Spectrochimica Acta Part A 60 (2004) 3269–3275 VCSEL based detection of water vapor near 940 nm Heidi Cattaneo ∗ , Toni Laurila, Rolf Hernberg Optics Laboratory, Institute of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland Received 1 October 2003; accepted 16 February 2004 Abstract A vertical-cavity surface-emitting laser (VCSEL) was used to study the absorption spectrum of water vapor in the 940nm region. Mea- surements were performed in ambient air at room temperature and in a hydrogen–oxygen flame over the temperature range of 1500–1800 K. Several rotational absorption lines within the 2ν 1 + ν 3 vibrational band were measured. The absorption spectra were well resolved, which demonstrates the feasibility of VCSEL-based spectroscopic measurements of water vapor at room and high-temperature in this spectral region. The results were in good agreement with the values obtained from the HITRAN-96 database. © 2004 Elsevier B.V. All rights reserved. Keywords: VCSEL; Gas detection; High-temperature; Water vapor 1. Introduction Monitoring of water vapor is an important tool for diag- nostics in many industrial processes. The concentration of water vapor yields information on performance parameters such as efficiency and heat release of the process. In addi- tion, information on gas dynamic parameters, e.g., gas tem- perature, pressure, and velocity, can be obtained from the absorption spectrum of water vapor. Water vapor has strong, temperature-sensitive absorption features in the near infrared. The strongest absorption lines occur in the 1.4 m region. For this region, line strengths and pressure broadening parameters have been thoroughly inves- tigated for a large temperature range [1–3]. Water absorbs notable amounts of light also in the 940 nm region, where several vibrational bands (of which the 2ν 1 + ν 3 band is one of the strongest) are present. However, significantly less at- tention has been given to determining spectral absorption parameters in this region, especially at high-temperatures. Line strengths, positions, and broadening for several at- mospheric molecules (including H 2 O) at room temperature can be found in molecular databases such as HITRAN-96 [4]. The databases can also be used to model the temperature dependence of the absorption features. Several authors have ∗ Corresponding author. Tel.: +358-3-3115-3417; fax: +358-3-3115-2090. E-mail address: heidi.cattaneo@tut.fi (H. Cattaneo). recently pointed out systematic errors in the database for water vapor. Giver et al. [5] suggested that the line strength values of HITRAN-96 should be corrected by a factor of 1.144 for the 940 nm region. Schermaul et al. [6] also found systematic differences between the measured line strengths and the values of the HITRAN-96 database. Tunable diode laser spectroscopy (TDLS) is a widely used and acknowledged method for sensor applications in the near-IR region. An important advantage of TDSL is the pos- sibility of performing in situ measurements without the need for sampling lines. TDLS has been applied to detect several gas species such as CO, CO 2 , CH 4 , OH, O 2 , and H 2 O in different environments [7–11]. With direct absorption tech- niques optical absorbances of 10 -3 to 10 -4 can be detected [7,8]. The detection limit can be improved by at least two orders of magnitude with more sensitive methods involving wavelength modulation and balanced detection, which have been successfully used with TDLS [11–13]. The vast majority of TDSL measurements on water vapor have made use of distributed feedback (DFB) semiconduc- tor lasers [1–3,13–15]. DFB lasers have very narrow line widths but are difficult to fabricate and, hence, are expensive devices. Vertical-cavity surface-emitting diode lasers (VC- SELs) have the potential of being low-cost devices, and can be fabricated in high volumes. They can operate single-mode with line widths that are narrow enough for most spec- troscopic applications. Their advantages include very good beam quality, large current tuning range, high modulation 1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.02.025