Preben Buchhave

Particle image velocimetry—status and trends

Abstract Particle image velocimetry (PIV) is presently attracting quite a lot of interest. PIV allows fluid velocities across a region of a flow to be measured at a single instant in time and is thus an alternative to the conventional hot-wire and laser anemometry, which measure a velocity-time record at a single point in space. This paper discusses the PIV technique as a fluid mechanics tool by comparison to other flow image processing methods and standard laser anemometry. Any implementation of the PIV technique offers a number of technical challenges that must be overcome before the method is truly useful as a measurement technique for fluid mechanics. These problems are reviewed, and work in progress to overcome or circumvent them is reported. The status of PIV is considered by reviewing some selected state-of-the-art applications, and the trends for future developments are discussed. The main emphasis is on the description of methods of processing the PIV images and on a discussion of the practical limitations of PIV as regards measurable velocities, obtainable accuracy, and spatial resolution.

Efficient all solid-state continuous-wave yellow-orange light source

We present highly efficient sum-frequency generation between two CW IR lasers using periodically poled KTP. The system is based on the 1064 and 1342 nm laser-lines of two Nd:YVO4 lasers. This is an all solid-state light source in the yellow-orange spectral range. The system is optimized in terms of efficiency as well as stability. We compare the performance of a singly and a doubly resonant system, and find that the stability of the singly resonant system is superior to that of the doubly resonant system. We find that the overall conversion efficiency of the single resonant system is higher than for the doubly resonant configuration.

Diode-pumped single-frequency Nd:YVO 4 laser with a set of coupled resonators

350 mW of single-frequency power from a diode-pumped solid-state Nd:YVO(4) laser has been obtained from a coupled resonator design without any intracavity elements. Single-frequency operation was obtained by use of a very short laser rod and a coupled resonator design. The two coupled resonators were formed by the two faces of a very short Nd:YVO(4) laser crystal and an output coupling mirror. The interaction of the two coupled cavities caused a modification of the eigenmodes supporting laser action in a single longitudinal mode. This design, which is extremely simple, represents a cost-eff icient way of obtaining single-frequency output.

THERMAL SELF-FREQUENCY LOCKING OF A DOUBLY RESONANT OPTICAL PARAMETRIC OSCILLATOR

The increase in the circulating signal and idler fields that occurs in a high- Q doubly resonant optical parametric oscillator (OPO) as it approaches resonance results in a small increase in the crystal temperature owing to absorption of the generated fields. The temperature change affects the refractive index of the crystal and alters the optical path length of the cavity. This effect may lead to self-frequency locking of the OPO to a specific resonance of the signal and idler fields, and it also results in peculiarities in the transient response of the system as it is scanned through resonance. We show that the experimentally observed effects are consistent with the results of a numerical model of the OPO.

Correction to the Beer-Lambert-Bouguer law for optical absorption

The Beer-Lambert-Bouguer absorption law, known as Beers law for absorption in an optical medium, is precise only at power densities lower than a few kW. At higher power densities this law fails because it neglects the processes of stimulated emission and spontaneous emission. In previous models that considered those processes, an analytical expression for the absorption law could not be obtained. We show here that by utilizing the Lambert W-function, the two-level energy rate equation model is solved analytically, and this leads into a general absorption law that is exact because it accounts for absorption as well as stimulated and spontaneous emission. The general absorption law reduces to Beers law at low power densities. A criterion for its application is given along with experimental examples.

Nonlinear combinations of gratings in Bi 12 SiO 20 : theory and experiments

Multiple optical interference patterns generate phase gratings in Bi12SiO20 as a result of the photorefractive effect. Because of the nonlinear response of the space-charge field to the optical interference pattern, these primary gratings interact and cause the formation of additional gratings at frequencies that are combinations of the primary grating frequencies. Only the diffusion regime is considered in the present investigation. In our setup, two primary gratings are induced as the result of interference among one reference and two closely situated object beams. The interference between the two object beams leads to a negligible photorefractive grating, because the photorefractive Bi12SiO20 crystal cannot respond to the corresponding large fringe spacing. We show theoretically that nonlinear combinations of gratings are introduced through this third interference pattern even though the corresponding grating itself is absent. We obtain analytical expressions for the fundamental frequency components of the space-charge field in the special case of a weak reference beam. In the general case, the frequency components of the space-charge field are calculated numerically by the use of Fourier transforms and are incorporated into the coupled-wave equations. The optical beam propagation method is then used for solving these equations numerically. The effect is probed in a three-wave mixing experiment that uses a sinusoidal phase modulation on one of the object beams to control the grating strength. With this technique one of the gratings is specifically canceled out, and the relative change of the diffraction efficiency in the remaining grating is measured. It is shown that nonlinear combinations of gratings may cause relative changes of more than 35% in the diffraction efficiency. The theoretical predictions are in excellent agreement with our experimental results and the cross talk observed in dynamic optical interconnects.

Tunable intra-cavity SHG of CW Ti:Sapphire lasers around 785 nm and 810 nm in BiBO-crystals

Phasematch curves as well as sensitivity to angular and wavelength misalignment for generation of second-harmonic of 785 nm and 810 nm in Bi(3)BO(6) crystal was calculated. Measurements were done for intra-cavity CW SHG in a Ti:Sapphire laser. The BiBO crystal was found to be excellent for this application. Temperature dependance was uncritical for both crystals, while power stability was good. Maximum blue output was 53 mW at 392 nm and 100 mW at 405 nm; corresponding to pump-to-blue optical conversion efficiencies of 0.96% and 1.82% respectively.

Dynamic spatial structure of spontaneous beams in photorefractive bismuth silicon oxide

We report the domain structure of spontaneously occurring beams (subharmonics) in photorefractive bismuth silicon oxide with an applied electric field from 1 to 6 kV/cm and a running grating. The subharmonic beams are generated in a pattern of domains that evolve dynamically as they move through the crystal. We find that the domains move as a whole with a speed approximately equal to that of the primary grating, but in the opposite direction. The domains are separated by narrow boundary regions, where the phase of the subharmonic waves changes by π. The domain motion is consistent with the group velocity for running space-charge waves.

Running gratings in photoconductive materials

Starting from the three-dimensional version of a standard photorefractive model (STPM), we obtain a reduced compact set of equations for an electric field based on the assumption of a quasi-steady-state fast recombination. The equations are suitable for evaluation of a current induced by running gratings at small-contrast approximation and also are applicable for the description of space-charge wave domains. We discuss spatial domain and subharmonic beam formation in bismuth silicon oxide (BSO) crystals in the framework of the small-contrast approximation of STPM. The experimental results confirming holographic current existence in BSO crystal are reported.

Generation of polarization squeezing with periodically poled KTP at 1064 nm.

We report the experimental demonstration of directly produced polarization squeezing at 1064 nm from a type I optical parametric amplifier (OPA) based on a periodically poled KTP crystal (PPKTP). The orthogonal polarization modes of the polarization squeezed state are both defined by the OPA cavity mode, and the birefringence induced by the PPKTP crystal is compensated by a second, but inactive, PPKTP crystal. Stokes parameter squeezing of 3.6 dB and anti squeezing of 9.4 dB is observed.