Adjusting the focus position of fiber collimators

The output of a single mode fiber collimator shall be a plane wave, which propagates as Gaussian beam with increasing diameter and varying curvature. One Rayleigh length behind the beam waist the curvature reaches its maximum. For much larger distances the beam diameter increases approximately linearly, and the radius of curvature roughly equals the distance. During the adjustment of the optics the axial fiber position is changed until the curvature of the exit wave vanishes. If the beam is observed at a large distance, its diameter is minimal for the best focus position. For small collimators with short focal length the Rayleigh length is only a few centimeters, and the minimum beam diameter on a distant screen is a good measure for the optimum focus. For larger collimators the Rayleigh length becomes too large for typical lab environments, and additional test optics is necessary. A test setup for an IR fiber collimator is presented and analysed with ABCD matrices. Some astigmatism is introduced to simplify the adjustment. The purely reflective test system is calibrated with visible light and requires only an imaging camera but no complex wavefront sensor or interferometer optics.