G. Tränkle

Ferdinand-Braun-Institut, Berlín, Berlin, Germany

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Publications (274)375.36 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a simple method to accurately measure the frequency noise power spectrum of lasers. It relies on creating the beat note between two lasers, capturing the corresponding signal in the time domain, and appropriately postprocessing the data to derive the frequency noise power spectrum. In contrast to methods already established, it does not require stabilization of the laser to an optical reference, i.e., a second laser, to an optical cavity or to an atomic transition. It further omits a frequency discriminator and hence avoids bandwidth limitation and nonlinearity effects common to high-resolution frequency discriminators.
    Applied Optics 10/2014; 53(30). · 1.69 Impact Factor
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    ABSTRACT: The defect distribution in thick AlN layers obtained by epitaxial lateral overgrowth (ELO-AlN) has been analyzed as a function of the miscut direction of the patterned sapphire substrate. A 0.25° miscut toward the sapphire a-plane leads to formation of smooth ELO-AlN layers containing vertical coalescence grain boundaries and exhibiting an almost homogeneous threading dislocation (TD) distribution with a TD density ranging from 5×108 cm−2 to 8×108 cm−2. In contrast, a 0.25° miscut toward the sapphire m-plane results in formation of periodically arranged macrosteps on the surface of the coalesced ELO-AlN layers as well as formation of inclined coalescence grain boundaries leading to an inhomogeneous TD distribution. A subsequent AlxGa1−xN deposition onto ELO-AlN template with surface macrosteps leads to Ga enrichment on the step sidewalls and, for lower Al-contents (e.g. x=0.5), even to additional defect formation. For higher Al contents (e.g. x=0.8) no additional threading dislocations are formed in the AlGaN layers and the observed TD density corresponds to that of the ELO-AlN template: less than 108 cm−2 in the wing regions and from 6×108 cm−2 to 9×108 cm−2 above the ridges. Compressive strain during growth of Al0.8Ga0.2N on ELO-AlN tends to be compensated by threading dislocation inclination. However, due to the low TD densities the inclination angles are more than 3 times larger than those observed in Al0.8Ga0.2N layers on planar AlN/sapphire templates.
    Journal of Crystal Growth 09/2014; 402:222–229. · 1.69 Impact Factor
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    ABSTRACT: A spectrally tunable, narrow linewidth master oscillator power amplifier system emitting ns pulses with high peak power is presented. The master oscillator is a distributed feedback ridge waveguide (DFB-RW) laser, which is operated in continuous wave (CW) mode and emits at about 975 nm with a spectral line width below 10 pm. The oscillator can be tuned over a range of 0.9 nm by varying the injection current. The tapered amplifier (TA) consists of an RW section and a flared gain-guided section. The RW section of the amplifier acts as an optical gate and converts the CW input beam emitted by the DFB-RW laser into a train of short optical pulses, which are subsequently amplified by the tapered section. The width of the pulses is 8 ns at a repetition rate of 25 kHz. The peak power is 16.3 W. The TA preserves the spectral properties of the emission of the DBR-RW laser. The amplified spontaneous emission is suppressed by about 40 dB.
    Optics Letters 09/2014; 39(17). · 3.39 Impact Factor
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    ABSTRACT: A dual-wavelength Y-branch distributed Bragg reflector (DBR) diode laser at 785 nm is presented as an excitation light source for shifted excitation Raman difference spectroscopy (SERDS). The monolithic device was realized with deeply etched surface DBR gratings using one-step epitaxy. An optical output power of 140 mW was obtained in continuous-wave (CW) operation for each laser cavity, with emission wavelengths of the device at 784.50 and 785.12 nm. A spectral width of the laser emission of 30 pm (0.5 cm−1), including 95% of optical power, was measured. The mean spectral distance of both excitation lines is 0.63 nm (10.2 cm−1) over the whole operating range. Raman experiments using polystyrene as the test sample and ambient light as the interference source were carried out and demonstrate the suitability of the dual-wavelength diode laser for SERDS.
    Applied Spectroscopy 08/2014; 68(8). · 2.01 Impact Factor
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    ABSTRACT: We analyze the influence of second and third order intracavity dispersion on a passively mode-locked diode laser by introducing a spatial light modulator (SLM) into the external cavity. The dispersion is optimized for chirped pulses with highest possible spectral bandwidth that can be externally compressed to the sub picosecond range. We demonstrate that the highest spectral bandwidth is achieved for a combination of second and third order dispersion. With subsequent external compression pulses with a duration of 437 fs are generated.
    Optics Express 07/2014; 22(15). · 3.53 Impact Factor
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    ABSTRACT: A high-power external cavity diode laser (ECDL) system with narrowband emission is presented. The system is based on a commercially available high-power GaN laser diode. For the ECDL, a maximum optical output power of 400 mW in continuous-wave operation with narrowband emission is achieved. Longitudinal mode selection is realized by using a surface diffraction grating in Littrow configuration. A spectral width of 20 pm at 445 nm with a side-mode suppression ratio larger than 40 dB is achieved. This concept enables diode laser systems suitable for subsequent nonlinear frequency conversion into the UV spectral range.
    Optics Letters 07/2014; 39(13):3794-3797. · 3.18 Impact Factor
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    ABSTRACT: An integrated Mach-Zehnder intensity modulator for laser radiation at the wavelength 780 nm is demonstrated for the first time. The device features a double heterostructure GaAs/AlGaAs electro-optic phase modulator. The estimated insertion loss is less than 2.5 dB and the extinction ratio is 3.3 dB.
    CLEO; 06/2014
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    ABSTRACT: An integrated Mach-Zehnder intensity modulator for laser radiation at the wavelength 780 nm is demonstrated for the first time. The device features a double heterostructure GaAs/AlGaAs electro-optic phase modulator. The estimated insertion loss is less than 2.5 dB and the extinction ratio is 3.3 dB.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: AlGaN/GaN HEMTs with low gate leakage current in the μA/mm range have been fabricated with a small-unpassivated region close to the gate foot. They showed considerably higher critical voltage values (average VCR = 60 V) if subjected to step stress testing at OFF-state conditions and room temperature as compared to standard devices with conventional gate technology. This is due to the fact that electrons injected from the gate can be accumulated at the unpassivated region and thus builds up negative charge. The lower gate leakage is due to virtual gate formation, which is reducing local electric field in the vicinity of the gate. In contrast to devices with standard gate technology, degradation during step stressing is not associated with a simultaneous gate leakage and drain leakage current increase but with a strong increase of drain current at OFF-state conditions while the gate leakage is practically not affected. Then a relatively higher critical voltage of around 60 V is achieved. An abrupt increase of subthreshold drain current implies the formation of a conductive channel bypassing the gate region without influencing gate leakage. It is believed that hopping conductivity via point defects formed during device stressing creates this channel. Once this degradation mode takes place, the drain current of affected devices significantly drops. This can be explained by negative trap formation in the channel region affecting the total charge balance in 2DEG region. Electroluminescence measurements on both fresh and degraded devices showed no hot spots at OFF-state conditions. However, there is additional emission at ON-state bias, which suggests additional energetic states that lead to radiative electron transition effects in the degraded devices, most possibly defect states in the buffer.
    Microelectronics Reliability 06/2014; · 1.21 Impact Factor
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    ABSTRACT: A master oscillator power amplifier (MOPA) system for the generation of ns-pulses with high peak power, narrow spectral line width, and stabilized emission wavelength will be presented. The master oscillator is a distributed feedback (DFB) ridge waveguide (RW) laser. The tapered amplifier consists of one RW section and one flared gain-guided section. The DFB laser is operated in continuous wave mode and emits at 973.5 nm with a spectral line width below 10 pm. The RW section of the amplifier acts as an optical gate. The tapered section amplifies the generated optical pulse. An optical peak power of 15.5 W for a pulse width of 8 ns is obtained. The emission wavelength remains constant at all output power levels of the MOPA system for a fixed current into the DFB laser. The spectral power density of the ASE is 37 dB smaller than the lasing spectral power density. The spectral line width is smaller than 10 pm, limited by the resolution of the optical spectrum analyzer. Keywords: ns-pulses, semiconductor lasers, narrow spectral line width, master oscillator power amplifier (MOPA), distributed feedback (DFB) laser, tapered power amplifier.
    proceedings.spiedigitallibrary.org/ on 08/14/2014 Terms of Use: http://spiedl.org/terms, Brussels, April 15th 2014, Paper 9134-28; 04/2014
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    ABSTRACT: We present a micro-integrated, extended cavity diode laser module for space-based experiments on potassium Bose-Einstein condensates and atom interferometry. The module emits at the wavelength of the potassium D2-line at 766.7 nm and provides 27.5 GHz of continuous tunability. It features sub-100 kHz short term (100 μs) emission linewidth. To qualify the extended cavity diode laser module for quantum optics experiments in space, vibration tests (8.1 g<sub>RMS</sub> and 21.4 g<sub>RMS</sub>) and mechanical shock tests (1500 g) were carried out. No degradation of the electro-optical performance was observed.
    Optics Express 04/2014; 22(7):7790-8. · 3.53 Impact Factor
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    ABSTRACT: Growth of Al x Ga1- x N layers by hydride vapor-phase epitaxy on patterned sapphire substrates is investigated. The pattern consists of honeycombs which by their orientation and size promote the formation of coalesced c-plane-oriented Al x Ga1- x N layers with reduced crack density. The orientation of parasitic crystallites in the honeycomb openings is investigated using scanning electron microscopy and electron back-scatter diffraction. Crystallites with their [ .0] and [52.3] directions parallel to the vertical growth direction of the Al0.3Ga0.7N layer are observed and successfully overgrown by a 20- μm-thick fully coalesced c-plane-oriented layer.
    Journal of Electronic Materials 03/2014; 43(4). · 1.68 Impact Factor
  • 03/2014
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    ABSTRACT: Wavelength stabilized distributed Bragg reflector (DBR) diode lasers at an emission wavelength of 785 nm will be presented. The devices have a 14 nm thick GaAsP single quantum well as active layer, which is embedded in Al0.65Ga0.35As waveguide layers and Al0.7Ga0.3As cladding layers. The DBR structures are realized as deeply etched tenth order gratings using I-line wafer stepper lithography. The devices have a stripe width of 2.2 µm and a cavity length of 3 mm including a DBR grating with a length of 500 µm. The devices are mounted p-side up on C-mounts. At a temperature of 25 °C a continuous wave output power of 215 mW and a conversion efficiency of 28% are measured. Up to 140 mW single mode operation with a small tuning range below 190 pm is observed. At 50 mW an aging test was performed showing reliable operation over 1000 h.
    Semiconductor Science and Technology 03/2014; 29(4):045025. · 2.21 Impact Factor
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    ABSTRACT: High power 9xxnm QCW- pump modules are very interesting for high- and ultra-high-energy laser systems. Main relevant issues beside price and power conversion efficiency are long term stability of the mounting scheme and stable fiber coupling. We present a design based on diode laser stacks with lateral heat removal. A single stack element consists of a diode laser, which is soldered on both sides to CuW carriers using AuSn. Life test over 1000 h showed no degradation. DCB coolers are subsequently soldered onto both outer sides of the stack. The thermal resistance of a single stack element is about 1.7 K/W. For >3 J pulse energy the stack contains 28 elements. >=60% power conversion efficiency of the used 940 nm diode laser chips at 120 W output power allows >=20% duty cycle without substantial heating (maximum measured output power >200 W). The light is collimated in vertical direction for each stack element. We choose a size for the FAC which allows staggering the beams of two stacks. The diode laser chips have an aperture width of 1.2 mm and a lateral divergence <14° (95 % power) at 120 W. Fiber coupling is performed by cylindrical lenses in both directions. For 6 J pump energy two stacks are used, coupled into 1.9 mm diameter fiber with a high optical coupling efficiency of >90 %. The principle design is very flexible to match other demands in fiber size and output power.
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    ABSTRACT: Raman lines are often obscured by background light or fluorescence especially when investigating biological samples or samples containing impurities. Shifted excitation Raman difference spectroscopy (SERDS) is a technique to overcome this. By exciting the sample with two slightly shifted wavelengths, it is possible to separate the Raman lines and distortions. In this paper, monolithic dual wavelength DBR diode lasers meeting the demands of Raman spectroscopy and SERDS will be presented. The wavelengths are stabilized and selected by using deeply-etched 10th order surface gratings with different periods manufactured using i-line wafer stepper lithography. Two possible resonator concepts, i.e. a mini-array of two parallel DBR RW-lasers and a Y-branch DBR laser, will be compared. Established excitation wavelengths for Raman spectroscopy at 671 nm and 785 nm are chosen. The total laser length is 3 mm; the ridge width is 2.2 μm for the 785 nm devices and 5 μm for the 671 nm lasers. The length of the DBR gratings is 500 μm. The devices at 671 nm reach output powers up to 100 mW having an emission width smaller than 12 pm (FWHM). The 785 nm lasers show output powers up to 200 mW and a narrow emission below 22 pm. For the dual wavelength lasers the spectral distance between the two excitation lines is about 0.5 nm as targeted. The power consumption at both wavelengths is below 1 W. These data proof that the devices are well suited for their application in portable Raman measurement systems such as handheld devices using SERDS.
    Proc SPIE 01/2014; 9002:900208.
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    ABSTRACT: Detailed experimental investigations of the generation of high-energy short infrared and green pulses with a mode-locked multi-section distributed Bragg reflector (DBR) laser in dependence on the lengths of the gain section and the saturableabsorber (SA) section as well the corresponding input currents and reverse voltages, respectively, are presented. The laser under investigation is 3.5 mm long and has a 500 μm long DBR section. The remaining cavity was divided into four 50 μm, four 100 μm, two 200 μm and eight 250 μm long electrically separated segments so that the lengths of the gain and SA sections can be simply varied by bonding. Thus, the dependence of the mode-locking behavior on the lengths of the gain and SA sections can be investigated on the same device. Optimal mode-locking was obtained for absorber lengths between LAbs = 200 μm and 300 μm and absorber voltages between UAbs= -2 V and -3 V. A pulse length of τ ≍ 10 ps, a repetition frequency of 13 GHz and a RF line width of less than 100 kHz were measured. An infrared peak pulse power of 900 mW was reached. The FWHM of the optical spectrum was about 150 pm. With an 11.5 mm long periodically poled MgO doped LiNbO3 crystal having a ridge geometry of 5 μm width and 4 μm height green light pulses were generated. With an infrared pump peak power of 900 mW a green pulse energy of 3.15 pJ was reached. The opto-optical conversion efficiency was about 31%.
    Proc SPIE 01/2014; 9002:90020F.
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    ABSTRACT: GaAs-based high power diode lasers are the most efficient source of optical energy, and are in wide use in industrial applications, either directly or as pump sources for other laser media. Increased output power per laser is required to enable new applications (increased optical power density) and to reduce cost (more output per component leads to lower cost in $/W). For example, laser bars in the 9xx nm wavelength range with the very highest power and efficiency are needed as pump sources for many high-energy-class solid-state laser systems. We here present latest performance progress using a novel design approach that leverages operation at temperatures below 0°C for increases in bar power and efficiency. We show experimentally that operation at -55°C increases conversion efficiency and suppresses thermal rollover, enabling peak quasi-continuous wave bar powers of Pout > 1.6 kW to be achieved (1.2 ms, 10 Hz), limited by the available current. The conversion efficiency at 1.6 kW is 53%. Following on from this demonstration work, the key open challenge is to develop designs that deliver higher efficiencies, targeting > 80% at 1.6 kW. We present an analysis of the limiting factors and show that low electrical resistance is crucial, meaning that long resonators and high fill factor are needed. We review also progress in epitaxial design developments that leverage low temperatures to enable both low resistance and high optical performance. Latest results will be presented, summarizing the impact on bar performance and options for further improvements to efficiency will also be reviewed.
    Proc SPIE 01/2014; 9002:90021l.
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    ABSTRACT: Shifted excitation Raman difference spectroscopy (SERDS) using a dual-wavelength laser diode laser as excitation light source at 671 nm is presented. This device has a size of 3 mm x 0.5 mm and contains two laser cavities with wavelengths adjusted by distributed Bragg reflector (DBR) gratings as rear side mirrors. An integrated Y-branch coupler guides the emission into a common output aperture. The two wavelengths are centered at 671 nm with a well-defined spectral spacing of about 10 cm-1. An output power up to 100 mW is achieved. Raman experiments using polystyrene as test sample and ambient light to disturb the Raman signals demonstrate the suitability of such light source for SERDS.
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    ABSTRACT: In this work we present a laser cavity with a spatial light modulator (SLM), which allows for arbitrary phase and amplitude manipulation. In comparison to previous setups, it allows the manipulation of spectral components inside the laser cavity without the introduction of spatial chirp. An electrically driven ultrafast semiconductor laser system is used for proper alignment of the laser cavity. We were able to demonstrate that the gain of the laser supports mode-locking operation over a spectral range greater than 12 nm with a central wavelength of 850 nm. This bandwidth has the potential to generate sub 150 fs pulses.

Publication Stats

2k Citations
375.36 Total Impact Points


  • 1970–2014
    • Ferdinand-Braun-Institut
      • Department of Optoelectronics
      Berlín, Berlin, Germany
  • 2010
    • Ruhr-Universität Bochum
      Bochum, North Rhine-Westphalia, Germany
    • Otto-von-Guericke-Universität Magdeburg
      • Institute of Experimental Physics (IEP)
      Magdeburg, Saxony-Anhalt, Germany
  • 2008
    • Friedrich-Alexander Universität Erlangen-Nürnberg
      Erlangen, Bavaria, Germany
  • 2007
    • Technische Universität Bergakademie Freiberg
      • Institute of Theoretical Physics
      Freiberg, Saxony, Germany
  • 1982–2007
    • University of Technology Munich
      • • Walter Schottky Institut (WSI)
      • • Faculty of Physics
      München, Bavaria, Germany
  • 1997
    • Fraunhofer Institute for Applied Solid State Physics IAF
      Freiburg, Baden-Württemberg, Germany
  • 1987–1988
    • Universität Stuttgart
      • Institute of Physics
      Stuttgart, Baden-Württemberg, Germany
    • University of California, Santa Barbara
      • Department of Electrical and Computer Engineering
      Santa Barbara, CA, United States