Feedback Regime Research Articles

Effect of Linewidth Enhancement Factor on Fringe in a Self-Mixing Signal and Improved Estimation of Feedback Factor in Laser Diode

The optical output power of a laser diode is modulated by the self-mixing effect when the optical beam is back-reflected or back-scattered into the laser cavity by a target. The optical feedback factor C is the most important one that defines the feature of the self-mixing signals (SMSs). And the estimation of C is indispensable for the displacement reconstruction by the phase unwrapping method because it is impossible to reconstruct the displacement with sub-wavelength resolution, provided the exact value of C is unknown. Unfortunately, C is subject to constant change during the measurement and, what is worse; its estimation is usually very time-consuming. This paper studies the feature of a high and low fringe of SMSs in moderate and strong feedback regime and the effect of the feedback factor and the linewidth enhancement factor on the fringes and presents a simple approach to the estimation of C when C > 1.5 based on the behavioral model proposed by Plantier et al. In particular, the novel approach enables fast direct estimation of C because it is based on analytic relations between C and the amplitudes of the high and low peak in SMSs.

Dark Side of Incentives: Evidence From a Randomized Control Trial in Uganda

Throughout our lives, we are routinely offered different incentives as a way to motivate us. Many researchers have studied the effects of incentives on people’s performance. There can also be important psychological outcomes in terms of stress and happiness. The current paper contributes to the literature by explicitly accounting for this performance-versus-well-being trade-off introduced by incentives. I implement two types of social comparative feedback regimes, within and across-class group comparisons, and two types of incentive regimes, financial and reputation rewards. The results show that rewards can improve performance up to 0.28 standard deviations, but at a cost of higher stress and lower happiness, whereas comparative feedback alone (without rewards) increases performance only mildly, by 0.09 to 0.13 standard deviations, but without impact on student’s stress and happiness. More stressed students exert less effort, perform worse and are absent by 29 percent more compared to those who are stressed minimally. The results also help to identify gender-specific responses to incentives. While boys strongly react to rewards, girls do so only if they are also provided with feedback. Final contribution comes from a rich dataset of more than 5000 primary and secondary school students in Uganda, who were repeatedly tested and interviewed over one academic year.

Semiconductor Laser in Distributed Optical Feedback Regime

In this work, we first analyse the behaviour of semiconductor laser in the presence of weak-to-moderate feedback from a single (lumped) and double external cavity setting. Analysis of laser diode in dynamic double cavity configuration shows that the dynamic double cavity does not disturb the stability of the laser device, but it introduces a considerable shift in the emission frequency that is directly related to the feedback strength and the relative phase difference between the phase of the two mirrors, and the phase of electric field inside the laser diode. Conditions for maximum and minimum impact of the laser emission wavelength have been derived. Addressing the issue of distributed optical feedback, optical feedback produced by more than two external mirrors has been also studied. To this aim we have first generalized the Lang and Kobayashi deterministic rate equations model to arbitrary number of external reflectors and provide the corresponding steady-state solution, which is done for the first time. To the best of our knowledge, this kind of study is carried out for the first time.

On the Effects of Relative Performance Feedback in a Risk Elicitation Task: Evidence From a Lab Experiment

This paper experimentally investigates the influence of different non-payoff-relevant feedback regimes on motivation, competitiveness and risk-taking behaviors in a risk elicitation task. We explore four feedback regimes: private feedback, full ranking feedback, top-5 ranking feedback, and non-ranking feedback based on the median performance of peers. We found that providing relative performance feedback increases risk-taking behavior, especially when using a top-5 ranking. Feedback regimes that provide a user with rank-based feedback have adverse effects on the users' attitudes as they increase competitiveness while failing to increase intrinsic motivation and performance motivation. In contrast, non-ranking feedback like the median feedback increases intrinsic motivation and does not influence competitiveness and risk-taking. Across all treatment groups, introducing relative performance feedback rules out the importance of a monetary compensation. From a management perspective, these results suggest that in situations where high risk decisions are problematic, non-ranking feedback should be preferred over rank-based feedback regimes.

A novel algorithm for laser self-mixing sensors used with the Kalman filter to measure displacement

This paper proposes a simple and effective method for estimating the feedback level factor C in a self-mixing interferometric sensor. It is used with a Kalman filter to retrieve the displacement. Without the complicated and onerous calculation process of the general C estimation method, a final equation is obtained. Thus, the estimation of C only involves a few simple calculations. It successfully retrieves the sinusoidal and aleatory displacement by means of simulated self-mixing signals in both weak and moderate feedback regimes. To deal with the errors resulting from noise and estimate bias of C and to further improve the retrieval precision, a Kalman filter is employed following the general phase unwrapping method. The simulation and experiment results show that the retrieved displacement using the C obtained with the proposed method is comparable to the joint estimation of C and α. Besides, the Kalman filter can significantly decrease measurement errors, especially the error caused by incorrectly locating the peak and valley positions of the signal.

Real-time gas sensing based on optical feedback in a terahertz quantum-cascade laser.

We report on real-time gas sensing with a terahertz quantum-cascade laser (QCL). The method is solely based on the modulation of the external cavity length, exploiting the intermediate optical feedback regime. While the QCL is operated in continuous-wave mode, optical feedback results in a change of the QCL frequency as well as its terminal voltage. The first effect is exploited to tune the lasing frequency across a molecular absorption line. The second effect is used for the detection of the self-mixing signal. This allows for fast measurement times on the order of 10 ms per spectrum and for real-time measurements of gas concentrations with a rate of 100 Hz. This technique is demonstrated with a mixture of D2O and CH3OD in an absorption cell.

All Analog Processing of Speckle Affected Self-Mixing Interferometric Signals

Self-mixing (SM) or optical feedback interferometry has been extensively used for high precision displacement and vibration sensing. However, presence of speckle can significantly degrade the SM interferometric signal and cause changes in signal amplitude as well as in the operating optical feedback regime, resulting in reduction in measurement precision. Previously, different advanced digital signal processing techniques have been proposed to undo the effects caused by speckle. However, their complex and computationally heavy nature inhibits their use for real-time, high bandwidth sensing applications. In this regard, an all analog signal processing algorithm has been presented in this paper, which allows real-time processing of speckle affected SM signal while using standard analog circuits. Various simulations indicated that it is able to correctly process speckle affected SM signals having amplitude variation of at least one order and optical feedback parameter $C$ reduction until 0.5. This proposed algorithm has been tested on experimentally acquired speckle affected SM signals and found capable of dealing with variations in optical feedback regime and amplitude modulation of SM signals, in accordance with simulation results. The developed hardware prototype circuit measures a maximum displacement amplitude of 0.4 mm at a maximum target velocity of 8 mm/s for an SM sensor with a laser wavelength of 785 nm as long as $C >0.5$ . The proposed all analog processing could be a significant step toward a robust, low-cost, integrated, real-time SM displacement sensor.

Looplessness in networks is linked to trophic coherence

Many natural, complex systems are remarkably stable thanks to an absence of feedback acting on their elements. When described as networks these exhibit few or no cycles, and associated matrices have small leading eigenvalues. It has been suggested that this architecture can confer advantages to the system as a whole, such as "qualitative stability," but this observation does not in itself explain how a loopless structure might arise. We show here that the number of feedback loops in a network, as well as the eigenvalues of associated matrices, is determined by a structural property called trophic coherence, a measure of how neatly nodes fall into distinct levels. Our theory correctly classifies a variety of networks-including those derived from genes, metabolites, species, neurons, words, computers, and trading nations-into two distinct regimes of high and low feedback and provides a null model to gauge the significance of related magnitudes. Because trophic coherence suppresses feedback, whereas an absence of feedback alone does not lead to coherence, our work suggests that the reasons for "looplessness" in nature should be sought in coherence-inducing mechanisms.

Atmospheric oxygen regulation at low Proterozoic levels by incomplete oxidative weathering of sedimentary organic carbon

It is unclear why atmospheric oxygen remained trapped at low levels for more than 1.5 billion years following the Paleoproterozoic Great Oxidation Event. Here, we use models for erosion, weathering and biogeochemical cycling to show that this can be explained by the tectonic recycling of previously accumulated sedimentary organic carbon, combined with the oxygen sensitivity of oxidative weathering. Our results indicate a strong negative feedback regime when atmospheric oxygen concentration is of order pO2∼0.1 PAL (present atmospheric level), but that stability is lost at pO2<0.01 PAL. Within these limits, the carbonate carbon isotope (δ13C) record becomes insensitive to changes in organic carbon burial rate, due to counterbalancing changes in the weathering of isotopically light organic carbon. This can explain the lack of secular trend in the Precambrian δ13C record, and reopens the possibility that increased biological productivity and resultant organic carbon burial drove the Great Oxidation Event.

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.

Anomalous feedback and negative domain wall resistance

Magnetic induction can be regarded as a negative feedback effect, where the motive-force opposes the change of magnetic flux that generates the motive-force. In artificial electromagnetics emerging from spintronics, however, this is not necessarily the case. By studying the current-induced domain wall dynamics in a cylindrical nanowire, we show that the spin motive-force exerting on electrons can either oppose or support the applied current that drives the domain wall. The switching into the anomalous feedback regime occurs when the strength of the dissipative torque β is about twice the value of the Gilbert damping constant α. The anomalous feedback manifests as a negative domain wall resistance, which has an analogy with the water turbine.

Improving teaching capacity to increase student achievement

Purpose– This paper argues that in a well-organised school with strong leadership and vision coupled with a concerted effort to improve the teaching performance of each teacher, student achievement can be enhanced. The purpose of this paper is to demonstrate that while macro-effect sizes such as “whole of school” metrics are useful for school leaders in their professional development roles, there are important micro-conditions that can be uncovered in a more detailed analysis of student achievement data.Design/methodology/approach– Evidence of student achievement in a variety of standardised and non-standardised assessment tasks was subjected to examination in apost hoc, case study design. The assessment tasks were the South Australian Spelling Test Waddington Reading Test, a school-wide diagnostic writing task, teacher running records and National Assessment Program for Literacy and Numeracy. Performance in selected classrooms was compared on these tests utilising a variety of parametric quantitative statistics.Findings– School-based reform initiatives require external criteria on which to base decision making. Without such criteria based on data and the capacity to interpret it, interactions in the school culture have unanticipated consequences that have the potential to neutralise school improvement strategies. Further, findings suggest that fewer but sharper and quicker data collection tools are more valuable in such teacher decision making, but these require expertise to produce and interpret them.Research limitations/implications– This paper provides insights from one school, but the reported data are embedded in a sustained five year school reform programme.Practical implications– This paper documents a whole school organisational reform model devised by a school head and leadership team to improve student academic performance. The paper offers a process for developing a data-based school reform strategy for professional development to enhance both student achievement and school outcomes.Social implications– The paper outlines a model for school reform that is focused on all students increasing their academic outcomes. By emphasising collaborative teacher work based on research-justified teaching approaches, the model shows that social inequalities can be reversed.Originality/value– The paper outlines a whole of school reform model focused through a combination of distributed leadership, data-driven decision making, within a context of a coaching, mentoring and feedback regime for teachers. Together this model is an application of theoretical ideas to school reform.

A Numerical Analysis for Optimizing the Gratings Length and Coupling Coefficient for High Reflectivity and Low Cavity Loss in Fiber Grating Fabry-Perot Laser Model

The effects of gratings length (kLFG) and amplitude coupling (Co) coefficients on the effective reflectivity (Reff) and the total cavity loss (αtot) of an external cavity laser source (ECLS) based fiber Bragg gratings (FBGs) are numerically analyzed for designing a laser source operating in strong feedback regime (Regime V). In this study, FBG is used as a wavelength selective element to control the properties of the laser output by controlling the fiber gratings reflectivity level. The study is performed by modifying full analytical expressions for the Reff and the αtot based on laser coupled-wave equations. Results show that Reff is strongly dependent on kLg and Co. Also, it is found that αtot has been reduced significantly with increasing Co, especially at high value for kLG (kLFG&gt; 1.5).

Theoretical approach to one-dimensional detonation instability

Detonation instability is a fundamental problem for understanding the micro-behavior of a detonation front. With the theoretical approach of shock dynamics, detonation instability can be mathematically described as a second-order ordinary difference equation. A one-dimensional detonation wave can be modelled as a type of oscillators. There are two different physical mechanisms controlling the behaviors of a detonation. If the shock Mach number is smaller than the equilibrium Mach number, the fluid will reach the sonic speed before the end of the chemical reaction. Then, thermal chock occurs, and the leading shock becomes stronger. If the shock Mach number is larger than the equilibrium Mach number, the fluid will be subsonic at the end of the chemical reaction. Then, the downstream rarefaction waves propagate upstream, and weaken the leading shock. The above two mechanisms are the basic recovery forces toward the equilibrium state for detonation sustenance and propagation. The detonation oscillator concept is helpful for understanding the oscillating and periodic behaviors of detonation waves. The shock dynamics theory of detonation instability gives a description of the feedback regime of the chemical reaction, which causes variations of the leading shock of the detonation.

Observation of photonic paramagnetic to spin-glass transition in a specially designed TiO2 particle-based dye-colloidal random laser.

Colloidal-based random lasers (RLs) are highly efficient and have been exploited in a wide range of geometries. However, in the particular case of ethanol solutions of rhodamines and TiO2 particles, the RL behavior is quite unstable due to the fast precipitation of the particles. In this Letter, specially designed amorphous TiO2 particles were synthesized by a sol-gel method, preventing the degradation of the RL for long operating lifetimes of over 105 shots. As a consequence, this modified colloidal RL allowed the observation of a clear replica-symmetry-breaking phase transition from the paramagnetic fluorescent to spin-glass RL behavior, which has not been observed in the system with nonfunctionalized TiO2 particles.

Detection of Zeptojoule Microwave Pulses Using Electrothermal Feedback in Proximity-Induced Josephson Junctions.

We experimentally investigate and utilize electrothermal feedback in a microwave nanobolometer based on a normal-metal (Au_{x}Pd_{1-x}) nanowire with proximity-induced superconductivity. The feedback couples the temperature and the electrical degrees of freedom in the nanowire, which both absorbs the incoming microwave radiation, and transduces the temperature change into a radio-frequency electrical signal. We tune the feedback insitu and access both positive and negative feedback regimes with rich nonlinear dynamics. In particular, strong positive feedback leads to the emergence of two metastable electron temperature states in the millikelvin range. We use these states for efficient threshold detection of coherent 8.4GHz microwave pulses containing approximately 200 photons on average, corresponding to 1.1×10^{-21} J≈7.0 meV of energy.

What if best practice is too expensive? Feedback on oral presentations and efficient use of resources

We evaluate feedback methods for oral presentations used in training non-quantitative research skills (literature review and various associated tasks). Training is provided through a credit-bearing module taught to MSc students of banking, economics and finance in the UK. Monitoring oral presentations and providing ‘best practice’ feedback is very resource-intensive. Do we withdraw oral presentations from the module, because best feedback practice is prohibitively expensive in a world of limited resources, or choose a second-best alternative? To what extent might the latter compromise intended learning outcomes? We used the same provision of video feedback for all students but used two verbal feedback regimes. For one regime, we decreased the amount of verbal feedback and increased the number of presentations. The impact was measured by academic outcome, rating scales and questionnaire. Overall satisfaction with the module was very high for both feedback regimes, and there were no statistically significant differences between regimes, suggesting that less resource-intensive methods need not compromise learning outcomes.

Nanoscale Displacement Sensing Based on Nonlinear Frequency Mixing in Quantum Cascade Lasers

We demonstrate a sensor scheme for nanoscale target displacement that relies on a single quantum cascade laser (QCL) subject to optical feedback. The system combines the inherent sensitivity of QCLs to optical reinjection and their ultrastability in the strong feedback regime where nonlinear frequency mixing phenomena are enhanced. An experimental proof of principle in the micrometer wavelength scale is provided. We perform real-time measurements of displacement with λ/100 resolution by inserting a fast-shifting reference etalon in the external cavity. The resulting signal dynamics at the QCL terminals shows a stroboscopic-like effect that relates the sensor resolution with the reference etalon speed. Intrinsic limits to the measurement algorithm and to the reference speed are discussed, disclosing that nanoscale ranges are attainable.

Land and atmospheric controls on initiation and intensity of moist convection:CAPEdynamics andLCLcrossings

AbstractThe local role that land‐atmosphere interactions play in the rainfall process has been often explored by investigating the initiation of moist convection as the top of the atmospheric boundary layer (ABL) crosses the lifting condensation level (LCL). However, this LCL crossing alone is not a sufficient indicator of the probability and intensity of subsequent convective precipitation, which is instead better characterized by the added consideration of the so‐called convective available potential energy (CAPE). In this study, both the LCL crossing and CAPE are jointly considered as the primary indicators of the occurrence and intensity of moist convection in order to analyze the land‐atmosphere interactions through a simple soil‐plant system and a zero‐dimensional mixed‐layer model. The approach is explored using the free atmospheric conditions observed at the Central Facility in the Southern Great Plains, where the ABL analysis shows both dry and wet soil can be conducive to early moist convection depending on atmospheric conditions but CAPE always tends to be larger under wetter soil conditions. The combination of the two indicators, LCL crossing and CAPE, further allows us to classify free atmosphere and soil moisture regimes into positive and negative feedback regimes for moist convection.

Robust Method of Stabilization of Optical Feedback Regime by Using Adaptive Optics for a Self-Mixing Micro-Interferometer Laser Displacement Sensor

A self-mixing (SM) micro-interferometer laser displacement sensor coupled with an adaptive liquid lens (ALL) system is proposed and implemented. This has been made possible by a new method of real-time estimation of the optical feedback coupling factor $C$ . It is shown that such an estimation of $C$ combined with an appropriate amplification of the SM signal $Gain$ allows the ALL system to seek and maintain the SM signal in the moderate optical feedback regime in spite of variations in the optical feedback. The ALL system thus enables robust real-time displacement sensing in an unmanned autonomous manner. The implemented system has provided measurement precision better than 90 nm for different target surfaces and distances. The paper also investigates the impact of the weighting attributed to $C$ and $Gain$ on the retrieved displacement precision. As this autofocus is presently only performed once during the sensor initialization, so maximum displacement span after achieving optical feedback regime locking has also been investigated and tabulated. This proof of concept, thus paves the way for the deployment of autonomous SM sensors.