Modular optofluidic platform based on femtosecond-laser-written interferometers and PDMS microchannels

This work presents an integrated optomicrofluidic platform combining Mach-Zehnder interferometers (MZIs) fabricated by femtosecond laser direct writing in fused silica with a detachable polydimethylsiloxane (PDMS) microfluidic system. A key feature of this platform is its fully detachable and reusable assembly, which avoids irreversible bonding processes and enables rapid reconfiguration of the microfluidic component without compromising optical alignment or interferometric stability. Unbalanced MZIs were implemented with the reference arm buried and the sensing arm inscribed at different depths to control evanescent field interaction. Optical characterization with liquid calibration samples revealed that shallower depths enhance sensitivity at the expense of higher optical losses. Spectral sensitivities of 216.36 nm/RIU and 1154 nm/RIU were measured for sensing arm depths of 5.7 μm (near-surface) and 4.7 μm (partially exposed), respectively. The partially exposed interferometer was integrated with a 500 μm-square microchannel. COMSOL simulations were used to investigate flow rates and determine optimal conditions for liquid circulation. Deionized water and urea served as analytical blank and model analyte. Flow tests with 10 mM urea showed that low flow rates below 1 μL/min produced measurable fringe shifts up to 450 pm, while higher rates suppressed the spectral response. Under continuous flow, urea (10−100mM) produced amplitude variations with a sensitivity of 0.0123 dB/mM and a detection limit of 18.76 mM. These findings demonstrate a transition from refractive index-based fringe shift detection at low Reynolds number flow to amplitude-based detection under continuous flow. The hybrid MZI-PDMS integration offers a modular, low-cost strategy for real-time biochemical sensing.