Apparatus for optically isolating a light beam from a laser, which light beam has an average power greater than 1 W and a pre-determined beam quality defined by an M 2 value between 1 and 25, and which apparatus comprises an optical input for the light beam, an input lens, an optical isolator, an output connector, a reference plane, and a first lens arrangement, wherein: • the optical isolator is selected to isolate the light beam; • the input lens is positioned between the optical input and the optical isolator, and is arranged to focus the light beam through the optical isolator to the output connector such that the beam diameter of the light beam varies by more than five percent within the optical isolator; • the first lens arrangement is positioned to receive the light beam from the optical isolator; • the optical isolator is a polarization insensitive isolator; • the apparatus is configured such that the light beam is not collimated in the optical isolator; • the input lens is selected based upon the pre-determined beam quality; • the input lens is configured to focus the light beam to a light beam diameter within the optical isolator of less than 0.5mm; • the output connector is formed with a connector interface configured such to be connectable to a collimator which is capable of being referenced to the reference plane; • the first lens arrangement is selected based upon the pre-determined beam quality of the light beam and is configured to provide an output light beam having a pre-determined divergence that is independent of the beam quality; • the first lens arrangement is positioned to form a beam waist at a pre-determined distance from the reference plane; and • the connector interface, the pre-determined divergence that is independent of the beam quality, and the pre-determined distance allow a family of collimators to be used in the apparatus, each collimator providing a different pre-determined beam diameter that is independent of the beam quality when the collimator is fitted to the output connector. • the optical isolator is selected to isolate the light beam; • the input lens is positioned between the optical input and the optical isolator, and is arranged to focus the light beam through the optical isolator to the output connector such that the beam diameter of the light beam varies by more than five percent within the optical isolator; • the first lens arrangement is positioned to receive the light beam from the optical isolator; • the optical isolator is a polarization insensitive isolator; • the apparatus is configured such that the light beam is not collimated in the optical isolator; • the input lens is selected based upon the pre-determined beam quality; • the input lens is configured to focus the light beam to a light beam diameter within the optical isolator of less than 0.5mm; • the output connector is formed with a connector interface configured such to be connectable to a collimator which is capable of being referenced to the reference plane; • the first lens arrangement is selected based upon the pre-determined beam quality of the light beam and is configured to provide an output light beam having a pre-determined divergence that is independent of the beam quality; • the first lens arrangement is positioned to form a beam waist at a pre-determined distance from the reference plane; and • the connector interface, the pre-determined divergence that is independent of the beam quality, and the pre-determined distance allow a family of collimators to be used in the apparatus, each collimator providing a different pre-determined beam diameter that is independent of the beam quality when the collimator is fitted to the output connector.