A needle comprising a shaft member having a first C-shaped cross-section of the elongate shaft and a second elongate member having a C-shaped cross-section. Said first shaft member and said second shaft member adapted to be detachably connected to one another. This is achieved by overlapping the ends of the respective first and second shaft elements. Within the channel formed by the probe shaft element can receive the light, separated from each other by the two shaft member, the shaft member may be separated from the optical probe.