Optical Output Power Research Articles

Design and Fabrication of Single-Mode Multi-Junction 905 nm VCSEL With Integrated Anti-Phase Mode Filter

In this paper, a record-high single-mode output power from a multi-junction vertical-cavity surface emitting laser (VCSEL) emitting at 905nm is achieved. Single spectral mode emission of the VCSEL is realized by using an anti-phase mode filter, placed at the top surface of the device, that acts as a relief layer to suppress the higher order transverse modes. Placement of such anti-phase layer at the surface areas in which the higher order modes are expected to emit, leads to reduction in the effective mirror reflectivity at those areas, increased optical losses, and consequently, increased threshold gain for the higher order modes compared to the fundamental mode. The room-temperature light-current-voltage characteristics of a 6-junction 905nm VCSEL with an oxide aperture size of ∼8μm and mode filter size of 5μm shows a single-mode optical power of 12.55mW and maximum power conversion efficiency of 56.48%. Moreover, the emission spectrum of this device shows a stable side-mode suppression ration of >30dB in the single-mode operation regime. The VCSEL remains reliably single-mode at higher temperatures, generating a maximum optical output power of 9.48mW at 60°C. Such remarkable single-mode performance can be mainly attributed to the multi-junction epitaxial structure of the device, in which the output optical power is expected to proportionally increase according to the number of bipolar cascade active regions.

Operation-induced degradation mechanisms of 275-nm-band AlGaN-based deep-ultraviolet light-emitting diodes fabricated on a sapphire substrate

Degradation mechanisms of 275-nm-band AlxGa1-xN multiple quantum well deep-ultraviolet light-emitting diodes fabricated on a (0001) sapphire substrate were investigated under hard operation conditions with the current of 350 mA and the junction temperature of 105 °C. The optical output power (Po) initially decreased by about 20% within the operating time less than 102 h and then gradually decreased to about 60% by 484 h. For elucidating the causes for the initial and subsequent degradations, complementary electrical, time-resolved photoluminescence (TRPL), and impurity characterizations were carried out making a connection with the energy band profiles. Because the degradation of the wells was less significant than the Po reduction, the initial degradation is attributed essentially to the decrease in carrier injection efficiency (ηinjection), not in internal quantum efficiency of the wells, most likely due to depassivation of initially H-passivated preexisting nonradiative recombination centers (NRCs) in a Mg-doped p-type Al0.85Ga0.15N electron blocking layer. The principal cause for the subsequent Po reduction until 484 h is attributed to further decrease in ηinjection due to the appearance of certain current bypasses in addition to continuous depassivation of the NRCs in p-type AlxGa1-xN layers. According to our database on the species of vacancy-type defects acting as NRCs in GaN and AlN, which have been identified using the combination of positron annihilation and TRPL measurements, vacancy clusters comprised of a cation vacancy (VIII) and nitrogen vacancies (VN), such as VIIIVN2∼4, are the most suspicious origins of the NRCs in Mg-doped p-type AlxGa1-xN layers.

224-Gbit/S 4-PAM Operation of Compact DC Block Circuit Integrated Hi-FIT AXEL Transmitter With Low Power Consumption

We previously developed a semiconductor optical amplifier (SOA) assisted extended reach electroabsorption modulator integrated distributed feedback (EADFB) laser (AXEL) to increase the optical modulation output power and a high-frequency integrated design based on the flip-chip interconnection technique (Hi-FIT) to obtain a high modulation bandwidth. We achieved a high-output power 224-Gbit/s 4-level pulse-amplitude-modulation (4-PAM) operation of a Hi-FIT AXEL module. In this paper, we developed a compact DC block circuit integrated into Hi-FIT AXEL module to decrease chip-power consumption without increasing the sub-assembly size and degrading the modulation bandwidth. We designed a Hi-FIT AXEL sub-assembly, that include a flip-chip interconnection board with a DC block circuit that affects the modulation bandwidth. We achieved a 3-dB bandwidth of up to 64 GHz for this designed sub-assembly. The fabricated Hi-FIT AXEL module has a 3-dB bandwidth of more than 62 GHz and a flat frequency response up to 50 GHz. For the 224-Gbit/s 4-PAM operation, the fabricated Hi-FIT AXEL module has a chip-out average output power of +10.0 dBm with a transmitter eye-closure quaternary (TECQ) of 1.9 dB. The integrated compact DC block circuit can reduce chip power consumption by up to 17%.

Achieving High Quantum Efficiency Broadband NIR Mg4 Ta2 O9 :Cr3+ Phosphor Through Lithium-Ion Compensation.

Ultra-efficient broadband near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are urgently needed to improve the detection sensitivity and spatial resolution of current smart NIR spectroscopy-based techniques. Nonetheless, the performance of NIR pc-LED has severely limited owing to the external quantum efficiency (EQE) bottleneck of NIR light-emitting materials. Herein, a blue LED excitable Cr3+ -doped tetramagnesium ditantalate (Mg4 Ta2 O9 , MT) phosphor is advantageously modified through lithium ion as a key efficient broadband NIR emitter to achieve high optical output power of the NIR light source. The emission spectrum encompasses the 700-1300nm electromagnetic spectrum of first biological window (λmax = 842nm) with a full-width at half-maximum (FWHM) of ≈2280 cm-1 (≈167nm), and achieves a record EQE of 61.25% detected at 450nm excitation through Li-ion compensation. A prototype NIR pc-LED is fabricated with MT:Cr3+ , Li+ to evaluate its potential practical application, which reveals an NIR output power of 53.22mW at a driving current of 100mA, and a photoelectric conversion efficiency of 25.09% at 10mA. This work provides an ultra-efficient broadband NIR luminescent material, which shows great promise in practical applications and presents a novel option for the next-generation high-power compact NIR light sources.

All-optical diode for terahertz optical power rectification based on two-dimensional photonic crystals

Photonic crystals are one of the most important structures used in the creation of optical devices. Photonic crystals are capable of being used to make optical diodes. The quantity of optical power transferred in the “on” state, from anode to cathode, is one of the most significant criteria in the design of an optical diode, while low optical power is the reverse. Another essential characteristic of an all-optical diode is its simple structure and compact size, which make it easy to manufacture and allow it to be incorporated into optical circuits. In this study, an all-optical diode is designed using the creation of a photonic crystal with a hexagonal lattice and tiny dimensions. To simplify the complexity of the structure, the crystal structure was created using simple line defects and a modest number of point defects. In addition to its rapid transfer of optical power, the suggested diode features a high optical power output while conducting and a negligible output when “off."

Fast Neutron Irradiation on GaN-Based Light-Emitting Diodes From Near-Ultraviolet to Green Spectral Ranges

Neutron irradiation effects were compared for GaN-based multi-quantum well (MQW) light emitting diodes (LEDs) with emission wavelength in the near-ultraviolet (NUV), blue and green spectral regions. Optical output power, current-voltage and peak wavelength were analyzed in detail before and after irradiation. Three colors of LEDs showed an increase and then a decrease in optical output power with the increase of neutron fluence ranging from 1.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> to 1.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . The enhanced light emission performance is attributed to the increase of carrier injection for neutron irradiated MQWs. In addition, the formation of tunneling conductivity channels in the p-doped AlGaN electron block layer (EBL) induced by neutrons is considered as the main factor that dominates the performance degradation. Among the three colors of LEDs, the order of severity for the optical and electrical properties influenced by this medium-fluence neutron irradiation is green, blue, and NUV LEDs, in turn becoming more serious. The NUV LEDs rely heavily on the EBL due to their poor carrier confinement in relatively shallow GaN/AlGaN MQWs, so an increase of carrier injection or leakage induced by neutrons has significant influence on device performance. As the green LEDs have deep quantum wells, the strong confinement of carriers leads to their being least sensitive to neutron irradiation. So we believe that design of LEDs with deep quantum wells to enhance the carrier confinement ability will be helpful to improve their radiation tolerance. The findings not only enrich the understanding of the influence mechanism of neutron irradiated GaN-based UV/visible LEDs, but are also helpful to radiation hardened design.

Performance analysis for 263 nm AlGaN DUV EELD by different EBL techniques

Theoretical calculations with LASTIP software for the electrically driven edge-emitting laser diode (EELD) composed of trinary Aluminum Gallium Nitride (AlGaN) with optimized parameters of Aluminum (Al) composition. Composition thickness is performed in the present work to reduce the threshold current and enhance the output performance. The simulation results for the designed structure lase at a target ultraviolet laser (UVC) wavelength of 263.8 nm. The maximum SE was about 1.43 W/A at an injection current of 763 mA for a conventional EBL with Al0.75Ga0.25, while the lowest threshold current of 725 mA was found for the same conventional structure with Al0.90Ga0.10N. Followed by a slightly higher threshold current of 784 mA when double-side linearly graded EBL techniques were adopted for all different grading levels. Opposing the worst threshold current was found to be 787 mA when simulating inversely tapered EBLs with a relatively lower slope efficiency of about 1.40 W/A for Al0.95–0.85Ga0.05–0.15N EBL. Opposing the worst threshold current was found to be 787 mA when simulating inversely tapered EBLs with a relatively lower slope efficiency of about 1.40 W/A for Al0.95–0.85GaN EBL. This result probably attributes to the sufficient conduction band barrier height for the designed EBL. This will help realize a higher optical output power and a lower threshold current in AlGaN-based ultraviolet EELDs.

2D-GaN/AlN Multiple Quantum Disks/Quantum Well Heterostructures for High-Power Electron-Beam Pumped UVC Emitters.

This article describes GaN/AlN heterostructures for ultraviolet-C (UVC) emitters with multiple (up to 400 periods) two-dimensional (2D)-quantum disk/quantum well structures with the same GaN nominal thicknesses of 1.5 and 16 ML-thick AlN barrier layers, which were grown by plasma-assisted molecular-beam epitaxy in a wide range of gallium and activated nitrogen flux ratios (Ga/N2*) on c-sapphire substrates. An increase in the Ga/N2* ratio from 1.1 to 2.2 made it possible to change the 2D-topography of the structures due to a transition from the mixed spiral and 2D-nucleation growth to a purely spiral growth. As a result, the emission energy (wavelength) could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) owing to the correspondingly increased carrier localization energy. Using electron-beam pumping with a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W.

Single-Mode High-Speed 1550 nm Wafer Fused VCSELs for Narrow WDM Systems

High power single-mode wafer fused 1550 nm VCSELs with an active region based on InGaAs quantum wells are fabricated. An InP-based optical cavity and two AlGaAs/GaAs distributed Bragg reflector heterostructures were grown by molecular-beam epita oy. The current and optical confinements are provided by a lateral-structured buried tunnel junction with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim ~6~\mu \text{m}$ </tex-math></inline-formula> diameter and etching depth of ~ 20 nm. The VCSELs demonstrate ~ 3.4 mW single-mode continuous-wave output optical power and a threshold current about 2 mA at 20°C. The output optical power exceeds 1 mW at 70°C. A −3dB modulation bandwidth > 13 GHz is obtained at 20°C. Non-return-to-zero data transmission under back-to-back condition of ~ 37 Gbps is shown.

Mode-Modulation Structure Based on 650 nm Ridge Waveguide Edge-Emitting Laser

Traditional laser diodes operating at 650 nm are more prone to high-order mode excitation, resulting in poorer beam quality. In this paper, we designed GaInP–AlGaInP laser diodes (LD) with a 650 nm range and a trench mode-modulation structure based on the structure of edge-emitting laser (EEL) diodes. The effect of the three-trench structure was investigated theoretically and experimentally. The right trench structure laser chips demonstrated good beam quality while maintaining a high power output. An electro-optical conversion efficiency of 56% was demonstrated with a slope efficiency of 1.32 W/A at a 40 mA current. The maximum optical output power reached 40.8 mW.

Fiber-Based Techniques to Suppress Stimulated Brillouin Scattering

Stimulated Brillouin scattering (SBS) is the major factor that limits the maximum optical fiber output power in narrow linewidth applications, which include important fields such as passive optical networks (PONs), high-power fiber amplifiers, and lasers. Great efforts have been dedicated to suppressing the SBS effect and increasing the maximum optical fiber output power. This paper focuses on key fiber-based techniques to suppress SBS. These techniques take advantages of the properties of optical fibers. We present how these properties (electric modes, acoustic modes, and material properties) could be utilized to suppress SBS. The fiber-based techniques are divided into transverse optical fiber design, longitudinal variant fiber design, and external perturbations (strain and temperature) on optical fibers. Transverse optical fiber design focuses on the mechanism electro-acoustic interaction. Large effective area fiber design and acoustic tailoring techniques have been discussed. Longitudinal variant fiber design considers the nonlinear SBS interaction along propagation distance, and various techniques related have been presented. External perturbations (strain and temperature) on optical fibers emphasize on how external static perturbations could modify the SBS effect.

A Segmental Measurement for Improving the Dynamic Range and Spatial Resolution of Φ-OTDR Fiber Vibration Detection

Phase-sensitive optical time domain reflectometry (Φ-OTDR) utilizes the back Rayleigh scattered light to realize long-distance and high spatial resolution vibration and acoustic waves detection in the whole sensing fiber. The Φ-OTDR has been widely used in structure health monitoring, gas pipelines preventing digging, etc. For practical applications, because of weak backscattering Rayleigh light and exponentially decreased injecting optical signal along the optical fiber, long distance Φ-OTDR detection is very difficult to realize, so how to extend detecting range meanwhile possessing a relatively high spatial resolution turns into a challenge in Φ-OTDR. We proposed a scheme of adjusting EDFA output optical power to use segmented measurement to improve the performance of Φ-OTDR, which can extend detecting range to several tens of kilometers. A segmental measurement scheme was proposed and the experimental results were presented.

High-SMSR wavelength-swept laser with a CL-band mode-hopping free tuning range

A high side-mode suppression ratio external-cavity wavelength-swept laser system with a wide mode-hopping free tuning range is developed. The proposed wavelength-swept laser is based on a single-angled facet gain chip in a classical Littman-Metcalf external-cavity configuration using a blazed diffraction grating as the feedback element. The output power, side-mode suppression ratio, and tunable range of the external-cavity wavelength-swept laser system are experimentally measured. Wide mode-hopping-free continuous wavelength tuning range about 100 nm in CL-band with a side-mode suppression ratio of more than 65.64 dB are realized. An output optical power of more than 14.12 dBm over the entire tunable region can also be achieved. The proposed external-cavity wavelength-swept laser system allows simultaneously high side-mode suppression ratio and no mode-hopping tunable radiation, thus opening a door for practical applications such as optical modulation schemes.

Low threshold current density and high power InGaN-based blue-violet laser diode with an asymmetric waveguide structure.

Performance of InGaN-based blue-violet laser diodes (LD) with different waveguide structure were investigated by simulation and experimental methods. Theoretical calculation demonstrated that threshold current (Ith) can be reduced and slope efficiency (SE) can be improved by using an asymmetric waveguide structure. Based on the simulation results, a LD with 80-nm-thick In0.03Ga0.97N lower waveguide (LWG) and 80-nm-thick GaN upper waveguide (UWG) is fabricated with flip chip package. Under continuous wave (CW) current injection at room temperature, its optical output power (OOP) reaches 4.5 W at an operating current of 3 A and the lasing wavelength of 403 nm. The threshold current density (Jth) is 0.97 kA/cm2 and the SE is about 1.9 W/A.

Simulation and analysis of high-brightness tapered ridge-waveguide lasers

In this work, a simulation-based analysis of a CW-driven tapered ridge-waveguide laser design is presented. Measurements of these devices delivered high lateral brightness values of {4}{hbox {W}cdot hbox {mm}^{-1} hbox {mrad}^{-1}} at {2.5},hbox {W} optical output power. First, active laser simulations are performed to reproduce these results. Next, the resulting complex valued intra-cavity refractive index distributions are the basis for a modal and beam propagation analysis, which demonstrates the working principle and limitation of the underlying lateral mode filter effect. Finally, the gained understanding is the foundation for further design improvements leading to lateral brightness values of up to {10}{hbox {W}cdot hbox {mm}^{-1} hbox {mrad}^{-1}} predicted by simulations.

A 200 MHz 14 W Pulsed Optical Illuminator With Laser Driver ASIC and On-Chip DLL-Based Time Interpolator for Indirect Time-of-Flight Applications

The applications requiring three-dimensional information have been growing rapidly, including facial recognition, gesture recognition, auto-focus for 2D camera, self-driving autonomous vehicles, etc. Indirect Time-of-Flight (iToF) technique has been adopted widely for 3D depth imaging due to its high spatial resolution and excellent depth accuracy performance. In this brief, a 200 MHz pulsed optical Illuminator for iToF applications is reported. A 940 nm GaAs-based triple-junction VCSEL array is adopted and an integrated pulsed VCSEL driver ASIC in 180 nm BCD process is realized. Optical pulses with peak power up to 14 W can be transmitted by the laser illuminator with a maximum driving current of 6.5 A by the driver ASIC. The modulation frequency can be up to 200 MHz. Automatic optical power control (AOPC) is realized for stable optical power output and laser eye safety protection. An on-chip DLL-based time interpolator is proposed to eliminate the mechanical moving components used during system calibration. The measured absolute accuracy of the time interpolator is +/−50 ps with a resolution of 800 ps and tuning range of 49.6 ns. This brief effectively extends the measurement distance of the iToF system. The calibration efficiency is also improved with the proposed method.

Pushing the Room Temperature Continuous‐Wave Operation Limit of GaSb‐Based Interband Cascade Lasers beyond 6 µm

AbstractGaSb‐based interband cascade lasers (ICLs) emitting at a center wavelength of 6.12 µm at 20 °C in continuous‐wave operation up to a maximum operating temperature of 40 °C are presented. Pulsed measurements based on broad area devices show improved performance by applying the recently published approach of adjusting the GaInxSb layer thickness in the active region to reduce the valence intersubband absorption. The W‐quantum well design adjustment and the optimization of the electron injector, to rebalance the electron and hole concentrations in the active quantum wells, improved the device performance, yielding room temperature current densities as low as 0.5 kA cm−2 for broad area devices under pulsed operation. As a direct result of this improvement together with optimizations of the waveguide design, the long wavelength limit for GaSb‐based ICLs in continuous‐wave operation could be extended. For an epi‐side down mounted 23 µm wide and 2 mm long device with nine active stages and high‐reflectivity back facet, the threshold power is below 1 W and the optical output power is over 25 mW at 20 °C in continuous‐wave mode. Such low‐threshold and low‐power consumption ICLs are especially attractive for mobile and compact sensing systems.

Single‐Site Occupancy of Eu2+ in Multiple Cations Enables Efficient Ultra‐Broadband Visible to Near‐Infrared Luminescence

AbstractAn important challenge in the research and development of compact near‐infrared (NIR) light sources is the discovery of new efficient ultra‐broadband NIR luminescent materials to replace the conventional Cr3+‐doped compounds. Herein, this work reports a divalent europium‐doped Ba3GeO4Br2 phosphor that exhibits a high photoluminescence quantum yield of 48.8% and an ultra‐broad emission band ranging continuously from 500 to 1100 nm (full width at half maximum = 202 nm) under near‐ultraviolet or blue light excitation. A strategy of embedding alumina crucible in high purity graphite crucible is devised and deployed during solid‐state sintering to facilitate the conversion of Eu3+ into Eu2+. Theoretical calculations, structural refinement, and spectral analysis demonstrate that the visible to NIR emission in Ba3GeO4Br2:Eu2+ originates from the Eu2+ ions occupying the distorted Ba3O3Br4 polyhedra in the lattice. The as‐prepared phosphor‐converted light‐emitting diode device achieves an optical output power of 30.1 mW@100 mA (520–1100 nm) and a photoelectric efficiency of 22%@100 mA (350–1100 nm). Experiments on tissue penetration and NIR imaging illustrate its application in spectral detection and food quality testing appears promising.

Multiple increase in the radiative efficiency of 1060-nm laser diodes based on heterostructures with tunnel-coupled injection and a single waveguide mode.

An approach aimed at increasing the radiative efficiency in heterostructures operating in a single vertical mode at 1060 nm has been studied. Two types of heterostructures-the STJH (single tunnel junction heterostructure) and DTJH (double tunnel junction heterostructure)-have been developed to obtain the operation of a single waveguide mode of the first and second order, respectively. A multiple increase in the injection efficiency is realized by using tunnel junctions (TJs) embedded in the wide-gap barriers and placed between the active regions. An internal quantum efficiency of close to 100% and a transparency current density of 78 A/cm2 is demonstrated for both types of heterostructures, while the internal optical loss is 5.2 and 3.8 cm-1 for STJH and DTJH, respectively. The maximum radiative efficiency (for the initial linear part of the light-current curve) and the output optical power (for the current pulse of 1 µs and 9.6 A) reach 1.6 W/A and 7.8 W for STJH and 2.2 W/A and 15.8 W for DTJH.

Strain induced power enhancement of far-UVC LEDs on high temperature annealed AlN templates

High temperature annealed AlN/sapphire templates exhibit a reduced in-plane lattice constant compared to conventional non-annealed AlN/sapphire grown by metalorganic vapor phase epitaxy (MOVPE). This leads to additional lattice mismatch between the template and the AlGaN-based ultraviolet-C light emitting diode (UVC LED) heterostructure grown on these templates. This mismatch introduces additional compressive strain in AlGaN quantum wells resulting in enhanced transverse electric polarization of the quantum well emission at wavelengths below 235 nm compared to layer structures deposited on conventional MOVPE-grown AlN templates, which exhibit mainly transverse magnetic polarized emission. In addition, high temperature annealed AlN/sapphire templates also feature reduced defect densities leading to reduced non-radiative recombination. Based on these two factors, i.e., better outcoupling efficiency of the transverse electric polarized light and an enhanced internal quantum efficiency, the performance characteristic of far-UVC LEDs emitting at 231 nm was further improved with a cw optical output power of 3.5 mW at 150 mA.