Maurizio Dabbicco

Intrinsic stability of quantum cascade lasers against optical feedback

We study the time dependence of the optical power emitted by terahertz and mid-IR quantum cascade lasers in presence of optical reinjection and demonstrate unprecedented continuous wave (CW) emission stability for strong feedback. We show that the absence of coherence collapse or other CW instabilities typical of diode lasers is inherently associated with the high value of the photon to carrier lifetime ratio and the negligible linewidth enhancement factor of quantum cascade lasers.

Laser‐Self‐Mixing Interferometry for Mechatronics Applications

We report on the development of an all-interferometric optomechatronic sensor for the detection of multi-degrees-of-freedom displacements of a remote target. The prototype system exploits the self-mixing technique and consists only of a laser head, equipped with six laser sources, and a suitably designed reflective target. The feasibility of the system was validated experimentally for both single or multi-degrees-of-freedom measurements, thus demonstrating a simple and inexpensive alternative to costly and bulky existing systems.

A Compact Three Degrees-of-Freedom Motion Sensor Based on the Laser-Self-Mixing Effect

The simultaneous measurement of the linear displacement and two rotation angles (yaw and pitch) of a moving object using a laser sensor based on the self-mixing effect is reported. The laser head includes three commercial diode lasers equipped with monitor photodiodes. The target is a plane mirror attached to the moving object. The linear and angular resolutions are 0.7 mum and 0.8times10-30 (2.7 arcsec), respectively. The linearity of the sensor response has been verified over a range of 1 m and plusmn0.4deg. Using three retroreflector prisms with a diameter of 10 mm instead of the plane mirror, the angular range of yaw and pitch has been improved by one order of magnitude.

High-resolution monitoring of the hole depth during ultrafast laser ablation drilling by diode laser self-mixing interferometry

We demonstrate that diode laser self-mixing interferometry can be exploited to instantaneously measure the ablation front displacement and the laser ablation rate during ultrafast microdrilling of metals. The proof of concept was obtained using a 50-μm-thick stainless steel plate as the target, a 120 ps/110 kHz microchip fiber laser as the machining source, and an 823 nm diode laser with an integrated photodiode as the probe. The time dependence of the hole penetration depth was measured with a 0.41 µm resolution.

Imaging of free carriers in semiconductors via optical feedback in terahertz quantum cascade lasers

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 (≅40 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.

Simultaneous measurement of multiple target displacements by self-mixing interferometry in a single laser diode

We demonstrate that a single all-optical sensor based on laser diode self-mixing interferometry can monitor the independent displacement of individual portions of a surface. The experimental evidence was achieved using a metallic sample in a translatory motion while partly ablated by a ps-pulsed fiber laser. A model based on the Lang-Kobayashi approach gives an excellent explanation of the experimental results.

Simultaneous measurement of linear and transverse displacements by laser self-mixing

We present a contactless optical sensor based on the laser-self-mixing effect for real-time measurement of linear and transverse displacements of a moving stage. The sensor is able to measure linear displacements of up to 400 mm along the main optical axis while simultaneously estimating straightness and flatness deviations up to 1 mm. The sensor exploits two identical coplanar nonparallel self-mixing interferometers and requires only one reference plane. The reduction in the number of optical elements allowed by the self-mixing configuration and the intrinsic stiffness of the adopted geometry result in a compact, low-cost, and easy-to-align setup.

Experimental determination of the temperature distribution in trench-confined oxide vertical-cavity surface-emitting lasers

We measured the temperature profile of operating vertical-cavity surface-emitting lasers by means of a contactless optical micro-probe technique, based on the analysis of spectral features in the spontaneous electroluminescence escaping the top Bragg mirror. Increasing the injected current above threshold in the range of 4-10 mA leads to a progressively nonuniform and bell-shaped distribution of temperatures with the laser center /spl sim/40% warmer than the edges. These results validate previous numerical calculations and show the usefulness of the proposed technique compared with the laser wavelength shift method.

Laser-self-mixing interferometry in the Gaussian beam approximation: experiments and theory.

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.

Real time ablation rate measurement during high aspect-ratio hole drilling with a 120-ps fiber laser

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.