Amplification of optical delay by use of matched linearly chirped fiber Bragg gratings

We describe the use of a matched linearly chirped fiber Bragg grating (FBG) pair as a key element in an adjustable optical delay line. This delay line has the unique property that the achievable optical group delay is orders of magnitude greater (factor of 10^2 in our experiment) than the actual physical displacement. We demonstrate operation of such an optical delay line over a delay range of 3.5 mm using a pair of matched 1300-nm chirped FBGs with a bandwidth of 20 nm each

Frequency estimation precision in Doppler optical coherence tomography using the Cramer-Rao lower bound

Doppler optical coherence tomography (DOCT) is a technique for simultaneous cross-sectional imaging of tissue structure and blood flow. We derive the fundamental uncertainty limits on frequency estimation precision in DOCT using the Cramer-Rao lower bound in the case of additive (e.g., thermal, shot) noise. Experimental results from a mirror and a scattering phantom are used to verify the theoretical limits. Our results demonstrate that the stochastic nature of frequency noise influences the precision of flow imaging, and that the noise model must be selected judiciously in order to estimate the frequency precision

Molecular contrast in optical coherence tomography using a pump-probe technique and a optical switch suppression technique

We describe two novel techniques for contrast enhancement in optical coherence tomography (OCT) which enables molecular specific imaging. The first, a pump-probe technique, is employed in which a pulsed pump laser is tuned to ground-state absorption in a molecule of interest. The location of the target molecule population is derived from the resulting transient absorption of OCT sample arm light acting as probe light. Preliminary results exhibiting contrast enhancement in cross-sectional OCT images using methylene blue dye are presented. The second method is an optical switch suppression technique based on the use of a transmembrane protein called bacteriorhodopsin. Initial experiments indicate that biochemical optical switches, such as bacteriorhodopsin, are excellent contrast agent candidates for molecular contrast OCT

Molecular contrast in optical coherence tomography using a pump-probe technique and a optical switch suppression technique

We describe two novel techniques for contrast enhancement in optical coherence tomography (OCT) which enables molecular specific imaging. The first, a pump-probe technique, is employed in which a pulsed pump laser is tuned to ground-state absorption in a molecule of interest. The location of the target molecule population is derived from the resulting transient absorption of OCT sample arm light acting as probe light. Preliminary results exhibiting contrast enhancement in cross-sectional OCT images using methylene blue dye are presented. The second method is an optical switch suppression technique based on the use of a transmembrane protein called bacteriorhodopsin. Initial experiments indicate that biochemical optical switches, such as bacteriorhodopsin, are excellent contrast agent candidates for molecular contrast OCT

Plasma Membrane Temperature Gradients and Multiple Cell Permeabilization Induced by Low Peak Power Density Femtosecond Lasers

Calculations indicate that selectively heating the extracellular media induces membrane temperature gradients that combine with electric fields and a temperature-induced reduction in the electro- permeabilization threshold to potentially facilitate exogenous molecular delivery. Experiments by a wide-field, pulsed femtosecond laser with peak power density far below typical single cell optical de- livery systems confirmed this hypothesis. Operating this laser in continuous wave mode at the same average power permeabilized many fewer cells, suggesting that bulk heating alone is insufficient and temperature gradients are crucial for permeabilization. This work suggests promising opportunities for a high throughput, low cost, contactless method for laser mediated exogenous molecule delivery without the complex optics of typical single cell optoinjection, for potential integration into microscope imaging and microfluidic systems

Autoconfocal transmission microscopy based on two-photon-induced photocurrent of Si photodiodes

We describe a simple, self-aligned confocal transmission microscopy technique based on two-photon-induced photocurrents of silicon photodiodes. Silicon detectors produce photocurrents in quadratic dependence on incident intensity under the pulsed illumination of light with wavelengths longer than 1.2 m. We exploit this nonlinear process to reject out-of-focus background and perform depth-sectioning microscopic imaging. We demonstrate a comparable background rejection capability of the technique to linear confocal detection and present three-dimensional imaging in biological specimens. © 2009 Optical Society of America OCIS codes: 180.0180, 180.1790 Laser-scanning confocal microscopy (LSCM) is capable of producing high-contrast, high-resolution images of biological and material specimens with optical sectioning capability. The improved image contrast and depth sectioning in LSCM is enabled by a physical pinhole placed in front of the image plane, which allows in-focus portion of light to be measured while rejecting stray light from out-of-focus background Here, we present another strategy for ACM based on two-photon-induced photocurrents of a silicon photodiode (Si-PD). Two-photon-induced photocurrents in solid-state devices have been observed and extensively used in many applications, including high-resolution defect imaging of integrated circuits We first examined a response of Si-PD under focused light illumination at ϳ1.55 m. Light from an erbium-doped fiber-based laser (Mercury 1000, PolarOnyx Inc., California) with a pulse duration ofϳ100 fs and repetition rate of ϳ50 MHz was focused onto an amplified Si-PD (PDA55, Thorlabs Inc., New Jersey) using an average incident optical power of ϳ30 mW and beam diameter of ϳ10 m. The Si-PD output was measured under continuous (CW) and pulsed wave (PW) illumination

Whole-body, real-time preclinical imaging of quantum dot fluorescence with time-gated detection

We describe a wide-field preclinical imaging system optimized for time-gated detection of quantum dot fluorescence emission. As compared to continuous wave measurements, image contrast was substantially improved by suppression of short-lifetime background autofluorescence. Real-time (8 frames∕s) biological imaging of subcutaneous quantum dot injections is demonstrated simultaneously in multiple living mice

Multiphoton microscopy with near infrared contrast agents

While multiphoton microscopy (MPM) has been performed with a wide range of excitation wavelengths, fluorescence emission has been limited to the visible spectrum. We introduce a paradigm for MPM of near-infrared (NIR) fluorescent molecular probes via nonlinear excitation at 1550 nm. This all-NIR system expands the range of available MPM fluorophores, virtually eliminates background autofluorescence, and allows for use of fiber-based, turnkey ultrafast lasers developed for telecommunications