## 2 Device Fabrication
### Fabrication of the Collimated VCSEL on PCB  
As low power consumption devices with a circular output beam shape and ease of coupling to optics and fibers, VCSEL chips offer unique advantages for fabricating compact optical sensors. We chose polarization-stable VCSEL chips (referred to as ULM850-A4-PL-S46 XZP Philips, CO., LTD) emitting at 850 nm. The natural full far-field beam divergence angle (1/e\({}^{2}\)) was measured to be about 14\({}^{\circ}\). To reduce this value, we integrated a polymer collimation microlens directly above the chip after the dicing step. It was composed of a microlens deposited on a pedestal with a high aspect ratio [20] and a typical height of at least 100 \(\upmu\)m. The micro-scale component dimensions (pedestal diameter and height, lens radius of curvature, and height) were optimized using the ZEMAX optical software. This was done to minimize laser beam divergence (\(<\)3\({}^{\circ}\)) and beam size (\(<\)60 \(\upmu\)m) at a distance of 1 mm (corresponding to the distance to the surface of the microfluidic channel). In the literature, the direct integration of microlens on VCSELs is only reported at the wafer-scale, mainly using standard photolithography [21; 22; 23; 24; 25; 26]. As these methods are not applicable to single chips, we developed an alternative method (as shown in Figure 2) to enable similar integration on single VCSELs chips of a very small size (200 \(\upmu\)m \(\times\) 200 \(\upmu\)m \(\times\) 150 \(\upmu\)m). It was based on three micro-optical fabrication steps: soft thermal printing (alternative to standard spin-coating), direct laser writing (alternative to standard UV-photolithography), and inkjet printing. First, the VCSEL chip was assembled on a printed circuit board (PCB) (Figure 2a). Three commercial dry thick photoresist films (DF-1050) were then transferred onto the VCSEL chip surface using soft-thermal-printing by means of nanoimprint equipment (Nanonex NX-2500). This equipment is suitable for handling brittle and small-sized samples (Figure 2a-d). This technique was previously developed by the authors to optimize the uniformity of thick resist layers on GaAs samples (a quarter of a wafer with a surface of a few cm\({}^{2}\)) [27]. As the nominal thickness of a single DF-1050 film is only 48 \(\upmu\)m, three films had to be successively printed on the chip to obtain a pedestal height greater than 100 \(\upmu\)m.  
Moreover, it is worth noting that due to the higher pressure applied during printing on the very small-sized VCSEL chip (compared to printing on a sample of a larger surface), the final film height measured after development was lower than the expected value (136 \(\upmu\)m instead of 144 \(\upmu\)m). Nevertheless, this height was found to be reproducible (standard deviation: \(\pm\)5 \(\upmu\)m). Moreover, this parameter is not critical as the lens curvature radius can be adjusted in the final step as a function of the effective pedestal height (see step (g)). Then, using direct UV laser writing (DILASE 750), high aspect-ratio cylindrical pedestals were photo-patterned in the printed films (typical diameter: 80 \(\upmu\)m). This technique provides the same alignment precision to standard photolithography (\(\pm\)1 \(\upmu\)m), leading to good pedestal centering with the VCSEL source located 136 \(\upmu\)m below (Figure 2e). After pedestal development and curing (Figure 2f), the inkjet printing technique was used to dispense commercial SUS-based liquid microdroplets of low viscosity (PriElex, MicroChem Co., Ltd.) on the  
Figure 1: Schematic of the miniaturized optical feedback interferometry (OFI) flowmetry sensor system. pedestal surface (Figure 2g). This technique leads to a self-centering of the polymer microlens obtained after UV-curing. The radius of the curvature of the lens was controlled precisely by the number of printed droplets, and it was previously calibrated using confocal optical microscopy. Finally, wire bonding could be performed to connect all the lensed-lasers (Figure 2h).