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We propose and experimentally demonstrate an approach for generating a wideband optical frequency comb (OFC) featuring multiple comb lines and wavelength tunability based on a gain-switched weak-resonant-cavity Fabry–Perot laser diode (WRC-FPLD) under multi-wavelength optical injection. The longitudinal mode interval of the utilized WRC-FPLD is about 0.28 nm (35.0 GHz), and its relaxation oscillation frequency is about 2.0 GHz at 1.15 times the threshold current. Under current modulation with a power of 20.00 dBm and a frequency of 2.0 GHz, the WRC-FPLD is driven into the gain-switched state. By further introducing multi-wavelength injection light (MWIL) containing four power equalization comb lines with an interval of 0.56 nm, a wideband OFC featuring multiple comb lines and wavelength tenability can be obtained. The experimental results demonstrate that by gradually increasing the injection’s optical power, the number of produced OFC lines initially increases and then decreases. By meticulously adjusting the wavelengths of the MWIL and carefully selecting the matched injection power, the broadband OFC can be tuned across an extensive spectral range. Under optimized operation parameters, an OFC with 147 lines, and a bandwidth of approximately 292 GHz within a 10 dB amplitude, variation is achieved. In this case, the measured single-sideband phase noise at the fundamental frequency is about −115 dBc/Hz @ 10 kHz, indicating that the comb lines possess good stability and strong coherence.

We experimentally demonstrate a scheme for generating massively parallel and real-time physical random bits (PRBs) by using weak-resonant-cavity Fabry-Perot laser diodes (WRC-FPLDs) with optical feedback. By using external optical feedback to modify the nonlinear dynamic behavior of the longitudinal modes in WRC-FPLDs, the chaotic behavior of each channel can be induced under suitable feedback strength. By filtering these longitudinal modes, a real-time PRBs at 10 Gbits/s can be generated by using field programmable gate array (FPGA) board for the real-time post-processing of a single-channel chaotic signal. Considering the presence of up to 70 longitudinal modes within a broad spectral range exceeding 40 nm, each of these modes can be used to extract chaotic time sequences for random number generation. Therefore, our PRB generation scheme has the potential to achieve a data throughput of over 700 Gbits/s.

Abstract Ultrafast physical random bit (PRB) generators and integrated schemes have proven to be valuable in a broad range of scientific and technological applications. In this study, we experimentally demonstrated a PRB scheme with a chaotic microcomb using a chip-scale integrated resonator. A microcomb contained hundreds of chaotic channels, and each comb tooth functioned as an entropy source for the PRB. First, a 12 Gbits/s PRB signal was obtained for each tooth channel with proper post-processing and passed the NIST Special Publication 800-22 statistical tests. The chaotic microcomb covered a wavelength range from 1430 to 1675 nm with a free spectral range (FSR) of 100 GHz. Consequently, the combined random bit sequence could achieve an ultra-high rate of about 4 Tbits/s (12 Gbits/s × 294 = 3.528 Tbits/s), with 294 teeth in the experimental microcomb. Additionally, denser microcombs were experimentally realized using an integrated resonator with 33.6 GHz FSR. A total of 805 chaotic comb teeth were observed and covered the wavelength range from 1430 to 1670 nm. In each tooth channel, 12 Gbits/s random sequences was generated, which passed the NIST test. Consequently, the total rate of the PRB was approximately 10 Tbits/s (12 Gbits/s × 805 = 9.66 Tbits/s). These results could offer potential chip solutions of Pbits/s PRB with the features of low cost and a high degree of parallelism. Graphical Abstract

This work proposes and experimentally demonstrates a high-performance polarization space parity-time (PT) symmetric fiber ring laser to achieve a low-noise, narrow-linewidth, and highly stable single-longitudinal-mode output. The gain/loss and coupling coefficients are regulated by adjusting a polarization controller (PC) and the pumping current of an erbium-doped fiber amplifier (EDFA) within the ring cavity. The results show that the single longitudinal mode oscillation of the laser can be implemented by PT symmetry breaking. The frequency noise spectral density and the linewidth characteristics of the laser are evaluated by the short-delay self-heterodyne method. The results reveal that excellent low-frequency noise (181 Hz2/Hz at a 10 kHz offset frequency) and narrow fundamental linewidth (68 Hz) can be achieved. Additionally, the laser exhibits outstanding stability with only 0.64 pm wavelength drift over 30 min. By tuning an optical tunable filter (OTF), the wavelength tunable range of the laser can cover the entire C-band. Furthermore, the impacts of different fiber length on the frequency noise spectral density and the filter bandwidth on stability are analyzed, offering guidance for component selection in such laser systems.

A stable and tunable parity-time (PT) symmetric single-longitudinal-mode fiber laser using a nonreciprocal Sagnac loop is proposed, which is experimentally and numerically demonstrated. The nonreciprocal Sagnac loop only including a 3-dB optical coupler (OC) and a polarization controller (PC) is incorporated into a fiber ring cavity, in which the nonreciprocal light transmission between frequency-degenerate clockwise (CW) and counterclockwise (CCW) resonator modes is induced to suppress multiple-longitudinal-mode oscillation of the laser. The two light paths along the CW and CCW directions in the Sagnac loop are respectively defined as the gain and loss loops of the PT-symmetric laser, due to a controllable birefringence induced by the PC. By controlling the polarization states of the two light waves, the PT symmetry is broken when the gain coefficient is larger than the coupling coefficient, single-mode lasing is thus achieved. Experimental results show that the optical signal to noise ratio (OSNR) of the single-mode lasing at 1550 nm is 43.0 dB, proving the great superiority of PT symmetry for lasing mode selection. By tuning an optical tunable band-pass filter incorporated into the fiber cavity, the wavelength of single-mode lasing varies from 1530 to 1560 nm, and their 3-dB Lorentzian linewidth varies within a range from 529 to 687 Hz. During a 30-min observation period, the variations of wavelength drift and OSNR of the single-mode lasing are less than 6 pm and 2.42 dB, respectively. The advantages of the proposed scheme are obvious, which include simple structure, low cost, simple operation, and good stability.

We propose and experimentally investigate a symmetric parity-time (PT) coupled optoelectronic oscillator (COEO) based on a polarization-dependent spatial structure. In such a COEO system, the gain/loss and coupling coefficients of two orthogonal polarization optical waves can be controlled by adjusting the polarization controller (PC) and the bias voltage of a Mach-Zehnder modulator (MZM). The single-mode selection of a microwave signal can be implemented by the PT symmetry breaking of a special mode. The performance of the proposed COEO is experimentally examined, and a 10.0 GHz microwave signal with a phase noise of −109.1 dBc/Hz @ 10 kHz and a side mode suppression ratio of 51.4 dB is generated. Moreover, an optical frequency comb with a comb tooth spacing of 10.0 GHz and a bandwidth of 100 GHz within a 10 dB amplitude variation can be simultaneously generated.

A simple schemefor generating chaotic signals with concealed time-delay signature (TDS) is proposed and experimentally demonstrated. The architecture of the system is based on a semiconductor laser (SL) under multi-path optical feedback (MPOF) provided by a 2 × 2 fiber coupler (FC) and a fiber mirror (FM). The results show that SLs with MPOF are more beneficial for achieving TDS suppressed chaotic output than those with single optical feedback (SOF) systems. In addition, the TDS of the chaotic signal generated by the SL under MPOF is insensitive to the length deviation of the feedback cavities, and therefore such a scheme is convenient for practical application. Furthermore, the influence of FC coupling ratio in the MPOF module, feedback strength, and bias current of the SL on the TDS is comprehensively studied. By selecting suitable parameters, the TDS of the chaotic signal generated by the SL under MPOF can be completely concealed. Finally, permutation entropy (PE) is adopted to evaluate the complexity of the generated chaotic signal, and the result demonstrates that the suppression of TDS does not lead to the degradation of complexity for the proposed scheme.

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

In this paper, we experimentally investigated the mode configuration of an excited-state quantum dot laser (ESQDL) under concave mirror optical feedback, and the influences of the feedback strength on the mode characteristics were analyzed. The results showed that after introducing concave mirror optical feedback, some longitudinal modes of the excited-state (ES) existing in a free-running ESQDL could be suppressed. When the feedback strength increased to a certain extent, the ground-state (GS) emission occurred and co-existed with the ES emission. By further increasing the feedback strength, all the longitudinal modes of the ES emission were suppressed, and only the longitudinal modes of the GS emission could be observed. As a result, the emission-state switching from the ES to GS emission was realized. When the ESQDL was biased at a larger current, the feedback strength required to achieve emission-state switching was stronger.

In this work, we experimentally investigate the nonlinear dynamics of an interband cascade laser (ICL) under variable-aperture optical feedback implemented by a gold mirror combining with a ring-actuated iris diaphragm (RAID). By continuously varying the diameter of RAID (DR), the evolution of the dynamical state of ICL with the aperture of the optical feedback can be inspected. The characteristics of each dynamical state are characterized by time series, power spectra, phase portraits, and Lyapunov exponents. The results show that, with the decrease of DR, the dynamical state of the ICL under variable-aperture optical feedback presents an evolution from complex, simple to stable. Diverse dynamical states including period one state (P1), period two state (P2), multi-period state (MP), quasi-period state (QP), low-frequency fluctuation (LFF), chaotic state (C), and hyperchaos have been observed. Through mapping the evolution of dynamical states with DR for the ICL biased at different currents, different evolved routes of the dynamical states are revealed.

In some previous reports about frequency-modulated continuous-wave (FMCW) Lidar, observing the longer waveform of a de-chirped signal is considered an effective scheme for further improving the ranging resolution. In this work, the ranging resolution of a FMCW Lidar is experimentally investigated, and the feasibility of such a scheme is tested. During the experiment, a FMCW signal is generated via a Mach–Zehnder modulator in the transmitted port. In the received port, the de-chirped signal is extracted based on a homodyne detection scheme and is analyzed by an electrical spectrum analyzer. Two different methods are adopted to determine the ranging resolution. One is based on a single target, for which the ranging resolution is obtained through inspecting the shift of spectral peak position as the target moves. The other is based on two targets, for which the ranging resolution is acquired through inspecting the variation of spectrum distribution as the spacing of two targets changes. The experimental results demonstrate that extending the observed duration of the de-chirped signal cannot improve the ranging resolution, and the corresponding physical mechanism is revealed.

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.

In this work, broadband optical frequency comb (OFC) generation by a gain-switching vertical-cavity surface-emitting laser (VCSEL) subject to optical injection is investigated experimentally. During implementing the experiment, a 1550 nm VCSEL under a large signal current modulation is driven into the gain-switching state with a broad noisy spectrum. By further introducing an optical injection, a high performance OFC can be produced. The experimental results demonstrate that the power and wavelength of the injection light seriously affect the performance of the produced OFC. Under proper optical injection parameters, two sub-combs originating from two orthogonal polarization components of the VCSEL can splice into a broadband total-OFC. By selecting optimized operation parameters, a high quality total-OFC can be acquired, with stable comb lines, high coherence, wide bandwidth of 70.0 GHz (56.0 GHz) within 10 dB (3 dB) amplitude variation and low single sideband phase noise at the fundamental frequency below −120.6 dBc/Hz @ 10 kHz.

High-repetition-frequency Q-switched laser is realized through adopting a Nd:LaMgAl11O19 (Nd:LMA) disordered crystal as the gain material, a laser diode lasing at 796 nm as the pumped source, and a semiconductor saturable absorber mirror (SESAM) as the Q-switched device. The output characteristics are analyzed under using different transmittance T plane mirrors as an output coupler. Without adopting SESAM, the laser is operating at a CW state, and a relatively high transmittance is helpful for achieving high output power, slope efficiency and light-to-light efficiency. For T = 7.5% and an absorbed power of 6.17 W, the output power arrives at its maximum of 1160 mW, and the corresponding slope efficiency and light-to-light efficiency are 20.71% and 18.78%, respectively. After introducing SESAM into the cavity, the laser operates at a passively Q-switched state, and the largest slope efficiency is 13.14% under T = 5.0%. Adopting five different output couplers, with the increase of the absorbed power, the pulse repetition frequencies, the pulse energies and the peak powers will ascend while the pulse widths will decline. The observed narrowest pulse width, the maximum pulse repetition frequency, the highest pulse energy and peak power are 1.745 μs, 175.88 kHz, 3.21 μJ and 1.84 W, respectively. [ABSTRACT FROM AUTHOR]

We theoretically investigate power-induced lasing state switching and bistability in a two-state quantum dot laser subject to optical injection. The simulated results show that, for a free-running two-state quantum dot laser operating at the ground state under low current, a power-induced lasing state switching between the ground state and the excited state can be achieved through introducing optical injection with a frequency (winj) close to the lasing frequency of excited state (wES). The injection power required for the state switching depends on the scanning route of injection power, i.e. there may exist state bistability for the injection power within a certain region. For forward scanning injection power, with the increase of frequency detuning (O = winj - wES), the injection power required for the state switching shows a decreasing trend accompanied by slight fluctuations. However, for backward scanning injection power, the injection power required for the state switching exhibits obvious fluctuations with the increase of O. The width of the hysteresis loop fluctuates with O, and the fluctuation amplitude is increased with the increase of the injection current. Additionally, the influences of the inhomogeneous broadening factor and the electron escape rate on the bistability performances are analyzed. [ABSTRACT FROM AUTHOR]

The dynamics of monolithically integrated amplified feedback lasers (AFL) is investigated through numerical simulation and experimental verification. The period-doubling route to chaos and high-frequency microwave generation are demonstrated through simulation. Then, we design and fabricate monolithically integrated AFLs. Mappings of dynamic states and oscillation frequency in the parameter space of phase section current IP and amplifier section current IA are depicted. For relative small IA, the period doubling evolution to chaos is presented with the increase of IP. For the relative large IA, a high-frequency mode-beating (M-B) pulsation can be observed under suitable value of IP. The oscillation frequency of period-one is about 10 GHz and the frequency of M-B pulsation is over 40 GHz for the device with a total length of 780 μm. [ABSTRACT FROM PUBLISHER]

Crosstalk noise is an intrinsic characteristic of photonic devices used by optical router which is used in optical networks-on-chip. It also adds a new dimension to the design of the optical router based on the photonic devices. We analyzed crosstalk noise at device level and router level. Based on the detailed analysis, we proposed a general analytical model to study the transmission loss, crosstalk noise, optical signal-to-noise ratio (OSNR), and bit error ratio in the 5 × 5 hitless optical router. For the first time, this paper used the crossing angles of 60° or 120° instead of the conventional 90° crossing angle to design the optical router. It is obtained that by using this method OSNR is improved by about 10 dB. [ABSTRACT FROM PUBLISHER]

Wavelength conversion based on the cross-polarization modulation (XPM) in a single semiconductor optical amplifier (SOA) has been numerically investigated by using the theoretical model in which the birefringence has been taken into account. The results show that the non-inverted and inverted wavelength conversion can be realized through selecting reasonably the parameters; there exit some differences in the chirp, extinction ratio (ER), and eye diagram of the converted signal between the non-inverted and inverted conversion; with the increase of the bit rate, the chirp increases and the eye opening ratio (EOR) decreases. [ABSTRACT FROM AUTHOR]

After taking into account the phase contribution of fiber grating (FG) to the longitudinal mode formation of the fiber grating external cavity semiconductor laser (FGECSL) and starting from the threshold condition of the FGECSL, the effect of the external cavity length on the lasing wavelength variation of the FGECSL with ambient temperature has been investigated theoretically. The numerical simulations show that with the increase of temperature, the lasing wavelength has a common tendency shifting towards longer wavelengths for short or long external cavity; for a short external cavity, the shift is accompanied by a periodic mode hopping; and for a long external cavity, the shift is approximately linear. The theoretical results, which are in agreement with reported experimental observations, have been analyzed and explained. [ABSTRACT FROM AUTHOR]

For the first time, we experimentally demonstrate phase chaos generation by introducing a fine-controllable dispersion module into the feedback-delayed loop. To study the effect of this specific nonlinearity, we demonstrate the nonlinear dynamics evolution route in terms of the dependence of dispersion and feedback gain parameters by both experiment and simulation, which show good agreement. We also investigate the synchronization error variation with the dispersion mismatch to evaluate the security level of the proposed phase chaos system. The results show that the proposed approach can simultaneously simplify the phase chaos synchronization and enhance the key space, which has potential to be used in high-speed chaotic optical communications. [ABSTRACT FROM AUTHOR]

Polarization dynamics in a 1550 nm vertical-cavity surface-emitting laser (1550 nm VCSEL) under parallel optical injection (POI) is investigated experimentally, and we experimentally observe the state bistability (SB) between pure- and mixed-mode states by scanning the injection power along different routes. Such a SB occurs only when the frequency of injection light is lower than that of the excited mode of a free-running 1550 nm VCSEL. Moreover, the effect of frequency detuning on the hysteresis loop width is analyzed. [ABSTRACT FROM AUTHOR]