Internal Optical Loss Research Articles

Ultranarrow-waveguide AlGaAs/GaAs/InGaAs lasers

We have designed, fabricated and studied ultranarrow-waveguide heterostructure lasers emitting in the spectral range 1000 – 1100 nm. The lasers have been characterised by current – voltage, light – current, far-field intensity distribution and internal optical loss measurements. The ultranarrow-waveguide lasers have been shown to have a threshold current density of ∼75 A cm−2, internal quantum efficiency near 100 % and internal optical loss near the lasing threshold under 1 cm−1, which corresponds to the level of standard heterostructures. We have demonstrated the possibility of obtaining up to 5 W of output power in continuous mode and up to 30 W in pulsed mode, with a beam convergence (FWHM) of 17.8°. The slope of the internal optical loss as a function of pump current for the ultranarrow-waveguide lasers can be markedly lower than that in lasers with a standard design, but internal quantum efficiency drops to 40 % with increasing pump current. The use of barrier layers in ultranarrow-waveguide lasers makes it possible to substantially reduce the drop in internal quantum efficiency.

Analysis of the Internal Optical Losses of the Vertical-Cavity Surface-Emitting Laser of the Spectral Range of 1.55 µm Formed by a Plate Sintering Technique

The results of a study of internal optical losses and current injection efficiency in vertical-emitting lasers of a spectral range of 1.55 µm obtained by sintering plates of high-q Bragg reflectors and the active region on the basis of thin strained InGaAs/InAlGaAs quantum wells have been presented. It has been shown that the proposed design of the laser provides a record low level of internal optical losses (less than 6.5 cm–1) and high efficiency of current injection (more than 90%) at room temperature, which allows the realization of submilliampere threshold currents. As the temperature rises to 85°C, the current injection efficiency drops to 70% due to the thermal emission of charge carriers from the active region, accompanied by an increase in internal optical losses to 9.1 cm–1 because of an increase in absorption on free carriers and/or intersubband absorption in the valence band.

Online Measurement of Internal Parameters in a Quasi-CW Diode-Pumped Nd:YAG Laser

We present a simple and practical online measurement method to estimate internal parameters (small-signal gain coefficient, internal optical losses, the saturation parameter, and transmittance of the coupling output mirror) in an neodymium doped yttrium aluminum garnet Nd:YAG laser system. A measurement cell, placed in the optical cavity of a laser, consists of a polarizer and a rotatable polarization analyzer. Internal parameters are measured conveniently by introducing variable polarization reflective loss. Online measurements of the internal parameters in a quasi-continuous-wave (CW) diode-pumped Nd:YAG laser are performed and the measured results, which are discussed and analyzed, indicate good measurement accuracy. Experiments to investigate validity are conducted and further verify the applicability of the measurement method. A measurement cell with a small volume that is easy to access has potential to be used in the online measurement, optimized design, and maintenance of a laser.

Effect of (Al)GaAs/AlGaAs quantum confinement region parameters on the threshold current density of laser diodes

An approach has been proposed for choosing parameters (width and depth) of an (Al)GaAs/AlGaAs quantum confinement region via calculation of the threshold current density of a semiconductor laser. A detailed assessment of its components has made it possible to find criteria for optimising the range of quantum well widths so as to minimise the threshold current for lasers with different heterostructure geometries. The data presented in this paper demonstrate the feasibility of further improving output characteristics of semiconductor lasers by optimising the design and technology of quantum well heterostructures. Owing to this, we have simultaneously reduced the threshold current, carrier escape and internal optical loss, which has allowed us to obtain high (60 % to 70 %) efficiencies of a semiconductor laser operating in the spectral range 800 – 850 nm.

Evolution of light‐current characteristic shape in high‐power semiconductor quantum well lasers

The light-current characteristic (LCC) of semiconductor quantum well lasers is theoretically studied. It is discussed here that, due to internal optical absorption loss, which depends on the electron and hole densities in the optical confinement layer, (i) roll-over of the LCC occurs with increasing injection current, and, (ii) depending on the parameters of laser structures, the LCC can have two branches, i.e. the optical emission at two different output powers will be possible within a certain range of injection currents.

High-performance 1.06-μm InGaAs/GaAs double-quantum-well semiconductor lasers with asymmetric heterostructure layers

High-performance 1.06-μm InGaAs/GaAs double-quantum-well (DQW) asymmetric herero- structure semiconductor lasers have been designed, fabricated and characterized. The laser structure, grown by metal-organic chemical vapor deposition (MOCVD), mainly consists of compositionally graded p-AlxGa1−xAs upper cladding layers, a p-AlxGa1−xAs upper waveguide layer, a 1.06-μm InGaAs/GaAs DQW active region, an un-doped In1−xGaxAsyP1−y lower waveguide layer, and compositionally graded n-doped In1−xGaxAsyP1−y lower cladding layers. Measurement results of the as-cleaved ridge waveguide (RWG) lasers with a contact ridge width of 25 μm and different cavity lengths (900 to 2765 μm) demonstrated state-of-the-art performances with a high internal quantum efficiency (ηi) of ∼98.4% and a low internal optical loss (αi) of ∼1.01 cm−1 at 20 °C. The laser has demonstrated high characteristic temperatures of 245 K (T0) and 663 K (T1) from 20 to 50 °C, and the cavity length dependent behaviour of T0 and T1 has also been investigated from 20 to 80 °C. Furthermore, a low transparency current density (Jtr) of 77 A/cm2/QW in this laser structure is obtained at 20 °C, which is among one of the lowest values reported so far for 1.06-μm InGaAs/GaAs semiconductor lasers.

Optical Gain and Lasing Properties of InP/AlGaInP Quantum-Dot Laser Diode Emitting at 660 nm

We investigated the optical gain properties and lasing characteristics of a pulsed electrically pumped laser structure, which consists of a single layer of self-assembled InP quantum dots (QDs) assembled in (Al 0.10 Ga 0.90 ) 0.52 In 0.48 P barriers. The optical gain and absorption spectra were obtained by analyzing the amplified spontaneous emission as a function of the excitation length. At a current density of 1.2 kA/cm 2 , an internal optical loss value of 5 ± 2 cm -1 and a peak modal gain of 39.3 cm -1 corresponding to a material gain of approximately 2675 cm -1 per QD layer were determined at room temperature. For a 2.24-mm-long laser with uncoated facets emitting at 660 nm, a low threshold current density of 281 A/cm 2 and an external differential quantum efficiency of 34.2% were determined. The internal quantum efficiency of 66% and the transparent current density of 65.8 A/cm 2 for a single layer of QDs were also demonstrated.

Suppression of higher-order modes in a large-optical-cavity waveguide structure for high-power high-efficiency 976-nm diode lasers

High-power high-efficiency broad-area diode lasers are highly efficient light sources. A study on the design of large-optical-cavity 976-nm diode lasers is presented here. Efforts were made to suppress the higher-order modes and to explore new design methods for high-power high-efficiency diode lasers. The mode distribution, confinement factor, internal optical loss and far-field pattern were numerically calculated to analyze the waveguide design. To suppress the higher-order transverse modes, both gain and loss discrimination mechanisms were experimentally investigated. By adjusting the position of the active region, successful suppression of higher-order modes can be achieved. Alternatively, higher-order modes can be suppressed by increasing the cavity length, which is attributed to the discrimination of modal loss between the higher-order modes and the fundamental mode. A high-efficiency 4-mm-long broad-area single emitter with an ultralow internal optical loss of 0.27 cm–1 and an internal quantum efficiency of 93.5% was designed and fabricated. A high slope efficiency of 1.1 W/A and >66% wall-plug efficiency was obtained at 25 °C under continuous current injection. The far-field divergence angles with 95% power content were 38° and 9.7° for the fast and slow axes, respectively, at an output power of 10.0 W.

Анализ внутренних оптических потерь вертикально-излучающего лазера спектрального диапазона 1.55 μm, сформированного методом спекания пластин

AbstractThe results of a study of internal optical losses and current injection efficiency in vertical-emitting lasers of a spectral range of 1.55 µm obtained by sintering plates of high-q Bragg reflectors and the active region on the basis of thin strained InGaAs/InAlGaAs quantum wells have been presented. It has been shown that the proposed design of the laser provides a record low level of internal optical losses (less than 6.5 cm^–1) and high efficiency of current injection (more than 90%) at room temperature, which allows the realization of submilliampere threshold currents. As the temperature rises to 85°C, the current injection efficiency drops to 70% due to the thermal emission of charge carriers from the active region, accompanied by an increase in internal optical losses to 9.1 cm^–1 because of an increase in absorption on free carriers and/or intersubband absorption in the valence band.

Theoretical Modeling of the Internal Power Flow and Absorption Loss of the Air Mode Based on the Proposed Poynting Vector Analysis in Top-emitting Organic Light-emitting Diodes

We propose the Poynting vector analysis of the air mode in a top-emitting organic light-emitting diode (OLED) by combining the transfer matrix method and dipole source term. The spatial profiles of the time-averaged optical power flow of the air mode are calculated inside and outside the multilayer structure of the OLED with respect to the thickness of the semi-transparent top cathode and capping layer (CPL). We elucidate how the micro-cavity effect controlled by the thickness variation of the semi-transparent top cathode or CPL affects the internal optical power and absorption loss inside the OLED multilayer and the external optical power coupled into the air. When the calculated absorption loss and external power obtained by the proposed Poynting vector and currently-used point dipole models are compared, two calculation results are identical, which demonstrates the validity of the two models.

Effect of carrier localization on performance of coupled large optical cavity diode lasers

Results are presented on a comparative study of InGaAs/GaAs/AlGaAs coupled large optical cavity (CLOC) laser heterostructures emitting at either 1.03 or 0.98 μm. The smaller energy of carrier localization in the active region of the latter structure leads to stronger carrier leakage out of the quantum well that in its turn results in a higher internal optical loss, higher threshold current density and more pronounced temperature sensitivity. In continuous wave regime, the maximal output power of 8 W in the 0.98-μm lasers is found to be limited by the thermal rollover, whereas in the longer wavelength laser it is 10.8 W being limited by the catastrophic optical mirror damage.

Reducing of thermal resistance of edge-emitting lasers based on coupled waveguides

We present a study on minimization of thermal resistance of edge-emitting diode lasers by placing the active region closer to the laser surface. We demonstrate that using coupled large optical cavity (CLOC) approach one can reduce the thickness of the p-cladding down to 0.5 μm due to the strong mode localization in a broad active waveguide (1.35 μm) coupled to a passive optical cavity. Also, the advanced laser design allowed us to put the active region closer to the cladding in comparison with conventional broad waveguide lasers. As a result, a record-low thermal resistance of 2 K/W for 3 mm device directly mounted on a copper heatsink in combination with a low optical internal loss of 0.4 cm−1 where demonstrated.

Omnidirectional Reflector with Total Internal Reflective Interface for Light Extraction Enhancement of Solid‐State Light Source

Light extraction efficiency is key to the performance of solid‐state light sources such as the light‐emitting diode. The reflectance of the bottom mirror has a significant impact on the light extraction efficiency because a large difference in the refractive index between the light source and air leads to multiple internal reflections and optical loss. Therefore, efficiency can be drastically improved with even a slight increase in the bottom mirror reflectance. This paper focuses on wide angular light incidences. An omnidirectional reflector (ODR) is then designed that included a low‐index thick layer (MgF2), a pair of quarter‐wavelength‐thick layers (Ta2O5/MgF2), and a metal layer (Ag) on a solid‐state light source with n = 2.4. The first thick layer works as a perfect reflector for wide angular and broadband incident light since total internal reflection (TIR) occurs at the interface. In the simulations, the angular integrated reflectance for our designed reflector (TI‐ODR) reaches 99.7%. The reflectance is higher than the typical ODR that is composed of a quarter‐wavelength‐thick layer and Ag (98.6%) or an Ag reflector alone (97.9%). The TI‐ODR is then fabricated onto a gallium nitride (GaN)‐based light‐emitting diode and obtained a light extraction efficiency enhancement factor of approximately 1.1.

Two-Valued Characteristics in Semiconductor Quantum Well Lasers

Due to internal optical absorption loss, which varies with carrier density in the optical confinement region, the operating characteristics can be two valued in semiconductor quantum well lasers. Particularly, there can be two lasing thresholds and two branches in the light–current characteristic. While the internal differential quantum efficiency for the first (conventional) branch of the light–current characteristic is less than 1 and decreases with increasing pump current, that for the second (unconventional) branch is greater than 1 and nonmonotonous.

High-power high-brightness 980 nm lasers with >50% wall-plug efficiency based on asymmetric super large optical cavity.

High-power high-brightness super large optical cavity laser diodes with an optimized epitaxial structure are investigated at the wavelength of 980 nm range. The thicknesses of P- and N-waveguides are prudently chosen based on a systematic consideration about mode characteristics and vertical far-field divergences. Broad area laser diodes show a high internal quantum efficiency of 98% and a low internal optical loss of 0.58 cm-1. The ridge-waveguide laser with 7 μm ridge and 3 mm cavity yields 1.9 W single spatial mode output with far-field divergence angles of 6.8° in lateral and 11.5° in vertical at full width at half maximum under 2 A CW operating current. The corresponding M2 values are 1.77 and 1.47 for lateral and vertical, respectively, and the corresponding brightness is 76.8 MW‧cm-2‧sr-1. The far-field divergence angles with 95% power content are in the range of 24.7° to 26.1° across the whole measured range.

Suppressed power saturation due to optimized optical confinement in 9xx nm high-power diode lasers that use extreme double asymmetric vertical designs

Broad area lasers with novel extreme double asymmetric structure (EDAS) vertical designs featuring increased optical confinement in the quantum well, Γ, are shown to have improved temperature stability without compromising series resistance, internal efficiency or losses. Specifically, we present here vertical design considerations for the improved continuous wave (CW) performance of devices operating at 940 nm, based on systematically increasing Γ from 0.26% to 1.1%, and discuss the impact on power saturation mechanisms. The results indicate that key power saturation mechanisms at high temperatures originate in high threshold carrier densities, which arise in the quantum well at low Γ. The characteristic temperatures, T0 and T1, are determined under short pulse conditions and are used to clarify the thermal contribution to power limiting mechanisms. Although increased Γ reduces thermal power saturation, it is accompanied by increased optical absorption losses in the active region, which has a significant impact on the differential external quantum efficiency, . To quantify the impact of internal optical losses contributed by the quantum well, a resonator length-dependent simulation of is performed and compared to the experiment, which also allows the estimation of experimental values for the light absorption cross sections of electrons and holes inside the quantum well. Overall, the analysis enables vertical designs to be developed, for devices with maximized power conversion efficiency at high CW optical power and high temperatures, in a trade-off between absorption in the well and power saturation. The best balance to date is achieved in devices using EDAS designs with , which deliver efficiencies of 50% at 14 W optical output power at an elevated junction temperature of 105 °C.

Ширина линии излучения и alpha-фактор одномодовых вертикально-излучающих лазеров спектрального диапазона 850 нм на основе квантовых ям InGaAs/AlGaAs -=SUP=-*-=/SUP=-

AbstractThe emission-line width for 850-nm single-mode vertical-cavity surface-emitting lasers based on InGaAs/AlGaAs quantum wells is studied. The width of the emission line for a laser with a 2-μm oxide current aperture attains it minimum (~110 MHz) at an output power of 0.8 mW. As the optical output power is further increased, anomalous broadening of the emission line is observed; this is apparently caused by an increase in the α-factor as a result of a decrease in the differential gain in the active region under conditions of increased concentration of charge carriers and of high internal optical losses in the microcavity. The α-factor is estimated using two independent methods.

Emission-Line Width and α-Factor of 850-nm Single-Mode Vertical-Cavity Surface-Emitting Lasers Based on InGaAs/AlGaAs Quantum Wells

The emission-line width for 850-nm single-mode vertical-cavity surface-emitting lasers based on InGaAs/AlGaAs quantum wells is studied. The width of the emission line for a laser with a 2-μm oxide current aperture attains it minimum (~110 MHz) at an output power of 0.8 mW. As the optical output power is further increased, anomalous broadening of the emission line is observed; this is apparently caused by an increase in the α-factor as a result of a decrease in the differential gain in the active region under conditions of increased concentration of charge carriers and of high internal optical losses in the microcavity. The α-factor is estimated using two independent methods.

Highly Reliable Low-Threshold InAs Quantum Dot Lasers on On-Axis (001) Si with 87% Injection Efficiency

Quantum dot lasers epitaxially grown on Si are promising for an efficient light source for silicon photonics. Recently, considerable progress has been made to migrate 1.3 μm quantum dot lasers from off-cut Si to on-axis (001) Si substrates. Here, we report significantly improved performance and reliability of quantum dot lasers enabled by a low threading dislocation density GaAs buffer layer. Continuous-wave threshold currents as low as 6.2 mA and output powers of 185 mW have been achieved at 20 °C. Reliability tests after 1500 h at 35 °C showed an extrapolated mean-time-to-failure of more than a million hours. Direct device transparency and amplified spontaneous emission measurements reveal an internal optical loss as low as 2.42 cm–1 and injection efficiency of 87%. This represents a significant stride toward efficient, scalable, and reliable III–V lasers on on-axis Si substrates for photonic integrate circuits that are fully compatible with CMOS foundries.

Strong doping of the n-optical confinement layer for increasing output power of high- power pulsed laser diodes in the eye safe wavelength range

An analytical model for internal optical losses at high power in a 1.5 μm laser diode with strong n-doping in the n-side of the optical confinement layer is created. The model includes intervalence band absorption by holes supplied by both current flow and two-photon absorption (TPA), as well as the direct TPA effect. The resulting losses are compared with those in an identical structure with a weakly doped waveguide, and shown to be substantially lower, resulting in a significant improvement in the output power and efficiency in the structure with a strongly doped waveguide.