Common frequency suppression method for fiber specklegram perimeter sensors

Abstract

In this work, a frequency based processing scheme has been employed to improve the sensitivity of Fiber Specklegram Sensors (FSSs) applied to perimeter fences. Based on 1.8 km of multimode fiber, two laser sources with different spectral properties have been employed to feed the optical fiber. These two laser sources provoke sensing systems with very different SNRs to which the common frequency suppression method has been applied. The final results suggest enough improvement on SNR to reduce the quality of the laser source on perimeter sensors, also decreasing the final cost.

Investigation on sensitivity enhancement for optical fiber speckle sensors

“…As discussed previously, the values of α and β were set to 1 by assuming the window completely covered the edge cylindrical modes. Based on the number of pixels occupied by the speckle field and the number of cylindrical modes estimated according to the formula given in [21], each pixel was estimated to correspond to 1 to 2 projected cylindrical modes on the image plane. In this case, we used the intensity difference between neighboring pixels as the approximated values for intensity difference (Δ (i)(i+1) ) between the neighboring modes.…”

“…In MMFs, the LP Ɩ,ν fiber modes can degenerate to cylindrical modes, which are also called the principal mode groups (PMG) with a new subscript m=Ɩ+2ν [21][22][23]. The transverse wavenumbers and propagation constants within the same PMG are similar to each other in value.…”

“…The transverse wavenumbers and propagation constants within the same PMG are similar to each other in value. Thus, for a step-index MMF, each cylindrical mode should correspond to a spatial ring in the MMF output image plane [21]. When the MMF is perturbed, the mode coupling occurs, and the following coupled equation [21,[24][25][26][27] is used to describe the mode coupling:…”

“…(1) shows the total power variation of the mth mode through mode coupling from the power of all the other modes. Given that the mode coupling is dominated between the neighboring modes [21,[27][28][29][30], Eq. (1) can be simplified as:…”

Investigation on refractive index sensing characteristics based on multimode fiber specklegram

“…That's why the detection and processing of fiber specklegram can be used in sensing. Therefore, the change of RI will make the propagation constant of each mode vary [26], resulting in phase deviation. In addition, mode coupling also plays a vital role in the modulation of the speckle field.…”