Johannes Koeth

DFB Lasers Between 760 nm and 16 µm for Sensing Applications

Recent years have shown the importance of tunable semiconductor lasers in optical sensing. We describe the status quo concerning DFB laser diodes between 760 nm and 3,000 nm as well as new developments aiming for up to 80 nm tuning range in this spectral region. Furthermore we report on QCL between 3 μm and 16 μm and present new developments. An overview of the most interesting applications using such devices is given at the end of this paper.

Sensing of formaldehyde using a distributed feedback interband cascade laser emitting around 3493 nm.

We have demonstrated sensing of formaldehyde (H(2)CO) using a room-temperature distributed feedback interband cascade laser (ICL) emitting around 3493 nm. The ICL has been characterized and proved to be very suitable for tunable laser spectroscopy (TLS). The H(2)CO TLS spectra were recorded in direct absorption mode and showed excellent agreement with the Pacific Northwest National Laboratory database. The measurements reported here were taken from a series of measurements of a mixture of H(2)CO in air obtained by vaporizing a solution also containing methanol and formic acid. We obtained a resolution limit better than 1 ppm × m assuming a relative absorption of 10(-3).

Demonstration of an Ethane Spectrometer for Methane Source Identification

Methane is an important greenhouse gas and tropospheric ozone precursor. Simultaneous observation of ethane with methane can help identify specific methane source types. Aerodyne Ethane-Mini spectrometers, employing recently available mid-infrared distributed feedback tunable diode lasers (DFB-TDL), provide 1 s ethane measurements with sub-ppb precision. In this work, an Ethane-Mini spectrometer has been integrated into two mobile sampling platforms, a ground vehicle and a small airplane, and used to measure ethane/methane enhancement ratios downwind of methane sources. Methane emissions with precisely known sources are shown to have ethane/methane enhancement ratios that differ greatly depending on the source type. Large differences between biogenic and thermogenic sources are observed. Variation within thermogenic sources are detected and tabulated. Methane emitters are classified by their expected ethane content. Categories include the following: biogenic (<0.2%), dry gas (1-6%), wet gas (>6%), pipeline grade natural gas (<15%), and processed natural gas liquids (>30%). Regional scale observations in the Dallas/Fort Worth area of Texas show two distinct ethane/methane enhancement ratios bridged by a transitional region. These results demonstrate the usefulness of continuous and fast ethane measurements in experimental studies of methane emissions, particularly in the oil and natural gas sector.

Monomode Interband Cascade Lasers at 5.2

We demonstrate application-grade monomode distributed feedback (DFB) laser devices with emission wavelength around 5.2 μm; suitable for nitric oxide sensing. The devices are based on interband cascade laser material that enables continuous wave operation in the midinfrared spectral region when mounted on a standard thermoelectrically cooled TO-style header. With etched vertical sidewall DFB gratings as wavelength selective elements, signal to noise ratios around 30 dB and typical tuning ranges greater than 12 nm were achieved, making the devices suitable for applications based on tunable laser absorption spectroscopy.

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We have developed GaSb/AlGaSb vertical-cavity surface-emitting lasers (VCSELs) by using solid source molecular beam epitaxy. The reflectivity spectra of the VCSELs show a very narrow (0.9 meV) cavity resonance at 1.5 μm. cw optical pumping results in lasing at the wavelength of the cavity resonance with a laser threshold density of 530 W/cm2 at 77 K.

for Nitric Oxide Sensing

This article demonstrates the integration of imprint lithography into nanoelectronic device fabrication. We present a quantum point contact (QPC) with split gates patterned by imprint lithography. The semiconductor substrate is a modulation-doped GaAs/AlGaAs heterostructure with the two-dimensional electron gas located about 90 nm below the surface. A Si mold with a split-gate pattern is embossed into a poly(methylmethacrylate) film located on top of the semiconductor. The Schottky gates are fabricated by metal evaporation and liftoff. The gate tip separation ranges from 120 to 600 nm. Transport studies performed at T=2 K show conductance quantization with varying gate voltages. Measurements performed on a reference QPC with gates defined by electron beam lithography show similar results. This indicates that the imprint does not affect the electronic performance of the semiconductor.

GaSb vertical-cavity surface-emitting lasers for the 1.5 μm range

Single mode distributed feedback (DFB) interband cascade lasers were realized by placing metal gratings laterally to dry etched ridges. A discrete tuning range of 104 nm could be realized on the same gain material by a variation of the grating period. At room temperature, a 2.4 mm long and 9.8 μm wide ridge with as-cleaved facets emitted more than 6 mW of single mode output power in continuous-wave (cw) mode at a wavelength around 3.8 μm. With typical temperature- and current-tuning rates of 0.31 nm/ °C and 0.065 nm/mA, respectively, a total tuning bandwidth of more than 10 nm could be covered with a single device.

Fabrication of quantum point contacts by imprint lithography and transport studies

The authors report the fabrication of widely tunable monolithic quantum cascade lasers (QCLs) with coupled Fabry–Perot (FP) cavities on indium phosphide. Quasicontinuous tuning of the single mode emission over a total spectral range of 242 nm was realized at two regions between 8.394 and 8.785 μm. An absorption experiment with ammonia shows principle feasibility of gas detection with multisegment QCL devices. Good agreement of the experimentally observed tuning behavior with the one expected from calculated FP mode-combs indicates that the change in the refractive index is mainly due to thermal heating as a result of current injection.

Single mode interband cascade lasers based on lateral metal gratings

We report the fabrication of single mode quantum cascade lasers using a shallow-etched distributed Bragg reflector as frequency selective element. Quasi-continuous single mode tuning over 15 cm-1 at room temperature and 25 cm-1 via temperature tuning at Peltier temperatures is demonstrated. The behavior of both electro-optic and spectral characteristics under variation of the segment currents is analyzed, showing a maximum peak output power at room temperature of 600 mW. Thermal crosstalk between the laser segments is investigated. The spectral resolution of a gas absorption experiment is determined to be better than 0.0078 cm-1.

Widely tunable quantum cascade lasers with coupled cavities for gas detection

The authors report on tunable, miniaturized, two-segment coupled cavity quantum cascade lasers fabricated in a process with a single dry etch step. They observed controllable vernier-effect-based mode switching, induced by the spectral shift of the short cavity’s Fabry-Perot mode comb due to Joule heating. With the help of this discontinuous tuning mechanism, single-mode operation with routinely ∼20dB side mode suppression ratio was achieved over a broad spectral range. The authors measured a quasicontinuous tuning range of 12cm−1 (138nm) and an entire single-mode tuning range of 16.5cm−1 (190nm), centered around 933cm−1 (10.7μm).