The application of precision interferometers is generally restricted to expensive and smooth high-quality surfaces. Here, we offer a route to ultimate miniaturization of interferometer by integrating beam splitter, reference mirror and light collector into a single optical element, an interference lens (iLens), which produces stable high-contrast fringes from in situ surface of paper, wood, plastic, rubber, unpolished metal, human skin, etc. The iLens splits the incoming beam to generate two beams, viz. reference beam and the object beam, at the front surface of the iLens. The iLens then collects back-scattered light from the sample and project towards the screen where it combines with the reference beam to produce high-contrast fringes. A self-referencing real-time precision of sub-20 picometer (∼λ/30000) is demonstrated with simple intensity detection under ambient conditions. The principle of iLens interferometry has been exploited to build a variety of compact devices, such as a paper-based optical pico-balance, with a weighing accuracy of sub-100 pg, having 1000 times higher sensitivity and speed when compared with a high-end seven-digit electronic balance. Furthermore, we used cloth, paper, polymer-films to readily construct broadband acoustic sensors possessing matched or higher sensitivity when compared with piezo and electromagnetic sensors. Our work opens path for a new class of ultra-affordable yet ultra-precise frugal photonic devices for diverse applications, such as material processing, high-resolution imaging, etc., in science and education.
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