P. M. W. French

Sub-100- mu m depth-resolved holographic imaging through scattering media in the near infrared.

We discuss the compromise between depth and transverse spatial resolution for photorefractive holographic imaging through turbid media. Results from an optimized geometry for a 45°-cut rhodium-doped barium titanate photorefractive crystal are presented, demonstrating two-dimensional imaging through turbid media with both sub-100-μm depth and transverse spatial resolution.

DEPTH-RESOLVED HOLOGRAPHIC IMAGING THROUGH SCATTERING MEDIA BY PHOTOREFRACTION

A depth-resolved near-infrared imaging system has been demonstrated for recording three-dimensional images of objects embedded in diffuse media. Time-gated holographic imaging employing rhodium-doped barium titanate as the recording medium is used to acquire whole depth-resolved two-dimensional images in 1 s. Millimeter depth resolution has been achieved with a transverse resolution of ~ 30 microm.

Holographic optical coherence imaging of tumor spheroids

We present depth-resolved coherence-domain images of living tissue using a dynamic holographic semiconductor film. An AlGaAs photorefractive quantum-well device is used in an adaptive interferometer that records coherent backscattered (image-bearing) light from inside rat osteogenic sarcoma tumor spheroids up to 1 mm in diameter in vitro. The data consist of sequential holographic image frames at successive depths through the tumor represented as a visual video “fly-through.” The images from the tumor spheroids reveal heterogeneous structures presumably caused by necrosis and microcalcifications characteristic of human tumors in their early avascular growth.

2-D fluorescence lifetime imaging using a time-gated image intensifier

Abstract We report a 2-D fluorescence lifetime imaging system based on a time-gated image intensifier and a Cr:LiSAF regenerative amplifier. We have demonstrated 185 ps temporal resolution. The deleterious effects of optical scattering are demonstrated.

Single-shot phase-stepped wide-field coherence-gated imaging

We present a single-shot wide-field CCD based coherence-gated imaging technique that utilizes spatially separated phase-stepped images and requires only one CCD camera to achieve simultaneous acquisition of four phase-stepped images. This technique provides a relatively low cost system for depth-resolved imaging of dynamic samples. We demonstrate real-time coherence-gated imaging of a moving watch cog, 3D reconstructions of a coin, phase measurements of the surface of a test-chart and depth-resolved imaging in a weakly scattering sample of onion.

Toward the clinical application of time-domain fluorescence lifetime imaging

High-speed (video-rate) fluorescence lifetime imaging (FLIM) through a flexible endoscope is reported based on gated optical image intensifier technology. The optimization and potential application of FLIM to tissue autofluorescence for clinical applications are discussed.

Real-time time-domain fluorescence lifetime imaging including single-shot acquisition with a segmented optical image intensifier

High-speed (video-rate) fluorescence lifetime imaging (FLIM) is reported using two different time-domain approaches based on gated optical image intensifier technology. The first approach utilizes a rapidly switchable variable delay generator with sequential image acquisition, while the second employs a novel segmented gated optical imager to acquire lifetime maps in a single shot. Lifetimes are fitted using both a non-linear least-squares fit analysis and the rapid lifetime determination method. Monte Carlo simulations were used to optimize the acquisition parameters and a comparison between theory and experiment is presented. The importance of single-shot imaging to minimize the deleterious impact of sample movements is highlighted. Real-time FLIM movies of multi-well plate samples and tissue autofluorescence are presented.

Fluorescence lifetime system for microscopy and multiwell plate imaging with a blue picosecond diode laser

We report a wide-field fluorescence lifetime imaging (FLIM) system that uses a blue picosecond pulsed diode laser as the excitation source. This represents a significant miniaturization and simplification compared with other time-domain FLIM instruments that should accelerate the development of clinical and real-world applications of FLIM. We have demonstrated this instrument in two configurations: a macroimaging setup applied to multiwell plate assays of chemically and biologically interesting fluorophores and a microscope system that has been applied to imaging of tissue sections. The importance of the adjustable repetition rate of this laser source is discussed with respect to noise reduction and precision in the lifetime determination, illustrating a further significant advantage over conventional mode-locked solid-state lasers.

Generation of 33-fs pulses from a passively mode-locked Cr 3+ :LiSrAlF 6 laser

Femtosecond and picosecond operating regimes of an argon-ion-pumped Cr(3+):LiSrAlF(6) laser, passively mode locked using a saturable absorber, are described. At low absorber concentrations, self-starting cw mode locking is demonstrated, which yields pulses as short as 33 fs after extracavity prism compensation. At higher concentrations, both cw mode-locked and Q-switched mode-locked operation is possible. the recovery time of the absorber is discussed.

Continuous-wave mode-locked Cr 4+ :YAG laser

A cw room-temperature Cr(4+):YAG laser, tuning from 1.37 to 1.51 microm, is described. Mode locking of this novel laser is reported for what is to our knowledge the first time.