Polyelectrolyte multilayer (PEM) thin films, fabricated at nano-scale by Layer-by-Layer (LbL) self-assembly techniques, have diverse nanotechnology applications. Precise thickness measurement during layer buildup is crucial in controlling the thickness. In this work, we present a novel optical measurement technique for in-situ analysis of the PEM film build-up by utilizing Etched Fiber Bragg Gratings (EFBG)-based sensors to quantify the deposited thickness. PEM films were deposited over EFBG by alternative deposition of weak polyelectrolytes Poly(allylamine hydrochloride)(PAH) and Poly(acrylic acid)(PAA) with quantitative analysis of adsorption and desorption steps at varying pH conditions. Further, the desorption process was observed in detail at the sub-nanometer scale, with PAA exhibiting a linear desorption while PAH follows an exponential desorption. This validates the inter-diffusive behavior of the low molecular weight polyelectrolytes during the PEM buildup, not only during the adsorption process but also during the desorption process. Thus, EFBG could be utilized as a precision tool to extract fundamental information during individual PEM layer build-up, thereby fine-tuning the nano-scale architecture of multilayers.