Low Internal Loss Research Articles

Hyperspectral imager for the mid-infrared spectral range using a single-mirror interferometer and a windowing method

In this paper, we present a static imaging Fourier transform spectrometer (sIFTS) for the mid-infrared spectral range. The system employs imaging optics, a single-mirror interferometer, and an uncooled broadband microbolometer array. As the hyperspectral data cube is acquired using a windowing method, a comparatively high light throughput in a spectral range from 2600 cm−1 to 800 cm−1, respectively 3.8 µm to 13 µm is achieved. The spectral resolution is 12 cm−1, and the spatial resolution amounts to 16 lp/mm, corresponding to a resolution of 62.5 µm at a design wavelength of 10.6 µm. As the employed spectrometer, in contrast to scanning Fourier transform infrared (FTIR) spectrometers, contains no moving parts, the measurement rate is mainly limited by the detector read-out and is currently 25 Hz. After an evaluation of the spatial resolution by both simulations and experimental results, the spatially resolved transmission spectra of a sample containing different polymer films are recorded and compared to a laboratory FTIR spectrometer. Thereby, the acquired spectra show good agreement. As the system combines both a spectrometer with low internal light loss and a windowing technique allowing high etendue, the presented hyperspectral imager shows significant potential especially for the mid-infrared.

Characterization of Carbon Felt Materials As Negative Electrode Material for Flow-Assisted Zn-Air Batteries

Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large-scale systems for energy storage. Such batteries use materials that are low cost: Zn is abundant in the earth’s crust and alkaline aqueous solutions are cheap and well understood. They also have low toxicity and high chemical stability. To make Zn-air batteries viable for large scale use, issues at the Zn electrode need to be addressed. Allowance must be made to prevent dendrite growth that eventually leads to cell shorting during charging. During the oxidation of Zn occurring in the discharge process in the battery, a sequence of chemical steps occurs producing a passive layer of ZnO on the electrode. Additionally, electrode morphology changes between each cycle produce inconsistent performance. These issues can be improved by using a three-dimensional support structure on which Zn is deposited. Carbon felt electrodes provide an ideal structure for the Zn anode in Zn-air systems as they are porous and highly conductive. These features allow for high surface area for deposition to occur, low mass transport constraints and low internal IR losses of the electrode. Furthermore, the carbon structure provides a consistent structure to return to minimize Zn morphology changes. These structures have been explored for their potential as the anode in Zn electrodes and showed significant promise. No issues with dendrite growth and passivation have been observed in the electrode during cycling. They can achieve 100mA/cm2 of current density at 20mV of overpotential during deposition and dissolution at a flow rate of 15ml/min of electrolyte through the electrode. Additionally, they have been shown to be stable for 100 cycles of cycling in a symmetric cell at 50mA/cm2 while maintain an average overpotential of less than 50mV during each step. Cycling at high flow rates improves the performance of the felt with an average step overpotential of 16mV at 28 ml/min and 28mV overpotential average at 10ml/min, all also at 4M NaOH saturated with Zincate. Increased state of charge of the electrode appears to increase the efficiency of the electrode. During charging steps in 10ml/min cycling experiments overpotential starts out near 40mV and trends towards 20mV during the step. Further, a 50mV CA step at 30ml/min shows current density increase from 100mA/cm2 at the start of the test to 400mA/cm2 at the end of 30 min. This is the result of the increased amount of Zn in the felt system providing more surface area of Zn for deposition to occur directly on. The potential during galvanostatic cycling experiments appears to be somewhat unstable indicating that the basic electrochemical performance of the electrode needs more study. Nucleation of Zn deposits occurs throughout the felt thickness regardless of the applied overpotential or current. As deposition progresses, the location of Zn growth depends on the current applied. At higher currents or overpotentials growth localizes near the membrane separator. It then begins to fill in back towards the current collector. The reverse occurs during discharge of the electrode, material is removed preferentially from the parts of the electrode near the current collector then proceeds towards the membrane separator. Experiments show that this is based on overpotential in the cell and under both potentiostatic and galvanostatic conditions; at low observed or applied potentials growth occurs throughout the cell felt and at higher overpotentials the growth becomes localized. IR drop measurements through the felt indicate that only 20% of the electrode is active during the growth phase of a 1hour, 40mA/cm2 deposition test. This suggests migration of the negatively charged zincate towards the membrane separator is dominant in these conditions. This ultimately reduces the efficiency of the electrode as it increases the overpotential required to achieved the desired current. Methods to counteract this effect with be discussed. Optimized electrode performance including flow rate, electrolyte conditions, and preconditioning of the electrode for cycling of symmetric and full cell setups will also be discussed further. We gratefully acknowledge the support of Dr. Imre Gyuk and the DOE Office of Electricity Delivery and Reliability. Figure 1

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.

Lasers Based on Quantum Well-Dots Emitting in the 980- and 1080-nm Optical Ranges

The main characteristics of edge-emitting lasers with active regions based on nanoheterostructures of a new type—quantum well-dots (QWDs) operating at various wavelengths—are compared. The QWD structures operating at 980- and 1080-nm wavelengths demonstrated minimum values of threshold current density (160 and 125 A/cm2), high internal quantum efficiency (74 and 85%), and low internal losses (1.1 and 0.9 cm–1), respectively.

Efficiency and Threshold Characteristics of Spectrally Beam Combined High-Power Diode Lasers

The efficiency, threshold characteristics, and the stability of spectral beam combining (SBC) of diode laser array had been studied based on different output couplers (OC). It was found that the lower OC reflectivity, lower internal loss, and shorter cavity-length of emitter were helpful for the realization of high efficiency SBC in an external cavity. However, too low OC reflectivity would also result in an unstable combining with the characteristic of side lobes appeared in far-field and the degraded efficiency. The influence of OC reflectivity on threshold current, lasing spectra, and the beam quality of SBC was investigated and analyzed.

Лазеры на основе квантовых яма-точек, излучающие в оптических диапазонах 980 и 1080 nm

AbstractThe main characteristics of edge-emitting lasers with active regions based on nanoheterostructures of a new type—quantum well-dots (QWDs) operating at various wavelengths—are compared. The QWD structures operating at 980- and 1080-nm wavelengths demonstrated minimum values of threshold current density (160 and 125 A/cm^2), high internal quantum efficiency (74 and 85%), and low internal losses (1.1 and 0.9 cm^–1), respectively.

Инжекционные лазеры InGaAlP/GaAs оранжевого оптического диапазона (~600 нм)

Lasing in orange spectral range(599−605 nm)in(AlxGa1−x)0.5In0.5P−GaAs laser diodes grown by MOVPE on GaAs substrates(211) and (322) was demonstrated. The active region consisted of 4 layers of InxGa1−xP vertically coupled quantum dots. Carrier leakage from the active area was suppressed by using barriers formed with 4 quantum-sized layers of InGaAlP with high Ga content. The maximum optical power in the pulsed regime was limited by catastrophic optical mirror degradation and reached 800 mW. Lower threshold current density, higher differential efficiency, and lower internal losses were demonstrated by lasers processed from epitaxial structures grown on(322)A substrates in comparison to those grown on(211)A. This fact is due to the higher energy barrier for electrons in the first case.

Edge-emitting lasers based on transitionally dimensional InGaAs/GaAs active region

We present a systematic study of basic characteristics of edge-emitting lasers based on a new type of quantum-sized InGaAs active medium grown on GaAs substrates. The active region referred to as quantum-well-dots (QWDs) comprises properties of quantum wells (QWs) and quantum dots (QDs). We have fabricated and investigated lasers with the active regions consisted of 1, 2, 5 and 10 layers of the QWDs. The low internal loss has allowed us to obtain maximal optical power as high as 8.8 W in the continuous wave (CW) regime. We have shown that QWD-based active media are very promising for devices requiring high gain, stacking a large number of layers in the active region, and suppressing of lateral carrier transport.

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.

Reduction of Internal Loss and Thermal Resistance in Diode Lasers with Coupled Waveguides

The results of studying edge-emitting quantum-well lasers with a layered design having coupled planar waveguides, capable of suppressing the generation of excited transverse modes, are presented. In this case, a low internal loss (0.4 cm–1) is provided, combined with a small depth (~0.9 μm) of the active region, which results in a low thermal resistance of 6.0 (K/W) mm without a submount.

A Resonant Magnetic Field Sensor With High Quality Factor Based on Quartz Crystal Resonator and Magnetostrictive Stress Coupling

A resonant magnetic field sensor is developed by using a piezoelectric quartz crystal double-ended tuning fork (DETF), two FeGa thin plates, and four quartz spacers. The double ends of the DETF are bonded to the ends of the magnetostrictive thin plates through four spacers with adhesive, allowing the magnetostrictive force to be converted into the longitudinal force in the DETF which leads to an increase of the resonant frequency of the DETF. This design brings the advantage of field-independent damping, i.e., the quality factor (Q-factor) of the sensor is found to be independent on the dc field, with an average value of about 3318 in the air due to the low internal loss in quartz. After an initially slow increase before the field reaching ~50 Oe, the resonant frequency of the sensor increases linearly with the increase of the applied dc field before reaching 280 Oe. The total frequency shift, sensitivity of the frequency shift per magnetic field, and field resolution in the linear region are 2.12% (~808.3 Hz), 3.5 Hz/Oe and 8 mOe, respectively. The results demonstrate a possibility and promising to realize a highly sensitive magnetic sensor with high-Q-factor quartz crystal as well as direct digital frequency output.

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.

Снижение внутренних потерь и теплового сопротивления в лазерных диодах со связанными волноводами

AbstractThe results of studying edge-emitting quantum-well lasers with a layered design having coupled planar waveguides, capable of suppressing the generation of excited transverse modes, are presented. In this case, a low internal loss (0.4 cm^–1) is provided, combined with a small depth (~0.9 μm) of the active region, which results in a low thermal resistance of 6.0 (K/W) mm without a submount.

1.06 μm high-power InGaAs/GaAsP quantum well lasers

The high power and low internal loss 1.06 μm InGaAs/GaAsP quantum well lasers with asymmetric waveguide structure were designed and fabricated. For a 4000 μm cavity length and 100 μm stripe width device, the maximum output power and conversion efficiency of the device are 7.13 W and 56.4%, respectively. The cavity length dependence of the threshold current density and conversion efficiency have been investigated theoretically and experimentally; the laser diode with 4000 μm cavity length shows better characteristics than that with 3000 and 4500 μm cavity length: the threshold current density is 132.5 A/cm2, the slope efficiency of 1.00 W/A and the junction temperature of 15.62 K were achieved.

High power, high efficiency continuous-wave 808 nm laser diode arrays

The continuous-wave 100W-class 808nm laser diode arrays with extremely high power conversion efficiency of 68% were reported at the heatsink temperature of 25°C. To the best of our knowledge, this was the highest power conversion efficiency at continuous-wave 106W 808nm laser diode array with 50% fill factor so far. An asymmetric broad waveguide epitaxial structure with very low internal optical loss of 0.5cm−1 was presented. In order to improve the efficiency, various fill factor devices were studied. The 50W laser diode array with 30% fill factoir and 1.0mm cavity length demonstrated power conversion efficiency of 71% at heatsink temperature of 15°C.

Static Fourier transform infrared spectrometer.

Fourier transform spectroscopy has established itself as the standard method for spectral analysis of infrared light. Here we present a robust and compact novel static Fourier transform spectrometer design without any moving parts. The design is well suited for measurements in the infrared as it works with extended light sources independent of their size. The design is experimentally evaluated in the mid-infrared wavelength region between 7.2 μm and 16 μm. Due to its large etendue, its low internal light loss, and its static design it enables high speed spectral analysis in the mid-infrared.

FeCo 자성 금속 중공형 섬유 고분자 복합재의 전자파 차폐 특성 연구

무전해 도금을 통해 제조한 FeCo 자성 금속 중공형 섬유와 EPDM 고분자를 이용하여 전자파 차폐 복합재를 제작하였다. 열처리 공정을 통하여 섬유의 종횡비를 제어하였으며 중공형 구조로 섬유를 제조한 후, 이를 EPDM 수지에 첨가하여 복합재를 제조하였다. 자성 금속 중공형 섬유의 종횡비가 클수록 낮은 표면 저항 특성과 우수한 전자파 차폐 성능을 나타내었다. 약 <TEX>$100{\mu}m$</TEX> 길이의 자성 금속 중공형 섬유를 이용한 두께 <TEX>$150{\mu}m$</TEX> 전자파 차폐 복합재의 경우 밀도 <TEX>$1.18g/cm^3$</TEX>, 약 30 ohm/sq의 표면 저항, 그리고 30 dB의 전자파 차폐 성능을 나타내었다. 이는 종횡비가 큰 섬유에 의한 퍼콜레이션 임계치 이상의 전도성 네트워크 형성과 더불어, 낮은 표면 저항에 기인한 임피던스 차이에 의한 반사 손실 증가, 흡수 손실, 그리고 다중 내부 반사 손실에 의하여 우수한 전자파 차폐 성능을 나타내는 것으로 판단된다. Electromagnetic interference shielding composite with low density (<TEX>$1.18g/cm^3$</TEX>) was fabricated using electroless plated FeCo magnetic metal hollow fibers and ethylene propylene diene monomer (EPDM) polymer. Aspect ratio of the fibers were controlled and their hollow structure was obtained by heat treatment process. The FeCo hollow fibers were then mixed with EPDM to manufacture the composite. The higher aspect ratio of the magnetic metal hollow fibers resulted in high electromagnetic interference shielding effectiveness (30 dB) of the composite due to its low sheet resistance (30 ohm/sq). The enhanced electromagnetic interference shielding effectiveness was mainly attributed to the formation of conducting network over the percolation threshold by high aspect ratio of fibers as well as an increase of the reflection loss by impedance mismatch owing to low sheet resistance, absorption loss, and multiple internal reflections loss.

Microchip laser operation of Yb-doped gallium garnets

Continuous-wave microchip laser operation and thermal lensing are studied for Yb-doped gallium garnets, Yb:LuGG, Yb:YGG, Yb:CNGG and Yb:CLNGG under diode-pumping at ~932 and 969 nm. It is shown that although thermal the conductivity of Ga garnets is lower than that of Yb:YAG, the compromised thermo-optic properties, high absorption in the zero-phonon line and low internal loss make the ordered Yb:YGG and Yb:LuGG crystals to be promising for compact highly efficient microchip lasers. In particular, Yb:LuGG microchip laser generated 8.97 W of output power with a slope efficiency η = 75% and 9.31 W with η = 65%, for pumping at 932 and 969 nm, respectively. Multi-watt output in the range 1039–1078 nm is emitted for different transmission of the output coupler. The sensitivity factor of the thermal lens for this crystal is 2.1 m−1/W (pumping at 969 nm with a pump waist radius of 100 μm) and the estimated thermal conductivity is 5.8 ± 0.5 W/mK. Power scaling of Yb:CNGG and Yb:CLNGG microchip lasers is limited by poor thermo-optic properties and high internal losses. Ordered Ga garnets show good prospects for the development of passively Q-switched microchip lasers with high pulse energies.

Reducing Thermal Carrier Spreading in InP Quantum Dot Lasers

Record low values in this material system of threshold current density, particularly at elevated temperature, are presented for InP quantum dot lasers. Lasers with Ga0.58In0.42P in the dot upper confining layer have the lowest threshold current densities, 138 A·cm−2 at 300 K, and 235 A·cm −2 at 350 K (77 °C) (2-mm lasers, uncoated facets). Gain-current density data suggests laser performance with an upper confining layer of ${\rm Ga}_{x}$ ${\rm In}_{1-x}$ P with ${\rm x} = 0.54$ , 0.56 or 0.58 would be similar if not for the very low internal optical mode loss, $\alpha _{{\rm i}}$ of samples with ${\rm x} = 0.56$ and 0.58. Gain measurements at fixed inversion level suggest that increasing x content in ${\rm Ga}_{x}$ ${\rm In}_{{1-x}}$ P increases gain at fixed inversion level but samples with ${\rm x} = 0.54$ also exhibit reduced recombination current density. The increasing recombination current density at elevated temperature due to thermal carrier spreading is significantly reduced in samples with ${\rm x} = 0.56$ and ${\rm x} = 0.58$ but measurements at common operating points attribute this largely to the reduced $\alpha _{{\rm i}}$ for these samples and given the same $\alpha _{{\rm I}}$ , samples with ${\rm x} = 0.54$ , 0.56 and 0.58 would all benefit from reduced effects due to thermal carrier spreading compared to ${\rm x} = 0.52$ .

Novel concepts for designing semiconductor lasers

We review novel concepts and demonstrate recent experimental data for edge- emitting semiconductor lasers with broad vertical waveguide. The ultimate case for waveguide extension in the vertical direction can be implemented by using the Tilted Wave Laser (TWL) approach. A TWL is composed of a thin active waveguide (typically 0.3-2 μm) optically coupled to a thick passive waveguide (10-150 μm). A TWL with a 26 μm-thick passive waveguide demonstrated low internal loss of 1.4 cm−1, maximum pulsed power 18 W and maximum CW power 4.7 W. Vertical far field of the TWL consists of two tilted narrow lobs of 2 degrees full width at half maximum each.