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We present surface micro-machined tunable vertical-cavity surface-emitting lasers (VCSELs) operating around 1550nm with tuning ranges up to 100nm and side mode suppression ratios beyond 40 dB. The output power reaches 3.5mW at 1555 nm. The electro-thermal and the electro-statical actuation of a micro electro-mechanical system (MEMS) movable distributed Bragg reflector (DBR) membrane increases/decreases the cavity length which shifts the resonant wavelength of the cavity to higher/lower values. The wavelength is modulated with 200 Hz/120 kHz. Both tuning mechanisms can be used simultaneously within the same device. The newly developed surface micro-machining technology uses competitive dielectric materials for the MEMS, deposited with low temperature plasma enhanced chemical vapor deposition (PECVD), which is cost effective and capable for on wafer mass production.

Long-wavelength VCSELs with emission wavelengths beyond 1.3 m have seen a remarkable progress over the last decade. This success has been accomplished by using highly advanced device concepts which effectively overcome the fundamental technological drawbacks related with long-wavelength VCSELs such as inferior thermal properties and allow for the realization of lasers with striking device performance. In this presentation, we will give an overview on the state of the technology for long-wavelength VCSELs in conjunction with their opportunities in applications for optical sensing. While VCSELs based on InP are limited to maximum emission wavelengths around 2.3 m, even longer wavelengths up to the mid-infrared range beyond 3 m can be achieved with VCSELs based on GaSb. For near-infrared InP-based VCSELs, the output characteristics include sub-mA threshold currents, up to several milliwatts of singlemode output power and ultralow power consumption. New concepts for widely tunable VCSELs with tuning ranges up to 100 nm independent from the material system for the active region are also presented. Today, optical sensing by Tunable Diode Laser Spectroscopy is a fast emerging market. Gas sensing systems are used for a wide range of applications such as industrial process control, environmental monitoring and safety applications. With their inherent and compared to other laser types superior properties including enhanced current tuning rates, wavelength tuning ranges, modulation frequencies and power consumption, long-wavelength VCSELs are regarded as key components for TDLS applications.

GaSb-based type-I quantum-well lasers, emitting in the spectral range from 2 to 4 m are promising light sources for various trace gas sensing systems by means of tunable diode laser absorption spectroscopy (TDLAS). Excellent device performance has been achieved so far in the spectral range from 2 to 3 m, however, room-temperature operation above 3 m is much more difficult to achieve. In this work we demonstrate the extension of room-temperature operation wavelength of GaSb-based type-I lasers up to 3.73 m by implementation of high-quality quinternary AlGaInAsSb heterostructures.

We report the investigation of the state of polarization (SOP) of a tunable vertical-cavity surface-emitting laser (VCSEL) operating near 850 nm with a mode-hop free single-mode tuning range of about 12 nm and an amplitude modulation bandwidth of about 5 GHz. In addition, the effect of a sub-wavelength grating on the device and its influence on the polarization stability and polarization switching has been investigated. The VCSEL with an integrated sub-wavelength grating shows a stable SOP with a polarization mode suppression ratio (PMSR) more than 35 dB during the tuning.

We present a micro electro-mechanical system (MEMS) tunable vertical-cavity surface-emitting laser (VCSEL) emitting around 1.55 m with single-mode output power of >2.5mW and high side-mode suppression-ratio (SMSR) of >50dB over the entire tuning range of >50nm. The small-signal modulation technique (S21) has been used to study intrinsic and parasitic influences on the modulation response of the device. Additionally, the static characteristics as well as electrical and thermal design of the device are discussed with respect to its high-speed modulation behavior. The tunable laser shows 3-dB direct modulation frequencies above 6.4 GHz.

In this paper we describe the design and performance of nonlinear quantum cascade laser sources for near-infrared and terahertz applications. Our devices are based on monolithically integrated mid-infrared quantum cascade lasers and passive nonlinear structures which provide a giant nonlinear response for the pumping frequency. Such design concept can be applied for both short-wavelength and long-wavelength generation. In our work, short-wavelength devices were based on the concept of second-harmonic generation whereas long-wavelength devices utilized difference-frequency generation. With this approach we demonstrate room-temperature operation down to 2.7 m for near-infrared devices and up to 70 m at 210 K for terahertz devices. The performance of our nonlinear devices is very sensitive to the resonance condition for the pump and nonlinearity. We demonstrate that, once resonant, nonlinear powers in the mW range are available.

Broad gain lasers attract much interest due to the practical gas sensing applications. A step well structure with double optical transitions is implemented in the active region of injectorless quantum-cascade lasers (QCLs) to achieve extended gain bandwidth. The step well is constructed by a "short period superlattice" which provides an equivalent GaInAlAs alloy quantum step well in each active module. The fabricated devices show an ultrabroadband gain spectrum of 480 cm-1(4 m) at room temperature in pulsed mode. The laser threshold current density is as low as 1.1 kA/cm2which is the lowest among the broad gain QCLs emitting at similar wavelengths and the slope efficiency is 1.6 W/A at room temperature.

Results of single mode and tunable GaSb-based VCSELs, with emission wavelengths above 2m, are presented. Devices are aimed at trace gas sensing applications and operate in two important spectral windows - 2.3 m and 2.6 m. The first one is suitable for CO detection and in the second one strong absorption lines of H2S and H2O lie. VCSELs emitting at 2.33 m operate in continuous-wave (CW) up to heatsink temperatures of 90 °C and deliver the maximum single-mode output power of 0.8 mW at 0°C with an aperture diameter of 6m. With the introduction of inverted surface relief on top of the processed device, single mode operation has been extended up to 12 m large aperture devices. The maximum wavelength tuning range of 20 nm has been achieved. VCSELs emitting at 2.6 m operate in CW mode up to 55 °C, with the maximum single-mode output power of 0.4 mW, at -20 °C. They offer single transverse mode emission up to 9 m large apertures. The maximum wavelength tuning of 10 nm is presented. Finally, first applications to trace gas sensing are also presented for 2.3 m GaSb-based VCSELs.

Injectorless quantum cascade lasers are lately getting more and more into focus of research, as they promise lower threshold current densities and higher output powers and efficiencies. Although the research on them started seven years later in 2001 and was less intensive, they have achieved much lower threshold current densities with 0.45 kA/cm2at 300 K compared to injectorbased devices. Additionally injectorless devices reached efficiencies and output powers in the important wavelength range between 8 and 12 m comparable to best performing injectorbased devices. With 2 W per facet and an overall efficiency of 7 % around 9 m, they show a strong potential regarding possible applications.

The latest results on InP based 1550 nm short-cavity VCSELs will be presented. Important design criteria will be discussed in order to manufacture high-power, high-speed or widely tunable VCSEL devices, respectively.

We discuss the design and performance of quantum cascade laser sources based on intra-cavity second harmonic generation operating in at wavelengths shorter than 3.7m. A passive heterostructure tailored for giant optical nonlinearity is integrated on top of an active region and patterned for quasi-phasematching. We demonstrate operation of 3.6m, 3.0m, and 2.6m devices based on lattice-matched and strain-compensated InGaAs/AlInAs/InP materials. Threshold current densities of typical devices with nonlinear sections are only 10-20% higher than that of the reference lasers without the nonlinear section. Our best devices have threshold current density of 2.2kA/cm2and provide approximately 35W of second-harmonic output at 2.95m at room temperature. The second-harmonic conversion efficiency is approximately 100W/W2. Up to two orders of magnitude higher conversion efficiencies are expected in fully-optimized devices.

Widely tunable vertical cavity surface emitting lasers (VCSEL) are of high interest for optical communications, gas spectroscopy and fiber-Bragg-grating measurements. In this paper we present tunable VCSEL operating at wavelength around 850 nm and 1550 nm with tuning ranges up to 20 nm and 76 nm respectively. The first versions of VCSEL operating at 1550 nm with 76 nm tuning range and an output power of 1.3mW were not designed for high speed modulation, but for applications where only stable continious tuning is essential (e.g. gas sensing). The next step was the design of non tunable VCSEL showing high speed modulation frequencies of 10 GHz with side mode supression ratios beyond 50 dB. The latest version of these devices show record output powers of 6.7mW at 20 °C and 3mW at 80 °C. The emphasis of our present and future work lies on the combination of both technologies. The tunable VCSEL operating in the 850 nm-region reaches a modulation bandwidth of 5.5GHz with an output power of 0.8mW.

We present long-wavelength buried tunnel junction (BTJ) VCSELs for emission wavelengths around 2.3 m. Two different device concepts have been realized utilizing either InP- or GaSb-based materials. The InP-VCSELs are based on a BTJ-design which has been well-proven for wavelengths up to 2 m in recent years. To extend this range up to emission wavelengths around 2.3 m, the main focus is set on an optimization of the active region. In this context, we use a graded and heavily strained quantum well design in conjunction with optimized growth conditions. The photoluminescence and x-ray characterization shows a very good material quality. Room-temperature operated VCSELs exhibit around 0.5 mW of output power with singlemode-emission at 2.36 m representing the longest wavelength that has been achieved with InP-based interband lasers so far. GaSb-based devices comprise an epitaxial back mirror and a dielectric output mirror while the basic BTJ-principle is maintained. Using GaInAsSb quantum wells, the active region reveals excellent gain characteristics at 2.3 m. Singlemode VCSELs show room temperature threshold currents around 1 mA and output powers of 0.7 mW, respectively. Both laser types have been implemented in a tuneable diode laser spectroscopy (TDLS) setup to evaluate their capability for sensing of carbon monoxide. Using an absorption path length of only 10 cm, concentration measurements down to a few ppm have been successfully demonstrated.