Optical-absorption spectroscopy can replace the use of finicky electrochemical or amperometric cells for measurement of atmospheric oxygen concentrations. Although distributed-feedback laser diodes with high mode-hop stability have been demonstrated for this use, such lasers are expensive. Scientists at the Centre Suisse d'Electronique et de Microtechnique (Zurich, Switzerland) have fabricated a narrow-linewidth vertical-cavity surface-emitting laser (VCSEL) emitting at 763 nm specifically for measuring oxygen concentrations. The laser is inexpensive to make and has a wide mode-hop-free tuning range.
The laser emits in a single transverse mode--a requirement for gas sensing. Such operation was assured by incorporating a 5-µm-diameter aperture. Threshold current is typically 2.5-3.0 mA with a linear slope of 0.147 W/A, while the maximum single-mode output power is 1 mW. The device has a 3.4-MHz linewidth and a tuning range of 763.2-764.1 nm, varied by sweeping the current. The center wavelength corresponds to the blue end of the P branch of the rotational transitions in the oxygen A band. Mounted in an instrument, the device reached a resolution of 0.2%/m, sufficient for medical use or industrial combustion monitoring. Contact Hans Zappe at [email protected].