BLSTM convolution and self-attention network enabled recursive and direct prediction for optical chaos.

Chaotic time series prediction has attracted much attention in recent years because of its important applications, such as security analysis for random number generators and chaos synchronization in private communications. Herein, we propose a BLSTM convolution and self-attention network model to predict the optical chaos. We validate the model's capability for direct and recursive prediction, and the model dramatically reduces the accumulation of errors. Moreover, the time duration prediction of optical chaos is increased with comparative accuracy where the predicted sequence length reaches 4 ns with normalized mean squared error (NMSE) of less than 0.01.

Experimental study of the influence of variable external optical feedback on visible laser diode speckle.

This study employs an experimental setup with a variable feedback fraction (fext), external cavity length (ECL), and pump current (J) to investigate their relationships with optical linewidth and external optical feedback-induced speckle contrast (SC) reduction in a visible laser diode. In total, speckle contrast and optical linewidth data for seven feedback fractions, two pump currents, and three external cavity lengths were collected. Achieving optical linewidths up to 3.90 nm (1/e2 width) and a reduction in speckle contrast of up to 41.6%, we also revealed negative correlations between speckle contrast and optical linewidth, and speckle contrast and feedback fraction, and a positive correlation between optical linewidth and feedback fraction.

Optical injection effects in electrically pumped semiconductor nano-laser arrays.

A theoretical analysis is performed on the response of electrically pumped nano-laser arrays to external optical injection. The response to both continuous wave and modulated optical injection is explored. Continuous wave injection is shown to excite several varieties of dynamical behaviour in the array elements including regular dynamics and quasi-periodic behaviour. The strength of the optical injection, the frequency detuning between the injected light and the target laser, and the magnitude of the Purcell spontaneous emission enhancement factor are shown to markedly affect the dynamics. When subject to modulated optical injection, the effects of frequency detuning and optical injection strength are the focus of attention. It is shown that the elements of the array subject to modulated optical injection exhibit oscillatory behaviour over broad regimes determined by the optical injection strength and the frequency detuning.

Solving the overlapped absorbance profile in gas detection by Lorentz distribution solution based on direct absorption spectroscopy method.

A novel method of Lorentz distribution solution (LDS) from overlapped absorbance profile in time domain (incomplete absorbance profile in frequency domain) based on the direct absorption spectroscopy method (DAS) was experimentally demonstrated. It utilized the ratio of the integral in a certain interval on the lower horizontal axis of the Lorentzian profile to the integral in the entire interval on the horizontal axis has a certain relationship and can be expressed by a formula. This method effectively solves the difficulties of extracting gas concentration from incomplete absorbance profile. Formulation and detection procedure were presented, experiments were carried out to prove the method on the extraction of gas concentration from different overlapped absorbance profile and different concentration. Compared with the conventional DAS (C-DAS), the maximum relative errors on the concentration extraction are minimized from 25.55% to 2.64% at different concentration and absorbance profile. Meanwhile, the experimental results show that the obtained gas concentration by LDS presents a good linear relationship while those measured by C-DAS are significantly different.

Transverse mode locking of different frequency-degenerate families based on annular beam pumping.

Optical vortex arrays have been achieved in an end-pumped Nd:YVO4 laser pumped by an annular beam. Spontaneous transverse mode locking of Laguerre-Gaussian modes in different frequency-degenerate families has been obtained by merely adjusting the pump power. A maximum output power of 0.88 W and optical conversion efficiency of 13.6% are achieved for optical vortex arrays. Optical vortex arrays formed in different frequency-degenerate families of Laguerre-Gaussian modes can be actively controlled by the position of the axicon. This work provides a way to research transverse mode locking of Laguerre-Gaussian modes in different frequency-degenerate families based on annular beam pumping.

Continuous In-Line Chromium Coating Thickness Measurement Methodologies: An Investigation of Current and Potential Technology.

Coatings or films are applied to a substrate for several applications, such as waterproofing, corrosion resistance, adhesion performance, cosmetic effects, and optical coatings. When applying a coating to a substrate, it is vital to monitor the coating thickness during the coating process to achieve a product to the desired specification via real time production control. There are several different coating thickness measurement methods that can be used, either in-line or off-line, which can determine the coating thickness relative to the material of the coating and the substrate. In-line coating thickness measurement methods are often very difficult to design and implement due to the nature of the harsh environmental conditions of typical production processes and the speed at which the process is run. This paper addresses the current and novel coating thickness methodologies for application to chromium coatings on a ferro-magnetic steel substrate with their advantages and limitations regarding in-line measurement. The most common in-line coating thickness measurement method utilized within the steel packaging industry is the X-ray Fluorescence (XRF) method, but these systems can become costly when implemented for a wide packaging product and pose health and safety concerns due to its ionizing radiation. As technology advances, nanometer-scale coatings are becoming more common, and here three methods are highlighted, which have been used extensively in other industries (with several variants in their design) which can potentially measure coatings of nanometer thickness in a production line, precisely, safely, and do so in a non-contact and non-destructive manner. These methods are optical reflectometry, ellipsometry and interferometry.

Design and optical characterization of an efficient polarized organic light emitting diode based on refractive index modulation in the emitting layer.

Luminescent liquid Crystal (LC) material is regarded as the most promising material for polarized organic light emission due to their intrinsic characteristics including orderly alignment and luminescence. Nevertheless, the optical extraction efficiency of LC based organic light emitting diodes (OLEDs) devices still requires significant effort and innovation towards real-world applications. In this paper, we propose the design of a highly linearly polarized light-emission from OLEDs with integrated refractive index nanograting in the emissive layer (EML) based on photo aligned luminescent liquid crystal material. The simulation results indicate that the geometrically optimized polarized device yields an external quantum efficiency (EQE) up to 47% with a polarized ratio up to 28 dB at a 550 nm emission wavelength. This conceptual design offers a new opportunity to achieve efficient polarized organic luminescence, and it is (to the best of our knowledge) the first approach that enhances the light extraction of OLEDs based on luminescent liquid crystal via index grating in the EML.

Features of a Self-Mixing Laser Diode Operating Near Relaxation Oscillation.

When a fraction of the light reflected by an external cavity re-enters the laser cavity, both the amplitude and the frequency of the lasing field can be modulated. This phenomenon is called the self-mixing effect (SME). A self-mixing laser diode (SM-LD) is a sensor using the SME. Usually, such LDs operate below the stability boundary where no relaxation oscillation happens. The boundary is determined by the operation condition including the injection current, optical feedback strength and external cavity length. This paper discovers the features of an SM-LD where the LD operates beyond the stability boundary, that is, near the relaxation oscillation (RO) status. We call the signals from such a SM-LD as RO-SM signals to differentiate them from the conventional SM signals reported in the literature. Firstly, simulations are made based on the well-known Lang and Kobayashi (L-K) equations. Then the experiments are conducted on different LDs to verify the simulation results. It shows that a RO-SM signal exhibits high frequency oscillation with its amplitude modulated by a slow time varying envelop which corresponds to the movement of the external target. The envelope has same fringe structure (half-wavelength displacement resolution) with the conventional SM signals. However, the amplitudes of the RO-SM signals are much higher compared to conventional SM signals. The results presented reveal that an SM-LD operating near the RO has potential for achieving sensing with improved sensitivity.

Simple method for measuring the linewidth enhancement factor of semiconductor lasers.

A simple method for measuring the linewidth enhancement factor (LEF) of semiconductor lasers (SLs) is proposed and demonstrated in this paper. This method is based on the self-mixing effect when a small portion of optical signal intensity emitted by the SL reflected by the moving target re-enters the SL cavity, leading to a modulation in the SL's output power intensity, in which the modulated envelope shape depends on the optical feedback strength as well as the LEF. By investigating the relationship between the light phase and power from the well-known Lang and Kobayashi equations, it was found that the LEF can be simply measured from the power value overlapped by two SLs' output power under two different optical feedback strengths. Our proposed method is verified by both simulations and experiments.

Dynamic stability analysis for a self-mixing interferometry system.

A self-mixing interferometry (SMI) system is a laser diode (LD) with an external cavity formed by a moving external target. The behavior of an SMI system is governed by the injection current J to the LD and the parameters associated with the external cavity mainly including optical feedback factor C, the initial external cavity length (L0) and the light phase (ϕ0) which is mapped to the movement of the target. In this paper, we investigate the dynamic behavior of an SMI system by using the Lang-Kobayashi model. The stability boundary of such system is presented in the plane of (C, ϕ0), from which a critical C (denoted as C(critical)) is derived. Both simulations and experiments show that the stability can be enhanced by increasing either L0 or J. Furthermore, three regions on the plane of (C, ϕ0) are proposed to characterize the behavior of an SMI system, including stable, semi-stable and unstable regions. We found that the existing SMI model is only valid for the stable region, and the semi-stable region has potential applications on sensing and measurement but needs re-modeling the system by considering the bandwidth of the detection components.

Improving the measurement performance for a self-mixing interferometry-based displacement sensing system.

Approaches that are, to our knowledge, novel, are proposed in this paper to improve the accuracy performance of self-mixing interferometry (SMI) for displacement measurement. First, the characteristics associated with signals observed in SMI systems are studied, based on which a new procedure is proposed for achieving accurate estimation of the laser phase. The studies also revealed the reasons for the inherent errors associated with the existing SMI-based techniques for displacement measurement. Then, this paper presents a new method for estimating the optical feedback level factor (denoted by C) in real time. Combining the new algorithms for estimating the laser phase and updating C value, the paper finally presents a novel technique for displacement measurement with improved accuracy performance in contrast to existing techniques. The proposed technique is verified by both simulation and experimental data.