Fast and compact focusing with shape-changing polymer lenses

Working principle of Optotune’s lens

Figure 1: Working principle of Optotune’s manually focus tunable lens ML-20-35.

August 2, 2012

Mark Blum, Optotune: co-founder & Chief Operating Officer.

Focus-tunable lenses based on elastic polymers open up new possibilities in adaptive optics. Thanks to these novel shape-changing lenses optical systems can be made more compact and reliable with response times of only a few milliseconds.

Less mechanics with focus tunable lenses
In traditional optical systems, changing the focus usually means moving one or several lenses along the optical axis. This requires mechanical actuators limiting not only compactness and response times but also robustness and lifetime. Today, focus tunable lenses offer a new degree of freedom: the variable curvature of the lens. A change in lens radius of several micrometers can have the same optical effect as moving the entire lens several centimeters. Optical systems can thus be designed more compact, oftentimes with less lenses and usually with less or no translational movement. This means that there is no more need for expensive mechanical actuators. Less movement also leads to a more robust design, which can be completely closed so that no dust can enter. Furthermore, the materials employed are all lighter than glass, saving overall weight. Less movement and weight also means less power consumption and that the response time of systems with tunable lenses can be very fast, in the order of milliseconds. The example of an auto-focus system below shows that the optical design can even be improved using shape-changing lenses.

Shape-changing lenses based on elastic polymers
A Swiss company called Optotune has developed a shape-changing lens that is based on elastic polymer materials. The focal length of the lens can be controlled mechanically or electrically. The core element of the lenses consists of a thin membrane, which builds the interface between two chambers, each of which can be filled with materials of different refractive indices. In the simplest case one of the chambers contains a liquid and the other contains air. The pressure difference between the two chambers determines the deflection of the membrane and with that the radius of the lens. The pressure difference can be controlled in many ways: mechanically (e.g. by using a thread ring to push a ring-shaped actuator down onto the membrane, see Figure 1), electromechanically (by using voice coils, piezo or stepper motors to exert the mechanical force) or pneumatically (by pumping liquid into or out of the chamber).

Large tuning ranges and short response times
The presented polymer lens technology has several advantages over alternative approaches of focus-tunable lenses. First and foremost is the large focal tuning range that can be achieved even with large apertures. Optotune’s ML-20-35-VIS-HR, for example, reaches optical powers from -25 to +25 diopters at a 20-mm aperture. The largest available focus-tunable lens, with an aperture of 55 mm, has a focal range of +60 mm to infinity.

The electrically tunable lenses operate at 0-5 V and can be easily driven with off-the-shelf current controllers. The optical materials used offer a large transmission over the range of 240 to 2500 nm and high damage thresholds (2.2 kW/cm2 for continuous wave (cw) operation and 10 J/cm2 for pulsed operation with ns-pulse duration). Furthermore, the lenses are polarization maintaining. Another big advantage is the short response time, which lies in the range of a few milliseconds (see Figure 2).

Response time of Optotune’s electrically focus tunable lenses
Figure 2: Response time of Optotune’s electrically focus tunable lenses.

A downside of this type of tunable lens is the influence of gravity, which can induce a coma when the lens is used in its upright position (optical axis horizontal). This effect can be minimized, however, by choosing an optimal membrane design.

Application examples

As mentioned above, focusing on objects at different working distances usually requires the movement of lenses. Figure 3 shows a simple example with a single lens. In 3a) the system is focused to infinity and achieves a good optical quality. The illustration shows the modulation transfer function (MTF) for the center field. If the object distance is reduced, in this example to 100 mm, the system must be adjusted to re-focus. In 3b) this is achieved by moving the lens away from the sensor, whereas in 3c) the radius of the lens is slightly increased. As can be seen, the optical quality of the system with a shape-changing lens is significantly better than in case of the moved lens.

Comparison of refocusing by translation versus tuning of a lens
Figure 3: Comparison of refocusing by translation versus tuning of a lens.

Another wide field of applications for focus-tunable lenses is illumination. Especially for LED-based lighting systems large focal tuning ranges and big apertures are required. Shape-changing condenser lenses are a perfect match for such conditions. Figure 4 shows an example of a spotlight that can adjust its beam angle from “flood” (45° FWHM) to “spot” (10° FWHM). The design includes an LED, secondary optics (a total-internal-reflection lens), a shape-changing condenser lens and a protective cover glass. The LED and the secondary optics together define the maximum beam angle of the spotlight, which is achieved in the flat state of the tunable lens. By tuning the condenser lens to a convex shape the light is focused to a smaller spot size.

Apart from imaging and illumination systems, focus-tunable lenses prove to be very useful for a number of other applications. Ophthalmic devices, for example, can benefit from automatic compensation of visual defects (including cylindrical correction). Thanks to the high damage thresholds the lenses are also suitable for focus control in laser processing systems. Furthermore, tunable lens technology has large potential for bio-medical applications in the form of spectacles or even intra-ocular lenses.

Adaptive illumination example with a shape-changing polymer lenss
Figure 4: Adaptive illumination example with a shape-changing polymer lens (Optotune Lumilens ML-25-50).

Fore more information:

Optotune®, Inc, develops and manufactures adaptive optical components based on elastic polymers. Optotune’s focus-tunable lenses and laser speckle reducers offer new solutions for several industries including mobile phone cameras, machine vision, laser processing, professional lighting and laser projection.

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