Janne Salo

Millimeter-wave beam shaping using holograms

We synthesize amplitude- and phase-type computer-generated holograms (diffractive gratings) for shaping millimeter-wave fields. We design holograms using quasi-optical back-propagation and rigorous optimization methods adopted from diffractive optics. We present experimental results from a plane-wave-generating hologram and a custom-designed field shaper at 310 GHz. Holograms can be applied, e.g., in a compact antenna test range and we propose their use for alignment purposes.

Holograms for shaping radio-wave fields

Holograms—diffractive elements—are designed and fabricated for shaping millimetre-wave radio fields. Methods for the synthesis of hologram elements are discussed and several beam shapes are tested: plane waves, radio-wave vortices and Bessel beams. Here we present an overview of the methods applied and results obtained with quasi-optical hologram techniques using both amplitude and phase holograms.

Orthogonal X waves

Nondiffracting pulses are spatially and temporally localized wave fields that undergo no diffractive spreading under propagation through homogeneous media. We introduce an orthogonality condition for nondiffracting pulses and present an orthogonal set of X waves which possess temporal spectra of the form (polynomial in ω) × e −αω . The newly introduced Bessel-X pulses and X-wave transforms are discussed in the framework of the orthogonal X-wave bases.

Subsonic nondiffracting waves

We introduce subsonic nondiffracting waves which—unlike the ordinary supersonic nondiffracting waves—evolve periodically under propagation. Such pulse-like waves have a subsonic uniformly propagating ‘core’, which is modulated by a supersonic plane wave. The subsonic core may also be considered an envelope for a truncated Bessel beam and subsonic nondiffracting waves may be used to describe signal propagation within Bessel beams.

Diffraction-free pulses at arbitrary speeds

We consider periodically propagating pulses, devoid of diffractive spreading. They may feature arbitrary velocities of propagation but their spectral characteristics vary according to whether they are luminal, subluminal or superluminal. The wave modes introduced are closely related to the X waves and the focus wave modes, but they allow a frequency-dependent cone angle and are not limited to the speed of light.

Radio-wave beam shaping using holograms

Millimetre-wave radio fields are shaped using both amplitude- and phase-type computer-generated holograms (diffractive elements). Methods for hologram element synthesis are described. Holograms that produce plane waves, radio-wave vortices, and Bessel beams at 310 GHz are fabricated and tested.

Experimental studies on radio holograms at mm- and submm-wavelengths

Computer-generated holograms can be used for shaping millimeter-wave beams. The radio holograms used are either amplitude or phase holograms. An amplitude hologram has been proven to be a feasible alternative as a focusing element in a compact antenna test range (CATR) in the mm-wavelength regime. Holograms in other mm- and submm-applications are also studied - especially for the creation of nondiffracting Bessel radio beams.

Subsonic nondiffracting waves in anisotropic crystals

We formulate nondiffracting waves (NDW) in a way which is directly applicable to isotropic as well as to anisotropic waves. Using this description, we show that anisotropic NDWs show both new qualitative and quantitative properties when compared with corresponding isotropic waves. In particular, we demonstrate that anisotropic materials can support subsonic NDWs while only supersonic nondiffracting solutions are known for isotropic wave motion.

Acoustic wave propagation: 2D Wigner and 3D wavefront simulations

Recently, we have applied an angular-spectrum based method, the thin-element decomposition (TED), to calculate SAW propagation in waveguide structures. However, the angular spectrum does not allow for reflections in the waveguide, which leads to discrepancies for long strips. This has lead us to use the Wigner-distribution function to describe the propagation of SAW in the paraxial limit. This approach leads to a ray-tracing type algorithm which is fast and easy to implement. We calculate wave propagation in a waveguide and compare the results to those given by the classical guided mode theory. We also discus the behaviour of Wigner distribution functions near sharp boundaries. We have also simulated expanding acoustic wavefronts produced by a point disturbance in a bulk. Due to elastic anisotropy of the solid, the energy flux associated with a plane wave is not collinear with the wave vector and, correspondingly, wave fronts (which correspond to the group-velocity surfaces) are not spherical.

Computer-generated holograms for mm- and submm-wave beam shaping

Computer-generated holograms (diffractive elements) can be used for shaping mm-wave beams and for producing complex field configurations. The radio holograms studied here are amplitude holograms. An amplitude hologram has been proven to be a feasible alternative as a focusing element in a compact antenna test range (CATR) at mm-wavelengths. Amplitude holograms have also been demonstrated in other mm- and submm-wave beamshaping applications - e.g., on the creation of nondiffracting radiowave Bessel beams, a radio wave vortex, and also a custom-designed beam pattern.