A Continuously Variable Optofluidic Aperture

Breck Hitz

Optofluidics technology brings to photonics the advantages of easily fabricated and inexpensive components, of miniaturization and of great flexibility. Recently, scientists at National University of Singapore reported fabricating an optofluidic version of one of the most common and useful optical devices: the variable aperture.

Figure 1. Initially, the deformable membrane across the top of the air pressure chamber is flat, and light incident on the device from the bottom is blocked by the dye, shown here as red (a). As air is forced into the air pressure chamber, the membrane expands into the dye chamber (b). At higher pressure, the membrane contacts the rigid plate, and the light incident from the bottom is transmitted through the aperture (c). In this sequence, the aperture has gone from closed in a and b to partially open in c (d). Images reprinted with permission of Optics Letters.

Although adjustable apertures have been in optics laboratories for decades, the traditional devices have been awkward and complex. One approach is the venerable aperture wheel, a disc with a series of various-size apertures near its edge. Although serviceable, these wheels are necessarily large and cumbersome, and they do not provide a continuously adjustable aperture. Another approach, especially in photography, is a complex mechanical system of metal blades that are adjusted to approximate a circular aperture.

Figure 2. The aperture diameter could be varied from 0 to 6.35 mm.

The optofluidic device, on the other hand, is simple, compact and continuously adjustable. Its aperture is a perfect circle. The device consists of a chamber filled with a light-absorbing dye. Conceptually, the aperture is created when the dye is forced aside by a deformable membrane that expands into the dye chamber (Figure 1).

The scientists fabricated the body of their variable aperture from polydimethylsiloxane (PDMS), a common optofluidic material that has high transmission from the near-ultraviolet to the near-infrared. They fabricated the 40-μm-thick membrane by spin-coating a thin layer of PDMS onto a silicon wafer and bonding it to the body of the device with an oxygen plasma.

Figure 3. The intensity profile of light transmitted through the aperture was recorded with a CCD camera.

Experimentally, they illuminated the aperture with an even distribution of white light and focused an image of the aperture onto a CCD camera. They varied the aperture’s diameter from 0 to 6.35 mm (Figure 2) and calculated its tuning speed, from closed to fully open, to be in the tens of microseconds. The intensity profile at various aperture sizes exhibited a relatively flat top and steep sides (Figure 3).

Optics Letters, March 5, 2008, pp. 548-550.