Accurate calibration of beam trajectories in scanning optical imaging systems.

We present a calibration method for finding the coordinates of points in the trajectory of the scanning beam in flying-spot imaging devices. Our method is based on laterally translating the field of view on the imaging object plane by introducing additional beam deflections. We show that laterally translating the field of view provides a series of images whose relative translations are equal to the distances between the points in the scanning pattern to be calibrated. We show how these distances are mapped to the coordinates of the trajectory points. As an example, we demonstrate the calibration of the scanning patterns in an optical system with two independent microelectromechanical system based scanners. Our method profits from a large collection of distance measurements to find the trajectory coordinates, thereby minimizing the effect of random sources of uncertainty in the positions of points in the scanning pattern. We have found that we are capable of finding the coordinates of points in the scanning patterns with accuracy greater than the optical resolution of the imaging system.

Morphological transformation of generalized spirally polarized beams by anisotropic media and its experimental characterization.

We present a generalization of the known spirally polarized beams (SPBs) which we will call generalized spirally polarized beams (GSPBs). We characterize in detail both theoretically and experimentally the streamline morphologies of the GSPBs and their transformation by arbitrary polarization optical systems described by complex Jones matrices. We find that the description of the passage of GSPBs through a polarization system is equivalent to the stability theory of autonomous systems of ordinary differential equations. While the streamlines of the GSPB exhibit a spiral geometry, the streamlines of the output field may exhibit spirals, saddles, nodes, ellipses, and stars as well. Using a novel experimental technique based on a Sagnac interferometer, we have been able to generate in the laboratory each one of the different cases of GSPBs and record their corresponding characteristic streamline morphologies.

On-demand tailored vector beams.

We introduce an effective optical system to produce optical beams with arbitrary, inhomogeneous polarization states. Using our system, we are capable of generating vector beams with discretionarily chosen transverse complex fields in a straightforward way. We generate several different instances of well-known vector beams and the less common spirally polarized vector beams, as well as a full Poincaré beam. We visually show the continual transition between azimuthally and radially polarized beams via a collection of spirally polarized beams. We experimentally determine the polarization states of the generated beams and quantitatively assess the performance of our system. We find that the measured polarization distributions accurately coincide with the intended input polarization distributions.

Refractive index measurement through image analysis with an optofluidic device.

We present a novel optical method, to our knowledge, to measure the refractive index of liquids by means of the images produced by an optofluidic lens. In addition we propose a new method to make optofluidic lenses.

Shaped nondiffracting beams.

We demonstrate that nondiffracting beams can be generated with an arbitrary transverse shape. In particular, we show that the azimuthal complex modulation of the angular spectra of Helmholtz-Gauss wave fields constitutes a degree of freedom sufficient to tailor nondiffracting beams with an intensity pattern of choice.

Optofluidic generation of Laguerre-Gaussian beams.

Laguerre-Gaussian (LG) beams have been extensively studied due to their unique structure, characterized by a phase singularity at the center of the beam. Common methods for generating such beams include the use of diffractive optical elements and spatial light modulators, which although offering excellent versatility, suffers from several drawbacks, including in many cases a low power damage threshold as well as complexity and expense. This paper presents a simple, low cost method for the generation of high-fidelity LG beams using rapid prototyping techniques. Our approach is based on a fluidic-hologram concept, whereby the properties of the LG beam can be finely controlled by varying the refractive-index of the fluid that flows through the hologram. This simple approach, while optimized here for LG beam generation, is also expected to find applications in the production of tunable fluidic optical trains.

Phase dynamics of continuous topological upconversion in vortex beams.

The vortex emergence process as an integer order Bessel field progresses continuously onto the contiguous higher order Bessel field is studied in detail. We assess the progressive migration of phase singularities and explain the predicted increase in fractional orbital angular momentum content of the beam in terms of this gradual process.

Direct detection of optical phase conjugation in a colloidal medium.

Degenerate four-wave mixing is demonstrated using an artificial Kerr medium and is evidenced by directly observing the phase conjugation of a vortex signal beam. The nonlinear susceptibility is produced by a refractive index grating created in a suspension of dielectric microscopic particles optically confined in the intensity grating distribution of two interfering laser beams.

Orbital angular momentum transfer in helical Mathieu beams.

We observe the transfer of orbital angular momentum to trapped particles in the azimuthally asymmetric transverse intensity distribution of a helical Mathieu beam. The average rotation rate, instantaneous angular displacement and terminal velocity of the trapped particles are measured experimentally. The angular dependence of these parameters is found to be in good agreement with the variation of the optical gradient force, the transfer of OAM from the wavefield and the Stokes drag force.

Orbital angular momentum transfer in helical Mathieu beams.

We observe the transfer of orbital angular momentum to trapped particles in the azimuthally asymmetric transverse intensity distribution of a helical Mathieu beam. The average rotation rate, instantaneous angular displacement and terminal velocity of the trapped particles are measured experimentally. The angular dependence of these parameters is found to be in good agreement with the variation of the optical gradient force, the transfer of OAM from the wavefield and the Stokes drag force.

Observation of parabolic nondiffracting optical fields.

We report the first experimental observation of parabolic non-diffracting beams, the fourth fundamental family of propagation-invariant optical fields of the Helmholtz equation. We generate the even and odd stationary parabolic beam and with them we are able to produce traveling parabolic beams. It is observed that these fields exhibit a number of unitary in-line vortices that do not interact on propagation. The experimental transverse patterns show an inherent parabolic structure in good agreement with the theoretical predictions. Our results exhibit a transverse energy flow of traveling beams never observed before.

Introduction: Optical trapping and applications feature issue.

The editors introduce the Biomedical Optics Express feature issue on "Optical Trapping and Applications." The works presented in the papers within this issue include were the focus of the third OTA Topical Meeting that was held on April 14-18, 2013, in Waikoloa, Hawaii.

Tunable optical tweezers for wavelength-dependent measurements.

Optical trapping forces depend on the difference between the trap wavelength and the extinction resonances of trapped particles. This leads to a wavelength-dependent trapping force, which should allow for the optimization of optical tweezers systems, simply by choosing the best trapping wavelength for a given application. Here we present an optical tweezer system with wavelength tunability, for the study of resonance effects. With this system, the optical trap stiffness is measured for single trapped particles that exhibit either single or multiple extinction resonances. We include discussions of wavelength-dependent effects, such as changes in temperature, and how to measure them.

Production of high-order Bessel beams with a Mach-Zehnder interferometer.

A new experimental setup is demonstrated to produce high-order Bessel beams. It is based on the field decomposition of the Bessel beam into its even and odd field components. The implementation is performed over the spectral components with a Mach-Zehnder interferometer that synthesizes the components into the desired Bessel beam. The main advantage of our setup is that the required annular transmittances have only discrete phase changes of pi radians instead of a continuous change of phase.