Comparison of InP- and GaSb-based VCSELs emitting at 2.3 μm suitable for carbon monoxide detection

摘要

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.