Reservoir computing based on transverse modes in a single optical waveguide.

In this work, a passive reservoir computing scheme is presented, which relies on the excitation and guiding of transverse modes in a large cross-section photonic waveguide. The optical modes act similar to conventional waveguide-based reservoir nodes, whereas random defects act as the operational equivalent of internode connections. Therefore, the proposed scheme's number of nodes and connections can scale with the waveguide's volume, contrary to planar implementations. The principle of operation, alongside the role of mode defects to performance, is validated through a numerical model, whereas the classification of time-dependent, 2D analog/digital image streams is used as an application-relevant benchmark test.

Physical Unclonable Function based on a Multi-Mode Optical Waveguide.

Physical unclonable functions are the physical equivalent of one-way mathematical transformations that, upon external excitation, can generate irreversible responses. Exceeding their mathematical counterparts, their inherent physical complexity renders them resilient to cloning and reverse engineering. When these features are combined with their time-invariant and deterministic operation, the necessity to store the responses (keys) in non-volatile means can be alleviated. This pivotal feature, makes them critical components for a wide range of cryptographic-authentication applications, where sensitive data storage is restricted. In this work, a physical unclonable function based on a single optical waveguide is experimentally and numerically validated. The system's responses consist of speckle-like images that stem from mode-mixing and scattering events of multiple guided transverse modes. The proposed configuration enables the system's response to be simultaneously governed by multiple physical scrambling mechanisms, thus offering a radical performance enhancement in terms of physical unclonability compared to conventional optical implementations. Additional features like physical re-configurability, render our scheme suitable for demanding authentication applications.

Artificial Neuron Based on Integrated Semiconductor Quantum Dot Mode-Locked Lasers.

Neuro-inspired implementations have attracted strong interest as a power efficient and robust alternative to the digital model of computation with a broad range of applications. Especially, neuro-mimetic systems able to produce and process spike-encoding schemes can offer merits like high noise-resiliency and increased computational efficiency. Towards this direction, integrated photonics can be an auspicious platform due to its multi-GHz bandwidth, its high wall-plug efficiency and the strong similarity of its dynamics under excitation with biological spiking neurons. Here, we propose an integrated all-optical neuron based on an InAs/InGaAs semiconductor quantum-dot passively mode-locked laser. The multi-band emission capabilities of these lasers allows, through waveband switching, the emulation of the excitation and inhibition modes of operation. Frequency-response effects, similar to biological neural circuits, are observed just as in a typical two-section excitable laser. The demonstrated optical building block can pave the way for high-speed photonic integrated systems able to address tasks ranging from pattern recognition to cognitive spectrum management and multi-sensory data processing.

Power losses in diffuse ultraviolet optical communications channels.

One of the most critical parameters in free-space optical communications systems operating in a non-line-of-sight regime are the optical losses. In this Letter, we numerically calculate these losses taking into account the scattering effects using the Monte Carlo simulation technique. The obtained results are compared with experimentally obtained data at 265 nm (solar-blind UV regime). A large set of measurements at distances up to 20 m, for different elevation angles of the transmitter (UV-LEDs) and receiver (photomultiplier tube) and for different atmospheric conditions has been taken for the characterization of the optical communications channel in terms of its loss properties.

Performance evaluation of modulation and multiple access schemes in ultraviolet optical wireless connections for two atmosphere thickness cases.

The exploitation of optical wireless communication channels in a non-line-of-sight regime is studied for point-to-point and networking configurations considering the use of light-emitting diodes. Two environments with different scattering center densities are considered, assuming operation at 265 nm. The bit error rate performance of both pulsed and multicarrier modulation schemes is examined, using numerical approaches. In the networking scenario, a central node only receives data, one node transmits useful data, and the rest of them act as interferers. The performance of the desirable node's transmissions is evaluated. The access to the medium is controlled by a code division multiple access scheme.

Broadband telecom to mid-infrared supercontinuum generation in a dispersion-engineered silicon germanium waveguide.

We demonstrate broadband supercontinuum generation (SCG) in a dispersion-engineered silicon-germanium waveguide. The 3 cm long waveguide is pumped by femtosecond pulses at 2.4 µm, and the generated supercontinuum extends from 1.45 to 2.79 µm (at the -30 dB point). The broadening is mainly driven by the generation of a dispersive wave in the 1.5-1.8 µm region and soliton fission. The SCG was modeled numerically, and excellent agreement with the experimental results was obtained.

High-speed all-optical pattern recognition of dispersive Fourier images through a photonic reservoir computing subsystem.

In this Letter, we present and fully model a photonic scheme that allows the high-speed identification of images acquired through the dispersive Fourier technique. The proposed setup consists of a photonic reservoir-computing scheme that is based on the nonlinear response of randomly interconnected InGaAsP microring resonators. This approach allowed classification errors of 0.6%, whereas it alleviates the need for complex high-cost optoelectronic sampling and digital processing.

Integrated semiconductor laser with optical feedback: transition from short to long cavity regime.

A 4-section semiconductor laser with integrated optical feedback has been shown experimentally to be capable of operating in either the short- or long-cavity regime, by controlling the device relaxation oscillation frequency relative to the external cavity frequency. Systematic increase of the laser injection current, and the resulting increase in relaxation oscillation frequency, allowed the transition between the two regimes of operation to be observed. The system displayed a gradual transition from a dynamic dominated by regular pulse packages in the short-cavity regime to one dominated by broadband chaotic output when operating in the long-cavity regime. This suggests that the "short cavity" regular pulse packages continue to co-exist with the "long cavity" broadband chaotic dynamic in the system studied. It is the relative power associated with each of these dynamics that changes. This may occur more generally in similar systems.

Two-mode injection-locked FP laser receiver: a regenerator for long-distance stable fiber delivery of radio-frequency standards.

We propose and experimentally validate a new cost-effective optical receiver-regenerator scheme for long-distance microwave-frequency standard dissemination, based on the properties of dual-wavelength injection locked Fabry-Perot (FP) lasers. The regenerator FP laser is injection locked to one of its longitudinal modes by the incoming intensity-modulated light carrying the microwave-frequency standard. The light of a local CW laser is also injected in the regenerator FP, locking it to an adjacent mode. The dual-injection locked laser reproduces the sinusoidal microwave-frequency standard on both wavelengths. The regenerated original signal is transmitted to the next node, whilst the local wavelength is fed back to the previous node for phase error extraction and link compensation. The performance of the proposed regenerator is demonstrated with Allan deviation and phase-noise measurements.

Picosecond pulse amplification up to a peak power of 42 W by a quantum-dot tapered optical amplifier and a mode-locked laser emitting at 1.26 µm.

We experimentally study the generation and amplification of stable picosecond-short optical pulses by a master oscillator power-amplifier configuration consisting of a monolithic quantum-dot-based gain-guided tapered laser and amplifier emitting at 1.26 µm without pulse compression, external cavity, gain- or Q-switched operation. We report a peak power of 42 W and a figure-of-merit for second-order nonlinear imaging of 38.5 W2 at a repetition rate of 16 GHz and an associated pulse width of 1.37 ps.

Towards nonlinear conversion from mid- to near-infrared wavelengths using Silicon Germanium waveguides.

We demonstrate the design, fabrication and characterization of a highly nonlinear graded-index SiGe waveguide for the conversion of mid-infrared signals to the near-infrared. Using phase-matched four-wave mixing, we report the conversion of a signal at 2.65 µm to 1.77 µm using a pump at 2.12 µm.

FWM-based wavelength conversion of 40 Gbaud PSK signals in a silicon germanium waveguide.

We demonstrate four wave mixing (FWM) based wavelength conversion of 40 Gbaud differential phase shift keyed (DPSK) and quadrature phase shift keyed (QPSK) signals in a 2.5 cm long silicon germanium waveguide. For a 290 mW pump power, bit error ratio (BER) measurements show approximately a 2-dB power penalty in both cases of DPSK (measured at a BER of 10(-9)) and QPSK (at a BER of 10(-3)) signals that we examined.

Optical properties of silicon germanium waveguides at telecommunication wavelengths.

We present a systematic experimental study of the linear and nonlinear optical properties of silicon-germanium (SiGe) waveguides, conducted on samples of varying cross-sectional dimensions and Ge concentrations. The evolution of the various optical properties for waveguide widths in the range 0.3 to 2 µm and Ge concentrations varying between 10 and 30% is considered. Finally, we comment on the comparative performance of the waveguides, when they are considered for nonlinear applications at telecommunications wavelengths.

Tapered InAs/InGaAs quantum dot semiconductor optical amplifier design for enhanced gain and beam quality.

In this Letter, a design for a tapered InAs/InGaAs quantum dot semiconductor optical amplifier is proposed and experimentally evaluated. The amplifier's geometry was optimized in order to reduce gain saturation effects and improve gain efficiency and beam quality. The experimental measurements confirm that the proposed amplifier allows for an elevated optical gain in the saturation regime, whereas a five-fold increase in the coupling efficiency to a standard single mode optical fiber is observed, due to the improvement in the beam quality factor M² of the emitted beam.

Phase synchronization scheme for a practical phase sensitive amplifier of ASK-NRZ signals.

We present a phase locking scheme that enables the demonstration of a practical dual pump degenerate phase sensitive amplifier for 10 Gbit/s non-return to zero amplitude shift keying signals. The scheme makes use of cascaded Mach Zehnder modulators for creating the pump frequencies as well as of injection locking for extracting the signal carrier and synchronizing the local lasers. An in depth optimization study has been performed, based on measured error rate performance, and the main degradation factors have been identified.

Space-time block code based MIMO encoding for large core step index plastic optical fiber transmission systems.

The performance of Space-Time Block Codes combined with Discrete MultiTone modulation applied in a Large Core Step-Index POF link is examined theoretically. A comparative study is performed considering several schemes that employ multiple transmitters/receivers and a fiber span of 100 m. The performance enhancement of the higher diversity order configurations is revealed by application of a Margin Adaptive Bit Loading technique that employs Chow's algorithm. Simulations results of the above schemes, in terms of Bit Error Rate as a function of the received Signal to Noise Ratio, are provided. An improvement of more than 6 dB for the required electrical SNR is observed for a 3 × 1 configuration, in order to achieve a 10(-3) BER value, as compared to a conventional Single Input Single output scheme.

Implementation of 140 Gb/s true random bit generator based on a chaotic photonic integrated circuit.

In the present work a photonic integrated circuit (PIC) that emits broadband chaotic signals is employed for ultra-fast generation of true random bit sequences. Chaotic dynamics emerge from a DFB laser, accompanied by a monolithic integrated 1-cm long external cavity (EC) that provides controllable optical feedback. The short length minimizes the existence of external cavity modes, so flattened broadband spectra with minimized intrinsic periodicities can emerge. After sampling and quantization--without including optical de-correlation techniques and using most significant bits (MSB) elimination post-processing--truly random bit streams with bit-rates as high as 140 Gb/s can be generated. Finally, the extreme robustness of the random bit generator for adaptive bit-rate operation and for various operating conditions of the PIC is demonstrated.

Dual-wavelength mode-locked quantum-dot laser, via ground and excited state transitions: experimental and theoretical investigation.

We report a dual-wavelength passive mode locking regime where picosecond pulses are generated from both ground (lambda = 1263 nm) and excited state transitions (lambda = 1180 nm), in a GaAs-based monolithic two-section quantum-dot laser. Moreover, these results are reproduced by numerical simulations which provide a better insight on the dual-wavelength mode-locked operation.

Chaos-on-a-chip secures data transmission in optical fiber links.

Security in information exchange plays a central role in the deployment of modern communication systems. Besides algorithms, chaos is exploited as a real-time high-speed data encryption technique which enhances the security at the hardware level of optical networks. In this work, compact, fully controllable and stably operating monolithic photonic integrated circuits (PICs) that generate broadband chaotic optical signals are incorporated in chaos-encoded optical transmission systems. Data sequences with rates up to 2.5 Gb/s with small amplitudes are completely encrypted within these chaotic carriers. Only authorized counterparts, supplied with identical chaos generating PICs that are able to synchronize and reproduce the same carriers, can benefit from data exchange with bit-rates up to 2.5Gb/s with error rates below 10(-12). Eavesdroppers with access to the communication link experience a 0.5 probability to detect correctly each bit by direct signal detection, while eavesdroppers supplied with even slightly unmatched hardware receivers are restricted to data extraction error rates well above 10(-3).

Hurst exponents and cyclic scenarios in a photonic integrated circuit.

We experimentally report the cyclic scenario of birth and annihilation of periodic orbits in a photonic integrated circuit as the feedback phase of the electric field varies. The latter is also shown to result in minimal alterations in the statistical properties of the chaotic attractor, with simultaneously transiting the Hurst exponent H , erratically, below and above the critical value of H=0.5 that indicates regular Brownian motion. Consequently there is an indication of the most effective operating regions with minimized predictability, which hinders eavesdropping and the progress of forecasting the development of the chaotic light carrier.