Accurate shape measurement of focusing microstructures in Fourier digital holographic microscopy

P-TDHM: Open-source portable telecentric digital holographic microscope

We present the design of a low-cost, portable telecentric digital holographic microscope (P-TDHM) that utilizes off-the-shelf components. We describe the system's hardware and software elements and evaluate its performance by imaging samples ranging from nano-printed targets to live HeLa cells, HEK293 cells, and Dolichospermum via both in-line and off-axis modes. Our results demonstrate that the system can acquire high quality quantitative phase images with nanometer axial and sub-micron lateral resolution in a small form factor, making it a promising candidate for resource-limited settings and remote locations. Our design represents a significant step forward in making telecentric digital holographic microscopy accessible and affordable to the broader community.

Spherical wave illumination scanning digital holographic profilometry

In this work, we proposed what we believe to be a novel scanning solution for the assessment of high-NA samples, referred to as spherical-wave illumination scanning digital holographic profilometry (SWS-DHP). This approach introduces a 2F optimization methodology, based on the measurement of the focal length of the object to determine the spherical component of the scanning. Furthermore, re-optimization of 2F, whether it needs to be operated depends on the measured object's NA to inspect more information. Meanwhile, utilizing phase space analysis shows SWS superiority in information transfer for high-NA samples compared to plane-wave illumination scanning. In addition, this method introduces a shape reconstruction algorithm with volumetric aberration compensation based on the propagation of the aberrated object and illumination waves to obtain high-quality measurements. Finally, the imaging merits of SWS-DHP were proved through simulations and were experimentally verified for the object of NA up to 0.87.

Speckle reduction in digital holography with non-local means filter based on the Pearson correlation coefficient and Butterworth filter

This Letter presents a non-local means filter based on the Pearson correlation coefficient and Butterworth high-pass filter. In the method, the new gray value of the denoising pixel is equal to the weighted sum of the surrounding pixel values. We use the Pearson correlation coefficient between the pixels to calculate the weight of the surrounding pixels to the denoising pixel, then use Butterworth high-pass filter to optimize. Experimental results show that the method effectively reduces the speckle noise of digital holography and the image details are also very rich. At the same time, its performance is still better when compared with methods such as BM3D.

Measurement algorithm for real front and back curved surfaces of contact lenses

The optical measurement algorithm for the real front and back surfaces of contact lenses from their center to periphery accurately and simultaneously is proposed. It is an algorithm that makes light incident vertically along the curved surfaces of contact lenses under the condition that the difference of curvature radii between the front and back surfaces is small enough within the NA of the optical probe. For this purpose, we adopted time-domain optical coherence tomography (OCT) with translation and rotation mechanisms. The shape, thickness distribution, and curvature radii of both surfaces were estimated with OCT signal analysis and circular approximation. The measured results were compared with the designed values and the measured data from a conventional shape measurement device. The curved shape of both surfaces and thickness were well matched with the designed values from lens center to periphery. In a curvature radius of the front surface, there was a proportional bias with a limit of agreement of -0.77% to -2.09%, and the correlation coefficient was 0.57. On the back surface, there was no systematic bias, and minimal detectable change was 0.178 mm, in a range of 95% confidential interval. The proposed algorithm well visualized the real shape and optical characteristics of the contact lens with enough accuracy to the design.

Spatial bandwidth-optimized compression of image plane off-axis holograms with image and video codecs

Image plane off-axis holograms (IP-OAH) are the most common data captured in digital holographic microscopy and tomography. Due to increasing storage and data transmission requirements, lossy compression of such holograms has been subject of earlier investigations. However, hologram compression can not be allowed to hinder the metrological capabilities of the measurement technique itself. In this work, we present lossy and lossless IP-OAH compression approaches that are based on conventional compression codecs, but optimized with regard to bandwidth of the signal. Both approaches outperform respective conventional codecs, while the lossy approach is shown to uphold the accuracy of holographic phase measurements.

Optimal modified lateral shearing interferometer with axial range extension by using a dual optical plate

We present a method to extend the axial range of digital holographic microscopy based on the optimal modified lateral shearing interferometer (MLSI). The proposed system can extend the axial range by using a dual optical plate. The interference pattern with two spatial wavelengths is generated by the plate with different thicknesses. These spatial wavelengths transfer a dual spatial frequency into the Fourier plane by using FFT. Two phases are extracted by a dual spatial frequency and combined to create a synthetic wavelength, which is applied to measure the micrometer-scale object without phase unwrapping. Also, the noise-reducing algorithm is used to reduce phase noise caused by the amplified noise of the synthetic wavelength. The experimental result confirms the feasibility of the optimal MLSI by using a dual optical plate.

Monitoring of photochemically induced changes in phase-modulating samples with digital holographic microscopy

This paper analyzes the performance of single-shot digital holographic microscopy for rapid characterization of static step-index structures in transparent polymer materials and for online monitoring of the photoinduced polymerization dynamics. The experiments are performed with a modified Mach-Zehnder transmission digital holographic microscope of high stability (phase accuracy of 0.69°) and of high magnification (of ≈90×). Use of near-infrared illumination allows both nondestructive examination of the manufactured samples and monitoring of optically induced processes in a photosensitive material concurrently with its excitation. The accuracy of the method for a precise sample's topography evaluation is studied on an example of microchannel sets fabricated via two-photon polymerization and is supported by reference measurements with an atomic force microscope. The applicability of the approach for dynamic measurements is proved via online monitoring of the refractive index evolution in a photoresin layer illuminated with a focused laser beam at 405 nm. High correlation between the experimental results and a kinetics model for the photopolymerization process is achieved.

Super-resolution microscopy for biological specimens: lensless phase retrieval in noisy conditions

The paper is devoted to a computational super-resolution microscopy. A complex-valued wavefront of a transparent biological cellular specimen is restored from multiple intensity diffraction patterns registered with noise. For this problem, the recently developed lensless super-resolution phase retrieval algorithm [Optica, 4(7), 786 (2017)] is modified and tuned. This algorithm is based on a random phase coding of the wavefront and on a sparse complex-domain approximation of the specimen. It is demonstrated in experiments, that the reliable phase and amplitude imaging of the specimen is achieved for the low signal-to-noise ratio provided a low dynamic range of observations. The filterings in the observation domain and specimen variables are specific features of the applied algorithm. If these filterings are omitted the algorithm becomes a super-resolution version of the standard iterative phase retrieval algorithms. In comparison with this simplified algorithm with no filterings, our algorithm shows a valuable improvement in imaging with much smaller number of observations and shorter exposure time. In this way, presented algorithm demonstrates ability to work in a low radiation photon-limited mode.

Laser interference-based technique for dynamic measurement of single cell deformation manipulated by optical tweezers

A laser interference-based method was proposed to measure the deformation response of cell manipulated by optical tweezers. This method was implemented experimentally by integrating a laser illuminating system and optical tweezers with an inverted microscope. Interference fringes generated by the transmitted and reflected lights were recorded by a complementary metal oxide semiconductor camera. From the acquired images, cell height was calculated and cell morphology was constructed. To further validate this method, the morphological analyses of HeLa cells were performed in static state and during detachment process. Subsequently, the dynamic deformation responses of red blood cells were measured during manipulation with optical tweezers. Collectively, this laser interference-based method precludes the requirement of complex optical alignment, allows easy integration with optical tweezers, and enables dynamic measurement of cell deformation response by using a conventional inverted microscope.