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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.

Chaos generation from a semiconductor laser under band-rejection optical feedback is proposed and investigated numerically and experimentally. The band-rejection feedback performs better than the conventional broadband mirror feedback in concealing the feedback delay time information measured by the time-delay signature (TDS) of optical field and intensity. In this work, the optical band-rejection filter (BRF) is built up based on the transmission of a Bragg grating. The best concealment of delay time information is achieved in the form of simultaneous TDS suppression in optical field and intensity when the BRF is negatively detuned from the free-running laser frequency. Such concealment is due to the suppression of feedback cavity modes by the band-rejection effect. The concealment prefers negative detuning frequency because the laser cavity resonance is red-shifted by optical feedback due to the antiguidance effect. The negatively detuned BRF suppresses the TDS of optical field and intensity by more than an order of magnitude than mirror feedback. The simulation results are qualitatively confirmed by the experimental measurements.

A tunable narrow-linewidth photonic terahertz (THz) carrier generation scheme using a distributed-Bragg-reflector-based mode-locked laser diode (MLLD) with optical feedback and mode filtering technique is proposed and experimentally demonstrated. Photonic terahertz carrier covering the D-band (110–170 GHz), and H-band (220–325 GHz) was obtained. With the assistance of the feedback loop, the optical linewidth of the MLLD was reduced from MHz to kHz level, and the heterodyning photonics carrier linewidth was reduced from 121.7 kHz to 2.1 kHz, satisfying the linewidth tolerance in the THz communication system with high-order modulation formats.

A simple method to regulate the single-mode lasing intensity of semiconductor lasers without wavelength shift is demonstrated in this paper. The basic component is a digital micro-mirror device (DMD), which can be taken as an adjustable two-dimensional grating by its special structure. The lasing peak wavelength and peak intensity are respectively regulated by the DMD angular position and the DMD setting. The lasing peak wavelength could be tuned over by rotating the DMD. Meanwhile, the lasing peak intensity could be regulated by the DMD setting, which generates optical feedback digitally. The regulating speed is very high (up to 10 kHz) and has no wavelength shift in the lasing mode, which is more stable and faster than the conventional method.

We report on a simple approach of time-delayed reservoir computing (RC) based on a two-element phased laser array for image identification. Here the phased laser array with optical feedback and injection is trained according to the representative characteristics extracted through histograms of oriented gradients. These characteristic vectors are multiplied by a random mask signal to form input data, which are subsequently trained in the reservoir. By optimizing the parameters of the RC, we achieve an identification accuracy of 97.44% on the MNIST dataset and 85.46% on the Fashion-MNIST dataset. These results indicate that our proposed RC indeed allows accurate classification of handwritten digit and fashion production. Moreover, we further forecast an RC scheme based on a larger-scale phased laser array, which is expected to tackle more complex tasks at a high speed. Our work offers a possibility for advanced image processing using highly integrated neuromorphic photonic systems. [ABSTRACT FROM AUTHOR]

Synchronization is a long-standing challenge in modern large-scale distributed systems, and newly deployed systems such as 5G and modern netted radars require more extensive synchronization. As the number of distributed stations increasing, new synchronization solution with much effort on extensibility is highly expected. In this paper we first analyze the requirements on extensibility of fiber-based sync systems, and then propose a phase-stabilized side-branch radio-over-fiber (RoF) link, which fully satisfies the extensibility requirements. The extensible side-branch link can be constructed at any intermediate point along the main link, without any reconfiguration at the central station, which is especially feasible when adding extra stations in system capacity expansion. The main link phase shift is first stabilized by tuning the optical carrier wavelength, while another phase-locked-loop constructed at the starting point of the branch link stabilizes the phase shift from the central station to the end of branch link, by phase locking two phase-varying signals. Moreover, the phase-stable side branch is inherently extensible to multiple frequency signals, which can be used in receivers requiring multiple local oscillators (LOs). Two frequency signals are simultaneously disseminated to the end of branch link in the experiment, with frequency stability optimization of two orders of magnitude on both LOs achieved. The proposed stable side-branch RoF link shows the potential of constructing a highly-extensible frequency dissemination network for the synchronization in modern distributed systems. [ABSTRACT FROM AUTHOR]

A tunable narrow-linewidth photonic terahertz (THz) carrier generation scheme using a distributed-Bragg-reflector-based mode-locked laser diode (MLLD) with optical feedback and mode filtering technique is proposed and experimentally demonstrated. Photonic terahertz carrier covering the D-band (110–170 GHz), and H-band (220–325 GHz) was obtained. With the assistance of the feedback loop, the optical linewidth of the MLLD was reduced from MHz to kHz level, and the heterodyning photonics carrier linewidth was reduced from 121.7 kHz to 2.1 kHz, satisfying the linewidth tolerance in the THz communication system with high-order modulation formats. [ABSTRACT FROM AUTHOR]

We experimentally investigate the performances of an optical reservoir computing (RC) system based on two parallel time-delay reservoirs composed of two semiconductor lasers (SLs) subject to optical feedback. In such a system, the information being processed is split into two parts to send into two reservoirs through directly modulating the pump currents of two SLs, and the temporal output of the two SLs are sampled and taken as the virtual node states for training and testing. Via Santa Fe time series prediction task and multi-waveform recognition task, the performances of the proposed RC system are investigated and compared with those of the system based on one reservoir. The results show that the system based on two parallel reservoirs behaves better performance and stronger parameter robustness than that based on one reservoir. Moreover, through analyzing the dependence of the system performances on the number of virtual node states actually used for readout, the potential data processing rate (DPR) of the system is evaluated. For processing a prediction task under guaranteeing the normalized mean square error below 0.1 and a recognition task under guaranteeing the signal error rate below 0.005, the potential DPR of the proposed RC system can achieve 200 MSa/s, which is twice the DPR of the system with only one reservoir. [ABSTRACT FROM AUTHOR]

In this work, we propose and numerically investigate a scheme for reservoir computing (RC) based on two parallel reservoirs under identical electrical message injection, in which two semiconductor lasers (SLs) under optical feedback are utilized as two parallel reservoirs. For simplifying the system, only one mask signal is employed in this scheme. After multiplying with input data, the masked information is injected into two parallel reservoir lasers (SL1 and SL2). The virtual node states can be obtained from the temporal outputs of two SLs. RC can be accomplished by three ways, namely RC1/RC2 (the virtual node states originating from SL1/SL2 are used for training and testing) and RCM (the merged virtual node states originating from two SLs are used for training and testing). Via chaotic time series prediction task and memory capacity (MC) test, the performance of the RC system is simulated and assessed. The results show that, for a given data processing rate, better prediction performance and higher MC can be realized by RCM through setting suitable mismatched parameters between the two SLs. Under satisfying the requirement for achieving a good performance, the highest data processing rate can be doubled for RCM with respect to that for RC1/RC2. [ABSTRACT FROM AUTHOR]

Light field manipulation is an important technology for improving system-level efficiency of optical wireless power transfer (OWPT). In this paper, we propose and compare the Gerchberg-Saxton algorithm, direct binary search and genetic algorithm enabled light field optimization for adapting photovoltaic cells in different geometric arrangements. Under static condition, photoelectric conversion efficiency can reach 33.14% through optimizing light field using Gerchberg-Saxton algorithm, almost 200% improvement over the case with Gaussian beams. Moreover, the resilience of power delivery can be improved by employing direct binary search and genetic algorithm in a dynamic fashion. By utilizing genetic algorithm optimization, initial photoelectric conversion efficiency is 14.11% with the help of random optimization process. The dynamic light field optimization technology will be an enabling technology of high efficient power delivery for mobile objects. [ABSTRACT FROM AUTHOR]

We propose and numerically demonstrate the generation of wideband millimeter-wave (mmW) flat chaos with controllable power spectrum by injection of chaotic signal from external cavity semiconductor laser (ECSL) into optical time lens with noise phase modulation. Simulation results indicate consistent elimination of the ECSL relaxation oscillation frequency domination over the RF spectrum for large scale parameters of the optical time lens module and a wideband flat chaos, whose efficient bandwidth rapidly increases with the bandwidth of the noise signal driving the phase modulator and phase modulation index. Besides, we show that the time delay signature suppression can be concurrently achieved for moderate values of the noise bandwidth and phase modulation index. The proposed wideband mmW flat chaos exhibits great potential applications for ultrahigh-speed chaos communications, mmW radars and macroscopic mmW noise source required for mmW research and design. [ABSTRACT FROM AUTHOR]

An InP-based, monolithically integrated multiwavelength laser with optical feedback is presented. We demonstrate the performance of the dual-wavelength operation. Single-mode lasing is obtained for each of the two channels within a temperature range from 16 to 30 °C and an injection current range from 30 to 70 mA without mode-hopping. This is due to the combined filtering functions of an arrayed waveguide grating and a distributed Bragg reflector. A method that makes use of weak optical feedback is demonstrated to improve the stability of each single-mode. With this method, sustained relaxation oscillations that are excited by on-chip parasitic reflections are damped and linewidths in both channels are narrowed to around 400 kHz compared to the linewidth of 1 MHz that is obtained when no feedback is applied. The suppression ratio of the relaxation-oscillation-induced side peaks is measured to be over 45 dB for two simultaneously operated channels.

In this work, a novel optoelectronic oscillator (OEO) structure is proposed for generating widely tunable photonic microwave. In this structure, an optical injection semiconductor laser (OISL) functions as an active tunable microwave photonic filter (MPF). Through controlling the injection power and frequency detuning, the OISL is driven into period-one dynamics whose oscillation frequency can be widely tuned from 10.43 to 65.82 GHz. As a result, the OISL can be regarded as a widely tunable active MPF. By introducing an optical feedback loop, the filtered bandwidth of the widely tunable active MPF can be further reduced. Taking the OISL under optical feedback as a widely tunable narrow-bandwidth MPF and a seeding laser source to establish an OEO structure, widely tunable narrow-linewidth photonic microwave can be generated. The experimental results demonstrate that widely tunable photonic microwaves ranging from 10.43 to 39.10 GHz with linewidths below 0.1 MHz and phase variances below 10-2 (rad2) can be achieved.

We demonstrate automated adaptation, and stabilization of a silicon photonic wavelength-division multiplexing (WDM), polarization-independent receiver. A two-channel, tunable WDM polarization-independent receiver is designed, and used to demonstrate automated WDM polarization control. Using a control algorithm based on Barzilai, and Borwein's two-point step size gradient descent method, we realize automated polarization adaptation, and wavelength stabilization for two arbitrarily polarized input data streams. 10 Gb/s on-off keying, and 20 Gb/s pulse-amplitude modulation 4-level formats are generated as the high-speed input data streams. In addition, we implement a long-duration experiment, in which we measure the bit-error-ratio for continuously varying polarization states, and changing chip temperatures. The experimental results show that, with the automated control, the WDM polarization-independent receiver can adapt, stabilize, and track the arbitrary input polarization states from a standard optical fiber into the transverse electric mode of a silicon waveguide, and simultaneously stabilize the transmitted wavelength channels at various chip temperatures. We also show how the presented WDM polarization-independent receiver scales with N channels, and propose an improved design for large-scale WDM applications. [ABSTRACT FROM AUTHOR]

A prism-based self-injection locked seamlessly tunable blue InGaN/GaN laser diode composite cavity system is presented. A rigorous analysis of this external cavity diode laser (ECDL) system is performed at two different optical feedback powers. At 130 mA low injection, the high reflection system (HRS) exhibits a record wideband tuning span of ∼12.11 nm with a side-mode-suppression-ratio (SMSR) ≥ 15 dB, measuring as high as 40 dB, linewidth ≤110 pm, and a working power of about 3 mW. Whereas, a tuning range of 8 nm with linewidth ≤88 pm, SMSR ≥ 13 dB, reaching a maximum of 35 dB with 14.5 mW usable power is achieved from the low reflection system (LRS) at the same injection current. Moreover, an inverse relationship between the optical power and the tunability is observed in both the systems with a value as high as 180 mW exhibited by LRS configuration and attaining a tunability of 4.5 nm. Both systems highlight high stability even at higher injection currents and temperature. Such a robust, simple, and compact system may serve as a crucial light source in a plethora of diverse applications besides visible optical communications. To the best of our knowledge, this is the first report on a continuously tunable self-injection locked tunable laser system. [ABSTRACT FROM AUTHOR]

In the present work, we report a path of RF stabilization versus delay subject to self-mode-locked (SML) two-section quantum-dash (QDash) lasers emitting at ∼1.55 μm and operating at ∼21 GHz repetition rate using a feedback ratio controlled and optical delay phase-dependent symmetric dual-loop optical feedback. For symmetric dual-loops (equal arms of external loops), we identify the three key parameters: power-split ratio through each cavity of the external feedback loop, optical delay phase settings, and overall feedback strength back into gain section, yields jitter stabilization on integer resonance as well as on full delay range tuning. Symmetric dual-loop feedback with two optical delay phase settings (weaker cavity set to integer resonance, fine-tuning of the stronger cavity and stronger cavity set to integer resonance, fine-tuning of a weaker cavity) and four chosen combinations of feedback ratios (−19.5:−29.03 dB, −20.6:−24.3 dB, −21:−22.7 dB, −21.3:−23 dB) has been demonstrated. Based on these four chosen combinations of feedback ratios and optical delay phase settings, a path of stabilization has been identified for SML QDash lasers using symmetric dual-loop optical feedback. Our proposed dual-loop feedback schemes provide a viable path towards the stabilization of mode-locked lasers, promising for various practical applications where ultra-stable optical pulses are highly desirable. [ABSTRACT FROM AUTHOR]

A simple method to regulate the single-mode lasing intensity of semiconductor lasers without wavelength shift is demonstrated in this paper. The basic component is a digital micro-mirror device (DMD), which can be taken as an adjustable two-dimensional grating by its special structure. The lasing peak wavelength and peak intensity are respectively regulated by the DMD angular position and the DMD setting. The lasing peak wavelength could be tuned over by rotating the DMD. Meanwhile, the lasing peak intensity could be regulated by the DMD setting, which generates optical feedback digitally. The regulating speed is very high (up to 10 kHz) and has no wavelength shift in the lasing mode, which is more stable and faster than the conventional method. [ABSTRACT FROM AUTHOR]

Optical techniques for information security have drawn a lot of attention in recent years. The development of cryptanalysis strategies will also have a positive promotion for the cryptosystems themselves. However, most of the present publications on cryptanalysis of optical cryptosystems are focusing only on theoretical analysis but not on experimental verification. In this paper, a practical optical cryptanalysis method using wavefront shaping is proposed. Considering that the confusion and diffusion of information in the optical cryptosystem are actually caused by scattering effect of random phase mask(s), the proposed method employs a point source function and the feedback-based optimization algorithm, which can obtain an equivalent key of the optical cryptosystem. In such a way, the confusion and diffusion of the random phase mask(s) in the cryptosystem could be removed successfully. To the best of our knowledge, this is the first time the attack strategy from the perspective of wavefront shaping can be seen, and one of the biggest advantages is that it could get an equivalent key of the cryptosystem without deducing its original keys.

In this paper, a chaotic secure communication system based on single feedback phase modulation is designed, in which phase modulation of feedback light is carried out by using random square wave as communication key, and chaotic light with wide spectrum and no delay signature is generated. Message is modulated into chaotic carrier at transmitter end, through chaotic synchronization and comparison, original message is successfully demodulated at receiver end. In addition, two kinds of message encryption methods, chaos modulation and chaos masking, are studied by comparing synchronization and bit error rate under different channel signal-to-noise rate. [ABSTRACT FROM AUTHOR]

We develop a graded-index plastic optical fiber (GI POF) that can significantly stabilize a multimode fiber (MMF) link with a vertical-cavity surface-emitting laser (VCSEL) subject to external optical feedback. It is demonstrated that the dominant mechanism for this stabilization effect is the strong mode coupling of the GI POF, which is closely related to the polymer-specific microscopic heterogeneities in the GI POF core material. Such mode coupling decreases correlation between the problematic reflected light field and VCSEL cavity field, increasing tolerance of the MMF link for external optical feedback. Our developed GI POF with specific microscopic heterogeneities prevents the external-optical-feedback-induced critical destabilization observed for silica GI MMFs. These results suggest that the stabilization effect can be controlled by microscopic properties regardless of the fiber attenuations and core refractive indices. [ABSTRACT FROM AUTHOR]

We propose a method with high spatial resolution to detect fiber faults in a time-division multiplexing passive optical network (TDM-PON). A semiconductor laser serving as the probe light source is subjected to self-feedback caused by reflection at fiber faults. The feedback will induce the laser to generate chaos. Autocorrelation of the output time series shows the external cavity signature and, therefore, indicates the distance between the laser and the fiber fault. Each branch is identified by the marker that is formed by the inserted fiber Bragg grating (FBG) providing the feedback in each branch, and the faulty branch can be distinguished by the marker that has disappeared. Our proof-of-concept experiment demonstrates the identification of the faulty branch and the location of the fault point in an optical network simultaneously. This is achieved with a 6-km feeder fiber and realizes the 8-mm spatial resolution.

In this work, a novel optoelectronic oscillator (OEO) structure is proposed for generating widely tunable photonic microwave. In this structure, an optical injection semiconductor laser (OISL) functions as an active tunable microwave photonic filter (MPF). Through controlling the injection power and frequency detuning, the OISL is driven into period-one dynamics whose oscillation frequency can be widely tuned from 10.43 to 65.82 GHz. As a result, the OISL can be regarded as a widely tunable active MPF. By introducing an optical feedback loop, the filtered bandwidth of the widely tunable active MPF can be further reduced. Taking the OISL under optical feedback as a widely tunable narrow-bandwidth MPF and a seeding laser source to establish an OEO structure, widely tunable narrow-linewidth photonic microwave can be generated. The experimental results demonstrate that widely tunable photonic microwaves ranging from 10.43 to 39.10 GHz with linewidths below 0.1 MHz and phase variances below 10−2 (rad2) can be achieved. [ABSTRACT FROM AUTHOR]

An InP-based, monolithically integrated multiwavelength laser with optical feedback is presented. We demonstrate the performance of the dual-wavelength operation. Single-mode lasing is obtained for each of the two channels within a temperature range from 16 to $30$  °C and an injection current range from 30 to 70 mA without mode-hopping. This is due to the combined filtering functions of an arrayed waveguide grating and a distributed Bragg reflector. A method that makes use of weak optical feedback is demonstrated to improve the stability of each single-mode. With this method, sustained relaxation oscillations that are excited by on-chip parasitic reflections are damped and linewidths in both channels are narrowed to around 400 kHz compared to the linewidth of 1 MHz that is obtained when no feedback is applied. The suppression ratio of the relaxation-oscillation-induced side peaks is measured to be over 45 dB for two simultaneously operated channels. [ABSTRACT FROM AUTHOR]

A novel bidirection secure communication scheme based on the digital key and chaotic random optical feedback is proposed and numerically verified. Digital sequence is used as a key to control random variation of external cavity optical feedback, and under the operation of this random feedback, high-dimensional chaotic light is generated with complex dynamics. It is proved that chaotic time delay signature is effectively suppressed and key space increases significantly, which enhances the security of chaotic secure communication. Meanwhile, the bidirection chaotic secure communication is successfully implemented with the message rate of 10 Gb/s. In addition, simulation results show that the proposed scheme increases system synchronization and reduces the bit-error rate of the transmitted message. [ABSTRACT FROM AUTHOR]

In this paper, a novel electro-optic feedback phase chaotic system based on nonlinear coupling of two delayed interfering branches is proposed and analyzed by numerical simulation. The feedback loop of the proposed system contains two delay branches and a Mach–Zehnder modulator. The chaotic voltages generated from two branches are nonlinearly converted in order to reduce their temporal correlations. The dynamic characteristics and the complexity of the generated chaotic signal are evaluated by the bifurcation diagram and permutation entropy, respectively. Besides, the simulated results show that the time-delay signature can be concealed effectively with a lower feedback gain, meanwhile the key space is extended considerably since more tunable parameters are nonlinearly coupled, thus the security can be improved greatly. Furthermore, the communication system based on the proposed scheme is discussed, and the results demonstrate that this system can be applied in secure communication. [ABSTRACT FROM AUTHOR]

We report on self-injection locking in InGaN/GaN (blue/green) and InGaP/AlGaInP (red) visible-light laser diodes. The free-space optical feedback path was accomplished via an external mirror. The effect of injection current, optical power injection ratio, and external cavity length on the spectral linewidth and modulation bandwidth of the lasers is investigated. Our results show that the laser performance was substantially improved. In particular, we achieved a significant increase of ∼57% (1.53–2.41 GHz) and ∼31% (1.72–2.26 GHz) in the modulation bandwidth, and ∼9 (1.0–0.11 nm) and ∼9 (0.63–0.07 nm) times reduction in spectral linewidth of the green and blue lasers, respectively. Consequently, side-mode-suppression-ratio was considerably increased in all the cases, reaching as high as ∼20 dB in self-injection locked blue laser diode, thus enabling a close to single-mode operation. This paper paves the way for attaining high-speed optical wireless communications by overcoming the challenges of limited modulation bandwidth and multimode operation of visible laser diodes with this simple scheme. [ABSTRACT FROM AUTHOR]

In this paper, we describe a generalized visible light communication (VLC) channel, which consists of the frequency responses of the light emitting diode (LED), the wireless optical channel, and the photo detector (PD). Based on the theoretical analysis of the generalized VLC channel, we found the response mask invariant characteristic (RMIC) under the condition of strong line of sight components, which means the shape of frequency response of generalized VLC channel is time-independent and invariant at different distances of the transceivers, different emitting angles of the LED and different incident angles of the PD. Depending on the RMIC, the shapes of received spectrum and signal-to-noise rate are also invariant. The experimental platform was set up to verify the proposed RMIC. Furthermore, a predefined VLC system with a new channel feedback method is described, both the complexity and time cost of feedback can greatly reduce through the proposed channel feedback method. [ABSTRACT FROM AUTHOR]

Most recent achievements in passive mode-locking of quantum-dot lasers, with the main focus on jitter reduction and frequency tuning, are described. Different techniques, leading to record values for integrated jitter of 121 fs and a locking range of 342 MHz, are presented for a 40-GHz laser. Optical feedback is observed to be the method of choice in this field. For the first time, five different optical-feedback regimes are discovered, including the resonant one yielding a radio-frequency linewidth reduction by 99%.

We propose a simple and effective even-power fast algorithm to obtain scalable fringe precision in self-mixing interference. In this letter, the principle of the even-power fast algorithm is introduced in detail. The validity of the proposed algorithm is demonstrated by means of simulated signals and confirmed by experimental measurements at different vibration amplitudes. Without adding any additional optical elements in the optical path, theoretical analysis shows that the resolution of the fringe could be enhanced to ${{\lambda }}/{2^{{\text{n}} + 3}} \text(n\,= \,0,\,1,\,2,\,3\ldots). Experimental results show a fringe precision of \lambda /32 is achieved at a sampling rate of 50 KS/s. The influence factors are also discussed when the algorithm is applied in actual measurements. [ABSTRACT FROM PUBLISHER]

We present a single stack active region design for terahertz emission by difference frequency generation in quantum cascade lasers. The active region contains a single design, which is based on multiple optical transitions within one period. This results in both a giant nonlinearity and an ultra-broad optical gain. The provided optical gain spectrum is broad enough to support two distinct mid-infrared modes with a significant spectral separation. For dual-emission, the waveguide contains a buried, index-coupled distributed feedback grating. This grating is based on a sampled approach to provide selective feedback for two mid-infrared modes at λ1=8.39  μm and $\lambda _{{\text{2 }}}= {\text{9.38}}\, \mu {\text{m}} . Simultaneously, the manifold of possible transitions within the active region is designed to provide a peak nonlinear susceptibility of \vert \chi ^(2)\rm\vert \, \,= \,\, \text29 nm \textV^- 1 at the conversion to 3.8 THz. The device emits up to 210-μW THz power at room temperature with a nonlinear conversion efficiency of \eta =\text 2.05 mW \textW^- 2. [ABSTRACT FROM PUBLISHER]

We present an optically coupled chaotic system involving three-phase modulated electro-optic nonlinear loops and an optical coupler. The dynamical properties and the time delay signature (TDS) suppressing performance of the system is analyzed in detail. Numerical results show that the TDS can be suppressed not only under statistical analysis of a single output, but also under mutual statistical analysis of the multiple outputs. Compared with the intensity modulated electrically coupled scheme, the presented system has less interior noise due to the simpler construction. [ABSTRACT FROM PUBLISHER]

A novel microwave photonic filter (MPF) based on carrier-suppressed single sideband (CS-SSB) injected distributed feedback (DFB) laser is proposed. The proposed MPF system exploits the frequency selective gain feature of optical injected DFB lasers, which can be tuned by adjusting the optical injection parameters or the working conditions of the laser. Therefore, the proposed MPF system can be tunable. The proposed system is verified by a proof-of-concept experiment, and the results show that the passband of the proposed MPF can be tuned by tuning the injection ratio and bias current of the slave laser, where the tuning range of 9–39 GHz is realized. At the same time, the out-of-band rejection of the proposed MPF can reach to 32 dB. Moreover, a dual-passband MPF response is also realized by the proposed system with a couple of master lasers, and the two passbands can be tuned independently from 10 to 39 GHz. [ABSTRACT FROM PUBLISHER]

A new method by utilizing Brillouin backscattering of optical fiber to suppress the time delay signature (TDS) of an external cavity semiconductor laser is proposed and experimentally demonstrated. When the average power of the injected chaotic laser is within the range of 400–1500 mW, the TDS can be significantly suppressed with the G.655 fiber of the 0.5, 2, 4.5, or 6 km length. The G.652 fiber has the same TDS suppression effect with the G.655 fiber. The TDS suppression effect is relatively stable among a number of measurements ranging from 1 to 50. Moreover, the TDS suppression has nothing to do with the length of the chaotic series by employing the Brillouin backscattering method. [ABSTRACT FROM PUBLISHER]

We experimentally demonstrate a real-time high-speed physical random bit generator using the time-interleaved sampling of broadband white chaos. The white chaos is generated by optical heterodyning of two external-cavity feedback laser diodes. It has a white-noise-like spectrum with a 3-dB bandwidth of 16.7 GHz and a symmetric amplitude distribution and has no time-delay signatures of the external feedback cavities. An analog-to-digital converter (ADC) chip integrated by two time-interleaved sub-ADCs with a maximum sampling rate of 7 GHz is utilized to discretize the white chaos into parallel binary streams. After real-time interleaving combination and exclusive-OR operation in the field-programmable gate array (FPGA), a 14-Gbps binary stream with verified randomness is achieved. [ABSTRACT FROM PUBLISHER]

The concealment of time delay signature is one of the most critical issues in delayed-feedback optical chaotic systems. In this paper, we present a method to address this problem without significantly complicating the original architecture. By properly introducing a dispersion module into the feedback loop, a different frequency component of chaotic spectrum will be subject to different time delay in feedback loop, thus destroying the generation mechanism of time delay signature (TDS). The concealment performance is verified by both time-domain and spectrum analysis methods. The relation between the amplitude of TDS and cavity dispersion value is evaluated in detail. Since the dispersion module has no filtering effect, the chaotic bandwidth will not be affected during the TDS concealment process. This proposal solves the security problem of feedback chaotic systems without introducing much extra effort in implementation, and the underlying principle is not restricted to all-optical chaotic systems but applies for other optelectrical chaotic dynamics, which include a feedback cavity. [ABSTRACT FROM PUBLISHER]

Residual facet reflection can significantly degrade the performance of electroabsorption modulated lasers (EMLs) operating at high data rates. This issue also complicates the fabrication and characterization of the highly demanded light sources for optical interconnects and transmission. It is desired to optimize the device structure to make EMLs robust and immune to residual facet reflection. EMLs with a partially corrugated-grating DFB section are designed and optimized to have much improved tolerance to the residual optical reflection from the modulator output facet. By designing the laser section with an appropriate grating length and linear gain coefficient, the EML can have good tolerance to residual facet reflection. The analysis indicates that 100% yield can be obtained with the optimal design. If the EML needs to operate over a wide ranges of gain coefficient and facet reflection, >70% of yield can still be obtained. [ABSTRACT FROM PUBLISHER]

Optical techniques for information security have drawn a lot of attention in recent years. The development of cryptanalysis strategies will also have a positive promotion for the cryptosystems themselves. However, most of the present publications on cryptanalysis of optical cryptosystems are focusing only on theoretical analysis but not on experimental verification. In this paper, a practical optical cryptanalysis method using wavefront shaping is proposed. Considering that the confusion and diffusion of information in the optical cryptosystem are actually caused by scattering effect of random phase mask(s), the proposed method employs a point source function and the feedback-based optimization algorithm, which can obtain an equivalent key of the optical cryptosystem. In such a way, the confusion and diffusion of the random phase mask(s) in the cryptosystem could be removed successfully. To the best of our knowledge, this is the first time the attack strategy from the perspective of wavefront shaping can be seen, and one of the biggest advantages is that it could get an equivalent key of the cryptosystem without deducing its original keys. [ABSTRACT FROM PUBLISHER]

An optoelectronic feedback reservoir computing (RC) system that can identify optical packet headers for an optical packet switching (OPS) network is proposed and implemented by the numerical simulation and the experiment in this paper. First, a simulation system of optical packet header recognition based on the optoelectronic feedback RC is established through optimizing its parameters. It is shown that the optoelectronic feedback RC system can correctly identify optical packet headers with lengths from 3 bits to 32 bits and that the recognition word-error rate (WER) is 0. Then, an experiment system consistent with the optimal simulation system is set up, whose WER for the 3-bit header recognition is only 1.25%. The experimental results also indicate that the bias voltage of the Mach–Zehnder modulator (MZM) has a significant impact on recognition results. When the RC system is in the critical state between the multiple-period oscillation and the chaos, the maximal Lyapunov exponent is slightly larger than 0, and the recognition normalized root-mean-square error (NRMSE) reaches its minimal value. [ABSTRACT FROM PUBLISHER]

A 103.12-Gb/s wavelength-division-multiplexing (WDM) four-level pulse amplitude modulation (PAM4) transmission based on 850-nm and 880-nm vertical-cavity surface-emitting lasers (VCSELs) with light injection and optoelectronic feedback techniques is proposed and experimentally demonstrated. Results show that two such 7.5-GHz VCSELs with light injection and optoelectronic feedback techniques are potent for 103.12-Gb/s WDM PAM4 transmissions. To the authors’ knowledge, it is the first one to successfully adopt two VCSEL transmitters with light injection and optoelectronic feedback techniques in a WDM PAM4 transmission. A total transmission rate of 103.12 Gb/s (51.56 Gb/s/λ × 2 λs) is achieved in the proposed WDM PAM4 transmissions. The link performances of the proposed WDM PAM4 transmissions have been evaluated in real time. Good real-time bit error rate performance and three independent clear eye diagrams are obtained at a 180-m OM4 multimode fiber operation. Such a proposed 103.12-Gb/s WDM PAM4 VCSEL-based transmission has great potential for providing efficient bandwidth in short-reach optical communications. [ABSTRACT FROM PUBLISHER]

The time-delay (TD) signature of the chaotic signal is an essential factor for eavedroppers and attackers to reconstruct the secure communication system. Based on three-cascaded vertical-cavity surface-emitting lasers (VCSELs) for TD signature suppression, we propose a novel long-distance secure communication system. We analyze the influence of the feedback time (i.e., the TD signature) on the security of the proposed system, and suppress the TD signatures of the outputs of the VCSELs under proper parameters. Based on chaos synchronization with TD signature suppression, we exploit the chaos masking encryption scheme for long-distance chaos secure communication. When four 10 Gbit/s messages transmit in 20 km single-mode fiber channel or 132 km SMF channel with dispersion compensating fiber, all bit error rates (BERs) keep smaller than 10−9, and Q-factors are larger than 6, which illustrates that the proposed system effectively realizes long-distance chaos secure communication. [ABSTRACT FROM PUBLISHER]

We investigate experimentally the state bistability driven by current or feedback in a 1550 nm vertical-cavity surface-emitting laser (1550 nm-VCSEL) with negative optoelectronic feedback (NOEF). The results show that through continuously varying the controlled parameters such as the bias current or the feedback strength along different routes, under a set of given system parameters, there may exist two kinds of output states, i.e., state bistability, which are critically dependent on the history of how the controlled parameters are varied to reach the given values. Within the varied range of bias current or feedback strength, the state bistability is shown via the time series, power spectrum, and phase portrait of the output signal, and the physical mechanisms for the state bistability appearing in the NOEF 1550 nm-VCSEL are analyzed. Furthermore, the complexities of the corresponding states are evaluated by calculating the characteristic value of the normalized permutation entropy of the output from the NOEF 1550 nm-VCSEL. Additionally, by revealing the evolution of the dynamical states under the increase and decrease in the controlled parameters, the ranges of the parameters required for generating current-induced or feedback-induced state bistability can be specified to certain degree. [ABSTRACT FROM PUBLISHER]

Laser diodes are very promising solid-state devices for the development of high-luminance white light sources. Currently, the most simple and efficient way to produce white light with solid-state sources is to combine them with color-converting luminescent materials or phosphors. The quantum yield (i.e., the potential to convert light to another color) of these materials has complex dependence on temperature. In the case of high-power white light sources based on laser diodes, high temperatures are reached inside the phosphor material, which decrease its quantum yield (thermal quenching) and result in a nonstable light output. While this thermal quenching effect is well known, the tools used today to design and optimize optical systems do not include it in a realistic way. This work fills that gap by proposing an automated framework that simulates both the optical and thermal effects and the interplay between them. Additionally, an adapted version of the framework is also proposed, which is more computationally efficient. The resulting shorter simulation time is crucial for the efficient optimization of the optothermal performance of lighting systems, which requires several iterations of the framework for each optimization variable. The optothermal simulation framework has been applied to some optical designs to study them by changing certain optical and thermal properties and evaluating how these affect the overall performance. It is clearly demonstrated that even for optical designs that use LED sources, neglecting the thermal effects of luminescent materials can lead to an incorrect assessment of the system's capabilities and performance. [ABSTRACT FROM PUBLISHER]

This paper investigates the use of a dual wavelength laser source, a frequency shifter, an erbium-doped fiber amplifier, and an optical filter to compose a multiwavelength fiber laser (MWFL) with an ultranarrow wavelength spacing. The frequency shifter can be achieved by modulating an optical carrier and an electrical sinusoidal signal in an optical single-sideband modulation format. The detuning range of the frequency shifting can be controlled by the applied electrical sinusoidal signal, and an MWFL with ultranarrow wavelength spacing can be achieved. Experimental results prove that 144 stable optical carriers can be generated simultaneously with a 10-GHz wavelength spacing, > 15-dB optical signal-to-noise ratio, and < 0.83-dB maximum peak power variation, if a dual wavelength laser source is fed into the MWFL and the frequency of the electrical sinusoidal signal is set to 10 GHz. Results prove that the ability of the proposed scheme practically and efficiently assist the progress of optical systems. [ABSTRACT FROM PUBLISHER]

We demonstrate a thulium-doped fiber ring laser (TDFRL) integrated with an amplifier, which has a watt-level single-frequency output at 2 \mu\textm with a 120-nm wavelength tunability. The TDFRL is constructed based on a hybrid ring-linear resonator configuration. An unpumped TDF is incorporated as a self-constructed saturable absorption grating. A blazed grating aligned in a Littrow configuration is employed for broadband wavelength-selective feedback. The characteristics of the laser output mode are analyzed by using an all-fiber scanning ring resonator. [ABSTRACT FROM PUBLISHER]

A monolithically integrated dual electroabsorption modulated laser (DEML), consisting of distributed feedback laser (DFB) and electroabsorption modulator (EAM) sections, is proposed as a transmitter in an optical network unit with phase-shift keying (PSK) modulation. The DFB is directly modulated in phase, and the EAM section is applied for mitigating the residual amplitude modulation. The DEML, as compared with external phase modulator, has advantages of low cost, simplified design, low footprint, and reasonable power consumption, thus making it attractive for an ultradense wavelength division multiplexing passive optical unit. The system is validated at 2.5 and 5 Gb/s with intradyne detection. [ABSTRACT FROM PUBLISHER]

We investigate the optical nonreciprocity induced by the parametrical amplification and the radiation trapping in an atomic photonic bandgap system both theoretically and experimentally. We use two pumping fields to generate entangled photon pairs from spontaneous parametric four-wave mixing, as well as the signals in which they can be enhanced by the feedback dressing effect. Therefore, the frequency difference and the shape change due to this feedback dressing on the two-arm ramps of one round trip are observed. Such optical nonreciprocity can be easily controlled by multiple experimental parameters (frequency detuning, power, and phase) of the dressing beams. In addition, the enhancement and suppression switching induced by the double dressing or triple dressing conditions are also studied. This optical nonreciprocity can contribute to the development of quantum information processing and quantum communications. [ABSTRACT FROM PUBLISHER]

In this paper, we report a novel single-mode laser with high immunity to external optical feedback following the principle of active-filter-tuned oscillator. Within the conventional Fabry-Pérot (FP) cavity, a high-Q bandpass filter formed by two tilted gratings with detuned Bragg wavelengths is embedded, and the resultant mode selection mechanism can purify the lasing spectrum by eliminating all but one of the FP cavity modes near the peak wavelength of the said filter. The proposed laser inherently bears the distinct feature of being robust against any small disturbances to oscillation loop. Simulation results from the large-signal dynamic model show that a stable single-mode operation has been achieved with a side-mode suppression ratio of as high as 50 dB. Further, compared with conventional index-coupled distributed feedback lasers, the proposed laser exhibits a much higher immunity to external optical feedback, even at a very high feedback level of −3 dB, and its single-mode yield is also less susceptible to variations of effective facet phases caused by cleaving in semiconductor laser fabrications. [ABSTRACT FROM PUBLISHER]

We propose a method with high spatial resolution to detect fiber faults in a time-division multiplexing passive optical network (TDM-PON). A semiconductor laser serving as the probe light source is subjected to self-feedback caused by reflection at fiber faults. The feedback will induce the laser to generate chaos. Autocorrelation of the output time series shows the external cavity signature and, therefore, indicates the distance between the laser and the fiber fault. Each branch is identified by the marker that is formed by the inserted fiber Bragg grating (FBG) providing the feedback in each branch, and the faulty branch can be distinguished by the marker that has disappeared. Our proof-of-concept experiment demonstrates the identification of the faulty branch and the location of the fault point in an optical network simultaneously. This is achieved with a 6-km feeder fiber and realizes the 8-mm spatial resolution. [ABSTRACT FROM PUBLISHER]