Wavefront sensing with an axicon

Direct wavefront reconstruction with the cone wavefront sensor using the inverse radon transform

The cone wavefront sensor consists of a cone (or axicon) placed at the focal plane of the imaging system, from which an annular intensity image is formed. Typically, the wavefront phase is estimated using inversion of an interaction matrix relating the intensity image to different aberration modes. In this paper, we show that the intensity image for the cone wavefront sensor is related to the radon transform of the wavefront phase. A reconstruction method using the inverse radon transform (filtered back-projection) is shown to be able to directly approximate the wavefront phase without the need for an interaction matrix for small wavefront aberrations.

Wavefront sensing using a liquid-filled photonic crystal fiber

A novel wavefront sensor based on a microstructural array of waveguides is proposed. The method is based on the sensitivity in light-coupling efficiency to the wavefront gradient present at the entrance aperture of each waveguide in an array, and hence the amount of incident light that couples is influenced by wavefront aberrations. The concept is illustrated with wavefront measurements that have been performed using a liquid-filled photonic crystal fiber (LF-PCF) working as a coherent fiber bundle. The pros and cons of the LF-PCF based sensor are discussed.

Digital pyramid wavefront sensor with tunable modulation

The pyramid wavefront sensor is known for its high sensitivity and dynamic range that can be tuned by mechanically altering its modulation amplitude. Here, a novel modulating digital scheme employing a reflecting phase only spatial light modulator is demonstrated. The use of the modulator allows an easy reconfigurable pyramid with digital control of the apex angle and modulation geometry without the need of any mechanically moving parts. Aberrations introduced by a 140-actuator deformable mirror were simultaneously sensed with the help of a commercial Hartmann-Shack wavefront sensor. The wavefronts reconstructed using the digital pyramid wavefront sensor matched very closely with those sensed by the Hartmann-Shack. It is noted that a tunable modulation is necessary to operate the wavefront sensor in the linear regime and to accurately sense aberrations. Through simulations, it is shown that the wavefront sensor can be extended to astronomical applications as well. This novel digital pyramid wavefront sensor has the potential to become an attractive option in both open and closed loop adaptive optics systems.

Tight focusing with a binary microaxicon

Using a near-field scanning microscope (NT-MDT) with a 100 nm aperture cantilever held 1 μm apart from a microaxicon of diameter 14 μm and period 800 nm, we measure a focal spot resulting from the illumination by a linearly polarized laser light of wavelength λ=532 nm, with its FWHM being equal to 0.58λ, and the depth of focus being 5.6λ. The rms deviation of the focal spot intensity from the calculated value is 6%. The focus intensity is five times larger than the maximal illumination beam intensity.

Generation of Bessel beams using a magnetic liquid deformable mirror

A deformable mirror made of a magnetic liquid has been used to produce conical surfaces with subwavelength (λ/4) accuracy. The surface profile of the liquid mirror is controlled by 91 small magnetic coils. The mirror exhibits a linear response with respect to the currents driving the coils, and it allows for real-time changes of its surface profile. The magnetic liquid deformable mirror has been used to produce reflected beams having a conical wavefront; the propagation of the reflected beams was verified to be consistent with that of Bessel beams in the near and far field. The large dynamic range of such a deformable mirror has made it possible to generate Bessel beams with a broad range of beam parameters.

Measuring the topological charge of optical vortices with an axicon

We analyzed the spatial spectrum of the diffraction intensity pattern of an ideal Bessel beam and found an implicit rule that the number of the bright rings in the spatial spectrum is equal to the topological charge of the Bessel beam. The radius of the bright and dark rings has some relation with the topological charge and can be determined accurately. It provides us with a new way for measuring the topological charge of an optical vortex through its diffraction intensity pattern after an axicon. The results of simulation coincide with the theory.