Water vapor is probably the most important trace gas in the atmosphere. It forms clouds, transports energy from the point of evaporation to the site of condensation, and is the strongest greenhouse gas. Water vapor concentrations are highly variable, so many instruments, called hygrometers, have been devised for its measurement.

Component side of the diode laser hygrometer (precipitation shield removed)

Diode laser hygrometry has been established as one of the most sensitive means of detecting atmospheric water vapor. However, past diode lasers systems have been uncompetitive with other balloon-borne hygrometry techniques in terms of cost, size, weight, and power consumption. This system represents a major advance in making diode laser sensing competitive in regard to these parameters. The system resides on a 12.7 x 16.5 cm (5" x 6.5") electronics board, weighs 230 g (8.5 oz, including batteries for a balloon flight), and has a component cost under $700. The system sensitivity is 2 ppmv at ground conditions and 16 ppmv at 20 km altitude conditions. The system accuracy is better than 5% for frostpoints greater than -60°C at all atmospheric conditions and the measurement precision is 1%.

The essential engineering advances in this system are the use of a 940 nm vertical cavity laser (VCSEL) and a custom electronics system based on a digital signal processor (DSP). The VCSEL is a low cost single mode diode laser that draws only 10 mW of power. The DSP chip that serves as the system central processor costs only $5, performs 100 million instructions per second, and requires only 150 mW of power. This chip performs all laser control functions, acquires the water vapor spectrum at a 1 kHz rate, and fits the data using principal component analysis. 1.9 m of optical sample path is provided by a 6 cm basepath multipass cell that is mounted directly on the electronics board. Currently, an upgrade to the system is planned by using recently developed 1850 nm VCSELs. This upgrade should improve the sensitivity by a factor of sixty.

For more information, contact Mark Paige.

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