Adaptive planar optics are software-reconfigurable: Page 2 of 3

September 10, 2019 //By Julien Happich
planar optics
Rather than modulating an optical path through mechanical deformation, like it is the case with commonly used spatial light modulators (SLMs), be it liquid-crystal or micromirror-based SLMs, researchers from the Institute of Photonic Sciences (ICFO, Barcelona) have devised a novel type of planar optics whose refractive index can be reconfigured locally and progressively.

The authors validated their approach experimentally, designing a number of SmartLenses able to create a wide range of wavefront modifications, including ring refocusing of a collimated laser beam and simultaneous refocusing across multiple planes (to bring in focus several coloured objects located at various distances from an imaging system). But they report that such SmartLenses can also be optimized to generate complex functions based on Zernike polynomials for dynamic beam-forming applications.

Rendering of a 5x5 array of spiral heaters.
Image credit: ICFO/M. Montagut.

Among the results reported in the paper, the authors note that the focal length decreases faster with the applied voltage V for smaller heaters, as the radius of curvature of the generated lens is shorter. One example, is a 10μm-diameter spiral delivering an f-number of 11 at 2.1V (from an infinite focal length at 0V). Here, the accuracy and precision of the focal length are only limited by the applied voltage accuracy and precision. The researchers tested spiral heaters as small as 10μm in diameter offering response times of around 0.5 milliseconds. In this research, the use of gold tracks as conductors slightly reduced the SmartLens’ transparency (down to 60%), but the authors anticipate this could be improved to over 90% using transparent conductive materials such as ITO. Anti-reflection coatings could further improve their design.

As well as being low-cost compared to today’s SLMs , the SmartLens is polarization-insensitive, and because it operates in a refractive rather than diffractive regime, it can also be used over a broad wavelength range without incurring chromatic aberrations or image distortion. Unlike deformable mirrors, SmartLens elements can also be used indifferently in transmission or reflection modes (with the appropriate optimized design).

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