Distributed Raman Sensing

A model for probe power stabilization in pulse coded distributed optical fiber sensors using EDFA dynamic gain control

“…A laser diode at 1550 nm is directly modulated to generate single pulses or code-words, according to the used specific coding theory (i.e. Simplex, Cyclic Simplex, Golay); the modulated light is coupled to a standard single mode (SMF) sensing fiber and the backscattered light components at the Stokes (S) and Anti-Stokes (AS) wavelengths, at around 1650 nm and 1450 nm respectively, are filtered and detected by avalanched photodetectors (APD), electrically amplified and processed to extract the temperature profile along the sensing fiber [9]. It is well known that, in order to overcome the trade-off between sensing distance and spatial resolution, which depends on the pulse duration, high peak power levels and pulse coding can be used simultaneously, without however exceeding the onset of nonlinear effects such stimulated Raman scattering (in SMF, fiber the maximum peak power should be below approximately 1 W).…”

“…It is well known that, in order to overcome the trade-off between sensing distance and spatial resolution, which depends on the pulse duration, high peak power levels and pulse coding can be used simultaneously, without however exceeding the onset of nonlinear effects such stimulated Raman scattering (in SMF, fiber the maximum peak power should be below approximately 1 W). The use of pulse coding consists in sending code-word sequences instead of single pulses, acquiring the coded traces and performing decoding [9]. This will provide a coding gain, or signal to noise ratio (SNR) enhancement with respect to the use of single pulses, which can be used to enhance the sensing distance, or improve the temperature resolution, ensuring the same spatial resolution (a 10 nsec pulse corresponds to 1 meter spatial resolution).…”

“…The power peak variation within the code-word ΔP is approximately 6.8 dB, not acceptable at all to perform decoding without distortion. The same analysis has also been performed considering cyclic Simplex coding, in which the code-word pulses are now distributed along the full fiber length [9]. Considering a sensing fiber distance of 40 km, corresponding to a transit time of 0.4 ms, Figs.…”

Swelling-Based Distributed Chemical Sensing with Standard Acrylate Coated Optical Fibers

“…Using the natural response of scattering processes (such as Brillouin, Raman, or Rayleigh scattering) to environmental changes, distinct distributed optical fiber sensors [3][4][5] have been exploited to monitor a variety of physical properties. The interrogating systems typically use optical frequency-domain reflectometry (OFDR) or optical time-domain reflectometry (OTDR) to obtain spatially resolved information about the measurand.…”