In-situ optical detection of mesoscale components in glass microfluidic channel with monolithic waveguide

Optical waveguides used as a local light source along a fluidic channel have proven to be an effective approach to detecting cells in the field of flow-cytometry. One challenge, however, has been a simple integration of optical waveguides with the fluidic channel. We employ the use of femtosecond laser-writing process to pattern a waveguide in the bulk of a fused-silica glass substrate housing a fluidic channel. We demonstrate an in-situ scheme for detecting sub-millimeter components based on such a monolithically fabricated device. By illuminating the waveguide and collecting the light signal past the channel, we detect opaque and transparent components between 300 - 500 µm in size, as each moves along the channel. Both an opaque square chip and a transparent bead attentuate the signal by more than 95% primarily due to reflection and refraction respectively. The signature of a transparent bead additionally shows attenuated peaks which we attribute to normal incidence of light from the waveguide. The projected sizes of the parts are determined with less than 1% uncertainty. We conclude that the femtosecond laser produced waveguides in fused-silica glass are a viable option for the detection of certain kinds of sub-millimeter components. This approach holds the prospects of fabricating complex three-dimensional networks of waveguides monolithically.