In this paper we present a lensless transmission digital holographic microscope for the investigation of transparent samples. The setup consists of a laser diode, an object positioned on a cover slip and a CMOS sensor. We use a laser diode for illumination which emits a divergent beam and acts as a point source, so that additional components such as a pinhole are not required. The laser diode is operated below the lasing threshold to decrease the coherence length and thus to reduce speckle noise. Due to the compact and small size of the setup, it requires minimized effort for applications in field operation. The lensless setup was characterized by using an USAF-target for determining the resolution of the system which is 2.2 μm. In the following, transparent or semitransparent samples are investigated. Microstructured plastic samples are placed on the specimen holder and characterized by the holographic microscope. By applying the angular spectrum method on the recorded images, we are able to reconstruct the investigated objects. The in-line geometry of the setup facilitates the simplicity of the setup but also induces optical errors, for instance twin images. Twin images superimpose with the object’s signal and require additional numerical reconstruction algorithms. For reducing the effect of the twin image problem, we apply an iterative phase retrieval algorithm. In the conclusion, we discuss the resolution and quality of the recorded images and evaluate the numerical reconstruction process.
Digital holography is capable of providing surface profiles of samples with axial resolution in the nanometer range. Lensless digital holography is a well-established microscopic method providing diffraction limited resolution of the order of the wavelength of the used light source. It is based on inline holography and usually allows imaging only in transmission geometry.
In this contribution we propose a compact low cost lensless digital holographic microscope capable of performing measurements on reflective microstructures. The novelty of the system consists on a direct use of a laser diode without any need of coupling optics as light source. This simplifies the setup and provides sufficient magnification to measure microstructures. We evaluate our setup by imaging reflective microstructures. We have achieved ̴ 6 mm2 field of view amplitude images with ̴ 2.5μm lateral resolution and phase images with axial resolution in nanometer range. The phase image provides a full-field profile measurement of the sample in nanometer range.
Wide field, lensless microscopes have been developed for telemedicine and for resource limited setting . They are based on in-line digital holography which is capable to provide amplitude and phase information resulting from numerical reconstruction. The phase information enables achieving axial resolution in the nanometer range. Hence, such microscopes provide a powerful tool to determine three-dimensional topologies of microstructures. In this contribution, a compact, low-cost, wide field, lensless microscope is presented, which is capable of providing topological profiles of microstructures in transparent material. Our setup consist only of two main components: a CMOSsensor chip and a laser diode without any need of a pinhole. We use this very simple setup to record holograms of microobjects. A wide field of view of ~24 mm², and a lateral resolution of ~2 μm are achieved. Moreover, amplitude and phase information are obtained from the numerical reconstruction of the holograms using a phase retrieval algorithm together with the angular spectrum propagation method. Topographic information of highly transparent micro-objects is obtained from the phase data. We evaluate our system by recording holograms of lines with different depths written by a focused laser beam. A reliable characterization of laser written microstructures is crucial for their functionality. Our results show that this system is valuable for determination of topological profiles of microstructures in transparent material.
Digital holography (DH) is capable of providing three-dimensional topological surface profiles with axial resolutions in the nanometer range. To achieve such high resolutions requires an analysis of the phase information of the reflected light by means of numerical reconstruction methods. Unfortunately, the phase analysis of structures located in scattering media is usually disturbed by interference with reflected light from different depths. In contrast, low-coherence interferometry and optical coherence tomography (OCT) use broadband light sources to investigate the sample with a coherence gate providing tomographic measurements in scattering samples with a poorer depth-resolution of a few micrometers. We propose a new approach that allows recovering the phase information even through scattering media. The approach combines both techniques by creating synthesized interference patterns from scanned spectra. After applying an inverse Fourier transform to each spectrum, we yield three-dimensional depth-resolved images. Subsequently, contributions of photons scattered from unwanted regions are suppressed by depth-filtering. The back-transformed data can be considered as multiple synthesized holograms and the corresponding phase information can be extracted directly from the depthfiltered spectra. We used this approach to record and reconstruct holograms of a reflective surface through a scattering layer. Our results demonstrate a proof-of-principle, as the quantitative phase-profile could be recovered and effectively separated from scattering influences. Moreover, additional processing steps could pave the way to further applications, i.e. spectroscopic analysis.