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The study shows that, in waveguide (WG) configuration, the coupling of a 2D plasmonic metamaterial grating (MMG) having a conventional Bragg period along the guide but a distinct period along the transverse axis can lead to Electromagnetically‐Induced‐Transparency (EIT) behavior. This epitomizes that metamaterials, as functional photonic building blocks, can lead to low losses in many standard devices if properly designed. The study reported the observation in passive WGs of a marked Fano‐type EIT resonance with record high‐quality factor: Q = 5000 and contrast >20 dB. Unlike any standard metal grating, MMG‐assisted waveguides exhibit strong grating coupling strength and low‐loss properties simultaneously. This concept is further applied to demonstrate single‐frequency‐emission electrically‐injected Distributed Feedback (DFB) lasers in the near‐infrared telecom domain. The key point is that laser emission occurs at the peak of EIT, i.e., the maximum in transmission. It therefore addresses one of the main critical issues of DFB lasers related to the single frequency yield. The laser performances are state‐of‐the‐art (Ith < 20 mA, Pmax > 20 mW at I = 200 mA, SMSR > 50 dB, optical feedback tolerance >−21 dB compliant with IEEE 802.3 standard). The presented approach, compatible with existing industrial technologies, is promising for real‐life telecom applications. [ABSTRACT FROM AUTHOR]

The new paradigm of parity-time symmetry in quantum mechanics has readily been applied in the field of optics with numerous demonstrations of exotic properties in photonic systems. In this work, we report on the implementation of single frequency electrically injected distributed feedback (DFB) laser diodes based on parity-time symmetric dual gratings in a standard ridge waveguide configuration. We demonstrate enhanced modal discrimination for these devices as compared with index or gain coupled ones, fabricated in the same technology run. Optical transmission probing experiments further show asymmetric amplification in the light propagation confirming the parity-time symmetry signature of unidirectional light behavior. Another asset of these complex coupled devices is further highlighted in terms of robustness to optical feedback.

Indium phosphide-based quantu m dash mode-locked lasers designed for optical telecomunications applications are designed. These generate picosecond pulses at different repetition frequencies with low timing jitter and can be used to achieve all-optical clock recovery.

Coherent reception becomes an interesting option when data rates in time-division-multiplexed (TDM) passive optical networks (PONs) grow beyond 50 Gbit/s. Controlling the wavelength, i.e., the optical frequency, and the phase of the laser acting as local oscillator (LO) is one of the main technical challenges in the design of coherent TDM PONs. In the optical network units (ONUs), low-cost lasers are required, which come at the expense of wavelength variations and drifts over multiple nanometers due to fabrication imperfections, and temperature variations. This contradicts the requirement of wavelength-stable LOs in coherent receivers. The use of a wavelength locker circuit and a temperature controller is considered as too complex for applications in access networks. In this work, we propose a novel colorless coherent architecture with high resilience to ONU laser wavelength drifts of up to $\pm$ 4 nm ($\pm$ 0.5 THz) for future 100 Gbit/s PON. It allows the use of distributed feedback lasers at the ONU side. This is rendered possible by generating a frequency comb with carefully chosen free spectral range in a quantum-dash mode-locked laser diode at the optical line terminal. In upstream operation, the frequency comb serves as an LO, whereas the same information is modulated onto all comb lines for the case of downstream. As a result, the ONU laser can drift over the entire comb bandwidth without substantial performance penalty. We experimentally demonstrate downstream and upstream operation with an aggregated raw data rate of 96 Gbit/s, respectively. We further introduce advanced digital signal processing (DSP) methods including a coarse frequency offset compensation (CFOC) and a multiple-input multiple-output (MIMO) equalizer to improve the performance of our concept. We show that the receiver sensitivity can be increased by 3 dB for a high-bandwidth receiver when using a 6 × 2 MIMO equalizer scheme. A 4 × 2 MIMO equalizer scheme enables colorless reception even with a limited-bandwidth receiver. [ABSTRACT FROM AUTHOR]

Single-mode diode lasers on an InP(001) substrate have been developed using InAs/In0.53Ga0.47As quantum dash (Qdash) active regions and etched lateral Bragg gratings. The lasers have been designed to operate at wavelengths near 2 lm and exhibit a threshold current of 65mA for a 600 lm long cavity, and a room temperature continuous wave output power per facet >5mW. Using our novel growth approach based on the low ternary In0.53Ga0.47As barriers, we also demonstrate ridge-waveguide lasers emitting up to 2.1 lm and underline the possibilities for further pushing the emission wavelength out towards longer wavelengths with this material system. By introducing experimentally the concept of high-duty-cycle lateral Bragg gratings, a side mode suppression ratio of >37 dB has been achieved, owing to an appreciably increased grating coupling coefficient of j~40 cm-1. These laterally coupled distributed feedback (LC-DFB) lasers combine the advantage of high and well-controlled coupling coefficients achieved in conventional DFB lasers, with the regrowth-free fabrication process of lateral gratings, and exhibit substantially lower optical losses compared to the conventional metal-based LC-DFB lasers. [ABSTRACT FROM AUTHOR]

GaN, with its wide bandgap of 3.4 eV, has emerged as an efficient material for designing high-efficiency betavoltaic batteries. An important part of designing efficient betavoltaic batteries involves a good understanding of the full process, from the behavior of the nuclear material and the creation of electron-hole pairs all the way through the collection of photo-generated carriers. This paper presents a detailed model based on Monte Carlo and Silvaco for a GaN-based betavoltaic battery device, modeled after Ni-63 as an energy source. The accuracy of the model is verified by comparing it with experimental values obtained for a GaN-based p-i-n structure under scanning electron microscope illumination. [ABSTRACT FROM AUTHOR]

Optical heterodyne analog radio-over-fiber (A-RoF) links provide an efficient solution for future millimeter wave (mm-wave) wireless systems. The phase noise of the photo-generated mm-wave carrier limits the performance of such links, especially, for the transmission of low subcarrier baud rate multi-carrier signals. In this work, we present three different techniques for the compensation of the laser frequency offset (FO) and phase noise (PN) in an optical heterodyne A-RoF system. The first approach advocates the use of an analog mm-wave receiver; the second approach uses standard digital signal processing (DSP) algorithms, while in the third approach, the use of a photonic integrated mode locked laser (MLL) with reduced DSP is advocated. The compensation of the FO and PN with these three approaches is demonstrated by successfully transmitting a 1.95 MHz subcarrier spaced orthogonal frequency division multiplexing (OFDM) signal over a 25 km 61 GHz mm-wave optical heterodyne A-RoF link. The advantages and limitations of these approaches are discussed in detail and with regard to recent 5G recommendations, highlighting their potential for deployment in next generation wireless systems. [ABSTRACT FROM AUTHOR]

Single section quantum dash (QDash) mode locked lasers (MLL) can provide a flat and broadband optical frequency comb with low energy consumption, operational simplicity and large-scale low-cost production possibilities. MLL longitudinal modes can be employed as single carriers with a regular spectral spacing in a dense wavelength division multiplexing (DWDM) link, making them promising components for next generation DWDM transceivers. However, individual modes of a MLL suffer from relatively high phase noise while high capacity coherent transmission requires carriers with low phase noise. Optical feedback, which is a well-known method to reduce the linewidth of a single mode laser, can be used to stabilize comb lasers. This article reports on the investigation of phase noise properties of comb lines delivered from a single section QDash MLL under optical feedback and shows that each MLL longitudinal mode optical linewidth can be drastically narrowed. It enables coherent transmission with extended link budget over 50 km at Tbit/s line rates. [ABSTRACT FROM AUTHOR]

We report on a mode locked semiconductor laser for high-power pulse generation in the 810-nm waveband. This is based on a novel GaAsP/GaInP single quantum-well structure, with Aluminum-free active region. Average output powers higher than 40 mW are reported in narrow ridge two-section devices. A 1.65-mm-long laser with 70 μm saturable absorber yields a stable mode locked regime at a 23-GHz repetition rate as evidenced by a record low RF linewidth of 75 kHz over a wide range of gain currents. Other laser dynamics are also reported at lower frequencies. [ABSTRACT FROM AUTHOR]

The (de)modulation of broadband orthogonal subchannels relying on all-analogue signal processing potentially achieves high-capacity orthogonal subcarrier multiplexing (OSCM) electro-optical transceivers with low power consumption and latency. Overall transmission rates can be multiplied by employing wavelength division multiplexing (WDM) technology. Mode-locked lasers (MLL) are relevant optical frequency combs, as they produce a high number of comb tones while presenting a small footprint and low power consumption. Unlike baseband transmission, the use of high-frequency subchannels in OSCM systems overcomes the high relative intensity noise that MLL comb tones present at low frequencies. This paper reports a direct-detection real-time all-analogue WDM/OSCM experiment that emulates a 432 Gbit/s (20 × 21.6 Gbit/s) electro-optical transceiver. The 20 optical carriers were generated by a state-of-the-art quantum-dash MLL. The net data rate after considering forward error correction overheads is still higher than 400 Gbit/s. This is the highest capacity achieved in real-time broadband all-analogue WDM/OSCM links to date. [ABSTRACT FROM PUBLISHER]

Temperature- and field-dependent electron and hole mobilities were simultaneously derived from admittance measurements on an InP/InGaAsP/InAs/InP long wavelength laser structure. Each carrier’s mobility was separated in the frequency domain due to the corresponding different relaxation times from C versus log f curves. Derived electron and hole mobilities followed the Poole–Frenkel-type field dependence, indicating the transport of carriers through hopping. Extracted thermal energy gap ( \Delta ) and disorder temperature ( T0 ) values were in mutual agreement with previous current density–voltage measurements and from the energy band diagram of the present structure, constructed from other studies in the literature by photoluminescence and photoluminescence excitation experiments. [ABSTRACT FROM PUBLISHER]

Laser diodes with optical feedback can exhibit periodic intensity oscillations at or near the relaxation-oscillation frequency. We demonstrate optoelectronic oscillators based on external-cavity semiconductor lasers in a periodic dynamical regime tunable over the entire $X$ -band. Moreover, unlike standard optoelectronic oscillators, we need not employ the time-dependent optical intensity incident on a photodiode to generate the microwave signal, but rather have the option of generating the electrical microwave signal directly as a voltage V(t)$ at the laser-diode injection terminals under constant current operation; no photodiode need be involved, thus circumventing optical-to-electrical conversion. We achieve a timing jitter of \lesssim 10$ ps and a quality factor of across the entire $X$ -band, that ranges from 6.79 to 11.48 GHz. Tuning is achieved by varying the injection current $J$ . [ABSTRACT FROM AUTHOR]

We investigate the amplitude and phase noise of an optical frequency comb based on InAs/InP quantum-dash mode-locked laser. The laser demonstrates low relative intensity noise (< −125 dB/Hz) and phase noise in the passive mode-locking regime. By actively mode-locking the laser, we observe a reduction in the flicker FM noise and timing jitter, as a result of which the optical linewidth decreases, and hence the effective bandwidth compatible with optical coherent systems increases by more than 50% to $\sim 1.1$ THz. [ABSTRACT FROM PUBLISHER]

A multi-GHz chaotic optoelectronic oscillator based on an external cavity semiconductor laser (ECL) is demonstrated. Unlike the standard optoelectronic oscillators for microwave applications, we do not employ the dynamic light output incident on a photodiode to generate the microwave signal, but instead generate the microwave signal directly by measuring the terminal voltage V(t) of the laser diode of the ECL under constant-current operation, thus obviating the photodiode entirely. [ABSTRACT FROM AUTHOR]

The properties of InP/InGaAsP/InAs/InP laser structure, mainly used in the fiber optic telecommunication industry, were investigated through current density-voltage- temperature ( $J$ – $V$ – $T$ ) measurement within dark and light conditions. Tunneling and generation-recombination carrier conduction mechanisms were identified within a junction bias voltage of 0–0.6 V. Above 0.6 V, Poole–Frenkel was the actual path in conduction and the mobility of carrier was determined quantitatively by means of temperature-dependent $J$ – $V$ curves in the space charge bias region. A thermal energy gap of 0.8 eV corresponded to the band gap of quantum dashes in which radiative recombination took place to emit a laser light at 1550 nm. [ABSTRACT FROM AUTHOR]

We demonstrate transmission of a 200-Gb/s superchannel over 3-km single-mode optical fiber (SMF) targeting intra-data center interconnect applications using a novel single-section quantum-well passive-mode-locked laser that generates an optical frequency comb. The transmitted optical signal consists of 16 subcarriers filtered from the optical frequency comb of the laser and modulated with an advanced modulation format of Baudrate-pilot-aided quadrature phase-shift keying (QPSK) modulation that enables direct detection. Bit error rates below the forward error correction limit have been confirmed for all subcarriers, and the detailed noise characteristics of the passive-mode-locked laser have been investigated to support the understanding of the system performance. [ABSTRACT FROM PUBLISHER]

Interfacial microstructure, elemental diffusion, and electrical properties of Pd/Au ohmic contactto p-type GaN have been investigated using scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy. The as-deposited sample exhibits the formation of anohmic contact which is fundamentally due to epitaxial relationship between the Pd, Au and GaNlayer. Pd nanocrystals are formed with crystallographic orientation with respect to the substrate described as follows (111)Pd//(0001) GaN , [ 11¯0 ]Pd//[ 112¯0 ] GaN . Thermal annealing at 800 °C leads to the formation of majority phase composed by crystalline Au rich Au6-Pd2-Ga composites, with crystallographic orientation of (11¯0)Au6−Pd2−Ga //(0001) GaN , and minority phases, which are composed of Au5-Pd2-Ga2, Au 3-Pd2-Ga3, and Ga2-Pd5, which is responsible for the formation of an ohmic contact with a specific contact resistance of 1.04 × 10−2 Ω cm2. [ABSTRACT FROM AUTHOR]

In this paper, we present a systematic investigation of passive mode-locked InAs/InP quantum-dash lasers in terms of frequency and timing stability. Mode-locking features are analyzed using the frequency domain approach based on the concept of supermodes. It is shown that the phase of adjacent longitudinal modes is correlated over 1.3-THz frequency span. The observed strong coherence between modes enables low timing jitter and a long-term stability of the repetition rate frequency within 5×10-8 over 100 s. [ABSTRACT FROM PUBLISHER]

We introduce a novel scheme for the simultaneous reduction of time-bandwidth product (TBP) and RF linewidth of quantum-dash two-section mode-locked lasers using optical injection-locking and filtered optical feedback. The optical injection-locked laser, double-locked with optical feedback showed 2\!\!\times TBP reduction and RF linewidth reduction by two orders of magnitude. This stabilization technique is implemented in an all-optical arrangement without optical/electrical conversion which is ideal for high-repetition-rate devices and photonic integration. [ABSTRACT FROM PUBLISHER]

Detailed investigation of Ohmic Cr/Ni/Au based contact formation to p-GaN was realized by scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy and compared to the Cr/Au bilayer metallization scheme. The authors found that it is essential to introduce a nickel film in Cr-based contacts and anneal the trilayer structure in air in order to suppress the Shottky barrier and thus obtain the Ohmic contact. Our findings also indicate that oxygen behaves as a dopant dispersed in chromium nitride matrix. Thus Ohmic trilayer Cr/Ni/Au contact to p-GaN annealed in air is formed by Ni-Ga-Au alloy mixed with Au-Ga-doped Cr2N crystalline composites. Possible ways for improvement of such types of contact are discussed. [ABSTRACT FROM AUTHOR]

This paper reports on recent results on passively mode-locked InAs/InP quantum-dot-based lasers. These low-dimensional structures have proved very attractive in improving most of the properties of these devices. Subpicosecond pulse generation at repetition rates up to beyond 300 GHz has readily been demonstrated. Ultranarrow RF linewidths reach record values of less than 1 kHz. Controlled optical feedback allows a further reduction of this linewidth yielding extremely low timing jitter. A comparison of single-section and standard two-section lasers is given for the first time. These performances open the way to various applications at 1.55 μm, including very high bit rate all-optical signal processing, frequency comb generation, radio over fiber, and low-noise all-optical oscillators. [ABSTRACT FROM PUBLISHER]

The ultrafast gain and refractive index dynamics in AlInAs/AlGaAs quantum dot (QD) based semiconductor optical amplifiers is reported. Measurements in the forward bias regime indicate a complete gain recovery timescale of \sim5~ps, while the phase dynamics occur over a much longer timescale. At increased pump powers, the impact of nonresonant carriers created by two-photon absorption is visible as an increased injection in both gain and phase dynamics. Reverse-biased measurements reveal a similar behavior to previous measurements on InAs QD devices. [ABSTRACT FROM PUBLISHER]