Date of Award
2016
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Optical Science and Engineering
Committee Chair
Patrick J. Reardon
Committee Member
Henry O. Everitt
Committee Member
Robert Lindquist
Committee Member
Dan Lawrence
Committee Member
John William
Committee Member
Laurie Joiner
Subject(s)
Holography, Optoelectronic devices, Photonics, Submillimeter waves
Abstract
Digital holography is explored for the first time in the terahertz frequency region (between 0.230 and 0.740 THz) with highly coherent, frequency tunable continuous wave sources. Terahertz digital holography is motivated by two characteristics: terahertz radiation penetrates through many dielectric materials, and the associated wavelengths are sufficiently short to generate and record holograms using optical components. However, these optics approach the size of the wavelength, a new regime in which an off-axis holographic imager differs significantly from its visible light equivalent. In this regime, even custom made large optics as they were used here are only hundreds of wavelengths in diameter and can cause undesirable aperture diffraction. Virtual diffraction due to the finite size of the hologram aperture will degrade the quality of the digital hologram reconstruction. These challenges were addressed in two ways, which fundamentally shaped the experimental setup. First, the imager was designed to minimize the need for optical components to only two large mirrors and one beamsplitter. The beamsplitter had to be large, thin, and non-polarizing. The solution came in the form of a 30 x 30 cm2 metallic paint coated dielectric thin film, which was stretched over a ridged frame. To collimate the expanding terahertz beam and generate planar reference and object waves, a 33 cm diameter off-axis paraboloidal mirror was used. Second, the virtual diffraction of the hologram aperture sides needed to be isolated, which was only possible with an out-of plane off-axis reference beam with respect to the source – beamsplitter – hologram plane. The reference beam was directed on a flat mirror roughly 30 cm above the aforementioned plane, before it was redirected back down to the hologram plane. The resulting off-axis angle was at an angle with respect to the horizontal and vertical sides of the hologram aperture, which allowed the reconstructed image to be separated from the broadband spatial frequency content of the sharp hologram aperture edges. However, the experiment had to be compact to minimize the object – hologram distance, which maximizes transverse resolution. In fact at approximately 18 cm it was comparable to the size of the object field of view (up to 20 x 20 cm2) resulting in a significant increase of the transverse resolution at the object edge compared to the object center, calling for a new description of the experiment’s transfer function. All holograms were recorded with a raster-scanned detector, which could take up to several days, so the use of highly stable coherent terahertz sources was essential. The irradiance of the large terahertz beam was on the order of nanowatts per square centimeter and picowatts per detector pixel. Therefore, highly sensitive super-heterodyne receivers were needed to acquire the signal at a high pixel resolution. This optimized experimental setup generated terahertz holograms, which were reconstructed with a variety of methods including the Fresnel diffraction, angular spectrum reconstruction, and dual wavelength methods. The reconstructions revealed a transverse resolution of about the size of the wavelength, which is comparable to the performance of analog visible holography. The longitudinal depth resolution of the imager is lambda/284, which is almost two times better than what has been achieved with visible light digital holographic microscopy. Visibly opaque dielectric structures were examined to demonstrate that terahertz digital holography is a very compelling nondestructive test, evaluation, and analysis tool. Other related coherent terahertz imaging techniques were investigated, including terahertz radar, terahertz tomography, and the use of novel terahertz diffractive optics. A terahertz radar was built and used to investigate the radar cross section characteristics of small targets. Measurements with Ronchi diffraction gratings revealed that one can measure the refractive index of the grating material with unprecedented accuracy to its third decimal place. Work on terahertz coherence tomography increased the depth of focus of a confocal imager by a factor of 12 and greatly reduced the need to scan in the axial dimension. The learned lessons from the development of a variety of coherent terahertz imaging experiments, conducting measurements, and analyzing the results are that the utilization of coherence is both a great opportunity but also a challenge for terahertz imaging research. Imaging with coherence terahertz radiation combined with sophisticated signal processing techniques can be used to achieve high resolution imagery in both transverse and longitudinal dimensions, and can be used to measure material properties or alleviate the scanning burden of single detector imaging systems. Furthermore, new transfer functions need to be developed to describe imaging systems that use no imaging lenses and provide significantly higher resolution for objects that are located off-axis than on-axis. Simultaneously, one will always have to battle the challenge associated with the finite size of practical optics. Optics that are 1000 or more wavelengths in diameter are simply unpractical in the terahertz region. Instead, one must account for inevitable diffraction from aperture edges by choosing beam paths and layouts that will allow these diffraction effects to be removed in image processing or build optics with apertures that form diffraction patterns, which are non-image degrading in the image plane.
Recommended Citation
Heimbeck, Martin Stephan, "Digital holography and coherent imaging at terahertz frequencies" (2016). Dissertations. 115.
https://louis.uah.edu/uah-dissertations/115