A photonic crystal fiber, PCF, assembly comprising a PCF and at least one ferrule structure, said PCF has a center axis and comprises a core region and a cladding region and a first fiber end section with a first fiber end, said ferrule structure has a center axis and is mounted to said first fiber end section, the PCF center axis at the first fiber end section and the ferrule structure center axis being parallel or coincident, said ferrule structure comprises an inner ferrule arrangement and an outer ferrule arrangement surrounding the first fiber end section, said inner ferrule arrangement comprises an inner ferrule front section proximally to said first fiber end and an inner ferrule rear section distally to said first fiber end, said inner ferrule front section and said inner ferrule rear section are not directly fixed to each other and the outer ferrule arrangement is configured to hold the inner ferrule front section and the inner ferrule rear section in position relative to each other, each of said inner ferrule sections (22, 23) have an inner diameter and in at least a length thereof fully surrounds the PCF, said inner ferrule rear section is anchored in an anchor length section to said first fiber end section and said inner ferrule front section supports said first fiber end section proximally to said first fiber end, wherein said first fiber end section has at least one mode stripper length section (27, 37) and at least one of said inner ferrule front section and said inner ferrule rear section has a carving (27a, 37a) exposing said at least one mode stripper length section (27, 37) of said PCF characterized in that said at least one mode stripper length section (27, 37) comprises a mode stripping high index material and/or a scattering layer applied in contact with the first fiber end section at said at least one mode stripper length section (27, 37) and/or said first fiber end section in said at least one mode stripper length section (27, 37) has a roughness Ra value of at least about 0.1 μm. one ferrule structure, said PCF has a center axis and comprises a core region and a cladding region and a first fiber end section with a first fiber end, said ferrule structure has a center axis and is mounted to said first fiber end section, the PCF center axis at the first fiber end section and the ferrule structure center axis being parallel or coincident, said ferrule structure comprises an inner ferrule arrangement and an outer ferrule arrangement surrounding the first fiber end section, said inner ferrule arrangement comprises an inner ferrule front section proximally to said first fiber end and an inner ferrule rear section distally to said first fiber end, said inner ferrule front section and said inner ferrule rear section are not directly fixed to each other and the outer ferrule arrangement is configured to hold the inner ferrule front section and the inner ferrule rear section in position relative to each other, each of said inner ferrule sections (22, 23) have an inner diameter and in at least a length thereof fully surrounds the PCF, said inner ferrule rear section is anchored in an anchor length section to said first fiber end section and said inner ferrule front section supports said first fiber end section proximally to said first fiber end, wherein said first fiber end section has at least one mode stripper length section (27, 37) and at least one of said inner ferrule front section and said inner ferrule rear section has a carving (27a, 37a) exposing said at least one mode stripper length section (27, 37) of said PCF characterized in that said at least one mode stripper length section (27, 37) comprises a mode stripping high index material and/or a scattering layer applied in contact with the first fiber end section at said at least one mode stripper length section (27, 37) and/or said first fiber end section in said at least one mode stripper length section (27, 37) has a roughness Ra value of at least about 0.1 μm.