A single femtosecond laser pulse waveform measurement device

Abstract

A single-shot femtosecond laser pulse waveform measurement device. The vertically polarized, spatially uniform femtosecond laser pulse to be measured is split into transmitted and reflected light by a beam splitter. The transmitted light is further split into two beams. These three beams are incident on a nonlinear crystal along the edges of a regular triangular prism at the same angle and with equal optical path lengths, undergoing third-order nonlinear harmonic transformation. The resulting third-harmonic beam is the second-order intensity-correlated signal of the measured pulse, after spatiotemporal transformation and double delay. The pulse waveform information is then obtained through image decoding. This invention improves time resolution, is low-cost, and easy to adjust.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast pulse laser testing technology, specifically relating to a single femtosecond laser pulse waveform measurement device. Background Technology

[0002] The time waveform of a femtosecond laser pulse can be accurately extracted using a dual-delay third-order intensity correlation signal. This method is different from FROG and SPIDER methods for measuring ultrafast pulse waveforms. Two invention patents, namely "Laser Pulse Waveform Measurement Device Based on Third-Order Correlation Method" (patent number: ZL 201610549016.7) and "An Ultrashort Laser Pulse Waveform Measurement Device" (patent number: ZL 201610548538.5), disclose a method for obtaining pulse waveforms by measuring a dual-delay third-order intensity correlation signal. These devices employ a cascaded second-order nonlinear harmonic transformation technique (frequency doubling followed by frequency summation) to obtain the dual-delay third-order intensity correlation signal. This requires two nonlinear transformations, which increases the difficulty of debugging and reduces the time resolution of the device. Summary of the Invention

[0003] To overcome the shortcomings of existing measurement technologies in femtosecond laser pulse waveform measurement, such as complex debugging and limited time resolution, this invention provides a single femtosecond laser pulse waveform measurement device.

[0004] The technical solution adopted by this invention to solve its technical problem is:

[0005] This invention discloses a single-shot femtosecond laser pulse waveform measurement device, characterized in that, in the measurement device, beam splitter I and beam splitter II are sequentially arranged along the incident direction of the spatially uniform femtosecond laser pulse under test with vertical polarization; the femtosecond laser pulse under test is split into transmitted light and reflected light by beam splitter I, and the transmitted light is further split into transmitted light and reflected light by beam splitter II; a delay adjuster I, a reflector I, a reflector II, a reflector III, a reflector IV, and a nonlinear crystal are sequentially arranged on the reflected light path of beam splitter I; the beam reflected from beam splitter I is adjusted by delay adjuster I and then sequentially reflected by reflector I, reflector II, reflector III, and reflector IV before being projected onto the nonlinear crystal; a reflector V, a delay adjuster II, a reflector VI, a reflector VII, and a nonlinear crystal are arranged on the reflected light path of beam splitter II; the beam reflected from beam splitter II is reflected by reflector V to delay adjuster II for optical path adjustment. The beam is reflected sequentially by mirrors VI and VII and then projected onto the nonlinear crystal. Mirror VIII, delay adjuster III, a beam guide assembly, and the nonlinear crystal are sequentially arranged along the transmission path of beam splitter II. The beam transmitted from beam splitter II is reflected by mirror VIII to delay adjuster III for optical path adjustment before being projected onto the beam guide assembly. The elevated beam exiting the beam guide assembly is projected obliquely downwards onto the nonlinear crystal. The beam from the beam guide assembly intersects with the beams from mirrors IV and VII within the nonlinear crystal and undergoes third-harmonic conversion. An attenuation filter, an imaging mirror assembly, and a CCD are sequentially arranged along the surface normal direction behind the nonlinear crystal. The third-harmonic beam output from the surface normal direction of the nonlinear crystal is attenuated and filtered by the attenuation filter before entering the imaging mirror assembly. The imaging mirror assembly images the third-harmonic beam from the surface of the nonlinear crystal onto the CCD in the near field. The CCD is connected to an external computer, and the signal from the CCD is finally processed by the computer.

[0006] The reflector IV is mounted at the center of the rotating stage I, and the rotating stage I is mounted at the center of the translation stage I. The combination of the rotating stage I and the translation stage I allows the reflector IV to rotate along the vertical y-axis and translate along the x-axis, ensuring the accuracy of the angle between the emitted beam and the vertical yz plane.

[0007] The reflector VII and reflector IV are placed in a mirror-symmetrical manner along the yz vertical plane, and their installation structure and function are the same as those of reflector IV.

[0008] The nonlinear crystal is mounted at the center of a rotating mirror frame, which is fixed at the center of a rotating stage III. The rotating stage III is mounted at the center of a translation stage III. The nonlinear crystal can rotate around the normal direction of its surface via the rotating mirror frame, rotate around the x-axis via the rotating stage III, and move up and down around the y-axis via the translation stage III. These measures ensure that the three incident beams intersect at the center of the nonlinear crystal and satisfy the angular position matching relationship.

[0009] The light guide mirror assembly consists of orthogonally positioned reflectors IX and X, a vertical guide rail, and a rotating stage IV. Reflectors IX and X are sequentially arranged along the fundamental frequency laser pulse transmission direction. Reflector IX reflects the incident beam from the horizontal x-axis direction vertically upward along the y-axis direction, while reflector X reflects the vertically upward reflected beam from reflector IX obliquely downward in the yz vertical plane to the nonlinear crystal. Reflector X is mounted at the center of rotating stage IV, which can rotate along the x-axis. Rotating stage IV is fixed on the vertical guide rail and can move up and down along the y-axis direction. The combination of rotating stage IV and the vertical guide rail allows adjustment of the orientation of reflector X, ensuring the angle α between the emitted beam and the xz horizontal plane.

[0010] The beneficial effects of this invention are:

[0011] 1. The measuring device of the present invention uses a single nonlinear crystal, which is low in cost. Using a single nonlinear crystal shortens the nonlinear transformation distance and increases the bandwidth, thereby improving the time resolution.

[0012] 2. The measuring device of the present invention uses a combination of a translation stage and a rotary stage to adjust the position and orientation of the incident beam in the nonlinear harmonic transformation, which is convenient for adjustment;

[0013] 3. The measuring device of the present invention uses a rotating nonlinear crystal to adapt to the polarization state of the incident beam being measured, eliminating the need for inserting polarization elements and improving the compactness and adaptability of the device. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the optical path of a single femtosecond laser pulse waveform measurement device according to the present invention;

[0015] Figure 2 This is a schematic diagram of the optical path of the light guide mirror assembly in this invention;

[0016] Figure 3 Uniaxial crystal coordinate representation;

[0017] In the diagram, 1. Beam splitter I 2. Beam splitter II 3. Delay adjuster I 4. Mirror I 5. Mirror II 6. Mirror III 7. Mirror IV 8. Nonlinear crystal 9. Mirror V 10. Delay adjuster II 11. Mirror VI 12. Mirror VII 13. Mirror VIII 14. Delay adjuster III 15. Light guide mirror group 16. Attenuation filter 17. Imaging mirror group 18. CCD 19. Rotating stage I 20. Translation stage I 21. Translation stage II 22. Rotating stage II 23. Rotating mirror frame 24. Rotating stage III 25. Translation stage III 15-1. Mirror IX 15-2. Mirror X 15-3. Vertical guide rail 15-4. Rotating stage IV. Detailed Implementation

[0018] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the scope of protection of the present invention.

[0019] Example 1

[0020] Figure 1 This is a schematic diagram of a single femtosecond laser pulse waveform measurement device according to the present invention; Figure 2 This is a schematic diagram of the optical path of the light guide mirror assembly in this invention. Figure 1 Side view of the light guide mirror assembly from direction A. Figure 1 , Figure 2 In a single-shot femtosecond laser pulse waveform measurement device of the present invention, beam splitters I1 and II2 are sequentially arranged along the incident direction of the spatially uniform femtosecond laser pulse under test with vertical polarization. The femtosecond laser pulse under test is split into transmitted light and reflected light by beam splitter I1, and the transmitted light is split into transmitted light and reflected light again by beam splitter II2. A delay adjuster I3, a reflector I4, a reflector II5, a reflector III6, a reflector IV7, and a nonlinear crystal 8 are sequentially arranged along the reflected light path of beam splitter I1. The beam reflected from beam splitter I1 is adjusted in optical path by delay adjuster I3, and then reflected sequentially by mirrors I4, II5, III6, and IV7 before being projected onto nonlinear crystal 8. Mirrors V9, II10, VI11, VII12, and 8 are arranged in the reflected light path of beam splitter II2. The beam reflected from beam splitter II2 is reflected by mirror V9 to delay adjuster II10 for optical path adjustment, and then sequentially reflected by mirrors VI11, VII12, and IV7 before being projected onto nonlinear crystal 8. 2. After reflection and transmission, the beam is projected onto the nonlinear crystal 8. A reflector VIII 13, a delay adjuster III 14, a light guide assembly 15, and the nonlinear crystal 8 are sequentially arranged along the transmission path of the beam splitter II 2. The beam transmitted from the beam splitter II 2 is reflected by the reflector VIII 13 to the delay adjuster III 14 for optical path adjustment before being projected onto the light guide assembly 15. The elevated beam exiting the light guide assembly 15 is projected obliquely downwards onto the nonlinear crystal 8. The beam from the light guide assembly 15 interacts with the beams from the reflector IV 7 and the reflector VII 12. The beams intersect and undergo third-harmonic conversion within the nonlinear crystal 8; after the nonlinear crystal 8, an attenuation filter 16, an imaging lens group 17, and a CCD 18 are sequentially arranged along the surface normal direction; the third-harmonic beam output from the surface normal direction of the nonlinear crystal 8 is attenuated and filtered by the attenuation filter 16 before entering the imaging lens group 17, which images the third-harmonic beam from the surface of the nonlinear crystal 8 onto the CCD 18 in the near field; the CCD 18 is connected to an external computer, and the signal from the CCD 18 is finally processed by the computer.

[0021] The reflector IV7 is mounted at the center of the rotating stage I19, which is mounted at the center of the translation stage I20. The combination of the rotating stage I19 and the translation stage I20 allows reflector IV7 to rotate along the vertical y-axis and translate along the x-axis, ensuring the accuracy of the angle between the emitted beam and the vertical yz plane. The reflector VII12 is placed symmetrically with reflector IV7 along the vertical yz plane, with the same mounting structure and function as reflector IV7.

[0022] The nonlinear crystal 8 is mounted at the center of the rotating mirror frame 23, which is fixed at the center of the rotating stage III 24. The rotating stage III 24 is mounted at the center of the translation stage III 25. The nonlinear crystal 8 can rotate around the normal direction of the crystal surface via the rotating mirror frame 23, rotate around the x-axis via the rotating stage III 24, and move up and down around the y-axis via the translation stage III 25. These measures ensure that the three incident beams intersect at the center of the nonlinear crystal 8 and satisfy the angular position matching relationship.

[0023] The light guide mirror assembly 15 consists of orthogonally aligned reflectors IX15-1 and X15-2, a vertical guide rail 15-3, and a rotating stage IV15-4. Reflectors IX15-1 and X15-2 are sequentially arranged along the fundamental frequency laser pulse transmission direction. Reflector IX15-1 reflects the incident beam from the horizontal x-axis direction vertically upward along the y-axis direction, and reflector X15-2 reflects the vertically upward beam from reflector IX15-1. The emitted beam is obliquely reflected downwards in the yz vertical plane to the nonlinear crystal 8; the reflector X15-2 is mounted at the center of the rotating stage IV15-4, which can rotate along the x-axis. The rotating stage IV15-4 is fixed on the vertical guide rail 15-3 and can move up and down along the y-axis. The combination of the rotating stage IV15-4 and the vertical guide rail 15-3 allows adjustment of the position and orientation of the reflector X15-2, ensuring the angle α between the emitted beam and the xz horizontal plane. Figure 2 As shown.

[0024] The basic principle of single-shot femtosecond laser pulse waveform measurement in this invention is as follows: utilizing the third-order nonlinear effect of a nonlinear crystal, and employing a three-dimensional non-collinear angular position matching method, a directly measurable dual-delay third-order intensity correlation signal G is obtained. (3) (x,y) is then transformed into a double-delay third-order intensity time correlation function G through a simple time-space coordinate transformation. (3) (τ1,τ2), the pulse time waveform I(t) can be extracted according to the recursive algorithm.

[0025] The basic principle for generating a double-delay third-order spatially correlated signal is:

[0026] For negative uniaxial crystals, there are three possible site matching conditions: Type I (ooo→e), Type II (ooe→e), and Type III (oee→e). Type I will be explained below.

[0027] like Figure 3 As shown, assuming R is chosen 3C The crystal with point groups is cut in the following way: x′y′z′ is the crystal coordinate system, and xyz is the beam coordinate system. The generated third-harmonic e-beam is output along the z-axis, and the crystal optical axis z′ is along the y-axis. The three beams are incident on the nonlinear crystal along the AO, CO, and DO directions, respectively, with corresponding wave vectors k. i The azimuth angle in the x′y′z′ coordinate system is In the xyz coordinate system The effective nonlinear coefficients are calculated to be:

[0028]

[0029] Select Since the generated third harmonic is relatively weak, it can be assumed that the three beams being measured are not consumed, that is, the input field strengths A1, A2, and A3 are assumed to be constant. Only the coupling equations generated by the third harmonic need to be solved:

[0030]

[0031] In the above equation, u1 is the group velocity of the fundamental frequency beam within the crystal. Considering the non-collinear position matching condition and the transverse group delay of the beam in the crystal, assuming that the incident angles i of the three beams are equal, and given that the crystal thickness d is sufficiently thin to neglect the longitudinal group delay, the solution to the above equation is:

[0032]

[0033] In the formula, c is the speed of light in a vacuum, then the signal recorded by the CCD is:

[0034]

[0035] Therefore, the signal recorded by the CCD is the double-delay third-order intensity-correlated signal of the measured pulse I(t).

[0036] For G (3) By performing a two-dimensional Fourier transform on (τ1, τ=τ2-τ1 / 2), the Fourier spectrum of the measured pulse I(t) can be obtained:

[0037]

[0038]

[0039] I know |I(ν)| and The pulse intensity distribution I(t) can be recovered by performing an inverse Fourier transform.

[0040]

[0041] In this embodiment, the incident laser pulse has a center wavelength of 1030 nm, a pulse width of approximately 0.1 ps, an energy of approximately 10 mJ, a beam aperture of 1 cm, vertical polarization, and the nonlinear crystal 8 is made of β-BBO material. The cutting method is selected as follows: Type I (ooo→e) position matching is adopted. The incident angle of the three beams is i≈30°, where the angle between the two horizontal beams is approximately 51°, and the angle between the third beam and the horizontal plane is α≈46°. The three beams overlap within the nonlinear crystal 8. By adjusting delay modulators I3, II10, and III14, the overlap center is made to achieve equal optical path length. The resulting third harmonic beam is output along the normal direction of the surface of the nonlinear crystal 8. At this time, the third harmonic beam recorded by CCD18 is the double-delay third-order intensity correlation signal of the measured pulse. Here, i ≈ 30°, c = 2.9979 × 10 8 m / s = 0.29979 μm / fs. Finally, the data is processed by computer according to the above principle to obtain the laser pulse waveform distribution I(t).

Claims

1. A single-shot femtosecond laser pulse waveform measurement device, characterized by: The device is characterized in that: a beam splitter I (1) and a beam splitter II (2) are arranged in the direction of the incident of the spatially uniform measured femtosecond laser pulse in the vertical polarization in sequence; the measured femtosecond laser pulse is divided into transmitted light and reflected light by the beam splitter I (1), and the transmitted light is divided into transmitted light and reflected light again by the beam splitter II (2); a delay adjuster I (3), a mirror I (4), a mirror II (5), a mirror III (6), a mirror IV (7) and a nonlinear crystal (8) are arranged in the reflected light path of the beam splitter I (1) in sequence; the light beam reflected from the beam splitter I (1) is adjusted in optical path by the delay adjuster I (3) and then transmitted by the mirror I (4), the mirror II (5), the mirror III (6) and the mirror IV (7) in sequence and then projected to the nonlinear crystal (8); a mirror V (9), a delay adjuster II (10), a mirror VI (11), a mirror VII (12) and the nonlinear crystal (8) are arranged in the reflected light path of the beam splitter II (2); the light beam reflected from the beam splitter II (2) is reflected to the delay adjuster II (10) by the mirror V (9) and then transmitted by the mirror VI (11) and the mirror VII (12) in sequence and then projected to the nonlinear crystal (8); a mirror VIII (13), a delay adjuster III (14), a light guide mirror group (15) and the nonlinear crystal (8) are arranged in the transmitted light path of the beam splitter II (2) in sequence; the light beam transmitted from the beam splitter II (2) is reflected to the delay adjuster III (14) by the mirror VIII (13) and then projected to the light guide mirror group (15), the light beam emitted from the light guide mirror group (15) is obliquely projected to the nonlinear crystal (8) in the direction of the surface normal of the nonlinear crystal (8), and the light beams from the light guide mirror group (15), the mirror IV (7) and the mirror VII (12) intersect in the nonlinear crystal (8) and are converted by three times frequency conversion; an attenuation filter (16), an imaging lens group (17) and a CCD (18) are arranged in the direction of the surface normal of the nonlinear crystal (8) in sequence; the three times frequency conversion light beam output from the nonlinear crystal (8) is attenuated by the attenuation filter (16) and then enters the imaging lens group (17), and the imaging lens group (17) images the three times frequency conversion light beam on the surface of the nonlinear crystal (8) to the CCD (18); a computer is connected to the CCD (18), and the signal from the CCD (18) is finally input to the computer for data processing. The nonlinear crystal (8) is a single nonlinear crystal; the mirror IV is installed at the center of a rotating table I, and the rotating table I is installed at the center of a translation table I; the mirror VII has the same structure and function as the mirror IV; the nonlinear crystal can be rotated around the direction of the surface normal of the crystal through a rotating mirror frame, rotated around the x-axis direction through a rotating table III and moved up and down around the y-axis direction through a translation table III.

2. The apparatus for measuring a waveform of a single femtosecond laser pulse according to claim 1, characterized by: The mirror IV (7) is installed at the center of the rotating table I (19), and the rotating table I (19) is installed at the center of the translation table I (20); the rotating table I (19) and the translation table I (20) are combined to enable the mirror IV (7) to rotate along the vertical y-axis direction and to translate along the x-axis direction; the mirror VII (12) is placed in mirror symmetry with the mirror IV (7) along the yz vertical plane, and has the same installation structure and the same function as the mirror IV (7).

3. The apparatus for measuring a waveform of a single femtosecond laser pulse according to claim 1, wherein: The nonlinear crystal (8) is installed at the center of the rotating mirror frame (23), the rotating mirror frame (23) is fixed at the center of the rotating table III (24), and the rotating table III (24) is installed at the center of the translation table III (25); the nonlinear crystal (8) can rotate around the normal direction of the crystal surface through the rotating mirror frame (23), rotate around the x-axis direction through the rotating table III (24), and move up and down around the y-axis direction through the translation table III (25).

4. The apparatus for measuring a waveform of a single femtosecond laser pulse according to claim 1, wherein: The light guide mirror group (15) is composed of the mirror IX (15-1), the mirror X (15-2), the vertical guide rail (15-3) and the rotating table IV (15-4) which are orthogonal to each other; the mirror IX (15-1) and the mirror X (15-2) are sequentially arranged in the transmission direction of the fundamental laser pulse; the mirror IX (15-1) reflects the incident light beam from the horizontal x-axis direction vertically upward along the y-axis direction, and the mirror X (15-2) reflects the vertically upward reflected light beam from the mirror IX (15-1) obliquely downward in the yz vertical plane to the nonlinear crystal (8); the mirror X (15-2) is installed at the center of the rotating table IV (15-4), the rotating table IV (15-4) can rotate along the x-axis direction, the rotating table IV (15-4) is fixed on the vertical guide rail (15-3) and can move up and down along the y-axis direction, and the rotating table IV (15-4) and the vertical guide rail (15-3) are combined to adjust the pose of the mirror X (15-2).

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