Single femtosecond laser pulse waveform measuring device

Abstract

The invention discloses a single femtosecond laser pulse waveform measuring device. A measured vertically-polarized space-uniform measured femtosecond laser pulse is divided into transmission light and reflected light after passing through a spectroscope, the transmission light is divided into two beams, and the three beams are incident to a nonlinear crystal at the same angle and equal optical path along the edge of a regular triangular prism for third-order nonlinear harmonic conversion; the generated third harmonic light beam is a double-delay third-order intensity related signal of the measured pulse after space-time transformation, and pulse waveform information is obtained through image decoding. The measuring device improves time resolution, and is low in cost and convenient to adjust.

Description

Technical Field

[0001] The invention belongs to the technical field of ultrafast pulse laser testing, and in particular relates to a single femtosecond laser pulse waveform measuring device. Background Art

[0002] The time waveform of the pulse can be accurately extracted by using the double-delayed third-order intensity correlation signal of the femtosecond laser pulse. This method is another ultrafast pulse waveform measurement method different from FROG and SPIDER. The invention patent named "Laser pulse waveform measurement device based on third-order correlation method" (patent number: ZL 201610549016.7) and the invention patent named "An ultrashort laser pulse waveform measurement device" (patent number: ZL 201610548538.5) disclose a method for obtaining a pulse waveform by measuring a double-delayed third-order intensity correlation signal. These devices use a cascaded second-order nonlinear harmonic transformation technology of first doubling the frequency and then summing the frequency to obtain a double-delayed third-order intensity correlation signal, which requires two nonlinear transformations, which increases the difficulty of debugging on the one hand and reduces the time resolution of the device measurement on the other. Summary of the invention

[0003] In order to overcome the shortcomings of the existing measurement technology in the measurement of femtosecond laser pulse waveform, such as complex debugging and limited time resolution, the present invention provides a single femtosecond laser pulse waveform measurement device.

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

[0005] The invention discloses a single-shot femtosecond laser pulse waveform measuring device, which is characterized in that, in the measuring device, a spectroscope I and a spectroscope II are sequentially arranged in the incident direction of a vertically polarized spatially uniform femtosecond laser pulse to be measured; the femtosecond laser pulse to be measured is divided into transmitted light and reflected light by the spectroscope I, and the transmitted light is again divided into transmitted light and reflected light by the spectroscope II; a delay regulator 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 the spectroscope I; a light beam reflected from the spectroscope I is adjusted for optical path length by the delay regulator I, and then sequentially transmitted through the reflectors I, II, III, and IV, and then projected to the nonlinear crystal; a reflector V, a delay regulator II, a reflector VI, a reflector VII, and a nonlinear crystal are arranged on the reflected light path of the spectroscope II; a light beam reflected from the spectroscope II is adjusted for optical path length by the reflector V, and then reflected and transmitted through the reflectors I, II, III, and IV, and then projected to the nonlinear crystal; a reflector V, a delay regulator II, a reflector VI, a reflector VII, and a nonlinear crystal are arranged on the reflected light path of the spectroscope II; a light beam reflected from the spectroscope II is adjusted for optical path length by the reflector V, and then reflected and transmitted through the reflectors I, II, III, and IV, and then projected to the nonlinear crystal. It is reflected and transmitted by reflectors VI and VII in sequence and then projected onto a nonlinear crystal; reflectors VIII, delay adjusters III, light guide mirrors and nonlinear crystals are arranged in sequence on the transmission light path of the beam splitter II; the light beam transmitted from the beam splitter II is reflected by reflectors VIII to delay adjusters III for optical path adjustment and then projected onto the light guide mirrors; the light beam emitted from the light guide mirrors and raised is projected obliquely downward onto the nonlinear crystal; the light beam from the light guide mirrors intersects with the light beams from reflectors IV and VII in the nonlinear crystal and undergoes triple frequency conversion; an attenuation filter, an imaging mirror group and a CCD are arranged in sequence behind the nonlinear crystal along the surface normal direction; the triple frequency light beam output from the surface normal direction of the nonlinear crystal is attenuated and filtered by the attenuation filter and then enters the imaging mirror group, and the imaging mirror group forms a near-field image of the triple frequency light beam on the surface of the nonlinear crystal onto the CCD; the CCD is externally connected to a computer, and the signal from the CCD finally enters the computer for data processing.

[0006] The reflector IV is installed at the center of the rotating table I, and the rotating table I is installed at the center of the translation table I; the combination of the rotating table I and the translation table I can make the reflector IV rotate along the vertical y-axis direction and translate along the x-axis direction, thereby ensuring the angle accuracy of the outgoing light beam and the yz vertical plane.

[0007] The reflector VII is placed in mirror symmetry with the reflector IV along the yz vertical plane, and has the same mounting structure and function as the reflector IV.

[0008] The nonlinear crystal is installed at the center of a rotating mirror frame, the rotating mirror frame is fixed at the center of a rotating platform III, and the rotating platform III is installed at the center of a translation platform III; the nonlinear crystal can be rotated around the normal direction of the crystal surface through the rotating mirror frame, can be rotated around the x-axis direction through the rotating platform III, and can be moved up and down around the y-axis direction through the translation platform III; the measures ensure that the three incident light beams intersect at the center of the nonlinear crystal and satisfy the angular phase matching relationship.

[0009] The light guide mirror group is composed of a reflector IX, a reflector X, a vertical guide rail, and a rotating platform IV which are orthogonal to each other. The reflector IX and the reflector X are arranged in sequence in the transmission direction of the fundamental frequency laser pulse. The reflector IX reflects the incident light beam from the horizontal x-axis direction vertically upward along the y-axis direction, and the reflector X reflects the vertically upward reflected light beam from the reflector IX obliquely downward in the yz vertical plane to the nonlinear crystal. The reflector X is installed at the center of the rotating platform IV, the rotating platform IV can rotate along the x-axis direction, the rotating platform IV is fixed on the vertical guide rail, and can move up and down along the y-axis direction. The rotating platform IV and the vertical guide rail are combined to adjust the posture of the reflector X to ensure the angle α between the outgoing light beam and the xz horizontal plane.

[0010] The beneficial effects of the present invention are:

[0011] 1. The measuring device of the present invention adopts a single nonlinear crystal, which is low in cost. The use of a single nonlinear crystal shortens the nonlinear conversion distance, increases the bandwidth, and thus improves the time resolution.

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

[0013] 3. The measuring device of the present invention adopts a rotating nonlinear crystal to adapt to the polarization state of the incident measured light beam, and there is no need to insert a polarization element, thereby improving the compactness and adaptability of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic diagram of the optical path of a single-shot femtosecond laser pulse waveform measurement device of the present invention;

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

[0016] Figure 3 Uniaxial crystal coordinate representation;

[0017] In the figure, 1. spectroscope I 2. spectroscope II 3. delay adjuster I 4. reflector I 5. reflector II 6. reflector III 7. reflector IV 8. nonlinear crystal 9. reflector V 10. delay adjuster II 11. reflector VI 12. reflector VII 13. reflector 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. reflector IX 15-2. reflector X 15-3. vertical guide rail 15-4. rotating stage IV. DETAILED DESCRIPTION

[0018] The present invention is further described below in conjunction with the accompanying drawings and embodiments, but the protection scope of the present invention should not be limited thereto.

[0019] Example 1

[0020] Figure 1 It is a schematic diagram of a single-shot femtosecond laser pulse waveform measurement device of the present invention; Figure 2 is a schematic diagram of the optical path of the light guide mirror assembly in the present invention, Figure 1 A side view of the light guide mirror assembly in FIG. Figure 1 , Figure 2 In a single-shot femtosecond laser pulse waveform measuring device of the present invention, a spectroscope Ⅰ1 and a spectroscope Ⅱ2 are sequentially arranged in the incident direction of a vertically polarized spatially uniform femtosecond laser pulse to be measured; the femtosecond laser pulse to be measured is divided into transmitted light and reflected light by the spectroscope Ⅰ1, and the transmitted light is further divided into transmitted light and reflected light by the spectroscope Ⅱ2; a delay regulator Ⅰ3, a reflector Ⅰ4, a reflector Ⅱ5, a reflector Ⅲ6, a reflector Ⅳ7, and a nonlinear crystal 8 are sequentially arranged on the reflected light path of the spectroscope Ⅰ1 ; The light beam reflected from the beam splitter Ⅰ1 is adjusted for optical path length by the delay adjuster Ⅰ3, and then is reflected and transmitted by the reflector Ⅰ4, the reflector Ⅱ5, the reflector Ⅲ6 and the reflector Ⅳ7 in sequence, and then is projected to the nonlinear crystal 8; the reflective light path of the beam splitter Ⅱ2 is provided with a reflector Ⅴ9, a delay adjuster Ⅱ10, a reflector Ⅵ11, a reflector Ⅶ12, and a nonlinear crystal 8; the light beam reflected from the beam splitter Ⅱ2 is reflected by the reflector Ⅴ9 to the delay adjuster Ⅱ10, and then is adjusted for optical path length by the reflector Ⅵ11, the reflector Ⅶ1 2 is reflected and transmitted, and then projected to the nonlinear crystal 8; on the transmission light path of the beam splitter II 2, a reflector VIII 13, a delay adjuster III 14, a light guide mirror group 15, and a nonlinear crystal 8 are arranged in sequence; the light 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, and then projected to the light guide mirror group 15, and the light beam emitted from the light guide mirror group 15 and raised is projected obliquely downward to the nonlinear crystal 8, and the light beam from the light guide mirror group 15 is combined with the light beams from the reflector IV 7 and the reflector VII 12. The beams intersect in the nonlinear crystal 8 and undergo triple frequency conversion; an attenuation filter 16, an imaging lens group 17, and a CCD 18 are sequentially arranged behind the nonlinear crystal 8 along the surface normal direction; the triple frequency beam output from the surface normal direction of the nonlinear crystal 8 enters the imaging lens group 17 after attenuation filtering by the attenuation filter 16, and the imaging lens group 17 forms a near-field image of the triple frequency beam on the surface of the nonlinear crystal 8 to the CCD 18; the CCD 18 is externally connected to a computer, and the signal from the CCD 18 finally enters the computer for data processing.

[0021] The reflector IV7 is installed at the center of the rotating platform I19, and the rotating platform I19 is installed at the center of the translation platform I20; the combination of the rotating platform I19 and the translation platform I20 can make the reflector IV7 rotate along the vertical y-axis direction and translate along the x-axis direction to ensure the angle accuracy between the outgoing light beam and the yz vertical plane. The reflector VII12 is placed in a mirror-symmetrical manner with the reflector IV7 along the yz vertical plane, and has the same installation structure and function as the reflector IV7.

[0022] 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, can rotate around the x-axis direction through the rotating table III 24, and can move up and down around the y-axis direction through the translation table III 25; the above measures ensure that the three incident light beams intersect at the center of the nonlinear crystal 8 and satisfy the angular phase matching relationship.

[0023] The light guide mirror group 15 is composed of a reflector IX 15-1, a reflector X 15-2, a vertical guide rail 15-3, and a rotating table IV 15-4, which are orthogonal to each other. The reflector IX 15-1 and the reflector X 15-2 are arranged in sequence in the transmission direction of the fundamental frequency laser pulse. The reflector IX 15-1 reflects the incident light beam from the horizontal x-axis direction vertically upward along the y-axis direction, and the reflector X 15-2 reflects the incident light beam from the reflector IX 15-1 vertically upward along the y-axis direction. The incident light beam is reflected obliquely downward in the yz vertical plane to the nonlinear crystal 8; the reflector X15-2 is installed at the center of the rotating table IV15-4, the rotating table IV15-4 can rotate along the x-axis direction, the rotating table IV15-4 is fixed on the vertical guide rail 15-3, and can move up and down along the y-axis direction. The rotating table IV15-4 and the vertical guide rail 15-3 are combined to adjust the position of the reflector X15-2 to ensure that the angle α between the outgoing light beam and the xz horizontal plane is, such as Figure 2 shown.

[0024] The basic principle of the single femtosecond laser pulse waveform measurement of the present invention is to utilize the third-order nonlinear effect of the nonlinear crystal and obtain a directly measurable double-delayed third-order intensity correlation signal G by means of three-dimensional non-collinear angle phase matching. (3) (x, y), and then converted into a double-delayed 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 of generating a double-delayed third-order spatial correlation signal is:

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

[0027] like Figure 3 As shown, assuming that R 3C The crystal of point group is cut in x′y′z′ is the crystal coordinate system, xyz is the beam coordinate system, the generated triple frequency e light is output along the z axis, the crystal optical axis z′ is along the y axis, and the three beams are incident on the nonlinear crystal along the AO, CO, and DO directions respectively. The corresponding wave vector k i The azimuth in the x′y′z′ coordinate system is In the xyz coordinate system The calculated effective nonlinear coefficient is:

[0028]

[0029] Select Since the generated third harmonic is weak, it can be considered that the three input beams to be measured are not consumed, that is, the input field strengths A1, A2, and A3 are assumed to be constants, and only the coupling equation for the third harmonic generation needs to be solved:

[0030]

[0031] In the above formula, u1 is the group velocity of the fundamental frequency measured beam in the crystal. Taking into account the non-collinear phase matching conditions and the transverse group delay of the beam in the crystal, assuming that the incident angles i of the three beams are equal, and when the crystal thickness d is thin enough to ignore the longitudinal group delay, the solution of the above formula is:

[0032]

[0033] Where 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-delayed third-order intensity correlation signal of the measured pulse I(t).

[0036] For G (3) (τ1,τ=τ2-τ1 / 2) is transformed into two dimensions, and the Fourier spectrum of the measured pulse I(t) can be obtained:

[0037]

[0038]

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

[0040]

[0041] In this embodiment, the incident laser pulse has a central wavelength of 1030 nm, a pulse width of about 0.1 ps, an energy of about 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) phase matching is adopted. The incident angle i of the three beams is ≈ 30°, of which the angle between the two horizontal beams is about 51°, and the angle between the third beam and the horizontal plane is α ≈ 46°. The three beams overlap in the nonlinear crystal 8. The delay regulator I3, delay regulator II10, and delay regulator III14 are adjusted to make the overlapping center reach equal optical path. The generated triple frequency beam is output along the normal direction of the surface of the nonlinear crystal 8. At this time, the triple frequency beam recorded by CCD18 is the double-delayed 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 in that: In the device, a beam splitter I (1) and a beam splitter II (2) are sequentially arranged in the incident direction of a vertically polarized spatially uniform femtosecond laser pulse to be measured; the femtosecond laser pulse to be measured is divided into a transmitted light and a reflected light by the beam splitter I (1), and the transmitted light is further divided into a transmitted light and a reflected light by the beam splitter II (2); a delay regulator I (3), a reflector I (4), a reflector II (5), a reflector III (6), a reflector IV (7), and a nonlinear crystal (8) are sequentially arranged on the reflected light path of the beam splitter I (1); the light beam reflected from the beam splitter I (1) is delayed by the delay regulator I (3). After adjusting the optical path of the beam splitter Ⅱ (2), the beam is reflected and transmitted through the reflector Ⅰ (4), the reflector Ⅱ (5), the reflector Ⅲ (6) and the reflector Ⅳ (7) in sequence and then projected to the nonlinear crystal (8); a reflector Ⅴ (9), a delay adjuster Ⅱ (10), a reflector Ⅵ (11), a reflector Ⅶ (12) and a nonlinear crystal (8) are arranged on the reflected optical path of the beam splitter Ⅱ (2); the beam reflected from the beam splitter Ⅱ (2) is reflected by the reflector Ⅴ (9) to the delay adjuster Ⅱ (10), and then is reflected and transmitted through the reflector VI (11) and the reflector VII (12) in sequence and then projected to the nonlinear crystal (8). to the nonlinear crystal (8); a reflector VIII (13), a delay adjuster III (14), a light guide mirror group (15), and a nonlinear crystal (8) are arranged in sequence on the transmission light path of the beam splitter II (2); the light 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 and then projected to the light guide mirror group (15); the light beam emitted from the light guide mirror group (15) and raised is projected obliquely downward to the nonlinear crystal (8); the light beam from the light guide mirror group (15) and the light beams from the reflector IV (7) and the reflector VII (12) are aligned in the beam path. The nonlinear crystal (8) intersects and performs triple frequency conversion; an attenuation filter (16), an imaging lens group (17), and a CCD (18) are sequentially arranged behind the nonlinear crystal (8) along the surface normal direction; a triple frequency light beam output from the surface normal direction of the nonlinear crystal (8) enters the imaging lens group (17) after attenuation filtering by the attenuation filter (16), and the imaging lens group (17) forms a near-field image of the triple frequency light beam on the surface of the nonlinear crystal (8) to the CCD (18); the CCD (18) is externally connected to a computer, and the signal from the CCD (18) finally enters the computer for data processing.

2. The single-shot femtosecond laser pulse waveform measuring device according to claim 1, characterized in that: The reflector IV (7) is mounted at the center of the rotating platform I (19), and the rotating platform I (19) is mounted at the center of the translation platform I (20); the rotating platform I (19) and the translation platform I (20) are combined to enable the reflector IV (7) to rotate along the vertical y-axis direction and translate along the x-axis direction; the reflector VII (12) and the reflector IV (7) are placed in a mirror-symmetrical manner along the yz vertical plane, and the mounting structure and function are the same as those of the reflector IV (7).

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

4. The single-shot femtosecond laser pulse waveform measuring device according to claim 1, characterized in that: The light guide mirror group (15) is composed of a reflector IX (15-1), a reflector X (15-2), a vertical guide rail (15-3), and a rotating table IV (15-4) which are orthogonal to each other. The reflector IX (15-1) and the reflector X (15-2) are arranged in sequence in the transmission direction of the fundamental frequency laser pulse. The reflector IX (15-1) reflects the incident light beam from the horizontal x-axis direction vertically upward along the y-axis direction, and the reflector X (15-2) reflects the incident light beam from the reflector IX (15-1) vertically upward along the y-axis direction. The vertically upward reflected light beam of the optical fiber 5-1 is reflected obliquely downward in the yz vertical plane to the nonlinear crystal (8); the reflector 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. The rotating table IV (15-4) and the vertical guide rail (15-3) are combined to adjust the position of the reflector X (15-2).

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