Laser machining presentation based on liming he et al paper

LASERS: Nd:YAG MACHINING

Wavelength optimization for machining metals with the harmonic generations

of a short pulsed Nd:YAG laser

Liming He, Yoshiharu Namba, Yuji Narita Received: 23rd August 1999 Revised: 13th December 1999 Accepted: 27th December 1999

WACHIRA J. NDUNG'U/MEMS/MECHATRONIC ENGINEERING/JKUAT

Introduction – Laser machining• Highly coherent light is directed towards the w/p for

machining• Lasers of different wavelengths are used in machining

variety of materials• It is important to select the optimum wavelength of

laser beam for machining various materials• Why lasers:• a) monochromatic• b) Parallel• Therefore, can be focused to a very small diameter

generating energy (as high as 100 MV/ mm2


Introduction- Nd:YAG Laser• Neodymium-doped yttrium aluminum garnet• Yttrium aluminium garnet (YAG, Y3Al5O12) is a synthetic crystalline material of

the garnet group.• Garnets are a group of silicate minerals• Garnets possess similar physical properties and crystal forms but different

chemical compositions. The different species are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series: pyrope-almandine-spessarite and uvarovite-grossular-andradite.

• Produces a collimated coherent beam in the near infrared region of λ= 1064 nm

• Can be run pulsed or continuously• Solid- state laser, safe to use and does not produce noxious gas• Can be made small in size and low in cost• Fundamental harmonic laser converted to higher harmonics by use of

nonlinear optical crystals


Introduction- Nonlinear optical crystals• Dielectric polarization responds non-linearly, P, to the

electric field, E, of the light• Examples: Potassium titanyl Phosphate (KTP),

Potassium dihydogen phosphate (KDP), Cesium dihydroarsenate (CDA) etc.

• Properties• Strongly bifringent (necessary to obtain phase

matching)• Have specific crystal symmetry• High damage threshold which make them resistant to

high intensity laser light


Presentation statement • An analytical method of wavelength optimization for

machining metals with various harmonic generations of a Nd:YAG

• From Absorptivity of metal and the conversion efficiency of laser apparatus, absorptivity efficiency is estimated for selecting an optimum machining wavelength

• As examples Gold, Silver, Copper, Nickel etc. are examined, and their optimum machining wavelengths are obtained


Absorption η for different H.G of a Nd:YAG• Conversion η

• The first, second, third and fourth-harmonic lasers from a Nd:YAG resonator by using nonlinear optical crystals

• λ1 – 1064 nm fundamental harmonic wavelength• λ2 – 532 nm- second harmonic wavelength obtained by use of KTP crystals• λ3 - 355 nm- third harmonic wavelength obtained by use of KDP crystals• λ4 – 266 nm - fourth harmonic wavelength obtained by use of CDA crystals

Nd:YAG Nonlinear crystalλ1 λ2

λ1

λ4Filter


Conversion η

• During frequency conversion by the nonlinear crystals there is loss in energy

• Therefore conversion efficiency () for the ith harmonic can be estimated as the ratio of the output fluence of the ith harmonic generation () to the input fluence of the fundamental generation ()

• ….eqtn 1• For Nd:YAG resonator, laser beam with the fundamental

frequency is produced when high voltage is applied to the flash lamp

• The fundamental wavelength laser is converted to it’s harmonic generations by nonlinear optical crystals

• From literature the frequency of output wave is the sum of input waves:

• ω+ ω 2ω (for second harmonic)


Conversion η• The third harmonic is obtained from

fundamental generation of second harmonic: ω +2ω ω

• Fourth harmonic is generated from fundamental generation of second harmonic: 2ω +2ω 4ω

• The output laser energy of the harmonic generations are measured with a power meter

• With the obtained data conversion η of the various harmonic generations can be estimated with equation 1


Results

• 41%, 25%, 13% for KTP, KD*P and CD*A respectively


Absorptivity• In metals the radiation is predominantly

absorbed by free electrons• The radiation does not penetrate metals to

any significant depth.• Metals are thus opaque and they appear shiny• For an opaque metal:• A= 1 – R , where R is reflectivity … 2• But for a perfectly flat clean surface:• R…3


Absorptivity

• Where n is the reflective index and k is the extinction coefficient for the material

• From literature this constants can be obtained to yield;

• from equation 2 & 3….4• Absorptivities of typical metals as a function of

wavelengths are as show below:


Absorptivity- Results


• It is evident that for most metals absorptivity are high in short wavelength region

• More energetic photons can be absorbed by a greater number of bound electrons in shorter wavelength region, the reflectivity falls at shorter wavelengths, and the absorptivity of surface is increased in the region of short wavelength.


Absorption efficiency• Energy loss in frequency conversion, conversion efficiency

lower in shorter wavelength region• Higher absorptivity of metals in shorter wavelength region

compensates for the energy loss in frequency conversion• Only the absorbed energy by the materials is used in

machining• … (5) • η is the absorption efficiency,β is the conversion efficiency

and A the absorptivity• Metals have different absorption efficiencies in various

wavelengths and thus their machining wavelengths are also different

• Results are calculated with data from figure 2 and table 2 to give results in table 3 using equation (5)


Results: Tables 2 and 3λ/ Materials 213 nm 266 nm 355 nm 532 nm 1064 nmGold 73. 85 63.78 63.66 23.47 2.05Silver 73.54 74.04 23.36 4.50 2.59Copper 61.2 66.28 58.03 38.93 2.75Nickel 58.15 57.49 57.35 40.08 27.41Molybdenum 31.50 33.69 45.69 41.63 32.44Platinum 70.64 58.92 48.26 50.40 25.18

λ/ Materials 213 nm 266 nm 355 nm 532 nm 1064 nmGold 4.80 8.29 15.92 9.62 2.05Silver 4.78 9.63 5.82 1.85 2.59Copper 3.98 8.62 14.51 16.43 27.41Nickel 3.78 7.47 14.34 16.43 27.41Molybdenum 2.05 4.38 11.42 17.07 32.44Platinum 4.59 7.66 12.06 20.66 25.18


Experimental Results and Discussions • Experimental setup:• Laser beam properties; diameter 6.0 mm• Duration pulses: 10-12 ns • Wavelengths: 1064 nm, 532 nm, 355 nm, and

266 nm using nonlinear crystals KTP, KD*P and CD*A for frequency generation respectively

• Results in table 3 were verified by machining gold as an experimental substrate

• Input fluence was varied from 0 to 500 J/cm2


Results

• Machined result is consistent with calculated results

• η is an aggregate parameter for examining machining parameter


• The absorptivity of materials for various harmonic generations of a Nd:YAG laser are definite and unchangeable

• Conversion efficiencies are changeable because of using different nonlinear crystals or using different laser fluences

• For the same material, the maximum absorption efficiency may be different, because various laser apparatus are used.


Conclusions• Optimum machining wavelength can be

estimated by calculating the absorption efficiency• For the various metals, the optimum machining

wavelength are different • Gold, silver and copper their highest absorption

efficiencies are the 3rd, 4th and 2nd harmonic generations respectively

• Nickel, platinum, and others optimum machining wavelengths are all in the fundamental wavelengths of the Nd:YAG laser

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Laser machining presentation based on liming he et al paper

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