High optical throughput interferometry with engineered diffusers

The use of interferometers is ubiquitous in optical measurements due the high measurement sensitivity with applications ranging from simple quality inspection applications to advanced applications in astronomy and small signal measurements. One of the defining features of any interferometer is the spatial and temporal coherence properties of the source. When temporal coherence is attained by use of narrowband coherent sources and possibly frequency stabilization systems, the spatial coherence of the interferometer becomes critical. This is the case for most of the commonly used short path length interferometers that use standard laser sources. Various applications require a different degree of acceptable spatial coherence. The use of free-space and fiber optic spatial filtering systems to attain an acceptable coherence is the most common practice. The trade-off is generally between the degree of spatial coherence and the optical loss in getting to the required coherence. While pinhole or fiber based spatial filtering systems afford excellent spatial coherence which comes at a significant loss (greater than 90% loss in most cases) in optical power. To compensate for this optical loss, this forces the use of higher power laser sources or detectors with higher sensitivity for interferometric applications. In this paper, we show that for small benchtop interferometers where path lengths of interferometer arms are less than a meter long, we can use engineered diffusers to implement an interferometer with adequate spatial coherence where the optical power efficiency – unlike pinhole-based systems – is in the order of 90%. We demonstrate, for the first time, a working diffuser-based Michelson interferometer setup exhibiting both tilt (straight) and bull-eye fringes.