Wide-field low coherence photorefractive holography has the potential to acquire coherence gated images at over 1000 frames/second, including through scattering media. We demonstrate photorefractive holography using a photorefractive multiple quantum well device and we demonstrate that it can be applied to real-time imaging of a moving watch cog. The use of higher frame rate of the CCD cameras will permit imaging at even greater frame.
We present a characterization of high-speed wide-field coherence-gated imaging using photorefractive holography with GaAs/AlGaAs Photorefractive Multiple Quantum Well (PRWQ) devices. Results are obtained with broadband c.w. laser illumination. The limitations of photorefractive holography using PRQW devices in our non-degenerate four-wave mixing geometry are discussed in terms of typical PRQW device parameters. We discuss the effect of the sensitivity and dynamic range of the CCD camera used to record the diffracted image and how the performance of PRQW devices may be improved in the future. We also present a spatial multiplexing technique for achieving phase-stepped single-shot wide-field coherence-gated imaging using a single CCD camera.
Holographic optical coherence imaging (OCI) has been used to acquire depth resolved images in tumor spheroids. OCI is a coherence-domain imaging technique that uses dynamic holography as the coherence gate. The technique is full-frame (en face) and background free, allowing real-time acquisition to a digital camera without motional reconstruction artifacts. We describe the method of operation of the holographic OCI on highly scattering specimens of tumor spheroids. Because of the sub-resolution structure in the sample, the holograms consist primarily of speckle fields. We present two kinds of volumetric data acquisition. One is uses fly-throughs with a stepping reference delay. Another is static holograms at a fixed reference delay with the coherence gate inside the tumor spheroids. At a fixed reference delay, the holograms consist of time-dependent speckle patterns. The method can be used to study cell motility inside tumor spheroids when metabolic or cross-linking poisons are delivered to the specimens.
Low coherence photorefractive holography is a wide-field technique for 3-D imaging that offers a unique mechanism to discriminate against a background of diffuse light. This provides a wide-field method to image through scattering materials that we have demonstrated may be implemented at frame rates as high as ~ 470/second. We present our recently developed low coherence photorefractive microscope and demonstrate how it may be realized using a spatially coherent broadband c.w. diode-pumped solid-state laser. This can provide real-time sectioned images of moving 3-D objects using only a simple uncooled 8-bit CCD camera. We also demonstrate a photorefractive 3-D imaging technique that exploits structured illumination and photorefractive holography to achieve a real-time wide-field sectioning microscope that may be applied to fluorescence, as well as reflected light. We also discuss issues for improving the sensitivity and spectral coverage of photorefractive holography using semi-insulating MQW devices.
When imaging through scattering media it is easiest to visualise a pulse propagating through the material. As the light propagates it is scattered away from its original trajectory and it becomes diffuse, both spatially and temporally. It is important to note that this picture is also valid for short coherence length c.w. light. In order to create a high resolution wide-field image, in an ideal case, it is necessary to select only the unscattered ballistic photons. This may be achieved using the coherence properties of the light. If the beam is initially split into an incident beam and a reference beam then only the unscattered ballistic photons will retain coherence with the reference beam. Therefore only this light will produce an interference pattern. In the wide-field case this interference of light from the object and light from a reference beam is termed holography.
We report a whole-field fluorescence imaging microscope that combines 3-D spatial resolution by optical sectioning, using structured illumination, with fluorescence lifetime imaging and spectrally-resolved imaging. We show the potential of this technique in the elimination of common artefacts in fluorescence lifetime imaging and apply it to study the dependence of the lifetime on the emission wavelength in biological tissue.
We report high speed (~ 470 frames/s) 3-D imaging using photorefractive holography with sources of diverse temporal and spatial coherence and discuss design considerations for real-world high bit-rate imaging systems. We also propose a new real-time optical sectioning technique based on structured illumination with photorefractive holography to detect fluorescence.
We present a rapid whole-field, 3-D imaging technique based on low coherence interferometry using photorefractive holography in semi-insulating Multiple Quantum Well (MQW) devices, which is capable of whole-field depth resolved 3-D imaging at frame rates exceeding 475 fps. Photorefractive holography provides a unique mechanism to discriminate against a diffuse light background, making it attractive for imaging through turbid media, e.g. for biomedical applications. We note that this whole-field technique can exploit sources of almost arbitrary spatial coherence, including LED's, fibre-coupled laser diode arrays, broadband c.w. lasers etc, as well as ultrafast laser pulses. The use of spatially incoherent light greatly reduces the deleterious impact of speckle.
Photorefractive holography is a whole-field, coherence-gating technique for 3-D imaging through turbid media that offers a unique mechanism to discriminate against a background of diffuse light. In contrast to the well-established technique of optical coherence tomography, it is a whole-field imaging technique and may be implemented with light sources of arbitrary spatial coherence, including low cost LEDs and broad-stripe, multimode diode lasers. One drawback of using broadband sources, such as LEDs, for off-axis holographic imaging is the 'walk-off' resulting from the short temporal coherence length that limits the field-of-view. Furthermore, the non-collinear geometry required for off-axis holography can introduce significant image aberration. In this paper we discuss these design considerations for various sources. We have addressed the issue of walk-off for sources of arbitrary bandwidth and have designed an off-axis holographic imaging system based on a Michelson interferometer with a collinear beam geometry that minimizes aberration. In this paper we review our work with high-powered LEDs and discuss these issues associated with spatially incoherent sources. Also, we present a novel, spatially coherent, broadband diode-pumped laser source that may also find application in OCT.