Depth-filtered digital holography

Quantitative phase imaging through an ultra-thin lensless fiber endoscope

Quantitative phase imaging (QPI) is a label-free technique providing both morphology and quantitative biophysical information in biomedicine. However, applying such a powerful technique to in vivo pathological diagnosis remains challenging. Multi-core fiber bundles (MCFs) enable ultra-thin probes for in vivo imaging, but current MCF imaging techniques are limited to amplitude imaging modalities. We demonstrate a computational lensless microendoscope that uses an ultra-thin bare MCF to perform quantitative phase imaging with microscale lateral resolution and nanoscale axial sensitivity of the optical path length. The incident complex light field at the measurement side is precisely reconstructed from the far-field speckle pattern at the detection side, enabling digital refocusing in a multi-layer sample without any mechanical movement. The accuracy of the quantitative phase reconstruction is validated by imaging the phase target and hydrogel beads through the MCF. With the proposed imaging modality, three-dimensional imaging of human cancer cells is achieved through the ultra-thin fiber endoscope, promising widespread clinical applications.

Digital Holographic Interferometry for the Measurement of Symmetrical Temperature Fields in Liquids

In this paper, we present a method of quantitatively measuring in real-time the dynamic temperature field change and visualization of volumetric temperature fields generated by a 2D axial-symmetric heated fluid from a pulsatile jet in a water tank through off-axis digital holographic interferometry. A Mach-Zehnder interferometer on portable platform was built for the experimental investigation. The pulsatile jet was submerged in a water tank and fed with water with higher temperature. Tomographic approach was used to reconstruct the temperature fields through the Abel Transform and the filtered back-projection. Averaged results, tomographic view, standard deviation and errors are presented. The presented results reveal digital holographic interferometry as a powerful technique to visualize temperature fields in flowing liquids and gases.

Confocal laser scanning holographic microscopy of buried structures

In this paper, we present a confocal laser scanning holographic microscope for the investigation of buried structures. The multimodal system combines high diffraction limited resolution and high signal-to-noise-ratio with the ability of phase acquisition. The amplitude and phase imaging capabilities of the system are shown on a test target. For the investigation of buried integrated semiconductor structures, we expand our system with an optical beam induced current modality that provides additional structure-sensitive contrast. We demonstrate the performance of the multimodal system by imaging the buried structures of a microcontroller through the silicon backside of its housing in reflection geometry.

Depth-filtering in common-path digital holographic microscopy

We demonstrate a method to select different layers in a sample using a low coherent gating approach combined with a stable common-path quantitative phase imaging microscopy setup. The depth-filtering technique allows us to suppress the negative effects generated by multiple interference patterns of overlaying optical interfaces in the sample. It maintains the compact and stable common-path setup, while enabling images with a high phase sensitivity and acquisition speed. We use a holographic microscope in reflective geometry with a non-tunable low coherence light source. First results of this technique are shown by imaging the hardware layer of a standard micro-controller through its thinned substrate.

Versatile spectral modulation of a broadband source for digital holographic microscopy

We demonstrate the potential of spatial light modulators for the spectral control of a broadband source in digital holographic microscopy. Used in a 'pulse-shaping' geometry, the spatial light modulator provides a versatile control over the bandwidth and wavelength of the light source. The control of these properties enables adaptation to various experimental conditions. As a first application, we show that the source bandwidth can be adapted to the off-axis geometry to provide quantitative phase imaging over the whole field of view. As a second application, we generate sequences of appropriate wavelengths for a hierarchical optical phase unwrapping algorithm, which enables the measurement of the topography of high-aspect ratio structures without phase ambiguity. Examples are given with step heights up to 50 µm.

Wavefront shaping for imaging-based flow velocity measurements through distortions using a Fresnel guide star

Imaging-based flow measurement techniques, like particle image velocimetry (PIV), are vulnerable to time-varying distortions like refractive index inhomogeneities or fluctuating phase boundaries. Such distortions strongly increase the velocity error, as the position assignment of the tracer particles and the decrease of image contrast exhibit significant uncertainties. We demonstrate that wavefront shaping based on spatially distributed guide stars has the potential to significantly reduce the measurement uncertainty. Proof of concept experiments show an improvement by more than one order of magnitude. Possible applications for the wavefront shaping PIV range from measurements in jets and film flows to biomedical applications.

Multiwavelength phase unwrapping and aberration correction using depth filtered digital holography

In this Letter, we present a new approach to processing data from a standard spectral domain optical coherence tomography (OCT) system using depth filtered digital holography (DFDH). Intensity-based OCT processing has an axial resolution of the order of a few micrometers. When the phase information is used to obtain optical path length differences, subwavelength accuracy can be achieved, but this limits the resolvable step heights to half of the wavelength of the system. Thus there is a metrology gap between phase- and intensity-based methods. Our concept addresses this metrology gap by combining DFHD with multiwavelength phase unwrapping. Additionally, the measurements are corrected for aberrations. Here, we present proof of concept measurements of a structured semiconductor sample.