In-process monitoring and feedback control are fundamental actions for stable and good quality laser welding process. In particular, penetration depth is one of the most critical features to be monitored. In this research, overlap welding of stainless steel is investigated to stably reproduce a fixed penetration depth using both CO 2 and Nd:YAG lasers. Plasma electron temperatures of Fe(I) and Cr(I) are evaluated as in process monitoring using the measurement of intensities of emission lines with fast spectrometers. The sensor system is calibrated using a quantitative relationship between electron temperature and penetration depth in different welding conditions. Finally closed loop control of the weld penetration depth is implemented by acquiring the electron temperature value and by adjusting the laser power to maintain a pre-set penetration depth. A PI controller is successfully used to stabilize the electron temperature around the set point corresponding to the right penetration depth starting from a wrong value of any initial laser power different than the set point. Optical inspection of the weld surface and macroscopic analyses of cross sections verify the results obtained with the proposed closed-loop system based on a spectroscopic controller and confirms the reliability of our system.

Algorithms for phase retrieval in spatial phase-shifting interferometry are examined. It is noticed that the classical Carre technique based on four neighbor pixels may introduce shaped phase residuals that affect the accuracy of the reconstruction. A variant of the technique is proposed, taking into consideration five pixels and steering the phase retrieval algorithm to the appropriate subset so that residuals are free of underlying errors. Computational results are presented illustrating the approach and demonstrating the accuracy improvement in a case study. (C) 2011 Elsevier B.V

Recent developments in laser joining show the applicability of spectral analysis of the plasma plume emission to monitor and control the quality of weld. The analysis of the complete spectra makes it possible to measure specific emission lines which reveal information about the welding process. The subsequent estimation of the electron temperature can be correlated with the quality of the corresponding weld seam. A typical quality parameter, for laser welds of stainless steel, is the achieved penetration depth of the weld. Furthermore adequate gas shielding of the welds has to be provided to avoid seam oxidation . In this paper monitoring and real-time control of the penetration depth during laser welding is demonstrated. Optical emissions in the range of 400nm and 560nm are collected by a fast spectrometer. The sensor data are used to determine the weld quality of overlap welds in AISI 304 stainless steel sheets performed both with CW Nd:YAG and CO2 lasers. A PI-controller adjusts the laser power aiming at a constant penetration. Optical inspection of the weld surface and microscopic analysis of weld cross sections were used to verify the results obtained with the proposed closed-loop system of spectroscopic sensor and controller.

Direct real-time measurements of the penetration depth during laser micromachining has been demonstrated by developing a novel ablation sensor based on laser diode feedback interferometry. Percussion drilling experiments have been performed by focusing a 120-ps pulsed fiber laser onto metallic targets with different thermal conductivity. In-situ monitoring of the material removal rate was achieved by coaxially aligning the beam probe with the ablating laser. The displacement of the ablation front was revealed with sub-micrometric resolution by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector system.

The in-process monitoring and real-time control of the penetration depth during laser welding is evaluated. An optical collimator collects the optical emission for measurement with a fast spectrometer. The sensor data are used to calculate the electron temperature and subsequently to determine the weld quality of overlap welds in AISI 304 stainless steel sheets performed both with CW Nd:YAG and CO2 lasers. A PI-controller adjusts the laser power aiming at a constant penetration depth and has been tested for Nd:YAG laser welding. Optical inspection of the weld verifies the results obtained with the proposed closed-loop system of spectroscopic sensor and controller.

Thanks to its robustness and reduced sensitivity to vibrations and air turbulence, spatial phase-shift interferometry (SPSI) is a measuring technique of particular value in industrial environments. Making use of a commercial CCD camera connected with a PC we have set up an essential system that acquires and processes the fringe pattern, extracting the relevant features of the phenomenon being observed. The basic algorithms for phase recovery are available from the literature. Here we present a variant of one of such algorithms and describe in detail its implementation in our SPSI system. Experimental results are presented, showing the effectiveness of the overall measuring chain.

The time stability of fused silica is investigated, reporting new data on two flats that were purchased in 1981. It is found that their present shape is no more plane, exhibiting instead a concavity that is compatible with a very slow laminar flow under the action of gravity. The data are examined altogether with those of prior observations, confirming the occurrence of a relaxation process whose time constant is estimated in the order of 10 years; during such a time period, viscosity approximately increases by a factor of three. Starting from the experimental data so far collected, the computations accounting for the estimates above are given in detail.

The use of wavelength-shifting interferometry with a He-Ne laser source is presented. The approach is based on the thermal expansion of the laser cavity during the warm-up time, in conjunction with an unequal path interferometric configuration. The optics principles are reviewed, and a demonstrative measurement is presented. The technique extends the capability of phase shift acquisition and processing to cases where conventional methods with piezoelectric transducers cannot be used, and particularly to external measurement cavities of large diameter.

Mono and polycrystalline Chemical Vapor Deposited (CVD) diamond is a promising material for several advanced topics: microchips substrate, biological applications, UV and particle detection. Commercial CVD diamonds are available in small square size, commonly 3-5 millimeters side and 0.5-1.5 millimeters thickness. To improve diamond reliability for described applications, it is important to have a quality control on diamond samples, not only for electrical constants but also for optical characteristics and surface roughness. In this paper we present an optical characterization method based on interferometric instruments, to measure surface structure and internal homogeneity of mono ad polycrystalline commercial CVD diamonds, with measurement examples.