A light absorption sensor is based on phenomena of light absorption between in two flow stream layers with different
refractive index in liquid/liquid (L<sup>2</sup>) waveguide in this letter. The diffusion between the core and the cladding forms the
tapered L<sup>2</sup> waveguide sensor. The taper can be controlled by the flow rates of the core and the cladding layers. The
absorption sensor is fabricated using PDMS onto a microfluidic chip, which chip size is 3.5cm × 3cm and the absorption
length is 2cm. Two different aqueous dye solutions such as rhodamin 6G and methylene blue are measured and
approved by the experimental results of the absorption sensor. The limitation of detection is 0.5ng/mL for rhodamin 6G
and 0.16ng/mL for methylene blue. The L<sup>2</sup> waveguide absorption sensor has high potential usage for the biological,
chemical and medical measurement applications.