Real time simultaneous measurement by remote laser source of long range longitudinal displacement and small angle wobbling of a sliding target is demonstrated. The compact self-aligned laser-self-mixing interferometer achieves 0.8 µm axial resolution over 1 m linear and 17 µrad of pitch and yaw resolution over 2 mrad angular ranges, respectively.

The recent development of ultrafast laser ablation technology in precision micromachining has dramatically increased the demand for reliable and real-time detection systems to characterize the material removal process. In particular, the laser percussion drilling of metals is lacking of non-invasive techniques able to monitor into the depth the spatial-and time-dependent evolution all through the ablation process. To understand the physical interaction between bulk material and high-energy light beam, accurate in-situ measurements of process parameters such as the penetration depth and the removal rate are crucial. We report on direct real time measurements of the ablation front displacement and the removal rate during ultrafast laser percussion drilling of metals by implementing a contactless sensing technique based on optical feedback interferometry. High aspect ratio micro-holes were drilled onto steel plates with different thermal properties (AISI 1095 and AISI 301) and Aluminum samples using 120-ps/110-kHz pulses delivered by a microchip laser fiber amplifier. Percussion drilling experiments have been performed by coaxially aligning the diode laser probe beam with the ablating laser. The displacement of the penetration front was instantaneously measured during the process with a resolution of 0.41 mu m by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector system.

To monitor the density of photo-generated charge carriers on a semiconductor surface, we demonstrate a detectorless imaging system based on the analysis of the optical feedback in terahertz quantum cascade lasers. Photo-excited free electron carriers are created in high resistivity n-type silicon wafers via low power (ffi40 mW/cm2) continuous wave pump laser in the near infrared spectral range. A spatial light modulator allows to directly reconfigure and control the photo-patterned intensity and the associated free-carrier density distribution. The experimental results are in good agreement with the numerical simulations.

We studied the laser ablation dynamics of steel in the thermal regime both experimentally and theoretically. The real-time monitoring of the process shows that the ablation rate depends on laser energy density and ambient pressure during the exposure time. We demonstrated that the ablation efficiency can be enhanced when the pressure is reduced with respect to the atmospheric pressure for a given laser fluence, reaching an upper limit despite of high-vacuum conditions. An analytical model based on the Hertz-Knudsen law reproduces all the experimental results.

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.

We analyze the laser self-mixing process in the Gaussian beam approximation and reformulate the expression of the feedback coefficient C in terms of the effective feedback power coupled back into the laser diode. Our model predicts a twenty-fold increase of the ratio between the maximum and the minimum measurable displacements judged against the current plane-wave model. By comparing the interaction of collimated or diverging Gaussian laser beams with a plane mirror target, we demonstrate that diverging beams tolerate larger wobbling during the target displacement and allow for measurement of off-axis target rotations up to the beam angular width. A novel method for reconstructing the phase front of the Gaussian beam by self-mixing scanning measurements is also presented.

We report on the instantaneous detection of the ablation rate as a function of depth during ultrafast microdrilling of metal targets. The displacement of the ablation front has been measured with a sub-wavelength resolution using an all-optical sensor based on the laser diode self-mixing interferometry. The time dependence of the laser ablation process within the depth of aluminum and stainless steel targets has been investigated to study the evolution of the material removal rate in high aspect-ratio micromachined holes. (C)2011 Optical Society of America

A general model is proposed for a Vertical Cavity Surface Emitting Laser (VCSEL) with medium aspect ratio whose field profile can be described by a limited set of Gauss-Laguerre modes. The model is adapted to self-mixing schemes by supposing that the output beam is reinjected into the laser cavity by an external target mirror. We show that the self-mixing interferometric signal exhibits features peculiar of the spatial distribution of the emitted field and the target-reflected field and we suggest an applicative scheme that could be exploited for experimental displacement measurements. In particular, regimes of transverse mode-locking are found, where we propose an operational scheme for a sensor that can be used to simultaneously measure independent components of the target displacement like target translations along the optical axis (longitudinal axis) and target rotations in a plane orthogonal to the optical axis (transverse plane). (C) 2012 Optical Society of America

Un sistema misura la tensione di un oggetto. Il sistema include una sorgente laser per generare una radiazione di uscita, una fibra ottica deformabile avente prima e seconda sfaccettatura e mezzi per calcolare una misura di una deformazione della fibra ottica. La prima faccetta è accoppiata alla sorgente laser per ricevere la radiazione di uscita e per trasmettere da essa una radiazione guidata sulla fibra ottica verso la seconda faccetta. La seconda faccetta è atta a ricevere la radiazione guidata e a riflettere una corrispondente radiazione riflessa verso la prima faccetta. La sorgente laser è un tipo di auto-miscelazione atto a ricevere almeno parte della radiazione riflessa e a miscelare la radiazione di uscita con la radiazione ricevuta. ; I mezzi di calcolo calcolano la misura della deformazione della fibra ottica attraverso lo spostamento lineare della seconda sezione misurata dall'effetto di auto-miscelazione nella sorgente laser.

Laser system and method for measuring in real time the variation of the laser ablation depth by means of self-mixing interferometry characterized by a reflecting surface subjected to ablation, a laser diode with an integrated photodiode for the self-mixing signal detecting, a lens for sending a portion of the radiation emitted by the laser diode on the surface subjected to ablation and for sending in the cavity of the laser diode a portion of the radiation emitted by the laser diode and back-reflected by the surface subjected to ablation.

A system is described for the measure of the strain of an object. The measurement system includes a laser source for generating an output radiation, includes a strainable optical fiber having a first and a second facet, wherein the first facet is coupled to the laser source for receiving at least part of the output radiation and for transmitting therefrom a guided radiation over the optical fiber towards the second facet, and includes means for calculating a measure of the strain of at least part of the optical fiber. The at least part of the optical fiber includes a first section and a second section, wherein the second section is movable by the strain of the at least part of the optical fiber according to a linear displacement of the second section respect to the first section along the at least part of the optical fiber. The second facet is adapted to receive the guided radiation and to reflect therefrom a corresponding reflected radiation towards the first facet. The laser source is a self-mixing type which is adapted to receive at least part of the reflected radiation guided from the second facet to the first facet and to mix the output radiation with the received radiation. The calculating means are adapted to calculate the measure of the strain of the at least part of the optical fiber through the linear displacement of the second section measured by the self-mixing effect in the laser source.