Improving Interferometry Instrumentation by Mixing Stereoscopy for 2π Ambiguity Solving

Improving Interferometry Instrumentation by Mixing Stereoscopy for 2π Ambiguity Solving

Avi Karsenty, Yaron Lichtenstadt, Sagi Naeim, Yoel Arieli
DOI: 10.4018/IJMTIE.2017070104
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Phase measurements obtained by high-coherence interferometry are restricted by the 2π ambiguity to height differences smaller than λ/2. A further restriction considers linear and nonlinear aberrations evolving in most interferometric systems due to the CCD-type array detectors. The authors present a new method to overcome the 2π ambiguity in interferometry when using a stereoscopic approach. In this method, a reconstructed wavefront reflected from an object was propagated into two different angles to obtain two different images of the object. These two different images were subsequently processed by stereo algorithms to resolve the 2π ambiguity. Such a method of wavefront propagation may enable several applications such as focusing and resolving the 2π ambiguity, as described in the article.
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1. Introduction

1.1. Coupling Techniques

In the past decades, the world of measurements and instrumentation is rapidly growing, adapting itself to existing needs, and enabling the application domains, such as nanotechnology, space and remote sensing, chemistry and applied physics, metrology and more, to progress in parallel. Combining measurement techniques is a smart way to improve the specifications of such instrumentation. One domain, which is constantly progressing, is the interferometry technique when mixing it with other components and methods. The interferometer’s architecture is usually dependent on the expected target to be measured, and on the final application. For example, improved accuracy and resolution could be obtained when mixing a Laser Diode (Norgia, 2007) to the interferometer system (Donati, 1996). On the other hand, amplitude fading phenomenon could be solved by self-mixing interferometry with piezo actuators, phase-sensing loop and liquid crystal attenuator (Norgia and Donati, 2003). More recently, displacement error phenomenon could be significantly decreased in displacement measuring interferometry by mixing FPGA Hardware-In-the-Loop (HIL) simulation (Wang, 2016). Another example is the coupling of laser Doppler sensors with interferometry enabling the study of the dynamic behaviour of a rotor (Dreier, 2013). Using existing proven techniques, and with the purpose of improving accuracy, we present a new approach to solve the 2π ambiguity in interferometry by coupling stereoscopy.

1.2. Interferometry’s Achilles Heel

The well-known 2π ambiguity concern in interferometry has been addressed and solved in various ways over the last two decades. Many techniques have been developed to resolve or prevent the 2π ambiguities in the interferometric measurements, such as multiple wavelength measurements (Lofdahl, 2001), phase unwrapping method (Yang, 2002), low-coherence interferometric differential phase measurements (Hitzenberger, 2001), phase-crossing technique (Yang, 2002), multiwavelength digital holography (Gass, 2003), terahertz phase imaging method with multiwavelengths (Zhang, 2006), and continuous-wave terahertz interferometry with multiwavelength phase unwrapping (Wang, 2010).

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