59 results
Search Results
Now showing 1 - 10 of 59
- ItemGeneralization of coupled S-parameter calculation to compute beam impedances in particle accelerators(College Park, MD : American Physical Society, 2020) Flisgen, Thomas; Gjonaj, Erion; Glock, Hans-Walter; Tsakanian, AndranikIn this article, a decomposition approach for the computation of beam coupling impedances is proposed. This approach can account for the mutual electromagnetic coupling in long accelerator structures consisting of several consecutive segments. The method is based on the description of the individual segments using a multimodal network matrix formulation in which the charged particle beam is considered as an additional port. Then, the generalized multimodal network matrices of all segments are combined to a multimodal network matrix of the complete structure. The beam coupling impedance as well as the scattering parameters of the full structure are recovered as particular matrix elements in this multimodal network matrix. The new method generalizes Coupled S-Parameter Calculation (CSC) introduced in earlier work such that charged particle beams are considered. Consequently, the introduced scheme is referred to as CSC. Application examples for realistic accelerator components such as the simulation of a full TESLA 1.3 GHz-cavity of the European XFEL are provided. These simulations demonstrate the high accuracy and numerical performance of the proposed method.
- ItemMode competition in broad-ridge-waveguide lasers(Bristol : IOP Publ., 2020) Koester, J.-P.; Putz, A.; Wenzel, H.; Wünsche, H.-J.; Radziunas, M.; Stephan, H.; Wilkens, M.; Zeghuzi, A.; Knigge, A.The lateral brightness achievable with high-power GaAs-based laser diodes having long and broad waveguides is commonly regarded to be limited by the onset of higher-order lateral modes. For the study of the lateral-mode competition two complementary simulation tools are applied, representing different classes of approximations. The first tool bases on a completely incoherent superposition of mode intensities and disregards longitudinal effects like spatial hole burning, whereas the second tool relies on a simplified carrier transport and current flow. Both tools yield agreeing power-current characteristics that fit the data measured for 5-23 µm wide ridges. Also, a similarly good qualitative conformance of the near and far fields is found. However, the threshold of individual modes, the partition of power between them at a given current, and details of the near and far fields show differences. These differences are the consequence of a high sensitivity of the mode competition to details of the models and of the device structure. Nevertheless, it can be concluded concordantly that the brightness rises with increasing ridge width irrespective of the onset of more and more lateral modes. The lateral brightness W mm-1at 10 MW cm-2 power density on the front facet of the investigated laser with widest ridge (23 µm) is comparable with best values known from much wider broad-area lasers. In addition, we show that one of the simulation tools is able to predict beam steering and coherent beam coupling without introducing any phenomenological coupling coefficient or asymmetries. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemCompact, Watt-class 785 nm dual-wavelength master oscillator power amplifiers(Bristol ; Philadelphia, PA : IOP Publishing Ltd., 2022) Müller, André; Maiwald, Martin; Sumpf, Bernd785 nm micro-integrated, dual-wavelength master oscillator power amplifiers with a footprint of 5 mm × 25 mm are presented. They are based on Y-branch distributed Bragg reflector ridge waveguide diode lasers and anti-reflection coated tapered amplifiers. In order to reduce the impact of potential optical feedback, devices with master oscillator front facet reflectivities of 5% and 30% as well as with an integrated miniaturized optical isolator have been realized. A comparison up to 1 W shows narrowband dual wavelength laser emission with a spectral distance of 0.6 nm (10 cm−1) and individual spectral widths <20 pm. As expected, a higher front facet reflectivity leads to a significant reduction of feedback related mode hops. Longitudinal modes corresponding to the master oscillator resonator length remain within spectral windows <0.15 nm (3 cm−1), suitable for applications such as Raman spectroscopy and especially shifted excitation Raman difference spectroscopy. Integrating a compact 30 dB optical isolator completely eliminates the observed optical feedback effects. Lateral beam propagation ratios of 1.2 (1/e2) enable easy beam shaping and fiber coupling. Outside of the experimental comparison, the developed MOPAs provide up to 2.7 W of optical output power available for applications.
- ItemInfluence of vacuum chamber port terminations on beam coupling impedances(College Park, MD : American Physical Society, 2023) Flisgen, Thomas; Gorgi Zadeh, Shahnam; Gjonaj, ErionVacuum chambers of particle accelerators are typically equipped with radio-frequency couplers. The couplers are employed to excite modes for particle acceleration, to extract the energy of higher-order modes, or for diagnostic purposes. From a network theory perspective, these couplers represent terminal ports by which means the structure can exchange energy with its exterior. Usually, these ports are terminated with fixed impedances corresponding to the characteristic impedances of the coaxial lines attached to them. In this paper, we investigate the influence of the termination conditions of vacuum chambers on beam coupling impedances. For this purpose, we introduce a novel approach that allows us to determine beam coupling impedances for arbitrary port terminations. A full-wave Maxwell solver is employed to determine a generalized scattering matrix of the vacuum chamber and its couplers terminated with prespecified reference impedances. Often, these impedances are chosen to be the characteristic line impedances of the waveguides so that coupler ports are free of reflection. Using the generalized scattering matrix, the beam coupling impedances can be readily determined by means of a computationally inexpensive postprocessing step that takes into account arbitrary impedance loads at the coupler ports. Thus, the influence of various port terminations on the beam coupling impedances can be conveniently examined. This is relevant to improve older structures that were designed when no sophisticated design tools were available or to improve the operation of existing structures for a purpose they were initially not designed for. Using the proposed approach, we investigate the 33-cell 200 MHz traveling-wave accelerating structures of the SPS at CERN. It is shown that port termination conditions do have an important influence on the beam coupling impedance and, therefore, must be taken into account in beam stability considerations.
- ItemHighly linear fundamental up-converter in InP DHBT technology for W-band applications(New York, NY [u.a.] : Wiley, 2020) Hossain, Maruf; Stoppel, Dimitri; Boppel, Sebastian; Heinrich, Wolfgang; Krozer, ViktorA fundamental up-converter with high linearity is presented, realized as full Gilbert cell (GC) mixer using a 800 nm transferred substrate (TS) InP-DHBT technology. The LO input of the Gilbert cell conducts from 75 to 100 GHz and requires 5 dBm of input power. The GC attains a single sideband (SSB) conversion gain of 10 ± 1 dB within the frequency from 82 to 95 GHz with a saturated output power of -1 dBm at 86 GHz and >5 dB conversion gain between 75 and 100 GHz. The up-converter exhibits 25 GHz of IF bandwidth. The DC power consumption is only 51 mW. © 2020 The Authors. Microwave and Optical Technology Letters published by Wiley Periodicals, Inc.
- ItemStructural and optical properties of (112̅2) InGaN quantum wells compared to (0001) and (112̅0)(Milton Park : Taylor & Francis, 2016) Pristovsek, Markus; Han, Yisong; Zhu, Tongtong; Oehler, Fabrice; Tang, Fengzai; Oliver, Rachel A.; Humphreys, Colin J.; Tytko, Darius; Choi, Pyuck-Pa; Raabe, Dierk; Brunner, Frank; Weyers, MarkusWe benchmarked growth, microstructure and photo luminescence (PL) of (112-2) InGaN quantum wells (QWs) against (0001) and (112-0). In incorporation, growth rate and the critical thickness of (112-2) QWs are slightly lower than (0001) QWs, while the In incorporation on (112-0) is reduced by a factor of three. A small step-bunching causes slight fluctuations of the emission wavelength. Transmission electron microscopy as well as atom probe tomography (APT) found very flat interfaces with little In segregation even for 20% In content. APT frequency distribution analysis revealed some deviation from a random InGaN alloy, but not as severe as for (112-0). The slight deviation of (112-2) QWs from an ideal random alloy did not broaden the 300 K PL, the line widths were similar for (112-2) and (0001) while (112-0) QWs were broader. Despite the high structural quality and narrow PL, the integrated PL signal at 300 K was about 4 lower on (112-2) and more than 10 lower on (112-0).
- ItemHigh-mobility 4 μm MOVPE-grown (100) β-Ga2O3 film by parasitic particles suppression(Bristol : IOP Publ., 2023) Chou, Ta-Shun; Seyidov, Palvan; Bin Anooz, Saud; Grüneberg, Raimund; Rehm, Jana; Tran, Thi Thuy Vi; Fiedler, Andreas; Tetzner, Kornelius; Galazka, Zbigniew; Albrecht, Martin; Popp, AndreasIn this work, we comprehensively investigate the development of unwanted parasitic particles in the MOVPE chamber while growing μm level films. The density of the parasitic particles is found to be pronounced at film thicknesses starting from >1.5 to 2 μm. These particles seem to induce structural defects such as twin lamellae, thereby harming the electrical properties of the grown film. The origin of the parasitic particle is attributed to the parasitic reactions within the chamber triggered by the promoted gas-phase reactions during the growth process, which can be largely reduced by increasing the total gas flow and decreasing the showerhead distance to the susceptor. A film thickness of up to 4 μm has been achieved after minimizing the density of parasitic particles. Thereby, RT Hall measurements reveal carrier mobilities of 160 cm2V−1s−1 at carrier concentrations of 5.7 × 1016cm−3
- ItemHybrid integrated mode-locked laser using a GaAs-based 1064 nm gain chip and a SiN external cavity(Washington, DC : Soc., 2022) Vissers, Ewoud; Poelman, Stijn; Wenzel, Hans; Christopher, Heike; Van Gasse, Kasper; Knigge, Andrea; Kuyken, BartExternal cavity mode-locked lasers could be used as comb sources for high volume application such as LIDAR and dual comb spectroscopy. Currently demonstrated chip scale integrated mode-locked lasers all operate in the C-band. In this paper, a hybrid-integrated external cavity mode-locked laser working at 1064 nm is demonstrated, a wavelength beneficial for optical coherence tomography or Raman spectroscopy applications. Additionally, optical injection locking is demonstrated, showing an improvement in the optical linewidth, and an increased stability of the comb spectrum.
- ItemTemperature-Dependent Charge Carrier Diffusion in [0001¯] Direction of GaN Determined by Luminescence Evaluation of Buried InGaN Quantum Wells(Weinheim : Wiley-VCH, 2020) Netzel, Carsten; Hoffmann, Veit; Tomm, Jens W.; Mahler, Felix; Einfeldt, Sven; Weyers, MarkusTemperature-dependent transport of photoexcited charge carriers through a nominally undoped, c-plane GaN layer toward buried InGaN quantum wells is investigated by continuous-wave and time-resolved photoluminescence spectroscopy. The excitation of the buried InGaN quantum wells is dominated by charge carrier diffusion through the GaN layer; photon recycling contributes only slightly. With temperature decreasing from 310 to 10 K, the diffusion length in [0001⎯⎯] direction increases from 250 to 600 nm in the GaN layer. The diffusion length at 300 K also increases from 100 to 300 nm when increasing the excitation power density from 20 to 500 W cm−2. The diffusion constant decreases from the low-temperature value of ∼7 to 1.5 cm2 s−1 at 310 K. The temperature dependence of the diffusion constant indicates that the diffusivity at room temperature is limited by optical phonon scattering. Consequently, higher diffusion constants in GaN-based devices require a reduced operation temperature. To increase diffusion lengths at a fixed temperature, the effective recombination time has to be prolonged by reducing the number of nonradiative recombination centers.
- ItemWavelength stabilized high pulse power 48 emitter laser bars for automotive light detection and ranging application(Bristol : IOP Publ., 2020) Klehr, Andreas; Liero, Armin; Christopher, Heike; Wenzel, Hans; Maaßdorf, Andre; Della Casa, Pietro; Fricke, Jörg; Ginolas, Arnim; Knigge, AndreaDiode lasers generating optical pulses with high peak power and lengths in the nanosecond range are key components for light detection and ranging systems, e.g. for autonomous driving and object detection. We present here an internally wavelength stabilized distributed Bragg reflector broad area laser bar with 48 emitters. The vertical structure based on AlGaAs (confinement and cladding layers) and InGaAs (active quantum well) is specifically optimized for wavelength-stabilized pulsed operation, applying a surface Bragg grating with high reflectivity. The bar is electrically driven by a new in-house developed high-speed driver based on GaN transistors providing current pulses with amplitudes of up to 1000 A and a repetition frequency of 10 kHz. The generated 4 ns to 10 ns long optical pulses are nearly rectangular shaped and reach a pulse peak power in excess of 600 Watts at 25 °C. The optical spectrum with a centre wavelength of about 900 nm has a width of 0.15 nm (FWHM) with a side mode suppression ratio > 30 dB. © 2020 IOP Publishing Ltd.