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Hybrid Optical Sensor for Combined Thermal and Dimensional Monitoring in Laser Processing
No full textOptical measurements enable non-contact and high-speed monitoring of physical processes, offering a noninvasive and versatile approach across a wide range of fields, from scientific research to industrial applications. This work presents an optical sensor capable of simultaneously measuring both the distance and thermal emission from surfaces, based on a simple laser diode (LD) probe. The principle of operation integrates triangulation with pyrometry in a single device, leveraging the monitor photodiode (PD) embedded in the probe module. By alternating the probe laser emission, the system can rapidly switch between dimensional and thermal measurements, resulting in combined data acquisition. The proposed method is compact, easy to integrate, and cost-effective. The hybrid sensor is demonstrated in a laser processing setup, where a metallic target is heated and melted by a high-power laser beam. Its inline operation allows for real-time dynamic measurements of melt pool distance and radiance, in a coaxial and self-aligning configuration. This innovative approach can be applied to various fields, such as remote environmental sensing and closed-loop control systems for stabilizing high-temperature processes, including laser welding and additive manufacturing
Combined optical system for dimensional and thermal measurements and operating method thereof
No full textA combined optical system is described for determining the temperature of the surface of an object or material and its distance with respect to a predetermined reference point associated with the system, which comprises an optical radiation source adapted to emit at least one optical probe radiation at a predetermined wavelength or in a predetermined wavelength range, a source control unit arranged to alternately control the switching of the source from an operative condition, in which it emits an optical probe radiation, to an inoperative condition, in which it does not emit the optical probe radiation, optical detectors adapted to acquire at least one scattered optical radiation and one thermally emitted optical radiation from the surface of the object or material, and a processing unit synchronized with the control unit and arranged to determine the distance of the surface of the object or of the material on the basis of the optical probe radiation scattered from the surface of the object or material and received by the detectors when the source is operative and to determine the local temperature of the surface of the object or material on the basis of the optical radiation thermally emitted from the surface of the object or material received by the detectors when the source is inoperative
Method and system for determining the local position of at least one optical element in a machine for laser processing of a material, using low-coherence optical interferometry techniques
No full textA method for determining local position of an optical element associated with an optical path for transporting a laser beam in a working head of a machine for laser processing a material, includes generating a measurement beam of low coherence optical radiation traveling a measurement optical path, leading the measurement beam towards the optical element and the reflected or diffused measurement beam towards an optical interferometric sensor arrangement, generating a reference beam of low coherence optical radiation traveling a reference optical path and leading the reference beam towards the interferometric optical sensor arrangement, superimposing the measurement and reference beams on a common region of incidence, detecting a position of a pattern of interference fringes between the measurement and reference beams, and determining a difference in optical length between the measurement and reference optical paths as a function of the position of the interference pattern along an illumination axis, or of the frequency of the interference pattern in the frequency domain
Method and system for determining the position of an element of an optical system in an assembly for processing or measuring an object, as well as the position of said object relative to said assembly, by parallel interferometric measurements
No full textA method and a system for determining relative position of an element of an optical system of an assembly for processing or measuring an object along a measurement line, involve generating a measurement beam and a reference beam of low coherence optical radiation. The measurement and reference beams, alternately or in combination, have a main beam and a multiplexed additional beam. The measurement beam, led toward the element of the optical system, and back-reflected, is superimposed on the reference beam in a region of common incidence of an interferometric optical sensor arrangement. Position or frequency of a main interference fringe pattern and an additional interference fringe pattern is detected
Method and system for determining the separation distance between a body and the surface of an object by means of low coherence optical interferometry techniques under distortion due to sub-sampling
No full textMethod and system for determining separation distance between an object and a processing or measuring tool involve generating a measurement beam of low coherence optical radiation, leading the measurement beam towards the object and the reflected measurement beam towards an optical interferometric sensor assembly in a first direction of incidence, generating a reference beam of low coherence optical radiation, and leading the reference beam towards the optical interferometric sensor assembly in a second direction of incidence, superimposing the measurement and reference beams on a common region of incidence, detecting position of a pattern of interference fringes between the measurement and reference beams on the region of incidence, and determining difference in optical length between a measurement optical path and a reference optical path on position of the pattern of interference fringes along an illumination axis to determine current separation distance between the processing or measuring tool and the object
Evolution of Laser Ablation Plume Measured by Self-Mixing Interferometry
No full textLaser ablation is the basis of most of the laser micromachining processes. Depending on the pulse duration, wavelength, intensity of the beam, as well as the material type, material removal may occur in the form of vaporization, melt expulsion, or direct sublimation. Most commonly material removal is accompanied by a plume formation. The characteristics of the plume can be analyzed to assess the ablation performance. Non-invasive optical methods for the purpose can be further exploited in industrial laser micromachining applications, if opportunely designed and implemented. This work uses an interferometric approach to investigate the optical path changes induced by the ablation plume formed during the laser percussion drilling of different metallic and ceramic materials. A self-mixing interferometer is installed inline to a laser micromachining setup composed of a ns-pulsed green fiber laser
Probing multipulse laser ablation by means of self-mixing interferometry
Full text linkIn this work, self-mixing interferometry (SMI) is implemented inline to a laser microdrilling system to monitor the machining process by probing the ablation-induced plume. An analytical model based on the Sedov–Taylor blast wave equation is developed for the expansion of the process plume under multiple-pulse laser percussion drilling conditions. Signals were acquired during laser microdrilling of blind holes on stainless steel, copper alloy, pure titanium, and titanium nitride ceramic coating. The maximum optical path difference was measured from the signals to estimate the refractive index changes. An amplitude coefficient was derived by fitting the analytical model to the measured optical path differences. The morphology of the drilled holes was investigated in terms of maximum hole depth and dross height. The results indicate that the SMI signal rises when the ablation process is dominated by vaporization, changing the refractive index of the processing zone significantly. Such ablation conditions correspond to limited formation of dross. The results imply that SMI can be used as a nonintrusive tool in laser micromachining applications for monitoring the process quality in an indirect way
Method and system for determining and controlling the separation distance between a working head of a laser processing machine and the surface of an object being processed by means of low coherence optical interferometry techniques
No full textA method and a system are described for determining and controlling the separation distance between a processing head of a machine tool and the surface of a material, comprising: - generating a measurement low coherence optical radiation beam, leading the measurement beam towards the material and leading the reflected or diffused measurement beam from the surface of the material towards an optical interferometric sensor arrangement in a first direction of incidence, - generating a reference low coherence optical radiation beam, and leading the reference beam towards the optical interferometric sensor arrangement in a second direction of incidence at a preset angle of incidence with respect to the first direction of incidence of the measurement beam; - superimposing the measurement beam and the reference beam on a common region of incidence of the sensor arrangement; - detecting the position of a pattern of interference fringes between the measurement beam and the reference beam on the region of incidence; and - determining a difference in optical length between the measurement optical path and the reference optical path on the basis of the position of the pattern of interference fringes along an illumination axis of the region of incidence, which is indicative of a difference between (a) the current separation distance between the working head and the surface of the material and (b) a predetermined nominal separation distance
Time-resolved quantification of plasma accumulation induced by multi-pulse laser ablation using self-mixing interferometry
Full text linkIn this work a method based on self-mixing interferometry (SMI) is presented for probing the concentration of plasma plumes induced by multi-pulse laser ablation. An analytical model is developed to interpret the single-arm interferometric signal in terms of plasma electron number density. Its time dependence follows a power-law scaling which is determined by concurrent effects of plume accumulation and propagation. The model has been applied for the experimental study of plume formation at variable laser pulse frequencies on different materials. The plume expansion dynamics has been observed with high-speed imaging, and the SMI measurements allowed for a time-resolved estimation of the electron number density. The intrinsic advantages of the SMI technique in terms of robustness and low intrusiveness would allow for its usage as a fast diagnostic tool for the dynamical scaling of laser-induced plumes. Moreover it can be easily applied in laser-based manufacturing technologies where plasma concentration monitoring and control is important
Interplay between powder catchment efficiency and layer height in self-stabilized laser metal deposition
Full text linkIn laser metal deposition (LMD) the height of the deposited track can vary within and between layers, causing significant deviations during the process evolution. Previous works have shown that in certain conditions a self-stabilizing mechanism occurs, maintaining a regular height growth and a constant standoff distance between the part and the deposition nozzle. Here we analyze the link between the powder catchment efficiency and the deposition height stability. To this purpose, a monitoring system was developed to study the deposition in different process conditions, using inline measurements of the specimen weight in combination with the layer height information obatined with coaxial optical triangulation. An analytical model was used to estimate the deposition efficiency in real-time from the height monitoring and the process parameters, which was verified by the direct mass measurements. The results show that the track height stabilization is associated to a reduction of the powder catchment efficiency, which is governed by the melt pool relative position with respect to the powder cone and the laser beam. For a given set of parameters, the standoff distance can be estimated to achieve the highest powder catchment efficiency and a regular height through the build direction
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