Performance enhancement in a modal wavefront sensor and its application to free space optical communication

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Wavefront is a fundamental feature of a light beam. It can be defined as the surface constructed by the loci of points having the same phase or the points that correspond to equal optical path lengths starting from the source. When a coherent light beam such as a laser beam passes through certain medium or gets reflected by certain surface, the wavefront of the light beam gets modified in such a way that the modification is characteristic of the transmitting medium or the reflecting surface. Thus a plane wavefront after being transmitted by the medium or after being reflected by the surface may get distorted. Such distortions which are nothing but deviations in the phase profile from an ideal plane wavefront are termed as wavefront aberrations. Wavefront aberration if introduced may result in loss of information carried by the beam and it requires removal of the aberration to extract the original information. On the other hand the aberrations introduced by a reflecting surface can provide information regarding the profile of the surface roughness. The setup to measure the wavefront of a light beam is known as wavefront sensor. There is an important type of wavefront sensor called the modal wavefront sensor that can provide a direct estimation of the strength of the orthogonal aberration modes present in a beam of light. One of the most widespread used modal wavefront sensors is the bias beam based modal wavefront sensor which is based on estimating the strength of an aberration mode by measuring the difference in the central focal intensities of two bias beams. The two bias beams are two copies of the same incident beam with the addition of a certain aberration to the phase profile in the case of one and the subtraction of the same aberration from the phase profile in the case of the other.
Supervisor: Bosanta Ranjan Boruah