PULSE DURATION MEASURING DEVICE

Abstract

Pulse duration measuring device (10) for measuring a pulse duration of a pulsed laser beam (LB0), comprising: a laser beam entrance element (12) for receiving the pulsed laser beam (LB0); a polarization beam splitter (13) for splitting the pulsed laser beam (LB0) from the laser beam entrance element (12) into a first laser beam (LB1) running along a first beam path (L1) and a second laser beam (LB2) running along a second beam path (L2); a first mirror (15A) for reflecting the first laser beam (LB1) traveling along the first beam path (L1) to the polarization beam splitter (13); a second mirror (15B) for reflecting the second laser beam (LB2) traveling along the second beam path (L2) to the polarization beam splitter (13); a first quarter-wave plate (14A) arranged between the polarization beam splitter (13) and the first mirror (15A); a second quarter-wave plate (14B) arranged between the polarization beam splitter (13) and the second mirror (15B); a beam path length changing unit (16) for moving the first mirror (15A) or the second mirror (15B) along the beam path of the first laser beam (LB1) or the second laser beam (LB2) to change a length of the beam path; a non-linear crystal (17) allowing a combined laser beam (LB3) to pass therethrough, the combined laser beam (LB3) including respective returning laser beams of the first and second laser beams (LB1, LB2) reflected by the first and second mirrors (15A, 15B) respectively and combined by the polarization beam splitter (13); and a photodetection device (18) for measuring an optical intensity of the combined laser beam (LB3) which has passed through the non-linear crystal (18), wherein the laser beam entrance element (12), the polarization beam splitter (13), the first mirror (15A), the second mirror (15B), the first quarter-wave plate (14A), the second quarter-wave plate (15B), the beam path length changing unit (16), the non-linear crystal (17) and the photodetection device (18) are arranged on a flat surface.

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The present invention relates to a pulse duration measuring device for measuring a pulse duration of a pulsed laser beam. Description of the state of the art

[0002] Wafers containing multiple components, such as integrated circuits (ICs) and large-scale integration (LSI) circuits, formed in respective regions defined on their front surfaces by a plurality of intersecting predefined dividing lines, are divided into individual component chips along the predefined dividing lines by a laser processing device. These component chips are used in various electronic devices, such as mobile phones and personal computers.

[0003] The laser processing apparatus includes a chuck table for holding a wafer thereon, a laser beam application unit for applying a pulsed laser beam to the wafer held on the chuck table, and a moving mechanism for moving the chuck table and the laser beam application unit relative to each other in the X-axis directions indicated by an arrow X and the Y-axis directions indicated by an arrow Y. The laser beam application unit includes a laser oscillator for emitting the pulsed laser beam, a beam condenser for converging the laser beam emitted from the laser oscillator onto the wafer held on the chuck table, and an optical system arranged between the laser oscillator and the beam condenser (see, for example, JP 2012-2604 A).

[0004] The following document contains further information helpful for understanding the present invention: Junbao Chen, Wei Xia, Ming Wang: Characteristic measurement for femtosecond laser pulses using a GaAs PIN photodiode as a two-photon photovoltaic receiver. In: J. Appl. Phys., 121, 2017, 121. 223103. (https: / / doi.org / 10.1063 / 1.4985244) PRESENTATION OF THE INVENTION

[0005] In the laser processing device described above, the optical system of the laser beam application unit is maintained periodically or as needed to maintain the required processing quality based on the pulsed laser beam. The various maintenance tasks to be performed include a verification process for measuring the pulse duration of the pulsed laser beam emitted by the laser oscillator and checking whether the measured pulse duration corresponds to the desired pulse duration.

[0006] The verification process for measuring the pulse duration of the pulsed laser beam is performed by a relatively large pulse duration measuring device. The pulse duration measuring device works as follows. First, the pulse duration measuring device is positioned near the laser processing device to be serviced. Then, a separate beam splitting device is used to split a laser beam from the pulsed laser beam emitted by the laser beam application unit of the laser processing device. The split laser beam is guided to the pulse duration measuring device, which measures the pulse duration of the split laser beam. The pulse duration measuring device is cumbersome and time-consuming because the pulse duration measuring device requires laborious installation and adjustment before it can actually perform measurements.

[0007] It is therefore an object of the present invention to provide a pulse duration measuring apparatus capable of efficiently measuring a pulse duration without splitting a laser beam from a pulsed laser beam from a laser processing apparatus.

[0008] According to one aspect of the present invention, a pulse duration measuring device for measuring a pulse duration of a pulsed laser beam is provided. The pulse duration measuring device comprises: a laser beam input element for receiving a pulsed laser beam; a polarization beam splitter for splitting the pulsed laser beam from the laser beam input element into a first laser beam traveling along a first beam path and a second laser beam traveling along a second beam path; a first mirror for reflecting the first laser beam traveling along the first beam path to the polarization beam splitter; a second mirror for reflecting the second laser beam traveling along the second beam path to the polarization beam splitter; a first quarter-wave plate arranged between the polarization beam splitter and the first mirror; a second quarter-wave plate;arranged between the polarization beam splitter and the second mirror, a beam path length changing unit for moving the first mirror or the second mirror along the beam path of the first laser beam or the second laser beam to change a length of the beam path, a nonlinear crystal (nonlinear crystal body) that allows a combined laser beam to pass through, the combined laser beam including respective returning laser beams of the first and second laser beams reflected by the first and second mirrors, respectively, and combined by the polarization beam splitter, and a photodetection device for measuring an optical intensity of the combined laser beam that has passed through the nonlinear crystal. The laser beam entrance element, the polarization beam splitter, the first mirror, the second mirror, the first quarter-wave plate, the second quarter-wave plate,the beam path length change unit, the nonlinear crystal and the photodetection device are arranged on a flat surface.

[0009] Since the pulse duration measuring device according to the present invention arranges the components from the laser beam entrance element to the photodetection device on a flat surface, the pulse duration measuring device, which has a flat configuration, can measure a pulse duration while being placed on a chuck table of a laser processing apparatus. The pulse duration measuring device according to the present invention eliminates the problems of the conventional pulse duration measuring device in that it is cumbersome and time-consuming to use, since the pulse duration of a pulsed laser beam must be measured by splitting a laser beam from the pulsed laser beam emitted from the optical system of the laser beam application unit.

[0010] The above and other objects, features and advantages of the present invention, as well as the mode for carrying them into effect, will best become apparent and the invention itself will be best understood by studying the following description and the appended claims with reference to the attached drawings which show a preferred embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a pulse width measuring device according to an embodiment of the present invention and a laser processing apparatus; Fig. 2 is an enlarged perspective view of the Fig. 1 shown pulse duration measuring device; and Fig. 3 is an enlarged perspective view illustrating the manner in which the Fig. 2 is used to measure a pulse duration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0011] A pulse width measuring device according to a preferred embodiment of the present invention will be described below with reference to the accompanying drawings.

[0012] Fig. 1 shows in perspective the pulse duration measuring device designated 10 according to the present embodiment and a laser processing device 1, wherein the pulse duration of a pulsed laser beam can be measured with the pulse duration measuring device 10.

[0013] As in Fig. 1, the laser processing apparatus 1 includes a holding unit 3 disposed on a base 2 and holding a plate-shaped workpiece thereon, a laser beam application unit 6 for applying a pulsed laser beam to the plate-shaped workpiece on the holding unit 3, an alignment means 7 having a camera 7a for taking an image of the workpiece on the holding unit 3, a moving assembly 4 for machining the laser beam application unit 6 and the holding unit 3 relative to each other and for moving the alignment means 7 and the holding unit 3 relative to each other, and a frame 5 having a vertical wall 5a erected on the base 2 rearward of the moving assembly 4 and a horizontal wall 5b extending horizontally from an upper end portion of the vertical wall 5a in an overhanging relationship with the holding unit 3.

[0014] The holding unit 3 includes a rectangular X-axis movable plate 31 movably mounted on the base 2 to move in the X-axis directions, a rectangular Y-axis movable plate 32 movably mounted on the X-axis movable plate 31 to move in the Y-axis directions perpendicular to the X-axis directions, a hollow cylindrical support post 33 fixedly attached to an upper surface of the Y-axis movable plate 32, and a rectangular cover plate 34 fixedly attached to an upper end of the support post 33. A chuck table 35 is disposed in the cover plate 34 and extends upward through an elongated hole formed in the cover plate 34. The clamping table 35 is rotatable about its vertical central axis by means not shown which is housed in the support post 33.The clamping table 35 has a circular suction clamp 35a made of an air-permeable porous material and positioned substantially horizontally. The suction clamp 35a is fluidly connected to the suction means (not shown) via a fluid channel extending through the support post 33.

[0015] The movement assembly 4 includes an X-axis feed mechanism 42 and a Y-axis feed mechanism 44. The X-axis feed mechanism 42 converts a rotary motion of an electric motor 42a into a linear motion with a ball screw 42b and transmits the linear motion to the X-axis movable plate 31. The X-axis movable plate 31 is moved in one or the other of the X-axis directions along a pair of guide rails 2a arranged on the base 2 and extending in the X-axis directions.The Y-axis feed mechanism 44 converts a rotary motion of an electric motor 44a into a linear motion with a ball screw 44b and transmits the linear motion to the Y-axis movable plate 32, thereby moving the Y-axis movable plate 32 in one or the other of the Y-axis directions along a pair of guide rails 36 arranged on the X-axis movable plate 31 and extending in the Y-axis directions.

[0016] The horizontal wall 5b of the frame 5 accommodates an unillustrated optical system of the laser beam application unit 6. The laser beam application unit 6 includes a beam condenser 6a disposed on the lower surface of a distal end portion of the horizontal wall 5b. The camera 7a of the alignment means 7 is positioned on the lower surface of the distal end portion of the horizontal wall 5b at a position adjacent to the beam condenser 6a in one of the X-axis directions.The camera 7a includes a conventional image pickup device such as a charge-coupled device (CCD) for picking up visible light images, an infrared ray applying means for applying infrared rays to a workpiece, an optical system for collecting infrared rays emitted from the infrared ray applying means, and an image pickup device such as an infrared CCD for outputting an electrical signal representing the infrared rays collected by the optical system.

[0017] The laser processing device 1 includes a control unit (not shown). The control unit comprises a computer and controls the movement assembly 4, the laser beam application unit 6, and the alignment means 7, which are electrically connected to the control unit. A display means 8 is arranged on an upper surface of the horizontal wall 5b and is electrically connected to the control unit. The control unit controls the display means 8 to display processing conditions and images captured by the camera 7a of the alignment means 7.

[0018] The optical system (not shown) of the laser beam application unit 6 includes a laser oscillator for emitting a pulsed laser beam with a predetermined pulse duration. When the laser processing device 1 is regularly maintained or as needed, the pulse duration measuring device 10 measures the pulse duration of the pulsed laser beam emitted by the laser oscillator and checks whether the measured pulse duration corresponds to the desired pulse duration.

[0019] Fig. 2 represents the Fig. 1 shows an enlarged view of the pulse duration measuring device 10. As shown in Fig. As shown in Figure 2, the pulse duration measuring device 10 comprises a circular base plate 11 with a flat lower surface. The pulse duration measuring device 10 further comprises, on a flat upper surface 11a of the base plate 11, a laser beam entrance element or a rectangular prism 12 onto which a pulsed laser beam is applied vertically from above, a polarization beam splitter 13 arranged centrally on the flat upper surface 11a for splitting the pulsed laser beam reflected by a reflecting surface 12a of the laser beam entrance element 12 into a first laser beam traveling along a first beam path L1 and a second laser beam traveling along a second beam path L2, with an inner reflecting surface 13a having a multilayer coating, a first mirror 15A for reflecting the first laser beam,which has traveled along the first beam path L1 from the polarization beam splitter 13 back to the polarization beam splitter 13, a second mirror 15B for reflecting the second laser beam which has traveled along the second beam path L2 from the polarization beam splitter 13 back to the polarization beam splitter 13, a first quarter-wave plate 14A arranged between the polarization beam splitter 13 and the first mirror 15A, and a second quarter-wave plate 14B arranged between the polarization beam splitter 13 and the second mirror 15B.

[0020] The pulse duration measuring device 10 further includes an optical path length changing unit 16 for moving the first mirror 15A along the optical path L1 of the first laser beam to change the length of the optical path L1, a nonlinear crystal (nonlinear crystal body) 17 for allowing a combined laser beam to pass therethrough, the combined laser beam including respective returning laser beams of the first and second laser beams reflected by the first and second mirrors 15A and 15B, respectively, and combined by the polarization beam splitter 13, and a photodetecting device 18 for measuring the optical intensity of the combined laser beam that has passed through the nonlinear crystal 17.According to the present embodiment, the first optical path L1 starts from the reflecting surface 13a of the polarization beam splitter 13 and returns from the first mirror 15A to the reflecting surface 13a of the polarization beam splitter 13, and the second optical path L2 starts from the reflecting surface 13a of the polarization beam splitter 13 and returns from the second mirror 15B to the reflecting surface 13a of the polarization beam splitter 13.

[0021] The laser beam entrance element 12 has the reflective surface 12a, which is inclined at a 45° angle to the beam path of the pulsed laser beam applied vertically from above. The reflective surface 12a reflects the pulsed laser beam applied vertically from above, causing it to spread horizontally, thereby changing the beam path of the pulsed laser beam toward the polarization beam splitter 13, which is arranged centrally on the flat upper surface 11a of the circular base plate 11.

[0022] The optical path length changing unit 16 includes a stepping motor 161, an externally threaded screw 162, one end of which is coupled to an output shaft of the stepping motor 161, and a nut 163 having an internally threaded hole operatively threaded over the externally threaded screw 162. The nut 163 is integrally connected to the first mirror 15A. When the stepping motor 161 is energized, it rotates the externally threaded screw 162 about its central axis, moving the first mirror 15A in one of the directions indicated by an arrow R, that is, along the first optical path L1.

[0023] The stepper motor 161 and the photodetection device 18 are electrically connected to a control unit 100. The control unit 100 can store the optical intensity of the combined laser beam, the optical intensity detected by the photodetection device 18, as an electrical signal and can also control the stepper motor 161. The control unit 100 sends an operating signal to the stepper motor 161 to move the first mirror 15A precisely along the pulsed first laser beam, which travels along the first beam path L1 in one of the directions indicated by the arrow R.The control unit 100 does not necessarily have to be included in the pulse duration measuring device 10, but the stepper motor 161 and the photodetection device 18 could be electrically connected to the unillustrated control unit of the laser processing device 1, so that the control unit of the laser processing device 1 can be used to control the pulse duration measuring device 10 to measure the pulse duration of the pulsed laser beam.

[0024] An operation and advantages of the pulse duration measuring device 10 as applied in the laser processing apparatus 1 will be described below with reference to Fig. 3 and the Fig. 1 and Fig. 2 described.

[0025] The circular base plate 11 of the pulse width measuring device 10 is set to such dimensions that it can be held by suction on the chuck table 35 of the laser processing device 1. For example, the circular base plate 11 is set to a size corresponding to the suction chuck 35a or larger.

[0026] To measure the pulse width, the pulse width measuring device 10 is placed on the suction chuck 35a of the chuck table 35, and the suction means (not shown) is actuated to generate a negative pressure, which is transmitted to the suction chuck 35a to hold the workpiece thereon under suction. Then, the moving assembly 4 is actuated to move the chuck table 35 and thus the pulse width measuring device 10 to a position directly below the camera 7a of the alignment means 7. The camera 7a is then energized to capture an image of the pulse width measuring device 10, thereby detecting the position of the center of the reflecting surface 12a of the laser beam entrance element 12 disposed on the flat upper surface 11a of the circular base plate 11.

[0027] After the alignment means 7 has detected the position of the center of the reflective surface 12a of the laser beam entrance element 12, the movement assembly 4 is actuated to move the pulse duration measuring device 10 until the center of the reflective surface 12a of the laser beam entrance element 12 is positioned directly below the beam condenser 6a of the laser beam application unit 6. Then, the laser beam application unit 6 is energized to emit a pulsed laser beam LB0 (see Fig. 3) from the beam condenser 6a. The pulsed laser beam LB0, for example, has a wavelength of 355 nm and an average output power level low enough not to process or damage the optical components of the pulse duration measuring device 10.

[0028] As in Fig. 3, the pulsed laser beam LB0 emitted from the beam condenser 6a is reflected by the reflecting surface 12a of the laser beam entrance element 12 and applied to the polarization beam splitter 13. The reflecting surface 13a of the polarization beam splitter 13 transmits the P-polarized light of the pulsed laser beam LB0 from the laser beam entrance element 12 as a first laser beam LB1 propagating along the first beam path L1, and reflects the S-polarized light of the pulsed laser beam LB0 as a second laser beam LB2 propagating along the second beam path L2, so that the P-polarized light and the S-polarized light are split as the first laser beam LB1 and the second laser beam LB2 from the pulsed laser beam LB0.

[0029] The P-polarized first laser beam LB1, which has passed through the reflecting surface 13a of the polarization beam splitter 13, is converted into a circularly polarized laser beam when it is transmitted through the first quarter-wave plate 14A arranged between the polarization beam splitter 13 and the first mirror 15A, and is then reflected by the first mirror 15A. When the first laser beam LB1 is reflected by the first mirror 15A, the rotation direction of the circularly polarized laser beam is reversed, and when the first laser beam LB1 passes through the first quarter-wave plate 14A, it is converted into an S-polarized laser beam.The S-polarized laser beam is then applied to the polarization beam splitter 13 as a returning laser beam LB1' of the first laser beam LB1 and reflected by its reflecting surface 13a to move toward the photodetecting device 18, whereupon the returning laser beam LB1' is applied to the nonlinear crystal 17.

[0030] The S-polarized second laser beam LB2 reflected by the reflecting surface 13a of the polarization beam splitter 13 is converted into a circularly polarized laser beam when it is transmitted through the second quarter-wave plate 14B disposed between the polarization beam splitter 13 and the second mirror 15B, and then reflected by the second mirror 15B. When the second laser beam LB2 is reflected by the second mirror 15B, the rotation direction of the circularly polarized laser beam is reversed, and when the second laser beam LB2 passes through the second quarter-wave plate 14B, it is converted into a P-polarized laser beam.The P-polarized laser beam is then applied to the polarization beam splitter 13 as a returning laser beam LB2' of the second laser beam LB2 and reflected by its reflecting surface 13a to travel toward the photodetecting device 18, whereupon the returning laser beam LB2' is applied to the nonlinear crystal 17.

[0031] As described above, the pulsed laser beam applied to the nonlinear crystal 17 is a combined laser beam LB3 including the returning first laser beam LB1' and the returning second laser beam LB2', which are combined by the polarization beam splitter 13. The combined laser beam LB3 includes the P-polarized laser beam and the S-polarized laser beam. According to the present embodiment, the nonlinear crystal 17 is arranged so that the ratio of the transmittances with which the P-polarized laser beam and the S-polarized laser beam of the combined laser beam LB3 are transmitted through the nonlinear crystal 17 varies depending on the ratio with which the waveforms of the P-polarized laser beam and the S-polarized laser beam overlap, thereby changing the optical intensity of a combined laser beam LB4 emitted from the nonlinear crystal 17.

[0032] The control unit 100 according to the present embodiment supplies power to the stepper motor 161 of the optical path length changing unit 16 to move the first mirror 15A along the pulsed first laser beam traveling along the first optical path L1, thereby changing the length of the first optical path L1 for the returning laser beam LB1' reflected by the first mirror 15A and returning to the polarization beam splitter 13, and controls the photodetection device 18 to detect a change in the optical intensity of the returning laser beam LB1'. The changes in the length of the first optical path L1 correspond to twice the distance traveled by the first mirror 15A along this path.

[0033] Fig. 3, in a lower section, represents a graph illustrating how the ratio of the transmittances with which the P-polarized laser beam and the S-polarized laser beam of the combined laser beam LB3 are transmitted through the nonlinear crystal 17 changes, plotted based on the optical intensity detected by the photodetection device 18. The graph has a vertical axis representing the ratio of transmittances and a horizontal axis representing time, i.e., picoseconds (ps), corresponding to the changes in the length of the first optical path L1 obtained by dividing the changes in the length of the first optical path L1 by the speed of light. The graph also represents an approximate curve S based on the recorded ratio values.

[0034] A point denoted by P0 on the graph is a point where the optical intensity detected by the photodetection device 18 is maximum and the waveforms of the P-polarized laser beam and the S-polarized laser beam of the combined laser beam LB3 do not overlap at all. The optical intensity detected at point P0 is considered a reference (0%) for the transmittance ratio at which the P-polarized laser beam and the S-polarized laser beam of the combined laser beam LB3 are transmitted through the nonlinear crystal 17. In other words, the optical path length changing unit 16 is operated so that the length of the first optical path L1 is smaller than the length of the second optical path L2.When the first mirror 15A is moved by the optical path length changing unit 16 to increase the length of the optical path, the waveforms of the P-polarized laser beam and the S-polarized laser beam of the combined laser beam LB3 begin to overlap. As the overlap ratio increases, the optical intensity of the combined laser beam LB4 emitted from the nonlinear crystal 17 decreases when the combined laser beam LB3 passes through the nonlinear crystal 17.

[0035] A minimum point P1 of the approximate curve S in the lower section of Fig. 3, a point is where the waveforms of the P-polarized laser beam and the S-polarized laser beam of the combined laser beam LB3 completely overlap, that is, a point where the length of the first optical path L1 and the length of the second optical path L2 coincide with each other, and the optical intensity of the combined laser beam LB4 emitted from the nonlinear crystal 17 when the combined laser beam LB3 passes through the nonlinear crystal 17 is the lowest. When the first mirror 15A is moved by the optical path length changing unit 16 to increase the length of the optical path, the ratio of the overlap between the waveforms of the P-polarized laser beam and the S-polarized laser beam of the combined laser beam LB3 is reduced, and the optical intensity of the combined laser beam LB4 detected by the photodetecting device 18 increases.When the waveforms of the P-polarized laser beam and the S-polarized laser beam of the combined laser beam LB3 cease to overlap, the detected optical intensity becomes maximum at a point P2 where the transmittance ratio is the same as the transmittance ratio at point P0. The waveform thus derived, represented by the approximate curve S, is highly correlated with the pulsed waveform of the pulsed laser beam LB0. The pulse duration of the pulsed laser beam LB0 is calculated from the approximate curve S according to the following calculation method.

[0036] According to the embodiment described above, a change in length X1 of the first beam path L1 from point P0 to point P2 of the approximation curve S is, for example, 12000 µm. Dividing 12000 µm by the speed of light (300 µm) per picosecond (ps), one obtains 40 ps. From the value shown in the graph in Fig. 3, a pulse duration-corresponding change X2 of the first optical path L1, denoted by W, is calculated as a pulse duration-corresponding change in length between points P3 and P4 at a transmittance ratio Q2 that is 1 / 2 of a transmittance ratio Q1 at the minimum point P1 of the approximate curve S. The change X2 detected according to the present embodiment is 3180 µm, and the pulse duration W measured according to the present embodiment is given with respect to a value obtained by dividing the change X2 by the speed of light, i.e., 300 µm / ps, as a reference value. More specifically, 3180 µm ÷ 300 µm / ps = 10.6 ps is calculated as a reference value, and 10.6 ps is divided by 2 1 / 2(≈ 1.41), which is a parameter based on the assumption that the pulsed waveform of the pulsed laser beam LB0 is a Gaussian waveform. The value thus obtained by the division (= 7.5 ps) is determined as the pulse duration W as a measured value, which is stored in the control unit 100. It is then determined whether the measured pulse duration W is a desired pulse duration or not. If the measured pulse duration W is not a desired pulse duration, the laser oscillator that emits the pulsed laser beam LB0 is adjusted or replaced.

[0037] The above method for measuring the pulse duration is merely exemplary, and various other methods for calculating pulse durations are known in the art. Therefore, an actual method of determining a pulse duration using the pulse duration measuring device 10 is not limited to the method according to the above embodiment.

[0038] According to the above configuration, the pulse duration measuring device 10, which has a planar configuration, can measure a pulse duration while being placed on the chuck table of the laser processing apparatus. The pulse duration measuring device 10 eliminates the problems of the conventional pulse duration measuring device, which is cumbersome and time-consuming to use because the pulse duration of a pulsed laser beam must be measured by splitting a laser beam from the pulsed laser beam emitted from the optical system of the laser beam application unit.

[0039] According to the above embodiment, the optical path length changing unit 16 is provided as a means for moving the first mirror 15A along the optical path. However, it could also be provided as a means for moving the second mirror 15B along the optical path. Such a modification also makes it possible to perform the same measurement as described above with reference to Fig.3 described.

Claims

[1] Pulse duration measuring device (10) for measuring a pulse duration of a pulsed laser beam (LB0), comprising: a laser beam entrance element (12) for receiving the pulsed laser beam (LB0); a polarization beam splitter (13) for splitting the pulsed laser beam (LB0) from the laser beam entrance element (12) into a first laser beam (LB1) running along a first beam path (L1) and a second laser beam (LB2) running along a second beam path (L2); a first mirror (15A) for reflecting the first laser beam (LB1) traveling along the first beam path (L1) to the polarization beam splitter (13); a second mirror (15B) for reflecting the second laser beam (LB2) traveling along the second beam path (L2) to the polarization beam splitter (13); a first quarter-wave plate (14A) arranged between the polarization beam splitter (13) and the first mirror (15A); a second quarter-wave plate (14B) arranged between the polarization beam splitter (13) and the second mirror (15B); a beam path length changing unit (16) for moving the first mirror (15A) or the second mirror (15B) along the beam path of the first laser beam (LB1) or the second laser beam (LB2) to change a length of the beam path; a non-linear crystal (17) allowing a combined laser beam (LB3) to pass therethrough, the combined laser beam (LB3) including respective returning laser beams of the first and second laser beams (LB1, LB2) reflected by the first and second mirrors (15A, 15B) respectively and combined by the polarization beam splitter (13); and a photodetection device (18) for measuring an optical intensity of the combined laser beam (LB3) which has passed through the non-linear crystal (18), wherein the laser beam entrance element (12), the polarization beam splitter (13), the first mirror (15A), the second mirror (15B), the first quarter-wave plate (14A), the second quarter-wave plate (15B), the beam path length changing unit (16), the non-linear crystal (17) and the photodetection device (18) are arranged on a flat surface.

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