CN111368243A - Method for determining defocusing amount of laser in femtosecond laser polishing process of optical element - Google Patents
Method for determining defocusing amount of laser in femtosecond laser polishing process of optical element Download PDFInfo
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- CN111368243A CN111368243A CN202010095083.2A CN202010095083A CN111368243A CN 111368243 A CN111368243 A CN 111368243A CN 202010095083 A CN202010095083 A CN 202010095083A CN 111368243 A CN111368243 A CN 111368243A
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- 230000003287 optical effect Effects 0.000 title claims abstract description 30
- 238000000034 method Methods 0.000 title claims abstract description 26
- 238000007517 polishing process Methods 0.000 title description 6
- 239000000463 material Substances 0.000 claims abstract description 37
- 238000005498 polishing Methods 0.000 claims abstract description 34
- 238000002679 ablation Methods 0.000 claims abstract description 24
- 238000013178 mathematical model Methods 0.000 claims abstract description 10
- 238000000608 laser ablation Methods 0.000 claims abstract description 8
- 230000008569 process Effects 0.000 claims abstract description 5
- 238000004364 calculation method Methods 0.000 claims abstract description 3
- 239000000969 carrier Substances 0.000 claims description 8
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 6
- 230000005684 electric field Effects 0.000 claims description 6
- 230000000694 effects Effects 0.000 claims description 4
- 229910052681 coesite Inorganic materials 0.000 claims description 3
- 229910052906 cristobalite Inorganic materials 0.000 claims description 3
- 239000000377 silicon dioxide Substances 0.000 claims description 3
- 229910052682 stishovite Inorganic materials 0.000 claims description 3
- 229910052905 tridymite Inorganic materials 0.000 claims description 3
- XOFYZVNMUHMLCC-ZPOLXVRWSA-N prednisone Chemical group O=C1C=C[C@]2(C)[C@H]3C(=O)C[C@](C)([C@@](CC4)(O)C(=O)CO)[C@@H]4[C@@H]3CCC2=C1 XOFYZVNMUHMLCC-ZPOLXVRWSA-N 0.000 claims description 2
- 230000005865 ionizing radiation Effects 0.000 claims 1
- 238000004519 manufacturing process Methods 0.000 abstract description 3
- 238000012545 processing Methods 0.000 description 8
- 238000003754 machining Methods 0.000 description 6
- 238000010884 ion-beam technique Methods 0.000 description 3
- 230000009471 action Effects 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 238000002474 experimental method Methods 0.000 description 2
- 230000007246 mechanism Effects 0.000 description 2
- 239000003990 capacitor Substances 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
- 230000003746 surface roughness Effects 0.000 description 1
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F17/00—Digital computing or data processing equipment or methods, specially adapted for specific functions
- G06F17/10—Complex mathematical operations
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K26/00—Working by laser beam, e.g. welding, cutting or boring
- B23K26/352—Working by laser beam, e.g. welding, cutting or boring for surface treatment
- B23K26/3568—Modifying rugosity
- B23K26/3576—Diminishing rugosity, e.g. grinding; Polishing; Smoothing
Abstract
The invention discloses a method for determining the defocusing amount of laser in the process of femtosecond laser polishing an optical element, which belongs to the technical field of mechanical precision manufacturing engineering and comprises the following steps: outputting laser wavelength by a femtosecond laser, focusing the radius of a light spot and calculating the Rayleigh length of the light beam; the laser outputs pulse energy to focus the radius of a light spot, and the energy density of the laser when the laser reaches the surface of the material is calculated out of the focus; calculating the ablation depth of the surface of the energy material by using a femtosecond laser ablation rate mathematical model; solving the ablation depth of the femtosecond laser to the material under different defocusing amounts by a laser ablation rate mathematical model; and measuring the average value of the surface of the optical element to be polished, and determining the optimal polishing defocusing amount by utilizing the height difference of the wave crest and the wave trough to carry out calculation. The defocusing amount of femtosecond laser polishing can be rapidly, efficiently and accurately determined, and the method is low in cost, high in efficiency and high in accuracy.
Description
Technical Field
The invention relates to the technical field of mechanical precision manufacturing engineering, in particular to a method for determining the defocusing amount of laser in a process of laser polishing an optical element in a femtosecond laser.
Background
The precision forming technology of optical materials focuses on the size, properties and surface roughness of the optical materials to meet correspondingly strict manufacturing requirements. The optical material has the processing characteristics of hard and brittle materials, and is characterized in that the optical material is not easy to deform under the action of external force and has low fracture toughness, the elastic limit and the strength of the material are close, and the polishing modes of optical elements are currently divided into three types. The first is conventional mechanical polishing, the second is plasma beam polishing, and the third is laser polishing. The traditional machining method is usually 'hard-to-hard', and the machining method usually generates defects such as cracks, surface damages and the like on the machined surface during machining, and the defects seriously affect the service characteristics of the optical material. Ion beam machining and laser machining, both of which avoid contact of the material with the tool during machining, with the energy beam instead of the material, are considered to have no contact stress. And the energy beam can be focused in a tiny action radius, so that the processing precision is greatly improved, and the nano-scale processing taking atoms as a measurement unit can be realized. Meanwhile, compared with ion beam polishing, laser polishing has lower requirements on processing environment, and meanwhile, compared with an ion beam generating device, the laser emitting device is relatively simple.
With the existing laser polishing method, the polishing result of the optical element by the femtosecond laser is optimal, and since the removing mechanism of the optical material by the femtosecond laser is a cold removing mechanism and the thermal influence on the rest of the material is small, there have been many researches on the femtosecond laser polishing of the optical element. However, since the laser polishing process has complex influencing factors and more processing parameters which are related to each other, it is complicated and tedious to find suitable polishing parameters at the present stage. Defocusing amount (distance between a laser focus and the surface of a material) is used as an important influence factor in the femtosecond laser polishing process, and how to determine the proper defocusing amount is an urgent problem to be solved in engineering. The existing solution is to estimate a proper defocus amount through experimental summary, a large amount of experiments are performed to determine the proper defocus amount after the structures of the processing material and the processing surface are changed, the scanning speed of laser polishing is limited, and the laser is preheated for a long time before the experiments, so that the process of selecting proper processing parameters becomes very slow. And no method can accurately determine the optimal defocus amount when the optical element is polished by the femtosecond laser at present.
Disclosure of Invention
1. Technical problem to be solved
In order to solve the problems that the cost is high, the efficiency is low and the accuracy is poor when the existing femtosecond laser polishing optical element determines the proper polishing defocusing amount, the invention provides a method for quickly and accurately calculating the size of the proper laser polishing defocusing amount under the specific process condition by combining a mathematical model and a calculation method.
2. Technical scheme
In order to solve the above problems, the present invention adopts the following technical solutions.
A method for determining the defocus of a laser during femtosecond laser polishing of an optical element, comprising the steps of:
s1, outputting laser wavelength lambda by a femtosecond laser, focusing the spot radius r and calculating the Rayleigh length Z of the light beamR；
S2, outputting pulse energy F by a laser to focus the radius r of a light spot, and calculating the energy density F' when the laser reaches the surface of the material by the defocusing amount Z;
s3, calculating the ablation depth L of the surface of the energy material by using a femtosecond laser ablation rate mathematical model;
s4, calculating different defocus Z by the laser ablation rate mathematical modelnAblation depth L (Z) of lower femtosecond laser to materialn)；
S5, measuring the average PV value of the surface of the optical element to be polished by using a plane digital interferometer, and dividing the wave crest into reference surfaces, namely the defocusing amount of the wave crest is ZnThe defocus amount of the trough is Zn+ PV, corresponding to a height difference between the peak and the trough of △ ═ L (Z)n)-L(Zn+ PV) corresponding to the respective defocus calculated above the ablation threshold of the material for the energy density of the laser applied to the material, the most suitable femtosecond laser polishing defocus when △ -PV, and the best polishing defocus when △ is constantly less than PV and △ reaches a maximum.
Further, in the step S1, the Rayleigh length ZRIs calculated by the formula
Further, in S2, the energy density F' is calculated by the formula
Further, in S3, the ablation rate of the femtosecond laser is mathematically modeled as
Where L is the ablation depth, h is the Planck constant, ω is the laser frequency, τ is the laser pulse width, FthAblation threshold, p, for materials receiving laser light of a particular pulse width and wavelengtha0Is the initial valence band electron density, ρc0For initial conduction band electron density, WPI(FTH)And η(FTH)Respectively photo ionization and impact ionization coefficients when the energy density reaches the ablation threshold.
Further, WF＝[F/FTH]nWPI(FTH)，HF＝[F/FTH](1/2)。
Further, WPI(FTH)The following equation is obtained:
wherein m is the effective mass of the cavity of the quartz glass material, and Delta is SiO2Phi is the Dorsen integral function, E is the laser field intensity, and U is delta-E2E2/(4mω2)。
Further, η(FTH)The following equation is obtained:
wherein v issAs the saturation drift rate of electrons, EiElectric field strength, E, required for the carriers to overcome ionizing radiationPTo carry currentElectric field strength required to overcome phonon scattering, EKTThe electric field strength required to overcome the thermal scattering effect for the carriers.
Further, in the S4, ZnN δ (n — 0,1,2, … …), where δ is the step length of each increment.
3. Advantageous effects
Compared with the prior art, the invention has the advantages that:
the femtosecond laser polishing device can realize the femtosecond laser polishing process of an optical element, quickly, efficiently and accurately determine the defocusing amount of femtosecond laser polishing, and has low cost, high efficiency and high accuracy.
Drawings
FIG. 1 is a flow chart of the present invention.
Detailed Description
The drawings in the embodiments of the invention will be combined; the technical scheme in the embodiment of the invention is clearly and completely described; obviously; the described embodiments are only some of the embodiments of the invention; but not all embodiments, are based on the embodiments of the invention; all other embodiments obtained by a person skilled in the art without making any inventive step; all fall within the scope of protection of the present invention.
Example 1:
referring to fig. 1, a method for determining a defocus amount of a laser in a femtosecond laser polishing process of an optical element, includes the steps of:
A. using femtosecond laser to output laser wavelength lambda (m), focusing spot radius r (m) to calculate Rayleigh length ZR (m) of the light beam,
B. the laser is used for outputting pulse energy F (J) and focusing spot radius r (um) and defocusing amount Z (um) to calculate the energy density F' (J/um) when the laser reaches the surface of the material2),
C. Calculating the ablation depth L (m) of the surface of the energy material by using a femtosecond laser ablation rate mathematical model, wherein the ablation rate mathematical model is as follows:
wherein L is the ablation depth; h is the Planck constant; omega is laser frequency; τ is the laser pulse width; fth is the ablation threshold of the material receiving laser with specific pulse width and wavelength (corresponding to the laser with the pulse width of 220fs and the wavelength of 800nm, the ablation threshold value is 2.5J/cm2)；ρa0The initial valence band electron density (1.76 × 1023 cm-3); pc0The initial conduction band electron density (108 cm-3); wF＝[F/FTH]nWPI(FTH)； HF＝[F/FTH](1/2)；WPI(FTH)And η(FTH)Photo ionization and impact ionization coefficients, respectively, at which the energy density reaches the ablation threshold.
WPI(FTH)Can be obtained by the following formula:
wherein m is the effective mass of the cavity of the quartz glass material; delta is SiO2The forbidden band width of the capacitor; phi is the Dorsen integral function, E is the laser field intensity; u ═ Δ -e2E2/(4mω2)。
η(FTH)Can be obtained by the following formula:
wherein vs is the saturation drift rate of the electron; eiElectric field strength, E, required for the carriers to overcome ionizing radiationPThe electric field strength required to overcome phonon scattering for the carriers; eKTThe electric field strength required to overcome the thermal scattering effect for the carriers.
D. By the mathematical model of laser ablation rate in S3Calculating different defocus amounts ZnAblation depth L (Z) of femtosecond laser to material at n δ (n is 0,1,2, … …)n) Where δ is the step size of each increment.
E. The average PV value (m) of the surface of the optical element to be polished is measured by using a plane digital interferometer, the peak is taken as a reference plane, namely the defocusing amount of the peak is Zn (m), the defocusing amount of the trough is Zn + PV (m), the height difference between the peak and the trough is △ ═ L (Zn) -L (Zn + PV), the defocusing amount corresponding to the energy density of the laser acting on the material corresponds to each defocusing amount above the ablation threshold of the material, the defocusing amount is calculated, the optimal femtosecond laser polishing defocusing amount is obtained when △ ═ PV is obtained, and the defocusing amount when △ reaches the maximum value is obtained when △ is constantly smaller than PV.
The above; but are merely preferred embodiments of the invention; the scope of the invention is not limited thereto; any person skilled in the art is within the technical scope of the present disclosure; according to the technical scheme and the improvement concept of the invention, equivalent substitutions or changes are made; are intended to be covered by the scope of the present invention.
Claims (8)
1. A method for determining the defocusing amount of laser in the process of polishing an optical element by femtosecond laser is characterized in that: the method comprises the following steps:
s1, outputting laser wavelength lambda by a femtosecond laser, focusing the spot radius r and calculating the Rayleigh length Z of the light beamR；
S2, outputting pulse energy F by a laser to focus the radius r of a light spot, and calculating the energy density F' when the laser reaches the surface of the material by the defocusing amount Z;
s3, calculating the ablation depth L of the surface of the energy material by using a femtosecond laser ablation rate mathematical model;
s4, calculating different defocus Z by the laser ablation rate mathematical modelnAblation depth L (Z) of lower femtosecond laser to materialn)；
S5, measuring the average PV value of the surface of the optical element to be polished by using a plane digital interferometer, and dividing the wave crest into reference surfaces, namely the defocusing amount of the wave crest is ZnTrough of waveDefocus amount of Zn+ PV, corresponding to a height difference between a peak and a trough of △ ═ L (Z)n)-L(Zn+ PV) corresponding to the respective defocus calculated above the ablation threshold of the material for the energy density of the laser applied to the material, the most suitable femtosecond laser polishing defocus when △ -PV, and the best polishing defocus when △ is constantly less than PV and △ reaches its maximum.
2. A method of determining defocus of a laser during femtosecond laser polishing of an optical element as set forth in claim 1, wherein: in the S1, the Rayleigh length ZRIs calculated by the formula
3. A method of determining defocus of a laser during femtosecond laser polishing of an optical element as set forth in claim 1, wherein: in S2, the calculation formula of the energy density F' is
4. A method of determining defocus of a laser during femtosecond laser polishing of an optical element as set forth in claim 1, wherein: in S3, the ablation rate of the femtosecond laser is mathematically modeled as
Where L is the ablation depth, h is the Planck constant, ω is the laser frequency, τ is the laser pulse width, FthAblation threshold, p, for materials receiving laser light of a particular pulse width and wavelengtha0Is the initial valence band electron density, ρc0For initial conduction band electron density, WPI(FTH)And η(FTH)Photo ionization and impact ionization coefficients, respectively, at which the energy density reaches the ablation threshold.
5. The method of claim 4, wherein the step of determining the defocus amount of the laser during the femtosecond laser polishing of the optical element comprises the following steps: wF＝[F/FTH]nWPI(FTH)，HF＝[F/FTH](1/2)。
6. The method of claim 4, wherein the step of determining the defocus amount of the laser during the femtosecond laser polishing of the optical element comprises the following steps: WPI(FTH)The following equation is obtained:
wherein m is the effective mass of the cavity of the quartz glass material, and Delta is SiO2Phi is the Dawson integral function, E is the laser field intensity, and U is delta-E2E2/(4mω2)。
7. The method of claim 4, wherein η is the amount of defocus of the laser during the femtosecond laser polishing of the optical element(FTH)The following equation is obtained:
wherein v issAs the saturation drift rate of electrons, EiThe electric field strength required for the carriers to overcome the ionizing radiation, EPThe electric field strength required for the carriers to overcome phonon scattering, EKTThe electric field strength required to overcome the thermal scattering effect for the carriers.
8. A method of determining defocus of a laser during femtosecond laser polishing of an optical element as set forth in claim 1, wherein: in said S4, ZnN δ (n ═ 0,1,2, … …), where δ is the step size per increment.
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Cited By (2)
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| CN113414889A (en) * | 2021-06-29 | 2021-09-21 | 华中科技大学 | Method and device for compounding laser-assisted diamond cutting and laser polishing in situ |
| CN113500711A (en) * | 2021-06-29 | 2021-10-15 | 华中科技大学 | High-precision composite energy field auxiliary cutting and finishing equipment and method |
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| CN109731968A (en) * | 2019-03-01 | 2019-05-10 | 大连理工大学 | A kind of arc groove laser bending shaping method based on plastic zone depth |
| CN109773340A (en) * | 2019-01-28 | 2019-05-21 | 广东工业大学 | A kind of laser cleaning for carbon steel surface and polishing combined machining method |
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| CN107498176A (en) * | 2017-08-02 | 2017-12-22 | 中国科学院光电研究院 | A kind of PRK polishing of porous ceramics and detection method |
| CN109773340A (en) * | 2019-01-28 | 2019-05-21 | 广东工业大学 | A kind of laser cleaning for carbon steel surface and polishing combined machining method |
| CN109731968A (en) * | 2019-03-01 | 2019-05-10 | 大连理工大学 | A kind of arc groove laser bending shaping method based on plastic zone depth |
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113414889A (en) * | 2021-06-29 | 2021-09-21 | 华中科技大学 | Method and device for compounding laser-assisted diamond cutting and laser polishing in situ |
| CN113500711A (en) * | 2021-06-29 | 2021-10-15 | 华中科技大学 | High-precision composite energy field auxiliary cutting and finishing equipment and method |
| CN113500711B (en) * | 2021-06-29 | 2022-09-13 | 华中科技大学 | High-precision composite energy field auxiliary cutting and finishing equipment and method |
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