Better understanding of particle-particle and particle-fluid interactions requires accurate 3D measurements of particle distributions and motions. We introduce the application of in-line digital holographic microscopy as a viable tool for measuring distributions of dense micrometer (3.2 microm) and submicrometer (0.75 microm) particles in a liquid solution with large depths of 1-10 mm. By recording a magnified hologram, we obtain a depth of field of approximately 1000 times the object diameter and a reduced depth of focus of approximately 10 particle diameters, both representing substantial improvements compared to a conventional microscope and in-line holography. Quantitative information on depth of field, depth of focus, and axial resolution is provided. We demonstrate that digital holographic microscopy can resolve the locations of several thousand particles and can measure their motions and trajectories using cinematographic holography. A sample trajectory and detailed morphological information of a free-swimming copepod nauplius are presented.

Digital holographic microscope for measuring three-dimensional particle distributions and motions

The quantification of three-dimensional (3D) flow structures and particle dynamics is crucial for unveiling complex interactions in turbulent flows. This review summarizes recent advances in volumetric particle detection and 3D flow velocimetry involving holography. We introduce the fundamental principle of holography and discuss the debilitating depth-of-focus problem, along with methods that have been implemented to circumvent it. The focus of this review is on recent advances in the development of in-line digital holography in general, and digital holographic microscopy in particular. A mathematical background for the numerical reconstruction of digital holograms is followed by a summary of recently introduced 3D particle tracking and velocity measurement techniques. The review concludes with sample applications, including 3D velocity measurements that fully resolve the flow in the inner part of a turbulent boundary layer, the diffusion of oil droplets in high–Reynolds number turbulence, and predator-p...

Applications of Holography in Fluid Mechanics and Particle Dynamics

The concept of numerical parametric lenses (NPL) is introduced to achieve wavefront reconstruction in digital holography. It is shown that operations usually performed by optical components and described in ray geometrical optics, such as image shifting, magnification, and especially complete aberration compensation (phase aberrations and image distortion), can be mimicked by numerical computation of a NPL. Furthermore, we demonstrate that automatic one-dimensional or two-dimensional fitting procedures allow adjustment of the NPL parameters as expressed in terms of standard or Zernike polynomial coefficients. These coefficients can provide a quantitative evaluation of the aberrations generated by the specimen. Demonstration is given of the reconstruction of the topology of a microlens.

Numerical parametric lens for shifting, magnification, and complete aberration compensation in digital holographic microscopy

Morphology and barrier properties of solvent cast composites of thermoplastic biopolymers and purified cellulose fibers

The recent results on the new laser material – Bi-doped glasses and optical fibers are reviewed. First, luminescence properties of various Bi-doped glasses are discussed. At last the results of investigations of Bi-doped fiber lasers covering a wavelength range of 1150 – 1550 nm are presented.

https://iopscience.iop.org/article/10.1002/lapl.200910025/pdf

Bi-doped fiber lasers

We present a digital holographic microscope that permits one to image polarization state. This technique results from the coupling of digital holographic microscopy and polarization digital holography. The interference between two orthogonally polarized reference waves and the wave transmitted by a microscopic sample, magnified by a microscope objective, is recorded on a CCD camera. The off-axis geometry permits one to reconstruct separately from this single hologram two wavefronts that are used to image the object-wave Jones vector. We applied this technique to image the birefringence of a bent fiber. To evaluate the precision of the phase-difference measurement, the birefringence induced by internal stress in an optical fiber is measured and compared to the birefringence profile captured by a standard method, which had been developed to obtain high-resolution birefringence profiles of optical fibers.

Polarization microscopy by use of digital holography: application to optical- fiber birefringence measurements

Long-period gratings (LPGs) have been inscribed by femtosecond laser radiation into fiber optics with Ge-doped and pure silica cores. Grating parameters of 16-dB rejection and 0.3-dB insertion loss are obtained for a Ge-doped fiber core. Numerical calculation of the transmission characteristics yielded a good correlation between the measured and synthetic data showing that an induced refractive index change up to 3/spl times/10/sup -4/ can be achieved. Inscription of an LPG in pure silica fiber core is demonstrated and thought to be first achieved in this fiber type. Greater laser intensity is required in pure silica compared with Ge-doped cores to produce a refractive index change of similar magnitude.

Inscription of long-period gratings in pure silica and Germano-silicate fiber cores by femtosecond laser irradiation

The tomographic measurement of the residual stress profile in femtosecond-laser irradiated standard SMF-28 germanium-doped telecommunication fiber is demonstrated. The fiber is irradiated with weakly focused pulses to realize long-period fiber gratings. In the irradiated grating regions, an asymmetrical increase in axial core stress up to 6.2 kg/mm2 is found. The increase in stress is attributed to a densification of the irradiated glass matrix. The stress-induced anisotropic index distribution is calculated and related to the absolute index change in the irradiated regions.

/pdf/tomographic-measurement-of-femtosecond-laser-induced-stress-1el61wsu4d.pdf

Tomographic measurement of femtosecond-laser induced stress changes in optical fibers

Characteristics of one-cascade (106 ⇒ 124 µ m) and two-cascade (106 ⇒ 124 ⇒ 148 µ m) phos- phorus-doped fiber Raman lasers are studied The cavities of both lasers are formed by Bragg gratings written directly in the active fiber Double-cladding Yb 3+ and Nd 3+ ion fiber lasers with λ ~ 106 µ m were employed for pumping These lasers were pumped, in their turn, by diode laser arrays with λ ~ 098 µ m (Yb) and λ ~ 081 µ m (Nd) The efficiency of the one-cascade laser with an unprecedentedly short length of 50 m was as high as 65% with 48-W pump radiation applied to the input of the laser The maximum efficiency of the two-cascade Raman fiber laser was 40% Gratings directly written in the active fiber reduced concentrated losses in laser cavities, which allowed the laser length to be decreased and the lasing threshold power of Raman fiber lasers to be low- ered

/pdf/highly-efficient-one-and-two-cascade-raman-lasers-based-on-16qrkpdu7z.pdf

Highly efficient one- and two-cascade Raman lasers based on phosphosilicate fibers

Long-period gratings were written using arc discharges in pristine nitrogen doped fibres. Comparison of the resulting gratings spectra show that the resonant positions of gratings in fibres pre-annealed at 1050°C for 30 min are shifted towards shorter wavelengths and their coupling strength is considerably higher. The refractive index and residual stress profiles were measured before and after annealing, as well as the two-dimensional stress profiles inside the grating region. Arc discharges induce periodic and strong asymmetric stresses in the fibre core of the pre-annealed nitrogen doped fibre.

/pdf/strong-asymmetric-stresses-arc-induced-in-pre-annealed-2uf1ce1jnr.pdf

F. Dürr

Papers

Polarization microscopy by use of digital holography: application to optical- fiber birefringence measurements

Inscription of long-period gratings in pure silica and Germano-silicate fiber cores by femtosecond laser irradiation

Tomographic measurement of femtosecond-laser induced stress changes in optical fibers

Highly efficient one- and two-cascade Raman lasers based on phosphosilicate fibers

Strong asymmetric stresses arc-induced in pre-annealed nitrogen-doped fibres