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The Self-Mixing Interformeter (SMI) is a self-aligned optical interferometer which has been used for acoustic wave sensing in air through the acousto-optic effect. This paper presents how to use a SMI for the measurement of Sound Pressure Level (SPL) in acoustic waveguides. To achieve this, the SMI is first calibrated in situ as a vibrometer. The optical feedback parameters C and α in the strong feedback regime (C≥4.6) are estimated from the SMI vibrometric signals and by the solving of non-linear equations governing the SMI behaviour. The calibration method is validated on synthetic SMI signals simulated from SMI governing equations for C ranging from 5 to 20 and α ranging from 4 to 10. Knowing C and α, the SMI is then used as an acoustic pressure sensor. The SPLs obtained using the SMI are compared with a reference microphone, and a maximal deviation of 2.2 dB is obtained for plane waves of amplitudes ranging from 20 to 860 Pa and frequencies from 614 to 17,900 Hz. The SPL measurements are carried out for C values ranging from 7.1 to 21.5.

In this paper, a method based on the inherent event-based sampling capability of laser optical feedback interferometry (OFI) is proposed to assess the optical feedback factor C when the laser operates in the moderate and strong feedback regimes. Most of the phase unwrapping open-loop OFI algorithms rely on the estimation of C to retrieve the displacement with nanometric precision. Here, the proposed method operates in open-loop configuration and relies only on OFI’s fringe detection, thereby improving its robustness and ease of use. The proposed method is able to estimate C with a precision of <5%. The obtained performances are compared to three different approaches previously published and the impacts of phase noise and sampling frequency are reported. We also show that this method can assess C on the fly even when C is varying due to speckle. To the best of the authors’ knowledge, these are the first reported results of time-varying C estimation. In addition, through C estimation over time, it could pave the way not only to higher performance phase unwrapping algorithms but also to a better control of the optical feedback level via the use of an adaptive lens and thus to better displacement retrieval performances.

In this study, we show the capability of integrated Micro-Ring Resonators (MRRs) to perform frequency-to-amplitude (FM-to-AM) conversion of optical feedback interferometry (OFI) signals with improved signal-to-noise ratio compared to conventional AM OFI signals. Further, contrary to traditional OFI FM-to-AM conversion techniques using gas cell-based edge filters and free-space or fiber Mach-Zehnder Interferometers (MZI), integrated photonic processing offers greater compactness and perturbation resilience, enhancing noise performance through improved temperature control and immunity to parasitic mechanical vibrations. The OFI FM-to-AM conversion was performed with a fabricated silicon nitride MRR of radius 120 μm and a quality factor of 130,000. The FM-to-AM conversion factor achieved was 0.61 GHz −1 , with a noise equivalent displacement of only 4.9 nm for a 1 kHz bandwidth. This demonstration highlights the potential of integrated edge filters to replace traditional freespace and fiber architectures, more prone to environmental perturbations, in OFI signal processing for vibrometric applications.

External optical feedback (EOF) has great impacts on the properties of lasers. It influences the stable operation of lasers. However, various applications based on lasers with EOF have been developed. One typical example is self-mixing interferometry technology, where modulated steady-state laser intensity is utilized for sensing and measurement. Other works show that laser dynamics can also be used for sensing, and the laser in this case is more sensitive to EOF. This paper reviews the sensing technology that uses the dynamics of lasers with EOF. We firstly introduce the basic operating principles of a laser with EOF and discuss the noise properties of and intensity modification in lasers induced by EOF. Then, sensing applications using laser dynamics are categorized and presented, including sensing by frequency-shifted optical feedback, relaxation oscillation frequency, and dynamics with self-mixing interferometry signals and laser optical chaos. Lastly, we present an analysis of the transient response waveform and spectrum of a laser with EOF, showing its potential for sensing.

In the filtered optical feedback system, a part of the emission of the laser is spectrally filtered, here by a Fabry–Perot (FP) filter, and then fed back into the quantum-dot (QD) laser cavity. This technique has been used in various applications such as inducing frequency dynamics in QD semiconductor lasers and studying the dynamical behavior of a QD semiconductor laser subject to delayed filtered optical feedback. We report on the systematical investigation of the steady-state regime and the dynamical behavior of a QD semiconductor laser subject to delayed FOF. The effects of the filter on the locking of the QD laser frequency to the external cavity modes are described. We report and observe hysteresis, bistability, and multistability and show that all these are well described by a set of rate equations for the coupled QD laser and FP cavity system. We also present a stability diagram that summarizes the dynamical behavior of the system.

Cold non-equilibrium atmospheric pressure plasma jets are increasingly applied in material processing and plasma medicine. However, their small dimensions make diagnosing the fluxes of generated species a challenge. Here we report on the detection of the hydroperoxyl radical, HO2, in the effluent of a plasma jet by the use of optical feedback cavity-enhanced absorption spectroscopy. The spectrometer has a minimum detectable absorption coefficient min of 2.25 × 10 − 10 cm−1 with a 100 second acquisition, equivalent to 5.5 × 10 12 cm − 3 of HO2 (under ideal conditions). Concentrations in the range of (3.1-7.8) × 1013 cm−3 were inferred in the 4 mm wide effluent of the plasma jet.

Microwave frequency comb (MFC) optimization for frequency-modulated continuous-wave (FMCW) generation by period-one (P1) dynamics with dual-loop optical feedback are numerically investigated. The linewidth, the side peak suppression (SPS) ratio, and the comb contrast are adopted to quantitatively evaluate the optimization performance, which directly influence the phase stability, spectral purity and repeatability of the MFC. The results show that intensity modulation of the optical injection can generate a sweepable FMCW signal after photodetection via the optical beat effect. When optical feedback loops are introduced, the single-loop configuration can reduce the phase noise of the FMCW signal whereas a dual-loop configuration exploits the Vernier effect to achieve further linewidth reduction and wide tolerance to the feedback strength. Finally, for both the SPS ratio and comb contrast, the dual-loop configuration achieves a higher SPS ratio and maintains high contrast across a wide range of optical feedback loop delays, which outperforms the loop time tolerance of the single-loop configuration.

The smart travel era has come and requirement for high precision lidar detection technology is higher and higher. The new solid-state lidars can meet the requirements of intelligent cars in the future, with the advantages of high resolution, strong anti-active jamming ability, small volume, light weight, low cost and so on. The narrow linewidth diode laser is the perfect light source of solid-state lidars. The progress and development of narrow linewidth diode laser technique can greatly improve the application of solidstate lidar. The technology and development status of narrow linewidth diode lasers has been described detailly in the paper. And the design ideas, key fabrication technologies and optical characteristics of various narrow linewidth diode lasers have been analyzed and discussed as well. Finally, the developments of narrow linewidth diode lasers are prospected.

This research reports a theoretical investigation on the role of filtered optical feedback (FOF) in the quantum dot light emitting diode (QD-LED). The underlying dynamics is affected by a sidle node, which returns to an elliptical shape when the wetting layer (WL) is neglected. Both filter width and time delay change the appearance of different dynamics (chaotic and mixed mode oscillations ,MMOs). The results agrees with the experimental observations. Here, the fixed point analysis for QDs was done for the first time. For QD-LED with FOF, the system transits from the coherence collapse (CC) case in conventional optical feedback (COF) to a coherent case with a filtered mode in FOF. It was found that the WL washes out the modes which is an unexpected result. This may attributed to the longer capture time of WL compared with that between QD states. Thus, WL reduces the chaotic behavior.

Self-mixing interferometry (SMI) is a promising sensing technology. As well as its compact structure, self-alignment and low implementation cost, it has an important advantage that conventional two-beam interferometry does not have, i.e., SMI signal fringe evolves into asymmetrical shape with increasing optical feedback level, which leads to discrimination of target movement directions for unambiguous displacement measurement possible by a single-channel interferometric signal. It is usually achieved by using SMI signals in moderate feedback regime, where the signals exhibit hysteresis and discontinuity. However, in some applications, e.g., in biomedical sensing where the target has a low reflectivity, it is hard for the SMI system to operate in a moderate feedback regime. In this work, we present comprehensive analyses on SMI signal waveforms for determining system parameters and movement directions by a single-channel weak feedback SMI signal. We first investigated the influence of two system parameters, i.e., linewidth enhancement factor and optical feedback factor, on the symmetry of SMI signals. Based on the analyses on signal waveform, we then proposed a method of estimating the system parameters and displacement directions. The method was finally verified by experiments. The results are helpful for developing sensing applications based on weak feedback SMI systems.