3.4 Issue no. 4

the path and band nature of AS and St signals, the above relation does not hold. The relation gets deteriorated at every stage in the path. For example, the cathode radiant sensitivity of a PMT for AS wavelength (1018 nm) and St wavelength (1109 nm) is 0.95 mA/W and 0.2mA/W, respectively, which causes St current to be approximately 5 times less compared to AS current. Nonideal performance of beam splitters and optical filters also does not support the above ideal relation. As a result, the cumulative effect of various components makes experimental values of R to be

Direct use of experimental values of R in Eq. (1) will yield highly erroneous and unacceptable temperature profile (T). Hence, Eq. (1) needs to be modified to obtain correct values. Modification is done by referencing the experimentally obtained ratio values with respect to the ratio value at some known temperature of calibra-

The second issue is the nonidentical fiber attenuation along the fiber length for Raman AS and St signals due to difference in their wavelengths [9, 11]. In a typical system using 1064 nm excitation laser, the difference between two wavelengths is 90 nm. The lower optical wavelength signal (AS) experiences higher attenuation in comparison to higher optical wavelength signal (St) while traveling in sensing optical fiber. This attenuation difference results in an unwanted downward slope in ratio (R) profile and finally in unknown temperature (T) profile with respect to fiber length. It may be noted that downward slope in ratio (R) profile causes additional errors in unknown temperature (T) profile of fiber and should be

While de-noising Raman AS and St signals for better signal-to-noise ratio (SNR), conventional finite impulse response/infinite impulse response (FIR/IIR)-based

different from the theoretical one.

Applications of Optical Fibers for Sensing

3.2 Issue no. 2

Figure 5.

corrected.

60

3.3 Issue no. 3

tion zone which is chosen from sensing fiber itself.

The anti-Stokes Raman signal profile at various temperatures.

The amplitude of AS and St signals varies with time due to slow variations/drifts in laser power and laser-fiber coupling. Also, the temperature of calibration zone itself may change unless it is controlled by a dedicated setup. Therefore, any previously stored reference values of AS and St signals and calibration zone temperature can no longer be used as a reference for temperature measurement at a later stage.
