DOI: 10.1007/s00340-006-2336-5 Appl. Phys. B 85, 337–341 (2006) Lasers and Optics Applied Physics B h. cattaneo ✉ t. laurila r. hernberg Photoacoustic detection of oxygen using cantilever enhanced technique Tampere University of Technology, Institute of Physics, Optics Laboratory, P.O. Box 692, 33101 Tampere, Finland Received: 24 April 2006/Revised version: 19 May 2006 Published online: 29 June 2006 • © Springer-Verlag 2006 ABSTRACT A tunable diode laser photoacoustic setup based on a recently demonstrated cantilever technique was used for sensitive detection of oxygen. As light sources, we used a distributed feedback (DFB) diode laser and a vertical-cavity surface-emitting (VCSEL) laser, both operating near 760 nm. With the 30 mW DFB laser a noise-equivalent detection limit of 20 ppm for oxygen was obtained, while a detection limit of 5 ‰ was achieved with the VCSEL having an output power of 0.5 mW. Our results yield a noise-equivalent sensitivity of 4.8 × 10 −9 cm −1 W Hz −1/2 and demonstrate the potential of this technique for compact and sensitive measurement of oxygen. PACS 42.62.Fi; 42.55.Px; 82.80.Kq 1 Introduction Sensitive detection of oxygen is essential in many industrial and research applications. These include medical, biological, and physiological applications (e.g., breath diag- nostics, studies on metabolism and fermentation, and moni- toring of sport activities), energy management and production control (e.g., gas purity in semiconductor industry, food stor- age, and power generation in combustion), as well as environ- mental monitoring. These applications necessitate detection limits ranging from sub-ppm levels at low pressures to sev- eral per cents at atmospheric conditions. Due to such diverse requirements, various types of oxygen sensors have been de- veloped, which are based on the electrochemical and physical properties of oxygen. Examples of commonly exploited tech- nologies include electro-chemical cells [1], ion conductivity of ceramics (mostly ZrO 2 ) [2], paramagnetism [3], adsorp- tion on semiconducting metal oxides [4], and fluorescence quenching [5]. Many of the applications listed above would benefit from a sensitive, small, portable, stable, non-intrusive, and possibly cheap oxygen sensor. All-optical oxygen sensors based on ab- sorption spectroscopy and near-infrared diode lasers have the ✉ Fax: +358 3 31152090, E-mail: heidi.cattaneo@tut.fi potential of meeting most of these stringent requirements and have been exploited widely in industrial applications [6–11]. Vertical-cavity surface-emitting lasers (VCSELs) are favored for tunable diode laser spectroscopy (TDLS) of oxygen be- cause of their advantages over traditional diode lasers, such as single-mode operation, good beam quality, high repeti- tion rate, relatively narrow line width, large current tuning range, good operability, and low power consumption. More- over, VCSELs have the potential of being low-cost devices due to their moderate fabrication costs. Unfortunately, small size is hardly achieved with oxy- gen sensors based on TDLS. This is due to the fact that the only absorption band of oxygen within the spectral range covered by diode lasers is a weak magnetic dipole tran- sition around 760 nm. As the minimum detectable optical density with TDSL is typically on the order of 10 −4 and the line strengths of oxygen transitions are on the order of 10 −24 cm 2 /(cm molec), an absorption path length of several tens of cm is needed to detect 1% oxygen levels at atmospheric conditions. The path length can be reduced by using wave- length modulation or balanced detection [11], which improve the sensitivity of TDLS sensors. Although the detection of optical densities on the order of 10 −6 –10 −7 has been demon- strated [11], such sensitivities are difficult to obtain in field measurements. Photoacoustic spectroscopy (PAS) is a well-known tech- nique for sensitive gas analysis. Recently, it has been com- bined with tunable diode lasers, allowing optical densities of ∼ 10 −7 to be detected [12, 13]. Due to the relatively high sensitivity, PAS can potentially yield smaller sensors com- pared to traditional absorption methods [14]. However, PAS is a power scalable technique and therefore calls for exci- tation sources with high output powers. As the power lev- els of VCSELs are typically below 1 mW, conventional PAS arrangements would only permit the measurement of rela- tively high optical densities with VCSEL based excitation. Therefore TDLPAS applications usually utilize distributed feedback (DFB) lasers, which typically provide more than 30 mW output power. Recently, we demonstrated a novel and sensitive TDLPAS setup [15,16], which is based on cantilever-enhanced detection [17]. A normalized sensitivity of 2.2 × 10 −9 cm −1 W Hz −1/2 for CO 2 detection at 1572 nm