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Resonance operation of an optical fibre cantilever-based endoscopic scanner is crucial for distortion-free reconstructed images. While feedback control systems have been developed to ensure resonance operation, accurately measuring the location of the vibrating fibre cantilever tip remains challenging. Therefore, we proposed a passive aperture reflector as a sensing mechanism, placed in front of the fibre cantilever for intermittent feedback control. Using the characteristic of the pulse signal reflected from the scan extremities, the driving frequency and the position of the fibre cantilever can be adjusted. For frequency adjustment, both non-proportional and proportional control were conducted, and their results were compared. Besides that, the pulse width and their location with respect to the driving signal were utilised for location adjustment. The findings show that the proportional method with a high coefficient of 5.0 achieves fewer iterations to adjust the driving frequency and closely match it with the resonance frequency. Additionally, the presence of a pulse signal at both positive- and negative-going zero-crossings of the driving signal with similar pulse width ensures the cantilever is centred horizontally. Vertical centring can also be achieved by minimizing the pulse width. The simplicity and passive nature of the apertured metal plate mechanism offer advantages in accurately determining frequency and position. [Display omitted] • Apertured metal plate provides passive, optical intermittent feedback mechanism. • Driving frequency adjustment using pulse signal centre alignment at zero-crossing. • Proportional control method achieves rapid frequency adjustment with iterations ≤ 5. • Horizontal position adjustment aligns the scan line to centre by matching pulse signal widths. • Minimizing pulse widths to centre the scan line vertically. [ABSTRACT FROM AUTHOR]

A vertical-cavity surface-emitting laser (VCSEL) under polarization selected and rotated optical feedback has been proposed and numerically investigated to generate neuron-like spiking dynamics and to show the potential functionalities in spiking information processing. The proposed optical-feedback VCSEL-neuron employs only one laser to simulate one neuron, while conventional optical-injection VCSEL-neurons often require multiple lasers. The input stimulus is conducted by modulating the feedback strength while the output response is monitored by measuring the intensity in the initially non-lasing polarization. On the one hand, the generation of tonic spiking in the form of periodic pulses is related to the frequency difference between the laser's two orthogonal polarization modes. On the other hand, the generation of phasic spiking in the form of a single abrupt pulse is attributed to the transient response at the onset of self-injection locking. Systematically investigations reveal that the frequency of tonic spiking is roughly proportional to the stimulus strength of input, while the critical stimulus strength of phasic spiking is roughly proportional to the birefringence rate of laser. The potential functionalities of proposed VCSEL-neuron are further verified by single neuron information processing tasks. • A novel single-VCSEL spiking neuron is proposed. • The proposed neuron needs only one laser thanks to the light stored in feedback loop. • The tonic spiking is due to the cavity anisotropies of VCSEL. • The phasic spiking is due to transient response at onset of self-injection locking. • The single-VCSEL neuron has similar performances with multi-VCSEL neurons. [ABSTRACT FROM AUTHOR]

• It is theoretically proved that the −1st comb tooth power is related with the modulation depth and the frequency difference between f m (the modulation frequency) and f rep (the free-running repetition rate). • An optical closed-loop injection locking technology is realized using the −1st comb tooth as error signal for the soliton microcomb repetition rate (f rep) long-term stabilization. • The SSB phase noise of the closed-loop locked f rep have no obvious change in long-term, which is about 20 dB lower compared with the open-loop locking case. And the Allan deviation of the closed-loop locked f rep maintains at 1.8 × 10-13@0.1 s for a long time, which improves by three orders of magnitude compared with the open-loop locking case. We demonstrate an optical closed-loop injection locking technology for the soliton microcomb repetition rate (f rep) stabilization. Using the power of the −1st comb line (−1st represents the first comb tooth on the left side of the pump laser) as an error signal, the pump laser frequency is auto-tuned to ensure f rep locked at an optimal level. After injection locking for 2 h, the single-sideband (SSB) phase noise of the closed-loop locked f rep decreases by 20 dB compared with the open-loop locked f rep within the offset frequency range of 20 Hz to 30 kHz. After locking one and a half hours, the Allan deviation of the closed-loop locked f rep reaches 1.8 × 10−13@0.1 s, which improves by three orders of magnitude. The experimental results prove the feasibility of the optical closed-loop injection technology for long-term f rep stabilization. The proposed scheme has excellent locking performance, simple structure and low cost, which has the potential application for stable microwave generation, precision ranging, etc. [ABSTRACT FROM AUTHOR]

Microwave photonic (MWP) sensing and measurement are envisioned to be a promising alternative to the conventional pure electronic or optical solutions. A semiconductor laser (SL) with external optical feedback (EOF) operating in a period-one (P1) dynamic state contributes a new implementation architecture for MWP systems. However, designing such a SL system to generate frequency-modulated MWP sensing signals through traditional Lang–Kobayashi (L–K) equations requires extensive computational effort to derive the system control parameters (SCP), making real-time adjustment of the SCP impossible in cases where it is needed. This paper proposes an effective design approach based on machine learning. A feedforward neural network (FNN), in conjunction with a gradient descent algorithm, is employed to fast and accurately ascertain the SCP, offering a solution readily applicable in the system design. Both simulation and experiment are conducted to validate the proposed approach. • Proposes a machine learning approach to optimize control for period-one state. • Solves computational bottlenecks of Lang–Kobayashi (L–K) equation method. • Uses a neural network with gradient descent to determine control parameters quickly. • Experimental validation confirmed the performance of the proposed method. [ABSTRACT FROM AUTHOR]

In allusion to the privacy and security problems, a secure multi-channel information protection scheme based on an integrated optical device is proposed in this paper. The information to be encrypted and shared is converted into a QR code. For supporting the multi-channel encryption, a 1- n w encryption algorithm is employed, which possesses the capability to map the original image information into n w seemingly random cipher images. In terms of hardware, an ultra-compact optical encryption device is designed, which consists of two rows of ring resonators coupled to the waveguide. This novel encryption device can output ten cipher images, each transmitted on a separate channel. Meanwhile, the vertical-cavity surface-emitting laser subject to optical injection and optical feedback is researched to generate optical chaos sequences, which are introduced for key generation and QR code encryption. Ultimately, combining the 1- n w encryption algorithm and optical chaos, information protection is implemented based on integrated optics. The simulation results demonstrate the practical feasibility and remarkable effectiveness of the proposed scheme. Furthermore, various security and robustness analyses are performed, and the results reveal that the proposed information protection scheme has a high level of security and robustness. This work is of great significance for the development of optical encryption chips and provides potential applications for secure data sharing. [ABSTRACT FROM AUTHOR]

This article presents a dual laser-based optical feedback cavity-enhanced absorption spectroscopy technique using polarization division multiplexing. The technique involves two laser beams with orthogonal polarization that enter a V-shaped cavity from two injection directions. This allows for independent control of feedback phase and eliminates the need for an optical component to combine the beams. The two lasers are detected separately at the two ends of the cavity. Additionally, a differential process is proposed to mitigate residual interference between the two lasers. By employing this technique, simultaneous detection of CH 4 , CO 2 and H 2 O is achieved, resulting in a detection sensitivity of approximately 2 × 10−11 cm−1. Despite using a moderate cavity finesse in our spectrometer, the minimal detectable concentration for the gases is comparable to that of superior gas analyzers using cavity ringdown spectroscopy (CRDS). • A dual laser-based optical feedback cavity-enhanced absorption spectroscopy technique using polarization division multiplexing is presented. • A differential process to mitigate residual interference between the two beams are proposed. • An excellent detection sensitivity of 2×10-11 cm-1 has been achieved, despite using a moderate cavity finesse. [ABSTRACT FROM AUTHOR]

Numerical simulation of generation of a random fiber laser proves that a model, considering only major physical effects, is capable of reproducing the characteristic line spectrum consisting of narrow localized spectral modes near the generation threshold. Lifetime of narrow spectral modes is shown to amount to tens of full cavity pass times or 1 ms by order of magnitude, that well corresponds to experimental observations. It was ascertained that addition of technical noise by implementation of a random phase of backscattered wave does not change the generation regime significantly. Possibility of narrow generation well above the threshold is demonstrated in case of negligible nonlinear effects. Essential role of Rayleigh backscattering in its formation is emphasized by comparison with corresponding ideal fiber amplifier with no feedback, characterized under the same conditions by smooth spectrum. [Display omitted] • Narrow modes emerge in radiation of a random fiber laser due to optical feedback. • Random fiber laser's spectrum acquires line pattern beyond the impact of nonlinearity. [ABSTRACT FROM AUTHOR]

Reservoir Computing (RC), an evolution from Recurrent Neural Networks (RNN), not only represents a unique machine learning paradigm, but also serves as a neuromorphic framework that mirrors the intricate cortical circuits of the human brain. This paper proposes another new photonic RC system based on four basic photonic reservoir computing architectures (single photonic RC system, the parallel photonic RC system, the dual-feedback loop-based photonic RC system and the mutually coupled photonic RC system). System proposed uses optical injection for signal input and retains two parallel responsive semiconductor lasers (R -SLs) with self-feedback loops. Meanwhile, two relatively independent R -SLs are mutually coupled via two coupling lines. The new photonic RC system adds only two sections of fiber compared to the parallel photonic RC system and the mutually coupled photonic RC system. The experiments show that the system proposed has significant advantages on the nonlinear auto regressive moving average series tasks, the chaotic time series prediction tasks and the waveform classification task. More importantly, the memory capacity of system proposed can be adjust by controlling the delay time of the self-feedback loops, so it has higher memory capacity to handle the higher order nonlinear auto regressive moving average tasks (NARMA20 and NARMA30) after optimizing the parameters. • The proposed system integrates the advantages of the dual-feedback loop-based and mutually coupled photonic RC systems. • The proposed system can adjust its memory capacity to suit various short-term memory needs. • The inclusion of mutual coupling and self-feedback structures in the proposed system enriches its dynamic characterization. [ABSTRACT FROM AUTHOR]

This paper presents a novel experimental technique for three-dimensional imaging of acoustic pressure waves by means of an optical feedback interferometer associated to standard Radon transformation algorithm for tomography. The imaging system is based on the sensing of the optical refractive index changes induced by the pressure variation in the medium where the acoustic wave propagates. Thanks to the compactness allowed by the laser optical feedback interferometer, this technique allows to image in three dimensions the variations of pressure of any acoustic source without any displacement of the source. • Development of new method for 3D observation of pressure waves. • Design of a compact, cost efficient, laser sensor to image pressure waves. • Development of a signal processing based on inverse Radon transform to compute 3D images from interferometric signals. • Validation of the imaging principle on pressure waves interferences. [ABSTRACT FROM AUTHOR]

[Display omitted] • Enable storage, calculation, and display functions in a single device. • AMSOF enables multi-stimulus perception of temperature and pressure. • AMSOF employs optical output imaging mode, enabling ease of human–machine interaction. • A fingerprint imaging system has been implemented based on AMSOF for sensing pressure and temperature stimulation. Artificial sensory systems require appropriate input–output modalities to enhance interaction efficiency and accuracy. It is crucial to mimic an organism to perceive and integrate multiple external stimuli and to construct systems with multiple sensory inputs. In addition, the application of optical output to the sensory system not only solves the problem of electrical signal transmission, but also enables direct imaging through optical output. Here, we propose an artificial multisensory system with optical feedback (AMSOF), which can simultaneously receive pressure and thermal stimuli and then generate neuromorphic optical responses along with feedback. Among them, the triboelectric nanogenerator (TENG) is responsible for receiving pressure stimuli, while the artificial light-emitting synaptic device utilizes the quantum dot thermal quenching mechanism to processes temperature signals and generate optical feedback. Additionally, if the AMOFS is applied to the fingerprint recognition system, it can enable fingerprint imaging through dual recognition of these two signals. In other words, it can enhance the security of biometric authentication by adding the capability to distinguish the live nature of objects on top of traditional fingerprint recognition. Therefore, this system aims to achieve more intelligent and efficient human–computer interaction and environmental perception capabilities, bringing forth a multitude of applications and innovations for smart systems. [ABSTRACT FROM AUTHOR]

This work presents a design of a flip-flop controlled by the nonlinear optical response exhibited by a dynamic distribution of nanostructures in a liquid. Silver decorated carbon nanotubes suspended in ethanol were employed. The flip-flop proposed corresponds to a probabilistic NAND latch. The nonlinear nanofluids were studied by two-wave mixing experiments with nanosecond pulses at 532 nm wavelength. The behavior of the system is based on the Brownian motion with influence over the nonlinear transmittance of the nanostructures associated with a probabilistic optical bit. The optical response of the latch system output states was defined by the irradiance dependent on nonlinear optical absorption of the nanofluid. Specifically, the optical logical outputs depend on the state present at the optical inputs and the previous optical output states regulated by the feedback of the system. The nonlinear optical evolution of the effects in the nanofluids was explored as probabilistic phenomena governed by a concentration gradient of nanoparticles. The evaluation of the stability of a nanofluid defines the variables in the probabilistic system proposed. This work highlights the potential of nanohybrid materials for processing probabilistic functions, switching quantum effects, and for developing optical sensors. • An all-optical flip-flop based on probabilistic phenomena and nonlinear effects was designed. • Third-order optical nonlinearities were induced in concentration gradients of suspended nanoparticles. • Plasmonic carbon/metal-based nanofluids were explored by a nanosecond two-wave mixing. • Optical feedback was proposed for encryption and multi-criteria logic functions. [ABSTRACT FROM AUTHOR]

• A highly sensitive interferometry with strong operational waveband anti laser jamming capability is proposed. • The particular laser structure and the unique detector placement method from optical feedback interferometry make the laser cavity operate as a high-stop-band filter to filter out laser jamming in the operational waveband. • The proposed anti-jamming laser feedback interferometric system can get the stopband optical attenuation of 44.8 dB in the operational waveband. • The comparison tests with traditional interferometry and Michelson heterodyne interferometry are performed, and the better anti-jamming capability of proposed system in both phase and intensity measurements is proved. • The proposed method shows promise for practical application in challenging optical environments with laser jamming. With the proliferation of diverse laser equipment, the optical environment in which highly sensitive optical systems operate has become harsh and intricate. Mitigating laser jamming originating from the operational waveband is a challenging problem encountered by laser interferometric measurement systems. In this paper, we set forth a robust anti-jamming optical interferometry with high sensitivity. This method is based on frequency shift optical feedback, namely anti-jamming laser feedback interferometry (AJLFI). A rear-end pumping Nd:YVO 4 microchip laser with bidirectional output channels is employed as the interferometric system source, and the photodetector is posterior to the laser cavity. This particular laser structure and the unique detector placement method from optical feedback interferometry make the laser cavity operate as a high-stop-band filter to filter out laser jamming in the operational waveband. In the AJLFI-based system we established, the jamming laser with a wavelength equaling that of the interferometric system suffers a notable attenuation of 44.8 dB. Concurrently, jamming laser falling within a range of ±10 nm from the wavelength of interferometric system gets an attenuation exceeding 44.43 dB. In addition, we set up the standard laser feedback interferometry(LFI)-based system and the Michelson heterodyne interferometry(MHI)-based system with the same experimental parameters as the AJLFI-based system to perform anti-jamming capability comparative experiments. In the experiments, a pulsed laser beam with a peak power of 1780.9 W is employed as laser jamming in the operating wavebands, which is precisely directed toward the target in conjunction with the probe light of the interferometric systems and scattered into the interferometric systems. The experimental findings demonstrate that the AJLFI-based system exhibits superior anti-jamming capability in both phase and amplitude measurements compared to conventional LFI-based and MHI-based systems. [ABSTRACT FROM AUTHOR]

This paper presents a fast tool servo (FTS) driven by a voice coil motor (VCM) with a function of self-sensing of cutting forces. Conventionally, cutting force measurement associated with a FTS system is made by integrating an additional force sensor or a dynamometer, which would make the system complicated and influence the dynamic performance of the FTS. Differing from the conventional method, the force measurement in the proposed system is achieved by detecting the current of the VCM and then obtaining the cutting force based on the electromagnetic field distribution of the VCM. Since it is not necessary to integrate additional force sensors, the main body of the FTS could be compact and the dynamics of the FTS would not be influenced by the added function of force measurement. The FTS mainly consists of an air bearing guide driven by a VCM with a stroke of 2.5 mm, an optical encoders feedback system for precision positioning and a hall current sensor for current measurement. To obtain forces from the measured currents, the magnetic field distribution of the VCM is figured out and the nonlinear relationship between the position and the magnetic field distribution is corrected. The basic performances of the FTS for positioning and force measurement were experimentally investigated. It is shown that the system could have a positioning resolution of 20 nm and a force self-sensing resolution of 5 mN based on the proposed method. The proposed method provides a new way for in-process cutting force measurement associated with FTS systems. • A VCM-FTS is developed with a function of self-sensing of cutting forces. • Cutting forces are measured by detecting the current of the VCM without using additional force sensors. • The force measurement resolution by the proposed method was evaluated to be 5 mN. [ABSTRACT FROM AUTHOR]

• The external laser based on femtosecond-written birefringent fiber Bragg grating (FBG) achieves excellent linewidth characteristics. • The hybrid laser based on ridge waveguide emission gain chip and femtosecond-written birefringent FBG attains high integration and low cost. • Linear polarization output is realized by the orthogonal polarization eigenmode and effective external cavity mode selection of the birefringent FBG. We demonstrate an external-cavity semiconductor laser (ECSL) with linear polarization and a narrow linewidth. A birefringent Bragg grating prepared by a femtosecond laser point-by-point technique in a polarization-maintaining fiber (PMF) is used to provide external-cavity feedback, and its polarization-dependent characteristics enable selection of the main polarization mode and linewidth narrowing of the ridge waveguide emission gain chip (GC). This compact and robust ECSL achieves an output power of > 60 mW and a polarization extinction ratio (PER) of > 30 dB. We simulate and calculate the linewidth and injection current, while a Lorentz linewidth of 2.58 kHz is obtained based on delayed self-heterodyne beat frequency measurement. This flexible and cost-effective solution allows realization of a compact ECSL with linear polarization and a narrow linewidth. [ABSTRACT FROM AUTHOR]

• An optical feedback cavity ring down spectrometer (OF-CRDS) is developed at 8.45 µm. • Water self-continuum cross-sections at 1185 cm−1 are measured between 296 and 308 K. • Our cross-section value is found about 20% smaller than the MT_CKD_4.1 value. • Available measurements in the 10 µm window seem to indicate that the MT_CKD_4.1 temperature dependence is overestimated. The accurate knowledge of the water vapor absorption in the 10 µm atmospheric window is of strong importance because this spectral range coincides with the maximum black body emission of Earth. Presently, the water vapor self-continuum is measured at the 1185 cm−1 (8.45 µm) spectral point, located in the most transparent interval of the 10 µm window. Measurements are performed at four temperatures ranging from 296 to 308 K using a newly developed Optical Feedback Cavity Ring Down Spectrometer (OF-CRDS). Self-continuum cross-sections, C S , are derived from the pressure dependence of the absorption during pressure ramps of pure water vapor up to 18 mbar. From the quadratic pressure dependence observed for the absorption coefficient at each temperature, we derived the value of the cross-section (C S = 0.996 (12) × 10 − 22 cm 2 molecule−1atm−1 at 296 K) and its temperature dependence. These results are discussed in relation with previous literature measurements available in the 10 µm window. Our cross-section is found about 20% smaller than the MT_CKD_4.1 value and the available experimental works seem to indicate that the MT_CKD_4.1 temperature dependence is overestimated. [Display omitted] [ABSTRACT FROM AUTHOR]

• A mathematical sensing model of a DOFI system working in P1 state was established. • Factors that determine the sensitivity enhancement were analyzed,and the operation region of a DOFI system in P1 state was investigated, based on which, design criteria of designing a DOFI-P1 sensing system were summarized. • Preliminary experiments have been performed to verify the high measurement sensitivity of a DOFI-P1 system as compared with the OFI system. • The results of this work provide helpful guidance for the construction of high-sensitivity DOFI sensing and measurement systems. Laser optical feedback interferometry (OFI) is a unique one-channel interferometric measurement technology with the advantages of compact structure, low implementation cost and easy alignment. However, it has a critical challenge that the visibility of interference fringe is often low and further induces low measurement sensitivity, especially in occasions when the optical feedback strength is feeble, e.g., biomedical and fluid detection. Recently, it has been demonstrated that period-one (P1) dynamics can enhance the measurement sensitivity by introducing extra optical feedback to form a dual optical feedback interferometry (DOFI) system. However, the details of the enhancement mechanism and how to design such a system are still required to unveil. In this paper, comprehensive discussions of designing a DOFI system with high measurement sensitivity enhanced by P1 dynamics are presented. First, based on the modified Lang-Kobayashi equations, a mathematical sensing model of a DOFI system working in P1 state was established. Then, simulations were carried out to verify the validity of the proposed model. After that, factors that determine the sensitivity enhancement were analyzed, based on which, theoretical analysis of achieving optimal sensitivity enhancement was performed. Next, we discussed the operation region of a DOFI system in P1 state and investigated the influence of the system parameters on the operation region. At last, experiments were conducted to verify the theoretical analyses. The results of this work provide helpful guidance for the construction of high-sensitivity DOFI sensing and measurement systems. [ABSTRACT FROM AUTHOR]

The smile effect in semiconductor laser arrays can cause spectral deterioration in spectral beam combining (SBC) systems. We analyze this effect using position error and pointing deviation and propose a coupled-cavity resonance model based on the external-cavity optical feedback and the multibeam interference theory to study locked spectra. Our results indicate that the central emitter exhibits excellent spectral purity, and the edge emitters show side peaks and a decrease in peak intensity. With the increase in the position error and pointing deviation, the emitters experience an obvious spectral shift and an expansion of the spectral width. Pointing deviation has a greater impact on wavelength locking and array spectral width than position errors. Increasing the focal length and incident angle can reduce shifts and spectral width. Furthermore, adopting a grating with a large line density can also decrease its center wavelength shift and compress the spectral width of the combined beam. • The smile effect of semiconductor laser array leads to spectral degradation in beam-combining systems. • A coupled-cavity resonance model is introduced to analyze spectral shift and stability. • Optimizing the external cavity parameters can improve the locked spectra and narrow the combined beam spectral width. • Giving measures to enhance laser sources with optimized spectra, and proposing future research directions. [ABSTRACT FROM AUTHOR]

Thin Cr films 28-nm thick deposited on glass substrates were processed by scanning low-intensity femtosecond laser pulses with energy well below single-pulse damage threshold. Two types of laser-induced periodic surface structures (LIPSS) were produced, depending on the scanning velocity, (1) parallel to laser light polarization with periodicity somewhat smaller than laser wavelength and (2) perpendicular to polarization with spatial period much smaller than wavelength. All structures are formed as protrusions above the initial film surface and exhibit a high degree of oxidation. To explain formation of the LIPSS and their conversion from one to another type, a rigorous numerical approach for modeling surface electromagnetic waves in thin-film geometry has been developed, which takes into account the change of optical properties of material due to laser-induced oxidation and porosity. The approach addresses the multiplicity of electromagnetic modes allowed for thin films. It has been found that the low spatial frequency LIPSS parallel to laser polarization, which are formed at low scanning velocities, are well described by the Sipe theory for surfaces of low roughness. The SEW mode responsible for high spatial frequency LIPSS formation at high scanning velocities has been identified. The mechanisms of optical feedback and transformation between types of LIPSS with scanning velocity have been proposed. Unlabelled Image • Two types of LIPSS were produced on Cr films depending on beam scanning velocity. • Metal oxidation processes responsible for formation of both LIPSS types are discussed. • High spatial frequency LIPSS are described based on a sophisticated theoretical model. • Mechanism of transition between LIPSS' types with scanning velocity is proposed. [ABSTRACT FROM AUTHOR]

• The FSR of a multi-longitudinal mode LD cavity to be measured is 78.95 GHz. • The applicable amplitude limitation range of the MLM-LD-SMVS system is 0 < A l ≤ 2. 0 53 × 1 0 - 3 . • The optimum optical feedback regime of the MLM-LD-SMVS system is 0 < C < 4. 6. The free spectral range of the resonant cavity is calculated to be 78.95 GHz based on 1.90 mm of the waveform separation period as a multi-longitudinal mode laser diode is employed as the light source of the self-mixing vibration measurement system. As such, the influence of the vibration amplitude of the loudspeaker to the self-mixing vibration signal waveform is further researched by means of the feasible theoretical analysis and experimental characterization. The primary experimental results demonstrate that the amplitude limitation ratio coefficient A l of this sensing system is required to design below 2.053 × 10−3 corresponding to an entire waveform separation period, of which is to prevent the self-mixing signal waveform from distorting effectively in the context of the integral order of the external cavity mode. Furthermore, the feedback levels of the multi-longitudinal mode laser self-mixing vibration system are studied in detail. It has been shown that the experimental result of the free spectral range fits well with theoretical value when this system is matched to the applicable amplitude limitation range 0 < A l ≤ 2. 0 53 × 1 0 - 3 and the optimum optical feedback regime 0 < C < 4. 6. [ABSTRACT FROM AUTHOR]

Based on trigonometric function identical deformation, an integral reconstruction method for the displacement measurement on the laser diode self-mixing interference is presented with strengths to avoid phase unwrapping, reduce the reconstruction error and improve the computation speed. The validity of the proposed method is demonstrated through experimental measurements, whose results show that the relative error is less than 3.2% and the computing time is reduced by half compared to PUM. [ABSTRACT FROM AUTHOR]

In this work, we consider an underlay cognitive radio (CR) RF/free space optical (FSO) network where the unlicensed secondary user (SU) wishes to communicate with a specific destination node with the aid of an optical path over a selected relay terminal without causing any interference to the primary user (PU). The proposed structure can be viable for metropolitans where an unlicensed user wishes to transfer its information to a specific destination node which is connected to the backbone network over an optical path. For the proposed structure, closed form and approximate outage probability and ergodic capacity expressions are derived for Nakagami- m / Exponentiated Weibull fading channels. The results show that the transmission capacity of the SU depends on the weather conditions and the number of relays in the system. [ABSTRACT FROM AUTHOR]

We experimentally show that Difference Frequency Generation (DFG) can efficiently occur in a graphene ink for a visible pump-probe radiation in consequence of the second order nonlinear optical response of graphene. DFG induces the local field enhancement over the graphene sheets and subsequently motivates Surface Plasmons (SPs). We then show that SPs can appear as internal feedback entities to realize Optical Bistability (OB) if a large nonlinear response is guaranteed. However, the figure of merit is the experimental demonstration of ultralow rise/fall threshold OB process, not only by using no external feedback procedure but also caused by a single visible guide beam (in contrast to the double pump-probe radiation). The reason is inferred as the presence of strong Raman scattering which in turn triggers a probe radiation interacting with the guide beam as the pump radiation. We repeat the experiment for different wavelengths/nonlinear medium lengths. We deduce that the smaller lengths as well as the larger wavelengths demonstrate better OB features. We previously bring a simple theory, yet mathematically rigorous to interpret the effects. Then, conforming to the Ishikawa's theory of the graphene nonlinear optical response as the interplay of both interband and intraband transitions, we indicate the agreement between the experimental observations and simulation results. Our experimental results can delineate a plain route for Raman scattering and the other elastic scattering mechanisms within the complex medium of graphene ink, breeding thus the connected theory development. On the other hand, since the low threshold OB can be used to sense SPs' formation elaborately, our proposed approach to control SPs via a single visible beam can be extended to the potential applications like the optical switching, plasmonic wave generation, sensing, etc. with the purpose of energy saving. Unlabelled Image • Observation of Intrinsic Optical Bistability at visible frequency in graphene. • Using no external feedback mechanism during the experiment. • Using a low power single visible guide beam instead of pump-probe approach. • Bringing a full-theory based on graphene nonlinear response. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Upgraded control with fast visual feedback using optical microscopy coupled to real-time object tracking. • Translational sensing: backward/forward translational motions and their substrate influences. • Rotary sensing: breakdown phenomenon provides an access to on-chip integrated local viscometry and magnetometry. Abstract Untethered mobile magnetic microrobots in liquids are under increased investigation with the intended goal to obtain in vivo transducing applications, mostly for micromanipulation or cargo transport. Using them as wireless sensors is less common though it offers a highly controllable and mobile sensing capability at micrometric scale with the capability to provide information about their dynamics and interaction with their environment. Here a system is proposed which is fully controllable by calibrated external magnetic fields and high temporal resolution (>5000 FPS) visual feedback. With the targeted applications being in microscale fluids, a mobile magnetic microrobot is integrated into an optically transparent microfluidic chip which demonstrates sensing capabilities. In these experiments, physical sensing is quantified through microrobot dynamics analysis of its elementary planar motions (rotary and translational) and these results are related to local viscosity of fluid and friction from the interfaces. These results also allow for the characterization of swimming performances, magnetization and thus help to improve the design of the microrobotic system. An analysis of transition dynamics also provides complementary measurement on the microrobot hydrodynamics and the interaction with its substrate. The proposed on-chip mobile microrobotic system provides an advantageous testing platform to further investigate the visual servo automation control towards their in-vitro or in-vivo applications. [ABSTRACT FROM AUTHOR]

Highlights • The fundamental and first overtone bands of N 2 are studied by CRDS and OFCEAS. • High sensitivity measurements are performed near 2492 and 4720 cm−1. • The collision-induced absorption (CIA) is measured at sub-atmospheric pressure. • The very weak 2–0 S(10) and S(11) are measured for the first time in the laboratory. • Comparison to literature, in particular to the HITRAN database, is discussed. Abstract Highly sensitive cavity enhanced laser techniques are used for accurate measurements of the absorption of dinitrogen (N 2) at two spectral points of the fundamental and first overtone bands. Spectra of pure N 2 and of a N 2 /O 2 mixture were recorded near 2491 and 4720 cm−1 by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) and Cavity Ring Down Spectroscopy (CRDS), respectively. The noise level of the OF-CEAS and CRDS spectra corresponds to a minimum detectable absorption, α min , of about 10-9 and 10-11 cm−1, respectively. Measurements involve both the N 2 collision-induced absorption (CIA) bands and the very weak 2–0 S (10) and S (11) electric quadrupolar transitions. Accurate CIA binary coefficients, B N 2 − N 2 and B N 2 − O 2 , are derived from the pressure-squared dependence of the baseline level of the spectra recorded at pressures below 1 atm. These CIA values are compared to previous determinations from high pressure spectra (up to 90 atm) and to recent classical molecular dynamics simulations (CMDS). An overall good agreement is obtained although a significant deviation is observed for the first overtone CIA near 4720 cm−1. The line position and intensity (on the order of a few 10−30 cm/molecule) of the 2–0 S (10) and S (11) electric quadrupolar transitions are determined experimentally for the first time and compared to theoretical values provided in the HITRAN2016 database. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract This paper investigates an observer-based neuro-adaptive prescribed performance control scheme for nonstrict feedback systems with consideration of unmeasurable states and expected output tracking performance constraint. Firstly, a novel simplified barrier Lyapunov function involving the prescribed performance function is proposed. Then, via combing the backstepping technique and neural network based approximation, an adaptive prescribed performance controller is developed along with a neural network based observer. To conquer the problem "explosion of complexity" inherent in backstepping technique, dynamic surface control technique is employed, wherein, an adaptive nonlinear filter is devised to compensate for the boundary layer errors. Compared with the existing works, the logarithmic function-based transformation for the prescribed performance constraint is avoided, which renders the developed scheme computationally simple. Finally, the application to a one-link manipulator is organized to verify the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Abstract The optoelectronic oscillation using the second harmonic of a period-one (P1) oscillation as the oscillation seed is proposed and experimentally demonstrated. The P1 oscillation was generated by a distributed feedback (DFB) laser subject to optical feedback. By using a bandpass electrical amplifier to provide sufficient gain for the second harmonic frequency of the P1 oscillation, frequency-doubled microwave signal was generated with the single sideband phase noise below -114.2 dBc/Hz at a 10-kHz frequency offset from the carrier. [ABSTRACT FROM AUTHOR]

Abstract In this contribution, the effects of two key internal parameters, i.e. the linewidth-enhancement factor (α) and the gain nonlinearity (ε), on time-delay signature (TDS) concealment of the optical injection chaos system with phase-conjugate feedback (PCF-OICS) are numerically investigated. The results show that, as the injection and feedback parameters (injection and feedback strength, frequency detuning) are varied, higher linewidth-enhancement factor and lower gain nonlinearity are useful for concealing the TDS, and our simulations uncover that combining the mechanism of PCF and chaotic optical injection is beneficial for TDS concealment in the slave semiconductor lasers (SSL). [ABSTRACT FROM AUTHOR]

Coupled-mode theory has been widely employed in various branches of photonics, such as nano-photonics and non-Hermitian optics. Here we show that it can also be used as an effective physical model for clarifying various optical mode lock-in phenomena, which have been revealed to essentially originate from non-Hermitian degeneracies. We discuss the lock-in effects in physical systems through prototypical examples of optical feedback self-injection frequency lock-in technology and lock-in phenomena of ring laser gyroscopes (RLGs). Seemingly unrelated effects have been unified through one general non-Hermitian mode coupling model. Furthermore, based on our theoretical framework, we have uncovered the underlying mechanism of lock-in in RLGs and analyzed how this effect can be systematically suppressed or even eliminated • Optical mode lock-in phenomena can be characterized through the coupling matrix in coupled-mode theory. • The non-Hermitian property of the coupling matrix can come from (but is not limited to) the existence of asymmetric complex elements in the diagonal elements and the non-conjugate of the off-diagonal elements. • Scattering phase has been introduced in the coupling matrix of the ring laser gyro, revealing the mechanisms for the change of lock-in region. • The lock-in of a ring laser gyro approach of that can potentially be eliminated by inserting an etalon through coupled-mode theory. [ABSTRACT FROM AUTHOR]

We studied two methods that quantify the orbital instability of a laser diode (LD) in two typical chaotic systems. In one method, a coefficient named the orbital expansion exponent (OEE) is calculated from only the time series of the laser intensity. In the other method, the Lyapunov exponent is calculated from the rate equations. Then we compared the OEE and the Lyapunov exponent to discuss the appropriateness of the OEE. In an optical feedback LD system, the OEE cannot represent the dynamics of the chaotic LD satisfactorily for the large feedback ratio. However, when the LD has a larger drive current and the inherent parameters of the LD are dominant in the dynamics, the OEE represents the dynamics of the chaotic LD as well as the Lyapunov exponent. On the other hand, in an optical injection LD system, the correlation coefficient between the OEE and the Lyapunov exponent is large even if the injection ratio is large. To find the reason for this, we replaced the optical feedback system with a multistage optical injection system, and showed that the feedback terms prevent the OEE from accurately representing the orbital instability of a chaotic laser. [ABSTRACT FROM AUTHOR]

Simultaneous suppression of time delay (TD) signature in all observables of optical chaos emission, i.e. in both intensity and phase, has been achieved using two or more semiconductor laser (SL) sources or using two independent polarization components of single vertical cavity surface-emitting laser (VCSEL). In this paper, we examine for the first time the simultaneous TD feature suppression using a single nanolaser of single polarization state. The influence of hybrid optical/electrooptic feedback schemes employing either conventional, phase-conjugate, or grating mirror is evaluated. The concealment of TD signature is then investigated by means of the autocorrelation function. The results reveal the operational regions in each case with well eliminated TD signature in all observables. A secure communications system utilizing chaotic nanolaser is presented. Security analysis for a message transmitted in the form of a colored image is carried out which verifies the immunity of the system against possible statistical, brute force and differential attacks. [ABSTRACT FROM AUTHOR]

Influences of line-width enhancement factor (α -Factor), combined optical phase (COP), and gain saturation (GS) on the onset of chaos (route to chaos (RTC)) of the short external cavity (EC) semiconductor lasers (LDs) are numerically investigated. The bifurcation diagrams (BDs), output photon number time variations, corresponding phase portraits, and fast Fourier transforms (FFT) are presented to examine the RTC of the laser in terms of GS, COP, and α -Factor. Calculated results displayed that variation of the α -Factor leads to RTC considerable modifications and these changes are positive by including the GS. The COP variation moves the optical feedback (OFB) intensity to higher values and causes fluctuations in the RTC of the LD with and without including the GS. The LD RTC is specified for four separate regions, oscillations with one frequency (PO), oscillations with two frequencies (PD), oscillations with three frequencies (3P), and oscillations with four frequencies (4P), in terms of GS, COP, and α -Factor. As the LD operation state moves from continuous wave (CW) to PO, PD, or chaos operation state the OFB intensity decreases / increases through α -Factor / GS increasing and fluctuates with increasing the COP within one period (0–2π). The results presented here can be useful in designing stable LD and directing chirp in integrated devices through COP and α -Factor changing. [ABSTRACT FROM AUTHOR]

Electro-optic (EO) delay oscillators have been widely investigated for secure optical communication. To improve complexity and security, an EO intensity chaotic system with an extra optical feedback is proposed. Simulation results show that the proposed system can improve complexity. Moreover, it can effectively suppress the time delay analysis, and the suppression can be enhanced by increasing the optical feedback strength of our system. In addition, synchronization of the communication system in a chaotic modulation encryption scheme based on the proposed system is discussed. [ABSTRACT FROM AUTHOR]

In a recent contribution [A. Campargue, S. Kassi, D. Mondelain, S. Vasilchenko, D. Romanini, Accurate laboratory determination of the near infrared water vapor self-continuum: A test of the MT_CKD model. J. Geophys. Res. Atmos., 121,13,180–13,203, doi:10.1002/2016JD025531], we reported accurate water vapor absorption continuum measurements by Cavity Ring-down Spectroscopy (CRDS) and Optical-Feedback-Cavity Enhanced Absorption Spectroscopy (OF-CEAS) at selected spectral points of 4 near infrared transparency windows. In the present work, the self-continuum cross-sections, C S , are determined for two new spectral points. The 2491 cm −1 spectral point in the region of maximum transparency of the 4.0 µm window was measured by OF-CEAS in the 23–52   °C temperature range. The 4435 cm −1 spectral point of the 2.1 µm window was measured by CRDS at room temperature. The self-continuum cross-sections were determined from the pressure squared dependence of the continuum absorption. Comparison to the literature shows a reasonable agreement with 1970 s and 1980 s measurements using a grating spectrograph in the 4.0 µm window and a very good consistency with our previous laser measurements in the 2.1 µm window. For both studied spectral points, our values are much smaller than previous room temperature measurements by Fourier Transform Spectroscopy. Significant deviations (up to about a factor 4) are noted compared to the widely used semi empirical MT_CKD model of the absorption continuum. The measured temperature dependence at 2491 cm −1 is consistent with previous high temperature measurements in the 4.0 µm window and follows an exp( D 0 /kT ) law, D 0 being the dissociation energy of the water dimer. [ABSTRACT FROM AUTHOR]

To study the spectral effect further, which exists in fold-type cavity ring-down or enhanced absorption spectrometer, a set of laser scattering measurement apparatus was presented and applied to evaluate the surface status of high-reflectivity coatings. Based on this apparatus and another set of optical-feedback cavity ring-down spectrometer with the equivalent noise absorption coefficient of 9.0×10 −9 cm −1 , some experiments are carried out to inspect some analyses about the effect's mechanism. It is determined that the spectral ripple originates from the interference fringe's movement of resonance light beams along the surface of folding mirror, which causes the folding mirror's loss to change with the laser wavelength's scanning. Based on this conclusion, the potential application and the processing methods of spectral ripple are proposed and discussed. [ABSTRACT FROM AUTHOR]

It has been an important research subject to find new nonlinear optical phenomena. In this paper, we report the experimental observation of stochastic, periodic, and localized light structures in a super long single-mode standard fiber with external optical feedback provided by the fiber end. The end facet reflection provides an analogous Fabry-Perot stimulated Brillouin resonator cavity. By increasing the pump power to exceed stimulated Brillouin scattering threshold, we observed light structures exhibiting extremely rich temporal-pulse characteristics that had never been reported in literature before, including supercontinuum background generation, the localization of periodic optical structure formation, fission, and compression. These optical structures are of period-doubling distribution and have different recurrence rates. What is more interesting is that we have observed sets of low frequency bipolar cycle-pulse trains that is often seen in the electrical field and hardly seen in pure optical system. Real-time specification of dynamical temporal regimes of laser operation may bring new insight into rich underlying nonlinear physics of practical fiber cavity systems. Therefore, some new nonlinear optical phenomena have been observed. [ABSTRACT FROM AUTHOR]

Optical feedback due to mutual relation between local field effect and intersubband transition in quantum dots is investigated for the first time. In this regard, dielectric function of quantum dots is considered up to the third order of nonlinearity. It is found that near the resonance, the intensity inside the dot is a function of frequency, which is determined by the optical feedback. This effect changes the magnitude of optical nonlinearity and its symmetry around the transition energy. The results indicate that the magnitude of the dielectric function decreases at frequencies below the transition frequency because of concentration of electric field inside the dot and vice versa. It is also shown that this effect is enhanced by increasing the intensity and resonance contribution in the dielectric function. [ABSTRACT FROM AUTHOR]

The weak absorption spectrum of water vapor in the important 2.2 μm transparency window is investigated with very high sensitivity. Overall, about 400 absorption lines were measured by Cavity Ring Down Spectroscopy (CRDS) and Optical-Feedback-Cavity Enhanced Laser Spectroscopy (OF-CEAS) in five spectral intervals: 4248.2–4257.3, 4298.4–4302.6, 4336.8.5-4367.5, 4422.4-4441.2 and 4514.6-4533.7 cm −1 . The achieved sensitivity of the recordings (noise equivalent absorption, α min , on the order of 2×10 −10 cm − 1 ) allowed detecting transitions with intensity values down to 1×10 − 28 cm/molecule, more than one order of magnitude better than previous studies by Fourier Transform spectroscopy. The rovibrational assignment was performed on the basis of variational calculations and of previously determined empirical energy values. Most of the newly assigned lines correspond to transitions of the ν 1 , ν 3 and 3ν 2 bands of H 2 17 O in natural isotopic abundance. Fourteen energy levels of H 2 17 O, H 2 18 O and HD 18 O are newly determined. An accurate and complete spectroscopic database is constructed for natural water in the 4190–4550 cm − 1 region (2.39–2.20 µm). The list includes about 4500 transitions with intensity greater than 1×10 − 29 cm/molecule, for the six most abundant isotopologues in natural isotopic abundance. Line positions were obtained by difference of empirical energy values determined from literature data and complemented with the present CRDS results. The list is made mostly complete by including weak transitions not yet detected, with positions calculated from empirical levels and variational intensities. The variational intensities computed by a collaboration between the University College London and the Institute of Applied Physics in Nizhny Novgorod are found to improve significantly previous results by Schwenke and Partridge. Examples of comparison of the constructed line list to CRDS spectra and to simulations based on the HITRAN2012 list illustrate the advantages of the new list. The absence of many HDO lines is identified as most important deficiency of the HITRAN2012 list in the region. [ABSTRACT FROM AUTHOR]

• Laser frequency-shifted feedback technology and polarization imaging technology are combined in this paper. • Laser frequency-shifted feedback technology and polarization imaging technology are combined in this paper. • According to the interaction principle of light and matter, the imaging principle is deeply analyzed. • This method has great potential in applications of the industrial and medical fields. The traditional laser feedback imaging techniques ignore the polarization effect, and only a single scattering information is obtained. In this paper, polarization imaging method based on frequency-shifted optical feedback is proposed, which is simple and suitable for most laser feedback imaging configurations. Using the polarization effect of self-mixed heterodyne interference, the separation and further processing of the target information can be realized. Taking the laser confocal feedback imaging system as an example, the effectiveness in information acquisition and anti-specular reflection noise is verified by experiments. This method, which combines polarization imaging with laser feedback imaging techniques, demonstrates great potential in applications of the industrial and medical fields. [ABSTRACT FROM AUTHOR]

We demonstrate a narrow-linewidth external cavity semiconductor laser (ECSL) based on permanent index modulated Bragg grating feedback for radiation resistance. The frequency selection device of the laser is a highly stable fiber Bragg grating (FBG) created with a femtosecond laser. The laser shows excellent radiation resistance after passing through radiation, which is assessed with a γ-ray total ionizing dose (TID) radiation test. The Lorentz linewidth and relative intensity noise (RIN) of the laser show almost no change after irradiation, and are less than 15 kHz and −150 dBc/Hz, respectively. The maximum output power and laser wavelength slightly change after irradiation increasing by 3.79% and redshifting by 9.10 p.m., respectively, compared with those before irradiation. The experimental results show that the compact-structure and low-cost semiconductor laser based on the permanent index modulated Bragg grating has outstanding stability and robustness against harsh-radiation environments, and it can meet the radiation resistance requirements of space laser communication applications for light sources. • The external cavity semiconductor laser (ECSL) based on permanent index modulated femtosecond-written fiber Bragg grating (FBG) achieves excellent radiation resistance. • A narrow linewidth output is realized by external optical feedback injection locking and apodized FBG mode selection. • The ECSL based on gain chip and FBG attains a high integration and low cost scheme. [ABSTRACT FROM AUTHOR]

Increasing the measurable distance and the displacement measurement limit of a self-mixing displacement sensor poses one of the most important problems to improve the sensor's performance. In order to solve the problem, there has to be found the estimation method of the optical feedback factor for a strong feedback regime. The hardship of the estimation in this regime is brought about by a strong speckle noise and the restriction of normalization methods presented hitherto. In the paper we have found an approach to improving the accuracy of the self-mixing sensor, which can work even for long range and large displacement, by deciding the region of the optical feedback factor based on the ratio of fringe height and using it and fringe counting method. • To present a new method of displacement reconstruction for long range and large displacement. • To present a new parameter estimation method in a self-mixing interferometer. • To verify the method proposed experimentally. [ABSTRACT FROM AUTHOR]

Alternating current-driven organic light-emitting diodes (AC-OLEDs) exhibit great potential for next-generation lighting and display applications owing to their advantage of direct integration into AC-based household electric systems, nevertheless, their low performance with high turn-on voltage and the inability of light emission in the entire cycle of AC signal remain major bottlenecks in their further development. Herein, the AC-OLEDs with coplanar electrodes (cpAC-OLED) are demonstrated, this new type AC-OLED architecture work in a broad voltage and frequency range, from 20 V/50 Hz to 200 V/10 kHz with low turn-on voltage and exhibits various optical feedback. The AC-OLEDs emit light in the whole cycle of applied AC signal, and manifest frequency-dependent color tunability. Moreover, these devices could be efficiently driven by a commonly used electric power supply (220 V/50 Hz, 110 V/50 Hz) without any additional back-end electronics. The working mechanism of developed cpAC-OLEDs based on a capacitor model is discussed. We anticipate that this work would accelerate the development of highly efficient color-tunable AC-driven lighting sources. [Display omitted] • AC-OLEDs with coplanar electrodes (cpAC-OLEDs) are demonstrated that could work in a broad range of voltage (V rms from 20 to 240V) and frequency (from 50Hz to 10kHz). • The fabricated cpAC-OLEDs emit light under both forward and reverse bias of the AC signal when the frequency is up to 300Hz, which would ultimately decrease the superfluous energy loss during operation. • Bi-color cpAC-OLEDs are also manifested that could achieve frequency dependent color tuning via controlling the transport of carriers in the device. • Notably, the developed cpAC-OLEDs could operate under 110V/220V at 50Hz, which is perfectly compatible with the household electric systems and hence, could greatly simplify the design of lighting systems. [ABSTRACT FROM AUTHOR]

• Phase difference is generated by a multi-longitudinal mode laser with a wave plate. • Quadrature phase difference is constructed by a phase-shifted quadrature-phase demodulation algorithm. • The proposed quadrature-phase demodulation is unaffected by the inherent phase shift error produced by other optical components in the optical path. • Simplified displacement demodulation process and might be applied in online displacement measurement. • The max error is 37 nm with a relative error of about 0.37 % for a target displacement of 10 μm. We present a novel method by a multi-longitudinal mode laser with a waveplate to generate two orthogonally polarized signals with stable and adjustable phase differences. The phase difference is observed from the output of the Wollaston prism, and is determined by the angle between the optical axes of the Wollaston prism and the waveplate. A phase-shifted quadrature-phase demodulation algorithm is proposed to construct a signal with a 90° phase-shift between one of the two polarized signals. This outcome allows us with low requirements on optical component adjustment accuracy and optical element manufacturing accuracy and also simplifies the processing of phase demodulation for displacement measurement under very weak optical feedback level. Experimental results show that the error is less than 38 nm when the displacement is 10.00 μm. Given its simplicity and efficiency, the proposed system has guiding significance for the design of the self-mixing interferometers and the high-precision online measurement. [ABSTRACT FROM AUTHOR]

This paper reports results on investigations of the dynamical behavior of a semiconductor laser with quantum dots active medium under the influence of a feedback from double external cavity. This configuration is treated in the framework of Lang-Kobayashi equations. The locus of external cavity modes is found to be elliptic, as in case of conventional optical feedback, but also represents different shapes, even with possible satellite bubbles. A bifurcation analysis is carried out revealing the points of saddle-node and Hopf bifurcations. Finally, the nature of bifurcations and the stability of steady state solutions are analyzed in dependence on different parameters. [ABSTRACT FROM AUTHOR]