G. Kaufel

Fraunhofer Institute for Applied Solid State Physics IAF, Freiburg, Baden-Württemberg, Germany

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Publications (81)71.6 Total impact

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    ABSTRACT: Using our 0.2 and 0.3 μm AlGaAs/GaAs/AlGaAs quantum well HEMT technology, we have developed a chip set for 20–40 Gbit/s fiber-optical digital transmission systems. In this paper we describe nine analog and digital receiver ICs: a 22 GHz high-gain transimpedance amplifier, a 20 Gbit/s OEIC front-end optical receiver, a 25 Gbit/s automatic-gain-control amplifier, a limiting amplifier with a differential gain of 26 dB and a bandwidth of 27.7 GHz, a 20–40 Gbit/s clock recovery, a 20 Gbit/s low-power Master-Slave-D-Flipflop with 24 mW power dissipation, a parallel data decision and a 1:4 demultiplexer, both for bit rates of 40 Gbit/s, and a 30 GHz static frequency divider, respectively. All chips were characterized on wafers with 50 Ω coplanar test probes.
    International Journal of High Speed Electronics and Systems 11/2011; 09(02).
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    ABSTRACT: We report on the short-pulse high-power operation of GaSb-based diode lasers emitting in the 1.95- to 2.2-mum wavelength range. Both epi-side-down mounted broad-area lasers and epi-side-up mounted ridge waveguide lasers, sharing the same active region and epitaxial layer structure, have been studied. They show drastically different output power characteristics and thermal limitations in continuous wave (CW) or quasi-CW operation (long current pulses > 1 mus) due to the vastly different laser geometries and mounting configurations. However, in short-pulse operation (pulse length les 50 ns), the maximum output power density was found to be almost independent of the device geometry and the mounting technique, even though still limited by thermal rollover due to self-heating of the device active region. With the heatsink temperature set to -30degC, a maximum single-emitter output power of 22.5 W was recorded at an injection current density of 110 kA/cm<sup>2</sup> (50-ns pulse length), and a maximum power density at the output facet of 26.7 MW/cm<sup>2</sup> (30-ns pulse length) was achieved, without encountering catastrophic optical mirror damage.
    IEEE Photonics Technology Letters 01/2010; · 2.04 Impact Factor
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    ABSTRACT: In this paper, we report on tapered amplifiers based on the GaInAs/AlGaAs-on-GaAs material system, where the active region consists of a 7 nm thick compressively strained InGaAs single quantum well with an indium content of 31%. The devices comprise an index-guided ridge waveguide section acting as a master oscillator which feeds a gain-guided tapered section. A novel split contact design enables us to separately adjust the currents in the ridge- and the taper section. Therefore, a modulation of the output power can be realized by varying the comparatively small ridge current only.
    Lasers and Electro-Optics 2009 and the European Quantum Electronics Conference. CLEO Europe - EQEC 2009. European Conference on; 07/2009
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    ABSTRACT: The realization of a compact green-emitting solid state laser source for applications like laser TV or head-up displays is still a challenging task. One way to generate green light with a solid state laser source is nonlinear frequency upconversion (frequency-doubling) of e.g. 1064 nm to 532 nm. In order to achieve good conversion efficiencies tunable laser sources with output powers of several watts, narrow bandwidth and good beam quality are required. We have realized tapered laser diodes based on the GaInAs/AlGaAs material system emitting at a central wavelength of 1064 nm. These devices have an AR-coating on the front facet as well as on the ridge facet. Therefore, these laser diodes can be frequency stabilized in an external cavity setup consisting either of a grating in Littrow mounting placed on the rear side or by an integrated Fiber Bragg grating. The latter configuration allows a compact low footprint integration of the laser diodes into compact laser modules. The optical output power of these devices frequency stabilized at 1064 nm exceeds 4 W with beam qualities suitable for frequency doubling (M2 < 2) and a tuning range from 1030 nm to 1070 nm. For laser diodes with a HR coating on the ridge facet even higher output powers of more than 8 W are achieved. The ridge and tapered section of the tapered diode amplifiers are contacted separately in order to enable the modulation of the light source by the variation of the ridge current. The rapid temporal modulation achieved this way is a prerequisite for the use of such lasers in flying spot display applications.
    Proc SPIE 02/2009;
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    ABSTRACT: A widely tunable (Deltalambda/lambda = 7%) micro-mechanical external cavity GaSb-based diode laser (muECL) emitting around 2.1 mum is presented. A micro-machined grating with a rectangular grating profile, which can be tilted electrostatically, is employed as wavelength selective element within the external cavity using a Littrow configuration. An optimized grating profile leads to a high diffraction efficiency in the -1st diffraction order and therefore to a broad tuning range of 152 nm. The maximum output power of the fiber coupled muECL system varied only moderately between 22 and 10 mW across the tuning range.
    IEEE Journal of Quantum Electronics 12/2008; · 2.11 Impact Factor
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    ABSTRACT: Tapered diode lasers combine high output power and a beam quality near to the diffraction limit resulting in very high brightness. Therefore, they are finding use in a wide range of applications today, such as pumping of rare-earth-doped fibre amplifiers, tunable frequency doubling of diode lasers for blue-green outputs, and non linear spectroscopy. Due to increasing brightness and lifetime tapered lasers even become attractive for material processing and for telecom applications like pumping of Er-doped fiber amplifiers or raman amplifiers. In order to further enhance the brightness of tapered diode lasers the output power has to be increased while simultaneously the beam quality has to be kept near the diffraction limit. For this purpose we have grown low modal gain, single quantum well InGaAs/AlGaAs devices emitting at 976 nm by molecular beam epitaxy. The lateral design of the investigated laser diodes consists of a tapered section and a ridge-waveguide section. Since it has been shown by previous simulations and experiments that longer tapered sections allow higher output power with unchanged beam quality, we use tapered section lengths of 2000 µm, 3000 µm and 4000 µm. The beam quality parameter M 2 and output powers as well as the nearfields of the different structures were carefully investigated. For longer devices we reach an optical output power of more than 10 W per single emitter in continuous wave mode (cw) without any distinct thermal rollover.
    Proc SPIE 03/2008;
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    ABSTRACT: We report on GaSb-based quantum-well diode lasers in a micro-machined external cavity setup using the Littrow configuration. An electrostatically-actuated silicon- grating, optimized to achieve a wide tuning range in the 2.3 mum wavelength range, is used as tuning element. A tuning range of 82 nm could be realized. The maximum output power of the micro external cavity laser (muECL) was above 15 mW over the entire tuning range.
    Micro Electro Mechanical Systems, 2007. MEMS. IEEE 20th International Conference on; 02/2007
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    ABSTRACT: Experimental results on direct and indirect optically controlled monolithically integrated oscillators will be presented. The oscillators had a free-running frequency around 7 GHz. The experimental results show that the indirect optically controlled circuits seem to be more effective in controlling the phase of the output signal. © 1993 John Wiley & sons, Inc.
    Microwave and Optical Technology Letters 01/2007; 6(1):36 - 38. · 0.59 Impact Factor
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    ABSTRACT: We report on a widely tunable external cavity GaSb-based diode laser (ECL) in Littrow-configuration. The low (44deg full-width at half-maximum) fast axis beam divergence of the quantum-well diode laser employed allowed an efficient coupling to the external cavity, which resulted in a wide tuning range of 177 nm around the central emission wavelength of 2.30 mum. The maximum output power of the fiber coupled ECL system varied only moderately between 16.5 mW at 2.23 mum and 9 mW at the long-wavelength limit at 2.39 mum
    IEEE Photonics Technology Letters 10/2006; · 2.04 Impact Factor
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    ABSTRACT: We report on GaSb-based quantum-well diode lasers in a micro-machined external cavity setup using the Littrow-configuration. An electrostatically actuated Si-grating, optimized to achieve a wide tuning range in the 2.3mum wavelength range, is used as a tuning element
    Optical MEMS and Their Applications Conference, 2006. IEEE/LEOS International Conference on; 09/2006
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    ABSTRACT: We have characterized 2.0 μm (aluminium-gallium-indium)(arsenide-antimonide) quantum-well diode lasers in pulsed operation (20–60 ns). A peak power of 1.25 W could be achieved. The near-field distribution on the output facet and the spectral output have been analyzed. Single transverse mode operation can only be maintained at low pulse currents. Above a certain current limit higher order modes occur and fluctuations between these modes have been resolved on a 10 ns time scale. The threshold for thermal and optical damage was investigated for ridge waveguide widths of 6, 8, and 16 μm. No systematic damage threshold could be determined up to current densities as high as 200 kA/cm2.
    Journal of Applied Physics 03/2006; 99(5):053105-053105-3. · 2.21 Impact Factor
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    ABSTRACT: GaSb based diode laser arrays emitting at wavelengths around 2 mum have a significant potential for a variety of applications including material processing, such as welding of transparent plastic materials, and optical pumping of mid-infrared solid state lasers. Even though high output power broad area single emitters and laser arrays have already been demonstrated, they all suffer from a large fast axis beam divergence of typically 67° FWHM due to the broadened waveguide design employed. Here we will present results on (AlGaIn) (AsSb) quantum-well diode laser single emitters and linear arrays consisting of 19 emitters on a 1 cm long bar emitting at around 1.9 mum. To improve on the poor fast axis beam divergence we abandoned the broadened waveguide concept and changed over to a novel waveguide design which features a rather narrow waveguide core. This results in a remarkable reduction in fast axis beam divergence to 44° FWHM for the new waveguide design. For single emitters a cw output power of more than 1.9 W have been observed. 16.9 W in continuous-wave mode at a heat sink temperature of 20 °C have been achieved for arrays. The maximum wall-plug efficiency amounts to 26% both for the single emitters and the laser arrays. These efficiencies are among the highest values reported so far for GaSb based diode lasers, and allow us to use passively cooled and thus less expensive heat sinking technologies.
    Proc SPIE 03/2006;
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    ABSTRACT: Today tapered diode lasers are mainly used in external resonator configuration for non-linear spectroscopy or frequency doubling for blue-green outputs. Now increased output power and brightness make tapered lasers even attractive for pumping of fibre amplifiers or lasers and fibre coupled modules. We have realized high-power ridge-waveguide tapered diode lasers emitting at 976 nm. The high material quality of the MBE-grown laser structures yields a high internal efficiency of more than 97% and low internal losses of 0.5 cm-1. Tapered single emitters consist of a ridge section with a length of 500 mum and a taper section with a length of 3000 mum. The taper angle was 6°. A threshold current of 1.07 A corresponding to a threshold current density of 222 A/cm2 has been obtained. The maximum slope efficiency of 1.09 W/A together with the low series resistance of 35 m Omega results in a high wall-plug efficiency of 58% at 5.5 W output power. This high wall-plug efficiency remains nearly constant up to operation currents of 9 A corresponding to output powers of more than 8 W. At an operation current of 15 A an output power of 12.5 W has been achieved, which is to our knowledge the highest output power in continuous wave mode for tapered diode lasers reported so far. At 8 W a nearly diffraction limited behavior with values for M2 of less than 1.5 have been observed resulting in a brightness of more than 660 MW/cm2.
    Proc SPIE 03/2006;
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    ABSTRACT: We report on GaSb-based 2.X μm diode lasers with an improved waveguide design, leading to a reduced beam divergence in the fast axis of 44° full width at half maximum (FWHM), compared to 67° FWHM of a conventional broadened waveguide design. 2.3 μm ridge-waveguide lasers with the improved epitaxial design showed, besides the narrow beam profile in the fast axis, an excellent slow axis beam quality [M2<1.1 up to 70 mW, continuous wave (cw)]. 2.0 μm broad-area lasers with the improved waveguide too, exhibit a maximum cw-output power of 1.96 W.
    Applied Physics Letters 02/2006; 88(8):081115-081115-3. · 3.52 Impact Factor
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    ABSTRACT: GaSb based diode laser arrays emitting at wavelengths around 2 μm have a significant potential for a variety of applications including material processing, such as welding of transparent plastic materials, and optical pumping of mid-infrared solid state lasers. Even though high output power broad area single emitters and laser arrays have already been demonstrated, they all suffer from a large fast axis beam divergence of typically 67° FWHM due to the broadened waveguide design employed. Here we will present results on (AlGaIn) (AsSb) quantum-well diode laser single emitters and linear arrays consisting of 19 emitters on a 1 cm long bar emitting at around 1.9 μm. To improve on the poor fast axis beam divergence we abandoned the broadened waveguide concept and changed over to a novel waveguide design which features a rather narrow waveguide core. This results in a remarkable reduction in fast axis beam divergence to 44° FWHM for the new waveguide design. For single emitters a cw output power of more than 1.9 W have been observed. 16.9 W in continuous-wave mode at a heat sink temperature of 20 °C have been achieved for arrays. The maximum wall-plug efficiency amounts to 26% both for the single emitters and the laser arrays. These efficiencies are among the highest values reported so far for GaSb based diode lasers, and allow us to use passively cooled and thus less expensive heat sinking technologies.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    02/2006;
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    ABSTRACT: High-power high-brightness 1.93-μm wavelength (AlGaIn)(AsSb) tapered diode lasers with a narrow vertical waveguide design are reported for the first time. A nearly diffraction-limited continuous-wave output power of 1.5 W together with a remarkable low fast axis divergence of 43° full-width at half-maximum have been demonstrated. The maximum brightness amounts to 32 MW/cm<sup>2</sup>sr.
    IEEE Photonics Technology Letters 02/2006; · 2.04 Impact Factor
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    ABSTRACT: We demonstrate a 1470-nm InGaAsP high-power tapered diode laser grown by all solid-source molecular beam epitaxy. Devices compare well to metal–organic vapor phase epitaxy grown lasers and reach 1.6 W of total power and 1 W of nearly diffraction-limited output power. First reliability tests indicate lifetimes exceeding 10 000 h.
    IEEE Photonics Technology Letters 02/2006; · 2.04 Impact Factor
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    ABSTRACT: High-power 1.91-mum (AlGaIn)(AsSb) quantum-well diode laser single emitters and linear arrays with improved waveguide design were fabricated and characterized. The use of a rather narrow waveguide core results in a remarkable low fast axis beam divergence of 44deg full-width at half-maximum. For single emitters, a continuous-wave (CW) output power of nearly 2 W has been observed. We have achieved 16.9 W in CW mode at a heat sink temperature of 20degC. The efficiencies of more than 25% are among the highest values reported so far for GaSb-based diode lasers, and allow the use of passively cooled and, thus, less expensive heat sink technologies
    IEEE Photonics Technology Letters 02/2006; · 2.04 Impact Factor
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    ABSTRACT: (AlGaIn)(AsSb) ridge-waveguide tapered diode lasers with separately contacted ridge and tapered sections, emitting at 1.93 mum, have been analyzed in pulsed mode with respect to their high-power capability and wavelength tunability. Operating the ridge section above saturation, a variation of the current through this section resulted in a change in lasing wavelength, while changing the current injected into the tapered section at a constant ridge current allowed to vary the output power at constant lasing wavelength. Furthermore, the optical power required to saturate the tapered amplifier section has been derived from a comparison of the experimental characteristics with beam propagation method calculations.
    Applied Physics Letters 01/2006; 89. · 3.52 Impact Factor
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    ABSTRACT: We report on the demonstration of continuous-wave (CW) operation of GaInAs-AlGaAsSb quantum cascade (QC) lasers. By placing a 2.5-μm-thick gold layer on both sides of the laser ridge to extract heat from the active region in the lateral direction, together with mounting the device epilayer down, we have achieved CW operation of GaInAs-AlGaAsSb QC lasers composed of 25 stages of active/injection regions. The maximum CW operating temperature of the lasers is 94 K, and the emission wavelength is around λ∼4.65 μm. For a device with the size of 10×2000 μm<sup>2</sup>, the CW optical output power per facet is 13 mW at 42 K and 4 mW at 94 K. The CW threshold current density is 1.99 kA/cm<sup>2</sup> at 42 K, and 2.08 kA/cm<sup>2</sup> at 94 K, respectively.
    IEEE Photonics Technology Letters 12/2005; · 2.04 Impact Factor