An automatic method for assembling a large synthetic aperture digital hologram

Complex image reconstruction and synthetic aperture laser imaging for moving targets based on direct-detection detector array

According to the principle of synthetic aperture ladar, high-resolution imaging can be achieved if the relative motion exists between the target and the ladar. The imaging system has characteristics including a large field of view, narrow-band laser signals applied, and easy engineering implementation. The complex image reconstruction and the synthetic aperture laser imaging method for moving targets based on the spatial light modulator and the direct-detection detector array are proposed. The far-field simulations and the near-field experiments for the stop-and-go target and the continuous-moving target were carried out. It is verified that the complex image reconstruction method can equivalently realize coherent detection for the target and reflect its phase information corresponding to the laser wavelength. Multi-frame complex images reconstructed can be applied to the synthetic aperture laser imaging, which forms high-resolution images for moving targets under far/near-field conditions.

Stitching sub-aperture in digital holography based on machine learning

Sub-aperture stitching in digital holography (DH) is a very important issue both for the spatial resolution improvement as well as for measuring larger aperture through synthetic enlargement of numerical aperture. In fact, sub-apertures stitching permits to greatly expand the capabilities of optical metrology thus allowing to accurately measure complex optical surfaces such as large spherical and aspheric. Stitching operations can be difficult and cumbersome depending on geometric parameters of specific objects under test. However, here we show that machine learning can definitively aid this process. In fact, here we propose for the first time, to the best of our knowledge, a novel sub-aperture stitching approach based on machine learning applied to an array of different phase-maps sub-apertures recorded by an off-axis digital holographic systems. Essentially, we construct a network according to computation model of sub-aperture stitching and remove the alignment errors and system aberration of sub-aperture maps by training the network. Correct measurement of the surface topography of hemisphere surface is demonstrated thus validating the proposed learning approach. Reported results demonstrate that machine learning can be a useful tool for simplifying the process and for making it a reliable and accurate tool in optical metrology.

Adaptive dual-exposure fusion-based transport of intensity phase microscopy

Via the transport of intensity phase microscopy, quantitative phase can be retrieved directly from captured multi-focal intensities. The accuracy of the retrieved phases depends highly on the quality of the recorded images; therefore, the exposure time should be carefully chosen for high-quality intensity captures. However, it is difficult to record well-exposure intensities to maintain rather a high signal to noise ratio and to avoid over-exposure due to the complex samples. In order to simplify the exposure determination, here the adaptive dual-exposure fusion-based transport of intensity phase microscopy is proposed: with captured short- and long-exposure images, the well-exposure multi-focal images can be numerically reconstructed, and then high-accurate phase can be computed from these reconstructed intensities. With both simulations and experiments provided in this paper, it is proved that the adaptive dual-exposure fusion-based transport of intensity phase microscopy not only provides numerically reconstructed well-exposure image with simple operation and fast speed but also extracts highly accurate retrieved phase. Moreover, the exposure time selection scope of the proposed method is much wider than that based on single exposure, and even though there is an over-exposure region in the long-exposure image, a well-exposure image can still be reconstructed with high precision. Considering its advantages of high accuracy, fast speed, simple operation, and wide application scope, the proposed technique can be adopted as quantitative phase microscopy for high-quality observations and measurements.

Wavefront-sensing-based autofocusing in microscopy

Massive image acquisition is required along the optical axis in the classical image-analysis-based autofocus method, which significantly decreases autofocus efficiency. A wavefront-sensing-based autofocus technique is proposed to increase the speed of autofocusing and obtain high localization accuracy. Intensities at different planes along the optical axis can be computed numerically after extracting the wavefront at defocus position with the help of the transport-of-intensity equation method. According to the focus criterion, the focal plane can then be determined, and after sample shifting to this plane, the in-focus image can be recorded. The proposed approach allows for fast, precise focus detection with fewer image acquisitions compared to classical image-analysis-based autofocus techniques, and it can be applied in commercial microscopes only with an extra illumination filter.

Ultra-high speed digital micro-mirror device based ptychographic iterative engine method

To reduce the long data acquisition time of the common mechanical scanning based Ptychographic Iterative Engine (PIE) technique, the digital micro-mirror device (DMD) is used to form the fast scanning illumination on the sample. Since the transverse mechanical scanning in the common PIE is replaced by the on/off switching of the micro-mirrors, the data acquisition time can be reduced from more than 15 minutes to less than 20 seconds for recording 12 × 10 diffraction patterns to cover the same field of 147.08 mm2. Furthermore, since the precision of DMD fabricated with the optical lithography is always higher than 10 nm (1 μm for the mechanical translation stage), the time consuming position-error-correction procedure is not required in the iterative reconstruction. These two improvements fundamentally speed up both the data acquisition and the reconstruction procedures in PIE, and relax its requirements on the stability of the imaging system, therefore remarkably improve its applicability for many practices. It is demonstrated experimentally with both USAF resolution target and biological sample that, the spatial resolution of 5.52 μm and the field of view of 147.08 mm2 can be reached with the DMD based PIE method. In a word, by using the DMD to replace the translation stage, we can effectively overcome the main shortcomings of common PIE related to the mechanical scanning, while keeping its advantages on both the high resolution and large field of view.

Rapid in-focus corrections on quantitative amplitude and phase imaging using transport of intensity equation method

Transport of intensity equation (TIE) method can acquire sample phase distributions with high speed and accuracy, offering another perspective for cellular observations and measurements. However, caused by incorrect focal plane determination, blurs and halos are induced, decreasing resolution and accuracy in both retrieved amplitude and phase information. In order to obtain high-accurate sample details, we propose TIE based in-focus correction technique for quantitative amplitude and phase imaging, which can locate focal plane and then retrieve both in-focus intensity and phase distributions combining with numerical wavefront extraction and propagation as well as physical image recorder translation. Certified by both numerical simulations and practical measurements, it is believed the proposed method not only captures high-accurate in-focus sample information, but also provides a potential way for fast autofocusing in microscopic system.

Laser Light-field Fusion for Wide-field Lensfree On-chip Phase Contrast Microscopy of Nanoparticles

Wide-field lensfree on-chip microscopy, which leverages holography principles to capture interferometric light-field encodings without lenses, is an emerging imaging modality with widespread interest given the large field-of-view compared to lens-based techniques. In this study, we introduce the idea of laser light-field fusion for lensfree on-chip phase contrast microscopy for detecting nanoparticles, where interferometric laser light-field encodings acquired using a lensfree, on-chip setup with laser pulsations at different wavelengths are fused to produce marker-free phase contrast images of particles at the nanometer scale. As a proof of concept, we demonstrate, for the first time, a wide-field lensfree on-chip instrument successfully detecting 300 nm particles across a large field-of-view of ~30 mm² without any specialized or intricate sample preparation, or the use of synthetic aperture- or shift-based techniques.

Propagation phasor approach for holographic image reconstruction

To achieve high-resolution and wide field-of-view, digital holographic imaging techniques need to tackle two major challenges: phase recovery and spatial undersampling. Previously, these challenges were separately addressed using phase retrieval and pixel super-resolution algorithms, which utilize the diversity of different imaging parameters. Although existing holographic imaging methods can achieve large space-bandwidth-products by performing pixel super-resolution and phase retrieval sequentially, they require large amounts of data, which might be a limitation in high-speed or cost-effective imaging applications. Here we report a propagation phasor approach, which for the first time combines phase retrieval and pixel super-resolution into a unified mathematical framework and enables the synthesis of new holographic image reconstruction methods with significantly improved data efficiency. In this approach, twin image and spatial aliasing signals, along with other digital artifacts, are interpreted as noise terms that are modulated by phasors that analytically depend on the lateral displacement between hologram and sensor planes, sample-to-sensor distance, wavelength, and the illumination angle. Compared to previous holographic reconstruction techniques, this new framework results in five- to seven-fold reduced number of raw measurements, while still achieving a competitive resolution and space-bandwidth-product. We also demonstrated the success of this approach by imaging biological specimens including Papanicolaou and blood smears.

Color holographic display with white light LED source and single phase only SLM

This work presents color holographic display, which is based on a single phase only spatial light modulator (SLM). In the display entire area of the SLM is illuminated by an on-axis white light beam generated by a single large LED. The holographic display fully utilizes SLM bandwidth and has capability of full-color, full frame rate imaging of outstanding quality. This is achieved through: (i) optimal use of the source coherence volume, (ii) application of the single white light LED source, (iii) a development of a novel concept of color multiplexing technique with color filter mask in Fourier plane of the SLM, (iv) and a complex coding with improved diffraction efficiency. Within experimental part of the paper we show single color, full-color holographic 2D and 3D images generated for reconstruction depth exceeding 10 cm.

Optimization of galvanometer scanning for optical coherence tomography

We study experimentally the effective duty cycle of galvanometer-based scanners (GSs) with regard to three main parameters of the scanning process: theoretical/imposed duty cycle (of the input signal), scan frequency, and scan amplitude. Sawtooth and triangular input signals for the device are considered. The effects of the mechanical inertia of the oscillatory element of the GS are analyzed and their consequences are discussed in the context of optical coherence tomography (OCT) imaging. When the theoretical duty cycle and the scan amplitude are increased to the limit, the saturation of the device is demonstrated for a useful range of scan frequencies by direct measurement of the position of the galvomirror. Investigations of OCT imaging of large samples also validate this saturation, as examplified by the gaps/blurred portions obtained between neighboring images when using both triangular and sawtooth scanning at high scan frequencies. For this latter aspect, the necessary overlap between neighboring B-scans, and therefore between the corresponding volumetric reconstructions of the sample, are evaluated and implemented with regard to the same parameters of the scanning process. OCT images that are free of these artifacts are thus obtained.

Field-portable pixel super-resolution colour microscope

Based on partially-coherent digital in-line holography, we report a field-portable microscope that can render lensfree colour images over a wide field-of-view of e.g., >20 mm(2). This computational holographic microscope weighs less than 145 grams with dimensions smaller than 17×6×5 cm, making it especially suitable for field settings and point-of-care use. In this lensfree imaging design, we merged a colorization algorithm with a source shifting based multi-height pixel super-resolution technique to mitigate 'rainbow' like colour artefacts that are typical in holographic imaging. This image processing scheme is based on transforming the colour components of an RGB image into YUV colour space, which separates colour information from brightness component of an image. The resolution of our super-resolution colour microscope was characterized using a USAF test chart to confirm sub-micron spatial resolution, even for reconstructions that employ multi-height phase recovery to handle dense and connected objects. To further demonstrate the performance of this colour microscope Papanicolaou (Pap) smears were also successfully imaged. This field-portable and wide-field computational colour microscope could be useful for tele-medicine applications in resource poor settings.

Increased space-bandwidth product in pixel super-resolved lensfree on-chip microscopy

Pixel-size limitation of lensfree on-chip microscopy can be circumvented by utilizing pixel-super-resolution techniques to synthesize a smaller effective pixel, improving the resolution. Here we report that by using the two-dimensional pixel-function of an image sensor-array as an input to lensfree image reconstruction, pixel-super-resolution can improve the numerical aperture of the reconstructed image by ~3 fold compared to a raw lensfree image. This improvement was confirmed using two different sensor-arrays that significantly vary in their pixel-sizes, circuit architectures and digital/optical readout mechanisms, empirically pointing to roughly the same space-bandwidth improvement factor regardless of the sensor-array employed in our set-up. Furthermore, such a pixel-count increase also renders our on-chip microscope into a Giga-pixel imager, where an effective pixel count of ~1.6–2.5 billion can be obtained with different sensors. Finally, using an ultra-violet light-emitting-diode, this platform resolves 225 nm grating lines and can be useful for wide-field on-chip imaging of nano-scale objects, e.g., multi-walled-carbon-nanotubes.

On-chip biomedical imaging

Lab-on-a-chip systems have been rapidly emerging to pave the way toward ultra-compact, efficient, mass producible and cost-effective biomedical research and diagnostic tools. Although such microfluidic and microelectromechanical systems have achieved high levels of integration, and are capable of performing various important tasks on the same chip, such as cell culturing, sorting and staining, they still rely on conventional microscopes for their imaging needs. Recently, several alternative on-chip optical imaging techniques have been introduced, which have the potential to substitute conventional microscopes for various lab-on-a-chip applications. Here we present a critical review of these recently emerging on-chip biomedical imaging modalities, including contact shadow imaging, lens-free holographic microscopy, fluorescent on-chip microscopy and lens-free optical tomography.