Sven-Göran Pettersson

Orientation along great circles by migrating birds using a sun compass

The time-compensated sun compass furnishes migrating birds with a possibility of orientating along great circle routes. When moving across longitudes, birds are exposed to a time shift and, if sun compass orientation is used, the course is expected to change along the migration route. These expected course changes are compared with the changing courses along great circle routes. At high latitudes, sun compass courses are expected to change by approximately 1° for each degree of longitudinal displacement, with little daily and seasonal variation. The resulting routes accord well with great circle routes as far away from the poles as lat. 50°–60°. At lower latitudes, birds may travel along approximate great circle routes if they let themselves be guided by sun compass orientation around the time of sunset/sunrise. Generally, sun compass routes at sunset/sunrise are exactly equal to great circles. This also applies to routes where birds, travelling during the night, are guided by stars close to the horizon using a time-compensated star compass. It remains to be elucidated if migrating birds make use of the possibilities, offered by their sun compass, for great circle orientation.

High-speed structured planar laser illumination for contrast improvement of two-phase flow images

A high-speed method to remove blurring effects caused by multiple scattering in planar laser images of two-phase flows is demonstrated. The technique is based on structured illumination and is for the first time to our knowledge applied on a dynamic medium. As structured illumination requires three successive images to be recorded and to freeze the flow motion in time, a high-speed laser and imaging system is employed. We show that by using a time delay of 55 micros between the images a single-shot representation of a dilute flow of water droplets can be achieved. By having an additional inner stream with known structure and composition, the efficiency of the method is quantitatively evaluated, showing an increase from 58% to 93% in image contrast. Such an improvement allows more accurate analysis and interpretation of scattering two-phase flow images.

Light deflection and convection in diffusion experiments using holographic interferometry

A study of the effect of light deflection during diffusion studies of ethanol into agarose gel using holographic laser interferometry is presented. Furthermore it also demonstrates how a diffusive flux could give rise to a convective flux in holographic laser interferometry experiments. The convective and diffusive mass transfer is also theoretically compared in both a liquid phase and a gel phase for the ethanol-agarose system used. The current study shows that errors due to light deflection in holographic laser interferometry are extremely small and can be neglected. It also shows the importance of designing the diffusion experiments to avoid natural convection. In gels the convective flow is cancelled by the friction forces between the liquid and the polymer network. However, in the liquid phase the natural convection could occur even though the density differences in the phase are small.

Determination of a diffusion coefficient in a membrane by electronic speckle pattern interferometry: a new method and a temperature sensitivity study

In this work, a method has been developed to easily determine the effective diffusion coefficient (De) of a solute in a permeable membrane using electronic speckle pattern interferometry. Fringes are introduced parallel to the direction of diffusion during the diffusion process and De can be calculated by simple measurements on the interference pattern. For a fast and convenient determination of De, a mathematical expression has been derived from the analytical solution of diffusion in two media separated by a resistance. The De obtained when fringes are introduced is in agreement with that obtained when fringes are not introduced. The effect of temperature variation on the optical path of the reference and the object beams has also been investigated. The error introduced into the calculation of De, when the temperature oscillation is not taken into account, has been compared for the case when fringes are not introduced during the diffusion experiment and the case when fringes are introduced. In the first case, the relative error can be greater than 100%. Interestingly, in the latter case, the error caused by temperature oscillation is considerably reduced, and no error is introduced if the temperature changes homogeneously over the whole diffusion cell used for the diffusion experiment.

Single-shot holography using high-order harmonics

This work demonstrates single-shot holography with ultrashort XUV pulses. The pulses are generated by high-harmonic generation in an Ar gas cell and focused to produce a strongly divergent reference beam suitable for in-line holography of micrometer size objects. The achieved intensity of the high-order harmonics in the XUV regime is sufficient for single-shot holography with good signal-to-noise ratio. A numerical algorithm is applied to reconstruct real images from the recorded holograms, which also includes an iterative process to reduce the twin-image blurring. Holograms and reconstructed images of microscope tips are presented and a description of the algorithm is given.

Spatially resolved, single-ended two-dimensional visualization of gas flow phenomena using structured illumination

A method for 3D mapping of scattering particle concentration in a gaseous medium based on the backscattered light in a single direction has been demonstrated. The technique is originally developed for microscopy but now implemented on larger-scale samples. The technique used is known as structured illumination, where a sinusoidal grid pattern is projected onto the medium, thus marking the in-focus plane. This makes it possible to discriminate against light originating from the out-of-focus parts of the sample, which usually makes it difficult to detect inner structures of the medium. In this study a flow of nitrogen was introduced into a flow of water droplets, with the aim to optically select only the plane where nitrogen was present. The results indicate that the technique could be used to study, e.g., combustion devices with limited optical access.

Light-in-flight recording. 5: Theory of slowing down the faster-than-light motion of the light shutter.

Light-in-flight recording by holography uses a picosecond pulse for the reference beam, which like a sheet of light intersects the hologram plate and produces a sensitivity area that with a speed faster than light moves over the plate like a light shutter. If, however, the front of the reference pulse by diffraction in a grating is tilted relative to its direction of motion, the velocity of the light shutter can be slowed down resulting in increased recording time. The practical result using a reflection grating was a true recording that corresponded to a time compression of two to one. To minimize distortions of the recorded pulse shape we studied intersections that are identical for apparent (ellipsoidal) and true (spheroidal) wavefronts.

Light-in-flight recording. 6: Experiment with view-time expansion using a skew reference wave

In normal light-in-flight holography the largest time difference that can be created in the image is when the reference light falls nearly parallel with the photographic plate. In that case, where the pulse front is perpendicular to the direction of propagation, the time scale on the plate is given by the speed of light. Thus the time it takes for the reference light to go from one side of the plate to the other sets the upper limit of the observable time interval. By using a reference light where the pulse front is leaning backward, this view time can be increased several times.

Digital in-line holography on amplitude and phase objects prepared with electron beam lithography.

We report on the fabrication and characterization of amplitude and phase samples consisting of well defined Au or Al features formed on ultrathin silicon nitride membranes. The samples were manufactured using electron beam lithography, metallization and a lift‐off  technique, which allow precise lateral control and thickness of the metal features. The fabricated specimens were evaluated by conventional microscopy, atomic force microscopy and with the digital in‐line holography set‐up at the Lund Laser Centre. The latter uses high‐order harmonic generation as a light source, and is capable of recovering both the shape and phase shifting properties of the samples. We report on the details of the sample production and on the imaging tests with the holography set‐up.

Applications of terawatt lasers

Experiments using high‐power laser radiation have recently become much more feasible through the development of chirped pulse amplification. By using titanium sapphire as the gain medium, very compact and high‐repetition rate systems reaching terawatt power levels can be constructed. Within the newly established Lund High Power Laser Facility we operate such a system (150 fs, 1.5 TW, 10 Hz, tunable 760–840 nm), which is being employed in a number of basic and applied studies requiring high optical powers. Detailed studies of high‐harmonic generation in inert gases are reported as well as the generation of hard x‐rays from microplasmas. Ultrasharp medical imaging has been achieved. Intense white‐light generation by self‐phase modulation in water has been used for biological tissue studies and for photon migration studies in green leaves.