Threshold Current Research Articles

Thermal Characterisation of Hybrid, Flip-Chip InP-Si DFB Lasers

WA detailed thermal analysis of a hybrid, flip-chip InP-Si DFB laser is presented in this work. The lasers were experimentally tested at different operating temperatures, which allowed for deriving their thermal performance characteristics: the temperature dependence of threshold current, lasing slope, and output spectrum. Using these data, the laser thermal resistance was calculated (Rth = 75.9 K/W), which allows for predicting the laser temperature during operation. This metric is also used to validate the thermal finite element models of the laser. A sensitivity study of the laser temperature was performed using these models, and multiple routes for minimising both the laser thermal resistance and thermal coupling to the carrier die are presented. The most effective way of decreasing the laser temperature is the direct attachment of a heat sink on the laser top surface.

Performance enhancement of an AlGaN-based deep-ultraviolet laser diode using a two-stepped doped lower waveguide

Subject of study. The performance of a deep-ultraviolet laser diode is improved using a two-stepped Si-doped lower waveguide. Method. The impact of the variations on AlGaN-based deep-ultraviolet laser diodes has been evaluated based on the simulated results after theoretical calculations. The two AlGaN-based ultraviolet laser diodes as a traditional device and the proposed device have been analyzed comparatively based on their performance within a nomination wavelength region range of 269–280 nm. Main results. Using a two-stepped Si-doped lower waveguide, the lasing threshold current decreases in the proposed device D2 in comparison to the traditional device D1. The operating threshold voltage of D1 is 13.8 V, and that of D2 is 4.24 V, respectively, with lasing threshold currents of 0.4 A and 0.002 A, respectively. Practical significance. The optimization of the design of AlGaN-based ultraviolet laser diodes following the perfect bulk aluminium nitride substrate in the traditional device is the crucial factor of the practical significance of the scientific field utilized in the proposed device. The performance of deep-ultraviolet laser diodes is improved when a suitably constructed two-stepped Si-doped lower waveguide substitutes the traditional lower waveguide. The noted improvements are the reduction in total optical loss with the improved optical confinement factor. These are primarily due to an increase in the hole injection current and a decrease in the electron leakage current.

Narrow linewidth electro-optically tuned multi-channel interference widely tunable semiconductor laser.

A narrow linewidth electro-optically tuned multi-channel interference (MCI) widely tunable semiconductor laser based on carrier injection is demonstrated in this paper. The MCI laser with a common phase section and a semiconductor optical amplifier (SOA) is packaged into a 16-pin butterfly box. The laser is characterized by a strategy: shifting the longitudinal mode and then aligning the reflection peak, which obtains a quasi-continuous tuning range over 48 nm. The corresponding side mode suppression ratios (SMSRs) are higher than 40 dB and frequency deviations from ITU-grid are less than ± 1 GHz. Threshold currents are less than 28 mA. Fiber coupled output powers are higher than 20 mW and power variations with fixed gain and SOA currents are less than 0.8 dB over the whole tuning range. Lorentzian linewidths are less than 320 kHz over the entire tuning range, which is one of the lowest results for monolithic widely tunable semiconductor lasers tuned by carrier injection. These results demonstrate the potential prospects of the MCI laser with carrier injection in the field of optical sensing and optical communications.

Characterization of Lateral and Longitudinal Mode Competition in Blue InGaN Broad‐Ridge Laser Diodes

In blue GaN‐based laser diodes with a 40 μm broad‐ridge waveguide, lateral modes of different order are selectively observed by high‐resolution spectroscopy combined with lateral near‐field scanning. Longitudinal mode clusters contribute to different lateral modes, close above the threshold current. Longitudinal mode competition is observed in each longitudinal mode cluster as intensity modulation and wavelength shift, repeating itself with a frequency of around 60–80 MHz. Despite the spectral separation of the different clusters and different lateral mode order, the mode competition processes share the same frequency and show a stable phase relation, especially at low currents. This indicates coupling of separate mode clusters via the charge carrier density. Due to the lateral modes occurring at separate wavelengths, lateral–longitudinal mode competition is found, which not only causes spectral effects but also time‐dependent periodic variations in the lateral near field.

Improving structure design of active region of InAs quantum dots by using InAs/GaAs digital alloy superlattice

A 1.3-μm InAs quantum dot laser has been successfully fabricated on a GaAs(100) substrate by molecular beam epitaxy (MBE) technique through using InAs/GaAs digital alloy superlattices instead of the conventional InGaAs layer. The samples grown by conventional growth method and the digital alloy superlattice growth method are characterized by atomic force microscope (AFM) and photoluminescence (PL) spectroscopy. It is found that 8-period sample possesses a low quantum dot density and poor luminescence performance. With the increase of the number of growth periods, the quantum dot density of the sample increases and the luminous performance improves. This indicates that the quality of the grown sample improves with the increase of InAs/GaAs period of the InGaAs layer. When the total InAs/GaAs period is 32, the quantum dot density of the sample is high and the luminescence performance is good. After the experimental measurement, the sample DAL-0 fabricated by conventional growth method and the sample DAL-32 (32-periods InAs/GaAs digital alloy superlattices) are utilized to fabricate quantum dot laser by standard process. The performances of two types of quantum dot lasers obtained with different growth methods are characterized. It is found that the InAs quantum dot lasers fabricated by the sample grown by digital alloy superlattice method have good performances. Under continuous wave operation mode, the threshold current is 24 mA corresponding to a threshold current density of 75 A/cm<sup>2</sup>. The highest operation-temperature reaches 120 ℃. In addition, InAs quantum dot laser using digital alloy superlattice has good temperature stability. Its characteristic temperature is 55.4 K. Compared with the traditional laser, the InAs quantum dot laser grown by InAs/GaAs digital alloy superlattice has good performance in terms of threshold current density, output power and temperature stability, which indicates that high-quality laser can be obtained by this growth method. Using the InAs/GaAs digital alloy superlattice growth method, the InGaAs composition can be changed without changing the temperature of the source oven. Thus InAs quantum dot lasers with different luminescence wavelengths can be obtained through this growth method. The InAs/GaAs digital alloy superlattice structure can be used to realize different averaging of In content in the growth structure. The method provides a new idea for designing and growing the active region of quantum dot laser.

Towards Efficient Electrically-Driven Deep UVC Lasing: Challenges and Opportunities

The major issues confronting the performance of deep-UV (DUV) laser diodes (LDs) are reviewed along with the different approaches aimed at performance improvement. The impact of threading dislocations on the laser threshold current, limitations on heavy n- and p-doping in Al-rich AlGaN alloys, unavoidable electron leakage into the p-layers of (0001) LD structures, implementation of tunnel junctions, and non-uniform hole injection into multiple quantum wells in the active region are discussed. Special attention is paid to the current status of n- and p-type doping and threading dislocation density reduction, both being the factors largely determining the performance of DUV-LDs. It is shown that most of the above problems originate from intrinsic properties of the wide-bandgap AlGaN semiconductors, which emphasizes their fundamental role in the limitation of deep-UV LD performance. Among various remedies, novel promising technological and design approaches, such as high-temperature face-to-face annealing and distributed polarization doping, are discussed. Whenever possible, we provided a comparison between the growth capabilities of MOVPE and MBE techniques to fabricate DUV-LD structures.

The Investigation of Carrier Mobility Effect on the Performance Characteristics of the InGaN-Based Vertical Cavity Surface Emitting Laser (VCSEL) by Solving the Rate Equations

Background: Among the parameters that play an important role in describing the performance of many devices is carrier mobility which is a criterion for the easy movement in semiconductor crystals. Objective: The effect of carrier mobility on the performance characteristics of InGaN quantum well vertical-cavity surface-emitting laser was analytically investigated. Methods: By solving the Poisson’s equation, current density equation, charge concentration continuity equation and carrier and photon rate equations, the variation of current density and carrier density with respect to the position and time and the effects of carrier mobility and temperature on these parameters were investigated. Furthermore, the effect of mobility on the variation of output power versus the injection current and on the time variation of photon and carrier density and the output power was investigated. Results: By increasing the carrier mobility, the threshold current is reduced and the output power is increased. In studying the effect of temperature on the desired parameters, the variation of carrier density with respect to time and position was affected by the temperature change. This phenomenon is due to the dependence of these parameters on the diffusion coefficients and consequently on the mobility of the carriers and the dependence of mobility on temperature. Conclusions: The output power increased, and the time delay in accruing the laser decreased. Consequently, the carrier recombination increased, further resulting in a rapid laser operation.

Threshold performance of pulse-operating quantum-cascade vertical-cavity surface-emitting lasers.

Quantum-cascade (QC) vertical-cavity surface-emitting lasers (VCSELs) could combine the single longitudinal mode operation, low threshold currents, circular output beam, and on-wafer testing associated with VCSEL configuration and the unprecedented flexibility of QCs in terms of wavelength emission tuning in the infrared spectral range. The key component of QC VCSEL is the monolithic high-contrast grating (MHCG) inducing light polarization, which is required for stimulated emission in unipolar quantum wells. In this paper, we demonstrate a numerical model of the threshold operation of a QC VCSEL under the pulse regime. We discuss the physical phenomena that determine the architecture of QC VCSELs. We also explore mechanisms that influence QC VCSEL operation, with particular emphasis on voltage-driven gain cumulation as the primary mechanism limiting QC VCSEL efficiency. By numerical simulations, we perform a thorough analysis of the threshold operation of QC VCSELs. We consider the influence of optical and electrical aperture dimensions and reveal the range of aperture values that enable single transversal mode operation as well as low threshold currents.

Single-mode interband cascade laser with a slotted waveguide

The design of a single-mode interband cascade laser (ICL) using a slotted waveguide is presented. This technique was explored as an inexpensive alternative to distributed feedback lasers since standard photolithography can be used in fabrication and complex techniques, such as e-beam lithography, re-growth steps, and/or metal gratings, can be avoided. The design of slotted waveguides must be carefully simulated before fabrication to ensure the efficacy of the photolithography masks with each ICL growth. Limitations and the behavior of key design parameters are discussed. Single-mode emission was achieved for certain temperature and injected current conditions, validating the operation of an Sb based slotted laser. The slotted ICLs were emitting from a single longitudinal mode at 3.5 μm and 2 mW of power per facet output at 20 °C with threshold currents around 80 mA.

850 nm VCSEL with sub quantum well and p-type [formula omitted]-doping in the active layers for improved high-speed and high-temperature performance

Thermal rollover is one of the fundamental limitations that affect the output power and modulation bandwidth of high-speed laser in monolithic laser integrated circuits (ICs). Herein, an 850 nm vertical cavity surface emitting laser (VCSEL) with p-doping and sub quantum well (sub QW) centers active layer is proposed and investigated. The p-doping and sub QW insert in the offset position of each barrier, which are designed for reducing the thermal rollover and increasing the maximum intrinsic modulation bandwidth of the laser. Numerical simulations show that adding the p-doping and sub QW in active region could reduce the threshold currents (Ith), improve the slope efficiency (SE) and delay the thermal rollover in VCSEL. Comparing with previous VCSEL, our proposed VCSEL has excellent high temperature output performance, and the maximum simulation 3-dB bandwidth is 43.9 GHz at 25 °C and 33.1 GHz at 85 °C, respectively. Furthermore, temperature-dependent time-resolved photoluminescence (PL) and PL excitation reveal this novel QW has strong radiative recombination rate, which is suitable for designing active region of high-speed and high power lasers.

Gain-Switched Short Pulse Generation from 1.55 µm InAs/InP/(113)B Quantum Dot Laser Modeled Using Multi-Population Rate Equations

The gain-switching properties of an InAs-InP (113)B quantum dot laser based on multi-population rate equations are examined theoretically in detail to generate shorter pulses with the application of a Gaussian pulse beam to the laser excited state. The numerical results demonstrated that as the homogeneous and the inhomogeneous broadening increase, the differential gain, the gain compression factor, and the threshold current of the excited state decrease while the threshold current of the ground state increases. It was also observed that the contribution of the excited state to gain-switched output pulses depend on not only the value of the inhomogeneous broadening but also the magnitude of the applied current. Additionally, it was found that without an optical beam, the output pulse has a long pulse width due to ground state emissions and the change in the parameters strongly affecting the output. However, under the optical beam, narrow pulses around 26 ps have high peak power owing to the excited state emission generated even at low currents and also the change in the laser parameters having a negligible effect. Finally, the gain-switching characteristics with and without a Gaussian pulse beam are shown to be similar for liner-gain and nonlinear-gain cases except that higher peak power and narrower output pulses are obtained for the linear-gain case.

Degradation Characteristics and Mechanism of High Speed 850 nm Vertical-Cavity Surface-Emitting Laser during Accelerated Aging

The degradation process of Vertical-cavity Surface-emitting lasers with high speed and a central wavelength at 850 nm is investigated via constant-current accelerated aging experiments. Degradation of the photoelectric performances under different operating conditions are characterized by optical output power, and forward and reverse bias current–voltage. The 1/f noise characteristics and formation mechanism are discussed in terms of the experimental results of low frequency noise below threshold current. The main composition of low frequency noise before and after aging, the bias current dependence and the origin of the noise are analyzed emphatically. The correlation between the degradation suggests that the loss of photoelectric performance and the fluctuation of low frequency noise characteristic can be attributed to the contagion of defects towards the active region of Vertical-cavity Surface-emitting lasers. Furthermore, the results of failure analysis confirmed the conclusion that the contagion of defects occurred towards the active region of the samples after aging.

Laser diode-pumped compact hybrid lithium niobate microring laser.

We demonstrate a compact hybrid lithium niobate microring laser by butt coupling a commercial 980-nm pump laser diode chip with a high-quality Er3+-doped lithium niobate microring chip. Single-mode lasing emission at 1531-nm wavelength from the Er3+-doped lithium niobate microring can be observed with the integrated 980-nm laser pumping. The compact hybrid lithium niobate microring laser occupies the chip size of 3 mm × 4 mm × 0.5 mm. The threshold pumping laser power is 6 mW and the threshold current is 0.5 A (operating voltage 1.64 V) at atmospheric temperature. The spectrum featuring single-mode lasing with small linewidth of 0.05 nm is observed. This work explores a robust hybrid lithium niobate microring laser source which has potential applications in coherent optical communication and precision metrology.

Impact of an AlGaN thin insertion layer in the p-GaN region on InGaN/GaN vertical-cavity surface-emitting laser diodes

In this work, we propose a p-GaN/p-AlxGa1-xN/p-GaN structure in p-type hole injection layer for InGaN/GaN vertical-cavity surface-emitting laser diodes (VCSEL), and this design can efficiently improve the hole injection into the multiple quantum wells (MQWs). In the p-GaN/p-AlxGa1-xN/p-GaN structure, the polarization induced charges generated at the heterojunction interface can produce the polarization induced electric field, which can accelerate holes. Meanwhile, the p-AlxGa1-xN layer also favors the current transport into the cavity center that can better enhance the device performance, i.e., the threshold current can be reduced and the lasing power can be enhanced. Moreover, we also find that the device performance is sensitive to the structural designs for the p-GaN/p-AlxGa1-xN/p-GaN junctions. Thus, the parametric investigations have also been conducted in this work.

Design of High-Power Red VCSEL on a Removable Substrate

In this work, the architecture of a high-power InAlGaP/InGaP vertical-cavity surface-emitting laser (VCSEL) with an emission wavelength of 680 nm was studied. The design of quantum well, including the well thickness, indium composition, and barrier aluminum composition targeting the emission wavelength, was elaborately optimized. Moreover, the influences of leakage current, temperature dependence of optical gain, and resonance mode gain to threshold current under different barrier aluminum compositions were investigated. Lastly, the temperature characteristics of InAlGaP/InGaP VCSEL with substrate removal have also been calculated with 24% and 40.6% improvement in thermal resistance and operating current range, respectively. It holds great promise for high-power red VCSEL application.

Design of a novel bilayered structure of ferromagnetic metal and nonmagnetic insulator wires while maintaining the distance between the constituent skyrmions

Magnetic-skyrmion-based racetrack memory is a high-density memory for storing information driven by low-power, wherein the arrangement of skyrmions is stored in the form of binary digit combinations. This arrangement is altered based on the skyrmion motion resulting from the influence of the spin-transfer torque. We propose a novel layered structure of a ferromagnetic metal (FM) wire/nonmagnetic insulator wire/FM wire in which the skyrmions move while maintaining the arrangement. It consists of two FM wires that are perpendicularly magnetized. One of these comprises several skyrmions, which act as binary digits (data skyrmions). The other wire constitutes a sufficient number of skyrmions that are closely packed and allow the spin-polarized current to flow. A dipole–dipole interaction acts between these two FM wires. The motion of the data skyrmion is induced by the current-induced motion of the packed skyrmion. The motion of skyrmions is simulated via micromagnetic approaches. The data skyrmions move while maintaining the distance between each skyrmion. The analog position of the data skyrmion is controlled by the current flowing through another wire. The stable data skyrmion motion has a threshold current. When the current is larger than the threshold current, the skyrmion breaks, and/or the distance between skyrmions breaks. The threshold current density is found to decrease with an increasing vacuum layer thickness and increase with increasing thickness of the FM wire flowing current. Our results show that the skyrmion motion in the proposed structure can drive skyrmions with a spin-current density lower than that of the coupled granular/continuous structure and the edge-notched nanowire structure. This structure can be used in developing low-power operational devices based on skyrmion motion.

Performance improvement of nitride semiconductor-based deep-ultraviolet laser diodes with superlattice cladding layers.

A deep-ultraviolet (DUV) laser diode (LD) consisting of specifically designed cladding layers involving superlattice nitride alloy has been proposed. Simulation studies of different cladding layers were carried out using Crosslight software. It was found that the proposed structure effectively suppresses the leakage of the optical field from the active region and the optical confinement coefficient is 1.45 times higher than that of the conventional structure. The proposed structure has a significant increase in laser power with a low threshold current. Moreover, the introduction of novel cladding layer suppresses the electron and hole leakage from the multiple quantum well (MQW) region, which provides an attractive solution for increasing the stimulated recombination rate in the MQW region leading to the improvement in the performance of the DUV LD.Graphical abstract

Anode Input Heat Flux Density in High-Current Vacuum Arcs as Anode Spot Formation

In vacuum interrupters, anode spot significantly decreases the current interruption capability. Generally, the anode spot is evaluated by the anode spot threshold current. Unfortunately, as a circuit parameter, arc current cannot explain the anode spot formation from the physical point of view. This work proposes the heat flux density as a new parameter to evaluate the anode spot. The objective of this study is to determine the anode input heat flux density as anode spot formation. The heat flux density is numerically obtained by a two-temperature magnetohydrodynamic model. Based on a previous systematic experimental work over a wide range of discharge conditions, the calculated critical anode input heat flux density located in a narrow range from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.3\times 10^{8}$ </tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$11.3\times 10^{8}$ </tex-math></inline-formula> W/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{2}$ </tex-math></inline-formula> . Compared with the arc current, the anode input heat flux density is a key parameter in the interaction between the vacuum arc column and the anode; moreover, the heat flux density has a close relationship with both the arc distribution and the anode activity. Therefore, heat flux density is a reasonable physical parameter to reveal the mechanism of the anode spot formation.

P‐4.6: High‐efficiency and high‐power 660 nm laser diode

A highly reliable and high-power laser diode with 110 um stripe width is designed and fabricated. At 25 °C, the lasing wavelength is 660.2 nm at 1.5 A and low threshold current of 506.4 mA can be obtained. At the output power of 2.5 W, the wall-plug efficiency (WPE) can achieve 48.7%. In addition, it can produce as high as 5.1 W at 25 °C and the maximum power can still reach over 4.2 W at 40 °C under CW. Attributed to the non-absorbing mirror, the catastrophic optical degradation (COD) power of this device can reach 6.5 W at 20°C. Till now, the laser diodes have stably operated for 510 hours at the operation current of 2.0 A and heatsink temperature of 40 °C.

Impact of a defect trapping layer on the reliability of 1.3 μm quantum dot laser diodes grown on silicon

In this paper, we studied the impact of a defect trapping layer on the degradation kinetics of 1.3 μm quantum dot laser diodes (QD LDs) for silicon photonics (SiPh). The purpose of the analysis is to prove how the inclusion of trapping layers (TLs) for misfit dislocations can improve the performance and reliability of QD LDs. To this aim we stressed two kinds of devices, that only differ from the inclusion of TLs. By submitting the samples to current step-stress, we tested the devices from mild to highly accelerated conditions. Our results demonstrate that a) the addition of TLs shifts the roll-off of the optical characteristics to higher current densities; b) in devices with TLs, excited state (ES) emission is reached at higher current densities, compared to standard devices; c) in terms of reliability, the addition of TLs results in a slower degradation of the threshold current, to a lower concentration of misfit dislocations in the active region; d) degradation is ascribed to the lowering of the injection efficiency, driven by the escape of carriers out of the active region, which causes an increase in the concentration of non-radiative recombination centers.