Zahra Monemhaghdoust

Dual wavelength full field imaging in low coherence digital holographic microscopy.

A diffractive optical element (DOE) is presented to simultaneously manipulate the coherence plane tilt of a beam containing a plurality of discrete wavelengths. The DOE is inserted into the reference arm of an off-axis dual wavelength low coherence digital holographic microscope (DHM) to provide a coherence plane tilt so that interference with the object beam generates fringes over the full detector area. The DOE maintains the propagation direction of the reference beam and thus it can be inserted in-line in existing DHM set-ups. We demonstrate full field imaging in a reflection commercial DHM with two wavelengths, 685 nm and 794 nm, resulting in an unambiguous range of 2.494 micrometers.

Off-axis digital holographic camera for quantitative phase microscopy.

We propose and experimentally demonstrate a digital holographic camera which can be attached to the camera port of a conventional microscope for obtaining digital holograms in a self-reference configuration, under short coherence illumination and in a single shot. A thick holographic grating filters the beam containing the sample information in two dimensions through diffraction. The filtered beam creates the reference arm of the interferometer. The spatial filtering method, based on the high angular selectivity of the thick grating, reduces the alignment sensitivity to angular displacements compared with pinhole based Fourier filtering. The addition of a thin holographic grating alters the coherence plane tilt introduced by the thick grating so as to create high-visibility interference over the entire field of view. The acquired full-field off-axis holograms are processed to retrieve the amplitude and phase information of the sample. The system produces phase images of cheek cells qualitatively similar to phase images extracted with a standard commercial DHM.

Portable advanced off-axis camera for quantitative phase microscopy

We propose and experimentally demonstrate a device in which common-path interferometry combined with off-axis holographic geometry is used to realize a digital holographic camera which can be attached to the camera port of a conventional transmission microscope for complex wavefront analysis. A thick transmission volume grating recorded holographically into thick photosensitive glass splits the beam containing the sample information in two beams. The untouched transmitted beam creates the sample arm of the interferometer. The Bragg diffracted order of the grating is spectrally and spatially filtered by diffraction to generate a clean reference beam. Double passing the diffracted order through the grating using a retroreflector device provides filtering in two dimensions. The spatial filtering done by the grating which works based on high angular selectivity of thick volume gratings, reduces the alignment spatial sensitivity which is an advantage over the conventional spatial filtering done by pinholes. Besides, using a second thick grating, we introduce a desired coherence plane tilt in the reference beam which is sufficient to create high-visibility interference over the entire field of view. The full-field off-axis interferograms are created from which the amplitude and phase can be reconstructed. The advantage of the proposed camera is the insensitivity to the alignment, thus can be the basis for a standalone camera mountable on a standard optical microscope.

Towards an incoherent off-axis digital holographic microscope

We propose and experimentally demonstrate a system in which off-axis digital holographic microscopy is realized using a broadband illumination source. Single-shot holographic measurements are enabled, while the coherence noise is removed thanks to the broad bandwidth of the illuminating source. The proposed digital holographic camera is portable and can be attached to the camera port of a conventional optical microscope. This camera is capable of obtaining the complex wavefront i.e the intensity and phase information of the light transmitted or reflected from a sample. A combination of a thick transmission volume grating recorded holographically into thick photosensitive glass and thin transmission phase gratings recorded holographically into thin photopolymers, spatially filters the beam of light containing the sample information in two dimensions through diffraction. This filtered beam creates the reference arm of the interferometer. The untouched transmitted beam creates the sample arm of the interferometer. The spatial filtering performed by the combination of gratings above reduces the alignment spatial sensitivity which is an advantage over conventional spatial filtering done by pinholes. Besides, using a second thin grating, we introduce a desired coherence plane tilt in the reference beam which is sufficient to create high-visibility interference over the entire field of view in off-axis configuration. Full-field off-axis interferograms are thus created from which the phase information can be extracted.

Portable Off-axis Camera for Quantitative Phase Microscopy

A portable camera system based on holographic elements is demonstrated which can be attached to the camera port of a conventional microscope for complex wavefront analysis. The full-field off-axis holograms provide the whole sample information.

Quantitative phase noise in two color low coherence digital holographic microscope

We have recently introduced a method based on a volume diffractive element (VDOE) to correct the coherence plane tilt in off-axis DHM. The VDOE consists of two phase gratings recorded on a photopolymer (BAYFOL® HX) from Bayer MaterialScience AG, each laminated on one side of a wedge prism. The VDOE is placed in the reference arm of an off-axis DHM to introduce a small tilt γ in the coherence plane of the reference without any effect on the propagation direction.

Quantitative phase noise in a two-color low coherence Digital Holographic Microscope

In digital holographic microscopy (DHM), the long coherence length of laser light causes parasitic interferences due to multiple reflections in and by optical components in the optical path of the microscope and thus degrades the image quality. The parasitic effects are greatly reduced by using a short coherence length light. The main drawback of using a short coherence light source in an off-axis digital holographic microscope, is the reduction of the interference fringe contrast occurring in the field of view. Previously, we introduced a volume diffractive optical element (VDOE) placed in the reference arm of a DHM to correct the coherence plane tilt so as to obtain a high interference contrast throughout the field of view . Here, we experimentally quantify the spatial and temporal phase noise in the extracted phase image caused by non-homogeneities and scattering of the VDOE element itself. The results over five VDOEs show that the temporal phase noise is unchanged and a slight increase (up to 20%) is observed in the spatial phase noise. These results show that even with a low coherence source, a full field of view can be obtained with an off-axis DHM thanks to the VDOE without introducing significant additional phase noise.

Surface Topography and Vertical Scanning in Two Color Low Coherence Digital Holographic Microscope

A Volume Diffractive Optical Element (VDOE) is placed in the reference arm of an off-axis short coherence DHM enabling nanometric-resolution surface topography in short coherence and high-speed vertical scanning, through field of view enlargement.

Single shot dual wavelength full field imaging in low coherence digital holographic microscopy

In off-axis digital holographic microscopy, short coherence length of the source results in an unwanted reduced field of view. A diffractive optical element (DOE) which combines two high efficiency transmission volume phase gratings holographically recorded into a thin photopolymer, is proposed to manipulate the coherence plane tilt of beam containing a plurality of wavelengths simultaneously. The DOE extends the interference pattern between object and reference beams in digital holographic microscope (DHM) over the whole physical beam overlap area. We experimentally demonstrate full field imaging in a commercial, two colors (685 nm and 794 nm) reflection digital holographic microscope (DHM). The synthetic wavelength created by the two colors extends the unambiguous depth range of the DHM from 0.39μm to 2.49μm .

Single-shot full field imaging in a dual-wavelength digital holographic microscope

We demonstrate volume diffractive structures to manipulate the coherence plane tilt at several two simultaneously to achieve full field 3D imaging in a low coherence digital off-axis holographic microscope (DHM) in a single shot.