Synthetic aperture in terahertz in-line digital holography for resolution enhancement

Abstract

Terahertz digital holography is a combination of terahertz technology and digital holography. In digital holography, the imaging resolution is the key parameter in determining the detailed quality of a reconstructed wavefront. In this paper, the synthetic aperture method is used in terahertz digital holography and the in-line arrangement is built to perform the detection. The resolved capability of previous terahertz digital holographic systems restricts this technique to meet the requirement of practical detection. In contrast, the experimental resolved power of the present method can reach 125 μm, which is the best resolution of terahertz digital holography to date. Furthermore, the basic detection of a biological specimen is conducted to show the practical application. In all, the results of the proposed method demonstrate the enhancement of experimental imaging resolution and that the amplitude and phase distributions of the fine structure of samples can be reconstructed by using terahertz digital holography.

THz coherent lensless imaging

“…According to them, the tested uncooled microbolometers are 40 times more sensitive than the pyroelectric camera Pyrocam III. As a consequence, the high detectivity of infrared microbolometers allows working with mW-level output sources such as QCLs or various FIRL lines [82,[110][111][112][113]. Their pixel pitch of 17-25 µm is ideal for off-line holography, as it allows large off-axis angles.…”

“…As an alternative, a synthetic aperture scheme uses real holograms to increase the resolution, and is an effective solution for THz in-line digital holography, as well. Examples were reported in [110,111], reconstructing features as low as λ. In Ref.…”

Advanced holographic wavefront sensor

“…where the   wave towards the recording plane is calculated using the angular spectrum propagation integral, as follows [21]:…”

“…The last two terms represents the real image and the virtual one, respectively, where * denotes the complex conjugation operation. The complex amplitude distribution of the object wave is obtained by the frequency spectrum filtering method and sequent angular spectrum reconstruction [21]:…”

Expanding the field-of-view and profile measurement of covered objects in continuous-wave terahertz reflective digital holography

“…11,12 THz digital holography records the interference pattern between the object beam and the reference beam and reconstructs the quantitative amplitude and phase distributions of the object wavefront through diffraction propagation. [13][14][15] CW THz off-axis reflective digital holography can be used to measure the profile of reflective objects, including covered or coated ones, from the reconstructed phase distribution. 16,17 Locatelli et al 18 proposed an off-axis reflective THz digital holographic approach obtaining the amplitude distribution of a metallic plate covered by a polypropylene mask coating, achieving a lateral resolution of 200 μm and a field-of-view (FOV) of 16 mm × 12 mm.…”