阅读说明：本技术 用于处理材料的激光系统和方法 (Laser system and method for processing materials ) 是由 潘乔·察内科夫 乔纳森·埃尔曼 杰弗里·克迈蒂克 阿列克谢·阿夫多欣 安德瑞·巴布什金 于 2018-11-20 设计创作，主要内容包括：多波长激光处理系统配置有多波长激光源,用于产生多波长的同轴激光处理光束。激光处理系统还包括多波长光学系统,以将同轴激光处理光束递送到工件表面上的激光-材料相互作用区,使得处理光束中的第一激光波长和第二激光波长中的每个激光波长作为相应的第一同心激光斑和第二同心激光斑至少照射相互作用区的一部分。多波长光学系统包括多波长光束准直器、可配置的色差光学器件和激光处理聚焦透镜,其中可配置的色差光学器件提供对第一激光波长和第二激光波长的相对焦距的调整。(A multi-wavelength laser processing system is configured with a multi-wavelength laser source for generating a multi-wavelength coaxial laser processing beam. The laser processing system also includes a multi-wavelength optical system to deliver the coaxial laser processing beam to a laser-material interaction region on the workpiece surface such that each of a first laser wavelength and a second laser wavelength in the processing beam illuminates at least a portion of the interaction region as respective first and second concentric laser spots. The multi-wavelength optical system includes a multi-wavelength beam collimator, configurable chromatic aberration optics, and a laser processing focusing lens, wherein the configurable chromatic aberration optics provide adjustment of relative focal lengths of the first and second laser wavelengths.)

1. A method of laser machining a workpiece, comprising:

partially converting a first broadband unpolarized fiber laser beam of fundamental wavelength into at least one second beam of harmonic wavelength;

directing the first beam and a second beam along a path, the first and second beams being coaxial and overlapping in time;

irradiating the first light beam and the one second light beam on a surface of the workpiece having different absorption coefficients at a fundamental wavelength and a harmonic wavelength, respectively;

controlling a flux ratio between the first beam and the one second beam to be at least equal to a reciprocal of an absorption coefficient ratio at which the second beam provides a material state change and has a focal plane closer to the surface than a focal plane of the first beam to increase an absorption coefficient of the workpiece for the first beam.

2. The laser processing method of claim 1, further comprising: an axial chromatic aberration is created between the first beam and the second beam such that a focal point of the first beam is at a distance from the surface within the workpiece and a second focal point of the second beam is spaced from the first focal point from upstream.

3. The laser machining method of claim 2, wherein controlling the fluence ratio comprises:

displacing non-achromatic optics, said displacement producing achromatism along said path, or

Replacing the non-achromatic optic with another non-achromatic optic configured with a different refractive curvature or made of a different optical material, such as fused silica, CaF₂Or MgF₂Or other material capable of changing the respective focal lengths of the first and second beams.

4. The laser processing method according to claim 2, wherein the reciprocal ratio I is a ratio of absorption coefficients at the fundamental wave wavelength and the harmonic wavelength, respectively.

5. The laser processing method according to claim 1 or 2, further comprising: controlling a harmonic conversion efficiency of the first beam to the second beam to adjust a fluence ratio on the workpiece surface above a reciprocal of the absorption coefficient ratio, wherein the machining method is achieved with a smaller total laser power at all wavelengths than would be required without the second beam.

6. The laser machining method of claim 1, wherein the harmonic wavelengths of the second beam comprise second, third and/or fourth harmonic wavelengths or independently tunable wavelengths by using optical parametric processes and/or raman processes. Note that all or a plurality of beams at these wavelengths may impinge on the workpiece surface at the same time and at the same location.

7. The laser processing method of claim 1, further comprising: a plasma is created to increase an absorption coefficient of the workpiece for the first beam.

8. The laser processing method of claim 1, further comprising: the coaxial first and second beams are achromatically collimated after the second beam is generated by using only one or more mirrors.

9. A method of laser machining a workpiece, comprising:

converting a portion of a first broadband unpolarized fiber laser beam of a fundamental wavelength to at least one second broadband beam of a harmonic wavelength, wherein the unconverted portion of the first beam and the second beam are coaxial and temporally overlapping;

directing the first and second beams along paths through non-achromatic optics to produce chromatic aberration between the beams such that when the beams are simultaneously impinged on a surface of the workpiece, the second beam having a focal point at the surface provides a material state change to increase the absorption coefficient of the workpiece to the first beam having a focal point spaced from the surface, either within or outside the workpiece, the workpiece being made of materials having different absorption coefficients at fundamental and harmonic wavelengths, respectively.

10. The method of claim 9, further comprising: adjusting flux ratios of the first and second light beams at the fundamental and harmonic wavelengths, respectively, to provide a material state change, the flux ratio being at least equal to an inverse of an absorption coefficient ratio at the fundamental and harmonic wavelengths.

11. The method of claim 9, further comprising: controlling a harmonic conversion efficiency of the first light beam to the second light beam, wherein a material state change is provided if the conversion efficiency is greater than an inverse of an absorption ratio at each of a fundamental wavelength and a harmonic wavelength.

12. The laser processing method of claim 11, wherein the wavelength conversion efficiency is controlled to prevent a gaussian intensity distribution of the first beam from degrading into an annular intensity distribution during conversion, the first beam being a single-mode beam or a multimode beam.

13. The laser machining method of claim 11 wherein the wavelength conversion efficiency is controlled to provide an annular intensity distribution of the first beam.

14. The laser processing method of claim 11, wherein a material state change is provided if the energy of each of the first beam and the second beam incident on the workpiece is as follows,

Eth(λ)/Ethall(λ)＞1，

wherein Eth (λ) is an energy threshold of a laser beam coupled into the workpiece, which is solely required for laser processing of the workpiece by a single beam at each individual fundamental or harmonic wavelength, and Ethall (λ) is an energy threshold of a laser beam at an individual wavelength when coaxial first and second beams are simultaneously irradiated on the workpiece.

15. The laser processing method of claim 9, wherein the second beam has a harmonic wavelength selected from the group consisting of the second harmonic wavelength, the third harmonic wavelength, and the fourth harmonic of the fundamental wavelength, and combinations of these wavelengths, and/or has an independently tunable wavelength generated by an optical parametric process or a raman process.

16. The laser processing method of claim 9, further comprising: generating electron plasma from the second beam while irradiating the surface of the workpiece made of a material having an absorption coefficient at a wavelength of the second beam higher than an absorption coefficient at a wavelength of a fundamental wave.

17. The laser processing method of claim 9, further comprising: heating the surface of the workpiece, the workpiece being made of a fundamental wavelength that significantly increases absorption at higher wavelengths.

18. The laser processing method according to claim 9, wherein the beam of fundamental wavelength comprises a plurality of single-mode and/or multimode lasers emitted in the wavelength range of 0.9-2.1 μm, said lasers having the following parameters:

pulse durations in the range of seconds to femtoseconds,

m between 1 and 100²The factor(s) is (are),

a broad spectral line between 2nm and several hundred nm,

pulse energy in the range of μ J to J, and

average power between single digit watts and several hundred kilowatts;

a spectral linewidth of the second beam of light exceeding 0.5 nm.

19. The laser processing method of claim 9, further comprising: the coaxial first and second beams are achromatically collimated after the second beam is generated by using only one or more mirrors.

20. The laser processing method of claim 9, wherein the first beam is a single mode beam or a multimode beam produced by a fiber laser operating in a pulsed, continuous wave CW, or quasi-continuous wave QCW regime.

21. A modular laser machining apparatus for a workpiece, comprising:

a tunable harmonic generator operative to partially convert a first broadband unpolarized fiber laser beam of a fundamental wavelength into at least one second broadband beam of a harmonic wavelength, the first and second beams being coaxial and temporally overlapping and simultaneously impinging on a surface of a workpiece having different absorption coefficients at the fundamental wavelength and the harmonic wavelength, respectively;

a system operative to adjust a fluence ratio between the first beam and the second beam to be at least equal to a threshold at which the second beam provides a material state change on or near the surface to increase an absorption coefficient of the first beam.

22. The laser processing apparatus of claim 21, further comprising: non-achromatic optics located between the tunable harmonic generator and the workpiece and operative to produce an axial chromatic aberration between the unconverted first and second beams such that a focal point of the first beam is at a distance from the surface within the workpiece and a second focal point of the second beam is spaced upstream from the first focal point and closer to the surface.

23. The laser processing apparatus according to claim 22, wherein the controller is configured with: a flux measurement unit operated to measure flux; a processing unit operable to determine the flux ratio and compare the measured value to a threshold value; and an actuator operative to displace the non-achromatic optics, or to replace the non-achromatic optics, or to displace and replace the non-achromatic optics.

24. The laser processing apparatus according to claim 21, wherein the threshold value is at least equal to an inverse of an absorption ratio at the fundamental wavelength and the harmonic wavelength.

25. The laser processing apparatus of claim 24, further comprising a system for controlling a harmonic conversion efficiency of the first beam to the second beam to be greater than an inverse of an absorption coefficient ratio at each of a fundamental wavelength and a harmonic wavelength.

26. The laser processing apparatus of claim 1, wherein the tunable harmonic generator is operative to convert the first beam to a second beam at a second harmonic wavelength, a third harmonic wavelength, and a fourth harmonic wavelength.

27. The laser processing method of claim 21, further comprising: the coaxial first and second beams are achromatically collimated after the second beam is generated by using only one or more mirrors.

28. A modular system for machining a workpiece, comprising:

a laser source outputting a broadband first beam of fundamental wavelength along a path;

a harmonic wavelength generator receiving the first light beam and operative to partially convert the first light beam into at least one second light beam of a harmonic wavelength, wherein the first and second light beams are coaxial and overlap in time; and

an adjustable non-achromatic optic illuminated by the coaxial first and second beams and configured to controllably produce chromatic aberration such that a focal plane of the second beam is at or near a surface of the workpiece, providing an initial material state change sufficient to increase temperature for subsequent effective absorption of the first beam, wherein the focal plane of the first beam is spaced from the surface within the body of the workpiece.

29. The modular system of claim 28, wherein the laser source is a fiber laser operating in a Continuous Wave (CW) regime or a quasi-CW regime or a pulsed regime and outputting the first beam in a single mode or a multimode.

30. The modular system of claim 28, wherein the harmonic generator is configured with:

one or more nonlinear crystals spaced apart from each other along the optical path by using reflection focusing only to selectively provide sequential conversion of the fundamental wavelength to the second, third and/or fourth harmonic wavelengths or independently tunable wavelengths by using an optical parametric process and/or a Raman process, and

single pass or multipass wavelength conversion schemes.

31. The modular system of claim 30, wherein the nonlinear crystals are each cut for type I or type II phase matching.

32. The modular system of claim 28, further comprising: a collimator including a parabolic mirror, the parabolic mirror positioned between the harmonic generator and the workpiece.

33. The modular system of claim 28, further comprising: a processing unit configured to determine flux ratios of the first and second light beams at the fundamental and harmonic wavelengths, respectively, which are greater than or at least equal to the inverse of the absorption coefficient ratios at the fundamental and harmonic wavelengths, and to adjust a non-achromatic processing lens or lens system such that the second light beam of higher wavelength generates a spark.

34. The modular system of claim 33, wherein the processing unit is operative to adjust a wavelength conversion efficiency of the harmonic generator to be greater than an inverse of an absorption coefficient ratio at each of a fundamental wavelength and a harmonic wavelength.

35. The modular system of claim 34, wherein the processing unit is operative to control the wavelength conversion efficiency to prevent a gaussian intensity profile of the first beam from degrading into an annular intensity profile during wavelength conversion.

36. The laser machining method of claim 34 wherein the wavelength conversion efficiency is controlled to provide an annular intensity distribution of the first beam, the first beam being a single mode beam or a multimode beam.

37. The laser processing method of claim 28, wherein the fiber laser source is operated such that the energy of each of the first and second beams incident on the workpiece is as follows,

Eth(λ)/Ethall(λ)＞1，

wherein Eth is an energy threshold of a single laser beam coupled into the workpiece, which is solely required for laser processing of the workpiece by the single laser beam at a separate wavelength beam, at each separate fundamental or harmonic wavelength, and Ethall (λ) is an energy threshold of a laser beam at a separate wavelength when the coaxial first and second beams are simultaneously irradiated together on the workpiece.

38. A method for laser machining, comprising:

determining a desired harmonic fluence level at the target surface to produce a state change in the surface material, wherein the desired fluence level will at least initiate the laser machining process,

setting a harmonic spot size, conversion efficiency, and unconverted beam power to achieve the desired harmonic flux level,

determining a fundamental spot size corresponding to a desired harmonic flux and harmonic spot size, the fundamental beam including residual unconverted radiation,

configuring a chromatic aberration optical system to deliver the harmonic spot size and the fundamental spot size coaxially to the target surface, and delivering laser radiation to the target work surface using the chromatic aberration optical system to effectively treat a target material.

39. A multi-wavelength laser processing system comprising:

a multi-wavelength laser source for generating a multi-wavelength coaxial laser processing beam,

a multi-wavelength optical system for delivering a coaxial laser processing beam to a laser material interaction region on a workpiece surface such that each of a first and second laser wavelength in the processing beam illuminates at least a portion of the interaction region as concentric first and second laser spots, respectively, the multi-wavelength optical system comprising a multi-wavelength beam collimator, configurable chromatic aberration optics and a laser processing focusing lens, wherein the configurable chromatic aberration optics provide adjustment of relative focal lengths of the first and second laser wavelengths.

40. The system claimed in claim 39 and wherein a diameter ratio of said first and second laser spots is adjusted by configuring said chromatic aberration optics.

41. The system of claim 39 wherein the diameter of the smaller concentric spot is less than 1/2 the diameter of the larger concentric spot.

42. The system of claim 39, wherein the configurable chromatic aberration optics are removable from the path of the on-axis beam, the system having a first set of respective focal lengths with the chromatic aberration optics installed and a second set of respective focal lengths when the chromatic aberration optics are removed.

43. The system of claim 39, wherein the configurable chromatic aberration optics comprise a first chromatic aberration optic interchangeable with a second chromatic aberration optic, the system having a first set of respective focal lengths corresponding to the first chromatic aberration optic and a second set of respective focal lengths corresponding to the second chromatic aberration optic.

44. The system of claim 39, wherein the configurable chromatic aberration optics comprise first and second removable chromatic aberration optics, the system having a plurality of sets of respective focal lengths corresponding to a plurality of chromatic aberration optics configurations.

Technical Field

The present disclosure relates to a cost-effective fiber laser system and method for processing materials with fundamental and harmonic wavelength beams propagating coaxially through a chromatic aberration producing lens system. The invention also relates to beams of light impinging on a material, wherein the respective beam foci of the beams of light are spaced apart such that the flux ratio of the beams of light is at least equal to the inverse of the absorption ratio of the material to the beams of light, wherein beams of harmonic wavelengths provide a material state change that increases the absorption coefficient of the material to the beam of fundamental wavelength.

Background

Laser processing is associated with a variety of materials including polymers, metals, glass, and ceramics. The type of laser used for each material is selected to match the optical absorption characteristics of the material. However, this is not straightforward for many materials because they have significantly different properties. Some materials have surfaces that reflect certain wavelengths, but under certain thermal conditions, allow light beams that would otherwise be reflected to propagate through. Other materials also selectively absorb specific wavelengths. In addition, some other materials are only ineffectively processed by one set of wavelengths, but another set of wavelengths is very efficient for processing these materials.

Many products are made from materials characterized as having high reflectivity in the Ultraviolet (UV) to Infrared (IR) wavelength range. This group of materials includes silicon (Si), copper (Cu), bronze, brass, aluminum (Al), stainless steel with mirror finish, silver (Ag), gold (Au), platinum (Pt), and alloys of the above materials. These materials can reflect up to 92% of visible light and up to 98% of infrared radiation at room temperature. It is needless to say that the above-mentioned materials and other similar materials are of vital importance in many industrial applications.

The published application US 2013/0134139 (US' 139) is a publication that recognizes the above-mentioned problems. It discloses a fiber laser system for treating highly reflective materials by laser generated light in the wavelength range of 700-1200 nm. This reference discloses a conceptually simple process involving simultaneously illuminating a material of interest with two beams at respective fundamental and dual frequencies. The green beam at the frequency doubling melts the illuminated surface, resulting in a more efficient absorption of the IR light at the fundamental frequency. Such irradiation of a material of interest at two different frequencies is well known in the art, e.g. US 5083007.

The solution disclosed in US' 139 comprises controlling the pulse shape by suppressing the IR light intensity within a short initial time of each pulse, which results in a higher wavelength conversion efficiency of the IR light beam into a green light beam. The resulting green beam raises the temperature of the irradiated material to the melting temperature, which increases the absorption of red light. The power distribution of each pulse after the initial peak power spike is controlled by minimizing the peak power of the IR light to the end of each pulse.

It is helpful to assess the cost effectiveness of the disclosed method and apparatus of US' 139 in view of its operation. On a large industrial scale, relatively low cost equipment that can operate efficiently translates into higher profits. In the context of laser processing of materials of interest, low cost and efficient operation of laser systems include a number of considerations that plague designers. For example, fiber lasers have a significant impact on the industrial manufacturing market due to their low cost, low maintenance and improved efficiency compared to the neodymium YAG laser of US 5083007. US' 139 teaching quasi-continuous fiber lasers may have some disadvantages that may raise its cost effectiveness. To achieve high conversion efficiency in a short time, this reference teaches a coherent narrow band laser associated with a desired high conversion efficiency. However, a narrowband fiber laser with a spectral width of less than 2nm may have a high cost and a low peak power. Controlling the pulse power profile requires complex control circuitry, which may only increase the cost of the disclosed apparatus. In sum, the disclosed apparatus may not be economically attractive for large-scale material laser processing operations.

There is therefore a need for a simple, cost-effective material processing laser system.

There is also a need for a simple, cost-effective material processing fiber laser system to efficiently process metals, dielectrics or composite materials that cannot be processed well or at excessive average power at the fundamental frequency of the fiber laser.

There is also a need for a simple, cost-effective modular material handling fiber laser system that is operable to limit the average laser power or pulse energy coupled into the material and typically dissipated as heat.

There is also a need for the fiber laser system described above, which is configured with beam directing optics operable to provide the necessary conditions for energy efficient processing.

Disclosure of Invention

In its basic configuration, the disclosed laser processing system is configured with a laser source that outputs a first beam of fundamental wavelength with a spectral width of at least 2 nm. The first light beam is converted one or more times with a high harmonic wavelength generator to produce at least one second light beam of at least one harmonic wavelength. The beams propagate coaxially through an optical system that passes them to the material, where one beam, typically a second beam of harmonic wavelength, is at least partially absorbed to cause a change in the state of the material that increases the absorption of the beam of fundamental wavelength. It should be noted that in limited cases, the cause of the change in the state of the material is caused by the second beam of light at the harmonic wavelength.

The material state changes may be induced on a macroscopic level, including temperature changes and transitions between the solid/liquid/gas/plasma phases of the irradiated material. Alternatively, the material state may occur on a microscopic level, switching the material from a ground state or excited state to another excited state or ionized or transition state, resulting in a change in chemical modification.

Typically, the focal point of the second beam is located on the surface of the material. However, depending on the material and/or harmonic wavelength, the focal point of the second beam may be closely spaced from the surface in the axial direction. Once the threshold is reached at which the material changes its state, the absorption of the first light beam is significantly increased, so that the efficiency of the task at hand is improved. In those rare cases where the first light beam of the fundamental wavelength heats the surface first to increase the absorption coefficient of the second light beam, the relationship between the focal points of the respective light beams remains unchanged.

One aspect of the disclosed system is the creation of chromatic aberrations, which are not common in the laser material processing arts where achromatic lenses or lens systems are commonly disclosed. The disclosed beam delivery system is therefore configured with a chromatic (chromatic) lens or lens system.

The chromatic aberration lens is configured to collect a plurality of different wavelengths of light and focus them at different focal heights relative to the surface. The chromatic aberration is generally axial, i.e., along the beam propagation path. The lateral chromatic aberration may be corrected in a chromatic aberration system or may remain corrected.

Another aspect of the disclosed system compensates for chromatic aberration and advantageously distinguishes the presently disclosed structure from known prior art structures. US' 139 is briefly modified and teaches that the energy balance is optimized by controlling the power distribution of each pulse. The pulse shape control is designed to: the IR peak power is initially increased to efficiently produce green light as soon as the melting temperature is reached, and then gradually decreased to the end of each pulse. In other words, the efficiency of the processing system to process the material is controlled by varying the wavelength conversion efficiency during each pulse.

Accordingly, another aspect of the present disclosure that includes the above features emphasizes controlling two or more coaxial beams at respective fundamental and harmonic wavelengths. In at least one embodiment, the disclosed system is configured with a processor for controlling a flux ratio between the beams at which the second beam provides a material state change to be at least equal to an inverse of an absorption coefficient ratio. In a brief description of the above advantages of the disclosed system, the typical 10kW fiber laser source of the prior art system is replaced by a significantly less powerful laser, such as a 1kW fiber laser, a fraction of which is converted to a different harmonic wavelength.

In practice, this control may be achieved by setting or adjusting the relative focal lengths of the first and second beams using chromatic aberration optics that are set and possibly placed along the optical path. By doing so, the diameter ratio of the first and second laser spots associated with the respective first and second beams at the surface is controlled to change the fluence ratio. The chromatic aberration optics may be removed from the path of the coaxial beam and subsequently replaced by another system configured to provide a different focal length, a different diameter ratio, thereby providing a different flux ratio.

In yet another aspect of the disclosed system, a beam delivery system includes an achromatic collimator provided having one or more reflective surfaces. This feature has been found to be particularly useful in the disclosed system with a beam of fundamental wavelength and multiple harmonic beams, such as green and Ultraviolet (UV) or green, UV and deep UV (duv). Maintaining the parallelism of the light beams plays a particularly important role in providing a chromatic aberration lens system of a desired flux ratio and a desired difference in focal length. Due to the extremely high tolerance of the lateral chromatic aberration effects, the collimator is advantageously configured without refractive elements. Any desired chromatic aberration in the system can be accommodated in the full optical system directly upstream of the chromatic aberration lens.

Yet another aspect of the disclosed system provides for the analytical determination of the energy threshold ratio of the material state change for each incident light beam. The determination of the energy threshold is studied by J.M Liu in the paper "Simple technique for measuring of pulsed Gaussian-beam spot size" Optics Letters, Vol.7, 5 months 1982, which is fully incorporated herein by reference. In particular, the energy of each of the plurality of beams delivered to the workpiece to be processed is determined as:

Eth(λ)/Ethall(λ)＞1，

where Eth (λ) is the energy threshold of each individual beam required to treat the workpiece alone without the aid of other wavelengths, and Ethall (λ) is the energy threshold of the same laser beam in the combined beam of the present disclosure (i.e., when all wavelengths are present at the same time).

The wavelength converter implemented in all aspects of the present disclosure is not limited to nonlinear crystals (NLCs). It may also be a raman crystal or even a raman fiber receiving a beam of fundamental wavelength from a fiber laser source-raman fiber amplifiers and oscillators. Alternatively, an optical parametric amplifier or oscillator may also be used. Combining parametric schemes and raman conversion schemes allows for the generation of spectrally tunable wavelengths that are more efficient at changing the surface state of some treatment materials than a limited number of harmonics at a fixed harmonic wavelength of the fundamental wavelength beam.

When a laser beam impinges on a material, the energy coupling is determined by absorption. The temperature of the material rises to increase its heat dissipation. For a strong laser, the temperature may rise above the melting and evaporation temperatures and the material becomes an ionized plasma. In this case, the subsequent laser absorption is determined by the plasma properties (e.g., density and temperature). In many material processing applications, creating a plasma helps absorb laser energy. This is the subject of another aspect of the present disclosure that may facilitate the operation of the disclosed system of each of the aspects discussed above.

The aspects discussed above include specific features of the disclosed laser system that implement the various steps of the disclosed method. Accordingly, all of the above aspects and some of the additional features disclosed below in the detailed description are directed to the disclosed methods. Each of the above disclosed aspects includes one or more features that may be practiced with any combination of the features of the described aspects.

Drawings

The above and other features and advantages of the present disclosure will become more readily apparent from the detailed description and claims, and the accompanying drawings in which:

FIG. 1 is an exemplary disclosed system.

Fig. 2 is the absorption of various materials depending on wavelength.

FIG. 3 is a table showing known absorption ratios of beams of fundamental and harmonic wavelengths for various materials.

Fig. 4A to 4D show the light pulse shapes of IR, green, UV and DUV wavelengths.

Fig. 5A to 5D and fig. 6A to 6C illustrate pulse shapes depending on wavelength conversion efficiency.

Fig. 7 is a flow chart showing a known laser processing process.

Fig. 8A and 8B are respective flow charts of the disclosed laser machining process.

Fig. 9 to 11 show respective wavelength conversion schematics based on nonlinear crystals and used in the laser system disclosed in fig. 1.

Fig. 12 and 13 show respective raman and parametric wavelength conversion diagrams.

Detailed Description

The basic concept of the present disclosure includes laser processing a workpiece with two or more laser beams of different wavelengths that are differentially absorbed by the material to be processed at room temperature. Energy of one of the beams at a wavelength that is more efficiently absorbed at room temperature than energy of the other beam is coupled into the material, thereby causing a change in state of the material. The workpiece effectively absorbs the further light beam at the corresponding wavelength as the material changes. Optimization of the disclosed processes allows the disclosed methods and systems to successfully process virtually any material. For example, it may be glass, sapphire, ceramic, copper, etched metal, thin metal, biological tissue, PCB, and silicon wafer.

Fig. 1 illustrates a general layout of the disclosed material processing system 10. The illustrated configuration includes a laser source 12 that outputs a broadband unpolarized beam 18 at the fundamental wavelength with spectral lines ranging between 2nm and several hundred nm. Although the laser source 12 may have various configurations, it is preferably locatedThere are fiber lasers operating in the following three regimes: continuous Wave (CW), quasi-CW (qcw), and pulsed. In the QCW or pulsed mode, the laser source 12 outputs a pulse train, each pulse having a pulse energy in the range μ J to J, a pulse duration in the range seconds to femtoseconds, and an average power between single digit watts and several hundred kilowatts. Although many industrial applications require the output beam 18 to have a maximum quality factor M of 1²However, the disclosed methods and systems may also utilize M having up to 100 in other applications²The multimode beam of (a) operates efficiently.

The generation of other light beams of respective wavelengths different from the wavelength of the light beam 18 is achieved by the harmonic frequency generator 14. The latter may operate based on a different physical mechanism, but ultimately regardless of the configuration, the generator 14 is operable to partially convert the beam 18 of that wavelength into a beam 20 of a different wavelength, the spectral linewidth of the beam 20 exceeding 0.5 nm. It is within the scope of the present disclosure that frequency harmonic generator 14 may be operable to use a variety of conversion processes, including frequency doubling, sum and difference frequency generation in nonlinear crystals, parametric oscillation and amplification in nonlinear crystal materials, and raman conversion in bulk crystals or optical fibers. Examples of specific optical schematics are discussed in detail below.

Propagating in a coaxial manner along the optical path, the beams 18 and 20 impinge on a chromatic aberration adjuster 16 equipped with one or more lenses, as described below. The chromatic aberration adjuster 16 produces an axial chromatic aberration between the first beam and the second beam that further illuminates the workpiece made of material 22 simultaneously. When the chromatic aberration adjuster 16 is used, the longer wavelength light beam 18 is focused farther on or near the surface of the material 22 than the shorter wavelength light beam 20. Thus, at the focus of the beam 20, the spot diameter of the beam 18 is larger than the spot diameter of the beam 20. Thus, the intensity of the longer wavelength beam 18 at the focus of the beam 20 is significantly lower than the intensity at its own focus. Because of the difference in spot diameter, the fluence between beams 20 and 18, which is the pulse energy per beam area in the case of QCW and pulsed lasers and the power per beam area for CW lasers, is 2 to 10 times higher than is typically the case for prior art achromatic lens systems, depending on the material of chromatic aberration system 16.

The flux ratio is important to the disclosed system 10, being configured to control the minimum amount of light beam 20 required to provide the desired material state change, thereby resulting in increased light beam 18 and more efficient use of the overall system. Note that with a high power laser source 12, typically a beam 18 such as an IR beam can laser process many materials, but using only one IR beam 18 would result in an unacceptably inefficient process. This also relates to any other single wavelength light that in principle can treat the material 22 alone but may render the laser treatment process inefficient. Using multiple beams instead of one beam to process the material can be represented analytically as:

Eth(λ)/Ethall(λ)＞1，

where Eth (λ) is the energy threshold of each individual beam required to treat the workpiece alone without the aid of other wavelengths, and Ethall (λ) is the energy threshold of the same laser beam in multiple beams (i.e., when all wavelengths are present simultaneously) of the present disclosure. When the above requirements are met, the efficiency of a process utilizing the disclosed system may be enhanced by several orders of magnitude in some applications. Typical pulse energies in applications with multiple beams are 4-5 times the threshold energy Eth of a single pulse.

The flux ratio between the beams 20 and 18 in the system 10 is obtained based on well-recorded absorption dependence at ambient temperature according to the wavelength for the various materials, as shown in fig. 2. The inventors have found that to cause the desired change in state of the material, the flux ratio should be at least equal to or greater than the inverse of the absorption ratio of the same beam in the material 22. Some absorbance ratios are shown in fig. 3, where IR is infrared, GR is green, UV is ultraviolet, and DUV is deep UV. Many techniques for measuring and controlling flux are known to those of ordinary skill in the art and are not disclosed in detail herein. It should be noted that in the disclosed system, the fluxes of the respective light beams are measured separately.

The flux ratio can be adjusted by several techniques. Techniques include manipulating the chromatic aberration of the chromatic aberration adjuster 16 by replacing a currently mounted lens group with a different lens group, which may be made of a different material, such as Fused Silica (FS), magnesium fluoride (MGF2), calcium fluoride (CAF2), or other materials. Yet another technique is provided for adjusting conversion efficiency. Selection of the chromatic aberration adjuster 16 suitable for the task at hand may be achieved by any mechanical method including an automatic lens delivery mechanism. Techniques for adjusting wavelength conversion parameters are also well known to those of ordinary skill in the laser art and may include controllably altering the geometry and temperature of the NL crystal or the length of the raman fiber, as well as many other parameters. Although the switching is controllably adjusted, the power profile of the pulses remains virtually unchanged as shown in fig. 4A-4D.

Typically, the lasers used in laser material processing are configured to be narrow band and polarized to provide good harmonic conversion efficiency. However, the high conversion efficiency is accompanied by significant deterioration of the fundamental wave beam distribution, as shown in fig. 6A to 6C, which show that the gaussian distribution is gradually transformed into a ring distribution by subsequent second and third harmonic conversions. The annular distribution is generally detrimental, except for a few applications. In most cases, a gaussian distribution beam is used. Both the low conversion efficiency and the broad spectral lines reduce the efficiency, which results in a relatively unmodified gaussian distribution, as shown in fig. 5A to 5C. Thus, as disclosed herein, low conversion efficiency facilitates the production of multiple beams from a single laser and provides good beam quality for all beams. For purposes of this disclosure, for green beams 20, the low conversion efficiency is less than 20%, while the high efficiency may be greater than 50%; for UV light, any efficiency below 10% is considered low, while conversion efficiency above 30% is high.

Fig. 7 shows an optical schematic with achromatization typically used in the known prior art. Note that both beams irradiating the workpiece are focused on the surface thereof.

Fig. 8A and 8B illustrate various optical configurations of the disclosed system 10. As described above, the light beam 18 of the first wavelength is converted in the harmonic converter 14 such that at least two coaxial light beams 18, 20 of respective longer and shorter wavelengths are collimated in the collimating optics 26. The collimated beam is received by a chromatic aberration adjuster 16.

With a broadband laser source 12 having multiple laser wavelengths, the monochromating lens design will typically exhibit so-called chromatic aberration. These deviations are the result of material dispersion, refractive index variation with wavelength. The focal length of the lens depends on the wavelength for different refractive indices and leads to axial chromatic aberration, where different wavelengths are focused at different focal lengths.

The color difference adjuster may have various configurations. For example, an air-spaced chromatic aberration doublet of suitable optical material may be used as a chromatic aberration corrector to adjust the beam collimation of one wavelength relative to another as an input to the processing lens to correct for axial chromatic aberration of the processing lens or to intentionally add chromatic aberration for focal length separation at multiple wavelengths.

The chromatic doublet lens may have a long focal length or an infinite focal length (i.e., no focus) at one wavelength to maintain beam collimation. The doublet lens may be air-spaced and utilize a combination of durable optical glass and crystalline laser material. Especially for high power lasers and ultraviolet wavelengths, a combination of FS, MGF2 and CAF2 may be used. In combination with the materials of the processing lens, two materials can correct or separate the focal lengths of two wavelengths, and three materials can correct or separate the focal lengths of three wavelengths. For example, to increase the focal length relative to shorter wavelengths at 1064nm, the doublet may include a positive element with relatively high dispersion and a negative element with relatively low dispersion. For example, a doublet may include a positive FS element and a negative CAF2 element having nearly equal focal lengths. It will be appreciated that more complex chromatic aberration corrector optical designs with more than 2 optical elements may be required to correct for other optical aberrations, such as spherical aberrations.

The beams 18, 20 are then focused by focusing optics 24 to have respective desired beam spot diameters. Delivered to the laser-material interaction region on the surface of the workpiece 22, the beams 18, 20 illuminate the surface as concentric first and second laser spots, respectively. In fig. 8A, the chromatic aberration adjuster 16 is configured such that the focal point 28 of the second beam 20 of the first beam is located upstream of the surface of the workpiece 22, while in fig. 8B the focal point 30 is located below the surface. In both configurations, the focal point 28 of the converted beam 20 is on the surface of the workpiece 22. Yet another configuration of the disclosed beam delivery optics may have the same configuration as shown in fig. 7, but the focusing optics of the prior art schematic are configured with chromatic aberrations.

The laser spot diameter ratio between beams 18 and 20 is adjusted by configuring chromatic aberration adjuster 16 such that the smaller concentric spot is smaller than 1/2 of the diameter of the larger concentric spot. The chromatic aberration adjuster may be configured with interchangeable sets of chromatic aberration optics 16, each set of optics 16 defining a respective focal length that is different from one another.

Fig. 9 to 13 disclose various configurations of the harmonic converter 14. The architecture of the illustrated converter 14 may include a single pass scheme as illustrated in fig. 9-11 or a multiple pass conversion scheme as illustrated in fig. 12-14.

Referring specifically to fig. 9-10, the converter 16 may be operated to generate the second harmonic frequency and the third harmonic frequency by utilizing nonlinear I- type crystals 32 and 34, such as lithium triborate (LBO). In operation, the beam 18 emitted from the laser 12 is incident on the collimating lens 36, rotated by the half-wavelength plate and focused within the body of the crystal 32 such that the beams 18 and 20 thereafter propagate in a coaxial manner. It will be readily appreciated that if the arrangement of figure 9 had only a crystal 32, the converter 14 would only output two beams, as shown in figure 12. But because of the incorporation of the second crystal 34 the schematic shown is also operable to generate third harmonics, and the lenses between the crystals are shown for convenience, although in practice it is one or more spherical mirrors.

All beams are concentric, the outer beam being beam 18 and the innermost beam being the third harmonic beam. The collimating optics 26 are positioned along the optical path downstream of the crystal 34 and may be configured as one or more spherical mirrors. The chromatic aberration adjuster 16 is shown directly downstream of the collimator 26. Fig. 10 has the same structure as that of fig. 9, but is operable to generate the fourth harmonic by the third crystal 40, except for the second harmonic and the third harmonic.

Fig. 11-13 illustrate a multi-pass conversion system. In fig. 11, the schematic is operable to generate the second harmonic using collimating optics configured as an achromatic total reflection collimator 26. Fig. 12 shows the generation of an alternative harmonic by using an Optical Parametric Amplifier (OPA) 44. The system is made to have a spectral tunability of 0.2-2 μm by varying one or more parameters of the system, such as, for example, temperature and/or crystal rotation and/or time delay. It is usually implemented in multiple nonlinear crystals in series or in a cavity that acts as an Optical Parametric Oscillator (OPO). Fig. 14 shows a raman amplifier that may be configured as a raman fiber or raman crystal 46 or raman fluid. Similar to OPO, it can be implemented in a cavity to act as a raman oscillator.

Embodiments of the present invention have been described with reference to the accompanying drawings, it being understood that the invention is not limited to those precise embodiments. For example, the system 10 may be configured with a long pulse IR source with coaxial, high intensity (ns/ps) pulses temporally leading the IR/green/UV/DUV, MM IR beam with coaxial single mode IR to create material state changes in focus and other aspects. It is therefore to be understood that various changes, modifications and adaptations may be made therein by one skilled in the art without departing from the scope or spirit of the present invention as defined in the appended claims.