Alexandre Locquet

Tunable lasers are essential and versatile tools in photonics, with applications including telecommunications, spectroscopy, and sensing. Advancements have aimed to precisely control the lasing wavelength, expand tuning ranges, suppress mode hopping, and enable photonic integration. In this work, we explore the adaptation of dynamic targeting, a technique originally developed to stabilize lasers under optical feedback, as a method for achieving agile, fast, and continuous wavelength tuning. This technique works by adjusting the feedback rate and phase, enabling a stable and controlled frequency shift. We experimentally demonstrate reliable and reproducible tuning over 2.1 GHz using a free-space optical setup. Simulations further suggest that this approach could extend the tuning range to tens of GHz, with a potential scan speed exceeding $10^{17}$ Hz/s. These results highlight dynamic targeting as a promising route toward agile frequency control in semiconductor lasers for photonic integrated circuits.

Conformal coatings are used to protect microelectronic circuitry and increasingly optoelectronics and photonics from detrimental effects of the environment, such as moisture, dust, gasses, and mechanical abrasion. The conventional approach to determine the mean time to failure of conformally coated microelectronic components is usually labor-intensive and time-consuming. We recently showed Shi H, et al. Terahertz Nondestructive Characterization of Conformal Coatings for Microelectronics Packaging, IEEE Trans. Compon. Packag. Manuf. Technol., vol. 14(1), pp. 3-9, 2024 that the quasi-optical approach terahertz (THz) time-of-flight tomography (TOFT) could in principle be used to map conformal-coating thickness over a sample of dimensions on the scale of square centimeters. In this study, we employ THz TOFT to characterize several conformal-coating types on microelectronic test samples in a nondestructive and noncontact manner. This study extends previous work on acrylic conformal coatings. THz TOFT is shown to be effective in the thickness characterization of silicone and acrylic conformal coatings, but not nanometric atomic-layer-deposition metal-oxide coating, which is too thin for the technique.

We propose a new mechanism for the generation of optical frequency combs (OFCs) in a semiconductor laser with nonlinear time-delayed optoelectronic feedback. We demonstrate, theoretically and experimentally, that a highly stable periodic regime exists in a closed region of the parameter space known as the Busse balloon. The boundary of the balloon is formed by the secondary Hopf bifurcation. The observed periodic regime results in a >30 GHz bandwidth OFC with microwave spacing between lines and a nearly parabolic distribution of narrow 10-20 MHz linewidths.

We report experimentally and theoretically a dynamical scenario involving the formation of pulse multiplet structures, i.e., configurations of two, three, and more pulses per round trip, in a semiconductor laser with positive optoelectronic feedback. The delayed feedback loop includes a cascade of two band-limited amplifiers with nonlinear saturation of sigmoid type. The multiplets appear subcritically in the vicinity of the lasing threshold, resembling gain-switched operation, and evolve with the injection current.

Measuring the thickness of conformal coatings and detecting defects in a nondestructive and noncontact fashion is important to ensure the reliability and performance of packaged electronic components and systems. In this study, we explore the application of terahertz time-of-flight tomography, a noncontact and nondestructive method here applied to conformal-coating thickness. We measure the time delay of terahertz pulses reflected (echoes) from fabricated structures covered by a conformal coating to ascertain coating thickness. Orthogonal matching pursuit is applied to deconvolve reflected terahertz signals from various interfaces, viz. air/conformal coating and conformal coating/substrate. By these means, we map the conformal-coating thickness distribution over significant areas. The conformal coating thickness found over the metal area is ~64.3 μm while it is ~56.2 μm over the metal-free area. Moreover, the Gaussian mixture model, a machine-learning algorithm, is used to detect a possible defect-rich region within the conformal coating itself.

We provide a comprehensive analysis of the resonant properties of the memory capacity of a reservoir computer based on a semiconductor laser subjected to time-delayed filtered optoelectronic feedback. Our analysis reveals first how the memory capacity decreases sharply when the input-data clock cycle is slightly time-shifted from the time delay or its multiples. We attribute this effect to the inertial properties of the laser. We also report on the damping of the memory-capacity drop at resonance with a decrease of the virtual-node density and its broadening with the filtering properties of the optoelectronic feedback. These results are interpretated using the eigenspectrum of the reservoir obtained from a linear stability analysis. Then, we unveil an invariance in the minimum value of the memory capacity at resonance with respect to a variation of the number of nodes if the number is big enough and quantify how the filtering properties impact the system memory in and out of resonance.