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We proposed a new reservoir computation (RC) structure using polarization dynamics of vertical cavity surface emitting lasers (VCSELs) with optoelectronic feedback. Santa-Fe time-series prediction and nonlinear channel equalization were used to evaluate its parallel processing capability. We numerically discussed the effects of feedback strength, injection strength, frequency detuning, injection current and scaling factors on the RC performance of the system. The results show that, the operation range of negative feedback is larger than that of positive feedback; under appropriate feedback and lager injection strengths, good RC performance can be achieved for a wider injection current; with continuous variation of feedback strength from negative to positive values, the optimized location for frequency detuning moves from positive to negative values. We also evaluated the processing rate of proposed RC structure. The related study is helpful for the design and optimization of VCSEL-based RC under optoelectronic feedback. [ABSTRACT FROM AUTHOR]

In this article, we propose an all-fiber self-mixing velocimeter based on a DBR fiber laser with adjustable injection intensity for the first time, and theoretically and experimentally explore the behavior of fiber lasers in detecting non-cooperative target under different injection intensities. When the ultra-strong injection level is attained, the linewidth of the DBR fiber laser will be narrowed to about 10 kHz, thus extending the coherence distance of laser tremendously. Simultaneously, the intensity noise can be also significantly suppressed. The non-cooperative target velocity measurement with 40 km single-channel transmission distance has been successfully achieved. This velocimeter is shown to be of great significance in the field of remote sensing with simple and compact configuration, and adjustable dynamical monitoring range. [ABSTRACT FROM AUTHOR]

Ultra-stable lasers serve as the backbone for the most advanced optical metrology scientific experiments, enabling the ability to laser interferometry or atomic spectroscopy at the highest levels of precision, thanks to the rapid development of laser wavelength stabilization techniques based on optical or electrical feedback from an external reference such as absorption line or optical cavity. With the increasing interest in making these scientific systems portable and applying it outside of the laboratory, it still remains an open question as to how to realize a laser source that can reach the high wavelength stabilization and still remain sufficiently compact and portable for field use. Here, we describe a wavelength stabilization fiber laser with intracavity reference mechanism, by utilizing the beat note of dual modes inside the laser cavity as the laser wavelength drift reference signal. A laser wavelength drift less than 5 kHz and ADEV 10−13 level at integration time scales from 0.01s to 1000s is achieved, which is to-date the best results for the laser wavelength stabilization without the external frequency reference. The outcome of this work presents a new way to achieve an ultra-stable laser with a simpler, lower cost feature. It offers new insights to promote the highest precision optical metrology scientific application outside of the laboratory, and also gives the possibilities of applying the ultra-stable laser source into a wider range of industrial application. [ABSTRACT FROM AUTHOR]

A DBR-laser-based SOA-assisted extended-reach EML (AXEL) was investigated in order to improve the optical feedback tolerance. The performance was tested in relative intensity noise measurement and fiber transmission experiment. A superior optical feedback tolerance was obtained in comparison with a conventional AXEL. The excess RIN induced by optical feedback is suppressed at 3-dB higher feedback power for the DBR-AXEL. 40 km fiber transmission of 10-Gbit/s signal with a feedback factor of −15 dB was achieved. The power budget was 41.3 dB. The feedback-induced power penalty was 0.52 dB. [ABSTRACT FROM AUTHOR]

In this work the optical feedback generated by a tapered amplifier (TPA) emitting at 1120 nm is characterized at different operational conditions. The TPA is operated in a master oscillator power amplifier (MOPA) configuration, while the influence of pump current or seed power on the emitted feedback is investigated. Measurements are also performed with the TPA exposed to defined external optical feedback. For an estimation of the feedback from the TPA a simple formula is derived and the calculations are compared to the measurement results. With the experimental and theoretical results the required feedback resistance of the master oscillator seed source can be defined and the MOPA system optimized for certain applications with unwanted optical feedback. [ABSTRACT FROM AUTHOR]

A semiconductor laser with optical feedback and optical injection is an appealing scheme to construct the time-delay reservoir computing (TDRC) networks. Quantum dot (QD) lasers are compatible to the silicon platform, and hence is helpful to develop fully on-chip TDRCs. This work theoretically demonstrates a parallel TDRC based on a Fabry-Perot QD laser with multiple longitudinal modes. These modes act as connected physical neurons, which process the input signal in parallel. The interaction strength of the modes is characterized by the cross-gain saturation effect. We show that the neuron interaction strength affects the performance of various benchmark tasks, including the memory capacity, time series prediction, nonlinear channel equalization, and spoken digit recognition. In comparison with the one-channel TDRC with the same number of nodes, the parallel TDRC runs faster and its performance is improved on multiple benchmark tasks. [ABSTRACT FROM AUTHOR]

Integrating photonics with CMOS electronics in silicon is essential to enable chip-scale, electronic-photonic systems that will revolutionize classical and quantum communication and computing systems. However, the lack of an on-silicon isolator, capable of blocking unwanted back reflections and ensuring the stable operation of the laser, precluded many previous demonstrations from providing single-chip solutions. For most optical systems employing a laser, magneto-optic isolators have been indispensable, but such isolators are incompatible with silicon. To stabilize on-chip lasers, reflections-cancellation circuits were proposed as a way to reduce the reflections going back to the laser. Yet, a stable laser against time-varying back reflections was never demonstrated. Here we demonstrate a stable quantum well-distributed feedback (QWDFB) laser against slowly time-varying reflections using a reflections-cancellation circuit (RCC) on a foundry-produced, silicon-photonic (SiP) chip. The optical spectrum and the relative intensity noise (RIN) of the laser when the RCC was running is comparable to when an isolator was used. By accurately locking the laser in a stable optical feedback regime, the RCC further enhances the QWDFB laser performance by reducing its linewidth by a factor of 100, down to 3 kHz. Both results are enabled using novel techniques in the design, calibration, tuning, and control of the proposed SiP RCC. The optical insertion loss of the RCC is less than 1.5 dB for reflections smaller than −20 dB and can yield isolation ranges of up to 64 dB. Our device paves the way towards the mass production of fully integrated, low-cost electronic-photonic silicon chips without attaching magneto-optic isolators between the laser and the SiP chip. [ABSTRACT FROM AUTHOR]

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]

We propose a new sensing method based on the measurement of the second-order autocorrelation of the output of micro- and nanolasers with intensity feedback. The sensing function is implemented through the feedback-induced threshold shift, whose photon statistics is controlled by the feedback level in a characteristic way for different laser sizes. The specific response offers performances which can be adapted to different kinds of sensors. We propose the implementation of two schemes capable of providing a quantitative sensing signal and covering a broad range of feedback levels: one is utilizing the evolution of $\text{g}^{(2)}$ (0), the other one is the ratio between central and side peaks in $\text{g}^{(2)}$ ($\tau $). Laser-threshold-based sensing could, thanks to its potential sensitivity, gain relevance in biomolecular diagnostics and security monitoring. [ABSTRACT FROM AUTHOR]

In this paper, a method is proposed to recover displacements with nanometric precision based on spectral analysis of the feedback phase of self-mixing laser interferometric signal. This method is an enhancement of the previously developed method called Time-domain Frequency-domain Signal Processing (TFSP) which could only recover target vibration(s) with greater than lambda/8 amplitude (where lambda is the laser’s wavelength) so long as at least one of the vibration’s components has an amplitude greater than or equal to lambda/2. Using TFSP reconstruction as a first approximation of the main target displacement combined with parameter (optical feedback factor C and linewidth enhancement factor alpha) estimation, the proposed method can provide a better in-depth analysis of the self-mixing signal spectrum and thus achieve the detection of even lower amplitude vibrations. Studies presented in the paper indicate that the proposed method does not require accurate parameter estimation. Experimental results obtained using a laser diode based self-mixing sensor with lambda = 785 nm show that the adding this method on top of TFSP allows to recover target vibrations with amplitude down to approximately lambda/32 with an average amplitude root-mean-square error of 1.56 nm, which corresponds to a 4 fold improvement over TFSP. [ABSTRACT FROM AUTHOR]

In this paper, a unified behavioral model of laser feedback based self-mixing interferometry (SMI) is proposed which is able to accurately model the SMI sensor signals encountered under experimental variable optical feedback conditions. The model provides correct SMI signals whether feedback is varied in a continuous or discrete manner, while spanning all major feedback regimes (such as weak-, moderate-, and strong-feedback regime) used for sensing applications. As a result, the proposed model allows the simulation of SMI signals in the presence of speckle which is of upmost importance to develop future efficient algorithms to reconstruct target displacements. The optical speckle usually occurs when the target of comparable surface roughness to the laser wavelength is moving as it induces variation of the optical feedback factor. The proposed model is shown to be able to address such cases and in particular to be able to reproduce very similar SMI signals to those acquired in the presence of speckle. It is thus anticipated that the proposed model would facilitate the design and testing of novel SMI algorithms and systems dedicated to the processing of variable optical feedback based SMI signals for metric sensing applications. [ABSTRACT FROM AUTHOR]

A photonic approach to generating an optoelectronic oscillator (OEO) based on an integrated mutually coupled (IMC) distributed feedback (DFB) laser is reported. Compared with the traditional construction of an OEO, the IMC-DFB has functions of the laser source, intensity modulator, and microwave photonic filters (MPFs), which could simplify the OEO scheme. An optical feedback loop (O-Loop) and an optoelectronic feedback loop (OE-Loop) are used simultaneously in the proposed scheme. The optical loop was used to reduce the optical linewidth of the IMC-DFB to improve the quality of the laser source in the OE loop. The IMC laser consisted of two distributed feedback (DFB) laser sections with a semiconductor optical amplifier (SOA) section in between. The optical linewidth of the IMC-DFB is reduced from 5.86 MHz to 3.94 kHz due to the feedback of the optical loop. By tuning the current of the SOA and DFB sections, tunable microwaves ranging from 24.9 to 46.5 GHz with single-sideband (SSB) phase noise below −110 dBc/Hz at a 10 kHz offset from the carrier were realized. [ABSTRACT FROM AUTHOR]

In a free-space optical (FSO) communication system, a conventional transmit laser selection (CTLS) scheme selects the best transmit aperture to send the information bits and offers the best possible error performance. However, its performance is highly dependent on the feedback link, and even a slight error in the feedback information can cause a significant loss to the diversity order and bit error rate (BER). To mitigate this problem, in this research, a novel clustering-based TLS scheme, named two TLS (TTLS), is proposed and analyzed. Note that the word two in TTLS corresponds to the transmission corresponding to the two transmit apertures. TTLS scheme divides the arbitrary number of transmit apertures/lasers into two clusters and then selects the best transmit aperture from each cluster. It is observed by analysis that the TTLS scheme offers a lesser loss in the diversity order as compared to CTLS under imperfect feedback link. Therefore, a significant performance gain can be achieved by TTLS as compared to CTLS under a practical (imperfect/erroneous feedback link) scenario. Further, to retain the full diversity order under the imperfect feedback link, a modified error-tolerant weighting scheme (METWS) is also proposed. It is noticed from the asymptotic analysis that METWS offers full diversity order and outperforms CTLS and repetition coding schemes under both perfect and imperfect feedback links. [ABSTRACT FROM AUTHOR]

Optoelectronic feedback on a laser diode is demonstrated to generate two distinct modes of periodic pulse-train formation depending on the injection current J of the laser leading to microwave combs in two distinct regimes. For J close to the threshold current Jth, the pulse repetition rate ƒrep is the inverse of the loop delay. This behavior is attributed to feedback-induced gain-switching and leads to comb spacings in the tens of MHz range. In contrast, for J ≅ 2Jth, the repetition rate ƒrep is observed to be related to the relaxation-oscillation frequency ƒRO. The potential to generate pulse trains and associated microwave combs may find use in metrology, optical communications, optical sampling, and spectroscopy. [ABSTRACT FROM AUTHOR]

Using a self-mixing laser interferometric sensor, the aim of this paper is to propose two new and distinct Fourier transform based methods for estimation of optical coupling factor and linewidth enhancement factor, as well as displacement of a target over all feedback regimes by processing the feedback phase. First algorithm deals only with band-limited smooth displacements, while second method deals with rather more general, smooth displacements. Under noiseless simulations over diverse target motion conditions and optical feedback regimes, these algorithms (first and second) have shown picometric displacement sensing precision of 1.1264 pm and 1.0996 pm respectively, for laser wavelength of 785 nm. The performance of proposed methods has also been quantified in case of additive noise. During experimental verification, by using a reference sensor with 2 nm resolution, their precision was measured to be 36.2142 nm and 34.4388 nm respectively, for weak-, moderate-, and strong-optical feedback regime signals. Importantly, for strong feedback regime, these methods have shown better performance than two previously reported methods. [ABSTRACT FROM AUTHOR]

This paper exploits the difference in amplitude spectrum of perturbed and unperturbed phase of self-mixing (SM) laser, to propose a new method for estimation of coupling factor (C), line-width enhancement factor (LEF), as well as displacement of the vibrating target in three major feedback regimes, i.e, weak-, moderate- and strong-feedback regimes. This result thus fills the gap in limitation of previous methods restricted to fewer regimes. The displacement of target is assumed to be bandlimited and sparse in frequency domain. The proposed algorithm showed an average of 1.0168 nm RMS error in displacement estimation corresponding to single- and multi-tone vibrations of amplitudes varying from 3 to 30 times that of laser’s wavelength, with percentage error of 1.15% and 3.31% in C and LEF estimations, from simulated SM laser signals. Using an SM sensor based on a laser diode emitting at 785 nm wavelength, the proposed method has achieved an average RMS error of 21 nm in weak-, moderate- and strong-feedback regimes, corresponding to a vibrating target of 5 micrometer peak-peak amplitude. [ABSTRACT FROM AUTHOR]

In multi-chip integrated photonic micro-systems, it is important to protect the laser source from reflection because the semiconductor laser is sensitive to the back reflection induced by the roughness of the waveguide sidewall or other on-chip optical structures. In order to maintain the stable operation of the light source, an additional micro-ring resonator coupled with the laser source is design before the optical circuit which enables the laser diode to enter a self-injection locking state and improves the reflection tolerance. It is experimentally demonstrated in the hybrid system that by tuning the phase shifter and the micro-ring resonate wavelength, the distributed feedback (DFB) laser enters a self-injection locking state, resulting significant improvement of the critical reflection ratio from −18.69 dB to −4.31 dB. The method can isolate the external reflection of laser diode without using heterogeneous integration, which provides a new idea for on-chip source isolation. The parameters design of the micro-ring and the coupling method with the laser are also introduced in the letter. [ABSTRACT FROM AUTHOR]

Two new dynamic-state switching phenomena are experimentally observed in a vertical-cavity surface-emitting laser with polarization-preserved external-cavity optical feedback. One is switching between a steady state and a quasi-periodic state, and the other is switching between two different steady states. Both switching phenomena occur in the same polarization and the switching period is equal to the round-trip time in the external feedback cavity. The evolution of the two switching phenomena is experimentally investigated in detail. This work not only enriches the understanding of laser nonlinear dynamics but also provides an all-optical alternative for generation of special signals for possible applications. [ABSTRACT FROM AUTHOR]

This paper, while focusing on post-perturbed phase processing of optical feedback based self-mixing laser displacement sensor, rigorously derives a general strategy to achieve the particular task of retrieving displacement from perturbed (feedback based) phase based on a proposition, which has been proved separately. From this, a new algorithm has been derived for estimation of optical feedback coupling factor and corresponding displacement over weak-, moderate-, and strong-optical feedback regimes, by using the concept of deformation. This method can work on all displacements so long as these are sufficiently smooth, i.e, have enough continuous derivatives. The algorithm’s steps are shown to be mathematically consistent. Furthermore, its performance has also been verified, and compared with a previous method, by using simulated and experimental signals for an SM sensor with emission wavelength of 785 nm. Displacement measurement precision has also been quantified by using a commercial transducer with 2 nm resolution showing that the proposed method provides comparable displacement precision even for fringe-loss affected, strong-feedback regime signals. [ABSTRACT FROM AUTHOR]

Coherent optical communication is considered as an indispensable solution to the ever-increasing demand for higher data rates. To reduce the cost and form factor of coherent transceivers, full integration of photonic devices including lasers, modulators, amplifiers, photodetectors, and other components is necessary. However, as fabricating optical isolators on chip remains extremely challenging, optical feedback, which can degrade the coherence of semiconductor lasers, becomes the main obstacle, thwarting large-scale photonic integration. An appealing solution to such a problem is to use semiconductor lasers with intrinsic insensitivity to optical feedback as the integrated light sources. The heterogenous Si/III-V lasers, with their built-in high-Q resonators, are expected to possess a robustness to optical feedback which exceeds by several orders of magnitude compared to commercial III-V distributed feedback (DFB) lasers, which will be validated here. We present data showing that the heterogeneous Si/III-V lasers can preserve their phase coherence under much larger optical feedback and therefore function without severe degradation in isolator-free coherent optical communication systems. [ABSTRACT FROM AUTHOR]

In this letter, we report the dynamics and performance of a 1.3- $\mu \text{m}$ InAs/GaAs directly modulated quantum-dot laser (QDL) under strong feedback. At a direct modulation speed of 10 Gbps, error-free back-to-back operation of the QDL under a feedback level of −12 dB was realized. Feedback-resistant fiber transmission over 20 km under a maximum optical feedback level of −9 dB was also demonstrated with a power penalty of less than 1 dB at a bit error ratio (BER) of 10−3. The performance of the QDL was compared with directly modulated quantum-well laser (QWL) with similar parameters, showing a clear advantage in feedback resistance of QDL. [ABSTRACT FROM AUTHOR]

Single section quantum dash (QDash) mode locked lasers (MLL) can provide a flat and broadband optical frequency comb with low energy consumption, operational simplicity and large-scale low-cost production possibilities. MLL longitudinal modes can be employed as single carriers with a regular spectral spacing in a dense wavelength division multiplexing (DWDM) link, making them promising components for next generation DWDM transceivers. However, individual modes of a MLL suffer from relatively high phase noise while high capacity coherent transmission requires carriers with low phase noise. Optical feedback, which is a well-known method to reduce the linewidth of a single mode laser, can be used to stabilize comb lasers. This article reports on the investigation of phase noise properties of comb lines delivered from a single section QDash MLL under optical feedback and shows that each MLL longitudinal mode optical linewidth can be drastically narrowed. It enables coherent transmission with extended link budget over 50 km at Tbit/s line rates. [ABSTRACT FROM AUTHOR]

Aiming at the problems of limited bandwidth and periodicity of chaotic laser, a monolithically integrated chaotic semiconductor laser subject to random feedback and mutual injection is proposed. The chaotic laser consists of two distributed feedback laser diodes (DFB-LD), two semiconductor optical amplifiers (SOA) and a passive optical waveguide. Optical injection of two coupled DFB-LDs achieves chaotic signal shaping and bandwidth enhancement. Random grating along the passive waveguide provides stochastic feedback to suppress the time delay signature (TDS). A theoretical model is established based on the Lang-Kobayashi model to explore the dynamics of the proposed chaotic laser. A chaotic signal with TDS of 0.06 and bandwidth of 13.12 GHz is obtained by simulation. Furthermore, the effects of parameters including feedback ratio, mutual injection ratio, and detuning frequency on the TDS and bandwidth of chaotic signal are investigated. The results show that the broadband chaotic laser signal with TDS suppression can be obtained by controlling parameters reasonably. [ABSTRACT FROM AUTHOR]

With external optical feedback (EOF), a laser diode (LD) can operate at different dynamic states. In this work, an LD with EOF is set at period-one (P1) oscillation state to generate microwave photonic (MWP) signal for sensing. Firstly, the P1 state boundary of the LD is determined and then the influence of the LD controllable parameters on the boundary is studied by solving the well-known Lang–Kobayashi equations. A set of parameters selection rule for designing an LD based MWP sensing system is obtained. In addition, a measurement algorithm for recovering the displacement from an MWP sensing signal is developed. By making full use of the sensing information carried in both amplitude and frequency of the MWP signal, displacement sensing with high resolution and large range can be achieved. Both simulations and experiments are conducted to verify the proposed method and show it is capable of realizing wide measurable range, high measurement sensitivity, and high resolution sensing. [ABSTRACT FROM AUTHOR]

Experimental Self-Mixing (SM) or optical feedback interferometric signals are usually affected by additive white Gaussian noise (AWGN) and impulsive noise. Depending on SM sensing set-up, these noises can significantly reduce the signal to noise ratio (SNR) of SM signals which in turn affects the measurement performance of signal processing algorithms employed for metric information retrieval. In this paper, adaptive line enhancement (ALE) technique is proposed to remove AWGN and impulsive noise from SM signals. Specifically, a recursive least squares (RLS) based ALE algorithm has been designed and the results have been compared with established methods such as high-order digital low-pass filtering and discrete wavelet transform. The comparison indicates better precision in case of use of RLS-ALE even when significant variations occur in the operating optical feedback regime and remote target velocity as well as in presence of speckle. The proposed algorithm can also estimate the SNR of SM signals belonging to weak-, moderate-, and strong-optical feedback regime with SNR ranging from 0 dB to 40 dB, with a mean absolute error of 1.35 dB and a 1.09 dB precision. Statistical analysis of noise recovered from different experimental SM signals attests the Gaussian- and impulsive-nature of noise. Thus, the proposed method also enables a simple and reliable quantitative analysis and comparison of different laser diode based SM laser sensors operating under variable optical conditions. [ABSTRACT FROM AUTHOR]

Optical feedback has an impact on the tunability of lasers. We created a model of a tunable distributed Bragg reflector (DBR) laser describing the effect of optical feedback from a constant reflector distance on the wavelength tuning. Theoretical and experimental results are in good agreement. A further discussion of the model sheds light on design rules to reduce the effect of optical feedback on the tuning behavior. We introduced a new parameter called mode loss difference (MLD) as a metric for the feedback tolerance of the tuning behavior. A large MLD indicates higher tolerance of the laser to cavity length variations. [ABSTRACT FROM AUTHOR]

Self-Mixing interferometry (SMI) signal characteristics are highly dependent on both the operating optical regime and target surface. In this paper, a method is proposed to overcome some identified limitations of the even power scalable algorithm (EPSA), such as the required operating regime to be weak feedback. In addition, by using the up-sampling techniques, the number of stages involved in the even-power scalable algorithm can be drastically increased without any data acquisition bandwidth modification. Here, 10 successive stages have been successfully implemented to achieve a theoretical resolution of ${\lambda } _{\sf 0}/ {\sf 2}^{\sf 13}$. It is further shown that the proposed method can handle and process weak, moderate and strong feedback regime as well as speckle affected SMI signals more efficiently. Lastly, FPGA based hardware emulation of EPSA is also done for later embedded implementation of this high-resolution algorithm. FPGA synthesis results show that the designed system can measure maximum target velocity up to 1.18 m/s while consuming total power of 1.4W. [ABSTRACT FROM AUTHOR]

We evaluate the effects of the external optical feedback on the on-off keying (OOK) and four-level pulse amplitude modulation (PAM4) signals in the single-mode fiber (SMF) link implemented by using a commercial 10G-class 1.55- $\mu \text{m}$ vertical-cavity surface-emitting laser (VCSEL). The results show that the PAM4 signal is much more sensitive to the external optical feedback than the OOK signal. For example, in the back-to-back condition, an error floor is observed at the bit-error rate (BER) of $\sim 8\times 10 ^{\mathbf {-3}}$ for the 10-Gbaud PAM4 signal when we set the optical feedback ratio to be −14 dB. In comparison, the 10-Gbaud OOK signal has a power penalty of only 0.4 dB (@BER $= 5\times 10 ^{-5}$) under the same conditions. The results also show that the PAM4 signal becomes more sensitive to the optical feedback after the SMF transmission due to the shallowed BER curve by the fiber dispersion. In addition, we confirm that the effects of the optical feedback are independent of the baud rate. From these results, we conclude that the error-free transmission of the high-speed PAM4 signal cannot be achieved if there is even one bad fiber connector having >2.5% reflection in the VCSEL-based SMF link. [ABSTRACT FROM AUTHOR]

We measure the noise of the converted frequency-modulated (cFM) signal in a self-mixing interferometer (SMI) and confirm the theoretical prediction of a dependence of rms noise from ($1+ C)^{-1}$ , where $C$ is the Acket feedback factor, up to a high value (i.e., $C=30$) in region III of Tkach and Chraplyvy feedback diagram. We also verify the minor dependence of noise from the phase factor 2ks + atan $\alpha $. Finally, we show that the cFM SMI easily attains an unprecedented limit of small-signal detection, with amplitudes down to 0.25 pm/ $\surd $ Hz. [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]

We present a design of a tunable hybrid laser based on III-V Reflective Semiconductor Optical Amplifier (RSOA) and Silicon Photonics (SiPh) external mirror that represents a good trade off between high Wall-Plug Efficiency (WPE) and high tolerance to optical feedback caused by unwanted back reflections from the rest of the SiPh chip. The sensitivity to optical feedback of different configurations, an important issue in many SiPh applications, is evaluated through the calculation of the critical feedback level based on an effective Lang-Kobayashi model and the main results are validated through numerical simulations of laser dynamics. We conclude that hybrid lasers with long effective external cavities typically designed to reduce the laser linewidth can be also exploited to improve the tolerance to spurious optical feedback. [ABSTRACT FROM AUTHOR]

In this work, we propose a new scheme of optical feedback using stimulated Brillouin scattering (SBS). A high-power laser diode emitting at 1450 nm stimulates Brillouin back-scattering in a 4km-long optical fibre and the back-scattered light is injected back into the laser diode for optical feedback. The experimental results with RF spectrum exhibit clearly the Brillouin frequency shift at 11.46 GHz. Although the frequency shift is very large, the laser diode subject to optical feedback using SBS shows abundant dynamics. RF spectrum maps are also established with respect to the laser drive current. The results are compared with the ones for conventional optical feedback and it is clearly shown that the dynamical behaviours for both the configurations are very different. The investigation with autocorrelation functions reveals that the time-delay signature vanishes completely using the SBS feedback scheme unlike the conventional feedback one. [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]

In this paper we explain how to use rate equations to describe a laser that includes integrated optical feedback. We find a relation between the threshold current, the voltage drop at the gain section, output power, linewidth, and side mode suppression ratio, and show experimental results. [ABSTRACT FROM AUTHOR]

In recent years, various methods, architectures, and implementations have been proposed to realize hardware-based reservoir computing (RC) for a range of classification and prediction tasks. Here we compare two photonic platforms that owe their computational nonlinearity to an optically injected semiconductor laser and to the optical transmission function of a Mach-Zehnder modulator, respectively. We numerically compare these platforms in a delay-based reservoir computing framework, in particular exploring their ability to perform equalization tasks on nonlinearly distorted signals at the output of a fiber-optic transmission line. Although the non-linear processing provided by the two systems is different, both produce a substantial reduction of the bit-error-rate (BER) for such signals of up to several orders of magnitude. We show that the obtained equalization performance depends significantly on the operating conditions of the physical systems, the size of the reservoir and the output layer training method. [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]

We propose and demonstrate an improved Talbot laser including a regenerative RF feedback loop. This architecture greatly enhances both amplitude and noise performances of the pulsed laser, while preserving the broadband tunability of its repetition rate. The phase noise power spectral density of the associated RF signal is found to be independent of the repetition rate. For a 4.9 GHz repetition rate, the timing jitter is as low as 54 fs (integrated between 10 kHz and 100 MHz offset frequency). This architecture paves the way towards tunable high performances coupled opto-electronic microwave oscillators. [ABSTRACT FROM AUTHOR]

This paper investigates the intensity noise and pulse oscillation characteristics of an InAs/GaAs quantum dot laser epitaxially grown on germanium. We show that the relative intensity noise of the free-running laser generally decreases with increasing pump current, and the minimum value reaches down to about $-$ 126 dB/Hz. The intensity noise is hardly affected by the optical feedback, unless there is a resonance or pulse oscillation in the noise spectrum. The laser pumped at a high current is more sensitive to the optical feedback. Interestingly, it is found that the free-running Ge-based quantum dot laser generates self-sustained pulse oscillations with one period or two periods upon the pump current, without incorporating saturable absorbers. This behavior is valuable for both self-generation of photonic microwaves and for understanding nonlinear dynamics of semiconductor lasers. [ABSTRACT FROM AUTHOR]

This study investigates chaotic and spiking dynamics of mid-infrared quantum cascade lasers operating under external optical feedback and emitting at 5.5 $\mu {\text{m}}$ and 9 $\mu {\text{m}}$. In order to deepen the understanding, the route to chaos is experimentally studied in the case of continuous-wave and current modulation operation. The non-linear dynamics are analyzed with bifurcation diagrams. While for quasi-continuous wave operation, chaos is found to be more complex, pure continuous wave pumping always leads to the generation of a regular spiking induced low-frequency fluctuations dynamics. In the latter, results show that by combining external optical feedback with periodic forcing and further induced current modulation allows a better control of the chaotic dropouts. This work provides a novel insight into the development of future secure free-space communications based on quantum cascade lasers or unpredictable optical countermeasure systems operating within the two transparency atmospheric windows hence between 3 $\mu {\text{m}}$ –5 $\mu {\text{m}}$ and 8.5  $\mu {\text{m}}$ –11 $\mu {\text{m}}$. [ABSTRACT FROM AUTHOR]

We present a route toward design and fabrication of a conceptually new type of optical feedback insensitive integrated semiconductor ring laser with a weak optical isolator included in the ring. Although the approach has not resulted in the clear advantages that were aimed at, this detailed account of the process we believe is valuable to readers active in the field. After a summary of optical-feedback sensitivity in semiconductor lasers, we review various attempts toward reducing this sensitivity and discuss the feasibility of the concept of weak intra-cavity isolation. The design, model and theory of the novel integrated ring laser device are presented. The observed 5 dB of isolation and 3 dB increase of tolerance for external optical feedback demonstrate the feasibility of the intra-cavity isolation concept. However, the observed output performance of the single-mode laser, both with and without activated intra-cavity isolation, suggests the presence of a spurious intra-cavity reflection, strongly coupling the directional modes. Due to this coupling, the sensitivity to external optical feedback is comparable to that of a Fabry–Pérot type laser. Therefore, an alternate route is indicated toward a less complex laser cavity with potentially less intra-cavity reflection and better tolerance for external optical feedback. [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]

Semiconductor lasers are notoriously sensitive to optical feedback, and their dynamics and coherence can be significantly modified through optical reinjection. We concentrate on the dynamical properties of a very small (i.e., microscale) Vertical Cavity Surface Emitting Laser (VCSEL) operated in the low coherence region between the emission of (partially) coherent pulses and ending below the accepted macroscopic lasing threshold, with the double objectives of: 1) studying the feedback influence in a regime of very low energy consumption and 2) using the micro-VCSEL as a surrogate for nanolasers, where measurements can only be based on photon statistics. The experimental investigation is based on time traces and radiofrequency spectra (common for macroscale devices) and correlation functions (required at the nanoscale). Comparison of these results confirms the ability of correlation functions to satisfactorily characterize the action of feedback on the laser dynamics. Numerical predictions obtained from a previously developed, fully stochastic modeling technique provide very close agreement with the experimental observations, thus, supporting the possible extension of our observations to the nanoscale. [ABSTRACT FROM AUTHOR]

We compare the timing jitter of a mode-locked InAs/InP quantum dash (QD) coherent comb laser (CCL) with and without an external cavity self-injection feedback locking (SIFL) system. The jitter is determined through a measurement of the first harmonic of the RF power spectrum of the laser output detected using a fast photodiode. A significant reduction in timing jitter is observed in the laser with the external cavity SIFL. A pulse to pulse root-mean-square time jitter of 1.56 fs is achieved for the laser with the external cavity SIFL. This low timing jitter is a promising light source for the next generation high-speed coherent networking systems and all-optical signal processing. We demonstrate results of a double polarization 16-QAM data format transmission system at a symbol rate of 25 GBd using more than 30 individual channels of the QD CCL. An error vector magnitude of 5.8% and bit error rate of 3.0 × 10–9 were obtained. And, 6.0 Tb/s (16QAM 30 × 25 Gbd PDM) coherent transmission is achieved. [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]

The fringes disappearance phenomenon in the self-mixing interferometry occurs whenever the external round-trip phase at free-running state is modulated by either external modulation such as external cavity length changes or internal modulation when the laser injection current is modulated with a high back-scattered light power. This phenomenon has been observed by many authors for harmonic motion or vibration application and more recently in the case of the absolute distance measurement scheme when the laser injection current is modulated in the triangle waveform. However, there are no clear explanations on the fringes disappearance phenomenon and its dependence on the self-mixing regimes. In this paper, a novel approach that depicts the mechanism of interferometric fringes disappearance is proposed that highlight with a new perspective the impact of the coupling strength between the laser and the external cavity on the number of missing fringes. The proposed modeling is validated by a set of measurements using the optical feedback interferometry absolute distance measurement scheme. [ABSTRACT FROM AUTHOR]

Semiconductor mode-locked lasers can be used in a variety of applications ranging from multi-carrier sources for wavelength-division-multiplexed communication systems to time-base references for metrology. Their packaging in compact chip- or module-level systems remains, however, burdened by their strong sensitivity to back-reflections, quickly destroying the coherence of the mode-locking. Here, we investigate the stabilization of mode-locked lasers directly edge coupled to a silicon photonic integrated circuit, with the objective of moving isolators downstream to the output of the photonic circuit. A 2.77-kHz 3-dB RF linewidth, substantially improved compared to the 15.01 kHz of the free running laser, is obtained in the best case. Even in the presence of detrimental reflections from the photonic circuit, substantial linewidth reductions from 20 to 8.82 kHz, from 572 to 14.8 kHz, and from 1.5 MHz to 40 kHz are realized. [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]

This letter reports on a 1.3- $\mu \text{m}$ reflection insensitive transmission with a quantum dot laser directly grown on silicon in the presence of strong optical feedback. These results show a penalty-free transmission at 10 GHz under external modulation with −7.4-dB optical feedback. The feedback insensitivity results from the low linewidth enhancement factor, the high damping, the absence of off-resonance emission states, and the shorter carrier lifetime. This letter paves the way for future on chip high-speed integrated circuits operating without optical isolators. [ABSTRACT FROM AUTHOR]