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.
In this paper, we present an optical setup which estimates the radius of curvature of spherical surfaces with the aid of Fizeau interferometry. While the use of Fizeau interferometry to achieve these measurements is wellunderstood in prior art and commercially deployed, we propose a variant of Fizeau interferometry for the same measurements. Our proposed method deploys electronically-controlled tunable focus lenses to perform cats-eye and confocal beam scans with a motion-free scanning system as opposed to a motion-based laser scanning and repositioning system. Eliminating motion from a surface scanning system mitigates system breakdowns related to bulk mechanical motion of optical elements. It also promises to reduce the system cost as well as bulkiness of such interferometric systems. We show the proposed system improvement via the use of a standard tunable focus lens on a legacy commercial Zygo surface curvature measuring system. We demonstrate the operation of the proposed system with experimental data and results using lenses and curved mirrors as samples. For all samples, we compare our measurements from the actively-tunable Fizeau interferometer to baseline measurements from the same original Zygo system using its own zoom lens. The experimental results show an excellent agreement between measurements from the motion-based legacy commercial system and the actively-tunable bulk motionfree system. Future work would focus on characterizing sample surface aberrations by subtracting wavefront aberrations imparted by the tunable focus lens piece.
In this paper, we explore the collimation quality of collimated Gaussian beams generated via the deployment of engineered diffusers. Various aspects of beam propagation through engineered diffusers are explored using results from carefully designed experiments. Raw Gaussian beam is incident on a sample engineered diffuser. The beam is then collimated and propagated over several meters to clearly estimate beam divergence for different test cases. These test cases include raw and focused beams incident on engineered diffusers, the effect on collimation with the use of speckle reducers, and the evolution of the collimated beam wavefront during propagation. To measure and document the spatial coherence properties of beam propagation and its spatial coherence properties after propagating through the engineered diffuser, we measure the beam profile with knife-edge measurement, CCD imaging, and Shack-Hartmann sensor-based wavefront measurements along the beam propagation path. We do so for all different type of beam conditioning before incidence at the engineered diffuser – this includes analyzing the effects of a speckle reducer in the system. We present detailed experimental results and parameters of the propagating collimated beam in the paper. We hope that this paper will lay foundations for our understanding of using engineered diffusers for short distance free-space optical links using a beam collimation approach which is several times optically efficient than a pinhole based spatial filtering-based beam collimation approach.
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