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

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

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

Chaos 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 systematic treatment of the influence of the optical feedback (OFB) and the ratio of the radiative and the non-radiative recombination lifetime (τr/τnr) on the relative intensity noise (RIN) and the system dynamics of a semiconductor laser (SL) is presented. We found that the ratio τr/τnr causes a significant change in the intensity of noise, states, and route to chaos of such a system. Laser transit from a continuous wave (CW) to a periodic oscillation (PO) becomes faster in terms of the OFB strength by increasing the ratio τr/τnr. The route to chaos was identified in three distinct operating regions, namely, PO, period doubling (PD), and sub-harmonics (SH), which are dependent on the ratio τr/τnr, and injection current. In the route to chaos regime, the ratio τr/τnr triggers a slight shift in the frequency with reference to the frequency of the solitary laser. At upmost levels of the current, the highest value of the ratio τr/τnr stabilizes the laser and stimulates it to operate in CW or PO. In the strong OFB region, when the ratio τr/τnr increases, the chaotic operation changed to CW or PO operation and RIN is suppressed close to the quantum noise level. [ABSTRACT FROM AUTHOR]

A wavelength tunable chaotic external cavity semiconductor laser (ECSL) composed by a distributed-feedback laser diode (DFB-LD) and a broadband chirped fiber Bragg grating (CFBG), is proposed and experimentally demonstrated for generating tunable chaos signals with time delay signature (TDS) suppression. The experimental results show that by using a CFBG with a 3-dB reflection bandwidth of approximately 30 nm and a low dispersion of approximately 1.25 ps/nm, the chaotic ECSL can be tuned over a wide wavelength range, and the generated chaos possesses a low TDS of about 0.04. With the increase of the bias current of the DFB-LD, the chaotic ECSL is tuned towards long-wavelength direction, the effective bandwidth of the chaotic signal is extended, and the TDS of the chaotic signal remains effectively suppressed. To highlight the advantages of the proposed scheme, relevant experiments for the DFB-LD under fiber mirror feedback and fiber Bragg grating feedback are also performed and compared. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a novel multi-channel chaos generation scheme, which can simultaneously produce multiple chaotic signals with wideband spectrum and suppressed time-delay signature (TDS). In this scheme, we introduce an external self-phase-modulated feedback (ESPMF) to improve the optical bandwidth of the initial chaos generated by a conventional external-cavity semiconductor laser (ECSL), then optical tunable filters (OTFs) are utilized to simultaneously extract three-channel chaotic outputs. The experimental results show that the proposed scheme has three main advantages. Firstly, it can simultaneously generate multiple chaotic outputs with different central wavelengths and low correlations. Secondly, the bandwidth of original ECSL-based chaos can be improved by several times, and the undesired TDS characteristics in the original chaos can be completely suppressed to an indistinguishable level (lower than 0.02). Thirdly, the bandwidths of these chaotic outputs are flexible and can be adjusted within a wide range of 20 GHz by controlling the filter bandwidths. In addition, we demonstrate an application of the proposed chaos generation scheme in random bit generation (RBG), and multi-channel high-speed random bit sequences with a total generation rate over Tb/s and NIST verified randomness are simultaneously obtained. [ABSTRACT FROM AUTHOR]

A method of generating flat broadband chaos by using a semiconductor laser subject to asymmetric dual-path optical feedback is proposed and demonstrated experimentally. In the asymmetric dual-path optical feedback, one path is connected with an optical band-pass filter (OBPF) to form filtered path, and the other is a traditional optical feedback path named non-filtered path. Through the beating frequency effect between the filtered mode and the non-filtered chaos mode, a chaotic signal with the standard bandwidth of 36.1 GHz and the spectrum flatness of 5.8 dB is generated experimentally. Furthermore, the effects of filter frequency detuning, filter bandwidth and feedback strength on the bandwidth and flatness of chaos are investigated. The results show that the bandwidth enhancement and the flat spectrum can be achieved simultaneously in a wide range of operating parameters. [ABSTRACT FROM AUTHOR]

A systematic treatment of the influence of the optical feedback (OFB) and the ratio of the radiative and the non-radiative recombination lifetime (τr/τnr) on the relative intensity noise (RIN) and the system dynamics of a semiconductor laser (SL) is presented. We found that the ratio τr/τnr causes a significant change in the intensity of noise, states, and route to chaos of such a system. Laser transit from a continuous wave (CW) to a periodic oscillation (PO) becomes faster in terms of the OFB strength by increasing the ratio τr/τnr. The route to chaos was identified in three distinct operating regions, namely, PO, period doubling (PD), and sub-harmonics (SH), which are dependent on the ratio τr/τnr, and injection current. In the route to chaos regime, the ratio τr/τnr triggers a slight shift in the frequency with reference to the frequency of the solitary laser. At upmost levels of the current, the highest value of the ratio τr/τnr stabilizes the laser and stimulates it to operate in CW or PO. In the strong OFB region, when the ratio τr/τnr increases, the chaotic operation changed to CW or PO operation and RIN is suppressed close to the quantum noise level.

We propose and demonstrate a chaos generation scheme that conceals the time-delay signature (TDS) by injecting the output of an external-cavity semiconductor laser (ECSL) into a two-element phased array (ECSL-TPA). We consider the case of the ECSL output with obvious TDS, and evaluate the performance of the ECSL-TPA with four waveguide structures in TDS suppression. The numerical results demonstrate that the ECSL-TPA allows for desired TDS suppression in wide parameter space regardless of the waveguide structures considered. However, different TDS behavior exists for these structures, which provides guidance for future design and optimization of the proposed ECSL-TPA. Additionally, the influence of some key parameters including the injection rate, frequency detuning, laser separation ratio, pump rate, linewidth enhancement factor, and frequency offset on TDS is systematically studied. Appropriate settings of these key parameters are beneficial for TDS suppression and thus optimizing optical chaos. The phased array can be extended to include a large number of elements and thus generates multiple independent chaotic signals with indistinguishable TDS in parallel, paying the way for chaos-based applications that require independent, high-quality chaos simultaneously. [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]

We describe a novel chaotic macropulse green laser designed employing the pulse modulation of a 520 nm laser diode with optical feedback. We analyze the characteristics of these chaotic macropulses, in view of effects of the feedback intensity, injection current, modulation voltage, duty cycle, and modulation frequency on the pulses, including their full width at half maximum (FWHM) values and peak side-lobe levels (PSLs). We determine an optimum feedback intensity of 11.7% based on the maximum peak optical power and the scope of the amplitude jitter of the chaotic macropulse. The optimum injection currents are approximately between 60 and 70 mA, and the modulation voltages between 2.0 and 3.0 V are limited by the radio-frequency (RF) power. We found that chaos also occurred at the bottom of the macropulse when the duty cycle was less than approximately 30%. The modulation frequencies were loaded from 4 to 20 MHz; this range was restricted by the performance of the RF input port on the laser diode mount. The range resolution of the chaotic macropulses is approximately 3 cm in seawater; it is achieved with lower power consumption than a continuous wave chaotic laser. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate high-speed correlated random bit generation in real time using synchronized chaotic lasers commonly driven by a laser with dispersive feedback. The dispersive feedback from a chirped fiber Bragg grating induces frequency-dependent feedback delay and thus no longer causes time-delay signature, and resultantly ensures the signal randomness and security of chaotic laser. Driven by the time-delay signature-free chaotic signal, the two response lasers are routed into chaotic states and establish a synchronization with correlation beyond 0.97 while they maintain a low correlation level with the drive signal. Through quantizing the synchronized laser chaos with a one-bit differential comparator, real-time 2.5-Gb/s correlated random bits with verified randomness are experimentally obtained with a bit error ratio of 0.07. Combining with a robust sampling method, the BER could be further decreased to $1\times 10^{-4}$ corresponding to an effective generation rate of 1.7 Gb/s. Bit error analysis indicates that the bit error ratio between the responses is lower than that between the drive and responses over a wide parameter region due to the synchronization superiority of the responses over the drive. [ABSTRACT FROM AUTHOR]

We propose a scheme for reservoir computing using multiple semiconductor lasers with optical feedback arranged in parallel on a photonic integrated circuit, and we investigate the performance of reservoir computing numerically. The virtual nodes are obtained from the temporal waveforms of the outputs of the parallel reservoir lasers. We test the chaotic time-series prediction task, memory capacity, and nonlinear channel equalization task to investigate the performance of reservoir computing. We found that our scheme using multiple lasers outperforms that using a single laser with multiple delay times. Large memory capacity can also be obtained for the multiple lasers. Finally, we investigate the effect of parameter mismatch of the multiple lasers on reservoir computing performance. [ABSTRACT FROM AUTHOR]

The dynamical behavior of semiconductor laser in the present of optical feedback is studied by varying number of control parameters that appeared in the adopted theoretical model viz, linewidth enhancement factor , pump level , delay time and feedback level . Various dynamics seen to occur such as steady state, periodic, aperiodic and chaotic in the output of the semiconductor laser.

Featured Application: This technique of improving the accuracy of g(2)(τ) measurement is useful to extract higher order coherence and achieve desired laser source for quantum imaging and secure communication. The second order photon correlation g(2)(τ) of a chaotic optical-feedback semiconductor laser is precisely measured using a Hanbury Brown–Twiss interferometer. The accurate g(2)(τ) with non-zero delay time is obtained experimentally from the photon pair time interval distribution through a ninth-order self-convolution correction. The experimental results agree well with the theoretical analysis. The relative error of g(2)(τ) is no more than 5‰ within 50 ns delay time. The bunching effect and coherence time of the chaotic laser are measured via the precise photon correlation technique. This technique provides a new tool to improve the accuracy of g(2)(τ) measurement and boost applications of quantum statistics and correlation. [ABSTRACT FROM AUTHOR]

Consistency is a powerful concept to characterize and classify the behavior of driven dynamical systems, therefore playing an essential role for the understanding of information processing in and by complex photonic systems. Here, we discuss two different experimental configurations of semiconductor laser systems with delayed optical feedback allowing for a unifying view via the detailed characterization and comparison of consistency and synchronization properties. The first configuration comprises two unidirectionally coupled semiconductor lasers, which has found considerable interest for applications in encrypted communication. The second configuration consists of a single laser with two optical delay loops of significantly different lengths, designed such that only one loop is active at a time and that allows to study replays of injected optical signals. We show that, in terms of three characteristic correlation measures—consistency correlation, transformation correlation, and spurious correlation—the dynamical features of the two different experiments are very similar and how the two configurations are connected. Our approach shows that consistency allows to understand and relate various chaos synchronization and stability properties. We particularly discuss the experimental results in the context of a previously established analytical framework, providing a common dynamical interpretation of consistency and chaos synchronization in delay-driven laser systems. [ABSTRACT FROM AUTHOR]

The statistical properties of chaotic signals generated from semiconductor lasers are important in different applications such as secure communications and random bit generation. While the laser dynamics determines the intrinsic statistics of the emission, the process of detection can significantly affect the statistics of the output signals. Using the rate-equation model, the role of detection on the statistical properties of the output signal is systematically investigated for a chaotic laser subject to optical injection. The chaotic emission is converted into electrical outputs using different detection schemes. The statistics of the output signals are evaluated through the probability density functions (PDFs). By low-passed direct detection, the PDF becomes nearly Gaussian when the cutoff frequency of the filter is lower than the relaxation resonance frequency of the laser. Alternatively, the PDF becomes nearly Gaussian when strong frequency components are avoided in band-passed direct detection, optical heterodyne detection, or electrical heterodyne detection. Additionally, through reconstructing the detected state space, the divergence of neighboring states is quantified by the time-dependent exponent (TDE). Gaussian PDFs are yielded using the abovementioned detection schemes without much affecting the TDE slopes. The results indicate the importance of detection on the statistics associated with chaotic lasers in random signal generation. [ABSTRACT FROM AUTHOR]

We propose a wideband chaos generation scheme that shows significant bandwidth enhancement and excellent time delay signature (TDS) suppression, by introducing self-phase modulation in the feedback loop of external-cavity semiconductor laser (ECSL) and passing the ECSL-generated chaos through a dispersive component. Proof-of-concept experiment and thorough complementary numerical simulations are carried out to study the efficient bandwidth and TDS characteristics of the generated chaos. The results demonstrate that with the joint contributions of self-phase modulation and dispersion, the efficient bandwidth of chaos can be enhanced by several times and the spectrum gets much flatter. Simultaneously, the TDS can be completely suppressed, in a wide operation parameter range. The proposed wideband chaos generation scheme shows great potential for the applications of fast random bit generation, high-resolution chaotic radar, and high-speed chaos communication. [ABSTRACT FROM AUTHOR]

We show numerically that dual-mode continues wave optical injection into a Fabry–Pérot laser diode subject to optical feedback generates a chaos bandwidth, which is four to six times that without optical injection. As a comparison, it is shown that single-mode optical injection can increase the chaos bandwidth threefold with careful choice of detuning. Furthermore, any combination of dual-mode injection will enhance the chaos bandwidth and especially for strong injection and over relatively large positive detuning ranges. Even, when the bias current of the Fabry–Pérot laser diode is relatively low, the bandwidth of chaos can reach 35 GHz after dual-mode optical injection, which is six times than that of without optical injection. The enhanced bandwidth with dual-beam optical injection is in accordance with our previous experiments. The present simulations provide detailed guidance on the choice of experimentally accessible parameters to generate chaotic signals with enhanced bandwidth by using single or dual-beam optical injection into a Fabry–Pérot laser diode with optical feedback. [ABSTRACT FROM AUTHOR]

The time delay signature (TDS) concealment properties of a novel semiconductor nanolaser subject to double chaotic optical injections are investigated numerically. The TDS concealment is evaluated by the peak size of the autocorrelation function (ACF). The effects of feedback rate, injection rate, frequency detuning, Purcell cavity-enhanced spontaneous emission factor, and spontaneous emission coupling factor on the TDS concealment are carefully examined. It is found that, with the double chaotic optical injections, the TDS can be concealed in wide ranges of feedback rate, injection rate, and frequency detuning. Moreover, better TDS concealment can be achieved for a smaller frequency detuning and a larger injection rate. The 2-D maps of peak size of ACF further show that the range of parameter space contributing to better TDS concealment is wider for a moderate Purcell cavity-enhanced spontaneous emission factor and a smaller spontaneous emission coupling factor. These results may be interesting for the enhanced chaotic sources on-a-chip based on novel semiconductor nanolasers. [ABSTRACT FROM AUTHOR]

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

This paper reviews experimental investigations of the route to chaos of a semiconductor laser subjected to optical feedback from a distant reflector. When the laser is biased close to threshold, as the feedback strength is increased, an alternation between stable continuous wave (CW) behavior and irregular, chaotic fluctuations, involving numerous external-cavity modes, is observed. CW operation occurs on an external-cavity mode whose optical frequency is significantly lower than that of the solitary laser. The scenario is significantly different for larger currents as the feedback level is increased. At low feedback, the laser displays periodic or quasiperiodic behavior, mostly around external-cavity modes whose frequency is slightly larger than that of the solitary laser. As the feedback level increases, the RF and optical frequencies involved progressively lock until complete locking is achieved in a mixed external-cavity mode state. In this regime, the optical intensity and voltage oscillate at a frequency that is also equal to the optical frequency spacing between the modes participating in the dynamics. For even higher feedback, the locking cannot be maintained and the laser displays fully developed coherence collapse.

An electrooptic chaos source is proposed based on phase-modulation-to-intensity-modulation conversion theory and an analog–digital hybrid time-delay feedback loop. The analog part takes the digital binary sequences from shift registers as input and converts them into an analog noise-like signal, from which new bits are determined. The dynamical characteristic of the generated waveform is studied in detail by means of spectrum analysis, periodicity analysis, correlation performance, and complexity analysis. Moreover, the evolution between chaos and stochastic noise is observed and measured by adopting the complexity–entropy curves when a random perturbation is introduced. Such a perturbation can help with recovering chaotic dynamics from degradation. Based on this chaotic source, a secure and robust synchronization scheme that can exactly reproduce the signal is put forward. The effect of parameters’ mismatch and noise level on the synchronization quality is also investigated. The scheme has the potential to be applied in various applications when a robust random source is needed. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the possibility of using optical injection to efficiently suppress the time-delay (TD) signatures of chaotic signals in a large experimentally accessible parameter range of semiconductor ring lasers (SRLs).We also study how this optical injection can improve the signal bandwidths. The injection signal is obtained from a master SRL with either optical self- or cross-feedback. For optical selffeedback configurations, it is found that the suppression of TD signatures is similar to what has been found in injected Fabry-Perot semiconductor lasers, i.e., narrow range of parameters with respect to the detuning and injection strengths. For cross-feedback configurations, however, the TD signatures can be suppressed in a wide range of parameters; meanwhile, the bandwidths are significantly improved for the same range of parameters. This is particularly useful for the security in chaos-based communications and also for generating random bits with improved properties. [ABSTRACT FROM AUTHOR]

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

The second order photon correlation g(2)(τ) of a chaotic optical-feedback semiconductor laser is precisely measured using a Hanbury Brown−Twiss interferometer. The accurate g(2)(τ) with non-zero delay time is obtained experimentally from the photon pair time interval distribution through a ninth-order self-convolution correction. The experimental results agree well with the theoretical analysis. The relative error of g(2)(τ) is no more than 5‱ within 50 ns delay time. The bunching effect and coherence time of the chaotic laser are measured via the precise photon correlation technique. This technique provides a new tool to improve the accuracy of g(2)(τ) measurement and boost applications of quantum statistics and correlation.

We propose a method to generate physical random numbers based on intermittent optical chaos. Intermittent chaotic output is produced in a semiconductor laser subjected to optical feedback embedded in a photonic integrated circuit. This dynamics is characterized by a temporal waveform organized in a succession of laminar regions of low amplitude and bursts of high amplitude. The temporal randomness ruling the alternation of successive laminar regions and bursts is used as an entropy source to generate sequences of random bits. We compare the performances of the exclusive OR and reverse methods implemented in the bit generation process and evaluate the quality of the random bits with the Rabbit test of TestU01 for different bit sequences lengths. [ABSTRACT FROM PUBLISHER]

The effect of bias current on the complexity and time-delay signature of chaotic signals in semiconductor lasers with polarization preserved optical feedback has been studied experimentally and theoretically. The peak value of the autocorrelation coefficient and the normalized permutation entropy at the feedback round trip time are used to quantify the time delay signature and complexity, respectively. The results show that the time-delay signature is approximately in an inverse relationship with the complexity of chaos when the semiconductor laser is subject to low or strong optical feedback. However, the inverse relationship disappears when the laser operates at higher bias currents with intermediate feedback strength. The simulation results are qualitatively agreed with the experimental results. [ABSTRACT FROM PUBLISHER]

A simple design of a hybrid integrated short-external-cavity chaotic semiconductor laser module is presented and fabricated. A distributed feedback laser chip is directed to the transflective mirror through a collimating micro-lens, and a part of the light is reflected by the mirror into the laser chip to induce chaos. Transmitted through the mirror, the generated chaotic light is coupled into a pigtail fiber as output by a focusing micro-lens. It should be pointed out that the part between the transflective mirror and the emitting surface of the chip works as a short external straight feedback cavity. All the components above together with chip submount, heat sink, and thermoelectric cooler are encapsulated by a commercial 14-pin butterfly package. In our experimental fabrication, the intensity reflectivity of the transflective mirror is optimized as 5% and the external-cavity length is 2 mm. Experimental tests show that chaotic light with a bandwidth larger than 4.5 GHz is readily obtained from the chaotic laser module by adjusting laser bias current. [ABSTRACT FROM PUBLISHER]

Laser chaos is promising for constructing high range-resolution and anti-jamming radars, whereas the range resolution and anti-jamming capability are unfortunately limited by laser relaxation oscillation and external-cavity resonance, respectively. Here, we theoretically propose using a broadband white chaos generated by optical heterodyne of two chaotic external-cavity semiconductor lasers as source signal to construct a radar system with enhanced range resolution and anti-jamming capability. The white chaos not only has a white-noise-like wide spectrum without relaxation oscillation, but also has no signature of external-cavity resonance. Benefitting from these merits, the proposed white-chaos radar shows a marvelous unambiguous detection performance with subcentimeter range resolution. Moreover, the elimination of external-cavity resonance enables the randomness improvement of source signal, and resultantly, the anti-jamming capability is enhanced as well. [ABSTRACT FROM PUBLISHER]

The important role of parameters in master laser with optical feedback for the elimination of time-delay (TD) signature in semiconductor laser subject to chaotic optical injection is investigated systemically. The experimental results show that TD signature suppressed chaotic signals can be credibly generated by increasing the feedback strength of the master laser, which is quite different from the trends observed in semiconductor laser (SL) with optical feedback. Systematically numerical analysis is also carried out as a validation, and it is shown that with low bias current and strong feedback strength, parameter regions contributing to successful TD suppression are much wider. Furthermore, it is shown that the influence of frequency detuning in TD concealment will change with the increase of feedback strength. All the numerical results are in perfect accordance with experimental observation. [ABSTRACT FROM AUTHOR]

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]

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 new secure key distribution scheme based on the dynamic chaos synchronization of two cascaded semiconductor laser systems (CSLSs) subject to common chaotic injection and random phase-modulated optical feedback is demonstrated. In this scheme, Alice and Bob adopt two independent random sequences to control the phase modulators of CSLSs, which induces a dynamic perturbation to the chaos synchronization. We thoroughly investigate the chaos synchronization performance under different phase-shift conditions with cross-correlation function, and systematically discuss the feasibility and security of the system. The results show that, with proper injection and feedback strength, the correlation coefficient gap between phase shift match and mismatch is clear and robust to the parameter mismatches in the CSLSs and those between the two CSLSs. Based on this, high-quality key distribution can be performed by picking out the identical random bits from the two independent random sequences according to the computational correlation. Moreover, the investigations on the information theoretic security and rate of the key distribution show that the security of the key distribution scheme can be further enhanced by properly increasing the number of layers in the CSLSs or employing high-order modulation format. [ABSTRACT FROM PUBLISHER]

A novel symmetric image encryption-then-transmission system based on optical chaos using semiconductor lasers is proposed. In this paper, with identical chaotic injection from a master laser, two slave lasers (SL1 and SL2) can output similar chaotic signals served as chaotic carrier to transmit image. Meanwhile, the chaotic signal of SL1 can be used to generate the key of the encryption scheme. After employing the three-dimensional (3D) cat map and logistic chaotic map, the positions of image pixels are shuffled, and the relationship between the cipher-image and the plain-image is confused simultaneously. Therefore, the system can resist the common attacks such as statistical attack, differential attack, and brute force attack. Through numerical simulations, the high quality chaos synchronization between SL1 and SL2 is obtained. When the chaos masking technique is adopted, the image encrypted by the proposed encryption scheme can be successfully transmitted and decrypted in a 10 km single mode fiber channel from SL1 to SL2, which is accompanied by a bit error rate of less than \rm6.18\,\times \,10^- 19. Exhaustive tests about security analysis are carried out, demonstrating the valuable feasibility and high security of the image encryption-then-transmission system. [ABSTRACT FROM PUBLISHER]

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]

The time-delay signature and the chaos bandwidth in three-cascaded vertical-cavity surface-emitting lasers have been investigated experimentally. A peak value of autocorrelation coefficient at the feedback round trip time and the ratio between this peak value and its background are used to quantitatively identify the time-delay (TD) signature of chaos. A new concept—peak to side-peak ratio is introduced for better quantification of the TD signature concealment. The peak to side-peak ratio is defined as the ratio between the peak value of autocorrelation coefficient at the TD and the peak value at a delay time other than the TD and zero delay time. Three injection cases, namely, small bandwidth, intermediate bandwidth, and wide bandwidth of the injecting chaos signals, have been used to study the effect of the bandwidth of the injecting chaos on the TD concealment. The experimental results show that the time-delay signature can be totally concealed in the slave laser subject to the intermediate bandwidth of chaotic optical injection over a wide frequency detuning range. [ABSTRACT FROM AUTHOR]

In this paper, an all-optical reservoir computing (RC) setup is proposed for identifying the types of optical packet headers in optical packet switching (OPS) network. The numerical simulation identification results of 3 bits and 32 bits optical headers with the bit rate of 10 Gbps are as low as 0.625% and 2.25%, respectively. The identification errors with the variation of the feedback strength and feedback delay are presented separately. Hence, the optimal feedback parameters are obtained. The all-optical feedback RC setup is robust to the white Gaussian noise. The recognition error is acceptable when the signal-to-noise ratio (SNR) is greater than 15 dB. [ABSTRACT FROM AUTHOR]

The effects of gain nonlinearity on the time delay signature (TDS) of both intensity and phase chaos in semiconductor lasers (SLs) with optical feedback are investigated numerically. The SLs subject to single-path feedback are mainly considered, and SLs with dual-path feedback are also discussed for the sake of generality. The TDS is evaluated via both auto-correlation function (ACF) and permutation entropy function. It is found that the TDS is more sensitive to nonlinear gain factor (NGF) for large injection current and low feedback strength. For given feedback strength, the peak heights of ACF for both intensity and phase chaos around TDS can be decreased by using SL with low NGF, especially when the injection current is relative high. Furthermore, with low NGF, successful TDS concealment can be achieved in wider region in the parameter planes of feedback strength and injection current. Hence, careful selection of an internal parameter is an effective candidate for TDS concealment in wider parameter region, which is complementary to the existing techniques by controlling the external parameters, and thus is highly desirable for expanding the key space for chaos-based secure communication. [ABSTRACT FROM AUTHOR]

We investigate experimentally and numerically the existence of fast chaotic spiking in the dynamics of a semiconductor laser with ac-coupled optical feedback. The observed dynamics is chaotic in the explored range of both bias current optical feedback strength. The effects of a modulation applied to the bias current are also investigated. We eventually indicate that the observed chaotic dynamics is a good candidate to hide information to satisfy secure communications. [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. [ABSTRACT FROM PUBLISHER]

We propose and demonstrate a novel broadband chaos generator using monolithic-integrated dual-mode amplified feedback laser (AFL) with an optical feedback. An external fiber loop is used to drive a free-running dual-mode AFL into a chaos state. The intercoupling between the two chaotic oscillation originated from the two laser modes produces a broadband chaos signal with a flat RF spectrum. A robust chaotic signal that covers an RF frequency of over 50 GHz, with a bandwidth of 32 GHz, an effective bandwidth of 28 GHz, and a flatness of 6.3 dB, is experimentally achieved. [ABSTRACT FROM PUBLISHER]

Influences of external optical injection on the nonlinear dynamics of a three-section monolithically integrated semiconductor laser (MISL) are investigated experimentally. The results show that, for the solitary three-section MISL, diversely dynamical states including the stable state as well as the so-called period-one, period-two, multi-period, and chaotic states can be observed through adjusting the currents of the gain section ( IG ) and the phase section ( IP ). However, the chaotic operation region of the solitary MISL in the parameter space of IG and IP is very small and found to exist when 21.28~\textrm mA< IG< 26.40 mA and 31~\textrm mA

We numerically investigate the complex and the broadband characteristics of semiconductor ring laser (SRL) with optical feedback. The results show that the SRL with optical feedback is brought into chaotic behaviors with proper parameters and the dynamic behavior of which is complex. The bifurcation diagrams of chaos generation processes and detailed states with different parameters are demonstrated. The bandwidth of the chaos is also studied in this paper and the widest bandwidth is about 18 GHz when the feedback coefficient is 0.5 which is wider than the traditional edge-emitting lasers with optical feedback. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel dual-channel optical chaos system with a time-delay (TD) feature simultaneously suppressed in all observables, i.e., in both intensity and phase. A hybrid optical and electro-optic feedback for a single vertical-cavity surface-emitting laser (VCSEL) is verified to induce simultaneous TD suppression for the polarization-resolved components of the chaotic output. A comprehensive mathematical model is developed to incorporate the optical and electro-optic time delays into the rate equations of the VCSEL. The suppression of TD signature is then examined by means of autocorrelation function and delayed mutual information. The results show that the output chaotic signal has the TD feature well eliminated in both the intensity and phase over certain regions of parameter space identified using the peak signal to mean ratio technique. The independent evolution of the two orthogonal VCSEL modes renders the polarization-resolved output modes appropriate for the enhanced dual-channel chaos applications. To the best of our knowledge, this is the first time that a dual-channel chaos communication system is reported with simultaneous suppression of the TD feature in all the transmitted observables. [ABSTRACT FROM PUBLISHER]

Low-cost vertical-cavity surface-emitting laser (VCSEL) and semiconductor optical amplifier (SOA) have been explored experimentally in a setup with a fiber ring resonator to generate flat broadband chaos. The effect of the frequency detuning between the fiber grating frequency and the freerunning VCSEL's frequency, and the bias current of the SOA on the bandwidth of chaos has been studied. The results show that the bandwidth of the chaos is strongly dependent on the frequency detuning and the bias current of the SOA. Using the conventional definition of chaos bandwidth, the bandwidth of chaos with the fiber ring resonator can be about three times greater than that with a conventional mirror. In order to consider the flatness of the chaos spectra, the flat bandwidth of chaos is redefined as the frequency range between dc and the frequency where the power fluctuation is within ±3 dB. With this definition, the flat bandwidth of chaos with the fiber ring resonator is >10 times higher than that with conventional mirror feedback. [ABSTRACT FROM AUTHOR]