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In this work, we have used a tunable VCSEL for high-speed optical data transmission. To obtain wide tunability, a MEMS-DBR is surface micromachined onto a short-cavity high-speed VCSEL operating at 1550 nm. Ultra-wide continuous tuning is realized with electro-thermal actuation of the MEMS with built-in stress gradient within SiOx/SiNy dielectric layers. The MEMS-VCSEL operates in single-mode with SMSR > 40 dB across the entire tuning range. Quasi-error-free transmission of direct-modulation at record 15 Gbps is reported for 20 nm tuning, showing the potential towards the standard requirements for the SFP+ modules in the tail-ends of the WDM transmission system.

A novel method for fabricating a single mode optical interconnection platform is presented. The method comprises the miniaturized assembly of optoelectronic single dies, the scalable fabrication of polymer single mode waveguides and the coupling to glass fiber arrays providing the I/O's. The low cost approach for the polymer waveguide fabrication is based on the nano-imprinting of a spin-coated waveguide core layer. The assembly of VCSELs and photodiodes is performed before waveguide layers are applied. By embedding these components in deep reactive ion etched pockets in the silicon substrate, the planarity of the substrate for subsequent layer processing is guaranteed and the thermal path of chip-to-substrate is minimized. Optical coupling of the embedded devices to the nano-imprinted waveguides is performed by laser ablating 45 degree trenches which act as optical mirror for 90 degree deviation of the light from VCSEL to waveguide. Laser ablation is also implemented for removing parts of the polymer stack in order to mount a custom fabricated connector containing glass fiber arrays. A demonstration device was built to show the proof of principle of the novel fabrication, packaging and optical coupling principles as described above, combined with a set of sub-demonstrators showing the functionality of the different techniques separately. The paper represents a significant part of the electro-photonic integration accomplishments in the European 7th Framework project "Firefly" and not only discusses the development of the different assembly processes described above, but the efforts on the complete integration of all process approaches into the single device demonstrator.

Long-wavelength VCSELs with emission wavelengths beyond 1.3 μm 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 μm, even longer wavelengths up to the mid-infrared range beyond 3 μm 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.

We present long-wavelength buried tunnel junction (BTJ) VCSELs for emission wavelengths around 2.3 μm. 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 μm in recent years. To extend this range up to emission wavelengths around 2.3 μm, 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 μm 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 μm. 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.

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 1.55 μm short-cavity buried-tunnel-junction VCSELs (Vertical-Cavity Surface-Emitting Lasers) with single mode output powers of 6.7 mW at 20°C and 3 mW at 80°C, respectively. Although the device had been predominantly optimized for high-power applications and a proper heat management, we are also observing a 3dB-cut-off frequency of more than 11 GHz and side mode suppression ratios (SMSRs) beyond 54 dB over the whole temperature range. The tuning range of the devices can be increased from 7 nm based on gain tuning to several tens of nanometers when replacing the top DBR by a micro-electro-mechanical system (MEMS) distributed Bragg reflector (DBR) composed of semiconductor or dielectric material being thermally actuated for changing the cavity length. These devices are perfectly suitable for telecommunication and gas sensing applications and represent outstanding devices for the so called tunable diode laser absorption spectroscopy (TDLAS) technique.

An investigation into the carrier and spectral dynamics of a 1.55 μm Buried Tunnel Junction (BTJ) VCSEL was carried out by examining the emission spectra under high resolution and the voltage across the junction as polarisation resolved light from a tunable laser source was injected into the cavity. The VCSEL combines an epitaxial InGaAlAs distributed Bragg reflector with a Si/ZnS dielectric reflector and an oval shaped BTJ leading to a predominantly single transverse polarisation mode and laser linewidths as low as 20 MHz. Around lasing threshold and injecting into the primary mode, the voltage required to maintain the current drops due to stimulated emission and a consequent reduction in the carrier density. Locking behaviour associated with this characteristic is measured with increased input power. Voltage drops as large as 6 mV are measured. Above threshold, injection locking is measured in addition to features associated with the relaxation oscillations of the carriers.

In this paper, a novel two-chip-concept for an electrically pumped and micro-mechanically tunable vertical-cavity surface-emitting laser (VCSEL) operating in the 1.55 μm wavelength range is presented. One chip contains the active region with 5 quantum wells based on the material system AlGaInAs/InP and a buried tunnel junction (BTJ) to provide current confinement and waveguiding. A dielectric mirror forms the back reflector. The second chip consists of a curved mirror membrane that can be displaced by electro-thermal heating. The main advantage of this approach is that both parts can be optimized separately. Packaged laser devices show continuous-wave operation at room temperature with an output power of up to 200 μW and very good side mode suppression in the order of 45 dB. Single-mode operation was observed across a tuning range of more than 30 nm.

InP-based VCSELs (Vertical Cavity Surface Emitting Lasers) are interesting light sources for applications in spectroscopy and fiberoptical communication. Reviewed are devices with a buried tunnel junction (BTJ) and a dielectric backside reflector directly integrated on a electroplated gold-heatsink in the InGaAlAs/InP material system covering the wavelength range from 1.3 to 2.0 μm. The BTJ accomplishes both current confinement to the active region and wave-guiding by the refractive index distribution to achieve low threshold currents. Furthermore it allows for substitution of p-doped device parts by more suitable n-doped material. This approach already proved excellent device performance such as 7 mW output power (multi-mode) and good high temperature characteristics such as 0.5 mW at 80°C for 1.55 μm. Modulation at 10 Gbit/s was also demonstrated. Since the BTJ VCSEL concept covers a wide wavelength range, there is a high-potential field of applications in Tunable Diode Laser Absorption Spectroscopy (TDLAS). Demonstrated are representative measurements of NH3 and HCl. A specialty of TDLAS with VCSELs is the ability for rapid concentration determination with a time resolution up to the megahertz regime. Recent results and further developments of the device structure are also discussed.

A new approach for InP-based long-wavelength VCSELs based on buried tunnel junctions: the buried-tunnel-junction (BTJ) VCSEL is reviewed. Excellent cw laser performance has been demonstrated for BTJ-VCSELs in the 1.55μm wavelength range, such as sub-mA threshold currents, 0.9 V threshold voltage (at λ=1.55μm), operation voltages below 1.4V, 10-70 Ω series resistance, differential efficiencies >25%, up to more than 7mW optical output power, >100°C cw operation, stable polarization and single-mode operation with SSR of the order 50 dB. Also, recent achievements on high-speed long-wavelength VCSELs are reported.