Low Cost Adaptive Optics

Background

Over recent years, adaptive optics has transformed astronomical imaging. An overview of this technique can be found here. The clickable image on the right has links to definitions of some of the components of a generic adaptive optic system. While these systems have achieved very good correction, they tend to be very expensive.

Recently, it has become possible to consider systems constructed out of much lower cost components than are used for astronomy. Such low cost systems inevitably cannot correct a wavefront as accurately as an astronomical system, but there are some situations where even low order correction can be very useful.

Sensor
Modifier
Control

 

 

 

Applications

A few of the most interesting non-astronomical applications of adaptive optics are discussed here.

Intracavity Laser Correction

Many lasers have their beam quality and total output power degraded due to optical aberrations that are present within the resonator. One possible solution to this problem is to have an adaptive mirror as one of the end (or one of the folding) mirrors inside the laser resonator, as shown below.

With this arrangement, the mode that builds up within the cavity depends strongly upon the precise nature of the intracavity aberrations and the current shape of the adaptive mirror. Experimental systems have been realised for a variety of types of laser, most notably CO2 and ND:YAG systems. The amount of improvement has been variable, and a completely reliable method of controlling the correction has yet to be conceived.

Extracavity Laser Beam Forming

There are several situations in which it is not appropriate to attempt intracavity correction. These are generally situations in which a very low order of correction (e.g. just focus control) are useful. The diagram on the right shows a system that has been realised experimentally: a laser machining head which is kept in focus using a single variable-curvature mirror.

As well as just focus control, a low-order system can correct for aberrations in a system that may vary with time. For example, many polygon scanner systems have a varying amount of astigmatism across the field, and the correction of this using adaptive optics is being investigated.

 

Work In Progress

At present, as well as considering complete systems in which it would be interesting to use low-cost adaptive optic systems, the construction of low-cost components is being undertaken. One of the most attractive types of wavefront corrector for low-systems is the bimorph mirror. This device consists of a thin layer of piezo-electric material glued to a layer of polished glass, as shown to the right.

When a voltage is applied across the piezo part, it expands laterally (in the plane) causing the whole structure to bend like a bimetallic strip. By applying different voltages to different areas of the piezo, one can control the shape. A typical layout of electrodes is shown on the left.

According to the theory, the deformation of the mirror solves Poisson's equation for the voltage distribution, i.e. , where Z(r) is the deflection, V(r) is the applied voltage and k is a constant. This enables the electrode pattern to be optimised for various applications.

A diagram of the bimorphs currently being made is shown on the left.

 

 

 

 

 

Applied Optics Group
Imperial College of Science, Technology and Medicine
Blackett Laboratory
Prince Consort Road
London SW7 2BZ
ENGLAND.