2011, Hempel, M., Tomm, J.W., Baeumler, M., Konstanzer, H., Mukherjee, J., Elsaesser, T.

Near-field properties of the emission of broad area semiconductor diode lasers under extremely high pumping of up to ∼50 times the threshold are investigated. A transition from a gain to thermally-induced index guiding is shown under operation with single pulses of 300 ns duration. At highest output powers, catastrophic optical damage is observed which is studied in conjunction with the evolution of time-averaged filamentary near-field properties. Dynamics of the process is resolved on a picosecond time scale.

2015, Peschka, D., Thomas, M., Glitzky, A., Nürnberg, R., Gärtner, K., Virgilio, M., Guha, S., Schroeder, T., Capellini, G., Koprucki, Th.

In this paper, we present a thorough modeling of an edge-emitting laser based on strained germanium (Ge) microstrips. The full-band structure of the tensile strained Ge layer enters the calculation of optical properties. Material gain for strained Ge is used in the 2D simulation of the carrier transport and of the optical field within a cross section of the microstrips orthogonal to the optical cavity. We study optoelectronic properties of the device for two different designs. The simulation results are very promising as they show feasible ways toward Ge emitter devices with lower threshold currents and higher efficiency as published insofar.

2010, Richter, H., Greiner-Bär, M., Pavlov, S.G., Semenov, A.D., Wienold, M., Schrottke, L., Giehler, M., Hey, R., Grahn, H.T., Hübers, H.-W.

We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum-cascade laser (QCL) operating at 3.1 THz with a compact, low-input-power Stirling cooler. The QCL, which is based on a two-miniband design, has been developed for high output and low electrical pump power. The amount of generated heat complies with the nominal cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cooler. The whole system weighs less than 15 kg including the cooler and power supplies. The maximum output power is 8 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser is fundamental Gaussian. The applicability of the system is demonstrated by imaging and molecular-spectroscopy experiments.

2014, Wienold, M., Röben, B., Schrottke, L., Sharma, R., Tahraoui, A., Biermann, K., Grahn, H.T.

Currently, different competing waveguide and resonator concepts exist for terahertz quantum-cascade lasers (THz QCLs). We examine the continuous-wave (cw) performance of THz QCLs with single-plasmon (SP) and metal-metal (MM) waveguides fabricated from the same wafer. While SP QCLs are superior in terms of output power, the maximum operating temperature for MM QCLs is typically much higher. For SP QCLs, we observed cw operation up to 73 K as compared to 129 K for narrow (≤ 15 μm) MM QCLs. In the latter case, single-mode operation and a narrow beam profile were achieved by applying third-order distributed-feedback gratings and contact pads which are optically insulated from the intended resonators. We present a quantitative analytic model for the beam profile, which is based on experimentally accessible parameters.

2010, Lichtenstein, N., Baettig, R., Brunner, R., Müller, J., Valk, B., Gawlik, A., Bergmann, J., Falk, F.

We present the design and performance of a diode laser module producing a high intensity line focus at 808 nm for material processing. The design is based on a linear array of 7 laser bars and beam forming optics featuring a micro-optic homogenizer. The module delivers a total output power of 900 W at 140 A and peak intensity created in the focus area of 10.3 kW/cm2. Two systems with line length of 5 cm and 10 cm at a large working distance of 110 mm have been realized. The chosen concept allows scaling in length by joining multiple modules which is of interest for material processing in industrial applications. Application results from laser crystallization of amorphous silicon seed layers used in the fabrication of photovoltaic cells for solar panels are given.

2016, Hagelschuer, Till, Rothbart, Nick, Richter, Heiko, Wienold, Martin, Schrottke, Lutz, Grahn, Holger T., Hübers, Heinz-Wilhelm

We report on a high-spectral-resolution terahertz imaging system operating with a multi-mode quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector. By tuning the frequency of the QCL, several spectra can be recorded in 1.5 s during the scan through a gas cell filled with methanol (CH3OH). These experiments yield information about the local absorption and the linewidth. Measurements with a faster frame rate of up to 3 Hz allow for the dynamic observation of CH3OH gas leaking from a terahertz-transparent tube into the evacuated cell. In addition to the relative absorption, the local pressure is mapped by exploiting the effect of pressure broadening.

2017, Röben, Benjamin, Lü, Xiang, Hempel, Martin, Biermann, Klaus, Schrottke, Lutz, Grahn, Holger T.

Terahertz (THz) quantum-cascade lasers (QCLs) are powerful radiation sources for high-resolution and high-sensitivity spectroscopy with a discrete spectrum between 2 and 5 THz as well as a continuous coverage of several GHz. However, for many applications, a radiation source with a continuous coverage of a substantially larger frequency range is required. We employed a multi-mode THz QCL operated with a fast ramped injection current, which leads to a collective tuning of equally-spaced Fabry-Pérot laser modes exceeding their separation. A continuous coverage over 72 GHz at about 4.7 THz was achieved. We demonstrate that the QCL is superior to conventional sources used in Fourier transform infrared spectroscopy in terms of the signal-to-noise ratio as well as the dynamic range by one to two orders of magnitude. Our results pave the way for versatile THz spectroscopic systems with unprecedented resolution and sensitivity across a wide frequency range.

2012, Otto, C., Lüdge, K., Vladimirov, A.G., Wolfrum, M., Schöll, E.

We study a passively mode-locked semiconductor ring laser subject to optical feedback from an external mirror. Using a delay differential equation model for the mode-locked laser, we are able to systematically investigate the resonance effects of the inter-spike interval time of the laser and the roundtrip time of the light in the external cavity (delay time) for intermediate and long delay times. We observe synchronization plateaus following the ordering of the well-known Farey sequence. Our results show that in agreement with the experimental results a reduction of the timing jitter is possible if the delay time is chosen close to an integer multiple of the inter-spike interval time of the laser without external feedback. Outside the main resonant regimes the timing jitter is drastically increased by the feedback.

2017, Hagelschuer, Till, Wienold, Martin, Richter, Heiko, Schrottke, Lutz, Grahn, Holger T., Hübers, Heinz-Wilhelm

We report on real-time gas sensing with a terahertz quantum-cascade laser (QCL). The method is solely based on the modulation of the external cavity length, exploiting the intermediate optical feedback regime. While the QCL is operated in continuous-wave mode, optical feedback results in a change of the QCL frequency as well as its terminal voltage. The first effect is exploited to tune the lasing frequency across a molecular absorption line. The second effect is used for the detection of the self-mixing signal. This allows for fast measurement times on the order of 10 ms per spectrum and for real-time measurements of gas concentrations with a rate of 100 Hz. This technique is demonstrated with a mixture of D2O and CH3OD in an absorption cell.

2014, Wienold, M., Röben, B., Schrottke, L., Grahn, H.T.

We report on a broad-band terahertz quantum-cascade laser (QCL) with a long Fabry-Pérot ridge cavity, for which the tuning range of the individual laser modes exceeds the mode spacing. While a spectral range of approximately 60 GHz (2 cm−1) is continuously covered by current and temperature tuning, the total emission range spans more than 270 GHz (9 cm−1). Within certain operating ranges, we found evidence for stable frequency comb operation of the QCL. An experimental technique is presented to characterize frequency comb operation, which is based on the self-mixing effect.