Instability of optical phase synchronization between chaotic semiconductor lasers.

The effect of periodic phase modulation of light on a chaotic external-cavity semiconductor laser working in a regime of low-frequency fluctuations (LFFs) is studied numerically. It is observed that the phase modulation changes the time period between consecutive dropouts in the emitted laser intensity. A new variable ΦL is defined as the phase of the laser's LFFs, which increases in time with 2π after each power dropout event. The phase ΦPM of the periodic phase modulator is unfolded on the real axis and increases linearly at a rate given by the modulating frequency. It is shown that the phase difference between the laser and the modulator ΔΦ(t)=mΦL(t)−nΦPM(t), where m and n are integers, remains constant in time, leading to phase-synchronized states, for specific values of the modulating frequency and amplitude.

Optical chaos generated by a semiconductor laser under cascaded optical injection is experimentally and numerically investigated. A semiconductor slave laser under continuous-wave optical injection from a master laser is employed as the chaotic laser. The chaotic output optically injects a third laser to enhance the chaotic properties. Using three semiconductor lasers coupled in a master-slave configuration without any delay-based components, optical chaos is generated without any time-delay signatures present. Flat broadband chaos is observed with standard and effective bandwidths reaching six and three times the relaxation resonance frequency of the semiconductor laser, respectively. For simultaneous flat and broadband chaos, the chaotic optical injection of the second stage is adjusted for weak injection strength and a high positively detuned frequency.

A compact three-laser source for optical sensing is presented. It is based on a low-noise implementation of the Pound-Drever-Hall method and comprises high-bandwidth optical phase-locked loops. The outputs from three semiconductor distributed feedback lasers, mounted on thermo-electric coolers (TEC), are coupled with micro-lenses into a silicon photonics (SiP) chip that performs beat note detection and several other functions. The chip comprises phase modulators, variable optical attenuators, multi-mode-interference couplers, variable ratio tap couplers, integrated photodiodes and optical fiber butt-couplers. Electrical connections between a metallized ceramic and the TECs, lasers and SiP chip are achieved by wirebonds. All these components stand within a 35 mm by 35 mm package which is interfaced with 90 electrical pins and two fiber pigtails. One pigtail carries the signals from a master and slave lasers, while another carries that from a second slave laser. The pins are soldered to a printed circuit board featuring a micro-processor that controls and monitors the system to ensure stable operation over fluctuating environmental conditions. This highly adaptable multi-laser source can address various sensing applications requiring the tracking of up to three narrow spectral features with a high bandwidth. It is used to sense a fiber-based ring resonator emulating a resonant fiber optics gyroscope. The master laser is locked to the resonator with a loop bandwidth greater than 1 MHz. The slave lasers are offset frequency locked to the master laser with loop bandwidths greater than 100 MHz. This high performance source is compact, automated, robust, and remains locked for days.

Photonic generation of wideband chaotic signals with time delay signature elimination is investigated experimentally and numerically based on a semiconductor laser (slave laser) with chaotic optical injection from a master laser. The master laser is subject to moderate optical feedback where the feedback strength and external-cavity length are fixed, while the slave laser stands alone. The experimental results show that wideband chaotic signals with successful time delay concealment can be generated in the slave laser by simply adjusting the coupling strength and frequency detuning between the two lasers. Furthermore, the numerical results are in accordance with the experimental observations. Finally, we propose a simple method for simultaneously generating multiple streams of high-quality chaotic signals using multichaotic lasers, where the time delay is effectively concealed in the autocorrelation function and delayed mutual information calculated from the chaotic time series.

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 ${\rm{6.18\,\times \,10}}^{- 19}$ . Exhaustive tests about security analysis are carried out, demonstrating the valuable feasibility and high security of the image encryption-then-transmission system.

Extreme events (EEs) are rare and unexpected events that emerge in many natural systems and are the object of intensive studies. Here, we experimentally investigate the evolution of deterministic optical EEs in a slave laser (SL) subject to chaotic optical injection from a master laser (ML) with optical feedback. Although the ML operates in the long-cavity regime, we observe occasional pulses that are high enough to be considered EEs, and the appearance probability of EEs can be significantly enhanced in the SL for certain injection parameters outside the injection-locking regions of the master-slave configuration. Additionally, the evolution of EEs can be readily identified from the two-dimensional maps. The numerical results show good qualitative agreement with the experiment. These results reveal the importance of injection parameters for the appearance of EEs in chaotic lasers and can be easily extended to other coupled systems.

Based on a master-slave frame, we propose and numerically simulate a scheme for generating frequency-modulated continuous-wave (FMCW) signals with broad bandwidth. In such a master-slave system, a semiconductor laser under current modulation with single-tone electrical signal is taken as the master laser (ML), and its output optical signal with modulated power is injected into another semiconductor laser (taken as the slave laser, SL) for generating FMCW. The simulated results show that, under suitable operating condition, the bandwidth and the sweep rate of generated FMCW signal can reach 15.4 GHz and 4.83 GHz/ns, respectively. Through further introducing optoelectronic feedback into SL for suppressing the phase noise, the contrast of the frequency comb in the FMCW signal can be increased by 20.22 dB.

We study the nonlinear dynamics of a vertical-cavity surface-emitting laser (VCSEL) subject to a repetitive optical pulse injection numerically using the SFM model. In our study, a linearly polarized slave laser is optically injected by a train of optical pulses from a master laser, where the polarization of the master laser is orthogonal to the polarization of the solitary slave laser (x-polarized). By varying the strength and the repetition frequency of the injected pulses, different dynamical states, including regular pulsations, period-doubled pulsations, chaotic pulsations, periodic oscillations, quasi-periodic oscillations, and chaotic oscillations, are found. Instead of having only one polarization mode at the slave laser output, both the y- and x-polarized modes are observed for the pulsation and oscillation states. While the pulsation states with y-polarization follow a period-doubling route to chaotic pulsations, the oscillation states with the x-polarization undergo a quasi-periodic route to chaos oscillations. Then, with adequate strength of the injection, the x-polarized mode will be suppressed (i.e. polarization switching) and eventually the slave laser will lock to the master laser with higher injection strength. Also, the switching points, the boundary of the injection-locked, and the regions of the chaotic states are found to be strongly influenced by the repetition frequency of the injection pulses and the detuning frequency between the two lasers.

A primary frequency standard based on cold Cs atoms is under development at the Time and Frequency Division of the Centre Nacional de Metrologia, CENAM. A Cs magneto-optical trap has been developed using the standard sigma+sigma$4-horizontal 2-vertical laser beams of 1.9 cm in diameter, with 5 mW each. We use 852 nm, 5 mW, DBR laser as a master laser which is stabilized by saturation spectroscopy. Linewidth of the master laser is 1 MHz. A 50 mW, 852 nm, semiconductor laser is used as slave laser in order to amplify the light of the master laser. The slave laser is stabilized by injection light technique. 12 MHz red shift of the light is performed by double pass through two AOM's. In this paper we present some preliminary results of our MOT characterization as function of MOT's parameters such as intensity of laser beams and gradient of the magnetic field.

Recent work has shown the Quantum Dot (QD) material system to be well-suited to support dual-mode lasing. In particular, optical injection from a master laser (ML) into the residual Fabry-Perot (FP) modes of a 1310 nm Quantum Dot Distributed Feedback (QD-DFB) laser has been recently demonstrated to offer a highly reliable platform for stable dual-mode lasing operation. External controls on the ML, such as operating temperature and bias current, can be used to precisely adjust the spacing between the two lasing modes. This tunability of modeseparation is very promising for a range of applications requiring the generation of microwave, millimeter wave and terahertz signals. Considering the versatility and utility of such a scheme, it is imperative to acquire a deeper understanding of the factors that influence the dual-mode lasing process, in order to optimize performance. Toward this end, this paper seeks to further our understanding of the optically-injected dual-mode lasing mechanism. For fixed values of optical power injected into each FP residual mode and wavelength detuning, the dual-mode lasing characteristics are analyzed with regard to important system parameters such as the position and the intensity of the injected residual mode (relative to the Bragg and the other residual FP modes of the device) for two similarly-fabricated QD-DFBs. Results indicate that for dual mode lasing spaced less than 5 nm apart, the relative intensity of the injected FP mode and intracavity noise levels are critical factors in determining dual mode lasing behavior. Insight into the dual-mode lasing characteristics could provide an important design guideline for the master and QD-DFB slave laser cavities.

A method of coherently summing the outputs of two diode-pumped ring lasers has been tested. The output of one such Nd:YAG laser (the master laser) at 1064 nm was mode-matched into a similar laser (the slave laser). The frequency of the slave laser was tuned toward that of the master laser by adjusting the crystal temperature (and thus the optical path length). The difference frequency was detected until the locking range was reached, when the free-running oscillation of the slave laser was completely injection-locked by the injected signal. With the master laser power of 15 mW and the slave laser power of 5 mW, the locking frequency occurred at ~23 MHz. The combined output power during locking ranged from ~20 to 21 mW with the maximum representing a 5% increase over the summation of the unlocked free-running lasers. Locking was held for periods >15 min. These results indicate the possibility of chaining together this type of laser to obtain coherent single-mode outputs with higher powers than currently possible.

Chaotic laser generated by semiconductor lasers has been attached great importance in areas of secure communication, random number generation, and optical sensing. This article mainly reviews the research progress in the performance improvement of chaotic laser, including bandwidth enhancement and time‐delay signature suppression. First, the methods of chaotic laser generation and the existing problems are briefly introduced. Then, innovative methods for the optimization of chaotic characteristics are analyzed and discussed. The integrated chaotic semiconductor lasers are also discussed. At last, avenues for the development trends and future applications of chaotic semiconductor lasers are prospected.

Ultra-wideband signal generator based on cross gain modulation effect in a distributed feedback laser

According to the DNA encryption algorithm and the double-chaotic system which contains the optical chaos and the coupled map lattice chaotic system, a novel image encryption-then-transmission system is proposed. In the system, with identical chaotic injection from a master laser with two optical feedbacks, two slave lasers (SL1 and SL2) can output similar chaotic signals served as chaotic carrier to transmit image and used to generate the core part of the encryption scheme. A 128-b key is selected to generate the original value of the double-chaotic system, which decides the DNA complementary rule, hence, the key is hypersensitive in encryption and decryption process. The security analysis demonstrates the effectiveness of the proposed encryption system. The simulation results verify that the cryptosystem is enough against the traditional attacks, such as statistical attack, differential attack, brute force attack, and entropy attack. Moreover, the encrypted image can be the optical message and transmitted in 10 km single-mode fiber channel from SL1 to SL2. In order to ensure the security, we use the chaos masking technique to modulate and demodulate the optical message. Through numerical simulations of the cross-correlation function, the chaos synchronization between SL1 and SL2 is desired. The Q-factor is 9.559 and the bit error rate is $5.771\times 10^{-22}$ .

The study of nonlinear dynamical systems is a subject of much interest, one application being in secure communication systems realized through the synchronization of chaotic dynamical systems. In this paper optical coupling is used to effect synchronization between two diode lasers in a master-slave configuration, and we study the effect of frequency detuning between the master and slave lasers on the character of the observed synchronization. Experimental conditions are found under which the synchronization plot (formed by plotting the output power of the slave laser against that of the master at each instant in time) makes a transition from a positive gradient to a negative gradient. The appearance of such a negative gradient is a new phenomenon, to our knowledge, which we term ``inverse synchronization.'' A rate-equation model is proposed which accounts for light injection into the slave laser, and which agrees with the experimental results. Using this model, we etablish that inverse synchronization is caused by nonresonant coupling between the master and slave lasers.