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Programmable Diffractive optics

Programmable diffractive optics may be defined as the area that concerns the manipulation of the properties of a spatially coherent beam such as a laser beam, in a programmable fashion, using the principle of holography. A simple but efficient way of controlling the properties such as amplitude, phase and polarisation of a laser beam is using a liquid crystal spatial light modulator (LCSLM). The LCSLM is a device comprising a 2D array of computer addressable pixels. Each pixel has a typical dimension of 10 µm and contains liquid crystal molecules. The birefringent properties of liquid crystal molecules can be changed on application of an external electric field. There are commercial LCSLM panels containing 1280x1024 or more number pixels which are addressed using the video signal from a PC. Thus with an appropriate video signal, the optical transmitivity of individual pixels can be set by the user in real-time.

A hologram is a record of the intereference pattern between a reference light beam and the object beam (the beam carrying the information of the object). The hologram has a spatial variation of transmissivity, due to which it will diffract a collimated beam incident on it. One of the diffracted beams carries the information of the object beam. If one knows the inteference pattern for a certain object beam, the right video signal may be sent to the LCSLM so that the device now acts as the corresponding hologram. Thus using the LCSLM one can generate arbitrary and reconfigurable object beam profiles. Details about the principle behind the working of such LCSLM assembly to generate an arbitrary wavefront are available here.

The movie in figure 1 shows two holograms (bottom row) to generate a helical wavefront (left one) and a plane wavefrtont (right one). The top row shows the intensity profiles of the respective focal spots. Ideally the plane wavefront will result in an Airy disc pattern while the helical wavefront will result in a doughnut shaped intesity pattern. However due to aberrations present in the beam the two profiles will be distorted. Frame 1 of the movie shows the distorted focal spots. It is seen that the distortions are more prominent on the helical beam. Using a LCSLM and CCD camera based setup one can detect the aberrations present by analysing the focal spot of the helical light beam. The holograms are then reconifigured to add the aberration compensating terms. Frame 2 onwards show the holograms and the corresponding focal spots as corrections are done in steps.

Figure 1: Movie showing aberration detection and correction using the sensitivity of helical light beam. (For more details see, Optics Express: 14 10377-10385.)