Modulation d'amplitude

La modulation d'amplitude ou MA (AM en anglais) est une technique utilisée pour moduler un signal. Elle consiste en la multiplication du signal à moduler par un signal de fréquence moins élevée. Définition La modulation d'amplitude consiste à faire varier l'amplitude d'un signal de fréquence élevée, le signal porteur, en fonction d'un signal de plus basse fréquence, le signal modulant. Ce dernier est celui qui contient l'information à transmettre (voix, par exemple, recueillie par un microphone). Allure du signal modulé Le signal porteur xp(t) est de fréquence élevée (fréquence radio dépassant 100 kHz). Son allure est la suivante : centre|thumb|397x397px|Signal porteur. Le signal modulant xm(t) est lui de fréquence relativement faible (par exemple fréquence audio inférieure à 20 kHz) : centre|thumb|395x395px|Signal modulant. Le signal modulé (ou signal de sortie) y(t), a l'allure suivante (pour un taux de modulation de 50 %) : centre|thumb|395x395px|Signal modul

Advanced Techniques in Optical Diffraction Tomography

Ahmed Bassam Sayed Ayoub Mohamed Emam

In this thesis, we study the 3 challenges described above. First, we study different reconstruction techniques and assess the fidelity of each reconstruction results by means of structured illumination and phase conjugation. By reconstructing the 3D refractive index of the sample using different algorithms (i.e. Born, Rytov, and Radon) and then perform a numerical back-propagation of experimentally measured structured illumination pattern we are able to assess the fidelity of each reconstruction algorithms without prior information about the 3D RI distribution of the sample.The second part of the thesis is concerned with the 3D reconstruction of samples using intensity-only measurements which the need to holographically acquire them. We show that using intensity-only measurements, we could still be able to reconstruct the 3D volume of the sample with edge-enhanced effects which was proven useful for drug delivery applications in which nano-particles were identified on the cell membrane of immune T-cells in a drug delivery studies. Such reconstruction technique would result in more robust imaging system where the commercial imaging microscope systems can be incorporated with LEDs for high-quality speckle noise-free imaging systems. In addition, we show that under certain conditions, we can be able to reconstruct the 3D refractive index distribution of different samples.The third part of the thesis is contributing to high-speed complex wave-front shaping using DMDs. In that part, new modulation technique is demonstrated that can boost the speed of the current time-multiplexing techniques by a factor of 32. The modulation technique is based on amplitude modulation where an amplitude modulator is synchronized withvthe DMD to modulate the intensity of each bit-plane of an 8-bit image and then all the modulated bit-planes are linearly added on the detector. Such modulation technique can be used not only for structured illumination microscopy but also for high-speed 3D printing applications as well as projectors.The last part is concerned with using deep learning approaches to solve the missing cone problem usually accompanied with optical imaging due to the limited numerical aperture of the imaging system. Two techniques are discussed; the first is based on using a physical model to enhance the quality of the 3D RI reconstruction and the second is based on using deep neural network to solve the missing cone problem.

EPFL2022

Low coherence digital holographic microscopy

Pia Massatsch

A new holographic technique combining digital holographic microscopy with the use of low coherence light has been developed and the potential of the resulting method for vision in turbid media and biomedical imaging has been evaluated. In digital holography the hologram is recorded on a CCD camera and reconstructed numerically on a PC, giving direct access to the phase and the amplitude of the reconstructed wave front. This technique is combined with low coherence light, creating the possibility to perform optical sectioning. Series of holograms corresponding to slices at different depths in the specimen are recorded and each of these holograms contain phase and amplitude information of the light originating from the corresponding tomographic slice in the object. Sub micrometer lateral resolution is obtained experimentally and the phase distribution of the reconstructed object corresponds to a depth resolution of approximately 10 nm for a depth range of a couple of micrometers. A second level depth resolution is achieved using low coherence illumination. The thickness of a tomographic layer in water is 11 µm. Vision through 18 mean free paths (mfp) of turbid media (micro sphere suspension) is demonstrated with depth resolved reconstruction of the phase. Both amplitude and phase is reconstructed through 17 mfp's of turbid media. Tomographic imaging of a USAF test target hidden behind a 100 µm thick onion cell membrane is performed with phase and amplitude reconstructions of the test target as well as the surface of the onion cell membrane. Tomography of a biological specimen is demonstrated by the images of an onion shell, with structures visible down to a depth of 1 mm and by the images of a St-Paulia leaf presenting two cellular layers separated by 200 µm. Two structures of the anterior eye, the iris and the cornea, are studied on enucleated porcine eyes. A tomography of the iris border close to the pupil was performed and the penetration into the iris tissue is good. The cornea is first imaged without contact medium giving images strongly dominated by the air-cornea reflection. These images contain valuable information on the superficial epithelial cells in spite the presence of the tear film on the cornea. A water immersion configuration is also applied to enable a tomography of the cornea. A z-scan profile of the cornea is obtained and phase and amplitude images are reconstructed for different locations in the cornea (epithelium, stroma and endothelium).

EPFL2003

Advanced Techniques in Optical Diffraction Tomography

Ahmed Bassam Sayed Ayoub Mohamed Emam

EPFL2022

Low coherence digital holographic microscopy

Pia Massatsch

EPFL2003

Advanced Techniques in Optical Diffraction Tomography

Low coherence digital holographic microscopy

Amplitude modulation of a low-coherence source; applications to distance and dynamic displacement sensing

Advanced Techniques in Optical Diffraction Tomography

Low coherence digital holographic microscopy

Amplitude modulation of a low-coherence source; applications to distance and dynamic displacement sensing