Hollow core optical fiber and a laser system

A hollow core photonic crystal fiber (PCF) comprising an outer cladding region and 7 hollow tubes surrounded by the outer cladding region, each of the hollow tubes having an average outer tube diameter d2 and an average inner tube diameter d1,

• wherein each of the hollow tubes is fused to the outer cladding region to form a ring defining an inner cladding region and a hollow core region surrounded by the inner cladding region,
• wherein the hollow tubes are not touching each other,
• wherein the hollow tubes are free of a nested sub-tube,
• wherein the hollow core region has a core diameter D of from about 10 μm to about 100 μm, and
• wherein d1/d2 is equal to or larger than about 0.8.