Bessel light beam for a surgical laser focusing telescope-a novel approach.

As the demand for CO[Formula: see text] laser surgeries continues to grow, the quality of their main instrument, the laser micromanipulator, becomes increasingly important. However, in many surgery systems, a large ratio of the laser power is wasted due to the reflection from the mirror of a telescopic system, like a Cassegrain telescope, back to the laser side, which not only decreases the system's efficiency but can also damage the system itself. In this article, we introduce a new design of the micromanipulator telescope for CO[Formula: see text] laser surgery, which employs a Bessel beam to improve the system efficiency. As in the propagation of a Bessel beam, the power of the light beam can be transferred from the center to a ring shape, the whole power reflected from the first mirror can reach the second mirror and no power goes back to the second mirror hole. The micromanipulator telescope design and optimization are carried out using Zemax Optics Studio, and the integration of the Bessel beam into the system is implemented using MATLAB. Our simulation results show that by applying the appropriate Bessel beam, the system efficiency can reach more than 96%, and the normalized peak irradiance can increase by 40 to 73% for various working distances. In addition to increasing the system efficiency and normalized peak irradiance, resulting in a sharper surgical blade, the use of the Bessel beam enhances the depth of focus, making the system less sensitive to depth misalignment.

Measuring source width and transverse coherence length using Fresnel diffraction from a phase step.

Measurement of the source size and specifying its effect on the spatial coherence of propagating light are important for characterizing distant sources such as stars, and imaging with partially coherent light. The common method for measuring spatial coherence is Young's two-pinhole experiment. For characterizing spatial coherence along a line, one needs to change the location of the pinholes over a large number of pairs of points. But it requires many measurements, which takes significant time. In this paper, we use Fresnel diffraction from a step in reflection to measure the source width and transverse coherence length. It is shown theoretically and experimentally that these quantities are determined by specifying the location of minimum visibility on the diffraction pattern. We utilize a sodium vapor lamp with a variable slit in front of it as an extended one-dimensional incoherent light source. The measurements are made through recording only one diffraction pattern formed by the step. The study is applicable in 2D, and one can characterize weak starlight using highly sensitive equipment.

Diffractometry-based vortex beams fractional topological charge measurement.

In this Letter, we investigate the Fresnel diffraction of vortex beams from a phase plate and propose a novel (to the best of our knowledge) method to determine the fractional part of the topological charge of vortex beams. When a vortex beam with a fractional topological charge illuminates the edge region of a transparent plate, the visibility of the diffraction pattern on two sides of the beam is different. Rotation of the phase plate changes the visibility on the left and right sides of the beam, periodically. By measuring three consecutive angles of the minimum visibilities, the fractional part of the topological charge is obtained. The proposed method is verified experimentally and is shown to be independent of the phase plate and vortex beam parameters. The precision of the method is obtained better than 0.01.

Fresnel diffraction due to phase gradient singularity.

In this Letter, a singularity in the phase gradient is introduced as a new origin of the diffraction from phase objects. The continuity in the amplitude and phase of the wave and the singularity in the phase gradient are assumed to describe this phenomenon. Fresnel diffraction of Fresnel double mirror and biprism are studied as practical examples, and the diffracted intensity distribution is calculated. We show that the intensity of the diffracted field varies with an almost constant period along the average propagation direction. Using this value and the fringe spacing of the interference pattern on a plane normal to the average propagation direction, the angle between two interfering beams and the wavelength of the incident light could be simultaneously derived. The diffraction pattern of the Fresnel double mirror and Fresnel biprism were obtained experimentally, which were in good agreement with the theoretical results.

Optical refractometry based on Fresnel diffraction from a phase wedge.

A method that utilizes the Fresnel diffraction of light from the phase step formed by a transparent wedge is introduced for measuring the refractive indices of transparent solids, liquids, and solutions. It is shown that, as a transparent wedge of small apex angle is illuminated perpendicular to its surface by a monochromatic parallel beam of light, the Fresnel fringes, caused by abrupt change in refractive index at the wedge lateral boundary, are formed on a screen held perpendicular to the beam propagation direction. The visibility of the fringes varies periodically between zero and 1 in the direction normal to the wedge apex. For a known or measured apex angle, the wedge refractive index is obtained by measuring the period length by a CCD. To measure the refractive index of a transparent liquid or solution, the wedge is installed in a transparent rectangle cell containing the sample. Then, the cell is illuminated perpendicularly and the visibility period is measured. By using modest optics, one can measure the refractive index at a relative uncertainty level of 10(-5). There is no limitation on the refractive index range. The method can be applied easily with no mechanical manipulation. The measuring apparatus can be very compact with low mechanical and optical noises.

Optical diffractometry.

Interference of light has numerous metrological applications because the optical path difference (OPD) can be varied at will between the interfering waves in the interferometers. We show how one can desirably change the optical path difference in diffraction. This leads to many novel and interesting metrological applications including high-precision measurements of displacement, phase change, refractive index profile, temperature gradient, diffusion coefficient, and coherence parameters, to name only a few. The subject fundamentally differs from interferometry in the sense that in the latter the measurement criterion is the change in intensity or fringe location, while in the former the criterion is the change in the visibility of fringes with an already known intensity profile. The visibility can vary from zero to one as the OPD changes by a half-wave. Therefore, measurements with the accuracy of a few nanometers are quite feasible. Also, the possibility of changing the OPD in diffraction allows us to use Fresnel diffraction in Fourier spectrometry, to enhance or suppress diffracted fields, and to build phase singularities that have many novel and useful applications.