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The measurement of trace concentrations of water vapor is important in many industrial applications, especially semiconductor fabrication. A variety of methods exists for measuring water vapor at concentrations approaching one nmole/mole (one ppb). We have chosen to focus on wavelength modulation spectroscopy (WMS) as a practical method to achieve sufficient sensitivity. WMS is a variation of transmission spectroscopy, and thus it shares the calibration benefits of other Beer's Law techniques. By rapidly modulating the laser wavelength, laser 1/f noise is avoided, so very small changes in laser power can be measured. When WMS is combined with a multiple pass absorption cell, ultrasensitive detection of water vapor is possible. This approach has the desired combination of sensitivity and accuracy while enjoying the benefits of rugged communications-type diode lasers.