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The effect of the optical polarized Feedback through feedback strength and delayed time on dynamics, polarization flipping points and chaotic of quantum dot semiconductor lasers is studied using polarization-rotated optical feedback from the transverse electric mode to the transverse magnetic mode. It was found that the change in the behavior of the laser output is little at the small values of the above parameters and perturbed by increasing them, but we got a different behavior that ends in chaos and an excessive increase in capacitance when changing those parameters. [ABSTRACT FROM AUTHOR]

This study quantifies the contribution of individual cloud feedbacks to the total short‐term cloud feedback in satellite observations over the period 2002–2014 and evaluates how they are represented in climate models. The observed positive total cloud feedback is primarily due to positive high‐cloud altitude, extratropical high‐ and low‐cloud optical depth, and land cloud amount feedbacks partially offset by negative tropical marine low‐cloud feedback. Seventeen models from the Atmosphere Model Intercomparison Project of the sixth Coupled Model Intercomparison Project are analyzed. The models generally reproduce the observed moderate positive short‐term cloud feedback. However, compared to satellite estimates, the models are systematically high‐biased in tropical marine low‐cloud and land cloud amount feedbacks and systematically low‐biased in high‐cloud altitude and extratropical high‐ and low‐cloud optical depth feedbacks. Errors in modeled short‐term cloud feedback components identified in this analysis highlight the need for improvements in model simulations of the response of high clouds and tropical marine low clouds. Our results suggest that skill in simulating interannual cloud feedback components may not indicate skill in simulating long‐term cloud feedback components. Plain Language Summary: Cloud feedback—the radiative response of clouds to changes in temperature—is determined by the contributions from various cloud types and exhibits large uncertainty. Here, we use satellite observations to evaluate how the individual cloud feedback components in response to interannual variability are represented in the latest generation of climate models. The total cloud feedback is positive in the observations, mainly driven by changes in the altitude of high clouds, changes in the cloud cover over land, and changes in the reflectivity of high and low clouds over extratropical regions. The climate models were driven by observed sea‐surface temperature, sea‐ice conditions, and radiative forcing, so they can be directly compared to the observations. We found the models simulate a total cloud feedback that agrees with the observed positive total cloud feedback in general. However, the models consistently overestimate the decrease of cloud amount over land and in oceanic regions of tropical descent and consistently underestimate the increase of high‐cloud altitude and the decrease of extratropical high‐ and low‐cloud optical depth. Models that better simulate cloud feedback in response to short‐term fluctuations do not perform better in simulating long‐term cloud feedback under global warming. Key Points: Individual short‐term cloud feedback components in climate models are assessed by comparing against satellite observationsModel biases are mainly driven by tropical marine low‐cloud, high‐cloud altitude, and extratropical high‐cloud optical depth feedbacksSkill in simulating short‐term cloud feedbacks is not correlated with skill in simulating long‐term cloud feedbacks [ABSTRACT FROM AUTHOR]

Ever since the mid-1960's, locking the phases of modes enabled the generation of laser pulses of duration limited only by the uncertainty principle, opening the field of ultrafast science. In contrast to conventional lasers, mode spacing in random lasers is ill-defined because optical feedback comes from scattering centres at random positions, making it hard to use mode locking in transform limited pulse generation. Here the generation of sub-nanosecond transform-limited pulses from a mode-locked random fibre laser is reported. Rayleigh backscattering from decimetre-long sections of telecom fibre serves as laser feedback, providing narrow spectral selectivity to the Fourier limit. The laser is adjustable in pulse duration (0.34–20 ns), repetition rate (0.714–1.22 MHz) and can be temperature tuned. The high spectral-efficiency pulses are applied in distributed temperature sensing with 9.0 cm and 3.3 × 10−3 K resolution, exemplifying how the results can drive advances in the fields of spectroscopy, telecommunications, and sensing. Pulses of adjustable duration are generated by a mode-locked random fibre laser that can drive advances in sensing. Rayleigh backscattering from cm-long sections of telecom fibre provides laser feedback and spectral selectivity to the Fourier limit. [ABSTRACT FROM AUTHOR]

Spectroscopic detection of radiocarbon dioxide (14CO2) offers several advantages, including real‐time detection, on‐site measurement capability, and ease of operation. Here, we propose a novel 14CO2 detection technique that utilizes optical feedback linear cavity enhanced absorption spectroscopy. The feedback phase is actively compensated by adjusting the light path length. The combination of cavity‐based technique and optical feedback enables the recording of stable and enhanced 14CO2 spectra. Through Allan deviation analysis, a minimal detectable 14CO2 concentration of 3.5 parts‐per‐trillion is achieved. This technique has the potential to contribute to the monitoring of the radiocarbon emissions in the air of nuclear power plants. [ABSTRACT FROM AUTHOR]

Radiative characteristics of microstripe laser diode bar in an external resonator based on an aspherical lens and a flat dielectric mirror were Investigated. The bar had total emitting aperture of 185 μm, formed by 10 stripes with a width of 6 μm separated by mesa-grooves. Operation in the external resonator of the entire emitting aperture was characterized by a multimode generation regime with a peak power of 3 W/6 A. Selection of lateral modal structures and transition to a single-mode regime is possible by limiting the number of stripes involved in optical feedback. Dependences of optical mode reconfiguration were studied by introducing limiting slits into the external cavity. It was shown that limiting the emitting aperture involved in the feedback to 65 μm allows us to demonstrate high-order single-mode operation with far-field divergence for the central lobe of 1°. [ABSTRACT FROM AUTHOR]

High-quality silicon-based lasers are necessary to achieve full integration of photonic and electronic circuits. Monolithic integration of III–Vmicron lasers on silicon by means of the aspect ratio trapping (ART) method is a promising solution. To obtain sufficient optical feedback to excite the laser without introducing complex fabricating processes, we have designed a high-order surface grating on micron lasers which was epitaxially grown by the ART method and can be fabricated by common UV lithography. The performance of the grating was analyzed by the finite-difference time-domain (FDTD) method and eigenmode expansion (EME) solver. After simulation optimization, the etching depth was set to 0.6 μm to obtain proper reflection. The width of the slots and the slot spacing were selected to be 1.12 μm and 5.59 μm, respectively. Finally, we obtained results of 4% reflectance and 82% transmittance at a 1.55 μm wavelength at 24 periods. [ABSTRACT FROM AUTHOR]

This document is a summary of a special topical collection in the journal APL Photonics on the advancements and applications of optical frequency combs. Optical frequency combs are a technology that combines ultrafast laser technology and single frequency laser stabilization techniques to generate stable optical and microwave signals for high-resolution time and frequency applications. The collection includes articles on various topics such as laser ranging, terahertz photonics, molecular referencing, optical communications, and space-based time/frequency dissemination. The articles highlight the impact and innovation that frequency comb technology brings to each field. The collection demonstrates the transformative potential of optical frequency combs and the ongoing research to further expand their applications. [Extracted from the article]

We experimentally investigate the dynamics characteristics of a two-state quantum dot laser (TSQDL) subject to optical feedback. Firstly, we inspect the impact of the temperature on the power-current characteristics of the ground state (GS) lasing and the excited state (ES) lasing in the TSQDL operating at free-running. The results demonstrate that with the decrease in the temperature, the threshold current for GS lasing (IthGS) and the threshold current for ES lasing (IthES) decrease very slowly. There exists a current for GS quenching (IQGS), which is gradually increased with the decrease in the temperature. After introducing optical feedback, the overall trend of change is similar to those obtained under free-running. Next, through inspecting the time series and power spectrum of the output from the TSQDL under optical feedback, the dynamical characteristics of the TSQDL are investigated under different feedback ratios, and diverse dynamical states including quasi-chaos pulse package, chaos state, regular pulse package, quasi-period two, quasi-regular pulsing, and chaos regular pulse package have been observed. Finally, for the TSQDL biased at three different cases: lower than IthES, slightly higher than IthES, and higher than IthES, nonlinear dynamic state evolutions with the increase in feedback ratio are inspected, respectively. The results show that, for the TSQDL biased at lower than IthES, it presents an evolution route of stable state—quasi-chaos pulse package—chaos state—regular pulse package. For the TSQDL biased at slightly larger than IthES, it presents an evolution route of stable state—quasi-regular pulsing—quasi-period two—chaos regular pulse package. For the TSQDL biased at higher than IthES, the TSQDL always behaves stable state within the range of feedback ratio that the experiment can achieve. However, with the increase in optical feedback ratio, the number of longitudinal modes for GS lasing and ES lasing are changed.

Due to its benefits in high stability and ability to speed up information processing in machine learning applications, quantum dot light-emitting diode (QDLED) has received a lot of attention for the next generation of local communication applications. Some optical applications for reinforcement learning in computers are disgusted by the mode-competition dynamics of multimode semiconductor QDLEDs. The chaotic modes-competition dynamics in a multimode semiconductor QDLED with optical feedback (OFB) are quantitatively assessed in this study.We can manage the chaotic mode-competition dynamics among the longitudinal modes by inserting an external optical signal into some of them. The dominant-mode ratio for the injected mode increases along with the increase in feedback strength. We discover that, as a result of the various OFB phases, the characteristics of the dominant mode ratio in terms of OFB intensity vary among the modes. We also assess the connection between the photon dynamic range and the region for side mode suppression ratio. The OFB range does not correlate to the region for the big dominant mode ratio. The intricate dynamics of mode competition in multimode semiconductor QDLEDs with OFB are the cause of this disparity. When the modulation frequency corresponded with a rational number of the optical feedback's round trip duration, the chaotic mode-competition dynamics remained high.

The results of the experimental study of the effect of optical feedback (OF) on intensity fluctuations at a selected polarization in a single-mode vertical-cavity surface-emitting laser (VCSEL) are presented. The aim of this study is to look for conditions of enhancement of the level of polarization noise for possible use as a source of randomness for fast random bit generation. It is shown for several VCSEL examples that strong optical feedback can lead to an increase polarization noise, the amplitude and frequency band of which is sufficient for the efficient generation of random bits at a speed of several Gbit/s. This approach makes it possible to implement a fairly compact and structurally simple high-speed random bit generator. [ABSTRACT FROM AUTHOR]

In this article, the dependence of the operation states, dynamics, and noise of laser diodes (LD) with external optical feedback (OFB) on the linewidth enhancement factor (LEF) and spontaneous emission factor (SEF) have been investigated. We systematically studied the classification of the laser dynamics based on the bifurcation diagrams (BDs) of the photon numbers and the relative intensity noise (RIN) spectra at different levels of OFB, LEF, and SEF. The simulation results show that variations in the LEF and SEF lead to significant changes in the laser operation states and dynamics, which vary from continuous wave (CW), pulsation, and chaos states. The Hopf bifurcation (HB) point moves toward increasing/decreasing OFB intensity by increasing/decreasing the SEF/LEF. The laser state becomes more stable through a wide range of OFB by increasing/decreasing the SEF/LEF. The RIN reduces the solitary laser noise level at higher/lower values of SEF/LEF when the laser is operated under OFB. The relaxation frequency of the laser shifts toward higher values by increasing/decreasing the SEF/LEF through most laser states, and the RIN peak is higher than solitary laser noise by four orders of magnitude, especially in the pulsation regions. In the low-frequency region, the RIN is enhanced from one to two orders by reducing the LEF and SEF through laser states. [ABSTRACT FROM AUTHOR]

Semiconductor lasers with optical feedback are stochastic nonlinear systems that can display complex dynamics and abrupt changes when their operation conditions change. Even very small changes can lead to large variations in the spatial and spectral properties of the laser emission. This makes a semiconductor laser with feedback an ideal system for conducting controlled experiments to test data analysis tools to detect and characterize transitions. Here, we identify feedback-induced transitions by analyzing speckle patterns that are generated after the laser light propagates in an optical fiber. Speckle patterns result from the interference of multiple modes, and their statistical properties are understood, but a direct mathematical model does not exist. Here we show the versatility of the correlation length and the permutation entropy as measures for characterizing speckle patterns. Combining entropy and correlation analysis with speckle contrast analysis, we uncover changes that occur when the laser current increases from below to well above the threshold, which unveils the effects of optical feedback on the coherence of the laser emission. [ABSTRACT FROM AUTHOR]

An investigation into the dynamic states and relative intensity noise of laser diodes subjected to double optical feedback has been conducted. We employed modified and improved time-delay rate equations to account for double external optical feedback. The dynamic states and noise of lasers will be investigated using bifurcation diagrams of the output photon number, its temporal variations, and the intensity noise of the laser. This analysis considers feedback strengths due to the double external cavity and their spacing from the front facet of the laser with and without phase due to feedback. The results reveal that considering phase causes significant variations in laser intensity and a phase shift in the temporal variations of the laser output. This results in relative intensity noise suppression and a frequency shift in the intensity noise spectrum. These findings represent new contributions to our understanding of the reliance of lasing frequency shift on the phase due to feedback, regardless of whether feedback originates from a single or double external cavity. We investigated the optimal conditions corresponding to stable dynamic states of the laser with the lowest noise level. Additionally, we identified conditions that result in chaotic dynamics, where the spectrum does not convey information about the laser system. These insights have potential applications in chaotic and secure optical data encryption. [ABSTRACT FROM AUTHOR]

Chaos synchronization of two quantum dot light-emitting diodes (QDLEDs) theoretically is studied, which are via a closed or open-loop and mutual coupling system. Whereas the synchronized-chaotic systems, the dynamics of there are identical to uncoupled dot light-emitting diodes (DLEDs) under optical feedback effect. Complete synchronization was obtained under certain conditions for the coupling parameters. We evaluated the range of the QDLED's chaos with extrinsic optical feedback in methods of the chaos synchronization residue diagram and discussion as well as the coherence for the optimal coupling strength range. With proper conditions of the coupling parameters and the evaluation methods, the synchronization was satisfactorily obtained between the transmitter and receiver. [ABSTRACT FROM AUTHOR]

The interaction of light with biological tissues is an intriguing area of research that has led to the development of numerous techniques and technologies. The randomness inherent in biological tissues can trap light through multiple scattering events and provide optical feedback to generate random lasing emission. The emerging random lasing signals carry sensitive information about the scattering dynamics of the medium, which can help in identifying abnormalities in tissues, while simultaneously functioning as an illumination source for imaging. The early detection and imaging of tumor regions are crucial for the successful treatment of cancer, which is one of the major causes of mortality worldwide. In this paper, a bimodal spectroscopic and imaging system, capable of identifying and imaging tumor polyps as small as 1 mm2, is proposed and illustrated using a phantom sample for the early diagnosis of tumor growth. The far-field imaging capabilities of the developed system can enable non-contact in vivo inspections. The integration of random lasing principles with sensing and imaging modalities has the potential to provide an efficient, minimally invasive, and cost-effective means of early detection and treatment of various diseases, including cancer. [ABSTRACT FROM AUTHOR]

In situ measurements of water vapour isotopic composition in Polar Regions has provided needed constrains of post-deposition processes involved in the archiving of the climatic signal in ice core records. During polar winter, the temperatures are so low that current commercial techniques are not able to measure the vapour isotopic composition with enough precision. Here, we make use of new developments in infrared spectroscopy and combine an optical feedback frequency stabilised laser source (OFFS technique) using a V-shaped optical cavity (VCOF) and a high-finesse cavity ring down cavity (CRDS) which yield sufficient precision to measure isotopic composition at water mixing ratios down to 1 ppmv. Indeed, thanks to the stabilisation of the laser by the VCOF, the instrument suffers extremely low drift and very high signal to noise ratio. Using new constrains on the fitting technique, the instrument is additionally not hindered by a large isotope-humidity response which low humidity can create extensive biases on commercial instruments. [ABSTRACT FROM AUTHOR]

A novel method for the generation of an optical frequency comb (OFC) is presented. The proposed approach uses a laser diode with optical feedback and operating at a specific nonlinear dynamic state named periodic window. In this case, the laser spectrum exhibits a feature with a series of discrete, equally spaced frequency components, and the repetition rate can be flexibly adjusted by varying the system parameters (e.g., external cavity length), which can provide many potential applications. As an application example, a dual-OFC system for distance measurement is presented. The results demonstrate the system's ability to achieve target distance detection, underscoring its potential for real-world applications in this field. [ABSTRACT FROM AUTHOR]

We study theoretically the mechanisms of square wave formation of a vertically emitting micro-cavity operated in the Gires–Tournois regime that contains a Kerr medium and that is subjected to strong time-delayed optical feedback and detuned optical injection. We show that in the limit of large delay, square wave solutions of the time-delayed system can be treated as relative homoclinic solutions of an equation with an advanced argument. Based on this, we use concepts of classical homoclinic bifurcation theory to study different types of square wave solutions. In particular, we unveil the mechanisms behind the collapsed snaking scenario of square waves and explain the formation of complex-shaped multistable square wave solutions through a Bykov T-point. Finally, we relate the position of the T-point to the position of the Maxwell point in the original time-delayed system. [ABSTRACT FROM AUTHOR]

This study investigates the interaction between frequency combs and optical feedback effects in Quantum Cascade Lasers (QCLs). The theoretical analysis reveals new phenomena arising from the interplay between comb generation and feedback. By considering the bias current corresponding to free-running single mode emission, the introduction of optical feedback can trigger the generation of frequency combs, including both fundamental and harmonic combs. This presents opportunities to extend the comb region and generate harmonic frequency combs with different orders through optimization of external cavity parameters, such as losses and length. Furthermore, this study demonstrates that optical feedback can selectively tune the harmonic order of a pre-existing free-running comb by adjusting the external cavity length, particularly for feedback ratios around 1%, which are readily achievable in experimental setups. Under strong feedback conditions (Acket parameter C > 4.6), mixed states emerge, displaying the features of both laser and external cavity dynamics. While this study is predominantly centered on terahertz QCLs, we have also confirmed that the described phenomena occur when utilizing mid-infrared QCL parameters. This work establishes a connection between comb technology and the utilization of optical feedback, providing new avenues for exploration and advancement in the field. In fact, the novel reported phenomena open a pathway toward new methodologies across various domains, such as the design of tunable comb sources, hyperspectral imaging, multi-mode coherent sensing, and multi-channel communication. [ABSTRACT FROM AUTHOR]

In contribution 2, a precision 3D measurement instrument integrating multiple laser range sensors was designed, which fuses the information of multiple redundant laser range sensors to obtain the coordinates of a 3D position. Laser-based measurement and sensing technology has been paid more and more attention by academia and industry because of its incomparable advantages, such as high sensitivity, fast response, and no contact. In contribution 8, an evaluation method based on fitting planes was proposed to evaluate laser plane attitude and determine the degree of laser coplanarity effectively. Using the method in this study, the laser plane attitude can be quantified and accurately assessed on both sides, effectively resolving the problem with traditional methods that can only assess the laser plane attitude qualitatively and roughly, thereby providing a solid foundation for calibration and error correction of the measurement system. [Extracted from the article]

We model dynamics of a quantum dot based micropillar laser array subject to the time-delayed optical feedback. The global coupling provided by the feedback generates a rich set of various instabilities including chaotic regimes with strong time-delay signature in the autocorrelation function. We demonstrate that the dispersion of the array coupling phases leads to effective suppression of the time-delay signature due to the dispersion of the system's internal timescales. We find that the transition to the complete suppression of the time-delay signature appears via a chimera state where highly correlated and non-correlated laser outputs coexist. The degree of correlation in the chimera state depends on the coupling phase dispersion. [ABSTRACT FROM AUTHOR]

Multiwavelength plasmonic nanolasers are central to the quest for ultradense and versatile photonic integrations. However, the insufficient optical feedback and mode selection strategies result in a high lasing threshold and multi‐mode operation with limited spectral purity and stability. Here, a multiwavelength near‐infrared plasmonic nanolaser with high mode purity by leveraging a nontrivial "H"‐shape plasmonic cavity is demonstrated. The nanolaser is constructed by inserting InGaAs/GaAs multi‐quantum‐disk (MQD) nanowires into a metallic Fabry‐Perot cavity consisting of a pair of Ag nanowires positioned on an ultrasmooth Au film. The design is endowed with strong optical feedback to enhance the energy transfer between the exciton and plasmon, rendering a significant reduction of the lasing threshold. More importantly, the "H"‐shape cavity is featured with a large flexibility in tuning the resonance wavelength, where multiwavelength lasing with the virtues of single‐mode is realized at room temperature. The results make a crucial step toward near‐infrared multiwavelength plasmonic nanolasers and open up exciting opportunities for applications such as ultracompact photonic integrated circuits and high‐throughput biochemical sensing. [ABSTRACT FROM AUTHOR]

Organic semiconductor lasers have shown great application potential in various fields, such as low-cost sensing, high-performance lighting and display, and lab-on-a-chip devices. Since the introduction of organic lasers in the 1960s, research on semiconductor laser devices has expanded to include various materials and structures. The organic laser has attracted much attention due to its wide range of emission spectrum and simple synthesis and processing. Researchers constantly pursue the goal of using organic semiconductors to fabricate low-threshold thin-film organic laser devices while retaining the characteristics of a wide luminescence spectrum of organic materials, simple and portable structure, and low cost. However, organic semiconductor lasers face challenges due to material stability under optical pumping and large optical losses under electrical pumping, making commercialization difficult. Many scholars have put great efforts into enhancing the performance of materials and optimizing the structure to minimize the threshold of organic semiconductor lasers. Herein, based on the basic principles of organic lasers, the main factors affecting the excitation threshold are summarized. A comprehensive analysis of the relevant factors and threshold conditions is performed, considering both positive and negative aspects of modal gain and modal loss that need to be addressed. We expect to provide a wide range of ideas for reducing the threshold of organic lasers and offer theoretical guidance for the practical industrial production of organic lasers. [ABSTRACT FROM AUTHOR]

The study of extreme events (EEs) in photonics has expanded significantly due to straightforward implementation conditions. EEs have not been discussed systematically, to the best of our knowledge, in the chaotic dynamics of a Fabry–Perot laser with optical feedback, so we address this in the current contribution. Herein, we not only find EEs in all modes but also divide the EEs in total output into two categories for further discussion. The two types of EEs have similar statistical features to conventional rogue waves. The occurrence probability of EEs undergoes a saturation effect as the feedback strength increases. Additionally, we analyze the influence of feedback strength, feedback delay, and pump current on the probability of EEs defined by two criteria of EEs and find similar trends. We hope that this work contributes to a deep understanding and serves as inspiration for further research into various multimode semiconductor laser systems.

In this work, we propose to employ strong optical feedback to narrow the spectral linewidth of quantum cascade lasers without using any phase control. Rate equation analysis demonstrates that optical feedback beyond a certain level always reduces the laser linewidth for any feedback phase. It is also found that the linewidth becomes less sensitive to the feedback phase for higher feedback strength. Simulations show that optical feedback with a feedback ratio of −10 dB can suppress the laser linewidth by about two orders of magnitude. This is in contrast to near-infrared laser diodes, which can be easily destabilized by strong feedback. [ABSTRACT FROM AUTHOR]

This article discusses the phenomenon of different colors displayed by a circular polarizing filter (CPL) when placed on different surfaces. The CPL is composed of a linear polarizer (LP) and a quarter-wave plate (QWP), and the principles of polarization can be used to explain the phenomenon. The article explains the process of light propagation and polarization states when the CPL is placed on a steel breadboard and a piece of paper. The article also discusses the wavelength dependence of the QWP and how it affects the colors observed. The phenomenon is further explored by analyzing the CPL on different reflective surfaces such as aluminum mirror and copper. The article concludes by highlighting the educational value of this phenomenon for teaching about polarization. [Extracted from the article]

Single‐frequency semiconductor lasers represent a critical role in optical communications, light detection and ranging systems, photonics integrated circuits, etc. Here, combining atom‐like quantum dot (QD) materials and advanced lateral gratings, a high‐power, ultra‐low‐noise 1.3 µm InAs/GaAs QD distributed feedback laser is demonstrated. Stable single‐longitudinal‐mode output power of 100 mW from 25 ℃ to 85 ℃ is achieved with a maximum side mode suppression ratio of 62.6 dB, and the variations of threshold current and slope efficiency over the temperature range are slight, indicting a high temperature stability. A record‐narrow intrinsic linewidth of 1.62 kHz is achieved at 55 ℃ with a white noise level of merely 515 Hz2 Hz–1, and a minimum averaged relative intensity noise of only –166 dB/Hz between 0.1 GHz and 20 GHz is obtained at 25 ℃. Furthermore, a strong tolerance to external optical feedback (〉 –14 dB) is demonstrated in the range from 25 ℃ to 85 ℃, with a maximum value of –8 dB at 85 ℃. This high‐quality single‐frequency laser fabricated with simplified processes and compact size paves the way for its future large‐scale applications such as high‐capacity optical communication, high‐precision optical detection, high‐speed optical interconnections, etc. [ABSTRACT FROM AUTHOR]

This paper presents a self-coherent optical feedback system that achieves resonance between the external cavity mode and the relaxation oscillation frequency of distributed feedback (DFB) laser and the repetition frequency of Fabry–Perot (FP) laser, all in an effort to achieve optical feedback self-locking. Results using this system show that for FP lasers, feedback is able to reduce the 3 dB radio frequency linewidth from 2 MHz to 600 kHz. In the case of DFB lasers, an electric frequency comb with an external cavity repetition rate of 17.25 MHz is achieved. This research presents a significant step toward the improvement of the performance of quantum dot lasers in optical communication systems. [ABSTRACT FROM AUTHOR]

In various optical feedback cavity-enhanced spectroscopies (OF-CESs) based on absorption or scattering, conventional phase-locking methods are constrained by their ability to handle only minor phase deviations. This limitation is due to the source of an error signal for phase adjustment. This paper introduces a robust approach for phase-locking, which combines the shape and intensity of cavity transmission profiles to identify phase deviations. The advantage of this combination is that it can always generate a suitable error signal, irrespective of the phase's position in the entire 2π period. The outstanding performance of the corresponding servo loop under severe airflow shocks demonstrates that our approach significantly increases the feasibility of applying various OF-CES setups for real-time, in situ gas detection in harsh environments. [ABSTRACT FROM AUTHOR]

When theoretically investigating the nonlinear dynamics of quantum dot lasers (QDLs), the parameter value of the electron escape rate (Ce) is sometimes approximated to zero to simplify the calculation. However, the value of Ce is dependent on the energy interval between the ground state (GS) and the excited state (ES) in the conduction band and is affected by the operation temperature. As a result, such simplified approximation treatments may lead to inaccurate results. In this study, after considering the effect of Ce, we investigate the nonlinear dynamics of QDLs with and without optical feedback based on the asymmetric electron-hole carrier rate equation model. The simulation results show that without optical feedback, the lasing conditions for ES and GS in free-running QDLs are dependent on the value of Ce. A larger Ce is more helpful for the ES emission, and the GS emission will stop lasing if Ce is large enough. Through analyzing the dynamical characteristics of GS and ES in QDLs with optical feedback under different Ce values, it can be found that the dynamical characteristics are strongly correlative with Ce. [ABSTRACT FROM AUTHOR]

Colloidal quantum wells (CQWs) have emerged as a promising class of gain material in various optical feedback configurations. This is due to their unique excitonic features arising from their 1D quantum confinement. However, existing methods for integrating CQW onto microresonators will cause low laser quality due to uneven CQW coating. To overcome this, the use of liquid‐interface kinetically driven self‐assembly is proposed to coat ultrathin, close‐packed layers of colloidal CdSe/Cd1−xZnxS core/shell CQWs between 7 and 14 nm onto the surface of silica microsphere cavities. The fabricated CQW‐whispering‐gallery‐mode microlasers possess a commendable high quality (Q) factor of 13 000 at room temperature. Stable single‐mode lasing output is demonstrated through evanescent field coupling between a CQW‐coated microsphere and a thin uncoated microfiber in a 2D‐3D microcavity configuration. These promising results highlight the suitability of the liquid‐interface kinetically driven self‐assembly method for realizing ultrathin CQW‐coated microlasers and its high compatibility for integrating colloidal nanocrystals onto complex 3D microstructures for future miniaturized colloidal optoelectronic and photonic applications. [ABSTRACT FROM AUTHOR]

Optical coherence tomography (OCT) imaging has become a useful tool in medical diagnosis over the past 25 years, because of its ability to visualize intracellular structures at high resolution. The main objective of this work is to add electronic feedback to the optical coherence tomography setup to increase its sensitivity. Noise added to the measured interferogram obscures some details of examined tissue layered structure. Adjusting signal power level in such a way to improve signal-to -noise ratio can help to enhance image quality. Electronic feedback is added to enhance system sensitivity. A logarithmic amplifier is included in the OCT setup to automatically adapt signal level. Moreover, the resolution of the optical spectrum analyzer is controlled according to the farthest layer detected in the A-scan. These techniques are tested showing an improvement in obtained image of a human nail. [ABSTRACT FROM AUTHOR]

Fiber fault detection is crucial for maintaining the quality of optical communication, especially in well-established optical access networks with extended distances and a growing number of subscribers. However, the increasing insertion loss in fiber links presents challenges for traditional fault-detection methods in capturing fault echoes. To overcome these limitations, we propose a modulation-enhanced external-cavity-resonant-frequency method that utilizes a laser for fault echo reception, providing improved sensitivity compared to traditional photodetector-based methods. Our previous work focused on analyzing key parameters, such as sensitivity and spatial resolution, but did not consider practical aspects of selecting optimal modulation parameters. In this study, we develop a model based on Lang–Kobayashi rate equations for current-modulated optical feedback lasers and validate it through experimental investigations. Our findings reveal that optimal detection performance is achieved with a modulation depth of 0.048, a frequency sweeping range of 0.6 times the laser relaxation oscillation frequency, and a frequency sweeping step of 0.1 times the external cavity resonant frequency. [ABSTRACT FROM AUTHOR]

Recent research results show that great progress has been made in improving the generation rate of physical random numbers. However, there are still some defects such as complex generation process and large volume. To solve these problems, this paper theoretically proposes a scheme for generating random numbers using mode hopping of Vertical Cavity Surface Emitting Laser (VCSEL) with optical feedback. The VCSEL is perturbed by the optical feedback to generate the mode hopping. The square wave signal then modulates the injection current, periodically restarts the VCSEL to switch from the non-emitting state to the mode hopping state, and finally outputs a random pulse sequence to realize the random number generation. The results of this paper show that unbiased random numbers with verified randomness can be generated continuously at a rate of the order of Gbit/s. Our scheme eliminates the need for optoelectronic conversion and post-processing operations, simplifing the current random number generation structure, which has the potential to realize a low-cost photonic integrated random number generator.

In this article, we propose and numerically demonstrate a novel dual-phase modulated time-delay signature (TDS) concealed communication system. The proposed system addresses the fatal disadvantage of conventional electro-optic chaotic systems in that TDS is vulnerable to detection. Combined with the dispersive devices ahead, which significantly expands the complexity of the signal, the scheme perturbs the transmitted data with the mutual injection of the feedforward and feedback branches in the phase, strengthening the nonlinear effect of the system considerably. In addition, the key space of the system against illegal users’ exhaustive attack within reasonable parameters reaches 1019, which is 14 orders of magnitude larger than that of the classical electro-optic system. A confidential 32 G/s OOK signal was transmitted over 100 km and successfully recovered. Numerical results demonstrate that the chaotic system can tolerate minor parameter mismatches which are controllable in practice, proving that the system can be used for secure communication.

Palpation is a simple but effective method to distinguish tumors from healthy tissues. The development of miniaturized tactile sensors embedded on endoscopic or robotic devices is key to achieving precise palpation diagnosis and subsequent timely treatment. This paper reports on the fabrication and characterization of a novel tactile sensor with mechanical flexibility and optical transparency that can be easily mounted on soft surgical endoscopes and robotics. By utilizing the pneumatic sensing mechanism, the sensor offers a high sensitivity of 1.25 mbar and negligible hysteresis, enabling the detection of phantom tissues with different stiffnesses ranging from 0 to 2.5 MPa. Our configuration, combining pneumatic sensing and hydraulic actuating, also eliminates electrical wiring from the functional elements located at the robot end-effector, thereby enhancing the system safety. The optical transparency path in the sensors together with its mechanical sensing capability open interesting possibilities in the early detection of solid tumor as well as in the development of all-in-one soft surgical robots that can perform visual/mechanical feedback and optical therapy. [ABSTRACT FROM AUTHOR]

Random fiber laser (RFL) utilizing stimulated Raman scattering and distributed random Rayleigh backscattering in fiber as optical gain and feedback mechanisms, respectively, has been developed rapidly in the past decade. Here, it is found, for the first time, that the combination of a high‐order pump and new transmission fiberthat has lower transmission loss, Rayleigh backscattering coefficient, and Raman gain coefficient than standard single‐mode fibercan break the length limits of both random fiber lasing and sensing. With a higher‐order pump and ultralow loss fiber (ULLF), the position of maximum intracavity lasing power can be shifted toward the far end of the fiber span resulting in more remote pump power for the erbium‐doped fiber (EDF) inserted in the fiber span, which plays a dominant role of cavity length extension in RFL. A 200 km ultralong single‐ended RFL and sensor are experimentally demonstrated, which are the longest single‐ended RFL and sensor with functional remote reflectors reported to date. It is believed that the proposed combination of high‐order random lasing and ULLF with EDF can become a general method to extend the working distance of various fiber sensing systems for safety monitoring of ultralong infrastructures, such as power transmission lines and oil/gas pipelines. [ABSTRACT FROM AUTHOR]

A two-dimensional (2D) solid-state random laser emitting in the visible is demonstrated, in which optical feedback is provided by a controlled disordered arrangement of air-holes in a dye-doped polymer film. We find an optimal scatterer density for which threshold is minimum and scattering is the strongest. We show that the laser emission can be red-shifted by either decreasing scatterer density or increasing pump area. We show that spatial coherence is easily controlled by varying pump area. Such a 2D random laser provides with a compact on-chip tunable laser source and a unique platform to explore non-Hermitian photonics in the visible. [ABSTRACT FROM AUTHOR]

Phototherapy, including photodynamic therapy (PDT) and photothermal therapy (PTT), is a non-invasive and effective approach used for cancer treatment, in which phototherapeutic agents are irradiated with an appropriate light source to produce cytotoxic reactive oxygen species (ROS) or heat to ablate cancer cells. Unfortunately, traditional phototherapy lacks a facile imaging method to monitor the therapeutic process and efficiency in real time, usually leading to severe side effects due to high levels of ROS and hyperthermia. To realize precise cancer treatment methods, it is highly desired to develop phototherapeutic agents possessing an imaging ability to evaluate the therapeutic process and efficacy in real time during cancer phototherapy. Recently, a series of self-reporting phototherapeutic agents were reported to monitor PDT and PTT processes by combining optical imaging technologies with phototherapy. Due to the real-time feedback provided by optical imaging technology, therapeutic responses or dynamic changes in the tumor microenvironment could be evaluated in a timely manner, thereby achieving personalized precision treatment and minimizing toxic side effects. In this review, we focus on the advances in the development of self-reporting phototherapeutic agents for a cancer phototherapy evaluation based on optical imaging technology to realize precision cancer treatments. Additionally, we propose the current challenges and future directions of self-reporting agents for precision medicine. [ABSTRACT FROM AUTHOR]

Diagnosing the root causes of cloud feedback in climate models and reasons for inter-model disagreement is a necessary first step in understanding their wide variation in climate sensitivities. Here we bring together two analysis techniques that illuminate complementary aspects of cloud feedback. The first quantifies feedbacks from changes in cloud amount, altitude, and optical depth, while the second separates feedbacks due to cloud property changes within specific cloud regimes from those due to regime occurrence frequency changes. We find that in the global mean, shortwave cloud feedback averaged across ten models comes solely from a positive within-regime cloud amount feedback countered slightly by a negative within-regime optical depth feedback. These within-regime feedbacks are highly uniform: In nearly all regimes, locations, and models, cloud amount decreases and cloud albedo increases with warming. In contrast, global-mean across-regime components vary widely across models but are very small on average. This component, however, is dominant in setting the geographic structure of the shortwave cloud feedback: Thicker, more extensive cloud types increase at the expense of thinner, less extensive cloud types in the extratropics, and vice versa at low latitudes. The prominent negative extratropical optical depth feedback has contributions from both within- and across-regime components, suggesting that thermodynamic processes affecting cloud properties as well as dynamical processes that favor thicker cloud regimes are important. The feedback breakdown presented herein may provide additional targets for observational constraints by isolating cloud property feedbacks within specific regimes without the obfuscating effects of changing dynamics that may differ across timescales. [ABSTRACT FROM AUTHOR]

We developed energy-efficient membrane III-V distributed-reflector lasers on silicon-based substrates for ultrafast short-reach communication links and neuromorphic computing applications. By leveraging high-speed photon-photon interactions enabled by integrated optical feedback and a high-opticalconfinement membrane structure, we demonstrated record-fast directly modulated laser bandwidths and spike-processing rates with ultralow operating energies. This is a step towards our goal of reducing the carbon footprint of information and communication technology and artificial intelligence hardware, while keeping pace with the increasing demand of processing speeds. [ABSTRACT FROM AUTHOR]

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Excitable cells can be stimulated or inhibited by optogenetics. Since optogenetic actuation regimes are often static, neurons and circuits can quickly adapt, allowing perturbation, but not true control. Hence, we established an optogenetic voltage-clamp (OVC). The voltage-indicator QuasAr2 provides information for fast, closed-loop optical feedback to the bidirectional optogenetic actuator BiPOLES. Voltage-dependent fluorescence is held within tight margins, thus clamping the cell to distinct potentials. We established the OVC in muscles and neurons of Caenorhabditis elegans, and transferred it to rat hippocampal neurons in slice culture. Fluorescence signals were calibrated to electrically measured potentials, and wavelengths to currents, enabling to determine optical I/V-relationships. The OVC reports on homeostatically altered cellular physiology in mutants and on Ca2+-channel properties, and can dynamically clamp spiking in C. elegans. Combining non-invasive imaging with control capabilities of electrophysiology, the OVC facilitates high-throughput, contact-less electrophysiology in individual cells and paves the way for true optogenetic control in behaving animals. Optogenetic actuation regimes are often static, which allows perturbation, but not true control of neuronal activity. Here, the authors describe an all-optical method for bidirectional steering of membrane potential, in closed loop, in C. elegans muscles and neurons, and rat hippocampal slice culture. The 'optogenetic voltage clamp' uses two microbial rhodopsin actuators and the rhodopsin voltage indicator QuasAr. [ABSTRACT FROM AUTHOR]

In conclusion, the papers collected in this Special Issue summarize significant advances in the field of optical sensors, with specific contributions in sensor technology and applications of fiber Bragg gratings, resonators and spectroscopy. The performance of this integrated sensor is demonstrated using glucose solutions (0.05-0.3 g/mL) and the sensor response is shown to be dynamic and reversible. The refractive index sensor exhibits a linear response in the refractive index range of 1.333-1.358 RIU (refractive index unit), with a sensitivity of 32.4 RIU SP -1 sp and a resolution of 2 × 10 SP -3 sp RIU at the refractive index of water. Today, optical sensors are the subject of a very significant number of studies and applications. [Extracted from the article]

In this paper, we study the dynamics of a vertically emitting micro-cavity operated in the Gires–Tournois regime that contains a semiconductor quantum-well and that is subjected to strong time-delayed optical feedback and detuned optical injection. Using a first principle time-delay model for the optical response, we disclose sets of multistable dark and bright temporal localized states coexisting on their respective bistable homogeneous backgrounds. In the case of anti-resonant optical feedback, we identify square-waves with a period of twice the round-trip in the external cavity. Finally, we perform a multiple time scale analysis in the good cavity limit. The resulting normal form is in good agreement with the original time-delayed model. [ABSTRACT FROM AUTHOR]

Quasi-2D lead halide perovskites have garnered increasing interest as lasing gain media. Relatively simple fabrication, high refractive index, and unique quantum well structure encourage their use in traditional cavity lasers and cavity-free systems called random lasers (RLs). Despite tremendous advances reported thus far, coherent random lasing in quasi-2D perovskite subwavelength films has not been reported. Consequently, coherent optical feedback mechanisms in quasi-2D perovskite systems are still unexplored. Here, this work reports the observation of coherent random lasing in subwavelength quasi-2D perovskite films. Statistical analysis of spectral measurements reveals Lévy-like intensity fluctuations, replica symmetry breaking confirms random lasing, and the coherent modes are studied with spectral and spatial correlation techniques. The observed coherent lasing modes are found to be extended states that arise from the random crystal grain structure during fabrication and span the entire pump volume. These modes out-compete diffusive lasing due to their coherence. [ABSTRACT FROM AUTHOR]

The experimental results were compared with the different perovskite quantum dot–doped materials, which explicitly include losses, gain, lasing, and threshold. The random lasers emerge multiple emission peaks at different central wavelengths, due to the recurrent optical gain feedback and the localized field resonance effect in vicinity of particles. By the operation of the applied different devices, the perovskite quantum dots can be better materials for generating photons and can create stronger light scattering for transition from incoherent lasing to coherent lasing. Contrary to conventional random lasers, we found that the nature of the random lasing is dictated by the perovskite quantum dots, and the quantum dots with a high refractive index can be advantageous for random lasing in mediums. The experiments also reveal a regime including the fundamental aspects of surface plasmon resonance light scattering for facile active lasing mode switching. These results will guide future design of different quantum dot random lasers with controlled lasing wavelength output. [ABSTRACT FROM AUTHOR]