Harmonically matched grating-based full-field quantitative high-resolution phase microscope for observing dynamics of transparent biological samples

We have developed a full-field high resolution quantitative phase imaging technique for observing dynamics of transparent biological samples. By using a harmonically matched diffraction grating pair (600 and 1200 lines/mm), we were able to obtain non-trivial phase difference (other than 0° or 180°) between the output ports of the gratings. Improving upon our previous design, our current system mitigates astigmatism artifacts and is capable of high resolution imaging. This system also employs an improved phase extraction algorithm. The system has a lateral resolution of 1.6 μm and a phase sensitivity of 62 mrad. We employed the system to acquire high resolution phase images of onion skin cells and a phase movie of amoeba proteus in motion

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

SNR enhancement through phase dependent signal reconstruction algorithms for phase separated interferometric signals

We report several signal reconstruction algorithms for processing phase separated homodyne interferometric signals. Methods that take advantage of the phase of the signal are experimentally shown to achieve a signal-to-noise ratio (SNR) improvement of up to 5 dB over commonly used algorithms. To begin, we present a derivation of the SNR resulting from five image reconstruction algorithms in the context of a 3x3 fiber-coupler based homodyne optical coherence tomography (OCT) system, and clearly show the improvement in SNR associated with phase-based algorithms. Finally, we experimentally verify this improvement and demonstrate the enhancement in contrast and improved image quality afforded by these algorithms through homodyne OCT imaging of a Xenopus laevis tadpole. These algorithms can be generally applied in signal extraction processing where multiple phase separated measurements are available

Low-loss wavelength-multiplexed optical scanners using volume Bragg gratings for transmit-receive lasercom systems

Low-loss no moving parts free-space wavelength-multiplexed optical scanner (W-MOS) modules for transmit-receive lasercom systems are proposed and experimentally demonstrated. The proposed scanners are realized using volume Bragg gratings stored in dichromated gelatin (DCG) coupled with high-speed wavelength selection such as by a fast tunable laser. The potential speed of these scanners is in the Gigahertz range using present-day state-of-the-art nanosecond tuning speed lasers. A 940-lines/mm volume Bragg grating stored in dichromated gelatin is used to demonstrate the scanners. Angular dispersion and diffraction efficiency of the volume Bragg grating used for demonstration are studied versus wavelength and angle of incidence to determine the free-space W-MOS angular scan and insertion loss, respectively. Experimental results show that a tunable bandwidth of 80 nm, centered at 1560 nm, delivers an angular scan of 6.25 deg. The study also indicates that an in-line scanner design realized using two similar Bragg gratings in DCG delivers 13.42 deg angular scan, which is more than double the angular scan available from the free-space W-MOS using single volume Bragg grating

Paired-angle-rotation scanning optical coherence tomography forward-imaging probe

We report a novel forward-imaging optical coherence tomography (OCT), needle-probe paired-angle-rotation scanning OCT (PARS-OCT) probe. The probe uses two rotating angled gradient-index lenses to scan the output OCT probe beam over a wide angular arc (∼19° half-angle) of the region forward of the probe. Among other advantages, this probe design is readily amenable to miniaturization and is capable of a variety of scan modes, including volumetric scans. To demonstrate the advantages of the probe design, we have constructed a prototype probe with an outer diameter of 1.65 mm and employed it to acquire four OCT images, with a 45° angle between adjacent images, of the gill structure of a Xenopus laevis tadpole. The system sensitivity was measured to be 93 dB by using the prototype probe with an illumination power of 450 μW on the sample. Moreover, the axial and the lateral resolutions of the probe are 9.3 and 10.3-12.5 μm, respectively

Pump-probe scheme for optical coherence tomography using indocyanine green mixed with albumin or human plasma

Use of indocyanine green (ICG) in a pump-probe scheme for OCT is proposed. The study illustrates that ICG in protein solution shows unusual pump-probe imaging potential, indicating its usefulness as a contrast agent for OCT

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

Slanted hole array beam profiler (SHArP)—a high-resolution portable beam profiler based on a linear aperture array

We demonstrate a novel high-resolution portable beam profiler based on a slanted linear array of small apertures, termed a slanted hole array beam profiler (SHArP). The apertures are directly fabricated on a metal-coated CMOS imaging sensor. With a single linear scan, the aperture array can establish a virtual grid of sampling points for beam profiling. With our prototype, we demonstrate beam profiling of Gaussian beams over an area of 66.5 μm×66.5 μm with a resolution of 0.8 μm (compare with the CMOS pixel size of 10 μm). The resolution can be improved into the range of submicrometers by fabricating smaller apertures. The good correspondence between the measured and calculated beam profiles proves the fidelity of our new beam profiling scheme