Nonlinear compression apparatus for low-energy ultrashort femtosecond pulse generation

Abstract

The utility model relates to a kind of nonlinear compression device for low-energy ultra-short femtosecond pulse generation, belong to nonlinear compression device field.The device includes: multi-cavity system, including chirped cavity mirror pair and nonlinear medium, the reflecting surface of chirped cavity mirror pair is designed as when laser pulse multiple reflection, the dispersion accumulation of nonlinear medium is synchronously cancelled;Incident coupling lens group, located in the laser incidence end of multi-cavity system, and for after spot control in suitable size and divergence angle injection into multi-cavity system;Ejection collimating lens group, located in the laser emission end of multi-cavity system, and for collimating the outgoing beam of multi-cavity system.The device by introducing the chirped cavity mirror pair with specific chirp characteristics, compensates the dispersion effect caused by nonlinear medium, without increasing the thickness of nonlinear medium, it can ensure that self-phase modulation effect always maintains higher intensity, to improve the broadening efficiency of pulse spectrum, and reduce the complexity and demand of subsequent dispersion compensation.

Description

Technical Field

[0001] This utility model relates to the field of nonlinear compression devices, and in particular to a nonlinear compression device for generating low-energy ultrashort femtosecond pulses. Background Technology

[0002] Conventional multi-cavity pulse compressors based on solid-state media are mainly used for laser pulses in the energy range of 1-30 µJ. They achieve pulse width compression through spectral broadening induced by the self-phase modulation (SPM) effect in the nonlinear solid-state medium. These compressors typically rely on the nonlinear medium, and the intensity of the SPM effect is closely related to the peak power density of the laser pulse. The higher the laser pulse energy, the stronger the SPM effect and the more pronounced the spectral broadening. Therefore, conventional multi-cavity systems perform well in pulse width compression for high-energy pulses. However, in the case of low-energy laser pulses (especially pulses below 1 µJ), traditional multi-cavity pulse compressors face challenges such as increased nonlinear medium thickness, weakened self-phase modulation effect, less pronounced pulse spectral broadening, and difficulties in dispersion compensation. Therefore, improvements are needed. Summary of the Invention

[0003] Therefore, it is necessary to provide a nonlinear compression device for generating low-energy ultrashort femtosecond pulses, addressing the problems faced by traditional multi-cavity pulse compressors in the case of low-energy laser pulses, such as increased nonlinear medium thickness, weakened self-phase modulation effect, less obvious pulse spectrum broadening, and difficulty in dispersion compensation.

[0004] This utility model provides a nonlinear compression device for generating low-energy ultrashort femtosecond pulses, comprising: A multi-cavity system includes a chirped mirror pair and a nonlinear medium, wherein the nonlinear medium is located between the chirped mirror pair and is a solid transparent material. The chirped mirror pair is used to pre-compensate for the dispersion effect caused by the nonlinear medium during laser pulse reflection. The reflective surface of the chirped mirror pair is designed to simultaneously cancel the dispersion accumulation of the nonlinear medium during multiple reflections of the laser pulse. An incident coupling lens group is located at the laser incident end of the multi-cavity system and is used to control the laser spot to a suitable size and divergence angle before injecting it into the multi-cavity system. The outgoing collimating lens group is located at the laser output end of the multi-cavity system and is used to collimate the outgoing beam of the multi-cavity system.

[0005] In one embodiment, the nonlinear medium is fused silica, calcium fluoride, or sapphire.

[0006] In one embodiment, the thickness of the nonlinear medium is 1 mm to 20 mm.

[0007] In one embodiment, the nonlinear compression device further includes a dispersion compensation module located at the laser emission end of the output collimating lens group.

[0008] In one embodiment, the dispersion compensation module is a chirped mirror pair, a prism pair, or a grating pair.

[0009] In one embodiment, the group delay dispersion of the chirped cavity mirror pair is matched to the thickness of the nonlinear medium and the number of reflections in the multi-channel cavity system.

[0010] In one embodiment, the group delay dispersion of the chirped cavity mirror pair ranges from -50 fs² to -1000 fs².

[0011] In one embodiment, the reflective surface of the chirped cavity mirror pair is coated with a chirped dielectric film, the dispersion curve of which is complementary to the dispersion characteristics of the nonlinear medium in the laser band.

[0012] The aforementioned nonlinear compression device for generating low-energy ultrashort femtosecond pulses introduces a pair of chirped cavity mirrors with specific chirped characteristics as the two end mirrors of a multi-cavity system to compensate for the dispersion effect caused by the nonlinear medium. This ensures that the self-phase modulation effect always maintains a high intensity without increasing the thickness of the nonlinear medium, thereby improving the pulse spectrum broadening efficiency and reducing the complexity and requirements of subsequent dispersion compensation. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in this utility model or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this utility model. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram of a nonlinear compression device for generating low-energy ultrashort femtosecond pulses in one embodiment. Figure 2 This is a B-integral evaluation curve after using the nonlinear compression device in one embodiment. Figure 3 The B-integral evaluation curve is shown after using a traditional nonlinear compression device. Figure 4 This is a graph showing the experimental data curves of the pulse width of this nonlinear compression device before compression. Figure 5 This is a graph showing the experimental data curves after pulse width compression of this nonlinear compression device.

[0015] Figure label: 100. Multi-cavity system; 110. Chirped cavity mirror pair; 120. Nonlinear medium; 200. Incident coupling lens group; 300. Outgoing collimating lens group; 400. Dispersion compensation module. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of this utility model clearer, the technical solutions of the embodiments of this utility model will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this utility model, not all embodiments. Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this utility model.

[0017] It should be noted that when a component is referred to as being "fixed to" or "set on" another component, it can be directly on the other component or there may be an intermediate component. When a component is considered to be "connected to" another component, it can be directly connected to the other component or there may be an intermediate component present. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used in this specification are for illustrative purposes only and do not represent the only possible implementation.

[0018] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this utility model, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0019] In this utility model, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature and the second feature are in indirect contact through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0020] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this specification belongs. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used in this specification includes any and all combinations of one or more of the associated listed items.

[0021] Conventional multi-cavity and solid-state dielectric-based pulse compressors are primarily used for laser pulses in the energy range of 1-30 µJ. They achieve pulse width compression through spectral broadening induced by the self-phase modulation (SPM) effect in the nonlinear solid-state dielectric. These compressors typically rely on the nonlinear dielectric, and the intensity of the SPM effect is closely related to the peak power density of the laser pulse. The higher the laser pulse energy, the stronger the SPM effect and the more pronounced the spectral broadening. Therefore, conventional multi-cavity systems perform well in pulse width compression for high-energy pulses. However, in the case of low-energy laser pulses (especially pulses below 1 µJ), traditional multi-cavity-based pulse compressors face the following problems and shortcomings: Increased thickness of nonlinear medium: Low-energy laser pulses usually require a thicker nonlinear medium to increase the interaction length between the laser pulse and the medium, which can lead to a very severe dispersion effect of the medium itself.

[0022] Pulse width broadening: As the thickness of the nonlinear medium increases and the number of multi-channel passes increases, the pulse width broadens rapidly, leading to a decrease in the peak power density of the pulse. In this case, the self-phase modulation effect weakens, and the pulse spectral broadening becomes less pronounced, failing to achieve the expected compression effect.

[0023] Dispersion compensation is difficult: Due to the inherent dispersion of the nonlinear medium, as the path number increases, the pulse requires a large amount of subsequent dispersion compensation. This process is very complex and inefficient, causing inconvenience to system design and operation.

[0024] Therefore, the following will combine Figures 1-5 This invention describes a nonlinear compression device for generating low-energy ultrashort femtosecond pulses.

[0025] like Figure 1 As shown, in one embodiment, a nonlinear compression device for generating low-energy ultrashort femtosecond pulses includes a multi-cavity system 100, an incident coupling lens group 200, and an exit collimating lens group 300.

[0026] The multi-cavity system 100 includes a chirped mirror pair 110 and a nonlinear medium 120. The nonlinear medium 120 is located between the chirped mirror pair 110 and is a solid transparent material. The chirped mirror pair 110 is used to pre-compensate for the dispersion effect caused by the nonlinear medium 120 during laser pulse reflection. The reflective surface of the chirped mirror pair 110 is designed to simultaneously cancel the dispersion accumulation of the nonlinear medium 120 during multiple reflections of the laser pulse.

[0027] It should be further explained that the chirped cavity mirror pair 110 is an optical element that can apply a specific chirp to the incident laser pulse. By properly designing the chirp amount of the chirped cavity mirror, the dispersion effect generated by the nonlinear medium 120 can be accurately compensated. The role of the chirped cavity mirror pair 110 in the multi-channel cavity system 100 is to adjust the temporal shape of the laser pulse in advance, reduce the pulse width expansion, and enable the pulse to maintain a high peak power and a continuous and effective self-phase modulation effect during multiple reflections.

[0028] Meanwhile, by using the chirped cavity mirror to compensate for the dispersion of 110, the thickness of the nonlinear medium 120 can be significantly reduced. In the case of low-energy laser pulses, traditional multi-cavity systems 100 usually require a thicker nonlinear medium 120 to enhance the self-phase modulation effect. However, a thick medium will lead to enhanced dispersion, which will affect the pulse width compression effect. By introducing the chirped cavity mirror to compensate for the dispersion of 110, a thinner nonlinear medium 120 can be used effectively, reducing the risk of pulse width expansion and improving the efficiency of the system.

[0029] In conventional devices, when the pulse energy is low, the dispersion effect of the nonlinear medium 120 is aggravated, which leads to the need for additional subsequent dispersion compensation operations. By using a chirped cavity mirror in the system to pre-compensate part of the dispersion of 110, the amount of dispersion required for subsequent compensation is greatly reduced, thereby simplifying the complexity of dispersion compensation and improving the stability of the pulse width compression process.

[0030] The incident coupling lens group 200 is located at the laser incident end of the multi-cavity system 100, and the incident coupling lens group 200 is used to inject the light spot into the multi-cavity system 100 after controlling the light spot to a suitable size and divergence angle. The exit collimating lens group 300 is located at the laser exit end of the multi-cavity system 100, and the exit collimating lens group 300 is used to collimate the exit beam of the multi-cavity system 100.

[0031] This nonlinear compression device for generating low-energy ultrashort femtosecond pulses introduces a pair of chirped cavity mirrors 110 with specific chirped characteristics as the two end mirrors of a multi-channel cavity system 100 to compensate for the dispersion effect caused by the nonlinear medium 120. Without increasing the thickness of the nonlinear medium 120, it can ensure that the self-phase modulation effect always maintains a high intensity, thereby improving the pulse spectrum broadening efficiency and reducing the complexity and requirements of subsequent dispersion compensation.

[0032] In this embodiment, the nonlinear medium 120 is fused silica, calcium fluoride, or sapphire, and the thickness of the nonlinear medium 120 is 1 mm to 20 mm.

[0033] In this embodiment, the nonlinear compression device further includes a dispersion compensation module 400, which is located at the laser emission end of the output collimating lens group 300. The dispersion compensation module 400 is a chirped mirror pair, a prism pair, or a grating pair.

[0034] In this embodiment, the group delay dispersion of the chirped cavity mirror pair 110 is matched with the thickness of the nonlinear medium 120 and the number of reflections in the multi-cavity system 100, and the group delay dispersion of the chirped cavity mirror pair 110 ranges from -50 fs² to -1000 fs².

[0035] In this embodiment, the reflective surface of the chirped cavity mirror 110 is coated with a chirped dielectric film, and the dispersion curve of the chirped dielectric film is complementary to the dispersion characteristics of the nonlinear medium 120 in the laser band.

[0036] A specific application example is as follows: Light source: A femtosecond laser is used, with an output pulse energy of 0.55 µJ and a pulse width of 250 fs.

[0037] Chirped mirror pair 110: A specially designed chirped mirror is used to compensate for the dispersion effect of the nonlinear medium 120. Its single-pass chirp compensation is -300 fs², used to compensate for the dispersion of the nonlinear medium 120.

[0038] Nonlinear medium 120: 20 mm thick fused silica was selected as the nonlinear medium 120.

[0039] Multi-cavity structure: A multi-cavity system 100 with 30 reflections was designed. Through precise calculations, it was ensured that the interaction between the laser pulse and the nonlinear medium 120 was optimized in each reflection.

[0040] Operating steps: Pulse input: After the laser pulse is output from the femtosecond laser, it passes through the incident coupling lens group 200 to control the spot size and divergence angle, and then is injected into the multi-pass cavity.

[0041] Multi-pass cavity reflection and pulse spectral broadening: The laser pulse is reflected 30 times within a multi-pass cavity. Due to the cavity's unique structure, the self-reproduction of the 30 reflection modes within the cavity is maintained. A nonlinear medium 120 is inserted into this multi-pass cavity. The self-phase modulation effect between the high-intensity laser and the nonlinear medium 120 broadens the laser pulse spectrum. Simultaneously, the dispersion effect of the nonlinear medium 120 gradually broadens the pulse width. In this multi-pass cavity, the cavity mirror uses a mirror with a built-in group delay dispersion of -300 fs. 2 The chirped cavity mirror pair 110 is used to compensate for the dispersion effect caused by the nonlinear medium 120, ensuring that the pulse width of the laser pulse is not rapidly broadened by the nonlinear medium 120.

[0042] Pulse collimation: The pulse is then output to the output collimating lens group 300 to collimate the beam.

[0043] Chirp compensation and compression: After the beam in the multi-pass cavity is collimated, it undergoes chirp compensation via an additional dispersion compensation module 400, resulting in a compressed femtosecond pulse. The compressed laser pulse output has a significantly reduced pulse width to <100 fs, and the peak power is improved.

[0044] Experimental data and results: Pulse width compression efficiency: After adopting this nonlinear compression device, see [reference needed]. Figure 4 and Figure 5 A 0.55 μJ laser pulse, with its pulse width compressed from 250 fs to 65 fs.

[0045] Traditional nonlinear compression devices can only compress up to 79 fs.

[0046] Laser pulse width variation: Compared with traditional multi-cavity compressors, traditional devices exhibit significant pulse width broadening in low-energy pulse processing, increasing from 250 fs to 740 fs. In contrast, this nonlinear compression device maintains high compression efficiency and a shorter pulse width even under low-energy pulse conditions, with pulse width broadening increasing from 250 fs to 362 fs.

[0047] Compensation for chirp variation: -15500 fs compensation is required using conventional devices. 2 The chirping; and using this nonlinear compression device, only -5800fs needs to be compensated. 2 A 64.7fs pulse can then be obtained.

[0048] The following is a specific application example two: 1. When using this nonlinear compression device to process low-energy laser pulses (e.g., 0.55 μJ injected into a multi-pass cavity), under the same number of multi-pass cavities and a nonlinear medium thickness of 120 mm, the compression result is reduced from 78.8 fs to 56 fs; the effect is 1.4 times that of traditional techniques. See the B-integral evaluation. Figure 2This device can achieve an integral B of 0.1375 during the final stage of a multi-channel circuit. (See [link]). Figure 3 The traditional device's pulse width is 0.06737, resulting in a 2-fold improvement in performance. Based on the pulse width evaluation of the multi-cavity output, its output pulse width increased from 250 fs to 362 fs, while the traditional device's pulse width increased from 250 fs to 740 fs.

[0049] in, Figure 2 The injection parameters were 0.55 uJ, 250 fs; the dielectric parameters were 20 mm UVFS; and the group delay dispersion of the chirped cavity was -300 fs. 2 Data obtained under the conditions of testing Figure 3 The data are from tests conducted using a traditional nonlinear compression device (i.e., the cavity mirror is a reflector) under the conditions of injection parameters of 0.55 uJ, 250 fs, and medium parameters of 20 mm UVFS.

[0050] Improved pulse width compression efficiency: This nonlinear compression device can maintain a high self-phase modulation effect even under low-energy laser pulses, thereby significantly improving pulse width compression efficiency. Through advance dispersion compensation of the chirped cavity mirror, the speed of spectral broadening is accelerated, thus achieving more efficient pulse width compression.

[0051] Using this nonlinear compression device, with the same injection energy (e.g., 0.55 uJ) and the same number of multi-pass cavities, to achieve the same pulse width (78 fs), the required thickness of the nonlinear medium 120 is 10 mm (chirped cavity pre-compensated single pass - 190 fs). 2 (dispersion) and 20mm (conventional device).

[0052] Reducing the thickness of the nonlinear medium: By introducing a chirped cavity mirror, this nonlinear compression device can effectively reduce the thickness of the nonlinear medium 120, avoiding the dispersion effect problem caused by a thick medium. A thinner nonlinear medium can reduce the amount of chirp compensation, increase the damage threshold of the nonlinear medium 120, mitigate the distortion caused by the medium on the mode, reduce the optical loss of the system, and simplify the design and implementation of the device.

[0053] We continue using the same parameters as in the simulation scenario described in section 2 above. Because most of the dispersion has been compensated in advance, the dispersion amount required by the subsequent dispersion compensation module 400 in the system is reduced from -15575 fs. 2 (Conventional device) reduced to 6140fs 2 (This nonlinear compression device). This not only simplifies the system and saves costs, but also improves the system's stability and reduces the difficulty of maintenance and adjustment.

[0054] Simplified dispersion compensation process: In traditional techniques, low-energy laser pulses often require extensive dispersion compensation, which complicates system design. By introducing a chirped cavity mirror, this nonlinear compression device can pre-compensate for dispersion, significantly reducing the complexity of subsequent dispersion compensation and simplifying the operation of the entire system.

[0055] According to simulation data, using a conventional device with a 20mm thick nonlinear medium 120, the pulse energy injected into the multi-cavity needs to be greater than or equal to 1uJ to avoid introducing significant spectral and pulse distortion. However, using this nonlinear compression device, with the same 20mm thick nonlinear medium 120 and the same multi-cavity shape, it can support a pulse energy of 0.55uJ. The improvement is even more pronounced with a thicker nonlinear medium 120.

[0056] Expanding Applicability and Compatibility with Low-Energy Laser Pulses: This nonlinear compression device expands the application range of existing pulse compressors, enabling them to effectively handle low-energy (below 1 µJ) laser pulses. Traditional multi-cavity pulse compressors often face problems of excessive pulse width broadening and insignificant compression effects under low-energy pulses. However, this nonlinear compression device maintains the high peak power required for low-energy laser pulse compression by introducing a chirped cavity mirror.

[0057] Improving system stability and reliability: By performing dispersion compensation in advance, this nonlinear compression device reduces the thickness of the nonlinear medium (120mm) and the need for subsequent dispersion compensation, thereby reducing system complexity and making the laser pulse compression system more stable and reliable. For high-power laser systems, system stability and reliability are key performance indicators.

[0058] Reduced system cost: By introducing a chirped cavity mirror, the reliance on a thicker nonlinear medium 120 is reduced, thus improving the efficiency of pulse width compression and lowering the system's material costs. The thinner nonlinear medium 120 and simplified dispersion compensation system further reduce the overall equipment cost.

[0059] Application Prospects: This nonlinear compression device is suitable for efficient compression of low-energy laser pulses, especially in low-power systems such as fiber lasers, and is compatible with almost all fiber lasers on the market. In certain applications (such as precision machining and medical diagnostics), the improved low-energy pulse compression effect significantly broadens the application scope of these technologies.

[0060] This nonlinear compression device for generating low-energy ultrashort femtosecond pulses solves the problems of pulse width broadening and weakened self-phase modulation effect in traditional multi-cavity pulse compressors by introducing a chirped cavity mirror to compensate for the dispersion of the nonlinear medium 120. Its advantages include: significantly improved pulse width compression efficiency, reduced thickness of the nonlinear medium 120, simplified dispersion compensation process, expanded system applicability, improved system stability and reliability, and reduced system cost. Through these technical effects, this nonlinear compression device has broad prospects in terms of technology, economy, and application.

[0061] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0062] The above-described embodiments are merely illustrative of several implementations of this utility model, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of this utility model. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this utility model, and these all fall within the protection scope of this utility model. Therefore, the protection scope of this utility model should be determined by the appended claims.

Claims

1. A nonlinear compression device for generating low-energy ultrashort femtosecond pulses, characterized in that, include: A multi-cavity system includes a chirped mirror pair and a nonlinear medium, wherein the nonlinear medium is located between the chirped mirror pair and is a solid transparent material. The chirped mirror pair is used to pre-compensate for the dispersion effect caused by the nonlinear medium during laser pulse reflection. The reflective surface of the chirped mirror pair is designed to simultaneously cancel the dispersion accumulation of the nonlinear medium during multiple reflections of the laser pulse. An incident coupling lens group is located at the laser incident end of the multi-cavity system and is used to control the laser spot to a suitable size and divergence angle before injecting it into the multi-cavity system. The outgoing collimating lens group is located at the laser output end of the multi-cavity system and is used to collimate the outgoing beam of the multi-cavity system.

2. The nonlinear compression device for generating low-energy ultrashort femtosecond pulses according to claim 1, characterized in that, The nonlinear medium is fused silica, calcium fluoride, or sapphire.

3. The nonlinear compression device for generating low-energy ultrashort femtosecond pulses according to claim 2, characterized in that, The thickness of the nonlinear medium is 1 mm to 20 mm.

4. The nonlinear compression device for generating low-energy ultrashort femtosecond pulses according to claim 3, characterized in that, The nonlinear compression device also includes a dispersion compensation module, which is located at the laser emission end of the output collimating lens group.

5. The nonlinear compression device for generating low-energy ultrashort femtosecond pulses according to claim 4, characterized in that, The dispersion compensation module is a chirped mirror pair, a prism pair, or a grating pair.

6. The nonlinear compression device for generating low-energy ultrashort femtosecond pulses according to claim 5, characterized in that, The group delay dispersion of the chirped cavity mirror pair is matched with the thickness of the nonlinear medium and the number of reflections in the multi-channel cavity system.

7. The nonlinear compression device for generating low-energy ultrashort femtosecond pulses according to claim 6, characterized in that, The group delay dispersion of the chirped cavity mirror pair ranges from -50 fs² to -1000 fs².

8. The nonlinear compression device for generating low-energy ultrashort femtosecond pulses according to any one of claims 1 to 7, characterized in that, The reflective surface of the chirped cavity mirror pair is coated with a chirped dielectric film, and the dispersion curve of the chirped dielectric film is complementary to the dispersion characteristics of the nonlinear medium in the laser band.

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