High-power laser spectrum shaping device and method based on multi-pass structure

Abstract

The invention discloses a high-power laser spectrum shaping device and method based on a multi-pass structure, and relates to the technical field of laser. The device comprises an input unit, an output unit and a multi-way cavity, the input unit comprises a mode coupling lens, a reflecting mirror and an input mirror; the multi-pass cavity comprises a first reflecting mirror, a polarization selection element, a birefringent crystal and a second reflecting mirror; the output unit comprises an output mirror and a reflecting mirror; the multi-way cavity is used for realizing space folding of a laser optical path; and the output mirror is used for controlling the propagation range number of the laser in the multi-way cavity. According to the invention, nonlinear phase shift is accumulated in the material to realize wide-range spectrum broadening, and meanwhile, the birefringent crystal is used for spectrum shaping, so that the precise shaping capability of spectrum broadening is obtained. In addition, according to the technical scheme, a multi-pass structure is adopted, high-power laser can repeatedly pass through the same element to achieve large-scale broadening and shaping of the spectrum, the space size of the device is compressed, the number of optical elements is reduced, and cost is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser technology, in particular to a high-power laser spectrum shaping device and method based on a multi-pass structure. BACKGROUND

[0002] The spectrum shaping technology of high-power laser has considerable research value in research and application fields. On the one hand, according to the time-bandwidth product theory, the pulse width of an ultrashort pulse is limited by the spectral width, and the effective spectral broadening is a prerequisite for obtaining shorter pulses; on the other hand, precise spectral shape control will greatly support the application of high-power laser in the fields of coherent imaging, spectral detection, laser filamentation and other frontiers. The existing high-power laser spectrum shaping methods mainly include a spectrum filtering method based on birefringence and a pulse shaping method based on Fourier transform, the former is mainly used to compensate for the gain narrowing effect in the laser amplification process, and the spectral width of the shaped spectrum is limited by the gain bandwidth of the medium; the latter is widely used to generate arbitrary waveforms, and the optical path is relatively complex, and expensive active devices such as spatial light modulators are used.

[0003] Therefore, it is necessary to further study a spectrum shaping system with simple structure and low cost to achieve the spectral broadening and precise modulation of high-power laser in the frequency domain. SUMMARY

[0004] The present application provides a high-power laser spectrum shaping device and method based on a multi-pass structure to achieve the spectral broadening and modulation of high-power incident laser.

[0005] The technical scheme adopted by the present application is as follows:

[0006] A high-power laser spectrum shaping device based on a multi-pass structure, the device comprising an input unit, an output unit and a multi-pass cavity; the input unit comprising a mode coupling lens 1, a mirror 2, an input mirror 3; the multi-pass cavity comprising a first mirror 6, a polarization selection element 4, a birefringent crystal 5 and a second mirror 7; the output unit comprising an output mirror 8 and a mirror 9; the multi-pass cavity is used to realize the spatial folding of the laser optical path; the incident laser passes through the mode coupling lens 1 to match the laser mode with the cavity mode of the multi-pass cavity, thereby reducing the diffraction loss of the incident laser during transmission in the multi-pass cavity, and then is injected into the multi-pass cavity by the mirror 2 and the input mirror 3, and propagates back and forth in the multi-pass cavity, and is output through the output mirror 8 and the mirror 9.

[0007] On the other hand, the present application also provides a high-power laser spectrum shaping method based on a multi-pass structure, the high-power laser spectrum shaping method being based on the aforementioned high-power laser spectrum shaping device, and the method comprising:

[0008] S1, using the first mirror 6 and the second mirror 7 to build a multi-pass cavity frame;

[0009] S2, taking the mode coupling lens 1, the mirror 2 and the input mirror 3 as a front-end input unit of the multi-pass cavity, taking the output mirror 8 and the mirror 9 as a rear-end output unit of the multi-pass cavity, and adjusting the pose of the input mirror 3;

[0010] S3, placing the spectral transmittance editable transmission element module composed of the polarization selection element 4 and the birefringent crystal 5 between the first mirror 6 and the second mirror 7 to form a multi-pass cavity for spatial folding of a laser optical path;

[0011] S4, adjusting the pitch attitude and the crystal angle of the birefringent crystal 5 to change the center wavelength and the depth of the filter curve;

[0012] S5, adjusting the position of the output mirror 8 to make the incident laser propagate in the multi-pass cavity for a target number of times and then be guided out, so as to control the spectral broadening and shaping effect.

[0013] As described above, due to the adoption of the technical solutions, the present application has the following advantages:

[0014] 1. The present application discloses a high-power laser spectrum shaping device and method based on a multi-pass structure, which can accumulate nonlinear phase shift in materials to achieve a wide range of spectral broadening, and use the adjustment of the birefringent crystal to shape the spectrum, thereby obtaining precise shaping ability of the broadened spectrum.

[0015] 2. The present application adopts a multi-pass structure to make high-power laser repeatedly pass through the same element to achieve a large amount of spectral broadening and shaping, thereby compressing the device space size, saving the number of optical elements and reducing the cost. BRIEF DESCRIPTION OF DRAWINGS

[0016] The present application will be described by examples and with reference to the accompanying drawings, in which:

[0017] Figure 1 is a structure schematic diagram of a laser spectrum shaping device provided by an embodiment of the present application.

[0018] Figure 2 is a spectral broadening schematic diagram provided by an embodiment of the present application.

[0019] Figure 3 is a spectral filtering schematic diagram provided by an embodiment of the present application.

[0020] Figure 4 is a spot acquisition schematic diagram provided by an embodiment of the present application.

[0021] Figure 5is a schematic diagram of a spectrum shaping result based on a multi-pass structure in the embodiment of the application.

[0022] In the figure: 1 - mode coupling lens, 2 - mirror, 3 - input mirror, 4 - polarization selection element, 5 - birefringent crystal, 6 - first mirror, 7 - second mirror, 8 - output mirror, 9 - mirror. DETAILED DESCRIPTION

[0023] In order for those skilled in the art to better understand the technical solutions of the present application, the technical solutions of the present application will be described clearly and completely below in combination with the drawings of the present application. Other similar embodiments obtained by those skilled in the art without making creative efforts on the premise that the embodiments in the present application should all belong to the scope of protection of the present application.

[0024] In the present application, the terms used are only for the purpose of describing specific example embodiments and are not intended to limit the present disclosure. Unless otherwise explicitly specified and limited, the terms "first", "second", etc. should be broadly understood, which does not represent the limitation of quantity and the fixation of order, but represents the existence of the technical features of the cited item. It is further understood that the terms "include" and / or "contain" are used in the specification to indicate the existence of the stated features, regions, integers, steps, operations, elements and / or components, but do not exclude the existence or addition of one or more other features, regions, integers, steps, operations, elements, components and / or combinations thereof. For those skilled in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances.

[0025] Embodiment 1

[0026] As shown in Figure 1 Embodiment 1 is a structural schematic diagram of a high-power laser spectrum shaping device based on a multi-pass structure, which includes a mode coupling lens 1, a mirror 2, an input mirror 3, a polarization selection element 4, a birefringent crystal 5, a first mirror 6, a second mirror 7, an output mirror 8 and a mirror 9.

[0027] The first mirror 6, the polarization selection element 4, the birefringent crystal 5 and the second mirror 7 constitute a multi-pass cavity for realizing spatial folding of the laser optical path. The eigenmode of the multi-pass cavity is determined by the curvature of the first mirror 6 and the second mirror 7, the Kerr lens at the polarization selection element 4 and the birefringent crystal 5, and the cavity length of the multi-pass cavity.

[0028] The polarization selection element 4 and the birefringent crystal 5 are located between the first mirror 6 and the second mirror 7 for spectrum broadening and spectrum shaping.

[0029] The incident laser passes through the mode coupling lens 1, the mode coupling lens 1 is adjusted to match the laser mode with the cavity mode of the multi-pass cavity, and the diffraction loss of the laser during transmission in the multi-pass cavity is reduced, and then the laser is injected into the multi-pass cavity by the mirror 2 and the input mirror 3.

[0030] The input mirror 3 and the output mirror 8 can be used to control the number of passes of the laser in the multi-pass cavity. By adjusting the position and attitude of the input mirror 3 and the output mirror 8, the incident laser is reflected multiple times between the first mirror 6 and the second mirror 7, so that the incident laser is transmitted in the multi-pass cavity for a target number of passes and then output through the output mirror 8.

[0031] As a necessary condition, in order to prevent the incident laser from interfering with the input mirror 3 during transmission in the multi-pass cavity, the physical size of the input mirror 3 and the matching mirror frame should be as small as possible under the premise of maintaining the integrity of the spot.

[0032] Embodiment 2

[0033] Specifically, for the aforementioned high-power laser spectrum shaping device based on the multi-pass structure, the high-power incident laser passes through the polarization selection element 4 and the birefringent crystal 5 during transmission in the multi-pass cavity, and a certain spectral broadening can be achieved due to the self-phase modulation effect (SPM) in the medium material.

[0034] The self-phase modulation effect is the phase shift caused by the light field itself when the light field is transmitted in the medium. In a nonlinear medium, such as the polarization selection element 4 and the birefringent crystal 5 in the present application, the refractive index of the medium is a function of the intensity of the incident light as follows:

[0035]

[0036] Where n is the refractive index of the medium, n0 is the linear refractive index of the medium, n2 is the nonlinear refractive index of the medium, and X (3) is related to the third-order tensor of the medium ε0 and c are the vacuum permittivity and the speed of light respectively, and I is the intensity of the laser light field. From the above formula, the new frequency generated by the self-phase modulation effect of the high-power laser passing through the nonlinear medium is:

[0037]

[0038] Where w0 is the angular frequency of the laser, and l is the length of the medium.

[0039] As can be seen from formula (2), the spectral broadening amount is positively related to the length l of the nonlinear medium passed through. Therefore, to obtain a large spectral broadening amount, the incident laser needs to pass through the multi-pass cavity for more passes, and after the broadening and shaping are accumulated after a certain number of passes, the spectrum is output by the output mirror 8.

[0040] Meanwhile, the polarization selective element 4 and the birefringent crystal 5 together form a transmissive element module with editable spectral transmittance, which filters and shapes the broadened spectrum. The filter curve is related to the thickness of the birefringent crystal 5, the angle (i.e. the rotation angle φ) between the principal axis of the birefringent crystal 5 and the polarization direction, and the tilt angle θ of the birefringent crystal 5. The optical path difference between o light and e light in the birefringent crystal 5 changes the polarization state of the incident linearly polarized light, and the optical path difference is related to the wavelength.

[0041] The birefringent crystal 5 in cooperation with the polarization selective element 4 can realize the function of spectral filtering. According to the crystal optical birefringence theory, the expression of the transmittance curve is as follows:

[0042]

[0043] where d is the thickness of the birefringent crystal 5, φ is the angle (rotation angle) between the principal axis of the birefringent crystal 5 and the polarization direction, θ is the tilt angle of the birefringent crystal 5, n is the refractive index of o light of the birefringent crystal 5, and λ is the wavelength. o o e is a function of n

[0044]

[0045] where n e is the refractive index of e light of the birefringent crystal 5, and n2 can be calculated from n o e

[0046]

[0047] As can be seen from the above formula, under the condition that the thickness and the crystal axis direction of the birefringent crystal 5 are fixed, the central wavelength and the depth of the filter curve can be changed by adjusting the pitch angle and the crystal angle of the birefringent crystal 5, so as to realize the precise modulation of the broadened spectrum in the multi-cavity.

[0048] Example 3

[0049] ​​​​​​For the foregoing spectral shaping device, the high-power incident laser propagates in the multi-pass cavity once, and a certain amount of spectral broadening and filtering shaping is generated. The shaped pulse is reflected by the first mirror 6 or the second mirror 7, and is input again as the next input laser to pass through the polarization selection element 4 and the birefringent crystal 5 for broadening and shaping. As can be seen from the formula in the foregoing embodiment, the spectral broadening amount is positively correlated with the length of the nonlinear medium passed through. That is, to obtain a large spectral broadening amount, the laser needs to propagate in the cavity for more passes. After a certain number of passes, the broadening and shaping are accumulated, and the spectral shaped pulse is output by the output mirror 8.

[0050] Therefore, embodiment 3 is used to describe the specific process of spectral shaping. In this embodiment, the optical parameters are designed as follows: the first mirror 6 and the second mirror 7 are concave mirrors with a curvature radius of 300 mm, and the distance between the first mirror 6 and the second mirror 7 is 500 mm; the thickness of the polarization selection element 4 is 5 mm; the thickness of the birefringent crystal 5 is 5 mm; the polarization selection element 4 and the birefringent crystal 5 are fused quartz crystals, and the distance between the two and the first mirror 6 and the second mirror 7 is 235 mm, considering the better spectral broadening effect and avoiding damage to the crystal caused by high laser power. In the birefringent crystal 5, n e is taken as 1.564299, and n o is taken as 1.53425. The incident laser is s linearly polarized, the center wavelength is 1030 nm, the pulse width is 500 fs, the spectral width is 4 nm, and the single pulse energy is 50 μJ.

[0051] The entire spectral shaping process includes: high-power laser is injected into the multi-pass cavity through the mode coupling lens 1 and the input mirror 3, and experiences SPM spectral broadening, spectral transmission shaping, and back-and-forth propagation on the medium and the cavity mirror.

[0052] Process one: spectral broadening is realized in the medium by SPM effect.

[0053] SPM effect is the phase shift of the light field caused by the light field itself during transmission in the medium. In this example, the laser will produce a nonlinear phase shift through the polarization selection element 4 and the birefringent crystal 5. The spectral broadening of the original spectrum after passing through the polarization selection element 4 and the birefringent crystal 5 once and after 6 times of cumulative passing is shown in Figure 2 The result is calculated by solving the nonlinear Schrödinger equation by the distributed Fourier algorithm, that is, the nonlinear Schrödinger equation is decomposed into a linear operator and a nonlinear operator in a very small space-time scale, and the evolution of the wave function is obtained in the Fourier space and the real space, respectively.

[0054] It can be seen that through appropriate optical design, SPM effect can achieve a large amount of spectral broadening of the incident laser.

[0055] Process two: spectral shaping is realized by birefringence effect.

[0056] The birefringent crystal 5 changes the polarization state of the incident light by using the optical path difference of o light and e light, and cooperates with the polarization selection element 4 to change the transmittance of the whole transmissive element module. Due to material dispersion, the optical path difference is related to the wavelength, so the transmittance of the transmissive element module changes with the wavelength of the incident light, thereby performing the function of spectral filtering. In the case of a certain thickness of the birefringent crystal 5, the spectral filtering curve can be edited by adjusting the angle (i.e. the rotation angle φ) between the crystal axis of the birefringent crystal 5 and the polarization direction, and the inclination angle θ of the birefringent crystal 5, and the adjustment effect is as shown in Figure 3 .

[0057] It should be noted that although the filtering curve is a complex function of the inclination angle θ and the rotation angle φ, there is a significant difference in the effect of the two on the filtering curve. Therefore, the filtering depth of the filtering curve can be changed by adjusting the crystal rotation angle φ, and the center wavelength of the filtering curve can be changed by adjusting the crystal inclination angle θ.

[0058] Process three: the laser propagates in the cavity in multiple passes.

[0059] The multi-pass cavity is designed based on the principle of the Herriott gas absorption cell, which greatly saves the space size and the number of optical elements. In this example, if the laser propagates in the multi-pass cavity for 14 passes, the simulated spot on the surface of the first mirror 6 and the second mirror 7 is as shown in Figure 4 , Figure 4 The number in the figure is the pass number of the laser propagation.

[0060] It can be seen that the physical position of the incident laser on the cavity mirror is different after different passes. Therefore, the position of the output mirror 8 can be adjusted to guide the laser out of the multi-pass cavity after a target number of passes in the multi-pass cavity, thereby controlling the spectral broadening and shaping effect.

[0061] In a preferred embodiment, the pass number and the spectral output are obtained by solving the nonlinear Schrödinger equation by the distribution Fourier algorithm, and the spectrum obtained after each pass through the SPM effect is multiplied by the filtering curve as the input for the next time to pass through the nonlinear medium again for spectral broadening, thereby obtaining by iteration.

[0062] In the improved design scheme of the present application, the high-power laser is incident on the multi-pass cavity, and the spectral broadening and filtering shaping are realized on the polarization selection element 4 and the birefringent crystal 5. After being reflected by the cavity mirror of the multi-pass cavity, the result of the previous spectral shaping is input again to pass through the polarization selection element 4 and the birefringent crystal 5, and the multi-pass cavity is guided out of the output mirror 8 after a target number of iterations. The spectral shaping result of the high-power laser obtained by the scheme provided in the embodiment of the present application is as shown in Figure 5As shown, by adjusting the position and posture of the relevant components in the device and the propagation distance of the incident laser in the multi-pass cavity, the spectral shaping result of the final output can be controlled, realizing large-range expansion and precise shaping of high-power laser.

[0063] Embodiment 4

[0064] Further, the embodiment makes a detailed description of the method for debugging and spectral shaping of the high-power laser spectral shaping device based on the multi-pass structure in the foregoing embodiments, including:

[0065] First, the first mirror 6 and the second mirror 7 form a multi-pass cavity frame;

[0066] Then, the input and output units of the multi-pass cavity structure are built, the mode coupling lens 1, the mirror 2, and the input mirror 3 are used as the front-end input unit of the multi-pass cavity, and the output mirror 8 and the mirror 9 are used as the rear-end output unit of the multi-pass cavity;

[0067] The mode coupling lens 1 is adjusted to match the incident laser with the multi-pass cavity eigenmode, so as to reduce the multi-pass transmission loss of the laser;

[0068] The pitch and roll of the mirror frame of the input mirror 3 are adjusted, so that the light spot is on the second mirror 7, and the incident laser is reflected multiple times between the first mirror 6 and the second mirror 7. When multiple reflection points are observed on the mirror surface 7 and the multiple reflection points form a circle, the position and posture of the input mirror 3 are optimal;

[0069] Then, the polarization selection element 4 and the birefringent crystal 5 are placed at the middle position between the first mirror 6 and the second mirror 7 at the Brewster angle, the pitch posture and crystal angle of the birefringent crystal 5 are adjusted, and the center wavelength and depth of the filter curve are changed.

[0070] The crystal angle refers to the angle between the crystal axis of the birefringent crystal 5 and the polarization direction, and at least includes the rotation angle and the inclination angle of the birefringent crystal (5);

[0071] The position of the output mirror 8 is adjusted, so that the incident laser is guided out after the target number of times of round trip propagation in the multi-pass cavity.

[0072] As a preferred embodiment, the output spectrum width can be adjusted by adjusting the rotation angle and / or the propagation distance of the birefringent crystal 5, and the center wavelength of the output spectrum can be controlled by adjusting the inclination angle of the birefringent crystal 5, so as to control the spectral expansion and shaping effect.

[0073] Preferably, when the multi-pass cavity is built, the first mirror 6 and the second mirror 7 are adjusted to be strictly parallel between the two mirror surfaces.

[0074] Compared with the prior art, the high-power laser spectrum shaping device and method based on a multi-pass structure and the high-precision laser cavity adjustment device and method provided in the embodiments of the present application can realize large-range spectrum broadening by accumulating nonlinear phase shift in a material, and can obtain precise shaping capability of the broadened spectrum by using a birefringent crystal for spectrum shaping. In addition, the present application adopts a multi-pass structure, so that the high-power laser repeatedly passes through the same element to realize large spectrum broadening and shaping, thereby compressing the device space size, saving the number of optical elements, and reducing the cost.

[0075] The above only describes the preferred embodiments of the present application, and does not limit the patent scope of the present application. Any equivalent structural transformation, direct / indirect application in other related technical fields, or the like made by using the content of the present application specification and drawings within the inventive concept of the present application is included in the patent protection scope of the present application.

[0076] Any feature disclosed in the present specification (including any accompanying claims, abstract) can be replaced by other equivalent or similar features having the same purpose, unless specifically stated otherwise. That is, each feature is only an example of a series of equivalent or similar features, unless specifically stated.

Claims

1. A high-power laser spectrum reshaping device based on a multi-pass structure, characterized in that, The device comprises an input unit, an output unit and a multi-pass cavity; the input unit comprises a mode coupling lens (1), a mirror (2) and an input mirror (3); the multi-pass cavity comprises a first mirror (6), a polarization selection element (4), a birefringent crystal (5) and a second mirror (7); the output unit comprises an output mirror (8) and a mirror (9); The multi-pass cavity is used for realizing spatial folding of a laser optical path; incident laser passes through the mode coupling lens (1) to match the laser mode with the cavity mode of the multi-pass cavity, is then injected into the multi-pass cavity by the mirror (2) and the input mirror (3) and propagates back and forth in the multi-pass cavity and is finally output through the output mirror (8) and the mirror (9).

2. The high power laser spectrum reshaping device based on multi-pass configuration according to claim 1, wherein, The polarization selection element (4) and the birefringent crystal (5) in the multi-pass cavity constitute a transmissive element module with editable spectral transmittance; the transmissive element module is located between the first mirror (6) and the second mirror (7) and is used for spectral widening and filtering.

3. A high power laser spectrum reshaping device based on a multi-pass configuration according to claim 2, wherein, High-power laser is incident from the input mirror (3), is spectrally widened and filtered by the transmissive element module, is reflected at the first mirror (6) / second mirror (7) of the multi-pass cavity, passes through the transmissive element module again, thereby propagates back and forth between the first mirror (6) / second mirror (7) of the multi-pass cavity and is guided out of the multi-pass cavity by the output mirror (8) after a target number of iterations of shaping the optical path.

4. A high power laser spectrum reshaping device based on a multi-pass configuration according to claim 3, wherein, When the high-power incident laser propagates in the multi-pass cavity once, the polarization selection element (4) and the birefringent crystal (5) of the transmissive element module cause a phase shift in the medium due to self-phase modulation, thereby realizing spectral widening.

5. A high power laser spectrum reshaping device based on a multi-pass configuration according to claim 3 or 4, characterized in that, The expression of the transmittance curve of the filtered spectrum through the transmissive element module is as follows: where d is the thickness of the birefringent crystal (5), is the angle of the principal axis of the birefringent crystal (5) and the polarizing direction (rotation angle), θ is the tilt angle of the birefringent crystal (5), n o is the refractive index of o light of the birefringent crystal (5), λ is the wavelength, f is n o , n e , θ and , which is expressed as follows: where n e is the refractive index of the e light of the birefringent crystal 5, n2is calculated from n o , n e , θ and ​ 6. A high power laser spectrum reshaping device based on a multi-pass configuration according to claim 2 or 3, wherein, Under the condition that the thickness and the crystal axis direction of the birefringent crystal (5) are fixed, the central wavelength and the depth of the filter curve are changed by adjusting the pitch angle and the crystal angle of the birefringent crystal (5), thereby realizing precise modulation of the widened spectrum in the multi-pass cavity.

7. A high power laser spectrum reshaping device based on a multi-pass configuration according to claim 6, wherein, The input mirror (3) and the output mirror (8) are used for controlling the number of passes of laser in the multi-pass cavity; by adjusting the relative pose of the mirror surface of the input mirror (3) and the cavity mirror, the output mirror (8) is used for reflecting the laser out of the multi-pass cavity after the laser propagates back and forth in the multi-pass cavity for a target number of times.

8. A high-power laser spectrum shaping method based on a multi-pass structure, characterized in that, The high-power laser spectrum shaping method is based on the high-power laser spectrum shaping device of any one of claims 1-7 and comprises the following steps: S1, using the first mirror (6) and the second mirror (7) to build a multi-pass cavity frame; S2, using the mode coupling lens (1), the mirror (2) and the input mirror (3) as the front-end input unit of the multi-pass cavity, using the output mirror (8) and the mirror (9) as the rear-end output unit of the multi-pass cavity and adjusting the pose of the input mirror (3); S3, placing the transmissive element module with editable spectral transmittance composed of the polarization selection element (4) and the birefringent crystal (5) between the first mirror (6) and the second mirror (7) to form a multi-pass cavity for spatial folding of a laser optical path; S4, adjusting the pitch attitude and the crystal angle of the birefringent crystal (5) to change the central wavelength and the depth of the filter curve; S5, adjusting the position of the output mirror (8) to make the incident laser propagate in the multi-pass cavity for a target number of round trips and then exit.

9. The method of claim 7, wherein the method further comprises: adjusting the laser cavity to achieve a desired laser cavity length. The crystal angle in step S6 refers to the rotation angle and the tilt angle of the birefringent crystal (5).

10. A high-precision laser cavity debugging method as described in claim 8, characterized in that, Step S2 further comprises adjusting the mode coupling lens (1) to match the incident laser with the eigenmodes of the multi-pass cavity to reduce the multi-pass transmission loss of the laser.

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