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 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 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.
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.
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.
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.
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 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.
We study numerically the synchronization regimes of optical-feedback-induced chaos in unidirectionally coupled semiconductor lasers. Depending on the operating conditions, the receiver laser intensity, IR(t), synchronizes with the intensity of the optically injected field, IT(t-tc), where tc is the flight time from the transmitter laser to the receiver laser, or synchronizes with IT(t-tc+t), where t is the is the external-cavity round-trip time. In the latter case, the intensity produced by the receiver anticipates the injected intensity IT(t-tc) by an anticipation time equal to t. We find that synchronization with a lag time tc-t is much more sensitive to frequency detuning and noise than synchronization with a lag time tc. Moreover, we also find that though synchronization with a lag time tc can be obtained using either an external-cavity or an open-loop receiver, the synchronization quality is always better when an external-cavity receiver is used.
We demonstrate numerically a secure communication scheme based on the synchronization of two chaotic laser diodes that are respectively subjected to incoherent optical feedback and incoherent optical injection. In this scheme, the optical fields emitted by the two lasers and the fields that are fed back and injected into these two lasers have orthogonal polarizations. Consequently, the external fields do not coherently interact with the lasing fields but only act on the population inversions. Synchronization of both lasers does not require fine tuning of their optical frequencies neither accurate control of the external cavity lengths contrary to the cryptographic systems based on conventional optical feedback. The message encoding/decoding is achieved by chaos shift keying.