The invention relates to an optical assembly (100) comprising a first optical fiber (101) propagating coherent light in a predetermined direction (P) into an input end (110) of the optical assembly (100), said optical fiber having a core and a cladding; a heat sink (111) surrounding the optical fiber (101) at the input end (110); and a lens (120) arranged after the heat sink (111) in the propagating direction (P). The optical assembly (100) further comprises a filter (130) arranged after the lens (120), wherein the filter (130) has a reflective surface (131) arranged to transmit light having one or more desired wavelengths and to reflect one or more undesired wavelengths back through the lens (120). The invention further relates to a method for separating desired and undesired wavelengths.