When acquiring information using imaging techniques, one wants to achieve the best possible resolution in order to obtain valuable results. In the ideal scenario the size of the smallest resolved object features is limited by the wave properties of light, with a number of techniques developed to overcome this limit. In real life measurements the image quality is additionally deteriorated by the non-uniformities in refractive index along the beam propagation path, called aberrations. Adaptive Optics, introduced in 1950’s independently by Horace Babcock and Vladimir Linnik is a set of methods used to overcome the effect of aberrations and thus improve the image quality. Initially used in military applications for tracking satellites, it later found civilian applications as advances in computing made it practical.
Adaptive Optics use principles of feedback control to correct aberrations, and thus a measurement of controlled variable is required. The most commonly measured quantity is first derivative of wavefront provided by the Shack-Hartmann sensor. Another possibility is to measure wavefront Laplacian, i.e. curvature, using the sensor proposed by Roddier in 1988. With the correct use of this sensor the deformable mirror actuators are decoupled from each other, enabling fast and accurate control of high order systems and potentially analog feedback loop.
In this thesis, the Roddier curvature wavefront sensor is studied. A number of modifications in the original sensor geometry and operating principles are made in order to make the system more robust and easy to implement in laboratory environment. Performance of the modified curvature wavefront sensor is evaluated in both simulation environment and physical setup.
Finally, application of the curvature sensor to measure wind velocity is demonstrated.

For wavefront reconstruction, often the combination of a Shack-Hartmann sensor and a reconstruction method utilizing the Cartesian derivatives of Zernike polynomials (the least-squares method) is used, which is known to introduce cross-talk. In Janssen (2014) a new wavefront reconstruction algorithm is introduced to be used with the Shack-Hartmann sensor that does not present cross-talk. To our knowledge, this method has never been demonstrated. In the current research, Janssen's method and the conventional least-squares method are compared on a modified Michelson interferometer setup with a spatial light modulator to first remove the aberrations of the complete system, and subsequently introduce specific aberrations for which the Zernike coefficients are reconstructed using both reconstruction methods. It is found that both methods work equally well with optimal fitting powers. When fitting fewer than the optimal amount of fitting powers, it was found that Janssen's method accurately recovered Zernike coeficient amn when n + 1 powers are fit, while the conventional method achieves the same accuracy only at n + 2 fitting powers, especially when aberrations introducing cross-talk are present. When more than the optimal amount Zernike powers are used, it is seen that Janssen's method presents aliasing at lower fitting powers. At the optimal fitting power, the RMS landscape for both methods is shown to be similar, and therefore they are equally susceptible to errors due to mis- estimation of the center position and radius of the beam on the ShackHartmann
sensor. Lastly, it is shown that theoretically, Janssen's method is at least 3.5 times slower than the conventional least-squares method due to the use of complex valued Zernike polynomials.