External cavity diode lasers (ECDLs) are a well-established laboratory tool due to their excellent emission properties. However, if the ECDLs are used outside the laboratory, they have limitations in terms of tuning speed and robustness. For overcoming these limitations, we developed a new micro-electro-mechanical system (MEMS) based ECDL cavity concept. The 1D MEMS actuator defines the angle of incidence at the diffraction grating as well as the cavity length of the ECDL. Due to the high resonance frequency of the MEMS actuator in the kHz range, the switching speed of the ECDL emission wavelength is drastically reduced. Furthermore, the MEMS actuator minimizes the sensitivity to external disturbance which opens a path to handheld wide mode-hop free tunable ECDLs in the near future. Therefore we have also optimized our curved waveguide concept based on GaSb for the ECDL design, whereby a wavelength range from NIR to the MIR range can be better covered. These features qualify the new developed MEMS tunable ECDL for the high demands of the high resolution multi-species molecular spectroscopy. Application examples of the MEMS based ECDL and the curved gain chips will be provided.
GaSb based types of diode lasers may cover the spectral regime from below 1.8 μm up to 5 μm. For the wavelength regime of 1.8 μm to 2.5 μm InGaAsSb/GaSb MQW material is used. For 2.5 μm to 3.4 μm InAlGaAsSb/GaSb MQW material is used. For above 3μm, an ICL type of design is required. We realized a growth campaign of 10 GaSb based wavers for covering the wavelength regime from 1.9μm to 3μm. We report on the test, performance and applications results in molecular gas sensing of both, gain chips within an external cavity laser as well as on digital DFB lasers.
Tunable laser sources are used in a wide range of novel applications such as spectroscopy, biomedicine or gas sensing techniques. New requirements in terms of size, tuning speed and output power are addressed with our work.
We present a miniaturized external cavity diode laser concept which will be compared with well-known laser systems such as distributed feedback (DFB) lasers. DFB lasers suffer high internal losses due to the overlap of the DFB grating with the optical waveguide. Our concept of Micro Electro Mechanical Systems (MEMS) based lasers are stabilized with a transmission grating, resulting in significantly less losses. Furthermore, the tuning of the diffraction efficiency of the gratings allows the optimization of the output power and the overall tuning range, which is measured to be one order of magnitude larger than what can be achieved with DFBs. It is also important to point out the tuning speed of the MEMS lasers due to the fast nature of the tilting capabilities of the MEMS actuators. Excellent relative intensity noise and narrow linewidth features are present in these laser systems due to the low noise driving electronics for both the diode lasers and the MEMS actuators.
The high output power and the low linewidth will enable a higher sensitivity and resolution for a wide range of applications. The performance of the MEMS laser systems will be presented, being suitable for applications such as Raman spectroscopy or tunable diode laser absorption spectroscopy (TDLAS) in the wavelength ranges of 780 nm and 920 nm.
The NIR/MIR region between 1.8μm and 3.5μm contains important absorption lines for gas detection. State of the art are InP laser based setups, which show poor gain above 1.8μm and cannot be applied beyond 2.1μm. GaSb laser show a significantly higher output power (100mW for Fabry-Perot, 30mW for DFB). The laser design is presented with simulation and actual performance data. The superior performance of the GaSb lasers is verified in gas sensing applications. TDLAS and QEPAS measurements at trace gases like CH4, CO2 and N2O are shown to prove the spectroscopy performance.