Vortex phase deterioration common-path interferometry

Question

1. How does OAM beam transmission study fluid media?

2. What is the average thickness of MoS2 nanoflakes?

3. What is the role of the binary amplitude fork grating in the experimental setup?

4. How does milk concentration affect speckle density in optical vortex beams?

Accepted Answer

Studying OAM beam transmission provides valuable information about the constituents of fluid media. By measuring the phase structure of optical fields using interferometers, researchers can analyze the interaction between OAM beams and particles in turbid media. This interaction is influenced by the orbital angular momentum carried by Laguerre Gaussian (LG) beams, which causes them to interact more with particles compared to optical fields without OAM. The phase maps extracted from the interference of beams with and without vortex phase structure can be used to study the relative interaction of beams with the fluid media, such as milk samples.

Accepted Answer

The average thickness of MoS2 nanoflakes is 153.5 nm with a standard deviation of 23 nm. These nanoflakes were prepared using a liquid phase exfoliation technique. The dispersion of MoS2 nanoflakes in a 3% w/w Polyvinylpyrrolidone (PVP) polymer solution was achieved to ensure a stable dispersion. The method of MoS2 nanofluid preparation is detailed in article [18]. UV-Vis spectra of the MoS2 nanofluid showed broadband absorption in the visible region, specifically between 400nm and 700nm. This information is crucial for researchers studying material synthesis and characterization, as it provides insights into the optical properties of MoS2 nanoflakes and their potential applications in various fields.

Accepted Answer

The binary amplitude fork grating in the experimental setup generates diffraction patterns, including a central 0th order diffraction (B0) with a Gaussian-like distribution and 1st order diffraction (B1) on either side. The 1st order diffraction pattern is a vortex beam with a topological charge corresponding to the fork pattern of the binary grating. The interference initiated by the grating is responsible for the topological charge, which is calculated as the number of fork prongs minus one. The diffraction patterns are crucial for the interference and measurement of the optical fields B0 and B1 in the experiment.

Accepted Answer

As the concentration of milk increases, there is higher scattering of the optical field, resulting in a proportional increase in speckle density. This is evident from Figure 5, which shows the intensity distribution and interferogram of optical vortices after transmission through milk samples of different concentrations. The interferograms recorded at a higher distance (32cm) after transmission through milk samples contain primarily two types of photons: ballistic photons with high mean free path and slightly scattered snake photons with lower mean free path. The phase information of these photons is encoded in the interferograms, providing unique insights into the interaction of milk particles with the optical vortex beams. This relationship between milk concentration and speckle density is crucial for understanding the scattering properties of milk and its impact on optical vortex beams.

Accepted Answer

Fourier transform is used to extract wrapped phase maps from single interferograms. It involves applying the Fourier transform method to interferograms, as shown in Fig. 5 [27], [28]. The Fast Fourier Transform (FFT) spectrum of interferograms is cropped to filter out 0th order Fourier spectra and include only higher order frequency spectra. The wrapped phase map is then extracted from this cropped FFT spectrum, as shown in Fig. 7. The frequency spectra become less sharp for optical vortices transmitted through milk samples of higher concentration, indicating higher scattering and distortion of phase structure. To unwrap the phase maps, an algorithm called the iterative transport of intensity equation (TIE) method using finite difference (FD) and FFT is used, as shown in Fig. 8. Unwrapped phase maps indicate that the quantity of vortex phase deterioration depends on the TC it carries, with higher TC undergoing more scattering and phase structure deterioration.

Accepted Answer

Phase deterioration is calculated by subtracting the phase map of the optical field after transmission through the sample from the phase map of the optical field without sample transmission. The beam centers in both phase maps should be aligned before subtraction. The result is the total phase deterioration (). The root mean square (RMS) azimuthal phase deterioration along the radial direction of the beam is then calculated using a formula that considers a circle of radius 'r' with its center over the vortex beam axis on the phase map. The phase values along the circumference of this circle are used to compute the phase deterioration at a distance 'r' from the center of the vortex beam. This analysis helps understand how phase deterioration varies across the beam radially and how it is affected by factors such as concentration of milk and topological charge of the beam.

Accepted Answer

In this work, the thermo-optic refraction interferometer (TORI) successfully demonstrates phase shifting interferometry by utilizing thermal lensing phenomena. TORI involves interfering two optical fields, where one is a vortex beam. By controlling the pump beam power, phase shifting interferometry can be achieved. Phase maps extracted from the interferograms reveal that the phase structure of higher topological charge (TC) beams experiences higher deterioration. The RMS azimuthal phase deterioration indicates that beams undergo varying levels of deterioration radially, depending on their TC. Consequently, higher topological charge beams (>2) are more sensitive to phase deterioration and can be used to probe very low concentrations of scatterers. This method opens up possibilities for a new kind of common path interferometry, involving relative phase changes between vortex and non-vortex beams.

Vortex phase deterioration common-path interferometry

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Most frequently asked questions

1. How does OAM beam transmission study fluid media?

2. What is the average thickness of MoS2 nanoflakes?

3. What is the role of the binary amplitude fork grating in the experimental setup?

4. How does milk concentration affect speckle density in optical vortex beams?

5. How is Fourier transform used in phase map extraction?

6. How is phase deterioration calculated in vortex phase analysis?

7. How does TORI demonstrate phase shifting interferometry?

References

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Vortex beam propagation through atmospheric turbulence and topological charge conservation

Spatial self-phase modulation of a laser beam propagating through liquids with self-induced natural convection flow

Deep transmission of Laguerre-Gaussian vortex beams through turbid scattering media.

Encoding information using Laguerre Gaussian modes over free space turbulence media.

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