Performance of quantum-dash mode-locked lasers (QD-MLLDs) for high-capacity coherent optical communications.

We investigate the capabilities and limitations of quantum-dash mode-locked lasers (QD-MLLDs) as optical frequency comb sources in coherent optical communication systems. We demonstrate that QD-MLLDs are on par with conventional single-wavelength narrow linewidth laser sources and can support high symbol rates and modulation formats. We manage to transmit 64 quadrature amplitude modulation (QAM) signals up to 80 GBd over 80 km of standard single-mode fiber (SSMF), which highlights the distinctive phase noise performance of the QD-MLLD. Using a 38.5 GHz (6 dB bandwidth) silicon photonic (SiP) modulator, we achieve a maximum symbol rate of 104 GBd with 16QAM signaling and a maximum net rate of 416 Gb/s per carrier in a single polarization setup and after 80 km-SSMF transmission. We also compare QD-MLLD performance with commercial narrow-linewidth integrable tunable laser assemblies (ITLAs) and explore their potential for use as local oscillators (LOs) and signal carriers. The QD-MLLD has 45 comb lines usable for transmission at a frequency spacing of 25 GHz, and an RF linewidth of 35 kHz.

Electrically reconfigurable waveguide Bragg grating filters.

We propose and demonstrate an electrically reconfigurable waveguide Bragg grating filters in silicon-on-insulator using a multiple-contact heater element. There are six electrical pads connected to the heater element in an equidistant manner. These electrical pads allow to create different heat, and corresponding refractive index, distributions across the grating so that the local Bragg wavelength corresponding to the heated segments can be controlled. In turn, this control over the heat distribution allows the device to be reconfigured to implement different filter spectral responses. These filters are applicable for both wavelength division multiplexing systems and optical signal processing applications. As a verification, we demonstrate the generation of two (or more) separate filter bands with a spacing up to 35 nm or a Fabry-Pérot cavity with a 1.6 nm free-spectral range. Moreover, we explain a firm and accurate simulation framework of the proposed device based on COMSOL Multiphysics and the transfer matrix method, which is in excellent agreement with our experimental measurements.

Standalone, CMOS-based Faraday rotation in a silicon photonic waveguide.

Nonreciprocity is a fundamental requirement of signal isolation in optical communication systems. However, on chip isolator designs require either post-processing steps or external magnetic biasing, which are impractical for commercial applications. This raises the need for standalone devices which support nonreciprocal functionality using standardized fabrication techniques. Here, we report the first design of an electromagnetic coil surrounding a waveguide which exclusively employed the complementary metal-oxide-semiconductor (CMOS) process flow. The coil supported an electric current up to 14 mA. In simulations, it generated an alternating magnetic flux density up to 1.16 mT inside a strip waveguide and thereby induced a rotation of 50.71 picodegrees for the fundamental transverse-magnetic mode at a wavelength of 1352 nm. Our analysis further revealed methods to increase the rotation by orders of magnitude. It demonstrated the scope of manufacturing processes and serves as a building block for the development of a commercially viable, on-chip optical isolator.

Net 400-Gbps/λ IMDD transmission using a single-DAC DSP-free transmitter and a thin-film lithium niobate MZM.

The insatiable growth of datacenter traffic mandates increasing the capacity of cost-effective intensity modulation direct detection (IMDD) systems to meet the foreseen demand. This Letter demonstrates the first, to the best of our knowledge, single-digital-to-analog converter (DAC) IMDD system achieving a net 400-Gbps transmission using a thin-film lithium niobate (TFLN) Mach-Zehnder modulator (MZM). Employing a driver-less DAC channel (128 GSa/s, 800 mVpp) with neither pulse-shaping nor pre-emphasis filtering, we transmit (1) 128-Gbaud PAM16 below the 25% overhead soft-decision forward error correction (SD-FEC) bit error rate (BER) threshold and (2) 128-Gbaud probabilistically shaped (PS)-PAM16 under the 20% overhead SD-FEC threshold, which respectively correspond to record net rates of 410 and 400 Gbps for single-DAC operation. Our results highlight the promise of operating 400-Gbps IMDD links with reduced digital signal processing (DSP) complexity and driving swing requirements.

Quantum size effect affecting environment assisted electron capture in quantum confinements.

Ultrafast inter-Coulombic electron capture (ICEC) has been established as an important energy-transfer process in open paired-quantum-dot systems which can mediate between entrapment of free-moving electrons and release of trapped ones elsewhere by long-range electron-electron interaction within nanowires. Previous studies indicated ICEC enhancement through population and secondary decay of two-center resonance states, the latter known as inter-Coulombic decay (ICD). This study investigates the quantum-size effect of single- and double-electron states in an established model of a quasi-one-dimensional nanowire with two embedded confinement sites, represented by a pair of Gaussian wells. We analyze the ICEC related electron flux density as a function of confinement size and are able to clearly identify two distinct capture channels: a direct long-range electron-electron impulse and a conversion of kinetic energy to electron-electron correlation energy with consecutive ICD. The overlay of both channels makes ICEC extremely likely, while nanowires are a strong candidate for the next miniaturization step of integrated-circuit components.

Predicting the performance of the inter-Coulombic electron capture from single-electron quantities.

The probability of the inter-Coulombic electron capture (ICEC) is studied for nanowire-embedded quantum-dot pairs where electron capture in one dot leads to electron emission from the other. Previous studies pointed to an interdependence of several ICEC pathways which can enhance the ICEC reaction probability. To identify favorable criteria for such synergies in a qualitative and quantitative manner, we conducted a considerable amount of simulations scanning multiple geometrical parameters. The focus of the paper is not only to find the geometries which are most favorable to ICEC but most importantly to explain the basic principles of the ICEC probability. We have thus derived a number of energy relations among solely single-electron level energies that explain the mechanisms of the multiple reaction pathways. Among them are direct ICEC, both slowing or accelerating the outgoing electron, as well as resonance-enhanced ICEC which captures into a two-electron resonance state that decays thereafter. These pathways may apply simultaneously for just one single geometric configuration and contribute constructively leading to an enhancement of the reaction probability. Likewise some conditions are found that clearly turn down the ICEC probability to zero. The results based on single-electron relations are so general that they can as well be used to predict the ICEC probability from the electronic structure in arbitrary physical systems such as atoms or molecules.