Can we create a tunable asphere with a liquid lens?
Electrowetting liquid lenses are a convenient tool to realise tunable optics without any moving parts. We focus on the question whether it is possible to extend the functionality from a spherical lens to more general optical surfaces by using the very same hardware, but operating it in a smarter way.
Since the earliest telescope has been patented by Hans Lippershey in the Netherlands in 1608, optical designers always combined lenses of different optical power to manufacture complex optical instruments. Until recently the optical properties of these glass/plastic lenses had been fixed, however a new technology is arising: the liquid lens.
A single optical component, that can change its optical properties by changing its shape. The most common technique is based on electrowetting. Manipulating the shape of a liquid droplet by changing its surface tension electrically is called electrowetting.
(insert this youtube video from Philips here: https://www.youtube.com/watch?v=JAZ3pJkgreI )
A lens with a tunable focal length is created. A lens that operates independently of gravity effects. A lens that can withstand many more operating cycles than a classical lens system, where lenses have to be moved mechanically to achieve the same effect. While liquid lenses are busy conquering markets like bar code scanners, ophthalmology, and autofocusing systems, research focusses on increasing the flexibility of the produced optical surfaces.
We study a new way to create optical surfaces by using the disadvantages of the used liquids. They cannot follow infinitely fast changes of the control voltage. Just like on any other liquid surface, waves are created, waves that have been considered to be the source of aberrations. While the industrial manufactures have been trying to suppress these waves, we try to stimulate them to create new controlled surface shapes.
The natural wave mode on a cylindrical liquid lens is a Bessel function. Bessel functions have one awesome property, their orthogonality. In other words, if you can create and scale Bessel waves on the liquid lens surface, you can create any surface shape by simply adding them up. And this is what this is all about: Studying whether this works and where the limitations of this technique are.
The Bessel modes have already been identified here:
Matthias Strauch, Yifeng Shao, Florian Bociort, and H. Paul Urbach, Study of surface modes on a vibrating electrowetting liquid lens, Applied Physics Letters, 111, 2017, https://doi.org/10.1063/1.4999562
And there is more to come: zernike polynomials, axicons, top hats, ptychographic measurements with the liquid lens, …
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