1. Introduction
A relatively cheap chemical material called chlorine is usually added to water flowing in long delivery pipelines and storing in fishery tanks in order to destroy the cell structure of organisms and remove some plant effluents [1,2]. One concern in the chlorination process is that although most organisms have been destroyed, there is some chlorine left in the water that can greatly affect our health as well as the growth rate of aquatic baby animals such as baby shrimps and baby fishes. Hence, the concentration of chlorine in water is a very important parameter used for quality control purpose in the chlorination process. Previously, several approaches were proposed and experimentally demonstrated for determining the amount of chlorine. These include the use of nanoparticles via their resonance scattering effect [3] and an evanescent optical field analysis via interferometric waveguide sensors [4,5]. However, these techniques require rigorous analysis, costly optical components, and very precise waveguide fabrication processes. Another optical approach is the differential absorption spectroscopy in which an expensive optical spectrometer is utilized for analyzing