High precision trace humidity measurements with a fibre-coupled diode laser absorption spectrometer at atmospheric pressure

Abstract

Laser absorption spectroscopy offers the potential for fast and precise trace moisture detection in gases at atmospheric pressure with a small cross sensitivity towards other molecules. We report on the development and calibration of a fibre-coupled tunable diode laser absorption spectrometer operating in the 1 - 100 ppm humidity range. The spectrometer was tested at three European humidity standards laboratories. The performance of the spectrometer was characterized by monitoring constant water vapour concentrations over several hours, yielding a good long-term stability, reproducibility and accuracy. The standard deviations of the measured water vapour concentrations for values above 5 ppm were below ±2%. Comparisons of the instrument with chilled mirror hygrometers demonstrated its fast response time. The dependences of the signal upon the flow rate and temperature are discussed.

Relative-Air Humidity Sensing Element Based on Heat Transfer of a Single Micromachined Floating Polysilicon Resistor

“…The water vapour can change both amplitude and polarization of the wavelength in the range 1 to 10 µm (infrared). An example of this sensor was presented by Schirmer et al [3]. Besides the infrared spectrum, we also find sensors operating in the ultraviolet range -called Lyman-Alpha sensors [4].…”

A tunable diode laser absorption spectrometer for moisture measurements in the low parts in 109range

“…Light from the DFB diode laser is directed through a Faraday isolator and then a half-wave plate to isolate the laser and provide light of the correct polarization for the EOM. The light is then focused through the EOM, which puts phase sidebands on the light at 9.19 and 9.21 GHz, and then enters a multi-pass White cell [14], with a 10 m optical path length and a volume of 0.75 l. The experimental arrangement has been described in detail elsewhere [8]. The light leaving the cell is re-focused by a spherical mirror onto the detector and the output from this detector is then de-modulated at 20 MHz by a double-balanced mixer (DBM 2 ) to give the TTFMS signal.…”

“…During a preliminary study comparing the effectiveness of various spectroscopic techniques for the detection of water vapour using a distributed-feedback diode laser at 1393 nm [7], two-tone frequency modulation spectroscopy (TTFMS) was identified as the most promising technique. A prototype fibre-coupled TTFMS spectrometer was initially developed and was tested at several European humidity laboratories [8]. Subsequently, a free-space spectrometer was also developed; its basic operation is briefly described in [9].…”

Quantitative Moisture Measurement with a Cavity Ring-down Spectrometer using Telecom Diode Lasers

“…In the last two decades, laser-based spectroscopy techniques become more prominent. Tunable diode laser absorption spectroscopy (TDLAS) is relatively simple and robust, therefore has been widely applied [8][9][10]. Cavity ringdown spectroscopy (CRDS) using cw diode lasers provides great enhancement in sensitivity and receives increasing interests in recent years [11][12][13][14][15].…”