CN115740792B - A method for generating a high-stability supercontinuum light source based on solid thin films - Google Patents
A method for generating a high-stability supercontinuum light source based on solid thin films Download PDFInfo
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Abstract
本发明公开了一种基于固体薄片的高稳定性超连续谱光源的产生方法，如下：S1：在二维或多维微位移平台上放置至少一个固体薄片；S2：超短激光脉冲会聚入射固体薄片前表面的某一辐照区域后，在固体薄片内传播过程中产生非线性光学作用而实现光谱展宽；S3：同时采用光功率探头对与固体薄片作用后的超短激光脉冲进行采样探测，实现对采样光束功率的实时测量及记录；S4：基于探测到的功率值，计算实时功率相对于初始功率的变化幅度，由此获得对固体薄片老化情况的定量评估；S5：主控系统发出控制指令控制微位移平台按预先设定运动模式进行运动，实现对固体薄片辐照区域的持续更新，由此实现产生高稳定性超连续谱的超短脉冲光源。
The invention discloses a method for generating a high-stability supercontinuum light source based on a solid slice, which is as follows: S1: placing at least one solid slice on a two-dimensional or multi-dimensional micro-displacement platform; S2: after an ultrashort laser pulse converges and enters a certain irradiation area on the front surface of the solid slice, a nonlinear optical effect is generated during the propagation process in the solid slice to achieve spectrum broadening; S3: an optical power probe is simultaneously used to sample and detect the ultrashort laser pulse after the action on the solid slice, so as to achieve real-time measurement and recording of the power of the sampled beam; S4: based on the detected power value, the variation amplitude of the real-time power relative to the initial power is calculated, thereby obtaining a quantitative evaluation of the aging condition of the solid slice; S5: a main control system issues a control instruction to control the micro-displacement platform to move according to a pre-set motion mode, so as to achieve continuous updating of the irradiation area of the solid slice, thereby achieving an ultrashort pulse light source for generating a high-stability supercontinuum.
Description
Technical Field
The invention relates to the technical field of ultrashort laser pulses, in particular to a method for generating a high-stability supercontinuum light source based on a solid sheet.
Background
For the field of ultra-short laser pulses, achieving shorter pulse time widths has been an important and difficult task for the development of the field. From the time-bandwidth product relationship of the fourier transform limited pulses, it is known that a shorter pulse time width is obtained, which is necessarily supported by a wider spectral bandwidth. In general, on the premise of keeping different spectrum components with high coherence, the laser pulse spectrum bandwidth is obviously widened by a certain technical method, and then the dispersion compensation of the pulse is carried out by a certain technical method, so that the obvious compression of the ultra-short laser pulse time width can be realized. That is, how to extend the spectral bandwidth of the ultrashort pulses as coherently as possible is a core technical problem to achieve narrower time widths for the ultrashort pulses. In general, since the gain bandwidth of a laser gain medium is relatively fixed, the ultimate pulse width of an ultrashort laser pulse based on a particular laser gain medium is also substantially uniform. In practice, however, the energy of the ultrashort laser pulses is significantly amplified, often at the expense of the temporal width of the pulses. For example, in the chirped pulse amplification technology, when the chirped ultrashort laser pulse is amplified by high-intensity pumping in a laser crystal, significant gain bandwidth narrowing occurs, so that the limit width of the high-energy ultrashort pulse output by the ultrashort laser amplifier is significantly larger than the limit width of the small-energy ultrashort pulse output by the ultrashort laser oscillator. For example, the limit pulse width of a titanium (Ti) sapphire ultrashort pulse oscillator may be less than 10fs, while the limit pulse width of a titanium sapphire ultrashort pulse amplifier is typically greater than 20fs. In addition, ytterbium (Yb) doped crystals have a much narrower gain bandwidth than titanium sapphire crystals, resulting in ultra-short laser pulses based on ytterbium doped crystals that are more difficult to obtain with pulse widths below 100 fs. Therefore, for both the titanium sapphire ultrashort pulse laser and the ytterbium-doped ultrashort pulse laser, the two main stream ultrashort pulse lasers need to obtain a low-period high-energy ultrashort pulse laser output, and further significant coherent broadening of the pulse spectrum bandwidth needs to be performed outside the laser resonant cavity, and then time domain compression is performed to obtain an extremely narrow pulse time width even approaching to a single period.
Currently, spectrum broadening of ultra-short pulses is mainly implemented based on the induction of third-order nonlinear effects of strong ultra-short pulses in a transparent medium, particularly the induction of optical kerr effects (such as self-phase modulation and cross-phase modulation) to cause pulse spectrum broadening. Typically, the transmission of strong ultrashort pulses in a transparent solid medium achieves significant broadening of the ultrashort pulse spectrum, resulting in a supercontinuum output. As in the 80 s of the last century, researchers have achieved significant spectral broadening of ultrashort pulses with bulk fused silica, compressing the pulse time width from 100fs to 20fs. However, although there have been some research follow-up for a long time later, this technique has not been widely used, particularly in commercial applications.
On the one hand, the transparent solid medium has a higher third-order nonlinear coefficient, the self-focusing threshold value is smaller, and the self-focusing is easy to be induced to form a multi-filament transmission state when the spectrum of the high-energy ultrashort pulse is widened, so that the disturbance of the transverse laser mode and the optical damage of the transparent medium are finally caused. Therefore, when the transparent solid medium is used for ultra-short pulse spectrum broadening, the laser spot size of the laser pulse incident on the medium surface must be well controlled according to the energy of the laser pulse, and the proper medium thickness is selected, so that the influence of self-focusing is not enough to destroy the transmission mode characteristics of the light beam. That is, there is a strong competition between the broad spectrum broadening and the good output mode, and it is very difficult to balance the two relationships to obtain a good spectrum broadening effect-this requires careful experimental investigation to obtain suitable laser irradiation conditions according to the characteristics of each sample (even each sample), and the technology is not practical for non-professional researchers.
On the other hand, the more serious problem is that the ultra-short pulse radiation with long time and high energy can cause the transparent solid medium to have obvious material aging phenomenon, namely, the strong laser radiation can cause the medium material to have greatly enhanced absorption of laser due to the increase of defects, meanwhile, the phenomena of transverse mode instability such as mitogenesis and mitosis can also be caused, positive feedback (hatching effect) can be formed by the phenomena along with the increase of radiation time, the phenomena become more and more obvious, so that the pulse spectrum broadening is obviously reduced, the beam mode is deteriorated, the energy is obviously reduced, permanent optical damage is gradually generated in the medium along with the hatching effect, and even the surface is finally severely ablated due to heat accumulation. The aging phenomenon of the material caused by the strong laser radiation is enhanced along with the approach of the pulse radiation power density to the laser damage threshold value of the material, namely, the aging speed of the transparent solid medium is increased along with the increase of the pulse energy, and the permanent optical damage is generated faster. However, the extent of spectral broadening of a pulse has a significant positive correlation with the radiated power density of the pulse, which necessarily results in the radiated power density approaching the material destruction threshold in pursuit of a broader spectral broadening. Therefore, the high performance of spectrum broadening and the long-term high stability of the output are not compatible with the conventional spectrum broadening technology based on transparent solid medium, and the excellent spectrum broadening characteristic is necessarily obtained at the cost of sacrificing the service life of the material, which is the most important obstacle for preventing the popularization and application of the technology, especially the commercial application.
Because of the above-mentioned problems with ultra-short pulse spectrum broadening techniques based on bulk solid media, such techniques have not shown clear application prospects and have not received significant attention in the field for a long time. In recent years, an ultrashort pulse spectrum broadening technology based on a solid slice (slice group) has reburned the research enthusiasm of researchers on the solid medium spectrum broadening technology. Unlike bulk solid medium-based methods, this solid sheet set-based method employs a plurality of wide-band, clear dielectric sheets (e.g., fused silica sheets) of about 0.1mm thickness placed at a distance near the converging focus of the ultrashort pulses to achieve strong field nonlinear optical effects of the ultrashort pulses and the solid sheet set, thereby achieving spectral broadening. Different from the bulk solid, the solid sheet can realize the separation of the spectrum broadening action process and the self-focusing filament forming (including multi-mitosis) action process, so that the actions of the ultra-short pulse and the sheet only induce remarkable spectrum broadening without causing self-focusing filament forming and multi-mitosis, the spectrum broadening mainly comes from self-phase and cross-phase modulation caused by optical kerr effect in time domain, the ultra-short pulse incident medium sheet can directly generate strong self-phase and cross-phase modulation to induce the spectrum broadening under the condition of high enough radiation power density without self-focusing, and the self-focusing comes from the spatial distribution of medium refractive index change caused by optical kerr effect in space domain, so that the remarkable effect can be generated by accumulating a certain action distance. Thus, when the thickness of the sheet is sufficiently small, the ultrashort pulses can produce significant spectral broadening while not producing significant self-focusing and filamentation effects. In addition to avoiding the problems associated with the self-focusing effect, solid sheet set spectral broadening techniques also provide a richer and flexible tuning dimension than bulk transparent media. For example, when a single solid sheet is insufficient to produce sufficient spectral broadening, the cumulative spectral broadening effect can be produced by placing multiple solid sheets at specific intervals as needed near the laser focus, ultimately resulting in a broadband supercontinuum that can be time-domain compressed to produce an ultrashort pulse output of periodic magnitude.
The above significant advantages of solid flake (flake group) spectral broadening technology have led to a great deal of interest to researchers in the relevant field. In recent years, the development of more related research and the presentation of results have presented some of the disadvantages of this technology while facilitating the understanding of the ultrashort pulse and solid flake action characteristics. The solid sheet technology realizes the separation of spectrum broadening and filamentization, but the sheet solid medium and the bulk solid medium are basically the same matrix material, so that the sheet solid medium and the bulk solid medium have similar aging characteristics under the same strong ultrashort pulse radiation, that is, the aging problem which needs to be faced by the bulk solid medium system is also encountered in the sheet solid medium system, which is an important technical problem unavoidable by the solid medium sheet spectrum broadening technology. In practice, the transmission of ultrashort pulses through a solid sheet corresponds to a shorter nonlinear action distance than a bulk solid, so that a higher pulse radiation power density is required to achieve significant spectral broadening when applying solid sheets for spectral broadening. Thus, to achieve a similar spectral broadening effect, the sheet medium is subjected to stronger radiation than the bulk solid medium, and thus ages more rapidly during irradiation. For the ultra-short laser pulse spectrum broadening technology based on a solid sheet, the continuous and rapid aging characteristic of the material under high radiation power density can cause the stability of the output supercontinuum to be adversely affected, such as continuous decrease of the output power of the supercontinuum, continuous narrowing of the spectrum broadening width, gradual deterioration of the fundamental transverse mode of the output beam, and the like.
In short, the continuous and rapid aging property of the dielectric material under the irradiation of the strong ultrashort pulse is an unavoidable inherent technical problem for realizing the practical long-time stable industrial application based on the solid sheet (sheet group) spectrum broadening technology, which is a direct technical bottleneck for preventing the technology from realizing the commercial application, and the new technology is needed to realize breakthrough.
Disclosure of Invention
The invention provides a method for generating a high-stability supercontinuum light source based on a solid sheet, which aims to solve the problem of material aging caused by strong laser radiation in the existing solid sheet supercontinuum technology.
In order to achieve the above purpose of the present invention, the following technical scheme is adopted:
a method for generating a high-stability supercontinuum light source based on a solid slice comprises the following steps:
S1, placing at least one solid slice on a two-dimensional or multi-dimensional micro-displacement platform;
s2, after the ultrashort laser pulse is converged and enters a certain irradiation area on the front surface of the solid sheet, nonlinear optical effect is generated in the propagation process in the solid sheet to realize spectrum broadening;
s3, sampling and detecting ultrashort laser pulses which act on the solid sheet by adopting an optical power probe at the same time, so as to realize real-time measurement and recording of sampling beam power;
S4, calculating the variation amplitude of the real-time power relative to the initial power based on the detected power value, thereby obtaining quantitative evaluation of the aging condition of the solid sheet;
s5, the main control system sends out a control instruction to control the micro-displacement platform to move according to a preset movement mode, so that the continuous updating of the solid sheet irradiation area is realized, and the ultra-short pulse light source for generating the high-stability ultra-continuous spectrum is realized.
The beneficial effects of the invention are as follows:
In the process of generating a supercontinuum by irradiating a solid sheet by ultrashort pulses, the ultrashort laser pulse irradiation with high intensity for a long time can cause a remarkable aging phenomenon of a solid sheet dielectric, wherein strong light irradiation induces random generation of lattice defects of a material and nonuniform change of refractive index, the random generation of the lattice defects and the nonuniform change of refractive index of the material are caused, the laser wire forming phenomenon is easily caused when the material is remarkably enhanced in laser absorption, the phenomena are more and more remarkable along with the increase of the strong laser irradiation time due to hatching effect, permanent optical damage is caused in the solid sheet dielectric or on the surface of the solid sheet dielectric, and even the severe ablation damage finally occurs on the surface of the dielectric due to heat accumulation caused by absorption. The continuous aging of the transparent dielectric material caused by the strong light irradiation can obviously reduce the spectrum broadening of the supercontinuum generated by the irradiation of the ultra-short pulse to the solid sheet, the beam transverse mode is deteriorated, and the pulse energy is obviously reduced, so that the long-time high-stability operation of the supercontinuum light source based on the solid sheet technology is greatly hindered. The method of the invention aims at the technical problems of the solid sheet supercontinuum source, obtains quantitative evaluation of the aging condition of the solid sheet by monitoring the variation amplitude of the beam power in the process of irradiating the solid sheet by ultrashort laser pulse to generate the supercontinuum, controls the solid sheet to move according to a preset movement mode based on the evaluation, and realizes continuous updating of the irradiation area of the solid sheet in the process of irradiating the intense laser for a long time, thereby effectively avoiding continuous accumulation of the aging process of the medium material in the irradiation area in time and space and creating proper substances and physical conditions for long-time stable output of the supercontinuum. The method and the device realize continuous updating of the sheet irradiation area by quantitatively evaluating the aging condition of the solid sheet irradiated by the strong ultrashort laser pulse in real time and introducing an automatic control technology, and effectively avoid serious aging of the dielectric material in the irradiation area, thereby greatly reducing the influence of the aging phenomenon of the material on the power stability, the spectrum broadening degree and the transverse mode characteristic in the supercontinuum generation process and finally realizing the long-time high-stability output of the supercontinuum light source based on the solid sheet technology.
Drawings
FIG. 1 is a flow chart of a method for producing a high stability supercontinuum light source based on a solid sheet according to the present invention.
FIG. 2 is a schematic diagram of an optical path of a method for generating a high stability supercontinuum light source according to the present invention.
FIG. 3 is a schematic diagram of another optical path of the method for generating a high stability supercontinuum light source according to the present invention.
Fig. 4 is a graph of three preset motion patterns for ultra-short pulse irradiation zone updating on a solid sheet.
Fig. 5 is a motion profile of an ultra-short pulse irradiation zone update on a square solid sheet.
Fig. 6 is another motion profile of an ultra-short pulse irradiation zone update on a square solid sheet.
Fig. 7 is a further motion profile of an ultra-short pulse irradiation zone update on a square solid sheet.
Fig. 8 is a motion profile of an ultra-short pulse irradiation zone update on a circular solid sheet.
Fig. 9 is another motion profile for ultra-short pulse irradiation zone updating on a circular solid sheet.
In the figure, a 1-femtosecond laser, a 2-continuous attenuation sheet, a 3-iris diaphragm, a 4-plano-convex lens, a 5-solid sheet, a 6-optical power probe, a 7-master control system, an 8-micro-displacement platform and a 9-beam splitting sheet are arranged.
Detailed Description
The invention is described in detail below with reference to the drawings and the detailed description.
Example 1
For the technology of generating the supercontinuum based on the ultra-short laser pulse irradiation solid thin sheet (thin sheet group), the continuous aging characteristic of the dielectric material under the high laser irradiation intensity can have important adverse effects on the stability of the supercontinuum output, such as continuous reduction of output power, continuous narrowing of spectrum broadening, continuous deterioration of the transverse mode characteristics of the output beam, and the like. That is, unavoidable aging of the dielectric material caused by strong laser irradiation is an important reason that hinders the realization of stable industrial application of such supercontinuum generation technology for a long period of time. The present invention provides a method for generating a high-stability supercontinuum light source based on a solid sheet around the inherent problem of the supercontinuum technology of the solid sheet, as shown in fig. 1, and specifically comprises the following steps:
S1, placing at least one solid slice on a two-dimensional or multi-dimensional micro-displacement platform;
s2, after the ultrashort laser pulse is converged and enters a certain irradiation area on the front surface of the solid sheet, nonlinear optical effect is generated in the propagation process in the solid sheet to realize spectrum broadening;
s3, sampling and detecting ultrashort laser pulses which act on the solid sheet by adopting an optical power probe at the same time, so as to realize real-time measurement and recording of sampling beam power;
S4, calculating the variation amplitude of the real-time power relative to the initial power based on the detected power value, thereby obtaining quantitative evaluation of the aging condition of the solid sheet;
s5, the main control system sends out a control instruction to control the micro-displacement platform to move according to a preset movement mode, so that the continuous updating of the solid sheet irradiation area is realized, and the ultra-short pulse light source for generating the high-stability ultra-continuous spectrum is realized.
In a specific embodiment, as shown in fig. 2, the output ultrashort laser pulse of the femtosecond laser 1 sequentially passes through a continuous attenuation sheet 2 for controlling the single pulse energy of the ultrashort laser pulse, an iris 3 for controlling the beam diameter of the incident ultrashort laser pulse, and a plano-convex lens 4 for focusing, and then is converged, and enters a certain irradiation area of the front surface of the solid slice 5. The irradiation light intensity generated by the ultrashort laser pulse on the surface of the solid sheet 5 can cause the incident ultrashort laser pulse and the solid sheet 5 to generate obvious nonlinear optical effect, and simultaneously can cause the aging time constant of the solid sheet 5 to be in an 'hour' time scale. In this embodiment, the ultrashort laser pulse output by the femtosecond laser 1 is converged by the plano-convex lens 4 and then irradiated on the front surface of the solid sheet 5, and the generated irradiation light intensity can make the incident ultrashort laser pulse and the solid sheet generate significant nonlinear optical effect. Specifically, the femtosecond laser 1 used in this embodiment has a center wavelength of 800nm, a pulse width of 50fs, a repetition frequency of 1KHz, a beam diameter of 8.5mm (1/e 2 width), and polarization as linear polarization in the horizontal direction. The solid sheet 5 used was an optically polished quartz glass sheet having front and rear surfaces, a thickness of 0.1mm, and square light-transmitting surfaces having dimensions of 10mm×10mm. Wherein the ultra-short laser pulse of the femtosecond laser output can be subjected to single pulse energy control through the continuous attenuation sheet 2, the beam transverse mode with Gaussian distribution can be subjected to further diameter control through the iris 3 arranged in the middle relative to the laser beam, and the subsequent converging operation is realized by selecting the plano-convex lens 4 with proper focal length according to the irradiation condition generated by the supercontinuum. In general, the beam diameter of a collimated beam converging focal spot is proportional to the focal length of the converging lens and inversely proportional to the incident beam diameter. Therefore, by adjusting the continuous attenuation sheet 2 to control the single pulse energy of the incident ultrashort pulse, adjusting the iris 3 to control the beam diameter of the incident ultrashort pulse, and selecting the plano-convex lens 4 with different focal lengths, the instantaneous light intensity irradiated on the solid sheet 5 can be flexibly controlled, so that the proper condition that the ultrashort laser pulse irradiates the solid sheet to generate a supercontinuum is obtained, that is, the converged ultrashort laser pulse can generate strong nonlinear optical effect with the solid sheet to remarkably broaden the spectrum of the solid sheet, and the irradiation intensity of the converged ultrashort laser pulse does not cause rapid laser induced damage to the solid sheet (although the medium material can generate accumulated structural change or even ablation damage under long-time irradiation of the laser with the intensity).
For example, in a typical irradiation parameter setting, the iris 3 is left fully open, i.e., the femtosecond laser gaussian beam is not affected by the iris 3, and the focal length of the plano-convex lens 4 is selected to be 2.0m, where the diameter of the converging gaussian beam waist is about 0.24mm. Under such converging conditions, typically when the laser irradiation power is 370mW, although the ultrashort laser pulse propagates in air and the self-focusing filamentation phenomenon does not occur, and the ultrashort laser pulse irradiates the solid sheet 5 to generate significant spectral broadening, the irradiated region of the solid sheet 5 rapidly generates ablation damage on a time scale approaching 1 minute (if the time interval from initial irradiation to ablation damage is defined as an aging time constant, at which the aging time constant is close to 1 minute). However, by adjusting the continuous attenuation sheet 2 to decrease the laser irradiation power to 250mW, the solid sheet 5 can be irradiated with laser for more than 1 hour without ablation destruction (aging time constant is more than 1 hour), and only a decrease in laser transmittance due to a cumulative structural change, that is, a decrease in supercontinuum output power, occurs. And, under such converging conditions, when the irradiation power is 250mW, the spectral width broadening of the supercontinuum generated by the irradiation of the solid-state thin sheet by the ultrashort laser pulse is nearly saturated. Thus, the irradiation conditions may be used as a suitable condition for irradiating a solid sheet with ultrashort laser pulses to produce a supercontinuum.
For another example, in another typical irradiation parameter setting, the iris 3 is adjusted to a clear aperture of 2mm, and the focal length of the plano-convex lens 4 is selected to be 0.10m, at which time the diameter of the Airy spot formed by converging the flat-top beam is about 0.10mm. Under such converging conditions, typically, when the laser irradiation power is 68mW, the irradiation of the solid sheet 5 by the ultra-short laser pulse may produce significant spectral broadening, but the irradiated region of the solid sheet 5 may rapidly undergo ablative destruction on a time scale of approximately 1 minute (aging time constant of approximately 1 minute). However, by adjusting the continuous attenuation sheet 2 to reduce the laser irradiation power to 46mW, the solid sheet 5 can be irradiated with laser for more than 1 hour without ablation destruction (aging time constant is more than 1 hour), and only a reduction in transmittance due to a cumulative structural change, that is, a reduction in supercontinuum output power, occurs. And, under such converging conditions, when the irradiation power is 46mW, the spectrum width broadening of the supercontinuum generated by the irradiation of the quartz thin sheet by the ultrashort laser pulse is also close to saturation. Therefore, the irradiation condition can also be used as a proper condition for generating a supercontinuum by irradiating the solid slice with ultrashort laser pulses.
In a specific embodiment, in step S1, a solid sheet 5 is placed on the two-dimensional micro-displacement platform 8, so as to achieve the purpose of continuously and automatically updating the laser irradiation area of the system during the irradiation of the sheet by the intense laser for a long time. In this embodiment, the displacement range of the two-dimensional micro-displacement platform 8 or the multi-dimensional micro-displacement platform should cover the entire light-passing surface of the solid sheet 5, and the displacement path of the two-dimensional or multi-dimensional micro-displacement platform may be preset and stored in the main control system, so that the ultra-short laser pulse scanning irradiation of the solid sheet 5 according to the set path may be realized. On the other hand, in this embodiment, the sampling and detection of the ultrashort laser pulse after the action with the solid slice is realized by directly detecting the power of the reflected laser beam after the action with the solid slice 5, so that the surface normal of the solid slice 5 needs to form an included angle of non-0 degrees with the transmission direction of the incident ultrashort laser pulse, so that the reflected ultrashort laser pulse beam and the incident ultrashort laser pulse beam are spatially separated, and further the detection of the reflected ultrashort laser pulse beam can be realized. In fact, the laser irradiation conditions of the solid sheet 5 are typically set to brewster's angle to minimize the reflection loss of the solid sheet 5. Considering the requirements of the sampling power and the above-mentioned anti-reflection requirements, in this embodiment, the angle between the surface normal of the solid sheet 5 and the transmission direction of the incident light is set to be close to the brewster angle, instead of the brewster angle, so as to keep the proper reflected light power for sampling monitoring. Under such a laser incident angle setting, in order to ensure that the spatial position of the laser irradiation sheet in the beam propagation direction is unchanged when the solid sheet 5 is updated to a new irradiation region (so that the laser irradiation light intensity is kept consistent after the solid sheet 5 is updated to the irradiation region), two mutually perpendicular displacement axis directions of the micro-displacement platform having the two-dimensional translation function should be set to lie within the light passing plane of the solid sheet 5, in this embodiment, along the horizontal direction (horizontal axis) and the vertical direction (vertical axis or y axis), respectively, to match the laser polarization conditions along the horizontal direction. For the solid sheet 5 with square light-passing surface in this embodiment, when it is placed on the micro-displacement platform, the ideal placement scheme for meeting the above requirements is to make two mutually perpendicular side directions of the solid sheet 5 respectively along two mutually perpendicular displacement directions of the micro-displacement platform, so that the light-passing area of the solid sheet 5 can be fully utilized in the subsequent irradiation region updating operation. The operation of the solid shim 5 to adjust the laser incidence angle is then achieved by rotating the micro-displacement stage directly along the y-axis.
The two-dimensional micro-displacement platform in the embodiment can also be expanded into a three-dimensional micro-displacement platform. For a three-dimensional micro-displacement platform, typically, three mutually perpendicular displacement axis directions thereof may be set along the incident light transmission direction (z-axis), and along the horizontal direction (x-axis) and the vertical direction (y-axis) within the plane of the perpendicular incident light transmission direction, respectively. Under the condition of three-dimensional motion capability, the micro-displacement platform can ensure that the space position of the laser irradiation sheet is unchanged in the beam propagation direction when the solid sheet 5 arbitrarily placed on the micro-displacement platform updates the irradiation area through one-axis, two-axis or three-axis motion, and the special direction association requirement between the two solid sheets is not needed to be considered when the two-dimensional micro-displacement platform places the solid sheet, so that the solid sheet has greater adjustment flexibility. For example, the operation of adjusting the laser incidence angle of the solid sheet 5 placed on the three-dimensional micro-displacement stage 8 can be achieved by rotating the solid sheet along the y-axis while keeping the micro-displacement stage stationary.
One solid sheet 5 in the present embodiment may be also expanded into a solid sheet group, that is, in addition to the case where one solid sheet 5 is placed near the convergence focus of the ultrashort laser pulse, two or more solid sheets may be placed near the convergence focus of the ultrashort laser pulse at a certain interval, so as to implement two-stage or multi-stage spectrum broadening of the ultrashort laser pulse irradiation solid sheet, thereby greatly improving the spectrum broadening capability of the solid sheet spectrum broadening technology. In the case of such a set of solid sheets, the micro-displacement platform is provided with two typical forms, namely that all the solid sheets are placed on the same micro-displacement platform, that is, all the solid sheets are subjected to the update of the radiation area position at the same time, and that each solid sheet is placed on one micro-displacement platform respectively, so that the individual update of the radiation area of each solid sheet can be realized by monitoring the aging condition of each sheet respectively.
In a specific embodiment, in step S2, the high-energy ultrashort laser pulse propagates in the solid slice 5 to generate a spectral broadening due to nonlinear optical effects, resulting in a supercontinuum ultrashort pulse output. Specifically, the ultrashort laser pulse will propagate in the solid sheet medium after it is incident on a certain irradiation area of the front surface of the solid sheet 5. By adjusting the continuous attenuator 2 and the iris 3, as described above, and selecting the plano-convex lenses 4 of different focal lengths, the instantaneous intensity of light irradiated on the solid sheet 5 can be controlled to be within a suitable window of irradiation conditions where ultrashort laser pulses interact with the solid sheet to produce a supercontinuum, as in the two typical irradiation parameter settings listed above. Under the irradiation conditions, as the laser irradiation intensity is in a physical action intensity window in which three-order nonlinear optical effects such as self-phase modulation, cross-phase modulation, four-wave mixing, stimulated Raman scattering and the like are remarkably generated, the incident ultra-short laser pulse generates remarkable spectrum broadening with the solid sheet medium in the propagation process of the solid sheet medium through the three-order nonlinear optical effect, and the ultra-continuous spectrum ultra-short pulse output is formed after the ultra-short laser pulse penetrates through the solid sheet.
In this embodiment, the two typical irradiation parameter settings can be used to expand the spectral bandwidth (full width at half maximum) of the ultra-short laser pulse from 30nm to 60nm after the incident ultra-short laser pulse and a solid sheet initially act, and the pulse energy utilization rate can be close to 95%. However, under the condition of fixed irradiation area, the spectrum bandwidth expansion amplitude and pulse energy utilization rate of the output super-continuous spectrum ultrashort pulse gradually decrease with the increase of irradiation time. When the irradiation area is updated by the moving solid sheet, the spectrum bandwidth expansion amplitude and the pulse energy utilization rate of the output super-continuous spectrum super-short pulse return to the initial values.
In a specific embodiment, in step S3, the reflected light beams generated by converging the high-energy ultrashort laser pulses on the front and rear surfaces of the solid sheet 5 are collected by the optical power probe 6, and transmitted to the master control system 7 after photoelectric conversion, so as to realize real-time measurement, recording and monitoring of the power of the reflected laser beams. In this embodiment, the high-energy ultrashort laser pulse after acting on the solid slice 5 needs to be sampled and detected, so as to realize real-time measurement and monitoring of the power of the sampled ultrashort laser pulse beam. In fact, the irradiation of the solid-state sheet by the ultrashort laser pulses causes reflections at the front and rear surfaces of the solid-state sheet, and the reflected beam can be used as a sampling beam for monitoring the variation of the laser power after interaction with the solid-state sheet 5. That is, the sampling detection of the present embodiment is realized by detecting the ultra-short laser pulse beam reflected after acting with the solid-state sheet 5.
Note that the reflected light beam detected in this embodiment includes reflected light of the front and rear surfaces of the solid sheet 5. The front surface reflected light reflects the reflection generated during the action of the incident ultrashort laser pulse and the front surface of the solid sheet, and the back surface reflected light reflects the reflection generated during the action of the spectrum-broadened supercontinuum ultrashort pulse and the back surface of the solid sheet, both of which can have reduced reflectivity due to aging or damage of the solid sheet on the surface and in the solid sheet. In the process of monitoring the power of the reflected light beam, the optical power probe 6 with a large enough detection area can completely collect the reflected light on the front surface and the rear surface so as to ensure the stability of the measured power data in the long-time detection process and reduce the influence of the light beam jitter on the accuracy of the data.
In this embodiment, the angle between the normal of the surface of the solid sheet 5 and the transmission direction of the incident light is set to 45 degrees, which is close to, but not equal to, the brewster angle (56 degrees) of the quartz glass, so as to ensure that the probe 6 can maintain proper reflected light power under the above typical irradiation parameter setting conditions to realize stable low-noise detection.
For example, under the above-mentioned conditions of the laser incidence angle and the first irradiation parameter setting, the power of the reflected beam generated by irradiating the solid sheet 5 with incident laser light of 250mW power is about 85 mu W, and under the above-mentioned conditions of the laser incidence angle and the second irradiation parameter setting, the power of the reflected beam generated by irradiating the solid sheet 5 with incident laser light of 46mW power is about 16 mu W, both of which ensure that the optical power probe 6 used in the present embodiment is in a high-sensitivity low-noise detection zone.
In the detection arrangement of this embodiment, a silicon opto-electronic power probe with an effective detection area of 10mm x 10mm is used to achieve complete collection of the reflected beam. In addition, as the reflected light beam is emitted by the converging light beam close to the beam waist, the reflected light beam has the characteristic of divergence, and the silicon photoelectric probe is arranged at a proper distance from the solid sheet, so that the situation that the detection signal has nonlinear or even saturated characteristics or the light beam diverges too much to realize complete acquisition due to the fact that the instantaneous irradiation light intensity of the ultra-short pulse on the surface of the probe is too high is avoided. In addition, in order to avoid the nonlinear or even saturated characteristics of the probe caused by the excessively high instantaneous irradiation light intensity, before the reflected ultra-short laser pulse is incident on the probe, the reflected ultra-short laser pulse can be attenuated by a neutral filter and then is incident on the probe.
For example, corresponding to the first irradiation parameter setting, the silicon photoelectric probe 6 is placed at a position 50cm away from the irradiation point of the solid slice 5 relative to the reflected light beam in the middle, and a suitable neutral filter is configured in front of the probe to attenuate the incident ultra-short laser pulse (the filter is placed close to the probe and can weaken the influence of ambient light), so as to avoid nonlinear characteristics of the signal detected by the probe.
The probe 6 converts the collected optical signals into electrical signals and transmits the electrical signals to the main control system 7, and then the main control system 7 records and presents the real-time collected power data, so that the real-time measurement, recording and monitoring of the reflected ultra-short laser pulse beam power are realized.
In a specific embodiment, in step S4, the master control system 7 further calculates the real-time variation amplitude of the detected real-time power with respect to the initial power based on the detected reflected laser beam power value, so as to obtain a real-time, quantitative assessment of the aging condition of the solid sheet 5 irradiated by the ultrashort laser pulse. In general, as the irradiation time of the strong ultrashort laser pulse increases, the irradiated region of the solid sheet will gradually age, resulting in a gradual increase in absorption of the irradiated laser by the material. This gradual increase in absorption of the irradiated laser light is fed back to the reflected beam, which results in a gradual decrease in the detected power of the reflected beam, and the magnitude of this decrease in reflected light is positively correlated with the magnitude of the increase in absorption of the irradiated light. Thus, by detecting the magnitude of the real-time drop in reflected light, the aging of the solid sheet can be assessed in real-time, quantitatively.
In theory, the aging speed of the solid sheet is directly related to the irradiation laser light intensity, and the aging speed of the solid sheet is higher and higher as the irradiation laser light intensity is gradually increased and approaches to the single pulse damage threshold, namely, the aging time constant (delta t d) is shorter and shorter. In fact, when the aging speed of the solid sheet is in a relatively rapid interval (for example, Δt d is in a "minute" time scale), the randomness of the aging process of the solid sheet becomes large (for example, uncertainty of Δt d is increased), which is unfavorable for stable output of the supercontinuum and closed-loop feedback control of the system. Therefore, in order to avoid various adverse effects caused by the excessively rapid aging of the solid sheet in the supercontinuum generation process, in practical application, the laser irradiation light intensity should be controlled in a range where the aging speed of the solid sheet is slower. The laser power selection corresponding to the typical irradiation parameter settings of the two examples can lead the solid sheet to be in a slow aging interval, and the delta t d of the solid sheet is in an 'hour' time scale.
In a specific embodiment, in step S5, based on the evaluation of the aging condition of the solid sheet, the master control system 7 issues an instruction for controlling the movement of the micro-displacement platform, so that the micro-displacement platform 8 moves according to a preset mode, and continuous update of the sheet irradiation area during the process of irradiating the solid sheet 5 with the intense laser for a long time is realized.
In this embodiment, based on the evaluation of the aging condition of the solid sheet, the present embodiment may implement the continuous position update of the irradiation area of the solid sheet by adopting a preset solid sheet motion control mode, as shown in a preset motion mode 1 in fig. 4, in which the main control system monitors the decreasing amplitude of the sampled beam power, and when the decreasing amplitude of the beam power is greater than a preset first threshold value, the main control system controls the micro-displacement platform to move along a preset path at an updated motion interval greater than the diameter of the irradiation spot.
The method comprises the steps of dividing a solid sheet into a plurality of radiation areas uniformly according to the size of the solid sheet, the irradiation spot diameter of an ultrashort laser pulse and the incident angle formed by the incident ultrashort laser pulse and the solid sheet, and controlling a micro-displacement platform to perform corresponding movement by a main control system by setting a first threshold value of the power reduction amplitude of a light beam as a triggering condition of the movement of the solid sheet when the power reduction amplitude of the light beam is larger than a preset first threshold value, so that the radiation area of the ultrashort laser pulse is switched from a current radiation area to an adjacent radiation area, and the space fixed point and time discrete updating of the radiation area of the solid sheet are realized.
In the solid sheet motion control mode according to this embodiment, when the real-time drop amplitude of the detected laser power is greater than the preset first threshold, the main control system 7 will issue a command for controlling the micro-displacement platform 8 to move, and the solid sheet irradiation area is updated by the micro-displacement platform moving a preset distance according to a preset path. Specifically, the first threshold is set with emphasis on two factors.
First, the first threshold should not be set too small because the beam power of the incident ultrashort pulse laser is not absolutely stable, which necessarily presents some random or systematic fluctuations over time, and such intrinsic beam power noise may cause false triggering of the threshold condition. Wherein for the case of random noise the measured power values statistically exhibit a normal distribution with a certain average power and power standard deviation. According to the characteristics of normal distribution, the probability of 1 time standard deviation is 68.26895%, the probability of 2 time standard deviation is 95.44997%, the probability of 3 time standard deviation is 99.73002%, the probability of 4 time standard deviation is 99.99367%, and the probability of 5 time standard deviation is 99.99994%. Considering the detection duration with the time scale of 'hour' and the sampling detection times of thousands times, it is known from the above data that when the first threshold is set to be equal to or greater than 4 times the standard deviation of the power of the incident beam, a lower probability of false triggering of the laser power can be obtained, and when the first threshold is set to be equal to or greater than 5 times the standard deviation of the power of the incident beam, the reason for ensuring that the first threshold of the triggering power is almost caused by the aging of the solid sheet, but not the power noise of the laser. Therefore, in order to avoid false triggering of the first threshold condition as much as possible, the first threshold value described in the present embodiment is set to be equal to or greater than the standard deviation of the 5 times ultrashort pulse laser power.
Specifically, when the first threshold is set to be 5 times the standard deviation of the ultrashort pulse laser power, the probability of random noise triggering the first threshold condition is only 0.00006% and less than the probability of one part per million, considering a single measurement. Considering 1000 measurements, the probability of random noise triggering the first threshold condition is only 0.06%, which is less than one thousandth. That is, for the ultra-short laser pulse light source with a power root mean square error of <0.5% (RMSE, 24 hours continuous measurement) used in this embodiment, when the set first threshold value is that the power drop amplitude is equal to 2.5%, that is, approximately equal to 5 times the standard deviation of the laser power, the probability that the laser power noise triggers the first threshold value condition is almost negligible.
In addition, in order to further reduce the noise of the input laser power, especially the influence of the long-time systematic fluctuation of the laser power caused by environmental change on the power measurement stability, thereby further reducing the probability of false triggering of the first threshold condition, the input laser beam power before acting with the solid slice can be subjected to real-time beam sampling detection. Based on the real-time measured pre-action input laser sampling beam power W I (t), the corrected post-action laser sampling power W Rr(t)＝WR(t)·WI(0)/WI (t) is obtained by multiplying the ratio of the post-action laser sampling power W R (t) measured in real time by the pre-action initial input laser sampling power W I (0) and the real-time input laser sampling power W I (t), so that the cancellation of laser power noise is realized, the influence of the input laser power noise on the post-action measuring power is obviously reduced, and the post-action real-time measuring power reflects the ageing degree of the solid sheet more truly.
On the other hand, when the probe 6 is adopted to detect the laser power, the influence of short-time random noise of the laser power can be effectively reduced by increasing the sampling integration time, and more stable laser power data can be obtained.
Secondly, under the condition that the requirement of the lower limit of the threshold setting is met, the threshold should be set in a section near the lower limit, such as a section 1.0 to 1.5 times lower limit, so as to avoid that the characteristic (power and spectrum broadening) of the supercontinuum output by the system before the irradiation area update is obviously changed due to the overlarge threshold. For the high stability supercontinuum light source technique of the embodiment for automatically updating the solid-sheet irradiation region, the stability of the supercontinuum long-term output is determined by the set threshold. For example, the threshold value set as described above is that the beam power drop amplitude is greater than 2.5%, and the maximum drop amplitude of the power curve of the supercontinuum output for a long time will be anchored at 2.5%, and will be characterized by an approximately periodic appearance, reflecting the curve period fluctuation caused by the systematic reason. The smaller the threshold setting is, the smaller the periodic fluctuation is, and the smaller the systematic deviation is obtained by the supercontinuum light source, so that the better high-stability working characteristic is realized.
Therefore, considering the above two factors of threshold setting, in this embodiment, the first threshold is directly set to the threshold lower limit required for almost completely avoiding false triggering of noise, that is, the threshold is the standard deviation of the 5 times ultrashort pulse laser power.
In this embodiment, the preset threshold is stored in the main control system, and when the main control system monitors that the drop amplitude of the probe beam power is greater than the threshold, the main control system 7 will issue an instruction for controlling the micro-displacement platform 8 to move. The instruction comprises displacement motion instructions of one or two or three displacement shafts in a two-dimensional or three-dimensional micro-displacement platform, and particularly relates to the motion direction (forward direction or reverse direction) and the motion distance of each displacement shaft needing to move. Based on the solid sheet light passing area, the irradiation spot diameter and the ultra-short laser pulse incident angle, the movement path of the solid sheet can be reasonably planned in advance so as to effectively utilize the solid sheet light passing area, and the movement path planned in advance is stored in a main control system so as to update the ultra-short laser pulse irradiation area of the solid sheet according to the set path. Generally, the distribution of laser irradiation points in the solid sheet can be arranged in the form of a rectangular lattice to facilitate scan path planning.
Specifically, according to the irradiation spot size and the laser incident angle size determined by the two typical irradiation parameter settings, the proper horizontal and vertical intervals of the center points of the two adjacent irradiation areas can be obtained, so that the light passing area of the solid sheet is fully utilized, and the rear irradiation area is not affected by the aging of the front adjacent irradiation area. After the horizontal and vertical intervals of the central points of the adjacent irradiation areas are determined, the movement path of the irradiation areas of the solid sheet in the long-time irradiation process and the movement parameters of each micro-displacement axis during each irradiation area update can be set according to the size of the solid sheet and the laser incident angle.
For example, for the 10mm×10mm quartz glass used in this example, when the irradiation spot diameter is 0.10mm and the incident angle is 45 degrees, it may be set such that irradiation is started near the point at the upper left corner of the solid sheet (e.g., 0.24mm and 0.20mm from the left and upper edges, respectively), and then the irradiation points are updated at intervals of 0.28mm on the sheet surface in the horizontal direction (corresponding to the setting of the two-dimensional stage, the horizontal direction displacement axis is moved by 0.28mm, and the x and z displacement axes are moved by 0.20mm, respectively) corresponding to the setting of the three-dimensional stage. After scanning 35 irradiation spots in the horizontal direction, the irradiation spot reached a position near the upper right corner of the solid sheet, and updating the next irradiation spot was completed by moving down the sheet surface in the vertical direction at intervals of 0.20mm (corresponding to the setting of the two-dimensional or three-dimensional platform, the y-displacement axes were each moved by 0.20 mm). The update operation of the second row of irradiation spots may then be similar to the first row, moving in the opposite direction until the left edge of the solid sheet is approached. At this time, the irradiation spot update operation will be returned to the first row case by moving downward in the vertical direction by 0.20mm interval. By repeating the above scanning operation, the light passing area of the entire solid sheet will be effectively utilized, and 35×49=1715 irradiation points can be finally obtained. The schematic diagram of the movement path of the solid sheet irradiation region is shown in fig. 5, which is preset and stored in the master control system. In fact, if the direction of movement or the initial position of the irradiated area of the solid sheet is changed, different movement paths can be obtained, which have the same technical effect as in the present embodiment, as shown in fig. 6 and 7, and the movement paths have various possibilities, which are not listed here.
In this embodiment, after the master control system 7 sends out the instruction for controlling the micro-displacement platform 8 to move, the micro-displacement platform moves a specific distance according to the preset path, so that the irradiation area of the solid sheet is updated. For the solid sheet, if the average use time of each irradiation point is controlled to be more than 1 hour, the total use time of the solid sheet can be more than 1715 hours, namely the time of long-time high-stability output of the super-continuous light source can be more than 1715 hours, which is far more than that of the super-continuous spectrum light source of the solid sheet without closed-loop control irradiation area update and single-point irradiation. In fact, if the irradiation and motion parameters are kept unchanged, the size of the solid sheet is only enlarged to 30mm×30mm, that is, the effective surface area is changed to 9 times that of the original solid sheet, the total service time of the corresponding sheet can exceed 1715×9= 15435 hours, that is, the time of long-time high-stability output of the super-continuous light source can exceed 15435 hours, and the service life requirement of the whole operation period of the general light source device can be met. That is, the solid-sheet supercontinuum light source produced by the method described in this embodiment does not require replacement of the supercontinuum generation medium throughout the operational life cycle, thereby significantly reducing the cost of operation and maintenance thereof.
In general, under certain irradiation parameter setting conditions, the average use time of each irradiation point is determined by the laser irradiation power (determining the aging speed of the solid sheet, i.e., the falling rate of the sampling power after the action) and the power falling threshold (determining the ultimate aging degree of the solid sheet, i.e., the ultimate falling amplitude of the sampling power after the action). Therefore, when the power drop threshold is set to a fixed value, as described above, equal to the standard deviation of the 5 times ultrashort pulse laser power, the irradiation point average use time is determined only by the laser irradiation power. For both of the above-described typical irradiation parameter settings of the present embodiment, the laser irradiation power is set in a power interval that brings the pulse spectrum broadening close to saturation. In fact, in this laser power interval, the variation of the laser power causes a significant variation of the solid sheet aging speed (aging time constant), i.e. the average time of use of the irradiation spot, but not the relative pulse spectrum broadening. Therefore, the embodiment can obtain the average service time of the irradiation point meeting the requirement by properly adjusting the laser irradiation power in the laser irradiation power interval, and can also meet the requirement of pulse spectrum broadening.
The displacement motion mode of the two-dimensional or three-dimensional micro-displacement platform according to the embodiment is not limited to the translational motion mode, but also includes a rotational motion mode, or a combination of the translational and rotational displacement motion modes. For example, a two-dimensional micro-displacement table consisting of a one-dimensional translation table and a one-dimensional rotation table can also realize continuous position updating of the solid sheet irradiation region through combined motion of two displacement dimensions. Particularly, when the solid sheet has a circular light-transmitting surface, but not the square light-transmitting surface, the two-dimensional micro-displacement platform formed by the one-dimensional translation platform and the one-dimensional rotary platform can make the irradiation light spot form a spiral scanning line on the wafer by setting a proper two-dimensional motion scheme, so that the full utilization of the irradiation area of the circular light-transmitting surface of the solid sheet is realized, as shown in fig. 8 and 9. That is, for a circular solid sheet, the two-dimensional displacement mode of the combination of rotation and translation is a displacement motion mode which is more matched with the wafer than the two-dimensional displacement mode of pure translation.
In addition, the materials of the solid sheet in this embodiment are not limited to quartz glass, but include various transparent solid media applicable to supercontinuum generation, such as various common wide band gap crystalline or amorphous materials. The thickness of the solid sheet in this embodiment is not limited to 0.10mm, and can be flexibly adjusted according to practical application requirements. The number of the solid sheets in this embodiment is not limited to 1, and can be flexibly set according to the actual application requirement.
Example 2
In this embodiment, in step S3, unlike embodiment 1 which performs sampling detection on the ultrashort laser pulse reflected after the interaction with the solid-state sheet, this embodiment performs beam-splitting sampling detection on the ultrashort laser pulse transmitted through the solid-state sheet after the interaction with the solid-state sheet, as shown in fig. 3. Specifically, when the ultra-short laser pulse transmitted through the solid sheet is further detected by beam splitting sampling, a beam splitting sheet 9 for beam splitting is arranged behind the solid sheet 5, the emergent ultra-short laser pulse is divided into two parts, one part is output as ultra-continuous spectrum ultra-short pulse, the other part is used as sampling beam, and the sampling beam is collected by an optical power probe and transmitted to a main control system after photoelectric conversion, so that the real-time measurement and monitoring of the power of the ultra-short laser pulse sampling beam are realized, and the aging condition of the solid sheet is further obtained.
Example 1 the reflected beam power of the high energy ultrashort laser pulse after interaction with the solid sheet was monitored in real time. Compared with the embodiment 2, the embodiment 1 can directly utilize the reflected light beam of the solid sheet to realize the real-time monitoring of the high-energy ultrashort laser pulse light beam power after the effect of the solid sheet, and does not need to add an additional optical element (the embodiment 2 needs to add an additional beam splitting sheet 9 in a light path), so that the light path design is more concise and efficient, meanwhile, the embodiment 1 adopts the reflected light beam of the solid sheet to carry out sampling detection monitoring, so that the characteristic (the output power and the frequency spectrum chirp characteristic) of the output supercontinuum is not influenced, the necessary influence on the characteristic of the output supercontinuum ultrashort pulse caused by the insertion of the optical element in an emergent light path is effectively avoided, and the practicability is higher. Thus, example 1 uses an ultrashort laser pulse that reflects after interaction with a solid sheet for sample detection is a preferred option.
Example 3
Based on embodiment 1 or embodiment 2, for the preset solid sheet motion control mode adopted in step S5 in embodiment 1, the present embodiment provides another preset solid sheet motion control mode, as shown in preset motion mode 2 in fig. 4, specifically, the main control system controls the micro-displacement platform to perform quasi-continuous motion, so that the ultra-short laser pulse performs quasi-continuous scanning on the solid sheet according to a preset scanning path, and continuous updating of the sheet irradiation area in the process of irradiating the solid sheet by the strong laser for a long time is realized, thereby realizing high stability output of the ultra-short pulse with a super-continuous spectrum.
The main control system 7 in this embodiment gives out the command for controlling the micro-displacement platform 8 to move in a quasi-continuous manner at intervals far smaller than the aging time constant, so that the micro-displacement platform 8 moves in a quasi-continuous manner along a predetermined path at a specific movement interval significantly smaller than the diameter of the irradiation spot, thereby enabling the irradiation area of the solid sheet to be updated in a near-continuous scanning manner in space and time. In the process of platform movement, based on the monitoring of the real-time sampling power dropping amplitude, the average movement speed (average movement speed=updating movement distance/updating time interval) of the platform is subjected to closed-loop optimization feedback regulation and control of the main control system 7, so that the dropping amplitude of the real-time sampling power is stabilized to be close to 0 or a smaller preset value relative to the initial sampling power in the process of long-time irradiation of the solid sheet, and the long-time stability of ultra-short pulse output of the ultra-continuous spectrum is improved. In particular, regulating the average movement speed can be achieved by regulating the update movement distance alone, regulating the update time interval alone, or regulating both the update movement distance and the update time interval.
Since the preset solid-state sheet motion control mode described in embodiment 1 is triggered by the power-down threshold, the irradiation area update has the characteristics of spatial fixed point and time dispersion, and can cause the power curve of the supercontinuum output to exhibit periodic jump determined by the threshold-down amplitude. In addition, since the threshold should be set to be greater than the standard deviation of the multiple incident laser power, such periodic fluctuations that determine the operational stability of the supercontinuum light source will result in the output supercontinuum stability being still significantly lower than the stability of the incident laser.
In contrast, the preset solid-state sheet motion control mode according to the present embodiment, whose irradiation area update is not triggered by the threshold condition, solves the problem of periodic power jump due to the presence of the threshold in embodiment 1, and is well suited to this embodiment. It can be seen that the irradiation region updating mode of this embodiment can bring about higher stability performance for the output supercontinuum. In fact, since the motion speed of the micro-displacement platform in the present embodiment is controlled by the closed-loop optimization feedback of the main control system 7, the monitored power can be reduced by a magnitude smaller than the second threshold, that is, the output of the supercontinuum can obtain very high stability.
In the preset solid sheet motion control mode of the present embodiment, the initial average motion speed is obtained based on the detection evaluation of the aging condition of the solid sheet. Specifically, through step S3 and step S4, data of the fixed-point irradiation power of the solid sheet changing with time can be measured, quantitative evaluation concerning the aging speed of the solid sheet can be obtained, such as obtaining a quantitative aging time constant, and then setting a suitable initial average movement speed (suitable irradiation update movement pitch and irradiation update time interval) based on the evaluation. Generally, the initial average movement speed should be set to at least satisfy the time interval of aging time constant, and the movement interval of the irradiation area is larger than the diameter of the laser irradiation spot, so as to avoid the aging accumulation of the irradiation area during the near continuous scanning process to be too serious, even the ablation damage occurs. For example, when the initial average moving speed is set to a value that satisfies the condition that the micro-displacement stage is quasi-continuously moved in the horizontal direction by 0.28mm in the time of 1 hour in this embodiment, that is, the initial average moving speed in the horizontal direction is set to 0.28 mm/hour, as compared with the case where the average use time of each irradiation point of embodiment 1 is 1 hour (the aging time constant is more than 1 hour), the embodiment can obtain similar aging characteristics of the solid sheet in the irradiation effect, and thus the initial average moving speed thus set can effectively avoid serious aging of the solid sheet.
In fact, since the quasi-continuous scanning irradiation mode (the front and rear irradiation spot areas are significantly overlapped) adopted in this embodiment can more fully utilize the area covered by the scanning path than the interval fixed-point irradiation mode (the front and rear irradiation spot areas are prevented from being overlapped) adopted in embodiment 1, a larger effective irradiation area is obtained. Thus, if the two solid sheet motion control modes employ the same average motion velocity, the material of this embodiment will exhibit weaker aging characteristics. That is, the present embodiment is advantageous in terms of fully utilizing the surface area of the sheet, and slowing down the aging of the material.
It should be noted that the solid sheet in this embodiment adopts a quasi-continuous motion mode, not a true continuous motion mode. Specifically, the master control system 7 sends out instructions for controlling the micro-displacement platform 8 to move according to the update time interval which is far smaller than the aging time constant, so that the micro-displacement platform 8 moves according to the update movement interval which is far smaller than the irradiation spot diameter. Since the update movement pitch is set to be much smaller than the irradiation spot diameter and the update time interval is set to be much smaller than the aging time constant, such movement can be regarded as quasi-continuous movement as viewed from the spatial scale of the irradiation spot or the time scale of the aging time constant.
Although the movement patterns of the embodiment 1 and the present embodiment are essentially discrete movements in time and space, the relationship between the time interval of the irradiation area update and the aging time constant, and the relationship between the movement interval of the irradiation area update and the irradiation spot diameter determine that the movement patterns have significantly different movement pattern characteristics and irradiation effects, and finally, significantly different effects are brought to the supercontinuum output stability. For example, in example 1, the time interval of irradiation region position update is determined by a threshold trigger condition, the typical value is 1 hour, and the corresponding horizontal movement interval is 0.28mm and is twice the irradiation spot diameter (under the condition of 45 degrees oblique incidence of irradiation laser), so that the movement mode is typical discrete movement in time and space, and the irradiation region update mutation caused by the movement mode can bring about significant periodic fluctuation of the ultra-short pulse output curve of the supercontinuum. Whereas the initial time interval and initial movement pitch (together determining the initial average movement speed) for the irradiation zone position update in this embodiment are determined by the movement parameters set by the master control system 7 (the parameters are obtained by evaluation of the aging condition of the solid sheet in steps S3 and S4), typically, when the update time interval is set to 45 seconds and the initial movement pitch is 0.0035mm, this embodiment can obtain not only the average movement speed (0.28 mm/hr) consistent with that of embodiment 1 but also an approximately quasi-continuous movement pattern. Because the time interval (45 seconds) is much smaller than the aging time constant (greater than 1 hour) and the movement pitch (0.0035 mm) is also much smaller than the irradiation spot diameter (0.10 mm). The quasi-continuous motion mode in the embodiment enables the material aging characteristic of the irradiation region to extend uniformly and slowly along the motion path, avoids the material characteristic mutation in the irradiation region updating process in embodiment 1, and is more beneficial to obtaining long-time high-stability output of the supercontinuum.
In the embodiment, the solid slice adopts a quasi-continuous motion mode rather than a continuous motion mode, which is beneficial to reducing the operation load of the micro-displacement platform and the control system on one hand and is beneficial to the real-time processing of feedback signals and the time sequence stabilization of closed-loop control in the closed-loop control process on the other hand. First, for the micro-displacement platform based on the stepper motor used in the present embodiment, the periodic intermittent operation is more advantageous for the long-time stable operation of the platform stepper motor than the long-time continuous operation. For example, for the quasi-continuous motion described above with 45 second time intervals and a motion pitch of 0.0035mm, the stepper motor micro-displacement table in this embodiment can be run continuously 7 steps in 1 second time to achieve a motion pitch of 0.0035mm in steps of 0.0005 mm. That is, in the irradiation area updating time interval of 45 seconds, only the micro-displacement platform is in an operating state in less than 1 second, and the micro-displacement platform is in a static state in more than 44 seconds, so that the heating problem caused by long-time continuous operation of the platform stepper motor can be remarkably reduced, and the working stability and the service life of the platform stepper motor are improved. On the other hand, the control module of the micro-displacement platform is in a standby state, so that the running load of the system control program in the quasi-continuous motion mode is low, the real-time processing of the feedback signal by the control program in the closed-loop control process is easy to realize, and the time sequence stability of the closed-loop control process is ensured.
The average motion velocity parameter in this embodiment is an important parameter for determining the output power of the supercontinuum source and the stability of the spectral broadening. In theory, if the average moving speed of the solid sheet is too slow, the laser irradiation flux received by each unit area is too high, the material is excessively aged, the output quality and stability of the supercontinuum are affected, and if the average moving speed of the solid sheet is too fast, the irradiation area is updated too fast, the solid sheet is scanned in a whole area in a short time, and the solid sheet needs to be replaced, so that the service life of the solid sheet is shortened. As described above, the initial average moving speed in the present embodiment is obtained from the data of the change in the fixed-point irradiation power with time by performing the detection evaluation of the aging condition of the solid sheet. Such a detection evaluation may ensure that the initial average movement speed is in a relatively suitable range, providing a parameter basis for the initial operation of the system. Then, during the quasi-continuous motion of the micro-displacement platform, based on the monitoring of the power reduction amplitude, the average motion speed of the platform is further subjected to closed-loop optimization feedback regulation and control by the main control system 7, so that the power reduction amplitude of the solid sheet stably approaches to 0 or a smaller preset value during the long-time irradiation. If the power drop width is gradually increased or kept at a larger value, it indicates that the material aging is aggravated or serious, that is, the irradiation area is updated too slowly, so that the master control system 7 will increase the average movement speed of the displacement platform (by increasing the update movement interval under the condition of keeping the update time interval unchanged, or by reducing the time interval under the condition of keeping the update movement interval unchanged), so as to accelerate the update of the irradiation area of the solid sheet, and reduce the power drop width to be close to 0 or a smaller preset value. By the closed-loop optimized feedback regulation, the supercontinuum source based on the solid slice can obtain long-time stable output, and the stability of the supercontinuum source can be close to the level of the input ultrashort pulse laser.
Further, if the initial average moving speed is directly set so that the aging degree (power drop amplitude) of the solid sheet in the near continuous scanning process is in a slight range, for example, the drop amplitude of the real-time laser power compared with the initial laser power is smaller than the standard deviation of the ultra-short pulse laser power by 1 time, at this time, the influence of the aging of the sheet on the stability and the spectrum broadening characteristic of the output ultra-short pulse power is almost negligible, and the noise of the output ultra-short pulse is mainly determined by the noise of the input ultra-short pulse, so that the stability close to the input ultra-short laser pulse and the spectrum broadening close to the sheet under the initial irradiation can be obtained. In fact, under the near-continuous scanning and weak irradiation aging conditions, the ultra-continuous spectrum ultra-short pulse output with high stability can be obtained without implementing the closed-loop optimization feedback regulation and control.
The movement path of the present embodiment can be implemented entirely in accordance with the movement path of embodiment 1 under the same laser irradiation parameter setting. Thus, solid flakes of the same area have the same useful life for both modes of motion under the same average velocity of motion. That is, for this example, a 30mm by 30mm solid sheet also achieved a service life in excess of 15435 hours, allowing for a system life cycle free of replacement of supercontinuum generating materials.
Example 4
For the preset solid sheet motion control mode adopted in step S5 in embodiment 1 or embodiment 3, the present embodiment provides another preset solid sheet motion control mode, as shown in preset motion mode 3 in fig. 4, specifically, the main control system controls the micro-displacement platform to continuously move along a preset path, so that the ultra-short laser pulse continuously scans the solid sheet according to the preset scanning path, and continuous update of the sheet irradiation area in the process of irradiating the solid sheet by the strong laser for a long time is realized, thereby realizing high stability output of the ultra-continuous spectrum ultra-short pulse.
In example 3, in step S5, the solid sheet adopts a quasi-continuous motion mode, not a true continuous motion mode. As described in embodiment 3, this motion pattern is compatible with the motion characteristics of the displacement platform based on the stepper motor in embodiment 3 (the motion of the stepper motor is discrete even if the stepper motor is continuously operated), and is also beneficial to implementing closed-loop optimization feedback regulation. In fact, if a micro-displacement platform with continuous displacement capability is used, such as a micro-displacement platform based on a linear motor, a voice coil motor, a piezoelectric actuator and other technologies, a solid sheet placed on the micro-displacement platform can realize real continuous motion, so that continuous scanning irradiation of ultra-short laser pulses on the solid sheet in space and time is realized. In this embodiment, by replacing the micro-displacement platform based on the stepper motor in embodiment 3 with the micro-displacement platform based on the linear motor, the main control system 7 can control the micro-displacement platform 8 to continuously move according to a preset path, so that the ultra-short laser pulse continuously scans the solid sheet along the preset scanning path, continuous updating of the irradiation area of the sheet is realized, and further high stability output of the ultra-short pulse with ultra-continuous spectrum is realized.
In this embodiment, in step S5, the solid sheet maintains a constant movement speed during the long-time continuous movement, and continuously moves at a constant speed according to a preset path. That is, the sheet does not realize the adjustment of the movement speed through closed loop optimization feedback regulation during long-time movement. Compared with the embodiment 1 and the embodiment 3, the solid slice motion control mode of the embodiment is simpler, the motion characteristic of the solid slice motion control mode is completely determined by the initially set motion parameters and is independent of feedback control, so that the equipment and technical requirements of the control module in the main control system are obviously reduced.
On the other hand, since the closed-loop feedback control is not implemented in the ultra-short pulse long-time irradiation process, the aging degree of the solid sheet in the long-time irradiation process in the embodiment is completely determined by the motion parameters of the micro-displacement platform under the condition that the irradiation parameters are set consistently. Therefore, obtaining the appropriate motion parameters of the micro-displacement platform is particularly important for the present embodiment. Similar to example 3, the solid sheet movement speed in this example is obtained based on the detection evaluation of the aging condition of the solid sheet, and by the steps S3 and S4, the data of the change of the irradiation power of the fixed point of the solid sheet with time can be measured, and the quantitative evaluation of the aging speed of the solid sheet, such as the quantitative aging time constant, can be obtained, and the proper movement speed can be set based on the evaluation. Generally, the set movement speed should at least satisfy the time interval of aging time constant, and the movement interval of the irradiation area is larger than the diameter of the laser irradiation spot, so as to avoid the aging accumulation of the irradiation area during continuous scanning to be too serious, even ablation damage occurs. For example, according to the evaluation of the aging condition of the solid sheet in the embodiment 1 and the embodiment 3, the micro-displacement platform is set to have the moving speed of 0.28 mm/hour (consistent with the average moving speed of the embodiment 1 and the embodiment 3), and the embodiment can effectively avoid the serious aging condition of the solid sheet under the condition that the irradiation parameter setting and the moving path are consistent with the embodiment 3, and obtain the irradiation aging characteristics of the solid sheet and the ultra-short pulse output characteristics of the ultra-continuous spectrum similar to those of the embodiment 3. In fact, the continuous motion mode adopted in this embodiment can completely avoid the irradiation characteristic jump (even if the jump amplitude is small) still existing in the quasi-continuous motion mode in the irradiation area updating in embodiment 3, so that the tiny system noise of the supercontinuum output power brought by the quasi-continuous motion mode can be eliminated theoretically.
In this embodiment, the moving speed is further increased on the basis of the moving speed, so that the aging degree (power reduction amplitude) of the solid sheet during the continuous scanning process can be in a more slight range. For example, similar to the case of the weak irradiation aging in example 3, the higher movement speed is set so that the decrease in the real-time laser power from the initial laser power is smaller than the standard deviation of the ultra-short pulse laser power by 1 time, and at this time, the influence of the sheet aging on the stability of the output ultra-short pulse power and the spectral broadening characteristic is almost negligible, and the noise of the output ultra-short pulse is mainly determined by the noise of the input ultra-short pulse, so that the stability close to the input ultra-short laser pulse and the spectral broadening close to the sheet under the initial irradiation can be obtained. It is worth noting that the improvement of the solid slice movement speed is at the expense of the service life of the solid slice, so that the advantages and disadvantages of the two aspects can be balanced to set the proper movement speed in practical application, and the proper application economy and service life can be obtained while the technical requirements of the supercontinuum output power stability and the spectrum broadening characteristic are met.
The movement path of the present embodiment can also be implemented entirely in accordance with the movement path of embodiment 1 under the same laser irradiation parameter setting. Therefore, under the condition of similar movement speed as in the embodiment 1 and the embodiment 3, the embodiment can also obtain the service life of more than 1 ten thousand hours by adopting a 30mm multiplied by 30mm solid sheet, and the replacement-free super-continuum spectrum generating material of the whole operation life cycle of the system is realized.
It is to be understood that the above examples of the present invention are provided by way of illustration only and not by way of limitation of the embodiments of the present invention. Any modification, equivalent replacement, improvement, etc. which come within the spirit and principles of the invention are desired to be protected by the following claims.
Claims (8)
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