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Carbon monoxide (CO) is a prominent and toxic gas mainly generated by imperfect burning in fires or ovens. For trace gas monitoring and fire detection a compact, long term stable and calibration-free CO sensor is desirable. One very interesting measuring method is Tuneable Diode Laser Absorption Spectroscopy (TDLAS), which utilizes the unique properties of Vertical-Cavity Surface-Emitting Lasers (VCSELs). Two approaches to reach the required wavelength range for the absorption lines of CO at around 2350 nm are discussed in this paper. The first approach is an expansion of the emission wavelength range of the now well-established lasers based on InP and a second and new one is fabricating VCSELs based on GaSb. From the epitaxial point of view the gain of the active material and the realization of a tunnel junction as well as optical, thermal and electrical characteristics of the mirror materials are important issues. For a proper choice of the device design the structuring of the used materials also plays a fundamental role— in particular the substrate removal. With simultaneous considerations of all these crucial issues, devices on InP and GaSb substrates have been fabricated. Both types work in continuous-wave mode, generating light in single-mode emission at the desired wavelength of the CO absorption line, which enables CO measurements down to a concentration limit of 2 ppm.

Vertical-cavity surface-emitting lasers are attractive light sources particularly for gas-sensing applications. Using the AlGaInAs/InP material system, an emission wavelength ranges from 1.3 to 2.05 μm has already been achieved [G. Boehm, M. Ortsiefer R. Shau, J. Rosskopf, C. Lauer, M. Maute, F. Kohler, F. Mederer, R. Meyer, M.-C. Amann, in: Proceedings of MBE XII, J. Crystal Growth (2002) pp. 748–753]. Within this range, absorption lines of gases like H 2 S (1590 nm), CH 4 (1654 nm), H 2 O (1877 nm) and CO 2 (2004 nm) are detectable [G. Totschnig, M. Lackner, R. Shau, M. Ortsiefer, J. Rosskopf, M.-C. Amann, F. Winter, Meas. Sci. Technol. 14 (2003) 472]. The aim of this work is a further expansion of the emission wavelength up to 2.3 μm, exploiting the well-known InP-based material system and the fabrication technique of buried tunnel junction (BTJ) VCSELs [R. Shau, M. Ortsiefer, J. Rosskopf, G. Boehm, C. Lauer, M. Maute, M.-C. Amann, in: Proceedings of SPIE, 5364 (2004) 1–15] to reach the technologically important absorption lines of carbon monoxide (CO) around 2.33 μm.

Buried tunnel junction vertical-cavity surface-emitting lasers (BTJ-VCSELs) are demonstrated as light sources in the wavelength range from 1.3 to 2.0 μm. Continuous-wave operation at room temperature could be achieved for the whole wavelength range. This emphasizes not only the sophisticated device design but also the excellent suitability of the material system AlGaInAs/InP. Advantages and restrictions are discussed in this paper. Transmission experiments with single-mode VCSELs at 1.55 μm show error-free data transmission at modulation frequencies up to 10 Gbit/s. At 1.68 and 1.80 μm, gas sensing experiments detecting methane and water, respectively, could be successfully performed.

Recently, we demonstrated InP-based VCSELs at 1.5 μm with record device performance, achieving low threshold currents ( 1 mW) at room temperature and under continuous wave conditions (Ortsiefer et al., Appl. Phys. Lett. 76 (2000) 2179). In this presentation, we describe the epitaxial growth methods applied for the realization of these high-performance devices. For growing suitable device structures, a very good reproducibility of device parameters, such as the center wavelength of the distributed Bragg reflector, the emission wavelength of the active region and the lasing mode, as well as the possibility to fabricate new tailored devices without great effort are required. An in-situ control technique using the standard pyrometer is a simple and efficient method to monitor growth rates and to enable an in-situ adjustment of growth parameters for the control of lattice matching and layer thicknesses. Using this technique, VCSELs in the 1.5–1.8 μm wavelength range with excellent performance have been fabricated.