Diode Stack Research Articles

Simple highly efficient pumping configuration in high-power thin-disk laser

A simple high-power thin-disk pumping configuration using a radiation combination of four commercially available laser–diode stacks is introduced. Two setup modifications are presented to compensate the nonsuitable shape of the pumping spot arising from low beam quality in our combination method. The effects of setup modifications on pumping spot shapes are confirmed by ray tracing simulations using Trace-pro™ software. All setups are arranged in the laboratory, and the experimental measurements show pumping spots improvements on the disk due to modifications in agreement with simulation results. Output power measurements show that by adapting the pumping spot size to the disk cooling capacity the modified setups can deliver higher output powers and efficiencies. Furthermore, the modifications reduce the laser threshold and improve output laser beam quality. Hence, the modifications make the simple four laser–diodes beam combination applicable for thin-disk laser pumping.

Orthogonal Solution-Processable Electron Transport Layers Based on Phenylpyridine Side-Chain Polystyrenes.

This article reports the synthesis and characterization of a series of polystyrenes containing phenylpyridine moieties as side chains. Methanol solubility of these polymers is induced if the relative pyridine content of the overall aromatic units of the side chains is larger than 0.5. This allows for orthogonal processing of multilayered organic light emitting diode (OLED) stacks fabricated from solutions. The polymers show high thermal stability due to their glass-transition temperatures ranging from 136 up to 247 °C. High triplet energies of up to 2.8 eV are obtained by combination of the side-chain aromatic rings in the meta position. The use of the methanol soluble side-chain polymers as an electron transport layer (ETL) is demonstrated in an orthogonally processed three-layer green-emitting OLED stack. When depositing the ETL from methanol, redissolution of the underlying emission layer does not occur.

Temperature insensitive high energy Q-switched Nd:YAG slab laser

A temperature insensitive high energy Nd:YAG Q-switched laser operating over a temperature variation range of 35 °C is presented. Multi-wavelength (λ) laser diode (LD) stacks and a hexagon Nd:YAG slab with an effective absorption length of 46 mm are adopted to diminish the output energy fluctuation due to the temperature changing. A couple of crossed Porro prisms are used to form a folded laser cavity with polarization coupled output, and actively Q-switched operation is realized by using a KD*P Pockels cell. Output pulse energy of about 128 mJ is obtained over the temperature range of 10–45 °C, corresponding to an optical-to-optical efficiency of 14% and a slope efficiency of 21%. The pulse duration and beam divergence are around 17 ns and 2.8 mrad respectively.

Slot Die Coated and Flexo Printed Highly Efficient SMOLEDs

A basic solution‐processable organic light‐emitting diode (OLED) stack comprising two organic layers is presented in this study. The hole injection layer (poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate), PEDOT:PSS) is slot die coated and flexographically printed for process comparison in terms of film homogeneity and the device performance. The emissive layer consisting of a small molecule host–guest system with a green triplet emitter is slot die coated. The single layer thicknesses are mapped by an in‐house developed imaging color reflectometry enabling the precise measurement of the layer thicknesses over the whole coating area (>50 cm2). Luminance, current density, and light and current efficiency are defined for two different pixel sizes with the active area of 6 × 4 and 14 × 14 mm2. A total of 100 OLEDs are measured showing a very low standard deviation for the different pixels. The solely slot die coated OLEDs show much higher current and power efficiency than the OLEDs with flexographically printed PEDOT:PSS. Small molecule OLEDs reach efficiencies up to 30 cd A−1 and 8 lm W−1. Additionally OLED demonstrators with an active area of ≈27 cm2 are fabricated.

Beam shaping for kilowatt fiber-coupled diode lasers by using one-step beam cutting-rotating of prisms.

The beam quality mismatch of laser diode stacks in both axes limits many direct applications for fiber or solid laser pumping and material processing. In this paper, a one-step cutting-rotating beam shaping system has been designed to homogenize the beam quality of two polarization-multiplexing laser diode stacks. Coupling laser diode stacks consisting of eight bars into a standard fiber with a core diameter of 600 μm and an NA of 0.22 is achieved. The simulative result shows that the system will have an output power over 1056 W. By using the technique, the production of compact and high brightness fiber-coupling diode lasers can be directly used for laser cladding and laser surface hardening processes.

Reconfigurable ultra-thin film GDNMOS device for ESD protection in 28 nm FD-SOI technology

We propose a novel ESD protection device (GDNMOS: Gated Diode merged NMOS) fabricated with 28nm UTBB FD-SOI high-k metal gate technology. By modifying the combination of the diode and transistor gate stacks, the robustness of the device is optimized, achieving a maximum breakdown voltage (VBR) of 4.9V. In addition, modifications of the gate length modulate the trigger voltage (Vt1) with a minimum value of 3.5V. Variable electrostatic doping (gate-induced) in diode and transistor body enables reconfigurable operation. A lower doping of the base enhances the bipolar gain, leading to thyristor behavior. This innovative architecture demonstrates excellent capability for high-voltage protection while maintaining a latch-up free behavior.

Focus on spectroscopy and photonics

Focus on spectroscopy and photonics

Quench Protection of Very Large, 50-GJ-Class, and High-Temperature-Superconductor-Based Detector Magnets

An investigation is performed on the quench behavior of a conceptual 50-GJ 8-T high-temperature-superconductor-based solenoid. In this design, a 50-kA conductor-on-round-core cable-in-conduit conductor utilizing ReBCO technology is envisioned, operating at 40 K. Various properties such as resistivity, thermal conductivity, and heat capacity are very different at this temperature, which affects the quench behavior. It is found that the envisioned conductor is very stable with a minimum quench energy of about 2 kJ. However, the quench propagation velocity is typically about 20 mm/s, so that creating a wide-spread normal zone throughout the coil is very challenging. Moreover, an extraction voltage exceeding 20 kV would be required to ensure a hot-spot temperature below 100 K once a thermal runaway occurs. A novel concept dubbed “rapid quench transformation” is proposed whereby the superconducting conductor is co-wound with a normal conductor to achieve a high degree of inductive coupling. This geometry allows for a significant electric noise reduction, thus enabling low-threshold quench detection. The secondary circuit is connected in series with a stack of diodes, not allowing current transfer during regular operation, but very fast current transfer once a quench is detected. With this approach, the hot-spot temperature can be kept within 20 K of the cold mass temperature at all times, the hot-spot temperature is well below 100 K, and just under 80% of the stored energy can be extracted during a quench.

Flexible Lamination-Fabricated Ultra-High Frequency Diodes Based on Self-Supporting Semiconducting Composite Film of Silicon Micro-Particles and Nano-Fibrillated Cellulose.

Low cost and flexible devices such as wearable electronics, e-labels and distributed sensors will make the future “internet of things” viable. To power and communicate with such systems, high frequency rectifiers are crucial components. We present a simple method to manufacture flexible diodes, operating at GHz frequencies, based on self-adhesive composite films of silicon micro-particles (Si-μPs) and glycerol dispersed in nanofibrillated cellulose (NFC). NFC, Si-μPs and glycerol are mixed in a water suspension, forming a self-supporting nanocellulose-silicon composite film after drying. This film is cut and laminated between a flexible pre-patterned Al bottom electrode and a conductive Ni-coated carbon tape top contact. A Schottky junction is established between the Al electrode and the Si-μPs. The resulting flexible diodes show current levels on the order of mA for an area of 2 mm2, a current rectification ratio up to 4 × 103 between 1 and 2 V bias and a cut-off frequency of 1.8 GHz. Energy harvesting experiments have been demonstrated using resistors as the load at 900 MHz and 1.8 GHz. The diode stack can be delaminated away from the Al electrode and then later on be transferred and reconfigured to another substrate. This provides us with reconfigurable GHz-operating diode circuits.

High-Voltage Ultra-Fast Pulse Diode Stack Based on 4H-SiC

The paper reports on the results of the studies of static and dynamic characteristics of 4H-SiC drift step recovery diodes (DSRDs) assembled in diode stacks. Switching performance of single dies has been simulated and experimentally confirmed. It was established that the switching process is determined primarily by the incomplete ionization of acceptors in 4H-SiC and by the bandgap narrowing in heavily doped emitters. Based on the simulation results the optimized die size has been selected. For DSRD stacks of 4 and 8 dies I-V and C-V measurements are reported. The stacks were dynamically tested in a special oscillator circuit. Repetitive voltage pulses of 10.5 kV with the leading edge length of 900 ps were demonstrated.

High Voltage Diffusion-Welded Stacks on the Basis of SiC Schottky Diodes

In the present work we have considered the prototype of the high-voltage diode stack made on the basis of commercial SiC Schottky diodes. Implementation of vertical integration for four diode chips yielded stack with the reverse current of 25 μA under reverse voltage of 6 kV. The capacitance of the stack at zero bias is reduced more than three times in comparison with initial diodes. Reverse recovery time of the stack was 8.0 ns. This paper proposes a convenient analytical approach to the estimation of parameters of modular compositions with vertical architecture.

Laser diode stacks: pulsed light power for nuclear fusion

Laser drivers are an enabling factor to inertial confinement fusion, because laser diodes must be used instead of flash lamps. We discuss the limitations of laser diode arrays and show what steps the industry is taking. The pump power requirements of large-scale projects such as LIFE or HiPER are within reach of semiconductor laser diode assemblies. Pulsed light output powers per laser bars have been around 300 W per bar, as in the Jenoptik 940 nm bars previously used for pumping the Yb:YAG slabs in the DiPOLE project. By redesigning the semiconductor laser structures 500 W per bar is now commercially available for 808, 880 and 940 nm pump wavelengths. The construction of one inertial fusion power plant will require an amount of semiconductor laser chips in excess of the current annual production by two orders of magnitude. This adds to the engineering task of improving the device characteristics a challenge to production capacity. While the industry benefits from the recent boost in solid-state lighting that acts as a technology driver, cooperation between manufacturers will be imperative, and to this end we propose standardization efforts.

高功率密度激光二极管叠层散热结构的热分析

High-power narrow spacing array stack is an effective solution to improve the power density of diode laser pumping sources,and its heat-sink structural design for package plays a vital role in cooling capacity management. Several important factors affecting thermal effect,such as stack spacing,ceramic thickness,the distance between ceramic and heat-sink thermostatic surface etc.,were analyzed in this paper. The maximum temperature rises of the devices under different structure parameters were simulated using ANSYS finite element analysis method to get the variation laws and optimize these three parameters. According to the optimization results,an efficient cooling heat-sink structure suitable for high power density laser diode stack pumping source was designed. It can significantly improve the cooling effect and reduce consumption of cooling water pump-power.

Diode-side-pumped, passively Q-switched Yb:LuAG laser

A high-gain, diode-side-pumped Yb:LuAG slab laser was designed and investigated for use at room temperature. Pumping occurred from a fast-axis collimated 2D laser diode stack emitting at a wavelength of 970 nm, with 0.8 J over a duration of 0.8 ms. The pump scheme, which enabled efficient mode matching and high gain, was analysed and experimentally verified for different dopant levels. An energy of 100 mJ with 23% slope efficiency in a near fundamental mode was achieved in the free-running regime. A peak power of 2.5 MW and a pulse energy of 10.1 mJ were demonstrated in passive Q-switching by means of a Cr:YAG saturable absorber with 39% initial transmission. The study defined the indications for optimizing such a system.

ANALYSIS OF HEAT PROPAGATION PROCESSES IN STRUCTERS OF PULSE POWERFUL DEVICES NEAR PLANES OF SOLDER OF SILICON PLATES IN HIGH VOLTAGE STACKS

Processes of heat transfer through interfaces of semiconductor diode stack structures of high voltage pulse switchers are investigated by laser thermal wave methods. Theoretical model of thermal wave propagation in such structures is developed. It takes into account specific features of preparation of semiconductor element surfaces, layers of solder or bonding. It is shown that laser thermal wave methods can be applied to diagnostics of thermal contact quality between elements of opening switchers for various technologies of their assembling.

高温硬焊料准连续半导体激光器巴条叠阵的性能研究

Based on the stack array of structure and assembly methods,a set of high temperature and hard solder quasi-continuous laser diode stack arrays were developed,and the related photoelectric performance and lifetime characteristics were studied. When the stack array is in condition of operating current 200 A,repetition frequency 250 Hz,pulse width 200 μs,the FWHM is less than 3 nm,electro-optical conversion efficiency is more than 50%,peak power per bar is more than200 W,lifetime is up to 4. 71 × 109 shots,and the peak power degradation is less than 15%. When the operating current is 150 A,the lifetime is up to 1. 5 × 1010 shots.

Laser diode double-end-direct-pumped slab laser with hybrid resonator

To reduce the influence of thermal lensing and attain the high power laser with high beam quality, a laser diode stack dual-end-direct-pumped Nd:YVO4 slab laser with negative branch confocal folded hybrid cavity has been developed. Its maximum output power is 416 W, optical conversion efficiency is 54.3%, and slope efficiency is 61.6%. When the output laser power is 370 W, M2 values in the unstable resonator direction and stable resonator direction are 3.9 and 4.7, respectively.

Quasicontinuous wave linearly polarized rubidium vapor laser pumped by a 5-bar laser diode stack

We report a quasicontinuous wave (CW) linearly polarized rubidium vapor laser. The pumping source consists of five laser diode bars and its linewidth is reduced from the raw 1.8 to 0.2 nm by a bulk volume Bragg grating. Instead of adopting the “quasi-waveguide structure” gain cell, the pumping light of the rubidium vapor laser propagates freely in the vapor cell. The pumping light with polarization perpendicular to one of the rubidium laser is coupled into the resonator cavity by the polarized beam splitter. This laser configuration is suitable for a convection-cooling diode-pumped alkali vapor laser.

Handheld probe integrating laser diode and ultrasound transducer array for ultrasound/photoacoustic dual modality imaging

Ultrasound and photoacoustics can be utilized as complementary imaging techniques to improve clinical diagnoses. Photoacoustics provides optical contrast and functional information while ultrasound provides structural and anatomical information. As of yet, photoacoustic imaging uses large and expensive systems, which limits their clinical application and makes the combination costly and impracticable. In this work we present and evaluate a compact and ergonomically designed handheld probe, connected to a portable ultrasound system for inexpensive, real-time dual-modality ultrasound/photoacoustic imaging. The probe integrates an ultrasound transducer array and a highly efficient diode stack laser emitting 130 ns pulses at 805 nm wavelength and a pulse energy of 0.56 mJ, with a high pulse repetition frequency of up to 10 kHz. The diodes are driven by a customized laser driver, which can be triggered externally with a high temporal stability necessary to synchronize the ultrasound detection and laser pulsing. The emitted beam is collimated with cylindrical micro-lenses and shaped using a diffractive optical element, delivering a homogenized rectangular light intensity distribution. The system performance was tested in vitro and in vivo by imaging a human finger joint.

Beam reshape of high power laser diode stack

Based on the ray tracing method the laser intensity distribution of single DL bar collimated with micro spherical cylindrical lens was analyzed in detail. We found that the intensity distribution of output laser have a better flat shape when the divergence angle is about 5. A beam reshape system was designed for a 25-bar DL stack. The system includes microlens of fast axis collimating, coupling lens of fast axis and coupling lens of slow axis. The laser medium to be pumped was designed at the focus plane of fast axis coupling lens and close to the imaging plane of slow axis coupling lens. A 7 mm8 mm pump beam was obtained with intensity uniformity of 90%. The coupling efficiency of the beam reshaping system reached 85%.