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Focus tunable lenses

Pacer has introduced a range of focus-tunable lenses based on elastic polymers, which 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.

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), 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).

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 20mm aperture. The largest available focus-tunable lens, with an aperture of 55mm, has a focal range of +60mm to infinity.

The electrically tunable lenses operate at 0-5V 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 2500nm and high damage thresholds (2.2kW/cm2 for continuous wave (cw) operation and 10J/cm2 for pulsed operation with ns-pulse duration). Furthermore, the lenses are polarisation maintaining. Another big advantage is the short response time, which lies in the range of a few milliseconds.

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 minimised, however, by choosing an optimal membrane design.

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