Apparatus (100) is provided comprising an optical micro-sensor (10) for directly measuring a fluid temperature and flow by thermoconvection, which suitable for medical applications, e.g. using minimally-invasive cardiovascular techniques. A multi-sensor apparatus (100) may take the form of a micro-catheter or steerable guidewire, equipped with a plurality of miniaturized optical sensors (10) arranged along a length of the distal end (101), each coupled via optical fibers (11) to a proximal end (102) comprising an input/output connector (112) to the control system (110), without the need for electrical connections. This enables direct measurement of blood flow, temperature and/or blood pressure simultaneously at several locations within the blood vessels or the heart, including transvalvular measurements. Preferably, the distal end portion has a diameter of 0.89 mm (0.035″) or less, and more preferably 0.46 mm (0.018″) or less, so that there is negligible effect on valve movement, transvalvular pressure gradient and flow during measurement.