COMBINING A THERMAL DETECTOR WITH A TUNABLE DIODE LASER TO PERFORM WAVELENGTH MODULATION SPECTROSCOPY DOWNHOLE
Publication Date: 2008-Jul-29
The IP.com Prior Art Database
US patent application 20060175547decribes using a thermal detector (e.g., pyroelectric detector) to collect optical spectra (e.g., mid-infrared spectra) in a high ambient temperature environment (e.g., a borehole tool) because it responds only to a change in light intensity but it does not respond to steady light (e.g., mid-infrared background radiation) and because it undergoes little change in optical sensitivity at elevated temperatures. The types of light source modulation explicitly described in 20060175547 include mechanically chopping the light beam or electrically pulsing the light source or steering the beam between different path lengths of sample or steering the beam between a reference cell (filled with a reference fluid or empty) and a sample-filled cell. However, another type of light modulation that could be used with such thermal detectors would be to modulate the wavelength the source light (e.g., a tunable diode laser) between a wavelength that the sample strongly absorbs and a nearby wavelength that the sample does not strongly absorb. Patent 7,196,786 describes using a tunable diode laser in a downhole tool for performing high resolution spectroscopy as well as for performing wavelength modulation spectroscopy. However, patent 7,196,786 does not specify any particular type of light detector to be used or describe the relative merits of using different types of light detectors. Use of a thermal detector in conjunction with wavelength modulation spectroscopy about a sample's absorption peak would allow direct measurement of the first derivative of the optical spectrum and thus remove any baseline offset in the sample's spectrum.