An optical arrangement, method and measurement system are disclosed. The arrangement includes a first input to receive a first beam from a frequency swept laser, a second input to receive a second beam from a fixed frequency pump laser source. A non-linear optical artifact receives and intermodulates the first and second beams to generate a third beam, the third being an inverted copy of the first beam mirrored relative to the fixed frequency of the pump laser source. A selective combining element outputs the first and third beams. The non-linear artifact or one or both of the lasers is selected or configured such that the optical frequency separation of the first and second beams satisfies the coherence length condition of the non-linear artifact.