Abstract: A beam coupler assembly for a fiber laser is disclosed. The assembly includes a housing having a sidewall with an interior surface, an exterior surface, a first end and a second end. A first seal extending from the interior surface of the tubular housing and dividing the housing into a first section and a second section is also provided. The first section and second section are environmentally isolated from one another. However, the first seal is substantially optically neutral. An input collimator unit received within the first end of the sidewall of the housing and into the first section and is releasably coupled thereto. An output collimator unit received within the second end of the sidewall of the housing and into the second section and is also releasably coupled thereto.