10-ns pulses of laser light Optical splitter
Laser Backscattered light processing unit Signal-
Optical fiber
Display unit
> DTS process. The DTS laser shoots bursts of light down the length of the optical fiber. Some light returns in the form of backscatter. The back scattered light is split from the incident pulses and filtered into discrete wavelengths. Because the speed of light is constant, a log of the backscattered light can be generated for each meter of the fiber.
DTS Basics
In its most basic form, a DTS system comprises a strand of optical fiber, a laser light source, an optical splitter, an optoelectronic signal- processing unit and a display unit (above). The fiber-optic strand is actually housed inside a protective tube, or carrier. A strand is hair thin— only about 100 microns thick—and has a central core of silica glass, some 5 to 50 microns in diameter. The core is surrounded by an outer layer of silica known as cladding. The silica composition of the cladding is doped with other components— such as germanium or fluorine—to alter its refractive index and light-dispersion properties. A laser launches 10-ns pulses of light (an interval equivalent to nearly 1 m) down the fiber strand. As each input pulse travels through the strand, its light is reflected along the boundary between the core of the fiber and its cladding, through a phenomenon known as total internal reflection. The core has a higher refractive index than the doped cladding, and light that strays from the centerline of the core will eventually strike the core/cladding boundary at an angle that guides the light beam back toward the center.
However, a fraction of that light is none - theless scattered as the pulse travels down the fiber. Light can be scattered by density fluctua - tions or minute compositional variations in the glass—through a process known as Rayleigh scattering—or by acoustic vibrations that change the refractive index of the optical fiber— known as Brillouin scattering.
For the purpose of DTS, the most important mode of light scatter is a third type called Raman scattering, which is caused by inelastic collisions of photons with molecules in the fiber medium. These collisions alter the molecules’ vibrational- energy states. A scattered photon may either lose energy to the molecule and raise it to a higher vibrational-energy state, called Stokes scattering, or it may gain energy by moving the molecule to a lower vibrational-energy state, called anti-Stokes scattering (next page, top).
A portion of this scattered light is reflected back along the fiber toward the laser source. Along the way, a directional coupler separates the input light pulse from the backscatter signal. The returning signal is then sent to a highly
sensitive receiver, where the Raman wavelengths are filtered from the dominant Rayleigh and Brillouin backscatter.
The energy transferred in Raman scattering between the scattering molecule and the photon is temperature dependent. The Raman signal comprises two components—the Stokes and anti-Stokes wavelengths. The longer-wavelength Stokes signal is very weakly temperature sensitive; however, backscattered light at the shorter anti-Stokes wavelength is strongly temperature sensitive. The ratio of these two signals is directly proportional to the temper - ature of the scattering medium.
The backscattered light is also analyzed to determine how far down the fiber it originated. Because each input pulse is 10 ns long, the interval from which the backscattered light originated will correspond directly to a specific meter-long segment of the fiber. Consequently, a log of temperature can be calculated along the length of the fiber by using only the laser source, the analyzer and a reference temperature in the surface system. There is no need for calibration points along the fiber or for calibration of the fiber before installation.
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Oilfield Review
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