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10.1109/JQE.1980.1070479 4 Schunk N., Petermann K. Numerical analysis of the feedback regimes for a single-mode semiconductor laser with external feedback. Colored Optical Feedback Turns Down the Noise in Chaotic Lasers Optical feedback always pushes the laser emission to change instantaneous values of frequency and power. 10.1109/JSTQE.2012.2234445 11 Jumpertz L., Carras M., Schires K., Grillot F. Regimes of external optical feedback in 5.6 m distributed feedback mid-infrared quantum cascade lasers. [Extracted from the article]

An InP-based mode-locked laser photonic integrated circuit with a repetition rate of 10 GHz is optically synchronized to a SiN microresonator-based dissipative Kerr soliton with a repetition rate of 305 GHz. The synchronization is achieved through regenerative harmonic injection locking assisted with electro-optic division which results in an optical frequency division factor of 18. The repetition rate of the dissipative Kerr soliton is stabilized through electro-optic division and transferred to the mode-locked laser, where we measure a fractional frequency instability in the repetition rate of $10^{-10}$ at 1 s with a $1/\tau$ trend. Furthermore, we also stabilize the repetition rate of the dissipative Kerr soliton using the mode-locked laser’s repetition rate beat as a feedback point. [ABSTRACT FROM AUTHOR]

Optical feedback is an efficient way to narrow and stabilise semi-conductor lasers. As a step forward for ultra-stable, yet highly tunable sources, we developed a new prototype of a three-mirror V-shaped optical cavity (VCOF). It is made of a precisely machined Zerodur spacer, that holds 3 optical contacted mirrors forming a high finesse V-shaped cavity. This arrangement allows the resonant light to be sent back to the seeding laser, triggering a drastic narrowing of the emission linewidth well below the cavity mode width without the need for high-bandwidth active electronic lock of the laser on the cavity. Low expansion material and precise temperature control of the reference cavity leads to a source with Hz level frequency drift and 70 Hz-level emission linewidth at 215 THz. We discuss the benefits of this new prototype compared to its predecessor for high-sensitivity cavity ring down spectroscopy (CRDS). To serve this purpose, the frequency stabilised laser was characterised on short, mid and long time scales, using a high finesse etalon, a self referenced optical frequency comb and a long term saturated CRDS absorption Lamb dip measurement on water. [ABSTRACT FROM AUTHOR]

Optical feedback is a powerful technique to stabilise and narrow semi-conductor lasers. As a step forward for field deployable, ultra-stable yet tunable sources, we analyse and model the opto-mechanical design of a V-shaped cavity optical feedback (VCOF) reference cavity. We estimate the relative contributions of sources of external disturbance on the optical resonance frequency of the cavity, and ultimately define the minimal thermal and mechanical shielding requirements to face field conditions. We attest of the robustness of the developed system, and show that external sources of disturbance are only secondary contributors to the overall stability of the cavity. The suite of analytical tools developed in the process paves the way for lighter and more compact cavity designs, more adapted to field deployment. [ABSTRACT FROM AUTHOR]

One such optic, the super-polished bimorph mirror, achieved sub-micron focusing of an X-ray beam and was able to expand the X-ray beam at the focal point by a factor of over 50. Reflective mirrors are used on most synchrotron and free electron laser (XFEL) beamlines to transport X-rays from the source to the sample. Now, the rapid, real-time control of X-ray beam shapes by fast, stable "adaptive" bimorph mirrors at Diamond appears within reach thanks to the ZPS displacement sensors, which are capable of measuring mirror surface errors with sub-nanometer resolution at kHz frequencies. Ex-situ testing is complemented by a range of in-situ, X-ray metrology techniques developed at Diamond's bending-magnet versatile optics Test Beamline B16 [[14]], and performed on beamlines throughout Diamond. [Extracted from the article]

In this paper, we analyze the effect of optical feedback on the dynamics of a passively mode-locked ring laser operating in the regime of temporal localized structures. This laser system is modeled by a set of delay differential equations, which include delay terms associated with the laser cavity and the feedback loop. Using a combination of direct numerical simulations and path-continuation techniques, we show that the feedback loop creates echoes of the main pulse whose position and size strongly depend on the feedback parameters. We demonstrate that in the long-cavity regime, these echoes can successively replace the main pulses, which defines their lifetime. This pulse instability mechanism originates from a global bifurcation of the saddle-node infinite-period type. In addition, we show that, under the influence of noise, the stable pulses exhibit forms of a behavior characteristic of excitable systems. Furthermore, for the harmonic solutions consisting of multiple equispaced pulses per round-trip, we show that if the location of the pulses coincides with the echo of another, the range of stability of these solutions is increased. Finally, it is shown that around these resonances, branches of different solutions are connected by period-doubling bifurcations. [ABSTRACT FROM AUTHOR]

The design of optical buffers for packet contention resolution has been recognized as a key issue in all-optical packet switching. One of the most general buffering schemes is priority queues, which includes first-in first-out (FIFO) queues and last-in first-out (LIFO) queues as special cases. In a priority queue, each packet is associated with a unique priority upon its arrival, the packet with the highest priority is sent out from the queue whenever there is a departure request and there are packets in the queue, and the packet with the lowest priority is dumped from the queue whenever there is a buffer overflow. In this paper, we consider the constructions of optical priority queues by using a feedback system consisting of an optical (bufferless) crossbar switch and multiple optical FIFO multiplexers with delay one (FM1’s) in the feedback path for buffering packets and feeding packets back to the switch. Such a feedback system is a generalization of that used in one of the authors’ earlier attempt for the constructions of optical priority queues in Tang et al. (2020). We fix the no-buffering problem in Tang et al. (2020) by using optical FM1’s to replace the optical FIFO multiplexers (FM’s) in Tang et al. (2020), which enables us to successfully achieve an exact emulation of a priority queue. We improve the utilization of buffering capacity over that in Tang et al. (2020) by routing packets to the optical FM1’s according to their buffering tags instead of their tags as used in Tang et al. (2020). We also extend and generalize the construction in Tang et al. (2020) and obtain a much larger class of constructions of optical priority queues. Our constructions are made possible by showing that the highest-priority (resp., lowest-priority) packet is always available at the input links of the switch whenever it needs to be routed to the departure (resp., loss) link, and by showing that there is no collision and there is no buffer overflow at any FM1 at any time so that there is no internal packet loss at any time. Our complexity analysis shows that by using a feedback system consisting of an optical $(M+2) \times (M+2)$ (bufferless) crossbar switch and $M$ fiber delay lines, we can achieve a buffer size of $2^{O(\sqrt {\alpha M})}$ , where $\alpha $ is a constant that depends on the parameters used in our constructions. Furthermore, we show that the best buffer size that we can achieve is $2^{O(\sqrt {4M/15})}$. Our result (exponential in $\sqrt {M}$) substantially improves on the best known result (polynomial in $M$) in the literature. Our numerical results show that the construction complexity of our constructions is lower than that of the construction in Tang et al. (2020), and the actual saving, in terms of the number of $2\times 2$ switches needed, by our constructions could be quite significant even in the tiny-buffer and small-buffer regimes. [ABSTRACT FROM AUTHOR]

An experiment for measuring the speed of light by measuring the distance between regularly generated optical pulses is described. The experiment utilizes optical feedback from a visible picosecond diode laser to accurately determine the distance between two successive laser pulses. The required setup is compact, and it does not implement either a fast detector or an oscilloscope. This experiment is suitable for an undergraduate physics instructional laboratory course or as an independent student project. [ABSTRACT FROM AUTHOR]

In this paper, microwave photonic signal generation based on the period-one dynamic of optically injected discrete mode (DM) semiconductor lasers has been experimentally demonstrated and numerically simulated. The results show that the frequency of the generated microwave increases linearly with the frequency detuning or optical injection ratio. In addition, a single optical feedback loop is sufficient to reduce the microwave linewidth without significantly deteriorating side mode suppression. The simulation results using a model considering the nonlinear dependencies of the carrier recombination agree well with the experimental results, which indicates that the nonlinear carrier recombination effect is important in determining the nonlinear dynamics of optically injected DM lasers. [ABSTRACT FROM AUTHOR]

With on-chip integration enabled, resonant integrated optical gyroscope (RIOG) is a promising choice of rotation rate sensor in the novel micro inertial navigation systems. However, various optical errors, especially the backward-light-related errors including backscattering error and back-reflection error caused by the manufacturing process and heterogeneous materials in the optical elements, limit the performance of RIOG. Here, we propose the frequency spectrum separation method to suppress the backward-light-related errors for improving the detection precision of RIOG. The method can separate the angular velocity signal from the backward-light-related errors in the frequency domain. And, we investigate the optimal strategy of modulation frequencies to obtain maximum spectrum gaps of clockwise and counter-clockwise light for suppressing the backward-light-related errors, and simultaneously maintain maximum demodulation gain and minimum nonlinearity of optical effect for improving detection sensitivity. With the frequency spectrum separation method, the long-term bias stability of RIOG achieves 0.3°/h based on Allan deviation, which is the best reported result to the best of our knowledge. The proposed method of backward-light-related errors suppression can significantly improve the detection precision of RIOG based on a photonic chip, which contributes to the application of RIOG in the novel micro inertial navigation field such as cluster unmanned aerial vehicles and micro-nano satellites. [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 new dither technique is presented to control the bias voltage of an intensity modulator of the Mach-Zehnder type to maintain the quadrature operating point. This method is based on the direct calculation of the operating point deviation. The calculation uses an exact mathematical model of the modulator. The data used for calculating the deviation value are obtained from an analysis based on a fast Fourier transform with a properly set level of vector averaging. Unlike other solutions, the proposed feedback system does not use an approach with a proportional-integral or proportional-integral-derivative controller. Moreover, the presented stabilization method is robust and invariant regarding changes in the input optical power of the modulator and immune to prospective fluctuations of most of the feedback loop parameters. With the proposed approach in its present form, a 3-hour stability of ±0.08° is achieved. This value reflects the limitations of the equipment used. Even better results can be expected using more powerful hardware. [ABSTRACT FROM AUTHOR]

Pure organic room‐temperature phosphorescence (RTP) materials can be used in optoelectronics, biochemical equipment, sensors and biological imaging. In the past few years, dynamic covalent chemistry has attracted significant attention because it provides a new way to produce intelligent optical materials with feedback functions. Two polymers are synthesized through supramolecular‐mediated [4+4] photocyclodimerization of anthracene, which can be reversibly transformed by photoreversible dynamic covalent bonds. The polymers show decent phosphorescence emission with different colors at room temperature, achieving the conversion between blue phosphorescence and cyan phosphorescence. This study provides a novel method for designing and synthesizing pure organic RTP materials with tunable emissions through dynamic covalent bonds. [ABSTRACT FROM AUTHOR]

Herein, recent progress in fiber optofluidic lasers and their applications in biochemical detection are reviewed. The unique properties of optical fibers are introduced for the development of optofluidic microlasers, including high quality factor, small mode volume, high aspect ratio, high reproducibility, and low cost. Optical fiber microresonators based on Fabry–Pérot cavity, microring resonator or photonic bandgap cavity are highlighted to provide optical feedback for optofluidic lasing. For applications, the capability of fiber optofluidic lasers is emphasized to perform biochemical analysis with ultrahigh sensitivity, fast response, high throughput, and disposable optical biodetection. The challenges and prospects for fiber optofluidic lasers are also discussed. [ABSTRACT FROM AUTHOR]

This work presents a 3-D-integrated opto-electrical receiver (RX) analog front end (AFE) operating up to 50 Gb/s. The electronic integrated circuit (EIC) is fabricated in ST SiGe BiCMOS-55-nm technology and flipped and mounted on top of the ST photonic integrated circuits (PICs) die through copper pillars (Cu-Pi). In the RX chain, a low-power fully differential shunt-feedback trans-impedance amplifier (FD SF-TIA) is exploited to reduce the input-referred noise. Following the TIA, a postamplifier (PA) based on a novel active feedback circuit topology extends the bandwidth (BW) and a buffer delivers the output electrical signal to the 100- $\Omega $ differential off-chip load. An automatic offset cancellation loop is included to protect the RX from any offset source at the input. The RX AFE consumes 56 mW from 1.8-V supply voltage and provides a trans-impedance (TI) gain of $78.7~\Omega $ with 27-GHz BW. By exploiting the FD SF-TIA with low parasitic capacitance of the germanium photodiodes (Ge-PD) in the photonic die as well as BW recovery by the PA, the RX achieves the sensitivity of −7.5-dBm OMA at Ge-PD and −2.3-dBm OMA at the single-mode fiber (SMF) optical output with bit error rate (BER) of <10−12 and PRBS-7. To the author’s best knowledge, among published state-of-the-art 50-Gb/s TIAs and RX exploiting SiGe BiCMOS technologies, this work proves the best energy efficiency ((pJ/bit)) and figure of merit (FoM) ($({\text {Gbps}}/{\mu \text {A.mW}})$) in terms of sensitivity and power consumption. [ABSTRACT FROM AUTHOR]

Analog photonic computing has been proposed and tested in recent years as an alternative approach for data recovery in fiber transmission systems. Photonic reservoir computing, performing nonlinear transformations of the transmitted signals and exhibiting internal fading memory, has been found advantageous for this kind of processing. In this work, we show that the effectiveness of the internal fading memory depends significantly on the properties of the signal to be processed. Specifically, we demonstrate two experimental photonic post-processing schemes for a 56 GBaud PAM-4 experimental transmission system, with 100 km uncompensated standard single-mode fiber and direct detection. We show that, for transmission systems with significant chromatic dispersion, the contribution of a photonic reservoir's fading memory to the computational performance is limited. In a comparison between the data recovery performances between a reservoir computing and an extreme learning machine fiber-based configuration, we find that both offer equivalent data recovery. The extreme learning machine approach eliminates the necessity of external recurrent connectivity, which simplifies the system and increases the computation speed. Above 31 dB OSNR, the photonics-based equalization exhibits a lower BER than the respective offline DSP-based KK receiver. [ABSTRACT FROM AUTHOR]

The effect of optical feedback on hysteresis loop (HL) is investigated. Hysteresis loop occurs between two eigenstates of the He-Ne laser (x , y) and it changes by optical feedback at angles from 0° to 180°. Hysteresis loop size decreases and increases with the rotation angle (θ) of the polarization. Hysteresis loop effects on vertical (⊥) and horizontal (//) polarization and the intensity for two eigenstates. When HL size increases, then the intensity (I⊥ or I//) decreases vice versa clear. Best results at 70-110 angles from the external cavity (optical feedback cavity), whereas the intensity very high and HL disappear.

Semiconductor lasers (SLs) that are subject to delayed optical feedback and external optical injection have been demonstrated to perform information processing using the photonic reservoir computing paradigm. Optical injection or optical feedback can under some conditions induce bandwidth-enhanced operation, expanding their modulation response up to several tens of GHz. However, these conditions may not always result in the best performance for computational tasks, since the dynamical and nonlinear properties of the reservoir might change as well. Here we show that by using strong optical injection we can obtain an increased frequency response and a significant acceleration in the information processing capability of this nonlinear system, without loss of performance. Specifically, we demonstrate numerically that the sampling time of the photonic reservoir can be as small as 12 ps while preserving the same computational performance when compared to a much slower sampling rate. We also show that strong optical injection expands the reservoir’s operating conditions for which we obtain improved task performance. The latter is validated experimentally for larger sampling times of 100 ps. The above attributes are demonstrated in a coherent optical communication decoding task.

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.

Optical feedback cavity ringdown spectroscopy is presented with a linear Fabry–Pérot cavity and a cost-effective DFB laser. To circumvent the low coupling efficiency caused by the broad laser linewidth, an optical feedback technique is used, and an enhanced coupling efficiency of 31%, mainly limited by impedance mismatch and mode mismatch, is obtained. The trigger of the ringdown event is realized by the shutoff of the laser driving current, and a novel method with the aid of one electronic switch is applied to avoid the ringdown events excited by the unexpected cavity modes during the process of laser current recovery. As a result, the ringdown signal with a signal-to-noise ratio of 2500 is achieved. Through continuous monitoring, the fractional uncertainty of the empty cavity ringdown times is assessed to be 0.04%. An Allan variance analysis indicates a detection sensitivity of 4.3 × 10−10 cm−1 is resulted at an integration time of 120 s, even with a moderate finesse cavity. To further improve the long-term stability, we regularly rectify the empty cavity ringdown time, and an improvement factor of 2.5 is demonstrated.

Spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) with birefringence-induced polarization oscillations have been proposed to generate desired photonic microwave signals. Here, we numerically investigate the generation of photonic microwave signals in an optically pumped quantum dot (QD) spin-VCSEL. First, the influence of intrinsic key parameters on period-one (P1) oscillations and microwave properties is discussed. Second, the difference between microwave generation based on the quantum well (QW) and QD spin-VCSELs is analyzed by controlling the carrier capture rate that is described in the spin-flip model. The QD spin-VCSEL shows superior microwave quality in the low-frequency range (e.g., 10 GHz~20 GHz) compared with the QW spin-VCSEL. Finally, to boost the performance of the generated photonic microwave signal, optical feedback is introduced. The results show that dual-loop feedback can simultaneously narrow the microwave linewidth and suppress the side modes that are derived from the external cavity mode.

Chaotic secure communication schemes encounter a conflict of key space enhancement between the consistency and complexity of chaotic transceivers. In this paper, we propose a monolithically integrated multi-section semiconductor laser (MIMSL), used as a compact chaotic transceiver with an enhanced key space. The MIMSL consists of a distributed feedback (DFB) laser section, a semiconductor optical amplifier (SOA) section, two phase (P) sections and a passive optical waveguide. We simulate the dynamics of the MIMSL by applying the time-dependent coupled-wave equations for traveling-wave optical fields. Further, we numerically demonstrate a security enhancement of the unidirectional chaotic communication scheme using the MIMSL transceivers with independent high-speed modulation in the phase sections of the MIMSL. The security of our scheme depends not only on the difficulty of identifying the MIMSL structural parameters and the bias current of each section, but also on the phase shifts in two phase sections providing the additional dimension of security key space. Final simulation results show that a total of 248 key spaces can be achieved with a data rate of 2.5 Gb/s and an injection strength of 0.36.

This research reports a theoretical investigation on the role of filtered optical feedback (FOF) in the quantum dot light emitting diode (QD-LED). The underlying dynamics is affected by a sidle node, which returns to an elliptical shape when the wetting layer (WL) is neglected. Both filter width and time delay change the appearance of different dynamics (chaotic and mixed mode oscillations, MMOs). The results agree with the experimental observations. Here, the fixed point analysis for QDs was done for the first time. For QD-LED with FOF, the system transits from the coherence collapse case in conventional optical feedback to a coherent case with a filtered mode in FOF. It was found that the WL washes out the modes which is an unexpected result. This may attributed to the longer capture time of WL compared with that between QD states. Thus, WL reduces the chaotic behavior. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers subject to tilted optical feedback produce periodic oscillations, quasi-periodic oscillations, and low-frequency oscillations. This work presents the modulation characteristics of period-one (P1) oscillations in a quantum cascade laser with tilted optical feedback. The electrical signal at the oscillation frequency is more than 50 dB higher than the noise level, and the electrical linewidth is less than 2.0 kHz. This electrical linewidth is about four orders of magnitude narrower than the optical linewidth (around 16 MHz) of the free-running laser, which suggests that the optical sidebands induced by the P1 oscillations are highly coherent with the main optical mode. In addition, the modulation depth of the optical signal is found to be in the range of 1% to 3.5%. In addition, it is verified in the simulations that the P1 oscillations induce not only amplitude modulation but also frequency modulation due to the phase-amplitude coupling effect. [ABSTRACT FROM AUTHOR]

A semi-analytical dimensionless formula is presented for predicting the period-one frequency of an optically-injected laser over a wide range of injection powers and positive detunings. The result shows that for any given injected optical power, there exists an optimal detuning at which the period-one frequency is maximized. The formula is applied to a tabletop optical injection system with a commercially-available quantum-well distributed feedback laser with previously unknown design parameters. By extrapolating the linearity of the period-one frequencies at zero-detuning and weak-injection regimes, the physical control parameters of the laser are approximated to the analytical formula for wide ranges of injection powers and positive detunings. The accuracy of the model is experimentally verified, achieving period-one frequencies as high as 84.7 GHz. [ABSTRACT FROM AUTHOR]

Injecting a spectrally pure oscillator signal into a noisy optoelectronic oscillator (OEO) can result in a drastic reduction of the phase noise; however, a unified explanation of this effect may be obscured by implementation-specific models. We show based on an analytically tractable theory that the power spectral density $S_f(\omega)$ of an ideal injection-locked (IL) oscillator signal is composed of a narrow Lorentzian peak at the center frequency lying on top of a broader roughly Lorentzian pedestal using an Ornstein-Uhlenbeck model obtained from the Adler equation in the weak-noise limit. The injected signal imposes its phase on the IL oscillator providing a restoring force for the phase, leading to this fundamentally different behavior of $S_f(\omega)$ compared with a self-sustaining oscillator. Our treatment is shown to be grounded in the physics-based approach for IL OEOs of A. F. Talla, R. Martinenghi, G. R. Goune Chengui, J. H. Talla Mbé, K. Saleh, A. Coillet, G. Lin, P. Woufo, and Y. K. Chembo, “Analysis of phase-locking in narrow-band optoelectronic oscillators with intermediate frequency,” IEEE J. Quantum Electron., vol. 51, 5000108, 2015. [ABSTRACT FROM AUTHOR]

A direct-modulation OEO (DM-OEO) generating stable 10 GHz and 20 GHz signals is presented. A single loop and a dual loop approach are implemented and compared. We show an output signal of 15 dBm RF power, and a phase noise as low as −135 dBc/Hz at 10 kHz offset from the 10 GHz carrier. The 20 GHz second harmonic exhibits a noise level of −127 dBc/Hz at 10 kHz. A high spur level reduction is also obtained in the dual loop architecture. [ABSTRACT FROM AUTHOR]

Photonics-based techniques spearhead the generation of high-frequency signals in the millimeter-and Terahertz wave, crucial for ultrabroadband mobile wireless link development. Photonic integration is enabling to provide the photonic approach with added advantages of energy-efficiency, flexibility and scalability, in addition to signal quality. We present an optical heterodyne system based on a novel dual laser module containing two InP-Si3N4 hybrid lasers with intracavity wavelength selective optical filters with output optical power per laser of up to 15 dBm (31 mW), wide wavelength tuning of about 60 nm, and narrow optical linewidth below 100 kHz. To the best of our knowledge, we present for the first time the continuous-wave generation of RF frequencies over a wide tuning range from C-band (4 GHz – 8-GHz) to W-band (75 GHz – 110 GHz) achieving record low RF electrical linewidth around 108 kHz and long-term drift < 12 MHz with two free-running lasers. This is the best beat-note linewidth obtained with such an integrated source in a free-runnning regime and with a wide tuning range ever reported. [ABSTRACT FROM AUTHOR]

Nonlinear spatiotemporal systems are the basis for countless physical phenomena in such diverse fields as ecology, optics, electronics, and neuroscience. The canonical approach to unify models originating from different fields is the normal form description, which determines the generic dynamical aspects and different bifurcation scenarios. Realizing different types of dynamical systems via one experimental platform that enables continuous transition between normal forms through tuning accessible system parameters is, therefore, highly relevant. Here, we show that a transmissive, optically addressed spatial light modulator under coherent optical illumination and optical feedback coupling allows tuning between pitchfork, transcritical, and saddle-node bifurcations of steady states. We demonstrate this by analytically deriving the system's dynamical equations in correspondence to the normal forms of the associated bifurcations and confirm these results via extensive numerical simulations. Our model describes a nematic liquid crystal device using nano-dimensional dichalcogenide (a-As 2 S 3) glassy thin films as photo sensors and alignment layers, and we use device parameters obtained from experimental characterization. Optical coupling, for example, using diffraction, holography, or integrated unitary maps allows implementing a variety of system topologies of technological relevance for neural networks and potentially Ising or XY-Hamiltonian models with ultralow energy consumption. [ABSTRACT FROM AUTHOR]

This letter presents the first vector network analyzer based subterahertz (sub-THz) phase-compensated channel sounder at 220–330 GHz using radio-over-fiber techniques that could enable long-range phase-coherent measurements. The optical cable solution enables long-range channel measurements at sub-THz bands, since it can effectively minimize the cable loss. This letter also proposes a novel phase compensation scheme to stabilize the phase variations introduced by optical fiber of the channel sounder to enable its application in multichannel/antenna measurements. This proposed channel sounder is validated in back-to-back measurements under two optical cable conditions, i.e., with presence of thermal changes and mechanical stress. The phase variation introduced by the cable effects in the system is shown to be over $400^\circ$ in 220–330 GHz, compared to $15^\circ$ at 220–288 GHz and $37^\circ$ in 288–330 GHz after compensation, respectively, demonstrating the robustness and effectiveness of the developed channel sounder in practice. The developed system, which has a dynamic range of 106.7 dB, can support measurement range up to 300 m (limited by the optical cable length in our system and subject to over-the-air signal transmission loss in practical environment). [ABSTRACT FROM AUTHOR]

This article presents design, fabrication, characterization, and experimental verification of a new sensing method (soft optical sensing) for pneumatic artificial muscle (PAM) actuators. Soft optical waveguides are designed and fabricated using the proposed modified stiffness concept for soft sensors. The design of the sensor allows us to measure both contraction and force of a PAM actuator under loaded conditions. This proposed method makes it possible to have a highly sensitive actuator system with linear contraction sensing. The sensorized PAMs are supposed to work in high-pressure applications with the ability to be used repeatedly without significant degradation in signal quality. To characterize and validate the proposed method, two PAM actuators make use of two separately designed test systems to plot and model the characteristic curves. Finally, the validation for control is provided using measured sensor data as feedback to the controller. [ABSTRACT FROM AUTHOR]

The dynamical properties of an optical neuron formed by a quantum dot semiconductor laser model subjected to optical injection and optical feedback are analyzed. The parameter space spanned by the injection strength and frequency detuning of the optical injection is systematically scanned and modulations of the bifurcation boundaries that induce complex scenarios are found, which enable new opportunities to introduce the optical setup as an optical neuron. The counterintuitive behavior of coherence resonance for different setups of a single‐driven optical neuron under optical feedback is also found. Following the results, the microscopically motivated quantum dot laser rate equation model is reduced to the normal form of a saddle‐node infinite period (SNIPER) bifurcation for low injection strengths and a network of four such SNIPER systems in a globally coupled setup is studied. A new phenomenon is observed, which is called collective coherence resonance. This novel dynamical behavior is connected to the coexistence of a network‐wide SNIPER bifurcation and a change in stability for the synchronized manifold, analyzed via the master stability function. [ABSTRACT FROM AUTHOR]

Parasitic motion corrupts the measurement of all non-contact sensor systems. It thus hinders deployment of these sensors for such real-world applications where it is impossible to keep the sensor perfectly stationary. In this paper, an adaptive algorithm is presented to remove parasitic vibrations corrupting the measurements of a self-mixing (SM) interferometric laser sensor. Previously, this was achieved by coupling a solid state accelerometer (SSA) with the SM sensor followed by either a pre-calibration based design or a time-domain adaptive filter based system. The proposed method is based on adaptive spectral filtering where filter coefficients are derived based on the parasitic vibrations present in the retrieved target motion. Importantly, it neither requires any pre-calibration procedure nor has any dependence on parameters such as filter-order, convergence criteria, or step-size (required in case of time-domain adaptive filters), thus making the proposed scheme a suitable choice for mass production of SSA-SM sensor systems. The proposed algorithm provides improved mean RMS error of 5.37 nm as opposed to 19.1 nm for least mean square (LMS) filter, 20.22 nm for recursive least square (RLS) filter, and 24.7 nm for pre-calibration based system. This method’s performance has also been characterized and an embedded, real-time, hardware design is also presented. At the end, hardware system results (on experimental data-set) and sensor bounds/bandwidth are also quantified. [ABSTRACT FROM AUTHOR]

Wavelength-dependent power excursions in gain-controlled erbium doped fiber amplifiers (EDFA) is a challenging issue in optical networks. We investigate a launch channel power control method using reinforcement learning (RL) to mitigate the power excursions of EDFA systems. A machine learning engine is developed, trained and evaluated with four different policy-gradient RL algorithms that are compared according to two main criteria: achieved power excursion reduction and learning time. Different scenarios are considered with 12-, 24-, 40- active channels at fixed wavelengths and with variable number of active channels (between 12 and 64) assigned randomly at different wavelengths during RL process. We show 62% power excursion reduction in the 40-channel scenario and 28% in the variable scenario, which demonstrates the promising role of online RL approach for controlling power excursion in EDFA systems. [ABSTRACT FROM AUTHOR]

Visuo-motor adaptation with optical prisms that displace the visual scene (prism adaptation, PA) has been widely used to study visuo-motor plasticity in healthy individuals and to decrease the lateralized bias of brain-damaged patients suffering from spatial neglect. Several factors may influence PA aftereffects, such as the degree of optical deviation (generally measured in dioptres of wedge prisms) or the direction of the prismatic shift (leftward vs. rightward). However, the mechanisms through which aftereffects of adaptation in healthy individuals and in neglect affect performance in tasks probing spatial cognition remain controversial. For example, some studies have reported positive effects of PA on auditory neglect, while other studies failed to obtain any changes of performance even in the visual modality. We here tested a new adaptation method in virtual reality to evaluate how sensory parameters influence PA aftereffects. Visual vs. auditory-verbal feedback of optical deviations were contrasted to assess whether rightward deviations influence manual and perceptual judgments in healthy individuals. Our results revealed that altered visual, but not altered auditory-verbal feedback induces aftereffects following adaptation to virtual prisms after 30-degrees of deviation. These findings refine current models of the mechanisms underlying the cognitive effects of virtual PA in emphasizing the importance of visual vs. auditory-verbal feedback during the adaptation phase on visuospatial judgments. Our study also specifies parameters which influence virtual PA and its aftereffect, such as the sensory modality used for the feedback. [ABSTRACT FROM AUTHOR]

Perovskite quantum dots (QDs) are of interest for solution‐processed lasers; however, their short Auger lifetime has limited lasing operation principally to the femtosecond temporal regime the photoexcitation levels to achieve optical gain threshold are up to two orders of magnitude higher in the nanosecond regime than in the femtosecond. Here the authors report QD superlattices in which the gain medium facilitates excitonic delocalization to decrease Auger recombination and in which the macroscopic dimensions of the structures provide the optical feedback required for lasing. The authors develope a self‐assembly strategy that relies on sodiumd—an assembly director that passivates the surface of the QDs and induces self‐assembly to form ordered three‐dimensional cubic structures. A density functional theory model that accounts for the attraction forces between QDs allows to explain self‐assembly and superlattice formation. Compared to conventional organic‐ligand‐passivated QDs, sodium enables higher attractive forces, ultimately leading to the formation of micron‐length scale structures and the optical faceting required for feedback. Simultaneously, the decreased inter‐dot distance enabled by the new ligand enhances exciton delocalization among QDs, as demonstrated by the dynamically red‐shifted photoluminescence. These structures function as the lasing cavity and the gain medium, enabling nanosecond‐sustained lasing with a threshold of 25 µJ cm–2. [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]

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]

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

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

To further improve the performance of cavity-enhanced spectroscopy systems, a high-quality U-cavity system was established. In the process of the experiment, an asymmetric ripple effect, which is different from the previous etalon effect, was found, which seriously affects the performance of the spectral system. This unique phenomenon mainly manifests in the different amplitudes of the fluctuations of the spectral curves measured by the folding mirror and the end mirror in the U-cavity system. Based on multi-beam interference theory, we analyzed the characteristics of the transmission spectrum of each mirror in the presence of the etalon effect at the end mirror, and obtained the following conclusions: for the U-cavity system, the strength of the etalon effect of each mirror is inversely proportional to its transmission loss value, that is, the larger the loss, the smaller the ripple of the transmission spectrum, and vice versa. In order to eliminate this effect, the most effective way is to eliminate the etalon effect caused by the light feedback of the end mirror. After improving the system, the minimum detectable absorption coefficient of αmin=8.33×10−9cm−1 is obtained with this U-shape Optical Feedback Cavity-Enhanced Absorption Spectroscopy. These works are valuable references for the design of folded Cavity-Enhanced Absorption Spectroscopy systems and have potential for laser wavelength calibration and measurement of a mirror’s reflectance.

In spite of its importance for the evaluation of the Earth radiative budget, thus for climate change, very few measurements of the water vapor continuum are available in the near infrared atmospheric windows especially at temperature conditions relevant for our atmosphere. In addition, as a result of the difficulty to measure weak broadband absorption signals, the few available measurements show large disagreements. We report here accurate measurements of the water vapor self-continuum absorption in the 2.1 μm window by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) for two spectral points located at the low energy edge and at the center of the 2.1 μm transparency window, at 4302 and 4723 cm-1, respectively. Self-continuum cross sections, CS, were retrieved with a few % relative uncertainty, from the quadratic dependence of the spectrum base line level measured as a function of water vapor pressure, between 0 and 16 Torr. At 296 K, the CS value at 4302 cm-1 is found 40% higher than predicted by the MT_CKD V2.5 model, while at 4723 cm-1, our value is 5 times larger than the MT_CKD value. On the other hand, these OF-CEAS CS values are significantly smaller than recent measurements by Fourier transform spectroscopy at room temperature. The temperature dependence of the self-continuum cross sections was also investigated for temperatures between 296 K and 323 K (23-50 °C). The derived temperature variation is found to be similar to that derived from previous Fourier transform spectrometer (FTS) measurements performed at higher temperatures, between 350 K and 472 K. The whole set of measurements spanning the 296-472 K temperature range follows a simple exponential law in 1/T with a slope close to the dissociation energy of the water dimer, D0 ≈ 1100 cm-1. [ABSTRACT FROM AUTHOR]

Doppler-free saturated-absorption Lamb dips were measured on weak rovibrational lines of 12C16O2 between 6189 and 6215 cm-1 at sub-Pa pressures using optical feedback frequency stabilized cavity ring-down spectroscopy. By referencing the laser source to an optical frequency comb, transition frequencies for ten lines of the 30013 ← 00001 band P-branch and two lines of the 31113 ← 01101 hot band R-branch were determined with an accuracy of a few parts in 1011. Involving rotational quantum numbers up to 42, the data were used for improving the upper level spectroscopic constants. These results provide a highly accurate reference frequency grid over the spectral interval from 1599 to 1616 nm. [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]

High-speed, low-power optical interconnects, such as intensity modulation direct detection (IMDD) optical links, are increasingly deployed in data centers to keep pace with the growing bandwidth requirements. High-sensitivity low-power optical receivers (RXs) are the key components that enable energy-efficient IMDD optical interconnects. This article presents a low-power nonreturn-to-zero (NRZ) optical RX using a combination of a limited-bandwidth trans-impedance amplifier (TIA) and duobinary sampling to improve RX sensitivity at high data rates. Duobinary sampling leverages the well-controlled TIA inter-symbol interference (ISI) to recover the transmitted data, making it much more hardware efficient than canceling the ISI using a decision feedback equalizer (DFE). The proposed optical RX employs a CMOS-based analog front-end (AFE) to achieve high linearity and excellent power efficiency. Fabricated in 65-nm CMOS process, the prototype RX achieves optical modulation amplitude (OMA) sensitivity of −11.6 dBm at 16 Gb/s with 0.7-pJ/bit efficiency. [ABSTRACT FROM AUTHOR]

The dynamics of semiconductor lasers with optical feedback and current modulation has been extensively studied, and it is, by now, well known that the interplay of modulation and feedback can produce a rich variety of nonlinear phenomena. Near threshold, in the so-called low frequency fluctuations regime, the intensity emitted by the laser, without modulation, exhibits feedback-induced spikes, which occur at irregular times. When the laser current is sinusoidally modulated, under appropriate conditions, the spikes lock to the modulation and become periodic. In previous works, we studied experimentally the locked behavior and found sub-harmonic locking (regular spike timing such that a spike is emitted every two or three modulation cycles), but we did not find spikes with regular timing, emitted every modulation cycle. To understand why 1:1 regular locking was not observed, here, we perform simulations of the well-known Lang–Kobayashi model. We find a good qualitative agreement with the experiments: with small modulation amplitudes, we find wide parameter regions in which the spikes are sub-harmonically locked to the modulation, while 1:1 locking occurs at much higher modulation amplitudes. [ABSTRACT FROM AUTHOR]