Laser wavelengths space significantly much shorter than microwave, and the laser beam remains narrow over lengthy distances.
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From: Sensor innovations for civil Infrastructures, 2014
K.T. Voisey, in substantial Materials Processing, 2014
9.07.2.2 Laser Drilling Parameters
The assorted parameters pertinent to laser drilling are noted below.
Laser wavelength and also frequency: these vital characteristics the the laser light are normally fixed for any given laser. This is in contrast to the various other parameters noted below, which space usually adjustable.
Pulse width: the time or term of one pulse. Pulse widths varying from millisecond to femtoseconds are generally used in laser drilling. Any given laser will normally only have the ability to vary the pulse width over one bespeak of magnitude; additional variation calls for a change of the laser system.
Pulse repetition rate or pulse frequency: this is the number of pulses per second. Relying on the laser system, this can range from a few Hertz to numerous kilohertz.
Average power: the mean power calculation of the laser is the output energy of the beam in a provided time divided by that time; the moment used should be large compared to the pulse term so that a representative power is determined.
Peak power: the power accomplished during a single laser pulse, i.e., pulse energy split by pulse width. This have the right to be orders of magnitude better than the typical power.
Fluence: the energy yielded per unit area.
Focal position: the plane in i beg your pardon the laser is focused. This commonly coincides with the height surface that the product being drilled however can be located below this surface or moved during the drilling process.
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A. Douplik, ... V.V. Tuchin, in Lasers for clinical Applications, 2013
3.4.1 Eye hazards
Laser wavelength, typical exposure time, and which framework absorbs the most radiant power per volume of tissue will influence the level of corneal and also retinal burns. Ocular impacts at the cornea are largely suffered if laser emissions room in the ultraviolet and far infrared spectral regions. Actinic ultraviolet in the variety of 180 nm to 315 nm is responsible because that ‘welder’s flash’ or photokeratitis.43 much longer wavelengths that the near ultraviolet (UV-A) spectrum (315 nm come 400 nm) are absorbed in the lens and also may add to particular forms the cataracts.
Retinal effects are feasible when the laser emissions wavelength occurs at clearly shows wavelengths, 400 nm come 780 nm, and also near infrared wavelengths, 780 nm to 1400 nm.44 Laser light directly entering the eye deserve to be focused to an extremely little spot on the retina, 10 μm to 20 μm in diameter. The focusing effect of the cornea and also lens will boost radiant exposure to the retina to about 100,000 time the incidental irradiance.44 This focusing result can reason intensities above the damage threshold. For this reason, laser radiation in the 400 nm to 1400 nm variety is termed the retinal peril region. For irreversible retinal exposures, or exposures lasting for hours, wavelengths in between 400 nm and also 550 nm are an especially detrimental. This is occasionally referred to together the blue irradiate hazard.43,44
Far infrared (IR-C) radiation v wavelengths of 3 μm to 1 mm is took in in the front surface ar of the eye.44 However, some center infrared (IR-B) radiation in between 1.4 μm and also 3 μm penetrates more deeply and also may add to ‘glassblower’s cataract’. Substantial exposure to close to infrared (IR-A) radiation may likewise contribute to together cataracts.
For a long-haul DWDM system, strength variation and wavelength drifting have the right to lead to system performance degradation. Tight power control and also accurate wavelength locking are forced to protect against degradation. The usual accuracy need for power regulate is 0.5 dB. Because that extra-long-distance transmission, the accuracy need for power manage would it is in 0.1 dB. The accuracy that wavelength locking depends on many parameters, such together the data rate (which will influence the bandwidth that modulated signals), channel spacing, and filter bandwidth. Because that 100-G and also 50-G spacing systems (which can support 40 and also 80 networks in the C-band, respectively), the wavelength accuracy of 0.1 nm is sufficient. However when the channel spacing reduces to 25 G, tighter wavelength manage is needed. Typical wavelength accuracy would certainly be determined by temperature-controlled feedback circuitry.
Reliability is another an important element that OPM. The failure rate of optical materials is number of orders greater than digital components. Optical equipment are forced to repetitively monitor power and wavelength sports in order to recognize the sign of symmetric degradation. Typically, data are conserved for part periods. Momentary data room stored in local devices and minimal by the memory size of this devices. Irreversible data are saved in far servers that can store records for days and also months. Numerous systems administer tools for data retrieval and performance analysis.
R. Fabbro, in Handbook of Laser Welding Technologies, 2013
3.2.4 Vapour plume behaviour
Compared to the CO2 laser wavelength, the 1.06 μm of the Nd-Yag laser has a really different behaviour worrying its communication with the vapour plume. Firstly, together the inverse Bremsstrahlung absorb coefficient of one ionized gas v a offered electronic thickness follows a λ2 scaling law, one can think about that this absorption device is not at all appropriate at 1.06 μm, for event laser intensities properties of welding processes. Together a consequence, the temperature that the vapour plume adheres to the temperature that the surface where the evaporation procedure occurs; in fact, this plume temperature might be also lower, because of its expansion into the approximately atmosphere and also because the reheating cannot occur because of the really low absorption coefficient, as questioned above. So these plume temperature are quite low, in the variety and over the evaporation temperature Tv that the material at atmospheric pressure. Spectroscopic dimensions have evidenced this fairly low temperature of the vapour plume and correspondingly its an extremely low ionization state (Greses, 2003; Greses et al., 2004; Katayama, 2010). This defines that, unequal CO2 laser welding whereby the plume temperature in the range of 6,000–10,000 K results from that reheating by the station Bremsstrahlung absorption, the nature that the shielding gas used, such together He, N2 or Ar, has no result on the final result of welding once 1 μm laser wavelength is used.
The second aspect the the laser–plume interaction comes to the important function of corpuscle ejected back towards the laser along the direction the the beam. These particles have a really wide variety of radius r that have the right to vary native a few nanometres when they an outcome from clusters of atoms, aggregates or ultra-fine particles generated by some complicated mechanism of local condensation of these metallic atoms, come micron dimension or above, as soon as they result from liquid droplets detached from the liquid wall surface by this violent widening vapour jet as disputed in Section 3.2.2. Since this variety of radius corpuscle is commonly smaller or about the 1.06 mm wavelength, it is meant that scattering and also absorption by these tiny particles may play a much more important duty than v the CO2 10.6 mm laser wavelength. It is recognized that Mie scattering becomes efficient when the ratio r/λ is around one and Rayleigh scattering scales through (r/λ)4, so this mechanisms need to be an extremely efficient with quick laser wavelengths. The analysis of the relative contribution that the beam absorption (that produces heater of this particles) and also of the scattering state (that cause a far-ranging defocusing result of the laser beam) mirrors that absorb is dominant for little particles, when scattering becomes crucial for larger ones (2πr/λ > 1).
Nevertheless, the application of this theories come experimental problems is no satisfying due to the lack of an accurate knowledge of experimental parameters such together the volume proportion (ratio of fragment volume to plume volume) or particle radius circulation along the vapour plume. However, Greses et al. (2004) has displayed that for welding problems at 1.06 mm, the particle mean size within the vapour plume differed from 20 to 50 nm when He or Ar shielding gas to be used, while for CO2 laser welding, this size was generally 10 times smaller; this most likely results from the solid difference the the plume temperature between these two varieties of welding. Moreover, attenuation and also scattering that a probe beam, of different wavelengths, transverse to the vapour plume, confirm the importance of this mechanism arising at much shorter laser wavelengths (Katayama, 2010). In fact, the real prominence of the perturbing result of these scattering mechanisms deserve to really it is in emphasized once the vapour plume is blown far by an extreme transverse gas jet located an extremely near the workpiece surface. Typically, the penetration depth and weld seam area are boosted by 15–20% when the vapour plume is effectively ‘cut’. The is most likely that the use of a next gas jet, as disputed in Section 3.2.3, additionally induces such effect by dispersing the vapour plume and also therefore reduce its scattering and/or absorbing effect. One should also include that, apart from these localized scattering mechanisms, radial table of contents variation top top the shielding gas above the surface ar induced through thermal heater from the workpiece or indigenous the vapour plume itself, deserve to perturb beam propagation and modify the focal point out intensity distribution, specifically for fibre far welding (Mitzutani and also Katayama, 2009). This results plainly show that the manage of the vapour plume and more particularly its dimension reduction or that is suppression by adjusted techniques, or the the shielding gas environment, boosts the quality and the power of the welding process. Moreover, due to the large relative variation of the focal clues by these perturbing mechanisms when little focal clues (and/or little apertures of concentrating optics) space used, one might expect that these improvements should be very effective as soon as high beam top quality lasers space operated.
K.-M. Winter, ... D. Koshel, in Thermochemical Surface design of Steels, 2015
3.13.4 procedure parameters and procedure set-up
There room multiple combinations of laser wavelengths, fluences, shoot durations and also repetition rates used for treatments. The chosen set of parameters determines the mechanism whereby the laser interacts v the material and also the framework of the result nitrided layer.
When an intense laser beam (~ 100 MW/cm2) is switched on, the event photons interact with the target metal’s conduction electrons. The took in energy is transferred to the lattice, melting and also vaporising a an extremely shallow, superficial layer. This communication depends critically ~ above laser wavelength, intensity, power and material surface ar properties (such together chemistry and roughness).
At high nitrogen pressure the warm laser plume is confined over the treated surface ar by shock waves. These problems generate a far-ranging increase in temperature (~ 1000s K) and also pressure (~ 100 s bar), producing plasma in this extremely localised volume. Pirri et al. (1977) propose a model of laser–plasma-surface interaction. It says three threshold intensities for the incident laser: Ie (evaporation threshold), Ilsc (laser supported combustion wave threshold) and also Ilsd (laser sustained detonation wave threshold). When occurrence laser intensity I > Ie yet I lsc, the produced plasma has low density and is transparent to the laser beam. The laser power conversion into plasma particle energy starts come be far-ranging at Ilsc lsd; the absorption wave propagates through subsonic velocity towards the treated surface, when the laser deserve to still reach the metal. Almost all laser energy is absorbed in the plasma if I > Ilsd, once the absorb front propagates supersonically. Under such conditions the treated surface ar is decoupled indigenous the laser beam and also nitriding becomes inefficient.
Shorter wavelengths room absorbed an ext efficiently by metal than much longer ones. Thus, follow to Thomann et al. (1996), titanium vaporisation in ~ 308 nm (ultraviolet, UV) is around 15 times more powerful than 10.6 μm (infrared, IR) irradiation for an tantamount laser strength density. However, IR radiation deserve to still be provided for nitriding. The IR laser is strong reflected indigenous metals however not native the laser plasma generated within the plume. Thus a significant amount the the energy in the occurrence IR laser is took in and went back to the metal via blackbody-like irradiation native the laser plasma (Illgner et al., 1998). The heating by IR outcomes in a deeper, boil layer and also more minimal metal evaporation contrasted with heating by UV laser. The IR-generated plasma contains much more nitrogen types while the UV-generated plasma is enriched in metal ions (Thomann et al., 1996).
The plasma acts together a resource of highly energetic nitrogen i m sorry diffuses into the molten steel surface. Schaaf et al. (1999) have analysed nitrogen profiles after laser nitriding that Armco Fe samples by an XeCl excimer laser (wavelength 308 nm, pulse term of 55 ns yielding 4 J/cm2 end a spot size 3.8 × 4.7 mm2). A temperature of 2445 K was got to over one pulse cycle (293 ns) v a melt depth of 800 nm. The nitrogen concentration acquired was around 12 at.% below the sample surface (melted zone). This number is much greater than the solubility of molecule nitrogen at this temperature, indicating plasma-enhanced nitrogen activity. An steel nitride layer with high nitrogen concentration (~ 30 at.%) was found on the surface ar of the treated sample. The authors explained it as fallout of the re-condensing plasma after the finish of the laser pulse complied with by ultra-fast cooling and clustering.
The typical depth of laser nitrided layers, minimal to a couple of microns, is as result of the highly localised activity of laser treatment. The framework is a result of a collection of rapid blend and solidification processes that have arisen within the superficial layer; a strong grain refinement effect is observed. In the instance of steel nitriding, the layer has mainly a mix that nitrogen austenite, ε nitride and also nitrogen martensite (Schaaf et al., 2002). The depth the the nitrided layer deserve to be enhanced by one annealing treatment. This reasons a flattening of the nitrogen concentration profile due to the diffusion the N atoms in the direction of the bulk. The annealing in ~ T > ~ 473 K activates the transformation of thermodynamically unfavourable nitrogen austenite, ε nitride and also supersaturated nitrogen solution in α-Fe come γ′ nitrides. When the annealing temperature above 523 K, the fast diffusion in the direction of the bulk leads come the dissolved of the γ′ nitrides and then only an α nitrogen equipment is reported (Han et al., 2001).
Annealing under low nitrogen potential environments results in the depletion of nitrogen through a degassing procedure (Han et al., 2001). The alloys containing strong nitride-forming elements (e.g., Ti alloys or stainless steels) have actually less propensity for degassing losses together the nitrogen i do not care trapped in nitrides (Han et al., 2001; Filip, 2008).
Laser nitriding offers low processing costs and short procedure times. The main control parameters space the laser power settings, irradiating beam diameter in ~ the surface, scanning speed and also sample temperature. The pulse duration boosts the time framework for the temperature-activated nitrogen diffusion procedure and rises the depth the the nitrogen enriched layer. Recent generations of lasers have actually overcome the pulse-energy instability issue. Thus, a high pulse-frequency therapy yielding a uniform dose circulation has become common practice.
To simplify the process, nitrogen is typically used at atmospheric pressure. Alternatively the process can it is in performed in one open-air atmosphere, through the nitrogen gift fed through a nozzle into the melt pool. The sample heating deserve to increase the nitriding depth native submicron values to numerous hundred microns. In such cases the nitrogen, delivered from the laser-induced plasma to the superficial layer, continues to diffusive in the mass of the material for the expression of the treatment. Boosting the power intensity of the laser, reduce laser scanning speed and focusing the point out diameter improve the melt depth thickness yet promote the erosion (evaporation) of the surface. For this reason laser gas-nitrided surfaces have a high surface roughness which modifies their tribological and corrosion behaviour, compared to surfaces obtained by classic nitriding approaches (Biswas et al., 2008).
A high frequency of treatments imposes really demanding requirements on process-control devices speed (communication, switching, analogue-to-digital conversion and signal processing). Carpene et al. (2005) carry out an instance with potential because that large-scale industrial application. They have performed nitriding of pure titanium v a high repetition price free-electron (FEL) laser under pure nitrogen conditions. The FEL beam contained a series of pulses in ~ a laser repetition rate of 37.4 MHz with mean pulse energy of 20 μJ. They obtained a 15 μm great of δ-TiNx, and also showed the a crack-free surface have the right to be accomplished with small spot overlap and long macro-pulses (1000 μs), if the roughness rises with large overlap and short macro-pulses (
P. Sharma, R.K. Vatsa, in products Under too much Conditions, 2017
6.1.4 role of inside Ionized electrons in magnified Ionization of swarm Constituents Under Giga-watt Laser strongness Pulses
As disputed in above sections, much longer laser wavelength and also larger cluster size facilitate amplified ionization of swarm constituents, under gigawatt laser conditions. This qualitatively concurs the three phase cluster ionization model. In bespeak to more substantiate the proposed mechanism, it to be pertinent to probe the fate the inner-ionized electrons produced upon subjecting clusters to nanosecond laser pulses. Thus, kinetic power of electrons ejected upon communication of methyl iodide clusters with nanosecond laser pulse was measured at various laser wavelengths. As a rule, the is taken into consideration that upon basic multiphoton ionization the kinetic energy of electron (Ee) is approximated by the relationship Ee = nhv − I.E., whereby n is the number of photons that laser frequency v compelled to cause ionization of the species and I.E. Is the ionization power of the species. Because that methyl iodide clusters ionization power is supposed to be ≤9.54 eV (as I.E. That methyl iodide clusters is intended to be reduced than the of its ingredient monomer and the lowering in I.E. Relies on the degree of clusterization). In ~ 532 nm, based on laser power dependency studies the number of photons (n) required to cause multiphoton ionization of methyl iodide clusters has actually been approximated to it is in ∼5. Thus it was supposed that electrons generated upon multiphoton ionization of methyl iodide cluster would get kinetic energy ≤2.11 eV. However, kinetic energy circulation measurement studies brought out top top methyl iodide clusters utilizing retarding field technique reveal that upon communication with 532-nm laser pulses electrons with kinetic power up come 30 eV room generated, top top laser-cluster interaction (Fig. 16.9d). Observation of electrons v kinetic energy up to 30 eV gives conclusive proof regarding energization of inner-ionized electrons together proposed in the three-stage swarm ionization design <96>. Further, for studies carried out at different laser wavelengths, researchers have actually observed enhancement in kinetic power of electrons at much longer laser wavelengths, which further confirms the proposed device <107>.
To further cross-check the electron heater mechanism, experiments were also carried the end on swarm doped with types having comparatively greater ionization energy. Due to the fact that ionization through quasi-free electrons is expected to be indiscriminate, that would also facilitate ionization of species having greater ionization energy. For this reason studies were lugged out ~ above water-doped methyl iodide clusters at 563 nm. Here water was favored as a dopant because of its greater ionization energy (12.62 eV) as contrasted to that of methyl iodide (9.54 eV). Additionally for studies carried out on pure water swarm under our experimental conditions, no ion signal could be it was observed in the time-of-flight mass spectrum, probably as result of lower multiphoton ionization probability the the water. However, once water-doped methyl iodide clusters were subjected to 563-nm laser radiation, in enhancement to multiply charged atomic ion of carbon and iodine, multiply fee atomic ion of oxygen (up to O4+) were additionally observed (Fig. 16.14). Thus, observation of multiply charged atomic ion of oxygen clearly demonstrates the significance of inner-ionized electron in laser-induced ionization of cluster constituents, under nanosecond laser conditions <110>. Thus, the previous set of studies offers concrete proof regarding duty of inner-ionized electrons and also their energization under the influence of extreme laser pulses.
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The confocal system has actually a spatial filter (called a confocal hole) — inserted usually in one image airplane between the microscope and the detector – which limits focal depth. Number 3 shows the rule of a confocal system. If the confocal hole were totally open, the irradiate scattered native the great P, P′, and P″ would reach the detector, bring about the sum of the spectra native all three layers. By to decrease the size of the confocal hole, one decreases the efficient depth that focus—and to reduce the contribution of surrounding layers to the Raman spectrum.