Device for collecting fluorescent light emitted by particles in a medium

A device for collecting fluorescent light emitted by particles in a medium, comprising:

• a substrate having a chamber for holding a medium including particles being capable of emitting fluorescent light,
• a first waveguide arranged at the substrate and extending into the chamber,
• wherein the first waveguide is arranged to receive excitation light from a light source and guide the excitation light through the chamber along a first direction of the first waveguide, and to collect fluorescent light emitted by the particles following an excitation of the particles by the excitation light, and
• a coupler arranged at the first waveguide,
• wherein the coupler is a directional coupler and includes a second waveguide arranged to output collected fluorescent light at one of its ends, and
• wherein the second waveguide is arranged in relation to the first waveguide such that collected fluorescent light travelling in a direction opposite to the first direction is coupled out from the first waveguide directly into the second waveguide and to output collected fluorescent light at a first end, and excitation light, coupled into the second waveguide from the first waveguide, at a second end.