JP2009006332A - Laser beam machining apparatus and laser beam machining method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser beam machining apparatus that is not affected by light emission from plasma or the like generated in laser beam machining and that can be made compact and inexpensive. <P>SOLUTION: A laser pulse emitted from a laser beam source (1) is made incident on a workpiece. A photodetector (16) detects reflected light of the laser pulse made incident on the workpiece. A controller (20) carries out the following steps: (a) a machining laser pulse for forming a hole in the workpiece is made incident on a machining position of the workpiece; (b) a laser pulse for confirmation having an intensity that forms no hole in the workpiece is made incident on the machining position; and (c) on the basis of the detection result of the reflected light of the laser pulse for confirmation by the photodetector, completion of hole formation is determined. If the hole formation is incomplete, the controller goes back to the step (a) to carry out from the step in which a new machining laser pulse is made incident on the machining position. If the hole formation is complete, the incidence of the machining laser pulse on the machining position is ended. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser processing apparatus and a laser processing method, and more particularly to a laser processing apparatus and a laser processing method suitable for forming a hole in a processing object in which an insulating layer and a metal layer are stacked.

  A laser processing technique is widely used in which holes are formed in a resin layer of a multilayer wiring board in which a metal layer and a resin layer are laminated using laser light to expose an inner metal layer.

  The laser processing method disclosed in the following Patent Document 1 will be described. A laser pulse is incident on the processing position of the processing object. A part of the laser pulse is reflected from the workpiece and returns along the incident path. The reflected light is branched from the incident path, and its intensity is detected by the reflected light detector. When the inner metal layer is exposed, the intensity of the reflected light increases, so that the time when the metal layer is exposed can be detected.

  The laser processing method disclosed in Patent Document 2 below will be described. The inspection light source is irradiated with inspection light from the inspection light source while performing processing by making the laser beam incident on the processing position of the object to be processed. By detecting the light reflected from the inspection light with the light receiving element, it is possible to detect the time when the inner metal layer is exposed.

  In the laser processing method disclosed in Patent Document 3 below, drilling is performed while scanning a laser beam using a beam scanner. Since the incident position of the laser beam moves on the object to be processed, the irradiation position of the inspection light must also be moved so as to coincide with the incident position of the laser beam. For this reason, the inspection light is made to enter the object to be processed after being merged with the path of the laser beam for processing.

JP-A-11-192570 JP 2000-137002 A JP 2002-290056 A

  In the laser processing methods disclosed in Patent Documents 1 and 2, drilling and detection of reflected light proceed simultaneously. During drilling, plasma is generated by the incidence of the laser beam. The detection accuracy of the reflected light intensity is lowered due to the influence of the light emission from the plasma.

  In the laser processing methods disclosed in Patent Documents 2 and 3, a laser light source for inspection light is required in addition to the laser light source for processing. For this reason, it is difficult to reduce the price of the apparatus while increasing the size of the apparatus.

  An object of the present invention is to provide a laser processing apparatus and a laser processing method that are not affected by light emission from plasma generated during laser processing, and that can reduce the size and price of the apparatus. is there.

According to one aspect of the invention,
A laser light source with variable pulse energy;
A stage for holding the workpiece,
A propagation optical system for causing a laser pulse emitted from the laser light source to be incident on a workpiece held by the stage;
A photodetector for detecting reflected light of a laser pulse incident on a workpiece held on the stage;
A control device for controlling the laser light source, the control device,
(A) a step of causing a machining laser pulse having a pulse energy of a size capable of forming a hole in the workpiece held on the stage to be incident on a workpiece position of the workpiece;
(B) a step of causing a confirmation laser pulse having a pulse energy of a size that does not form a hole in the workpiece held on the stage to be incident on the workpiece position;
(C) Based on the detection result of the reflected light of the confirmation laser pulse by the photodetector, it is determined whether or not hole formation is complete. If hole formation is not complete, the process returns to step (a), The process is performed so that a new laser pulse for machining is incident on the machining position and the process of ending the incidence of the machining laser pulse on the machining position is executed when the hole formation is completed. A laser processing apparatus for controlling a laser light source is provided.

According to another aspect of the invention,
(A) A processing laser pulse having a pulse energy of a size capable of forming a hole in the processing object is emitted from a laser light source and incident on the processing object, thereby forming a hole in the processing object. Process,
(B) A confirmation laser pulse having a pulse energy of a size that does not form a hole in the workpiece is emitted from the laser light source and incident on the position of the hole formed in the step (a). Detecting the intensity of the reflected light of the confirmation laser pulse;
(C) Based on the intensity of the reflected light detected in the step (b), it is determined whether or not the hole processing is completed. When the hole processing is completed, the irradiation of the processing laser pulse to the hole is finished. If the hole machining is not completed, the process returns to the step (a) and includes a process of executing a process laser beam newly incident on the position of the hole formed halfway. A method is provided.

  By detecting the reflected light of the confirmation laser pulse, it is possible to detect a change in the reflectance of the workpiece. This makes it possible to determine whether or not a hole has been formed to a desired position. Each time the processing laser pulse is incident, the confirmation laser pulse is incident and the determination is performed, so that it is possible to prevent insufficient formation of holes and excessive laser pulse irradiation. Since the irradiation of the confirmation laser pulse is performed after the irradiation of the processing laser pulse, it is possible to detect the intensity of the reflected light with high accuracy while avoiding the influence of light emission from the plasma generated during the processing. Since the confirmation laser pulse is emitted from the same laser light source as the processing laser pulse, it is not necessary to prepare a dedicated laser light source for generating the confirmation laser pulse.

  FIG. 1 shows a schematic diagram of a laser processing apparatus according to an embodiment. The laser light source 1 emits a pulse laser beam. As the laser light source 1, for example, a carbon dioxide laser oscillator can be used. The laser light source 1 emits a laser pulse while receiving the trigger signal trg from the control device 20. By changing the time for transmitting the trigger signal trg, the pulse width of the laser pulse emitted from the laser light source 1 can be changed. As the pulse width changes, the energy per pulse (pulse energy) changes.

  The laser beam emitted from the laser light source 1 is converted into a parallel beam whose beam diameter is expanded by the beam expander 2. A beam cross section of a parallel beam having an enlarged beam diameter is shaped by the mask 3.

  The laser beam transmitted through the mask 3 is incident on the first partial reflection mirror 4. The first partial reflection mirror 4 reflects a part of the incident laser beam and transmits the rest. The transmittance and reflectance of the first partial reflection mirror 4 are, for example, about 99% and about 1%, respectively. The laser beam that has passed through the first partial mirror 4 enters the beam distributor 5. The beam splitter 5 receives the control signal scon from the control device 20, and makes the incident laser beam go straight and enter the beam damper 6, and deflects the laser beam and propagates it along the machining path. And can be switched. For example, an acousto-optic deflection element (AOD) can be used as the beam distributor 5.

  The laser beam reflected by the first partial reflection mirror 4 enters the first photodetector 15. The first photodetector 15 converts the light intensity of the incident laser beam into an electrical signal, and transmits the light intensity signal det 1 to the control device 20.

  The laser beam distributed to the processing path by the beam distributor 5 is reflected by the folding mirror 7 and enters the second partial reflection mirror 8. The transmittance and reflectance of the second partial reflection mirror 8 are, for example, about 99% and about 1%, respectively. The laser beam transmitted through the second partial reflection mirror 8 is incident on the beam scanner 9. The beam scanner 9 receives the control signal gcon from the control device 20 and scans the incident laser beam in a two-dimensional direction. As the beam scanner 9, for example, a galvano scanner including an X oscillating mirror and a Y oscillating mirror can be used.

  The laser beam scanned by the beam scanner 9 is converged by the fθ lens 10 and enters the processing position of the processing object 50. The workpiece 50 is held on the XY stage 11. The fθ lens 10 images the position of the mask 3 on the surface of the workpiece 50. Instead of the method of processing at the imaging position, it is possible to adopt a focus processing method of processing at the rear focal position of the fθ lens 10. By scanning the laser beam with the beam scanner 9, the incident position of the laser beam can be moved on the surface of the workpiece 50.

  A part of the laser beam incident on the workpiece 50 is reflected by the surface of the workpiece 50 and travels backward through the laser beam incident path. The reflected light passes through the fθ lens 10 and the beam scanner 9, is reflected by the second partial reflection mirror 8, and enters the second photodetector 16. The second photodetector 16 converts the light intensity of the incident laser beam into an electric signal and transmits the light intensity signal det 2 to the control device 20. As the first photodetector 15 and the second photodetector 16, for example, an infrared sensor can be used.

  FIG. 2A shows a cross-sectional view of the workpiece 50. An inner conductive pattern 53 made of copper or the like is formed on the surface of the core substrate 52 made of glass fiber-containing epoxy resin or the like. An insulating film 54 made of an epoxy resin 54 a including glass fibers 54 b is disposed on the core substrate 52 so as to cover the inner layer conductive pattern 53. A metal film 57 made of copper or the like is formed on the surface of the insulating film 54. For example, the thickness of the insulating film 54 is 30 to 60 μm, the thickness of the metal film 57 is 9 to 12 μm, and the thickness of the inner metal pattern 53 is 18 to 36 μm.

  FIG. 2B shows a plan sectional view of the insulating film 54. The glass fibers 54b are not uniformly dispersed in the epoxy resin 54a, but are, for example, dispersed in a square lattice shape in which vertical stripes and horizontal stripes intersect. For this reason, the glass fiber density is highest at the position of the lattice point P3, and there is no glass fiber at the position P1 where neither vertical stripes nor horizontal stripes pass. The glass fiber density at the position P2 through which only one of the vertical stripes or the horizontal stripes passes is intermediate between the glass fiber density at the position P1 and the glass fiber density at the position P3.

Thus, the glass fiber density varies depending on the position in the surface of the insulating film 54. Glass fiber is harder to process with a laser beam than epoxy resin. Consider a case where a laser pulse is incident on the metal film 57 to form a hole reaching the bottom surface of the insulating film 54. In the position P3 where the glass fiber density is the highest, for example, a laser pulse having a pulse energy density of 20 to 30 J / cm ² is incident on four shots, thereby forming a hole reaching the inner metal pattern 53. However, it is sufficient if two shots of laser pulses having the same pulse energy density are incident on the position P1 where the glass fiber density is the lowest. When a four-shot laser pulse is incident on the position P1, excessive energy is input, and the inner layer metal pattern 53 is damaged or the hole shape becomes a barrel shape. Conversely, at positions P2 and P3 where the glass fiber density is high, a hole reaching the inner metal pattern 53 is not formed by the two-shot laser pulse.

  Next, the laser processing method according to the embodiment will be described with reference to FIGS. FIG. 3 shows a flowchart of the laser processing method according to the embodiment, FIG. 4 shows a timing chart of the trigger signal trg, and the control signals gcon and con, and FIGS. 5A to 5D are in the middle stage of processing of one hole. Cross-sectional views of the object to be processed are shown, and FIGS. 5E and 5F are cross-sectional views when processing is completed.

  In step St <b> 1 of FIG. 3, the beam splitter 5 is controlled so that the laser beam enters the beam damper 6, and emission of the heat pulse HP from the laser light source 1 is started. The heat pulse HP has a pulse energy density of such a size that a hole cannot be formed in the metal film or insulating film of the workpiece 50 on the surface of the workpiece. The pulse frequency of the heat pulse HP is 4 kHz, for example.

  In step St2, the beam scanner 9 is controlled so that the laser beam is incident on the position of the hole to be processed next. When the control of the beam scanner 9 is completed, the process proceeds to step St3, the emission of the heat pulse HP is stopped, and the control signal scon is transmitted to the beam distributor 5 so that the laser beam propagates through the machining path. To do.

  Proceeding to step St4, the metal processing laser pulse MP is emitted. Since the beam splitter 5 is in a state of propagating the laser beam to the machining path, the metal machining laser pulse MP is incident on the workpiece 1 at a position where a hole is to be formed.

FIG. 5A shows a cross-sectional view of the workpiece 50 after the metal processing laser pulse MP is incident thereon. A hole is formed in the metal film 57, and the surface layer portion of the insulating film 54 therebelow is also shaved. The pulse energy and beam spot size of the metal processing laser pulse MP are adjusted so that the surface of the workpiece 50 has a pulse energy density of about 40 J / cm ² . For example, the pulse width of the metal processing laser pulse MP is about 30 to 100 μs, and the diameter of the hole formed in the metal film 57 is 100 to 125 μm.

In step St5, a confirmation laser pulse CP (0) is emitted. The pulse energy density of the confirmation laser pulse CP (0) on the surface of the workpiece 50 is, for example, 5 J / cm ² , and is lower than the minimum value (threshold value) of the pulse energy density that can open a hole in the insulating film 54. Is also small. For example, the pulse width of the confirmation laser pulse CP (0) is about several μs to 10 μs. In such a short time, the laser pulse falls before the light intensity of the laser pulse emitted from the laser light source 1 reaches a steady state. That is, the peak value of the light intensity of the confirmation laser pulse CP (0) is smaller than the peak value of the light intensity of the metal processing laser pulse MP.

  Since the confirmation laser pulse CP (0) does not process the insulating film 54, the depth of the hole hardly changes even when the confirmation laser pulse CP (0) is irradiated as shown in FIG. 5B.

  Proceeding to step St6, it is determined whether or not one hole machining is completed. Specifically, the second photodetector 16 detects the light intensity of the reflected light of the confirmation laser pulse CP (0), and compares the detection result with the determination reference value. When the inner layer metal pattern 53 is exposed, the reflectance is increased, so that the intensity of the reflected light is increased. The determination reference value is set to be larger than the reflected light intensity when the inner metal pattern 53 is not exposed and smaller than the reflected light intensity when the inner metal pattern 53 is exposed. Therefore, it is possible to determine whether or not the inner metal pattern 53 is exposed by comparing the detection result of the reflected light with the determination reference value. When the metal processing laser pulse MP is irradiated, as shown in FIG. 5A, the inner layer metal pattern 53 is not exposed, and therefore it is determined as “processing incomplete” in step St6.

Proceeding to step St7, the resin processing laser pulse RP (1) is emitted. As shown in FIG. 5C, the resin processing laser pulse RP (1) is incident on the position of the hole formed in the metal film 57 in step St4. The pulse energy density of the resin processing laser pulse RP (1) on the surface of the workpiece 50 is large enough to form a hole in the insulating film 54, and is, for example, 20 to 30 J / cm ² . The pulse width is about 20 to 30 μs. As shown in FIG. 5C, the hole formed in the workpiece 50 is deepened by the incidence of the resin processing laser pulse RP (1). The formed hole does not reach the inner metal pattern 53.

  Proceeding to step St8, the confirmation laser pulse CP (1) is emitted. The pulse energy of the confirmation laser pulse CP (1) is the same as the pulse energy of the confirmation laser pulse CP (0) emitted in step St5. For this reason, as shown in FIG. 5D, even when the confirmation laser pulse CP (1) is irradiated, the shape of the hole formed in the workpiece 50 hardly changes.

  Proceeding to step St9, it is determined whether or not one hole machining is completed. This determination is performed in the same procedure as that in step St6. As shown in FIG. 5D, since the formed hole does not reach the inner metal pattern 53, it is determined that “processing is not completed”.

  The process returns to step St7. As shown in FIG. 5E, it is assumed that the inner metal pattern 53 is exposed by the third shot of the resin processing laser pulse RP (3). In step St8, the confirmation laser pulse CP (3) is irradiated. As shown in FIG. 5F, the inner layer metal pattern 53 is exposed on the bottom surface of the hole, so that the reflectivity increases, and it is determined in step St9 that “one hole processing is completed”.

  Proceeding to step St10, it is determined whether or not all holes have been processed. In the case of “all hole machining completed”, the process is terminated. Thereafter, for example, the XY stage 11 shown in FIG. 1 is moved to move the unprocessed area of the processing object 50 within the scanable range of the laser beam, and the same procedure as the laser processing described above is executed.

  If it is determined that “all hole machining is incomplete”, the process returns to step St1, the control signal scon transmitted to the beam distributor 5 is stopped, and the laser beam enters the beam damper 6, and the heat The emission of the pulse HP is started. Thereafter, drilling is performed at the position of the hole to be processed next.

  When the insulating film 54 is thin and no glass fiber is disposed at a position where a hole is to be formed, a hole reaching the inner metal pattern 53 may be formed by irradiation with the metal processing laser pulse MP in step St4. is there. In this case, in Step St6, it is determined that “one hole machining is completed”, and the process proceeds to Step St10. If the inner metal pattern 53 is not exposed by the irradiation of the metal processing laser pulse MP, steps St5 and St6 may be omitted.

  In the laser processing method according to the above embodiment, every time a processing laser pulse is incident on a position where a hole is to be formed, a confirmation laser pulse is incident on the position where the hole is formed, and the intensity of the reflected light is detected. It is determined whether or not the pattern is exposed. For this reason, insufficient processing and excessive laser pulse irradiation can be avoided.

  Further, since the processing laser pulse and the confirmation laser pulse are emitted from the same laser light source, it is not necessary to prepare a dedicated laser light source for generating the confirmation laser pulse.

  Furthermore, in the first embodiment, since the confirmation laser pulse is incident after the processing laser pulse has been incident, it is possible to avoid the influence of light emission from the plasma generated during the processing. Thereby, the detection accuracy of the reflected light intensity of the confirmation laser pulse can be increased.

  Time T0 from emission of the last heat pulse HP shown in FIG. 4 to emission of the metal processing laser pulse MP, from emission of the metal processing laser pulse MP to emission of the first confirmation laser pulse CP (0). Time T1, time T2 from emission of the confirmation laser pulse CP to emission of the next resin processing laser pulse RP, and time from emission of the resin processing laser pulse RP to emission of the next confirmation laser pulse CP T3 are all equal, for example 250 μs. That is, the repetition frequency of the laser pulse emitted from the laser light source 1 is constant at 4 kHz.

  When the repetition frequency of the laser pulse emitted from the carbon dioxide laser oscillator is changed, even if the pulse width is constant, the pulse energy and the light intensity distribution in the beam cross section are changed. In the embodiment, since the pulse repetition frequency is kept constant, fluctuations in pulse energy and light intensity distribution can be suppressed and high-quality processing can be performed. In the above embodiment, since the confirmation laser pulse CP and the heat pulse HP have short pulse widths, the light intensity of the laser pulse falls before the light intensity reaches a steady state. As described above, it has been experimentally confirmed that even a short laser pulse that falls before reaching a steady state can suppress fluctuations in pulse energy and light intensity distribution in the beam cross section by inserting at a constant frequency.

  In the above embodiment, the reflected light from the workpiece 50 is branched from the path of the laser beam by the second partial reflection mirror 8, but can also be branched by other methods. For example, a polarization beam splitter may be used. In this case, a quarter-wave plate is inserted in the beam path between the polarization beam splitter and the beam scanner 9, and the polarization direction of the forward beam and that of the return beam are made to differ by 90 °, thereby reflecting the reflected light. Can be branched.

  In the above embodiment, the absolute value of the intensity of the reflected light is compared with the determination reference value. However, the reflected light is reflected from the light intensity of the confirmation laser pulse CP detected by the first photodetector 15 and the light intensity of the reflected light. The reflectance may be obtained, and the calculated reflectance may be compared with the reflectance criterion value. By determining whether or not hole formation is complete based on the calculated reflectance, it becomes possible to eliminate the influence of the fluctuation of the light intensity of the incident confirmation laser pulse itself and perform more accurate determination. .

  In the above embodiment, a carbon dioxide laser oscillator is used as the laser light source 1, but another laser oscillator with variable pulse energy may be used. For example, an excimer laser oscillator may be used.

  In the above embodiment, the method of forming a hole in the metal film 57 on the surface by laser processing and subsequently forming a hole in the insulating film thereunder (so-called “direct processing”) has been described as an example. The laser processing method according to the above embodiment can also be applied to a method in which a hole is formed in the metal film in advance and a hole is formed in the insulating film at the position of the hole by laser processing (so-called “conformal processing”). Is possible. In this case, steps St4 to St6 are omitted.

  Further, in the above embodiment, the case where the hole reaching the inner layer metal pattern 53 disposed under the insulating film 54 has been described, but other materials whose reflectivity changes when the inner layer pattern is exposed are used. Also in the case of processing, the above-described embodiment can be applied.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

It is the schematic of the laser processing apparatus by an Example. (2A) is a cross-sectional view of the workpiece, and (2B) is a flat cross-sectional view thereof. It is a flowchart of the laser processing method by an Example. It is a timing chart of the laser processing method by an Example. (5A)-(5D) are sectional views in the middle of processing of an object to be processed by the laser processing method according to the embodiment. (5E) and (5F) are sectional views of the object to be processed at the time when one hole processing is completed by the laser processing method according to the embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source 2 Beam expander 3 Mask 4 1st partial reflection mirror 5 Beam distributor 6 Beam damper 7 Folding mirror 8 2nd partial reflection mirror 9 Beam scanner 10 f (theta) lens 11 XY stage 15 1st photodetector 16 Second optical detector 20 Controller 50 Processing object 52 Core substrate 53 Inner layer metal pattern 54 Insulating film 54a Epoxy resin 54b Glass fiber MP Laser pulse CP for metal processing Laser pulse RP for confirmation Laser pulse HP for resin processing Heat pulse

Claims (8)

A laser light source with variable pulse energy;
A stage for holding the workpiece,
A propagation optical system for causing a laser pulse emitted from the laser light source to be incident on a workpiece held by the stage;
A photodetector for detecting reflected light of a laser pulse incident on a workpiece held on the stage;
A control device for controlling the laser light source, the control device,
(A) a step of causing a machining laser pulse having a pulse energy of a size capable of forming a hole in the workpiece held on the stage to be incident on a workpiece position of the workpiece;
(B) a step of causing a confirmation laser pulse having a pulse energy of a size that does not form a hole in the workpiece held on the stage to be incident on the workpiece position;
(C) Based on the detection result of the reflected light of the confirmation laser pulse by the photodetector, it is determined whether or not hole formation is complete. If hole formation is not complete, the process returns to step (a), The process is performed so that a new laser pulse for machining is incident on the machining position and the process of ending the incidence of the machining laser pulse on the machining position is executed when the hole formation is completed. Laser processing device that controls the laser light source. The propagation optical system includes a scanner that scans the laser beam so that the incident position of the laser beam moves on the surface of the workpiece held on the stage,
When it is determined in the step (c) that the hole formation is completed, the control device further
(D) controlling the scanner so that the laser beam is incident on a processing position where a hole is to be formed next;
(E) After the control of the scanner, the laser light source and the scanner are controlled such that the steps (a) to (c) are repeated for a new processing position. The laser processing apparatus according to claim 1.   In the step (d), during the period when the position where the laser beam should be incident is moved by the scanner, the control device applies pulse energy of a size that does not form a hole in the workpiece held by the stage. The laser processing apparatus according to claim 2, wherein the laser light source is controlled such that a heat pulse is emitted from the laser light source.   The control device controls the laser light source so that a series of laser pulses including the processing laser pulse, the confirmation laser pulse, and the heat pulse are emitted from the laser light source at a constant repetition frequency. The laser processing apparatus as described. (A) A processing laser pulse having a pulse energy of a size capable of forming a hole in the processing object is emitted from a laser light source and incident on the processing object, thereby forming a hole in the processing object. Process,
(B) A confirmation laser pulse having a pulse energy of a size that does not form a hole in the workpiece is emitted from the laser light source and incident on the position of the hole formed in the step (a). Detecting the intensity of the reflected light of the confirmation laser pulse;
(C) Based on the intensity of the reflected light detected in the step (b), it is determined whether or not the hole processing is completed. When the hole processing is completed, the irradiation of the processing laser pulse to the hole is finished. If the hole machining is not completed, the process returns to the step (a) and includes a process of executing a process laser beam newly incident on the position of the hole formed halfway. Method. If it is determined in the step (c) that the hole processing is completed,
(D) moving the incident position of the laser pulse emitted from the laser light source within the surface of the workpiece to a position where a new hole is to be formed;
The laser processing method according to claim 5, further comprising: (e) repeating steps (a) to (c) in a state where a laser pulse is incident on a position where a new hole is to be formed.   7. In the step (d), a heat pulse having a pulse energy of such a size that a hole is not formed in the object to be processed is emitted from the laser light source while the incident position of the laser pulse is moving. Laser processing method.   The laser processing method according to claim 7, wherein a series of laser pulses including the processing laser pulse, a confirmation laser pulse, and a heat pulse are emitted from the laser light source at a constant repetition frequency.

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