Self-referencing collimation metrology using path-tunable single-beam interferometry

Optical beams of different coherence lengths are used in many optical precision metrological instruments. Knowledge about the collimation properties of these beams is of immense importance for achieving accurate quantitative measurements. Most of the precise collimation testing techniques are based on interferometry. To generate high contrast interference fringes, the optical path difference between the interfering beams must be less than the coherence length of the test beam. Most of the interferometric setups reported for collimation testing are of fixed path length, making it cumbersome to test beams of different coherence lengths. We report a collimation measurement technique based on path-tunable single-beam interferometry using just two axially aligned holographic optical elements (HOEs). This simple lateral shearing system could test the collimation properties of beams having different coherence lengths. The path length between the interfering beams, according to the coherence length of the test beam, is varied by simply translating one of the HOEs along the direction of beam propagation. This system also generates an inherent self-reference fringe pattern for visual collimation inspection. The system is described and validated by testing the collimation properties of three beams having different coherence lengths.