows another example of the waveforms of the excitation command signal Ec and the pulse laser beam Lp. Each time T1 of the excitation command signal Ec state Ec1 and state Ec2 shown in Fig. 5 is shorter than the time T1 of the excitation command signal Ec shown in Fig. 5A. The period T2 of the excitation command signal Ec is the same as the period T2 of the excitation command signal Ec shown in Fig. 5A. As a result, the duty cycle Dcc of the excitation command signal Ec shown in Fig. 5B is smaller than the duty cycle Dcc of the excitation command signal Ec shown in Fig. 5A. The switching frequency fs of the excitation command signal Ec shown in Fig. 5B is the same as the switching frequency fs of the excitation command signal Ec shown in Fig. 5A. When the duty cycle Dcc of the excitation command signal Ec becomes small, the high frequency power supplied to the discharge electrode 12 (Fig. 3) decreases, and the average power in the pulse width of the pulse laser beam Lp decreases. Therefore, the average power Wa2 in the pulse width of the pulse laser beam Lp when the duty cycle Dcc of the excitation command signal Ec and the switching frequency fs are the conditions shown in Fig. 5B, Is smaller than the average power Wa1 in the pulse width of the indicated pulse laser beam Lp.

5A and 5B, the duty cycle information instructed to the laser controller 15 by the excitation pattern command signal Ep is changed to change the average power within the pulse width of the pulse laser beam Lp .

The average power within the pulse width of the pulse laser beam Lp can be controlled by varying the high frequency power supplied to the discharge electrode 12 by changing the output voltage of the DC power supply 13 (Fig. 1). However, the output voltage of the direct current power supply 13 is smoothed by a smoothing capacitor or the like. This makes it difficult to change the output voltage of the DC power supply 13 with a short time constant of the pulse interval of the pulse laser beam.

In the above embodiment, the duty cycle Dcc of the excitation command signal Ec is changed while maintaining the DC voltage applied to the high-frequency power supply 14 (Fig. 1) Power is changing. The duty cycle Dcc of the excitation command signal Ec can be changed for each output of the pulse output timing signal Pt. Thus, it is possible to control the average power in the pulse width for each laser pulse.

5A to 5B, the average power within the pulse width was varied by changing the duty cycle Dcc while keeping the switching frequency fs of the excitation command signal Ec constant. Conversely, even when the duty cycle Dcc is kept constant and the switching frequency fs is changed, the average power within the pulse width can be changed.

The switching frequency fs of the excitation command signal Ec is set near the resonance frequency fr of the resonance circuit including the transformer 14C and the discharge electrode 12 shown in Fig. As a result, high-frequency power can be efficiently supplied to the laser oscillator 10. If the switching frequency fs differs from the resonance frequency fr, the high-frequency power supplied to the laser oscillator 10 lowers.

6 shows an example of the relationship between the switching frequency fs and the average power Wa in the pulse width of the pulse laser beam Lp. Since the high frequency power is efficiently supplied to the laser oscillator 10 when the switching frequency fs is equal to the resonance frequency fr, the average power Wa in the pulse width exhibits the maximum value. If the switching frequency fs differs from the resonance frequency fr, the high-frequency power supplied to the laser oscillator 10 decreases and the average power Wa in the pulse width decreases. Therefore, the average power Wa in the pulse width can also be changed by changing the switching frequency fs.

Both the duty cycle Dcc of the excitation command signal Ec and the switching frequency fs may be changed in order to change the average power Wa in the pulse width. The duty cycle Dcc and the switching frequency fs of the excitation command signal Ec are instructed by the excitation pattern command signal Ep transmitted from the machine control device 35 to the laser control device 15. [

When the laser machining apparatus according to the embodiment is used for punching a printed substrate, it is possible to improve the machining quality by controlling the average power within the pulse width for each laser pulse. The duty cycle information and the switching frequency information are input to the machine control device 35 via the input device 36. [

In the above embodiment, a carbon dioxide gas laser oscillator is used as the laser oscillator 10 (Fig. 1), but it is also possible to use another gas laser oscillator.

Although the laser control device 15 and the machine control device 35 are implemented by different apparatuses in the above embodiment, the laser control device 15 and the machine control device 35 may be integrated into one device You can. In this case, the integrated control device gives the excitation command signal Ec to the laser power source 11 based on the duty cycle information inputted through the input device 36. [

While the present invention has been described with reference to the above embodiments, the present invention is not limited thereto. For example, it is apparent to those skilled in the art that various modifications, improvements, combinations, and the like are possible.

10 laser oscillator
11 Laser power
12 discharge electrode
13 DC power source
14 High frequency power
14a switching element
14A, 14B bridge arm
14C transformer
15 Laser control unit
20 photodetector
21 partial reflector
22 Spot position stabilization optical system
23 Aspheric Lens
24 collimate lens
25 Mask
26 field lens
27 fold mirror
28 beam syringe
29 f? Lens
30 stages
31 Object to be processed
35 Machine control unit
36 input device
40 blowers
42 discharge space
43 conductive member
44 ceramic member
46 Heat exchanger
50 laser chamber
51 terminal
52 Current in chamber
55 out of chamber current
Cv control value
Dcc duty cycle
Dv detection value
Ec excitation signal
Ep Exposed pattern command signal
Ie discharge current
Lp pulsed laser beam
Pt pulse output timing signal
And high frequency voltage
Average power in Wa pulse width
fr resonant frequency
fs switching frequency

Claims (9)

A laser oscillator including a pair of discharge electrodes and outputting a pulsed laser beam;
A laser power source for applying a high-frequency voltage to the discharge electrode in accordance with a duty cycle of an excitation command signal to be input;
Wherein when the excitation pattern command signal including the duty cycle information and the pulse output timing signal for commanding the output timing of the pulse laser beam are input to the laser power source based on the duty cycle information included in the excitation pattern command signal, A laser control device for giving an excitation command signal,
A processor control device for applying the pulse output timing signal and the excitation pattern command signal to the laser control device,
A stage for holding an object to be processed,
And an optical system for guiding the pulsed laser beam output from the laser oscillator to an object to be processed held on the stage,
The excitation command signal is applied to the laser power source every time the pulse output timing signal is inputted,
The ratio of the conduction time to the non-conduction time is changed by changing the duty cycle in a cycle in which the conduction time and the non-conduction time of the excitation command signal are alternately repeated while the pulse output timing signal is being input, And changes the average power within the pulse width of the pulse laser beam. The method according to claim 1,
Add to,
And an input device for inputting the duty cycle information to the machine control device,
Wherein the machine control device applies the excitation pattern command signal to the laser power source based on the duty cycle information input through the input device. The method according to claim 1 or 2,
Wherein the machine control device changes the excitation pattern command signal for each output of the pulse output timing signal. The method according to claim 1 or 2,
Wherein the laser oscillator is a carbon dioxide gas laser oscillator. The method according to claim 1 or 2,
The laser power source
An H bridge circuit including two bridge arms,
A DC power supply for applying a DC voltage to the H-
/ RTI >
Wherein each of the bridge arms includes two switching elements connected in series to each other,
A pair of the discharge electrodes are connected to the respective intermediate points of the two bridge arms,
Wherein the excitation command signal performs ON / OFF control of the switching element of the H bridge circuit. The method of claim 5,
Wherein the excitation pattern command signal further includes switching frequency information for performing switching of the H bridge circuit,
Wherein the laser control device applies the excitation command signal to the laser power source based on the duty cycle information and the switching frequency information included in the excitation pattern command signal. A laser oscillator including a pair of discharge electrodes and outputting a pulsed laser beam;
A laser power source for applying a high-frequency voltage to the discharge electrode in accordance with a duty cycle of an excitation command signal to be input;
A control device for applying the excitation command signal to the laser power source based on the duty cycle information inputted when the duty cycle information is inputted,
An input device for inputting the duty cycle information to the control device,
A stage for holding an object to be processed,
And an optical system for guiding the pulsed laser beam output from the laser oscillator to an object to be processed held on the stage,
The ratio of the conduction time to the non-conduction time is changed by changing the duty cycle in a cycle in which the conduction time and the non-conduction time of the excitation command signal are alternately repeated, so that the average power in the pulse width of the pulse laser beam is Laser processing apparatus. A method of outputting a pulsed laser beam by exciting a plasma by applying a high frequency voltage pulsed to a discharge electrode of a gas laser oscillator through an H bridge circuit,
Wherein a ratio of the conduction time to the non-conduction time is changed for each of the laser pulses by changing the duty cycle in a cycle in which the conduction time and the non-conduction time of the H bridge circuit are alternately repeated, A method of outputting a pulsed laser beam that changes an average power. The method of claim 8,
Further comprising changing the switching frequency of the H bridge circuit for each of the laser pulses to change the average power in the pulse width of the laser pulse.

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