Laser Optical Feedback Turns 60.

As soon as a laser is fired, some of the emitted light is scattered backward and coupled with the cavity modes, causing instability [...].

Scanless optical feedback imaging principle by single-pixel compressed sensing.

Optical feedback in lasers is being used for unconventional imaging of fluid dynamics, pressure fields, material properties, and free-carrier distribution, especially in spectral regions where two-dimensional detectors are not yet available. As this technique requires scanning the laser spot across the target, the resulting image contrast is often hampered by the speckle effect. Compressed sensing is becoming a workhorse technique for signal analysis, allowing the reconstruction of complex images from a relatively small number of integrated (single-pixel) measurements, and is being efficiently adapted to a number of single-pixel detector cameras. We applied compressed sensing algorithms to the inherently single-pixel optical feedback in a semiconductor diode laser, demonstrating for the first time, to the best of our knowledge, scanless and detectorless speckle-free imaging of a simple binary object.

Versatile Multimodality Imaging System Based on Detectorless and Scanless Optical Feedback Interferometry-A Retrospective Overview for A Prospective Vision.

In this retrospective compendium, we attempt to draw a "fil rouge" along fifteen years of our research in the field of optical feedback interferometry aimed at guiding the readers to the verge of new developments in the field. The general reader will be moved at appreciating the versatility and the still largely uncovered potential of the optical feedback interferometry, for both sensing and imaging applications. By discovering the broad range of available wavelengths (0.4-120 µm), the different types of suitable semiconductor lasers (Fabry-Perot, distributed feedback, vertical-cavity, quantum-cascade), and a number of unconventional tenders in multi-axis displacement, ablation front progression, self-referenced measurements, multispectral, structured light feedback imaging and compressive sensing, the specialist also could find inspirational suggestions to expand his field of research.

Detectorless measurements of the operational linewidth of NIR VCSELs by self-mixing interferometry.

Self-mixing based on vertical-cavity surface-emitting lasers (VCSELs) offers a compact and low-cost coherent detection scheme for interferometric accessible measurements. The direct detection of the change in the junction voltage, in contrast to the traditional optical detection method by means of an external photodiode, further simplifies the setup by adding detector-less capability. The linewidth of an NIR VCSEL was estimated by using the method based on the statistical analysis of the laser self-mixing fringe period in the moderate feedback regime. We investigate the junction voltage noise and optical power noise, simultaneously acquired, in order to establish the best operational condition for both detection schemes. When comparing the laser linewidth measured by the traditional optical power modulation with that of the detector-less voltage self-mixing signal, the agreement is within the experimental errors.

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.

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.

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.

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.

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.

Photo-generated metamaterials induce modulation of CW terahertz quantum cascade lasers.

Periodic patterns of photo-excited carriers on a semiconductor surface profoundly modifies its effective permittivity, creating a stationary all-optical quasi-metallic metamaterial. Intriguingly, one can tailor its artificial birefringence to modulate with unprecedented degrees of freedom both the amplitude and phase of a quantum cascade laser (QCL) subject to optical feedback from such an anisotropic reflector. Here, we conceive and devise a reconfigurable photo-designed Terahertz (THz) modulator and exploit it in a proof-of-concept experiment to control the emission properties of THz QCLs. Photo-exciting sub-wavelength metastructures on silicon, we induce polarization-dependent changes in the intra-cavity THz field, that can be probed by monitoring the voltage across the QCL terminals. This inherently flexible approach promises groundbreaking impact on THz photonics applications, including THz phase modulators, fast switches, and active hyperbolic media.