Optical square waves (SWs) have been extensively investigated in semiconductor laser diodes (LDs) like VCSELs or EELs under optical feedback and/or optical injection. In this abstract, we discuss optical SW generation in a delay-driven optoelectronic (OE) feedback system. We have found that at high J, the SWs originate from the same branches of the dynamical regime as the gain-switched pulsing found close to the injection threshold (J_th) of a positive optoelectronic feedback system. A single-mode DFB multi-quantum-well (MQW) InGaAsP LD (3SP Technologies-1953LCV1) with J_th of 20 mA is used for this experiment. The origin of the feedback signal is the photodetector output, which is appropriately boosted in the amplifiers/attenuator cascade before feeding it to the radio frequency input arm of the Bias Tee. An oscilloscope measures the optical intensity after the PD. The delay in the feedback loop is τ=10.64 ns. The first appearance of the SW for this particular configuration is recorded at 48.20 mA. The SW appears with a repetition rate of f_τ=τ^(-1)=(10.64 ns)^(-1)=94 MHz. The optical spectrum shows two peaks separated by a frequency related to the duty cycle of the SWs. At higher feedback delay, the SWs appear at harmonics of the fundamental delay frequency. Theoretical analysis based on a delay-differential model and accounting for the multilevel amplification, multistage filtering, and saturable nonlinearity attributes the origin of the SWs to the same branches of dynamical regimes as those observed for the gain-switched pulse-train generation near the J_th and confirms the experimental observation of SW harmonics for higher feedback delays. In conclusion, we experimentally demonstrate SWG in a laser diode subjected to OE delayed feedback on its injection current.
We study delay-based photonic reservoir computing using a semiconductor laser with an optoelectronic feedback. A rate-equation model for a laser with an optoelectronic filtered feedback is used. The filter allows only high-frequency signals to pass through the feedback loop. The delay-differential equation model consists of three equations for the normalized electric field intensity I(t), the carrier density N(t); and the filtered intensity signal IF (t). The stability boundaries which correspond to the Hopf bifurcation condition are determined analytically, showing multiple Hopf bifurcation branches in the dynamics, and the parity asymmetry with relation to the feedback sign.
We use the Santa Fe time-series prediction task to evaluate the performance of reservoir computing. Our objective is to determine location of the optimal operating point defined as corresponding to minimal normalized means square error (NMSE) and relate it to the stability properties of the system. We use 3000 points for training and 1000 for testing, number of virtual nodes is chosen in regard to the relaxation oscillation frequency. Single-point prediction of the chaotic data is performed. Input signal is determined by the chaotic waveform
having n sampling points, and three cases are investigated: prediction of n + 1 ,n + 2 or n + 3 sampling point. The best NMSE value order of 10^7 for n + 1 point prediction task is obtained in the absence of feedback and the rapid increase in NMSE is observed in the vicinity of Hopf bifurcation without regard to the feedback sign. On the contrary, the minimum values of NMSE for n + 2 and n + 3 point prediction task correspond to the Hopf bifurcation, and only for the positive feedback. We discuss whether the parity asymmetry can explain strongly asymmetric reservoir computing results.
Time-domain terahertz reflectometry has long been of interest to obtain three-dimensional information about objects transparent in the 100 GHz-3 THz frequency range. The application of such techniques has been limited by the conductivity of the materials and by conventional axial resolution criteria. Specifically, I discuss work on detecting subsurface damage in carbon fiber composites and in measuring paint-layer thicknesses down to ~20 microns. We employ a typical commercial broadband time-domain source with bandwidth from ~100 GHz to ~3 THz. Yet terahertz imaging faces two important limitations. (1) The first is that in materials with a significant conductivity, the attenuation of incident terahertz electromagnetic waves is very rapid, limiting the penetration into the object. (2) The second limitation can best be illustrated by a stratified medium. Due to the Fresnel coefficients associated with the variation of the refractive index across material interfaces, one expects a reflected signal that is composed of various echoes associated with the interfaces (as well as multiple reflections which in practice are often weak). If the layers are sufficiently thin, then the echoes from successive layers will overlap, and thus multiple echoes will not be visually evident in the reflected signal.
In this talk I discuss my group’s recent work to circumvent these limitations. The approaches rely upon applying advanced signal-processing techniques to extract the maximum information from the detected signals. Specifically, I provide case studies of coatings on metals, fiber composite laminates, and an oil painting on canvas with multiple paint layers.
We explore experimentally and theoretically the dynamics of a DFB quantum well laser subject to external optical feedback from a mirror. With increasing feedback, the system exhibits the following dynamical scenario: an extremely small limit cycle appears first and is followed by a quasi-periodic regime, and then by three subsequent limit cycles with different repetition rates. This sequence of limit cycles can be associated with the change of phase of the reflected field which reveals translational symmetry and the fact of periodic solutions coexistence which we confirm numerically. The results can be useful for applications in reservoir computing with phase space of coexisting limit cycles acting as a nonlinear reservoir as well as for other applications.
Terahertz pulsed imaging has attracted considerable interest for revealing the stratigraphy and hidden features of art paintings. The reconstruction of the stratigraphy is based on the precise extraction of THz echo parameters from the reflected signals. Several historical panel paintings and wall paintings have been well studied by THz reflective imaging, in which the detailed stratigraphy has been successfully revealed. To our knowledge, however, the stratigraphy of oil paintings has not been clearly uncovered by THz imaging, since the paint layers in an oil painting on canvas, especially for the 16th and 17th century art works, are usually very thin (~10 μm) in the THz regime. Therefore, in order to improve the performance of THz imaging, advanced signal-processing techniques with higher depth-resolution are still needed. In this study, THz reflective imaging is employed to reveal for the first time the detailed stratigraphy of a 17th century Italian oil painting on canvas. The paint layers on the supporting canvas are very thin in the THz regime, as the THz echoes corresponding to the stratigraphy totally overlap in the first cycle of the reflected THz signal. THz sparse deconvolution based on an iterative shrinkage algorithm is utilized to resolve the overlapping echoes. Based on the deconvolved signals, the detailed stratigraphy of this oil painting on canvas, including the varnish, pictorial, underdrawing, and ground layers, is successfully revealed. The THz C- and B-scans based on the THz deconvolved signals also enable us to reveal the features of each layer. Our results thus enhance the capability of terahertz imaging to perform detailed analysis and diagnostics of historical oil paintings on canvas with foreseen applications for the study of the artist’s technique and for authentication.
The stabilization of a relatively simple optoelectronic oscillator tunable across the X-band based on a laser subjected to optical feedback is achieved. Specifically, a resonance effect based on locking the two inherent frequencies of the system, as well as, self-modulation were utilized to achieve a sub-ps phase jitter.
In the last ten years terahertz techniques have become increasingly common laboratory and industrial tools. This progress has been made possible by over thirty years of concentrated effort. In this talk we discuss our recent work combining time-domain terahertz imaging with advanced signal-processing to obtain unprecedented depth information in a nondestructive fashion about subsurface damage in both glass and carbon fiber composites and in coatings on metals. In addition, we present an example characterizing the stratigraphy of an art painting to illustrate the technique to measure thicknesses in a multilayer coating. Other optically opaque materials, including polymers, glass, textiles, paper, and ceramics, are transparent to terahertz radiation, and thus terahertz imaging may access information in these materials below the surface.
Signal processing techniques are needed to unleash the power of terahertz imaging to measure thin layers of thickness on the order of 10 microns. These approaches permit us to gain information about thin layers that are obscured in the raw signals. That is, when the time duration of the terahertz pulses is longer than the optical delay to traverse a given layer, the terahertz echoes associated with reflections off the various interfaces may temporally overlap. Specifically, we have successfully employed frequency-wavelet domain deconvolution, sparse deconvolution, and autoregressive deconvolution for a range of problems.
We explore both experimentally and numerically the dynamics of semiconductor lasers subject to delayed optical feedback and show that the external cavity repetition rate can be resonant with the relaxation oscillations leading to a discretisation of the relaxation oscillation frequency which evolves in a series of discrete steps, remaining almost constant along each step. Numerically, the steps are found to result from different Hopf bifurcation branches.
Periodic arrays of proximate but noncontacting noble metal nanoparticles provide the ability to electromagnetically couple the surface plasmons on various nanoparticles to produce collective delocalized surface-plasmon-polariton modes with well characterized mode structure. Such structures provide intrinsic interest as well as provides an attractive concept for optical sensing in potentially open structures permitting gas or liquid to percolate through the structure. In this talk, I review our work on modeling the surface-plasmon-polariton modes of various nanoparticle arrays. I discuss one-dimensional chains and two-dimensional nanoparticle slabs. Ways of dealing with losses will be discussed.
Provided the ratio of nanoparticle spacing to nanoparticle diameter (for spherical nanoparticles) exceeds a critical value, the electromagnetic interactions between nanoparticles are dominated by dynamic dipole-dipole coupling. In other words, one nanoparticle acts on the others (and on itself) by means of the retarded electromagnetic dipole-dipole interactions between all nanoparticles. The resulting collective modes delocalized over the structure are surface-plasmon polaritons. We consider various structures including infinite and finite chains, ring-shaped arrangements of nanoparticles, and two-dimensional slab arrays. Given the dispersion of surface-plasmon polaritons, we can then obtain the linear optical properties (e.g., reflection and transmission spectra, angle-dependence of reflectivity and transmittivity), which in turn provide important information on the design of nanoparticle-array-based devices.
Surface-plasmon polaritons are strongly attenuated by homogeneous and inhomogenous broadening as well as by intrinsic radiative loss. We discuss strategies for mitigating these losses.
Terahertz (THz) reflective imaging is applied to the stratigraphic and subsurface investigation of oil paintings, with a focus on the mid-20th century Italian painting, ‘After Fishing’, by Ausonio Tanda. THz frequency-wavelet domain deconvolution, which is an enhanced deconvolution technique combining frequency-domain filtering and stationary wavelet shrinkage, is utilized to resolve the optically thin paint layers or brush strokes. Based on the deconvolved terahertz data, the stratigraphy of the painting including the paint layers is reconstructed and subsurface features are clearly revealed. Specifically, THz C-scans and B-scans are analyzed based on different types of deconvolved signals to investigate the subsurface features of the painting, including the identification of regions with more than one paint layer, the refractive-index difference between paint layers, and the distribution of the paint-layer thickness. In addition, THz images are compared with X-ray images. The THz image of the thickness distribution of the paint exhibits a high degree of correlation with the X-ray transmission image, but THz images also reveal defects in the paperboard that cannot be identified in the X-ray image. Therefore, our results demonstrate that THz imaging can be considered as an effective tool for the stratigraphic and subsurface investigation of art paintings. They also open up the way for the use of non-ionizing THz imaging as a potential substitute for ionizing X-ray analysis in nondestructive evaluation of art paintings.
We use a laser diode from a commercial CD/DVD-ROM drive to detect changes in the surface of a diffraction grating without a photodiode. Specifically, we exploit the changing terminal voltage in the laser-diode due to changing feedback strength as the laser is rastered across the grating's surface.
We demonstrate experimentally that optical chaos generated by a laser diode with optical feedback is suitable for compressive sensing of sparse signals. Specifically, we find that the coherence collapse regime guarantees that the generation of a sensing matrix, necessary for sparse reconstruction, has a comparable level of performance to those constructed with Gaussian random sequences. Our result opens new avenues for the use of optical chaotic devices for signal processing applications at ultra-high speed.
We report experimental bifurcation diagrams (BDs) of an external-cavity semiconductor laser (ECSL). We have focused on the case of the ECSL biased just above threshold to moderate and subjected to feedback from a distant reflector and observed a sequence of bifurcations involving bifurcation cascade as well as intermittency between multiple coexisting attractors. More importantly, we reiterate: the results map out, for the first time to our knowledge, detailed BDs of the ECSL as a function of feedback strength for various external cavity lengths and currents, thus covering a significant portion of parameter space. We have grounded our discussion in extensive theoretical studies based on the Lang-Kobayashi equations and simulated BDs in accordance with our experimental results.
Random bit generation (RBG) with chaotic semiconductor lasers has been extensively studied because of its potential applications in secure communications and high-speed numerical simulations. Researchers in this field have mainly focused on the improvement of the generation rate and the compactness of the random bit generators. In this paper, we experimentally demonstrate the existence of two regimes of fast RBG using a single chaotic laser subjected to delayed optical feedback: the first one is based on the extraction of all min-entropy contained in each random sample, and the second one is to demonstrate a possibility of increasing the generation rate by extracting 55 bits from each variable.
By employing the enhanced phase transfer of single microring resonator, the enhanced intensity sensing is obtained
based on the dual microring resonator with coupled mode theory. The two times higher sensitivity than the conventional
microring sensor is demonstrated based on the FDTD simulation.
Optical chaos-based cryptosystems hide an information-bearing message within the chaotic dynamics of a laser system.
On-off phase-shift keying (OOPSK) is considered to be a particularly efficient encryption technique. It is based on the
modulation of the feedback phase of a chaotic external-cavity emitter laser at the rhythm of a digital message. At the
receiving end, message values are decrypted by observing the synchronization and de-synchronization of an external-cavity
receiver which has a constant feedback phase. This cryptosystem is popular because so far it has been thought to
be impossible to find the message by analyzing the chaotic optical field transmitted from the emitter to the receiver laser,
thus, it is hitherto thought, providing high security. We demonstrate that the phase modulation produces a displacement
of the chaotic attractor which is detectable by analyzing low-dimensional projections or sections of the high-dimensional
attractor. This leads to the successful decryption of the message value based on an analysis of the chaotic optical field
sent to the receiver only. We show that the bit-error-rate (BER) of the decrypted message varies with the modulation
depth and speed. Though small depths and large bit rates lead to an increase of the BER, we find it possible to extract the
message for most operating conditions of an on/off phase- shift keying-based cryptosystem.
We present an architecture tailored for the multiplexing of multiple optical chaotic carriers generated by semiconductor
lasers with external optical cavities. Our setup can discriminate multiple chaotic signals with high spectral overlap. The
various emitters are mutually globally coupled thanks to a shared optical feedback, which creates a multiplexed optical
field. This field is then coherently and unidirectionally injected in the decoupled receivers, and allows each of them to
synchronize on their respective emitter. Using this setup, it would be possible to transmit several messages and make a
better use of the wide chaotic spectrum. In this paper, we demonstrate theoretically and numerically the possibility to
synchronize two optical chaotic fields as a premise for the transmission of two messages. We also study the robustness of
synchronization to parameter mismatch and noise, which are important issues in real field experiments.
Fabrication of silicon microring resonators was optimized by using electron-beam lithography (EBL) and inductively coupled plasma (ICP) etching with different mask materials. Sidewall roughness of less than 10 nm was revealed by high-resolution scanning electron microscopy (SEM) without any post-etch process. The fabrication processes are described in detail, and comparisons are made in consideration of process complexity, process latitude, and sidewall roughness.
We investigate theoretically the identification of the
external-cavity roundtrip time of an external-cavity semiconductor
laser (ECSL). The time-delay identification is performed by analyzing the laser-intensity time series with conventional
techniques based on the autocorrelation function or mutual information. We find that a weak feedback rate and a time-delay
close to the laser's intrinsic relaxation-oscillation period are two conditions leading to difficult delay identification.
This arduous time-delay identification is of particular interest for the security improvement of chaos-based
communications schemes using ECSLs.
We study mid-infrared quantum-cascade lasers (QCL's) with a pair of triply harmonic resonant levels. Potential
applications of such designs are discussed based on the resonant third-order nonlinear susceptibility χ(3) at the third-harmonic
(TH) frequency, χ(3)(3ω; ω, ω, ω), and that at fundamental mode (FM) frequency χ(3)( ω; ω, ω,-ω). Mode
overlap and the phase mismatch effect are considered in the TH power evaluation. In addition to single-photon
processes, resonant two-photon processes are included in the evaluation of χ(3)( ω; ω, ω,-ω), which results in the
enhancement of the positive nonlinear (Kerr) refractive index, and thus induce stronger self-phase modulation (SPM). It
is concluded that QCL's with multiple-resonance design are not only favorable for tunable light emission in the near- to
mid-infrared region, but are also promising candidates for spectrum broadening by SPM.
We describe two Si based optical-electric modulators based on photonic crystals (PC), which are capable of monolithically integrated with Si photonic integrated circuits. One is a modulator based on Mach-Zehnder interferometer (MZI), the other is a modulator based on photonic band gap. These devices may enable the deployment of ultra-compact (-200 μm) devices with high extinction ration and low insertion loss.
Theoretical calculations of the optical properties of InGaAsP quantum well (QW) electroabsorption modulators (EAM's)
operating at c-band (~1550 nm wavelength) is presented. Absorption coefficients of QW's are obtained from the linear
optical susceptibility. Excitons are calculated in momentum space, which includes valence-band mixing, mixing of
excitons originating in different subband pairs, and exciton spin-related optical selection rules. Various line-broadening
mechanisms relevant to InGaAsP-QW's are also included. Investigations on asymmetric double QW's (ADQW's) show
that the small-signal modulation efficiency, which is an important figure of merit for analog application, can be
enhanced significantly at substantially reduced operating bias voltage. Simple optimization of ADQW band structure
results in a maximum slope efficiency ~3.8 times larger than that of SQW EAM's at a reduced operating bias field of 34
kV/cm compared with ~70 kV/cm for comparable SQW's.
We present two-dimensional photonic-crystal waveguides for fluid-sensing applications in the sub-terahertz range. The
structures are produced using a standard machining processes and are characterized in the frequency range from 67 to
110 GHz using a vector network analyzer. The photonic crystal consists of an air-hole array drilled into a high-density
polyethylene block. A waveguide is introduced by reducing the diameter of the holes in one row. The holes can be
loaded with liquid samples. For all structures we observe photonic band gaps between 97 and 109 GHz. While the pure
photonic crystal shows the deepest stop band (28 dB), its depth is reduced by 5 dB when inserting a waveguiding
structure. The depth of the photonic band gap is further reduced by several decibels depending on the refractive index of
the liquid that is inserted. With this type of fluid sensor we can clearly distinguish between cyclohexane and
tetrachloromethane with refractive indices of 1.42 and 1.51, respectively. The results are in good agreement with
theoretical calculations based on the 2D finite-difference time-domain (FDTD) method.
We study the identification of the delays of several chaotic optical cryptosystems subjected to one or two delayed feedbacks. We show that the delay of a single-delay system can be identified, even if highly complex chaos is used. For certain types of systems with two delays, the same identification techniques that work for single-delay systems also work for multiple-delay systems. These systems thus do not provide a significant increase of the security level. A careful choice of the architecture of multiple-delay system can, however, make these techniques fail. We propose some higher-dimensional techniques that lead to the identification of the delays for these architectures too. The increased complexity of these techniques means, however, that it takes a significantly longer time to identify the delays.
Dephasing of optical excitations in semiconductor quantum dots (QDs) has recently received much attention. A common model used for understanding such processes is a two-level electronic system interacting with phonons. In our work we construct a consistent non-perturbative theory of the ZPL homogeneous broadening and resolve the contradictions by pointing out the limits of validity of the theoretical papers mentioned.
We outline an engineering approach to modeling the optical properties of semiconductor quantum wells which are driven by a growth-direction polarized electric field at frequency in the THz range. The approach is based on solving the Schroedinger equation for the electron-hole envelope wavefunction with inclusion of the excitonic effects. Unlike the usual case of a dc applied field when the optical response is a time-independent function, the presence of the THz field requires introduction of a response function with periodicity given by the THz period. Our focus is on the linear, with respect to the optical power, regime while the THz field can be strong and thus must be accounted nonperturbatively.
We investigate the electro-optic effect in quantumwire (QWR) arrays and discuss possible applications for self-electro-optic-effect devices. By application of a static electric field normal to the growth planes the interband optical selection rules are modulated between one and two dimensional due to heavy-holelight-hole mixing. I.
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