Optoelectronic coherent Ising machine for combinatorial optimization problems.

Hopfield networks are iterative procedures able to solve combinatorial optimization problems. New studies regarding algorithm-architecture adequacy are fostered by the re-emergence of hardware implementations of such methods in the form of Ising machines. In this work, we propose an optoelectronic architecture suitable for fast processing and low energy consumption. We show that our approach allows effective optimization relevant to statistical image denoising.

Low-κ, narrow linewidth III-V-on-SOI distributed feedback lasers with backside sampled Bragg gratings.

We demonstrate a heterogeneously integrated III-V-on-SOI distributed feedback laser with a low grating strength (κ < 40 cm-1) and a narrow linewidth of Δν = 118 kHz. The laser operates single mode with a side-mode suppression ratio over 45 dB, provides a single-sided waveguide-coupled output power of 22 mW (13.4 dBm) and has a wall-plug efficiency of 17%. The dynamic characteristics were also evaluated, obtaining an intrinsic 3 dB modulation bandwidth of 14 GHz and a photon lifetime of 8 ps. Large-signal intensity modulation using a 231-1 PRBS pattern length revealed open eye diagrams up to 25 Gb/s and a penalty on the dynamic extinction ratio lower than 1 dB after transmission over a 2 km standard single mode optical fiber.

32QAM WDM transmission at 12 Tbit/s using a quantum-dash mode-locked laser diode (QD-MLLD) with external-cavity feedback.

Chip-scale frequency comb generators lend themselves as multi-wavelength light sources in highly scalable wavelength-division multiplexing (WDM) transmitters and coherent receivers. Among different options, quantum-dash (QD) mode-locked laser diodes (MLLD) stand out due to their compactness and simple operation along with the ability to provide a flat and broadband comb spectrum with dozens of equally spaced optical tones. However, the devices suffer from strong phase noise, which impairs transmission performance of coherent links, in particular when higher-order modulation formats are to be used. Here we exploit coherent feedback from an external cavity to drastically reduce the phase noise of QD-MLLD tones, thereby greatly improving the transmission performance. In our experiments, we demonstrate 32QAM WDM transmission on 60 carriers derived from a single QD-MLLD, leading to an aggregate line rate (net data rate) of 12 Tbit/s (11.215 Tbit/s) at a net spectral efficiency (SE) of 7.5 bit/s/Hz. To the best of our knowledge, this is the first time that a QD-MLLD optical frequency comb has been used to transmit an optical 32QAM signal. Based on our experimental findings, we perform simulations that show that feedback-stabilized QD-MLLD should also support 64QAM transmission with a performance close to the theoretical optimum across a wide range of technically relevant symbol rates.

Comb-based WDM transmission at 10 Tbit/s using a DC-driven quantum-dash mode-locked laser diode.

Chip-scale frequency comb generators have the potential to become key building blocks of compact wavelength-division multiplexing (WDM) transceivers in future metropolitan or campus-area networks. Among the various comb generator concepts, quantum-dash (QD) mode-locked laser diodes (MLLD) stand out as a particularly promising option, combining small footprint with simple operation by a DC current and offering flat broadband comb spectra. However, the data transmission performance achieved with QD-MLLD was so far limited by strong phase noise of the individual comb tones, restricting experiments to rather simple modulation formats such as quadrature phase shift keying (QPSK) or requiring hardware-based compensation schemes. Here we demonstrate that these limitations can be overcome by digital symbol-wise blind phase search (BPS) techniques, avoiding any hardware-based phase-noise compensation. We demonstrate 16QAM dual-polarization WDM transmission on 38 channels at an aggregate net data rate of 10.68 Tbit/s over 75 km of standard single-mode fiber. To the best of our knowledge, this corresponds to the highest data rate achieved through a DC-driven chip-scale comb generator without any hardware-based phase-noise reduction schemes.

Coherent WDM transmission using quantum-dash mode-locked laser diodes as multi-wavelength source and local oscillator.

Quantum-dash (QD) mode-locked laser diodes (MLLD) lend themselves as chip-scale frequency comb generators for highly scalable wavelength-division multiplexing (WDM) links in future data-center, campus-area, or metropolitan networks. Driven by a simple DC current, the devices generate flat broadband frequency combs, containing tens of equidistant optical tones with line spacings of tens of GHz. Here we show that QD-MLLDs can not only be used as multi-wavelength light sources at a WDM transmitter, but also as multi-wavelength local oscillators (LO) for parallel coherent reception. In our experiments, we demonstrate transmission of an aggregate net data rate of 3.9 Tbit/s (23 × 45 GBd PDM-QPSK, 7% FEC overhead) over 75 km standard single-mode fiber (SSMF). To the best of our knowledge, this represents the first demonstration of a coherent WDM link that relies on QD-MLLD both at the transmitter and the receiver.

On the Mechanism of In Nanoparticle Formation by Exposing ITO Thin Films to Hydrogen Plasmas.

We present our systematic work on the in situ generation of In nanoparticles (NPs) from the reduction of ITO thin films by hydrogen (H2) plasma exposure. In contrast to NP deposition from the vapor phase (i.e., evaporation), the ITO surface can be considered to be a solid reservoir of In atoms thanks to H2 plasma reduction. On one hand, below the In melting temperature, solid In NP formation is governed by the island-growth mode, which is a self-limiting process because the H2 plasma/ITO interaction will be gradually eliminated by the growing In NPs that cover the ITO surface. On the other hand, we show that above the melting temperature In droplets prefer to grow along the grain boundaries on the ITO surface and dramatic coalescence occurs when the growing NPs connect with each other. This growth-connection-coalescence behavior is even strengthened on In/ITO bilayers, where In particles larger than 10 µm can be formed, which are made of evaporated In atoms and in situ released ones. Thanks to this understanding, we manage to disperse dense evaporated In NPs under H2 plasma exposure when inserting an ITO layer between them and substrate like c-Si wafer or glass by modifying the substrate surface chemistry. Further studies are needed for more precise control of this self-assembling method. We expect that our findings are not limited to ITO thin films but could be applicable to various metal NPs generation from the corresponding metal oxide thin films.

Publisher's Note: Quantized Vortices and Four-Component Superfluidity of Semiconductor Excitons [Phys. Rev. Lett. 118, 127402 (2017)].

This corrects the article DOI: 10.1103/PhysRevLett.118.127402.

Quantized Vortices and Four-Component Superfluidity of Semiconductor Excitons.

We study spatially indirect excitons of GaAs quantum wells, confined in a 10 µm electrostatic trap. Below a critical temperature of about 1 K, we detect macroscopic spatial coherence and quantized vortices in the weak photoluminescence emitted from the trap. These quantum signatures are restricted to a narrow range of density, in a dilute regime. They manifest the formation of a four-component superfluid, made by a low population of optically bright excitons coherently coupled to a dominant fraction of optically dark excitons.

Mitigation of relative intensity noise of quantum dash mode-locked lasers for PAM4 based optical interconnects using encoding techniques.

Quantum dash (Q-Dash) passively mode-locked lasers (PMLLs) exhibit significant low frequency relative intensity noise (RIN), due to the high mode partition noise (MPN), which prevents the implementation of multilevel amplitude modulation formats such as PAM4. The authors demonstrate low frequency RIN mitigation by employing 8B/10B and Manchester encoding with PAM4 modulation format. These encoding techniques reduce the overlap between the modulation spectral content and the low-frequency RIN of the Q-dash devices, at the expense of increased overhead. The RIN of the 33.6 GHz free spectral range Q-dash PMLL was characterized, and the results obtained show very high levels of RIN from DC to 4 GHz, but low levels for higher frequencies. The performance improvement for 28 GBaud 8B/10B and Manchester encoded PAM4 signal has been demonstrated compared to the case when no encoding is used. Finally, the effect of RIN on the system performance was demonstrated by comparing the bit error rate (BER) performance of the PAM4 signaling obtained with an external cavity laser (ECL) to those obtained with Q-dash PMLL.

Correlation coefficient measurement of the mode-locked laser tones using four-wave mixing.

We use four-wave mixing to measure the correlation coefficient of comb tones in a quantum-dash mode-locked laser under passive and active locked regimes. We study the uncertainty in the measurement of the correlation coefficient of the proposed method.

Single-section quantum well mode-locked laser for 400 Gb/s SSB-OFDM transmission.

Successful use of a single-section quantum well (QW) passively mode-locked laser (MLL) as a comb source for optical interconnects is demonstrated for the first time. Sixteen comb lines spaced by 37.6 GHz are modulated using 25 Gb/s compatible single sideband orthogonal frequency division multiplexed (SSB-OFDM) signals and transmitted over 50 km of standard single-mode fiber with bit error ratio below the 7% forward error correction limit. The system performance, analyzed on the basis of the relative intensity noise of the device, reveal the suitability of single-section QW MLLs as inexpensive comb sources for inter- and intra-data center communication scenarios.

Timing jitter from the optical spectrum in semiconductor passively mode locked lasers.

An analysis of the passively mode locked regime in semiconductor lasers is presented, leading to an explicit expression relating the timing jitter diffusion constant to the optical linewidths in these devices. Experimental results for single section quantum-dash based lasers validating the theoretical analysis are presented for the first time. Timing jitter of mode locked lasers at rates of up to 130 GHz has been experimentally estimated from the optical spectra without requiring fast photodetection.