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There has been much research performed in recent years on tunable diode laser spectroscopy for detection of gases such as methane, carbon dioxide, acetylene, etc., which possess near-IR absorption lines. To attain adequate sensitivity with weak near-IR lines, wavelength modulation spectroscopy with phase-sensitive detection is normally employed. However injection current modulation of diode lasers produces both wavelength and amplitude modulation, with a phase shift dependent on the modulation frequency. This results in residual amplitude modulation on the output and in distortion of the harmonic signals derived from the absorption line. These are important issues for calibration and where it is desired to accurately recover the line-shape function in order to make simultaneous measurements of gas concentration, pressure or temperature in industrial applications. Here we discuss how calibration-free measurements may be obtained with diode lasers and explore the implications for fibre laser based systems for spectroscopy which conventionally employ thermal or piezoelectric tuning of the wavelength. In particular, we consider modulation techniques which may be applied to ring fibre lasers which use un-pumped erbium fibre as a saturable absorber to prevent mode-hopping or to DFB fibre lasers which use a short cavity with a Bragg grating to ensure single mode operation.
|Number of pages||10|
|Journal||Proceedings of SPIE: The International Society for Optical Engineering|
|Publication status||Published - 2008|
|Event||SPIE Photonics Europe International Symposium - Strasbourg, France|
Duration: 7 Apr 2008 → 11 Apr 2008
- fibre sensors
- fibre lasers
- DFB lasers
- diode laser spectroscopy
- wavelength modulation spectroscopy
- gas sensors
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1/07/07 → 31/07/10