Wavefront image sensor chip

We report the implementation of an image sensor chip, termed wavefront image sensor chip (WIS), that can measure both intensity/amplitude and phase front variations of a light wave separately and quantitatively. By monitoring the tightly confined transmitted light spots through a circular aperture grid in a high Fresnel number regime, we can measure both intensity and phase front variations with a high sampling density (11 µm) and high sensitivity (the sensitivity of normalized phase gradient measurement is 0.1 mrad under the typical working condition). By using WIS in a standard microscope, we can collect both bright-field (transmitted light intensity) and normalized phase gradient images. Our experiments further demonstrate that the normalized phase gradient images of polystyrene microspheres, unstained and stained starfish embryos, and strongly birefringent potato starch granules are improved versions of their corresponding differential interference contrast (DIC) microscope images in that they are artifact-free and quantitative. Besides phase microscopy, WIS can benefit machine recognition, object ranging, and texture assessment for a variety of applications

A generalized noise variance analysis model and its application to the characterization of 1/f noise

We present a novel generalized model for the analysis of noise with a known spectral density. This model is particularly appropriate for the analysis of noise with a 1/f^a distribution in a homodyne interferometer. The noise model reveals that, for α>1, 1/f^a noise significantly impacts the homodyne signal-to-noise ratio (SNR) for integration times that near a characteristic time, beyond which the SNR will no longer significantly improve with increasing integration time. We experimentally verify our theoretical findings with a set of experiments employing a quadrature homodyne optical coherence tomography (OCT) system, finding good agreement. The characteristic integration time is measured to be approximately 2 ms for our system. Additionally, we find that the 1/f noise characteristics, including the exponent, α, as well as the characteristic integration time, are system and photodetector dependent

The application of Fresnel zone plate based projection in optofluidic microscopy

Optofluidic microscopy (OFM) is a novel technique for low-cost, high-resolution on-chip microscopy imaging. In this paper we report the use of the Fresnel zone plate (FZP) based projection in OFM as a cost-effective and compact means for projecting the transmission through an OFM's aperture array onto a sensor grid. We demonstrate this approach by employing a FZP (diameter = 255 µm, focal length = 800 µm) that has been patterned onto a glass slide to project the transmission from an array of apertures (diameter = 1 µm, separation = 10 µm) onto a CMOS sensor. We are able to resolve the contributions from 44 apertures on the sensor under the illumination from a HeNe laser (wavelength = 633 nm). The imaging quality of the FZP determines the effective field-of-view (related to the number of resolvable transmissions from apertures) but not the image resolution of such an OFM system -- a key distinction from conventional microscope systems. We demonstrate the capability of the integrated system by flowing the protist Euglena gracilis across the aperture array microfluidically and performing OFM imaging of the samples

Harmonically-related diffraction gratings-based interferometer for quadrature phase measurements

We demonstrate the use of shallow diffraction gratings for quadrature phase interferometry. A single shallow diffraction grating-based Michelson interferometer yields only trivial (0° or 180°) phase shift between different output ports. In comparison, a combination of two parallel shallow diffraction gratings can be useful to achieve desired phase shifts (e.g., 90° for quadrature phase interferometry). We show that the phase at different output ports of a grating-pair based interferometer can be adjusted by shearing the two gratings with respect to each other. Two harmonically-related diffraction gratings are used to demonstrate phase shift control at the output ports of a modified Michelson interferometer. Our experimental data is in good agreement with theory

Quantitative surface normal measurement by a wavefront camera

A compact wavefront camera that allows users to quantitatively measure the intensity and wavefront at a remote object plane is reported. The camera is built from a chip-scale wavefront sensor that we previously developed. By measuring the wavefront of the image and calibrating the wavefront relationship between the image and object planes, the wavefront at the object plane can be computed and the surface normal of the object can be derived. We built a prototype camera and calibrated the wavefront relationship. In a proof-of-concept experiment, a set of concave mirrors with different focal lengths (50–200 mm), were imaged. The results agree well with their expected values. To demonstrate the application of the camera, we applied this method to measure the deformation of a microfluidic channel under pressure

Optofluidic microscope: a complete on-chip imaging device

This paper reports a complete on-chip high resolution lensless imaging device based on the optofluidic microscopy method, which can form a vital optical microscopy component in a wide range of lab-on-a-chip systems. This imaging device does not use any lens elements and yet is capable of resolution comparable to that of a conventional microscope with a 20× objective. We demonstrate the use of the device for Caenorhabditis elegans and microsphere imaging at a resolution of ~ 1 μm with an imaging time of ~2 sec. The fabrication of this on-chip imaging device is fully compatible with existing semiconductor and microfluidic technologies, so the device can be massively fabricated and integrated into microsystems to form compact and low-cost total analysis systems for biological and colloidal studies

Investigation of the Probe-Sample Interaction in the Ultrasonic/Shear-Force Microscope: The Phononic Friction Mechanism

The dissipative and conservative interactions between a sharp probe and a flat Si sample in the ultrasonic/shear-force microscope are investigated. It is shown that, when working in the ambient condition, there are two distinct probe-sample interaction regions: the pure dissipative interaction region in the relatively far probe-sample distance, and the highly correlated dissipative and conservative interaction region in the close probe-sample distance. The ultrasonic data suggest that the phonon generation is a dissipative channel for the probe-sample interaction in the shear force microscope. A shaking potential model is proposed to explain the phononic friction mechanis

A wide field-of-view microscope based on holographic focus grid

We have developed a novel microscope technique that can achieve wide field-of-view (FOV) imaging and yet possess resolution that is comparable to conventional microscope. The principle of wide FOV microscope system breaks the link between resolution and FOV magnitude of traditional microscopes. Furthermore, by eliminating bulky optical elements from its design and utilizing holographic optical elements, the wide FOV microscope system is more cost-effective. In our system, a hologram was made to focus incoming collimated beam into a focus grid. The sample is put in the focal plane and the transmissions of the focuses are detected by an imaging sensor. By scanning the incident angle of the incoming beam, the focus grid will scan across the sample and the time-varying transmission can be detected. We can then reconstruct the transmission image of the sample. The resolution of microscopic image is limited by the size of the focus formed by the hologram. The scanning area of each focus spot is determined by the separation of the focus spots and can be made small for fast imaging speed. We have fabricated a prototype system with a 2.4-mm FOV and 1-μm resolution. The prototype system was used to image onion skin cells for a demonstration. The preliminary experiments prove the feasibility of the wide FOV microscope technique, and the possibility of a wider FOV system with better resolution

Full field phase imaging using a harmonically matched diffraction grating pair based homodyne quadrature interferometer

In this letter, the authors present a novel quadrature interferometry method based on the use of a harmonically matched shallow grating pair. Unlike a simple beam splitter or single shallow grating, the grating pair can confer a nontrivial interference phase shift (other than 0° or 180°) between the output ports of the interferometer. Using the grating pair as the beam splitter/combiner, the authors implement a homodyne quadrature full field phase interferometer and demonstrate the system's capability to acquire phase and amplitude images

Two-dimensional differential interference contrast microscopy based on four-hole variation of Young's interference

We demonstrate a novel method of two-dimensional differential interference contrast (DIC) microscopy. Our method is cheaper, more compact, and more robust compared to conventional DIC microscopes; since it uses a simple variation of Young's double-slit geometry, no expensive or complex optical components are needed. In addition, our method quantitatively measures differential phase, unlike conventional DIC, which makes our device useful for optical metrology and cell biology applications. The device consists of four circular holes arranged in a "plus" pattern, milled into a metal layer 80 μm above a complimentary metal-oxide semiconductor (CMOS) image sensor. Light incident upon the four-hole aperture is transmitted through the holes and creates an interference pattern on the CMOS sensor. This pattern shifts as a function of the spatial phase gradient of the incident light. By capturing the amplitude and location of the zero-order fringe of the interference pattern, the amplitude and differential phase of the incident light can be measured simultaneously. In this article, we model the response of the device using both geometric optics and Huygens principle. We then verify these models by experimentally measuring the responsivity of our device. A short analysis on the algorithm used to calculate the fringe location follows. We then show a beam profiling application by measuring the amplitude and spatial phase gradient of a Gaussian laser beam and an optical vortex. Finally, we show a DIC microscope application; we image a phase mask of the letters "CIT"