Self-accelerating beams—shape-preserving bending beams—are attracting great interest, offering applications in many areas such as particle micromanipulation, microscopy, induction of plasma channels, surface plasmons, laser machining, nonlinear frequency conversion and electron beams. Most of these applications involve light-matter interactions, hence their propagation range is limited by absorption. We propose loss-proof accelerating beams that overcome linear and nonlinear losses. These beams, as analytic solutions of Maxwell’s equations with losses, propagate in absorbing media while maintaining their peak intensity. While the power such beams carry decays during propagation, the peak intensity and the structure of their main lobe region are maintained over large distances. We use these beams for manipulation of particles in fluids, steering the particles to steeper angles than ever demonstrated. Such beams offer many additional applications, such as loss-proof self-bending plasmons. In transparent media these beams show exponential intensity growth, which facilitates other novel applications in micromanipulation and ignition of nonlinear processes. Self-accelerating beams are attractive for light-matter interaction applications but their propagation has been limited by absorption. Here, Schley et al.demonstrate self-healing in shape-maintaining, accelerating beams where the central peak intensity is preserved despite losses and apply these beams to particle manipulation.

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Loss-proof self-accelerating beams and their use in non-paraxial manipulation of particles’ trajectories

We show that it is feasible to design and manufacture a refractive logarithmic axicon that generates a quasi-diffraction-free/Bessel beam with nearly constant beam size and intensity over a predetermined range. The novel optical element was characterized with both coherent and incoherent light, and good correspondence with the predicted behavior of the intensity distribution and spot size was found. The energy flow was also found to be nearly constant over most of the designed range. Logarithmic axicons may find applications in situations where large depth of field and uniform axial intensity/energy distributions are important.

Characterization of a refractive logarithmic axicon.

We demonstrate a class of nondiffracting beams, called frozen waves, with a central spot that can be made to maintain a predefined intensity profile while propagating in an absorbing fluid. Frozen waves are composed of Bessel beams with different transverse and longitudinal wavenumbers, and are generated using a programmable spatial light modulator. The attenuation-resistant frozen waves demonstrated here address the problem of propagation losses in absorbing media. This development can be beneficial for many applications in particle micro-manipulation, data communications, remote sensing, and imaging.

Generating attenuation-resistant frozen waves in absorbing fluid.

In 3D, diffraction-free or Bessel beams are well known and have found applications in diverse fields. An analog in 2D, or pseudonondiffracting (PND) beams, is a nontrivial problem, and existing methods suffer from deficiencies. For example, Airy beams are not highly localized, some PND beams have significant side lobes, and a cosine beam has to be truncated by a very narrow aperture thus discarding most of the energy. We show, both theoretically and experimentally, that it is possible to generate a quasi-nondiffracting 2D light beam in a simple and efficient fashion. This is achieved by placing a mask consisting of a pair of double slits on a cylindrical lens. The applications include light sheet microscopy/optical sectioning and particle manipulation.

Toward the optical “magic carpet”: reducing the divergence of a light sheet below the diffraction limit

Very compact focal spot in the near-field of the fractional axicon

We report the first observation (to our best knowledge) of a constant intensity, quasi-Bessel/nondiffracting beam in an absorbing medium generated by a novel optical element, “exicon,” or exponential intensity axicon. Such absorption-compensated and diffraction-resistant beams can find applications in illumination, remote sensing, free-space communications, imaging in biological tissues, nonlinear optics, and other situations where absorption and diffraction hinder light propagation.

The taming of absorption: generating a constant intensity beam in a lossy medium

Phase engineering of an axicon's phase transmission function results in an exponential growth of an on-axis intensity of a diffraction-free or Bessel beam. We show that in a lossy medium obeying Beer's absorption law/exponential intensity decrease, a beam focused by such an optical element propagates with nearly constant intensity while preserving the quasi-diffraction-free property. Absorption-free beams may find applications in illumination, atmospheric propagation, biomedicine/light diffusion in live tissues, and other situations where light absorption hinders beam propagation.

Absorption-free beam generated by a phase-engineered optical element.

This paper summarizes the factors that limit the performance of moving-magnet galvo scanners driven by
closed-loop digital servo amplifiers: torsional resonances, drifts, nonlinearities, feedback noise and friction.
Then it describes a detailed Simulink® simulator that takes into account these factors and can be used to
automatically tune the controller for best results with given galvo type and trajectory patterns. It allows for rapid testing
of different control schemes, for instance combined position/velocity PID loops and displays the corresponding output in
terms of torque, angular position and feedback sensor signal. The tool is configurable and can either use a dynamical
state-space model of galvo's open-loop response, or can import the experimentally measured frequency domain transfer
function.
Next a drive signal digital pre-filtering technique is discussed. By performing a real-time Fourier analysis of the
raw command signal it can be pre-warped to minimize all harmonics around the torsional resonances while boosting
other non-resonant high frequencies. The optimized waveform results in much smaller overshoot and better settling time.
Similar performance gain cannot be extracted from the servo controller alone.

Theodore Mirtchev

Papers

The taming of absorption: generating a constant intensity beam in a lossy medium

Absorption-free beam generated by a phase-engineered optical element.

Optimizing the feedback control of Galvo scanners for laser manufacturing systems