Enhancement Of Optical Power Research Articles

Output Optical Power Enhancement of Push-Pull Modulated DFB Laser With Asymmetric Structure

Output Optical Power Enhancement of Push-Pull Modulated DFB Laser With Asymmetric Structure

Spectral extension and synchronization of microcombs in a single microresonator

Broadband optical frequency combs are extremely versatile tools for precision spectroscopy, ultrafast ranging, as channel generators for telecom networks, and for many other metrology applications. Here, we demonstrate that the optical spectrum of a soliton microcomb generated in a microresonator can be extended by bichromatic pumping: one laser with a wavelength in the anomalous dispersion regime of the microresonator generates a bright soliton microcomb while another laser in the normal dispersion regime both compensates the thermal effect of the microresonator and generates a repetition-rate-synchronized second frequency comb. Numerical simulations agree well with experimental results and reveal that a bright optical pulse from the second pump is passively formed in the normal dispersion regime and trapped by the primary soliton. In addition, we demonstrate that a dispersive wave can be generated and influenced by cross-phase-modulation-mediated repetition-rate synchronization of the two combs. The demonstrated technique provides an alternative way to generate broadband microcombs and enables the selective enhancement of optical power in specific parts of a comb spectrum.

Effect of solvent absorption on the optical properties of 3D printed methacrylate waveguide

Methacrylate-based photopolymer waveguides are 3D printed using stereolithography as a single-step fabrication technique and the light propagation through these waveguides is investigated in the presence of various solvents. Solvent absorption on the photopolymer surface is found to play a significant role in the optical output power of the waveguide. Swelling of the photopolymer caused by the solvent absorption is found to increase the optical power transmission through the waveguide. Mixing isopropanol and glycerol with DI water separately at specific concentrations so that their respective refractive indices remain similar, it is observed that isopropanol samples increase the optical power by ~36% compared to glycerol samples, which showed optical power enhancement around 4.9%. A first-order optical model is presented that shows the increase in optical power has a spontaneous response due to the change in the refractive index of the waveguide surface upon solvent contact, followed by a dynamic response due to the formation of the swelling layer as the absorbed solvent diffuse inside the polymer. The methacrylate-based waveguide also showed selectivity towards different concentrations of isopropanol. Results from our study show that the optical properties of the given methacrylate-based photopolymer are largely affected by the absorption affinity of the solvent towards the photopolymer rather than the solvent’s refractive index, which can be crucial for designing integrated optical devices using methacrylate-based photopolymers that work in a different solvent condition.

Unravelling the synergistic effect of reduced graphene oxide on optical, phonon and optical power limiting properties of rGO/α-MoO3 nanohybrids

Reduced graphene oxide (rGO) and semiconducting metal oxide nanohybrids is an emerging class of attractive candidates towards many optical and optoelectronic applications. Herein, we investigated the nonlinear optical properties of rGO/α-MoO3 nanohybrids through open aperture z scan technique under nanosecond pulses at 532 nm excitation which shows the two-photon absorption (2PA) behavior. We found that the synergistic effect between rGO and α-MoO3 plays a significant role in the enhancement of optical power limiting properties. The variation in refractive index (n) and extinction coefficient (k) of α-MoO3 and rGO/α-MoO3 were calculated using the Kramers–Kronig (K–K) equations from the optical reflectance spectroscopic studies. The effect of rGO concentration on the lifetime phonons of singly, doubly, and triply coordinated stretching vibrations of terminal oxygen was investigated from Raman spectroscopic studies. Our experimental results imply that rGO/α-MoO3 has better optical power limiting property with the highest 2PA coefficient compared to previous literature (listed in the table).

Real-Time 2.2-Gb/s Water-Air OFDM-OWC System With Low-Complexity Transmitter-Side DSP

Underwater optical wireless communication (UOWC) is of great interest to the academic and the industry community. In this article, we propose a low-complexity and effective joint transmitter-side digital signal processing (DSP) including geometric shaping, time-domain tone reservation (TR), and clipping. The peak-to-average power ratio reduction performance and implementation complexity of the proposed time-domain TR are extensively analyzed. We then develop a real-time 2.2-Gb/s system, which is optimized for its shaping ratio and clipping ratio. With the help of the efficient DSPs, an 8-dB received optical power enhancement is realized under the bit-error-rate threshold of 3.8 × 10−3. We successfully demonstrate a time-multiplexed four 4K video transmission in real-time using the proposed scheme over a 3.6-m underwater and 8-m free-space channel. The implementation details, as well as the analyses of system stability, resource utilization, and latency, are presented. The results validate the feasibility and effectiveness of the proposed scheme and a 2.2-Gbit/s real-time OWC system is demonstrated for a water-air communication link.

Излучательная рекомбинация и ударная ионизация в полупроводниковых наноструктурах (О б з о р)

In this review we discuss processes of radiative recombination and impact ionization in light emitting structures based on bulk semiconductors, heterostructures with high potential barriers, nanostructures with deep quantum wells, as well as in nanocrystals with quantum dots. It is demonstrated that enhancement of quantum efficiency and optical power of luminescence in all of the structures under study is determined by the common physical mechanism. This mechanism is creation of additional electron-hole pairs during impact ionization process by hot carriers heated at high band offset on the heterointerface at current pumping or due to carrier multiplication in the process of multiexciton generation in nanocrystals at high-energy photons illumination.

Performance-Enhanced 365 nm UV LEDs with Electrochemically Etched Nanoporous AlGaN Distributed Bragg Reflectors.

A 365-nm UV LED was fabricated based on embedded nanoporous AlGaN distributed Bragg reflectors (DBR) by electrochemical etching. The porous DBR had a reflectance of 93.5% at the central wavelength of 365 nm; this is the highest value of porous AlGaN DBRs below 370 nm which has been reported so far. An innovative two-step etching method with a SiO2 sidewall protection layer (SPL) was proposed to protect the n-AlGaN layer and active region of UV LED from being etched by the electrolyte. The DBR-LED with SPL showed 54.3% improvement of maximal external quantum efficiency (EQE) and 65.7% enhancement of optical power at 100 mA without any degeneration in electrical properties, compared with the un-etched standard LED sample. This work has paved the way for the application of electrically-pumped UV LEDs and VCSELs based on nanoporous AlGaN DBRs.

Enhanced the Optical Power of AlGaN-Based Deep Ultraviolet Light-Emitting Diode by Optimizing Mesa Sidewall Angle

In this work, we propose and optimize the sidewalls for the mesa to enhance the optical power for AlGaN-based deep ultraviolet light-emitting diodes (LEDs). We obtain the mesa with the inclined sidewalls by conducting dry etching. The optical performance for LED devices with different inclination angles are tested by putting the integrated sphere on the sapphire side of the devices. The DUV LED with the optimal angle of 37.83° yields the 48% optical power enhancement at the current density of 35 A/cm2 as compared with the reference device. The physical mechanism for the impact of the mesa inclination angle on the light extraction efficiency is also discussed both theoretically and experimentally. Then, we find that once the Al reflector is evaporated on the insulated silica dioxide that is coated on the sidewall of the mesa, the light extraction efficiency for the TM-polarized DUV photons and the optical power can be further improved.

Organic Light-Emitting Transistors with Simultaneous Enhancement of Optical Power and External Quantum Efficiency via Conjugated Polar Polymer Interlayers.

Organic light-emitting transistors (OLETs) show the fascinating combination of electrical switching characteristics and light generation capability. However, to ensure an effective device operation, an efficient injection of charges into the emissive layer is required. The introduction of solution-processed conjugated polyelectrolyte (CPE) films at the emissive layer/electrode interface represents a promising strategy to improve the electron injection process by dipole formation. However, their use in optoelectronic devices also involves some limitations because of the ionic nature of CPEs. In this context, neutral conjugated polar polymers (CPPs) represent a valid alternative to CPEs because the conjugated backbones of CPPs are functionalized with polar nonionic side groups, thus avoiding ion-dependent drawbacks. By introducing a layer of polyfluorene-containing phosphonate groups underneath the metal electrodes, we here demonstrate a substantial improvement of the electron injection properties into the OLET-emissive layer and, accordingly, a more than 2-fold increased light power and a 5 times higher external quantum efficiency of p-type OLETs in comparison with reference devices without any interlayer. The great benefit of using a transparent glass substrate allowed to selectively investigate the morphological and photoluminescent characteristics of both CPE- and CPP-buried interlayers within complete OLETs by means of an optical scanning probe technique. This, together with a thorough optoelectronic characterization of the figures of merit of working light-emitting devices, allowed to disclose the origin of the improved optical performance of CPP-based devices as well as the operation mechanisms of the investigated interlayer in the corresponding OLETs.

Built-in random inverted micro-cone arrays: Nanosecond laser-induced surface texturing for optical outcoupling enhanced flexible white organic light-emitting diodes

Flexible white organic light-emitting diodes (FWOLEDs) have garnered progressively increasing attention in recent years due to their unique capacity to be integrated with bendable or curvilinear optoelectronics. However, highly efficient FWOLEDs are still challenging to make for application in general solid-state lighting because of its inherent optical confinement and imperfect device architectures. Here, in-depth investigation is performed on designing an alternative route for wide-viewing angle optical power enhancement of FWOLEDs based on a simple synergetic electrode architecture. The key feature is the electrode architecture that takes a full advantage of the synergetic collaboration between the tunable density of built-in random inverted micro-cone arrays (IMCAs) and embedded silver nanowires controlling both optical outcoupling enhancement and electrical shorts suppression. The maximum power efficiency (PE) and external quantum efficiency (EQE) reach 58.7 lm W−1 and 28.9% (@ 1000 cd m−2), respectively, which are 70% and 80% enhancement compared with a similarly structured device fabricated on traditional indium-tin-oxide (ITO) glass. Furthermore, the proposed method is directly compatible with inexpensive and scalable roll-to-roll manufacture technology, offering additional appealing features for next-generation higher-valued-added functional solid-state lighting apparatus.

Strong light extraction enhancement using TiO2 nanoparticles-based microcone arrays embossed on III-Nitride light emitting diodes

Colloidal TiO2 nanoparticles were used for embossing of composite microcone arrays on III-Nitride vertical-thin-film blue light emitting diodes (LEDs) as well as on silicon, glass, gallium arsenide, and gallium nitride surfaces. Ray tracing simulations were performed to optimize the design of microcones for light extraction and to explain the experimental results. An optical power enhancement of ∼2.08 was measured on III-Nitride blue LEDs embossed with a hexagonal array of TiO2 microcones of ∼1.35 μm in height and ∼2.6 μm in base width, without epoxy encapsulation. A voltage increase in ∼70 mV at an operating current density of ∼35 A/cm2 was measured for the embossed LEDs. The TiO2 microcone arrays were embossed on functioning LEDs, using low pressures (∼100 g/cm2) and temperatures ≤100 °C.

Enhanced Optical Output Power of Blue Light-Emitting Diode Grown on Sapphire Substrate with Patterned Distributed Bragg Reflector

We demonstrate InGaN-based blue light-emitting diodes (LEDs) with embedded SiO2/SiNx distributed Bragg reflectors (DBRs). Five pairs of circular SiO2/SiNx DBRs were fabricated on a sapphire substrate by depositing multiple dielectric layers and patterning. At an injection current of 20 mA, the optical output power of the LEDs with circular DBRs was 33% higher than that of conventional LEDs without DBRs. The enhancement of optical output power is attributed to the improved light extraction efficiency and internal quantum efficiency of an LED caused by inserting the highly reflective circular DBRs, which increase the reflection of light from the substrate and increase the internal quantum efficiency by reducing threading dislocations in the GaN epilayer.

Mode matching of a laser-beam to a compact high finesse bow-tie optical cavity for quartz enhanced photoacoustic gas sensing

We report on the optical characterization of a compact bow-tie cavity composed of two flat mirrors and two concave mirrors, all having a reflectance >99.99% in the spectral range between 4.8μm and 5.3μm, mounted in a stainless-steel enclosure. The cavity was designed for the implementation of an intracavity-quartz enhanced photoacoustic sensor system. The propagation parameters of the intra-cavity beam were determined using the ABCD-matrix method, allowing the analytical formulation of the size of two beam waists occurring inside the cavity. A collimated mid-infrared laser beam was optically coupled and mode matched into the bow-tie cavity via a focusing lens. A cavity finesse of∼2000 was measured at a pressure of 90Torr inside the cavity, corresponding to an optical power enhancement factor of∼320.

Improved performance of InGaN/GaN MQW LEDs with trapezoidal wells and gradually thinned barrier layers towards anode

We design and evaluate the performance of three InGaN/GaN multiple quantum well blue LEDs – A. rectangular quantum wells with a fixed barrier width, B. trapezoidal quantum wells with a fixed barrier width, and C. trapezoidal quantum wells with a decreasing barrier width towards the anode end – in terms of efficiency droop and power output. We obtain band diagram, electric field, emission spectra and carrier concentration using well calibrated APSYS simulation. Use of trapezoidal quantum wells increases better overlapping between electron and hole wavefunctions thereby increasing radiative recombination events. Furthermore decreasing barrier width from n- to p- regions shortens hole transport path which results in better hole transport and distribution in the wells and hence larger radiative recombination rate. Our proposed structure C exhibits efficiency droop reduction of 2.1% and enhancement of optical power of 236.7% compared to conventional rectangular quantum well structure at injection current of 120 mA.

Enhanced optical output power of InGaN/GaN light-emitting diodes grown on a silicon (111) substrate with a nanoporous GaN layer.

We report the growth of InGaN/GaN multiple quantum wells blue light-emitting diodes (LEDs) on a silicon (111) substrate with an embedded nanoporous (NP) GaN layer. The NP GaN layer is fabricated by electrochemical etching of n-type GaN on the silicon substrate. The crystalline quality of crack-free GaN grown on the NP GaN layer is remarkably improved and the residual tensile stress is also decreased. The optical output power is increased by 120% at an injection current of 20 mA compared with that of conventional LEDs without a NP GaN layer. The large enhancement of optical output power is attributed to the reduction of threading dislocation, effective scattering of light in the LED, and the suppression of light propagation into the silicon substrate by the NP GaN layer.

Slab waveguide theory for general multi-slot waveguide

Optical devices based on slot waveguide are of considerable interest in numerous applications due to the distinct feature of strong electric field confinement in a low-refractive index region. A theoretical model based on multi-slab waveguide theory is used to reveal the physical mechanism of the slot waveguide. The calculation results derived from the basic Helmholtz equation for the conventional single-slot waveguide with a ~2% validation of the effective refractive index are compared to the former experiment results by the Cornell University group. Moreover, we extend the theoretical model to a general multi-slot waveguide. Its electric field distribution and key properties such as optical power confinement factor and enhancement factor in slot are deduced theoretically and fully discussed.

The beneficial effects of a p-type GaInN spacer layer on the efficiency of GaInN/GaN light-emitting diodes

Light-emitting diodes (LEDs) with a Mg-doped p-type Ga1−xInxN (0 ≤ x ≤ 0.07) spacer layer located between an undoped GaN spacer layer and the electron blocking layer are investigated. The LEDs are found to have comparable peak efficiency but less efficiency droop when the crystal quality of the p-type Ga1−xInxN spacer layer is well-controlled by lowering the growth temperature and by using a suitable In composition and Mg doping concentration. All LED samples with the p-type spacer layer show a smaller efficiency droop compared to a reference LED having an undoped GaN spacer. Among the sample sets investigated, an optical power enhancement of 12% at 111 A/cm2 is obtained when inserting a 5 nm-thick p-type Ga0.97In0.03N spacer layer. The results support that carrier transport is the key factor in the efficiency droop observed in GaN-based LEDs.

Improved light extraction efficiency of GaN-based vertical LEDs using hierarchical micro/subwavelength structures

We demonstrate the hierarchical micro- and subwavelength-structures on the surface of GaN based green vertical light-emitting diodes (VLEDs) to enhance the light extraction efficiency. Hierarchical structures (HSs), which were fabricated by simple two-step process consisting of pattern transfer of reflowed photoresist for micro structures (MSs) and self-masked dry etching method for subwavelength structures (SWSs), play an important role in enhancing the light extraction efficiency by reducing both the total internal reflection and the Fresnel reflection. HSs lead to an enhancement of optical output power of ∼200% compared to the conventional VLEDs with flat surface as a result of reflection suppression and transmittance enhancement, without any serious degradation of electrical characteristics and a change of peak emission wavelength as function of injection current. Furthermore, optical simulations by ray-tracing and the rigorous coupled-wave analysis methods provide design guidelines for MSs and SWSs, respectively.

Regimes of external optical feedback in 5.6 μm distributed feedback mid-infrared quantum cascade lasers

External optical feedback is studied experimentally in mid-infrared quantum cascade lasers. These structures exhibit a dynamical response close to that observed in interband lasers, with threshold reduction and optical power enhancement when increasing the feedback ratio. The study of the optical spectrum proves that the laser undergoes five distinct regimes depending on the phase and amplitude of the reinjected field. These regimes are mapped in the plane of external cavity length and feedback strength, revealing unstable behavior only for a very narrow range of operation, making quantum cascade lasers much more stable than their interband counterparts.

Two-band superlinear electroluminescence in GaSb based nanoheterostructures with AlSb/InAs1−x Sbx/AlSb deep quantum well

We report on superlinear electroluminescent structures based on AlSb/InAs1−xSbx/AlSb deep quantum wells grown by MOVPE on n-GaSb:Te substrates. Dependence of the electroluminescence (EL) spectra and optical power on the drive current in nanoheterostructures with AlSb/InAs1−xSbx/AlSb quantum well at 77–300 K temperature range was studied. Intensive two-band superlinear EL in the 0.5–0.8 eV photon energy range was observed. Optical power enhancement with the increasing drive current at room temperature is caused by the contribution of the additional electron-hole pairs due to the impact ionization by the electrons heated at the high energy difference between AlSb and the first electron level Ee1 in the InAsSb QW. Study of the EL temperature dependence at 90–300 K range enabled us to define the role of the first and second heavy hole levels in the radiative recombination process. It was shown that with the temperature decrease, the relation between the energies of the valence band offset and the second heavy hole energy level changes due to the temperature transformation of the energy band diagram. That is the reason why the EL spectrum revealed radiative transitions from the first electron level Ee1 to the first hole level Eh1 in the whole temperature range (90–300 K), while the emission band related with the transitions to the second hole level occurred only at T > 200 K. Comparative examination of the nanostructures with high band offsets and different interface types (AlAs-like and InSb-like) reveals more intense EL and optical power enhancement at room temperature in the case of AlAs-like interface that could be explained by the better quality of the heterointerface and more efficient hole localization.