Using the optical feedback interferometry (OFI) technique, we demonstrated a miniaturized and compact sensor system based on a dedicated optical source for flowmetry at the micro-scale. In the system, polymer microlenses were integrated directly on a VCSEL (vertical-cavity surface-emitting laser) chip and the microfluidic channel chip surface using polymer-based micro-fabrication technologies. In particular, at a post-process stage, we integrated a collimation lens on a VCSEL chip of small dimensions (200 µm × 200 µm × 150 µm). This process was enabled by the soft-printing of dry thick resist films and through direct laser writing technology. We performed flow rate measurements using this new compact system, with a conventional bulk glass lens configuration for system performance evaluation. A maximum 33 dB signal-to-noise ratio was achieved from this novel ultra-compact system. To our knowledge, this is the highest signal level achieved by existing OFI based flowmetry sensors.

In this work, two-photon polymerization three-dimensional laser writing is used to integrate a microlens on the surface of a single mode polarization-stable verticalcavity surface-emitting laser (VCSEL) to be used as a current-driven tunable source in a compact optical guided-wave gas sensor. The writing conditions are optimized to enable on-demand room temperature and single-step fabrication at a post-mounting stage. We show that a writing time of 5 min is sufficient to fabricate a microlens that efficiently reduces the VCSEL beam divergence, without significant change on its emitted power or polarization stability. The lens addition reduces the spectral available range at high injection currents. A two-dimensional optical modeling of the gain characteristics is used to explain this effect and a new transverse design is proposed to avoid this issue.