Pertti Pääkkönen

Partially coherent Gaussian pulses

The concept of a plane-wave pulse with a Gaussian spectrum and a Gaussian distribution of correlations between different frequency components is introduced. The temporal coherence properties of such a pulse are related to the spectral coherence properties and equivalence relations between pulses with different spectral and temporal correlation parameters are established.

An analysis of the angular momentum of a light field in terms of angular harmonics

Abstract This paper proposes an optical interpretation for the Lie algebras symmetry operators of the paraxial wave equation. In particular, the angular momentum operator is used to derive a relation for the expression of the angular momentum of an arbitrary light field in terms of angular harmonics. Furthermore, experimental results are presented demonstrating a filter that extracts angular harmonics from different Gauss-Laguerre modes.

Resonators with Bessel-Gauss modes

Abstract Laser resonators for active generation of bright Bessel–Gauss fields are designed, which employ aspheric (conventional or diffractive) radially phase-conjugating mirrors. The planar semi-transparent mirror is located at the waist of the Bessel–Gauss mode, but the second mirror is necessarily aspheric unless an exact Fourier-transform relationship between the fields at the two mirrors is established with, e.g. an intracavity lens. Conventional Gaussian-beam resonators and Fabry–Perot resonators are obtained as natural limiting cases of pure Gaussian and Bessel modes, respectively, and modal discrimination is studied with the Fox–Li algorithm.

Rotating optical fields : experimental demonstration with diffractive optics

Transversally sharply structured optical fields are discussed, which rotate upon propagation without any lateral expansion of the intensity profile. Finite-aperture approximations of such fields, realizable with phase-only and complex-amplitude recording, are demonstrated. Recording of at least some of the amplitude information (rather than neglecting it completely) is shown to improve the field quality considerably, in particular close to the element. Lohmann-coded binary-phase diffractive elements with restricted amplitude recording are fabricated by electron beam lithography and reactive ion etching. The experimental results are in good agreement with theory.

Detection of porosity of pharmaceutical compacts by terahertz radiation transmission and light reflection measurement techniques

We report on the non-destructive quantification of the porosity of pharmaceutical compacts (microcrystalline cellulose tablets) by using both optical and terahertz techniques. For the full analysis of the porosity of pharmaceutical tablets, the results obtained in both cases have shown that optical and terahertz techniques are complementary. The intrinsic refractive index of microcrystalline cellulose was estimated using the effective refractive index obtained from the time delay of the THz pulse together with the Bruggeman model for effective media. Once this intrinsic refractive index is known, the unknown porosity of the tablet can be estimated with the aid of the measured effective refractive index as well as the thickness of the pharmaceutical tablet. The method was tested using a set of thirteen tablets having different porosities. It is shown that the error in the estimation of the unknown tablets porosity is less than 1%. In addition, surface roughness was measured by using an optical interferometer and gloss by using a diffractive-optical-element based glossmeter. The measurement was achieved by scanning the tablets with a probe beam and detecting the reflected light. The surface roughness and gloss data show relatively good correlation with the porosities of the tablets.

General vectorial decomposition of electromagnetic fields with application to propagation-invariant and rotating fields.

A novel decomposition of the transversal part of the electric field vector of a general non-paraxial electromagnetic field is presented, which is an extension of the radial/aximuthal decomposition and is known as gammazeta decomposition. Purely gamma and zeta polarized fields are examined and the decomposition is applied to propagation-invariant, rotating, and self-imaging electromagnetic fields. An experimental example on the effect of state of polarization in the propagation characteristics of the field: its is shown that a simple modification of the polarization conditions of the angular spectrum converts a self-imaging field into a propagation-invariant field.

Diffractive shaping of excimer laser beams

Abstract We address the problem of shaping the intensity distribution of a highly directional partially coherent field, such as an excimer laser beam, by means of diffractive optics. Our theoretical analysis is based on modelling the multi-transverse-mode laser beam as a Gaussian Schell-model beam. It is shown numerically that a periodic element, which is unsuitable for the shaping of a coherent laser beam, works well with an excimer laser beam because of its partial spatial coherence. The conversion of an approximately Gaussian excimer laser beam into a flat-top beam in the Fourier plane of a lens is demonstrated with a diffractive beam shaper fabricated as a multilevel profile in SiOl by electron-beam lithography and proportional reactive-ion etching.

Astigmatic bessel laser beams

Abstract The oblique incidence of a He-Ne laser beam onto a phase-only diffractive optical element (DOE) that simultaneously produces several unimode different-order Bessel beams propagating at various angles with respect to the optical axis is studied theoretically and experimentally. It is shown that, under obliquely incident illumination of a DOE that forms Bessel beams, the resulting astigmatic diffraction pattern can be used to unambiguously identify the direction of the Bessel beams phase rotation and the order of the Bessel mode.

Thermo-Optical Properties of Thin-Film TiO2–Al2O3 Bilayers Fabricated by Atomic Layer Deposition

We investigate the optical and thermo-optical properties of amorphous TiO2–Al2O3 thin-film bilayers fabricated by atomic layer deposition (ALD). Seven samples of TiO2–Al2O3 bilayers are fabricated by growing Al2O3 films of different thicknesses on the surface of TiO2 films of constant thickness (100 nm). Temperature-induced changes in the optical refractive indices of these thin-film bilayers are measured by a variable angle spectroscopic ellipsometer VASE®. The optical data and the thermo-optic coefficients of the films are retrieved and calculated by applying the Cauchy model and the linear fitting regression algorithm, in order to evaluate the surface porosity model of TiO2 films. The effects of TiO2 surface defects on the films’ thermo-optic properties are reduced and modified by depositing ultra-thin ALD-Al2O3 diffusion barrier layers. Increasing the ALD-Al2O3 thickness from 20 nm to 30 nm results in a sign change of the thermo-optic coefficient of the ALD-TiO2. The thermo-optic coefficients of the 100 nm-thick ALD-TiO2 film and 30 nm-thick ALD-Al2O3 film in a bilayer are (0.048 ± 0.134) × 10−4 °C−1 and (0.680 ± 0.313) × 10−4 °C−1, respectively, at a temperature T = 62 °C.

Fabrication of terahertz wire-grid polarizers

Wire-grid polarizers for terahertz region were fabricated by manufacturing triangular grating using a ruling-based, ultraprecision diamond machining process and replicating the pattern into polymethylpentene (TPX) and cyklo-olefin copolymer (COC) sheets using hot embossing. On top of the imprinted structures, aluminum was evaporated in an oblique angle, forming an aluminum wire grid. The achieved extinction rate was over 150 for TPX polarizers and near 1000 for COC polarizers.