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&Prime) or less, and more preferably 0.46 mm (0.018&Prime) or less, so that there is negligible effect on valve movement, transvalvular pressure gradient and flow during measurement.
