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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]

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]

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]

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]

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]

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]

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]

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]

An improved chaotic laser system, which has a slave semiconductor laser (SL) injected by a master SL with double optical feedback (DOF), is proposed, so that the time delay (TD) signature can be successfully concealed from both intensity and phase chaos via choosing appropriate parameters. The TD signature is investigated by employing autocorrelation function (ACF) and mutual information (MI) function. Through analyzing the influence on TD signature in the region of injection current and injection strength for the slave SL, we find that, for both intensity chaos and phase chaos, the TD signature can be easily concealed under weak injection strength. When the injection strength is strong, we can not only successfully conceal TD signature, but also improve the bandwidth of chaotic laser output by choosing the optimal detuning frequency. [ABSTRACT FROM AUTHOR]

Time delay (TD) signatures are studied experimentally in orthogonal polarizations and in individual transverse modes respectively in a VCSEL operating with three transverse modes. Different types of concealment of the TD signatures are observed when the polarization of feedback is rotated through large angles. Effects of feedback strength and external cavity length on the TD signatures are investigated. Weak feedback leads to better concealment of the TD signatures in the dominant polarization. When the round-trip time difference between the two external cavities is close to a half of the relaxation oscillation period, the TD signatures are minimized. [ABSTRACT FROM AUTHOR]

A novel long-distance multi-channel bidirectional chaos communication system over multiple paths based on two synchronized 1550 nm vertical-cavity surface-emitting lasers (VCSELs) is proposed and studied theoretically. These two responding VCSELs (R-VCSELs) can output similar chaotic signals served as chaotic carrier in two linear polarization (LP) modes with identical signal injection from a driving VCSEL (D-VCSEL), which is subject to optical feedback and optical injection, simultaneously. Through the numerical simulations, high quality chaos synchronization between the two R-VCSELs can be obtained. Besides, the effects of varied qualities of chaos synchronization on communication performances in 20 km single mode fiber (SMF) channels are investigated by regulating different internal parameters mismatch after adopting chaos masking (CMS) technique. With the decrease of the maximum cross correlation coefficient (Max-C) between the two R-VCSELs, the bit error rate (BER) of decoded message increase. Meanwhile, the BER can still be less than 10 −9 when the Max-C degrades to 0.982. Based on high quality synchronization, when the dispersion compensating fiber (DCF) links are introduced, 4 n messages of 10 Gbit/s can transmit in 180 km SMF channels over n coupling paths, bidirectionally and simultaneously. Thorough tests are carried out with detailed analysis, demonstrating long-distance, multi-channel, bidirectional chaos communication based on VCSELs with chaotic signal injection. [ABSTRACT FROM AUTHOR]

We propose a novel algorithm for Multiple Self-Mixing Interferometry (MSMI). The algorithm is able to measure nanometer scale vibration by the power spectrum analysis. In the paper, the principles of the method are introduced in detail. The experimental setup has been built. The validity of the proposed algorithm was confirmed by conducting a series of experimental measurements at different reflection times, feedback factors, and vibrational frequencies using PZT as a reference. Experimental results showed that the method can quickly demodulate parameters of vibration and good correspondence between theory and experiment. The proposed algorithm, thus, furnishes nanometer measurements with a very high resolution using a self-aligned, cost effective and compact experimental setup. [ABSTRACT FROM AUTHOR]

We study self-injection locking of DFB laser obtained with a feedback loop that comprises a ring optical fiber cavity. The cavity is tuned to operate in the under-coupled, critically coupled, and over-coupled regimes. The laser operation is found to depend on the cavity coupling regimes. For the locked laser the experimental dependences of the transmitted and reflected from the ring cavity powers are well described by the analytical expressions for the ring cavity only. The best laser stability is observed with critical coupling. In this case the laser power is accumulated inside the cavity providing strong feedback for laser locking and leading to significant narrowing of the laser emitting spectrum. Reduction of the laser linewidth more than 1000 times is demonstrated with the ring cavity operating in under-coupled and critically coupled regimes. [ABSTRACT FROM AUTHOR]

We propose a neuromorphic photonic time-delayed reservoir computing (RC) scheme based on a large-scale laterally coupled laser array with optical feedback and injection. Here, the feasibility of this RC system for dealing with a complex task-image recognition is proved numerically. By the offline pre-processing using randomly initialized convolutional neural network, two common datasets in the visual domain, i.e., MNIST and Fashion-MNIST, are employed to evaluate the performance of the proposed RC. After enlarging the laser array size and optimizing the parameters, our RC system achieves identification accuracy of 98.2% for the MNIST dataset and of 89.9% for the Fashion-MNIST dataset. Since the feedback delay time and information mask period of each laser in the array are coincident and maintained at 0.2 ns, the information processing rate of the RC system reaches 5 Gbps. Besides, the potential situations of parameter mismatch are also analyzed. • Time-delayed reservoir computing based on a large-scale laterally coupled laser array for image classification task. • Identification accuracies of 98.2% and 89.9% respectively for the MNIST and Fashion-MNIST dataset. • High information processing rate of 5 Gbps. • Influence of parameter mismatch in reservoir computing based on the coupled laser array. [ABSTRACT FROM AUTHOR]

Nonlinear dynamical characteristics of a slave semiconductor laser (S-SL) subject to optical injection from a master SL (M-SL) and optical feedback from a fiber Bragg grating (FBG) are investigated experimentally. First, we investigate the nonlinear dynamics of the S-SL subject to only optical injection. Through varying the injection coefficient and fixing the frequency detuning between the M-SL and S-SL, some dynamical states with typical characteristics are recorded and identified. Next, the variations of these dynamical states are further investigated with the introduction of a FBG feedback, and the according results show that these dynamical states may be changed after a FBG feedback is introduced into the S-SL. Finally, after collecting the mappings of the dynamical states of S-SL in the parameter space of injection coefficient and frequency detuning under different FBG feedback coefficients, the influences of FBG feedback on the nonlinear dynamics of the S-SL are analyzed. [ABSTRACT FROM AUTHOR]

Dynamics of a multi-transverse mode vertical-cavity surface-emitting laser is studied experimentally in a wide parameter range of optical feedback and current modulation. While the orthogonal polarizations manifest anticorrelated feedback dynamics, dynamics of different transverse modes with orthogonal polarizations do not exhibit a clear correlation property. This may be attributed to spatial hole burning effect. As the current modulation becomes strong, both polarization and modal dynamics are modulation dominated. When the modulation frequency is close to the external cavity resonance frequency or its harmonics, feedback dynamics is enhanced. For the modulation frequency close to half integer multiples of the external cavity resonance frequency, feedback feature can be suppressed. The minimum modulation amplitude for suppressing feedback dynamics is measured for each polarization and one of the transverse modes in the polarization, and the results are discussed. Interplay of relaxation oscillation, optical feedback, and current modulation is observed and measured. Our results are compared to the theoretical predictions. [ABSTRACT FROM AUTHOR]

We report results on the asymptotic, bifurcation and numerical analysis of semiconductor ring lasers with negative optoelectronic feedback or incoherent optical feedback. We find that the dynamical behavior of both systems can be adequately described by two differential equations and one map with time delay on time-scales longer than the relaxation oscillations. On these timescales, the dynamics for both types of feedback is identical. We study the stationary solutions of this model and are able to obtain the analytical expressions of all local bifurcations. As we vary the feedback strength for small delay times comparable to the period of relaxation oscillations, the devices under consideration in the paper display both continuous wave operation and a period-doubling route to chaos. The intensities of two counter propagating modes of both systems exhibit in-phase chaotic behavior similar to single mode semiconductor lasers. However, for long delay times, the counter propagating modes show anti-phase chaotic oscillations. This anti-phase chaotic regime does not involve carrier dynamics and is a result of the bistable character of the semiconductor ring laser. [ABSTRACT FROM AUTHOR]

The monolithic integration of 1.5-µm four channels phase shift distributed feedback lasers array (DFB-LD array) with 4×1 multi-mode interference (MMI) optical combiner is demonstrated. A home developed process mainly consists of butt-joint regrowth (BJR) and simultaneous thermal and ultraviolet nanoimprint lithography (STU-NIL) is implemented to fabricate gratings and integrated devices. The threshold currents of the lasers are less than 10 mA and the side mode suppression ratios (SMSR) are better than 40 dB for all channels. Quasi-continuous tuning is realized over 7.5 nm wavelength region with the 30 °C temperature variation. The results indicate that the integration device we proposed can be used in wavelength division multiplexing passive optical networks (WDM-PON). [ABSTRACT FROM AUTHOR]

We report here a random lasing from the dye-doped media with the lnP/ZnS quantum dots. The lnP/ZnS quantum dots assistant optical feedback for random lasing emission through multiple scattering centers. Changing the energies of pump, the tunable lasing operation ranging from single photoluminescence mode to many lasing modes. The tunability of the central wavelength is realized by altering the constraint structure of the devices, and the wavelength ranges are changed from 600 nm to 635 nm. Besides, it is the first time, our quantum dots liquid crystal random lasers design holds great potential for the liquid crystal display of lasers, as well as for the promising material of liquid crystal lasers. [ABSTRACT FROM AUTHOR]

Abstract: This works presents an efficient technique for generating multi-wavelength sources, which consist of an expanded optical frequency comb based on the nonlinear process of multiple four-wave mixing. The nonlinear efficiency is significantly enhanced by the use of the proposed scheme based on optical feedback, highly nonlinear and erbium-doped fibers. Numerical and experimental results illustrate its efficiency and applicability by expanding a comb from 20 to over 200 coherent lines spaced by only 12.5GHz. [Copyright &y& Elsevier]

This paper reports an optical injection locking (OIL) assisted all-optical microwave oscillator (AOMO) scheme for the photonic generation of a microwave (MW) signal and demonstrates a 40th harmonic single-mode signal generation with an external low-frequency radio-frequency (RF) signal. In this all-optical single-loop oscillator that only depends on optical devices, the stimulated Brillouin scattering (SBS) effect of a coil of single-mode fiber (SMF) and a semiconductor optical amplifier (SOA) are used to select the oscillation frequency and modulate the optical feedback signal, respectively. The OIL effect in a distributed feedback (DFB) laser multiplies the frequency of the external RF signal. Thanks to the OIL module, it only requires an optical loop to acquire a single-mode oscillation and the oscillation signal is phase-related to the low-frequency RF source. The experiment demonstrates the generation of a single-mode MW signal of 10.71 GHz with an external RF signal of 267.8 MHz, with a frequency multiplication factor of 40. Mode-hopping is not evident in the output signal, and a moderately low-quality OIL signal is acceptable since unwanted harmonics and the phase noise floor of the OIL signal have little impact on the generated MW signal. • Microwave oscillation is established with a purely optical loop. • Optical side-band injection locking multiplies the frequency of the external signal. • A 40th harmonic (10.7 GHz) single-mode signal generation is performed experimentally. • It's potential for all-optical chip-based sources at high frequencies (over 100 GHz). • It has potential applications for the purpose of synchronization. [ABSTRACT FROM AUTHOR]

Although it appears that there have been made enough researches as regards the estimation the optical feedback factor C of a self-mixing interferometer, any method of the C estimation cannot ensure its reliability if the operation of the self-mixing displacement sensor covers the strong feedback regime with fringe loss due to large displacement of a target. That is the main reason why the precision of the sensor degenerates for the measurement of large displacement. In the paper we propose a novel C estimation method called region tracking for the C value, which is indispensable for the measurement of long-range displacement, and demonstrate its fidelity experimentally. The experimental results in the paper were given when the target distance was about 60 cm and the target displacement 25 mm. The displacement reconstruction based on the proposed region tracking method ensures a better precision than a half wavelength, depending on the method of displacement retrieval. The approach presented will make a great contribution to further improvement of the accuracy of the self-mixing displacement sensor for large displacement. • To clarify the mechanism of fringe loss in strong feedback regime. • To find how to enlarge the dynamic range of SM displacement sensor. • To propose a new algorithm for displacement retrieval for all regimes. [ABSTRACT FROM AUTHOR]

In self-mixing interferometry, the fast and accurate estimation of fundamental feedback parameters, such as optical feedback coupling factor C and linewidth enhancement factor α , is critical for high-resolution real-time displacement sensing using an SMI sensor. In this work, a Marquardt's algorithm (MA) based fast and highly accurate method is proposed to estimate C and α. The proposed method utilizes the SMI signal phase and applies a simple numerical analysis to estimate C and α. The proposed method can accurately estimate C and α for all three major feedback regimes (i.e., week, moderate and strong). A detailed comparison is also made with previously proposed algorithms to show the superiority of the proposed algorithm in terms of initial conditions, number of iterations, accuracy, and ability to work under all feedback regimes. The results have shown that our method has a negligible average error of around 0.0023 and 0.0006 for C and α estimation, respectively. The proposed method only requires approximately five iterations for accurate estimation. The method's simplicity, accuracy, and fast nature enable compact and cost-effective SMI sensors with high-resolution real-time displacement sensing. • A fast and accurate method for the estimation of feedback factor and linewidth enhancement factor has been proposed. • A detailed comparison of the proposed method is made with previously proposed estimation methods. • It provides the possibility of developing a fast and cost-effective setup for self-mixing interferometry based displacement sensing. [ABSTRACT FROM AUTHOR]

Abstract: An efficient multiwavelength Brillouin–Erbium fiber laser (BEFL) was developed by improving the feedback mechanism of Brillouin Stokes lines. The BEFL performance in terms of threshold power and wavelengths count was improved by using a new structure of double pass amplification cavity. The new structure utilized the variable optical coupler (VOA) not only as the input and output ports but also to form a fiber loop mirror that reduces the number of optical components, thus, only three optical components were needed. By optimizing the Brillouin and 1480nm pump power, up to 41 channels and 26 channels were obtained using 0.6km and 0.3km long of standard single mode fiber (SSMF), respectively. [Copyright &y& Elsevier]

Abstract: Synchronisation of two unidirectionally coupled semiconductor ring lasers working in a chaotic regime (transmitter and receiver) is studied numerically. Two different configurations: open and closed loops, and two different ways of light injection: one- and two-mode injection are investigated. We show that synchronisation is more efficient when the closed loop is used compared to the open loop. When one directional mode is injected, very high coupling between the lasers is required to get the lasers synchronised, while in case of injection of both modes synchronisation with high cross-correlation coefficient is obtained at moderate values of coupling. The ON/OFF phase shift keying method is successfully applied to encrypt a message at a high bitrate. [Copyright &y& Elsevier]

Abstract: We have discussed numerically the characterization of time delay (TD) signature in a chaotic vertical-cavity surface-emitting laser (VCSEL) subject to optical feedback, where the results for polarization-preserved optical feedback (PPOF) and polarization-rotated optical feedback (PROF) are presented comparatively. It is found that, when the feedback strength is moderate, TD signature can be retrieved successfully by analyzing the total intensity, intensities of x-linearly polarized (x-LP) mode, and y-LP mode in a VCSEL with PPOF; while for PROF case, the intensity amplitude peaks in autocorrelation function (ACF)/delayed mutual information (DMI) of the two orthogonal LP modes are located at multiple values of TD. Furthermore, the influence of the feedback strength and time delay has been investigated. The results show that, a further increase of the feedback strength (roughly>40ns−1) leads to the successful identification of the TD signature in the two LP mode intensities of VCSELs with PROF. It is also observed that the estimation of TD depends significantly on the value of the spin–flip relaxation rate (γ s ). For low relaxation rates the TD signature could be much easier to be revealed by the two LP modes than that for large values of relaxation rate in VCSELs with PROF. Finally the influence of injection current and the device linear anisotropies is investigated, and the results of TD estimation are confirmed by calculating permutation entropy (PE). [Copyright &y& Elsevier]

Abstract: We propose a sub-THz continuous wave (CW) generation scheme using a high-order harmonics modulated lightwave (HML) to reduce an electronic dependency of a conventional double sideband suppressed carrier (DSB-SC) scheme. The electronic dependency should be overcome to increase frequency tunability of the conventional DSB-SC scheme. This is because the frequency of a local oscillator (LO), f LO, should be one-half frequency of the frequency of a desired sub-THz CW in the conventional DSB-SC scheme. The proposed scheme is formed by adding an optical feedback loop to the conventional DSB-SC scheme. In order to verify our proposed scheme, a 120GHz CW is generated using the LO with f LO =20GHz. Based on our experimental results, we have found that the frequency of the LO can be reduced by our proposed scheme up to one-sixth (20GHz) of 120GHz. The 120GHz CW generated by the proposed scheme has 52dB higher photomixed output power with narrow spectral linewidth than that of the 120GHz CW generated by the conventional DSB-SC scheme using the LO with f LO =20GHz. Consequently, our proposed scheme can be helpful to reduce the electronic dependency of the conventional DSB-SC scheme. [Copyright &y& Elsevier]

Abstract: Synchronization between laser diodes subject to multiple incoherent, delayed, optical feedback has been recently investigated [Shahverdiev, Shore, Opt. Commun. 282 (2009) 3568] through numerical simulations. Here, I explain that the model that was proposed and used is not suitable when the lasers are subject to multiple feedbacks of this kind. [Copyright &y& Elsevier]

Abstract: We study experimentally and numerically the dynamic states of chaotic oscillations in vertical-cavity surface-emitting semiconductor lasers (VCSELs) with polarization-selected optical feedback. We identify the regimes of fully-developed chaotic states, low-frequency fluctuations (LFFs), and coexistent states of LFFs and stable oscillation for the variations of the bias injection current and the optical feedback ratio. In particular, coexistent states of LFFs and stable oscillations are observed at higher optical feedback ratio and lower bias injection current. We draw maps of dynamic states in the space of the bias injection current and the optical feedback ratio. The qualitative agreement between the theory and the experiment is found. [ABSTRACT FROM AUTHOR]

Abstract: We have experimentally studied polarization instabilities in the multi-transverse-mode regime of a vertical-cavity surface-emitting laser subject to polarized optical feedback. A dynamical regime that is similar to the transition from the so called low frequency fluctuation (LFF) to coherence collapse (CC) in the single-transverse-mode VCSEL is found and investigated. The role of higher order transverse modes is to increase the irregularity of the dynamics as shown experimentally and numerically. [Copyright &y& Elsevier]

Abstract: Intensity modulation and multi modes hopping in Nd:YAG lasers subjected to optical feedback have been studied experimentally and theoretically. Their dependence on the external-cavity-phase has been studied. The results are compared with the predictions of the model of threshold gain modulation. As the initial length of external cavity is changed the number of hopping modes also changes. [Copyright &y& Elsevier]

Abstract: In this research, single-mode 3.76GHz optical pulses were generated in a Fabry–Perot type single-mode semiconductor laser with polarization-rotated optical feedback (PROF) at a round-trip feedback distance of 50.8cm, corresponding to a feedback frequency of 0.59GHz. Experimental results and numerical simulations revealed that the pulse generation mechanism involved a self-modulation of the laser’s relaxation oscillation frequency so that the oscillation frequency approximated to an integer multiple of the PROF round-trip feedback frequency. This effect is very different from similar experiments reported by many researchers before, in which the laser’s output was amplitude modulated by the feedback frequency and an ultra-short feedback distance was required to generate giga-hertz optical pulses. Investigations about some characteristics of the self-modulation mechanism will be reported in the paper. [Copyright &y& Elsevier]

Abstract: We present a systematic study of external optical feedback effects on the output characteristics of the two orthogonally polarized modes in a frequency locking dual frequency laser. Accompanied by the change of the external cavity length, the length of the resonance cavity is changed very slowly to set the oscillating modes sweeping across the laser gain curve. Comprehensive results are obtained and a strong mode competition is observed. Particularly, when the laser runs in the middle region of the gain curve, which mode can oscillate is determined by the movement direction of the external feedback mirror. Based on the phenomenon, a displacement sensor with directional discrimination is proposed and its capability is also discussed. [Copyright &y& Elsevier]

Abstract: We investigate a system consisting of multiple ring lasers coupled by a single gain medium. All the ring lasers share a common feedback arm. The output power of an individual laser shows periodic oscillations as a function of time. The periodicity of the oscillation is determined by the ratio of the roundtrip times of the feedback arm and the ring cavity. In the case that two of such ring lasers are coupled, either their oscillation periodicities are synchronized, or the system is bi-stable. In the latter operation regime, the system can act as an optical flip-flop memory whose state be switched by injection of external light. The concept can be extended to multi-state operations; an eight-state optical flip-flop memory is experimentally demonstrated. [Copyright &y& Elsevier]

Abstract: We have studied experimentally spectral characteristics of a multi-mode vertical-cavity surface-emitting laser that is subject to optical feedback. Appropriate alignment of the feedback mirror can suppress higher-order modes and significantly decrease the spectral linewidth of the laser. [Copyright &y& Elsevier]

Abstract: We describe a diode laser system at 635 nm, which has been developed for a colour laser display. The poor beam quality of conventional high power lasers has lead to the conception that these lasers are not suited for laser displays. In this paper, we introduce a new laser system that eliminates some of the previous problems. The system is based on a broad area laser diode with external optical feedback, which improves the spatial coherence of the beam emitted from the diode. We investigate the operation of the laser system in connection with acousto-optic intensity modulation. A diffraction efficiency from 0th to 1st order of 92% and 60% is achieved for modulation rise times of 80 and 10 ns, respectively. The advantages of the feedback scheme are the ability to use compact diode lasers for display purposes while simultaneously obtaining fast rise times and high output power. The results demonstrate that the new red source is suitable for laser display applications. [Copyright &y& Elsevier]

The output characteristics of two orthogonally polarized frequencies in a Zeeman-birefringent dual frequency laser are studied under three different optical feedback cases: (a) only the perpendicular light is fed back; (b) only the parallel light is fed back; (c) both frequencies are fed back. Laser intensity and the free running frequency’s intensity are also studied at the same time. Sign inversion of self-mixing interference of either frequency is observed. When one frequency is fed back into the laser cavity, the modes competition between these two frequencies is more obvious than that both two frequencies are fed back. The modulation depth of perpendicular light depends on which frequency is fed back into the laser cavity. [Copyright &y& Elsevier]

The output characteristics of two orthogonally polarized modes are studied in a birefringence dual frequency He–Ne laser with optical feedback. The frequency difference of the dual frequency laser is 450 MHz. Modes competition is fair when the two modes’ maximal intensities are equal with optical feedback. But it is quite unfair modes competition when the two modes’ maximal intensities are quite different with optical feedback and one mode nearly extinguishes within the whole optical feedback loop. Theory analysis is presented. The experimental results are in good agreement with the theory analysis. The potential use of the experimental results is also discussed. [Copyright &y& Elsevier]

This research investigated the characteristics of single-wavelength optical pulses generated with orthogonal-polarization optical feedback in semiconductor lasers. Optical pulses with a pulse rate of 470 MHz and a pulse width around 700 ps were observed. A model was also proposed to solve the inconsistency between the experimental results and the computer simulations of Otsuka and Chern’s model. Furthermore, the new model’s predictions explore that single-wavelength Giga-Hertz optical pulses could be generated using this method with conventional technology. [Copyright &y& Elsevier]

This paper reports on a novel design for a high-brightness quantum-well AlGaInP laser-diode system operating at 635 nm. The spatial coherence of two broad area laser diodes is improved using spatial filtering and optical feedback from a narrow mirror stripe on each laser. To increase power, the outputs from the two systems are passively combined using a polarizing beam splitter. The result is a compact system providing a high-brightness beam. The beam-quality parameter is improved in one direction from M2=12 and M2=16 for the two lasers, to M2=2.1 for the combined beam. The other direction of the beam is left unchanged due to its natural high quality with an M2 value around 1.5. A procedure for efficient coupling of the two lasers into a small core-diameter optical fiber is given. This capability facilitates a wide range of applications including interstitial medical therapies, where delivery of the treatment light through thin optical fibers is essential. [Copyright &y& Elsevier]

The frequency characteristics of a VCSEL with a quarter-wave plate (QWP) and an external reflector are investigated with the translation matrix of the vectorial field. Two series of eigenmode with a shift of half the free spectrum range are linearly polarized, respectively, along the neutral axes of QWP. We also numerically explore the polarization self-modulation phenomenon by using a vectorial laser equation and considering the inhomogeneous broadening of the gain medium. If the external cavity is so short that the shift is bigger than the homogeneous broadening, two stable longitudinal modes oscillate, respectively, on the neutral axes of QWP because they consume different carriers. With a long external cavity, the competition of the modes for the common carriers causes the intensity fluctuation of the modes with a period of one round-trip time of the external cavity. [Copyright &y& Elsevier]

We describe our system for frequency calibrating high resolution laser induced fluorescence spectra. The system utilizes two diode lasers that are spectrally narrowed with optical feedback from external resonators. One laser acts as a reference and is locked to a cesium transition with an external resonator containing a vapor cell in a Faraday filter configuration. Additional electronic feedback fixes the length of the resonator. The second laser receives its feedback from a confocal optical cavity and has the capability of scanning over several hundred megahertz. A portion of the scanning-laser output excites cesium atoms in a thermal atomic beam producing a fluorescence signal. Portions of each laser output are combined on a fast photo diode that produces a heterodyne signal. A counter reads the RF beat note between the laser frequencies and provides the frequency calibration. [Copyright &y& Elsevier]

A new cavity design for the clad-pumped fiber laser (CPFL) was proposed and experimentally demonstrated in a Nd-doped silica fiber with the single transverse mode output and an enhancement in the slope efficiency for butt-coupled end pumping. In a fiber cavity two concatenated adiabatic rectangular tapers were formed to remove the optical feedback in higher order modes as well as to provide a low splice loss to standard single mode fibers. Mode propagation along the composite cavity was theoretically analyzed using beam propagation method and continuous wave oscillation in the LP01 mode at 1.06 μm was achieved with the output power over 1.3 W at the pump power of 3.67 W. [Copyright &y& Elsevier]

Noise figure characteristics of erbium-doped fibre amplifiers (EDFAs) with different optical feedback directions, namely counter- and co-feedback, and without feedback are presented. It was found that the noise figure exhibited the pump power and the feedback direction dependence although the population has been clamped after the onset of laser oscillation. Departure of the noise figure for two different feedback directions was found to be increasing with the pump power. The dependency of the depth of dip for the noise figure on the feedback direction at the saturated input signal power was also observed. [Copyright &y& Elsevier]

Frequency-stable operation of an extended-cavity GaN diode laser was achieved by resonant optical feedback from a confocal cavity. The observed frequency fluctuations were smaller than 300 kHz peak-to-peak in one second, and the upper limit of the linewidth was measured to be 880 kHz. A continuous frequency scan up to 5 GHz was realized by simultaneous servo control on the optical feedback phase and on the extended-cavity diode laser frequency. The violet laser was used to perform saturation spectroscopy on the 52S1/2–62P3/2 transition of rubidium at 420 nm in a gas cell, and resulted in spectra with a linewidth of 2.3 MHz. [Copyright &y& Elsevier]

In this paper, we propose a nonlinear interferometer based on two-port feedback nondegenerate optical parametric amplification (NITFPA), which employs two linear beam splitters to feed the two output beams of a nondegenerate optical parametric amplifier back to its input ports. By comparing the NITFPA with the SU(1,1) interferometer (SUI), which is another typical nonlinear interferometer, we demonstrate that the NITFPA has its unique advantages in the precision enhancement of the phase estimation. In addition, we demonstrate that the NITFPA is also advantageous over the SUI in the robustness against the losses. Such a NITFPA may find its potential applications in quantum metrology. • We propose a nonlinear interferometer based on two-port feedback nondegenerate optical parametric amplification (NITFPA), which cyclically utilizes the internal entanglement sources to boost the phase sensitive photon number inside the interferometer, with the help of the feedback structure. • Given the same sources of system, our NITFPA is able to boost the internal photon number of the interferometer, and thus makes the phase sensitivity of the NITFPA enhanced compared with the traditional SU(1,1) interferometer. • Considering the effect of losses, we found that NITFPA performs better in the robustness against the losses than SU(1,1). [ABSTRACT FROM AUTHOR]

A high-side mode suppression ratio (SMSR) and high-stability external-cavity diode laser array employing optical feedback from a volume Bragg grating (VBG) with 15% diffraction efficiency at 976 nm was investigated. Based on the transfer matrix method and on the multimode rate equation, the theoretical model for an external-cavity diode laser at 976 nm was built for different temperature and current values. Both the simulated and the experimental results show that the temperature and the current range of such laser array with a front end reflectivity (R f) of 0.5% are 10 °C and 15 A broader than that for an external cavity with R f = 2%. As a result, the external-cavity diode laser array at 976 nm with a R f value of 0.5% exhibits a high stability in the temperature range of 15–35 °C and from 15 to 50 A. Moreover, it shows an SMSR higher than 30 dB and a power larger than 33.9 W at 50A. • The model for an external-cavity laser diode (ECLD) at 976 nm is built. • ECLD with R f = 0.5% shows a higher SMSR in a wide temperature and current range. • ECLD with R f = 0.5% should work with good stability in operating environment. • The fiber coupled laser diode system with higher SMSR is fabricated. [ABSTRACT FROM AUTHOR]