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Liquid Deformable Mirror Promises Fast Wavefront Correction

Photonics Spectra
Jul 2006
Device has millisecond response time and multimicron surface movement.

Breck Hitz

Deformable mirrors offer the possibility of compensating for atmospheric optical distortion, an important function both for astronomy and for directed-energy military applications. Conventional deformable mirrors, which rely on the bending and twisting of a thin reflective membrane, have several disadvantages — including low spatial resolution and high cost — that might be overcome with a liquid mirror. Recently, researchers at Delft University of Technology in the Netherlands demonstrated a deformable liquid mirror that holds promise for dynamic wavefront correction.


Figure 1. The meniscus of the liquid that overfills the microchannel array forms the reflective surface of the deformable mirror. To enhance the reflectivity of the liquid surface, the researchers envision floating a reflective membrane atop the liquid (a). The inner surface of each microchannel is coated with several layers, including an electrode (b). Images ©OSA.

Conceptually, the mirror is formed by the surface of a thin liquid layer sitting atop a microchannel array (Figure 1a). The liquid surface is deformed when liquid in the microchannels is forced upward or downward. The force is applied by electrocapillary pressure — a change in surface tension between the wall of a microchannel and the liquid within the channel — induced by an applied voltage. The liquid flows toward the region with lower surface tension.

The inner surfaces of the microchannels are coated with multiple layers, including a conducting electrode, a dielectric layer and a hydrophobic material (Figure 1b). By applying a voltage between the electrode coated onto the wall of the capillary and the electrolyte in the microchannel, the scientists induce the electrocapillary pressure that produces a deformation of the liquid surface atop the microchannel array.

Figure 2. The 64 × 64-element microchannel array is the silver square at the center of the image.

Because most liquids have intrinsically low reflectivity, the investigators envision floating a reflective membrane — or perhaps a dye-coated liquid — on top of the thin, deformable liquid layer. In their feasibility demonstration, however, they omitted this and simply measured the deformation of the bare liquid surface.

The microchannel system in the demonstration consisted of a 64 × 64 array of 350-μm-diameter capillaries in a hexagonal structure (Figure 2). The researchers addressed the channels electrically in groups of eight, resulting in eight-channel columns of movable liquid. In the next stage of their work, they plan to address the microchannels individually.

Figure 3. The voltage applied to the microchannels was a square wave (a). The detector response was proportional to the displacement of the liquid surface (b). The inset in the lower graph is an expanded view of the boxed region.

A fundamental problem with a liquid mirror is the generation of surface waves, little ripples that would destroy the mirror’s utility. The investigators considered this problem and calculated that these surface waves would be damped out in less than a millisecond with the appropriate liquid viscosity and thus would not pose a problem to the mirror’s operation.

In their demonstration, they filled the microchannel array with two immiscible fluids: an aqueous electrolyte topped with a viscous dielectric. The dielectric overfilled the channels and formed the thin liquid layer atop the array. The nature of this deformable mirror limited it to a vertical orientation.

The researchers measured both the response time of the deformable surface and the magnitude of its deformation. For the response time measurement, they applied a 196-V square-wave pulse that lasted 100 ms and observed the displacement of the liquid (Figure 3). The initial spike, corresponding to a displacement of ~1 mm, had a rise time of approximately 7 ms, which they interpreted to mean that the mirror could reach an operating frequency of 500 Hz or higher.

Figure 4. Interference fringes were created by light reflected from the deformed surface of the liquid (a). The researchers constructed a 3-D plot of the surface from the interferometric data (b).

They measured the precise physical displacement of the liquid surface using a Mach-Zehnder interferometer and interpreted the fringe pattern with a plot of the deformed surface (Figure 4). The displacement was smaller in this case than in the temporal measurements because different liquids were used in the capillaries.

Optics Letters, June 1, 2006, pp. 1717-1719.

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