Double-stage picosecond Kerr gate for ballistic time-gated optical imaging in turbid media

Efficiency estimates and practical aspects of an optical Kerr gate for time-resolved luminescence spectroscopy

We report experience of assembling an optical Kerr gate setup at the Femtosecond Laser Center for collective use at the Institute of Physics of the National Academy of Sciences of Ukraine. This offers an inexpensive solution to the problem of time-resolved luminescence spectroscopy. Practical aspects of its design and alignment are discussed and its main characteristics are evaluated. Theoretical analysis and numerical estimates are performed to evaluate the efficiency and the response time of an optical Kerr gate setup for fluorescence spectroscopy with subpicosecond time resolution. The theoretically calculated efficiency is compared with the experimentally measured one of ~12% for Crown 5 glass and ~2% for fused silica. Other characteristics of the Kerr gate are analyzed and ways to improve them are discussed. A method of compensation for the refractive index dispersion in a Kerr gate medium is suggested. Examples of the application of the optical Kerr gate setup for measurements of the time-resolved luminescence of Astra Phloxine and Coumarin 30 dyes and both linear and nonlinear chirp parameters of a supercontinuum are presented.

Imaging of the magnetic field structure in megagauss plasmas by combining pulsed polarimetry with an optical Kerr effect shutter technique

Pulsed polarimetry in combination with a high speed photographic technique based on the optical Kerr effect is described. The backscatter in a pulsed polarimeter is directed through a scattering cell and photographed using an ∼1 ps shutter, essentially freezing the intensity pattern. The image provides both the local electron density and magnetic field distributions along and transverse to the laser sightline. Submillimeter spatial resolution is possible for probing wavelengths in the visible due to the high densities and strong optical activity. Pulsed polarimetry is thereby extended to centimeter-sized plasmas with n(e)>10(19)-10(20) cm(-3) and B>20-100 T (MG) produced by multiterawatt, multimega-ampere electrical drivers, wire Z pinches, and liner imploded magnetized plasmas.

Ultrafast time-gated ballistic-photon imaging and shadowgraphy in optically dense rocket sprays

Time-gated ballistic-photon imaging is a form of shadowgraphy in which an ultrashort, optical-Kerr-effect (order 2 ps) time gate is used to enhance the relative intensity of ballistic versus multiply scattered photons. In the current work, this technique is adapted for what is believed to be the first time for use in the moderately dense environment (optical density approximately 1.5 to 2) of a high-speed 5 to 15 mm diameter rocket spray to improve image contrast and observe liquid-breakup phenomena. Unlike coherence gating, which is another form of ballistic imaging, the time-gating approach allows sufficient signal levels from ballistic and near-ballistic photons to enable time-resolved single-shot imaging. Direct comparisons with non-time-gated shadowgraphy indicate that the two techniques are sensitive to different features of the flowfield, with regions composed of a dense field of droplets being highly attenuated in conventional shadowgrams but appearing transparent to ballistic photons. This enables significant image contrast enhancement (approximately 6.6:1) of liquid-core structures and facilitates improved understanding of the primary and secondary breakup processes in sprays of moderate optical density.

Polarization gating in ultrafast-optics imaging of skeletal muscle tissues

By comparing the results of polarization-dependent, time-resolved intensity profiles of photons transmitted through diluted milk, chicken breast tissue, and chopped chicken breast tissue, we found that the inherent anisotropic optical property of skeletal muscle tissue resulted in coherent coupling between two mutually perpendicular polarization directions. This coupling process led to difficulty in using the conventional polarization gating method for imaging unless the anisotropy characteristics were well understood. However, imaging based on polarization gating in diluted milk and chopped chicken breast tissue, which had an isotropic random-scattering nature, was quite effective.

Role of higher-order scattering in solutions to the forward and inverse optical-imaging problems in random media

From analytical and numerical solutions that predict the scattering of diffuse photon density waves and from experimental measurements of changes in phase shift theta and ac amplitude demodulation M caused by the presence of single and double cylindrical heterogeneities, we show that second- and higher-order perturbations can affect the prediction of the propagation characteristics of diffuse photon density waves. Our experimental results for perfect absorbers in a lossless medium suggest that the performance of fast inverse-imaging algorithms that use first-order Born or Rytov approximations might have inherent limitations compared with inverse solutions that use iterative solutions of a linear perturbation equation or numerical solutions of the diffusion equation.

Fluorescence tomographic imaging in turbid media using early-arriving photons and Laplace transforms

We present a multichannel tomographic technique to detect fluorescent objects embedded in thick (6.4 cm) tissue-like turbid media using early-arriving photons. The experiments use picosecond laser pulses and a streak camera with single photon counting capability to provide short time resolution and high signal-to-noise ratio. The tomographic algorithm is based on the Laplace transform of an analytical diffusion approximation of the photon migration process and provides excellent agreement between the actual positions of the fluorescent objects and the experimental estimates. Submillimeter localization accuracy and 4- to 5-mm resolution are demonstrated. Moreover, objects can be accurately localized when fluorescence background is present. The results show the feasibility of using early-arriving photons to image fluorescent objects embedded in a turbid medium and its potential in clinical applications such as breast tumor detection.

Time-gated viewing studies on tissuelike phantoms

A time-gated technique to enhance viewing through highly scattering media such as tissue is discussed. Experiments have been performed on tissuelike plastic phantoms to determine the possibilities and limitations of the technique. The effects of the time-gate width and the localization, size, and optical properties of hidden objects have been studied. A computer model to simulate light propagation in tissue is also presented. The predictions of the model are compared with experimental results.

Image quality in time-resolved transillumination of highly scattering media

Using a photon-counting setup and a streak-camera arrangement with time resolutions of 35 and 6 ps, respectively, we have investigated the spatial resolution of a time-gated transillumin tion technique applied to turbid media. In the case of large relative amounts of unscattered light, it is found that small detection angles improve the spatial resolution. For large concentrations of scatterers and large sample thicknesses, i.e., when the amount of unscattered light is negligible, the best time-gate position is found to be at times that are later than the minimum transit time. In this case (minimum transit time), temporal resolutions from small values up to approximately 50 ps yield almost the same image resolution. The only advantage of measuring systems with a higher than 50-ps temporal resolution is their ability to distinguish the diffused from the unscattered light, when a significant amount of the latter is present.

Three-dimensional imaging of objects embedded in turbid media with fluorescence and Raman spectroscopy

We present a single-ended technique for three-dimensional imaging of objects embedded in a turbid medium by the use of time-resolved fluorescence emission or Raman scattering. The technique uses the earliest arriving photons, which we show are not sensitive to the relatively long fluorescence lifetime, and thus can be used to extract the desired spatial information accurately, even at a distance equivalent to 100 mean free paths. The results also demonstrate the feasibility and the potential of one's combining time-resolved optical tomography with fluorescence or Raman spectroscopy to localize and identify the embedded objects. This technique may be valuable for the diagnosis of disease in highly scattering human tissue because it can provide spatial and biochemical information about the composition of embedded lesions.

Time-resolved multichannel imaging of fluorescent objects embedded in turbid media

We present a multichannel detection technique for three-dimensional imaging of objects embedded in turbid media by using time-resolved fluorescence. By using a streak camera, we can obtain the experimental data in a single measurement. The data, analyzed by means of a triangulation algorithm, provide accurate localization of a fluorescent object for path lengths of up to 120 scattering mean free paths. The results demonstrate the feasibility of combining fluorescence spectroscopy with time-resolved optical tomography for localizing and identifying embedded objects.

Localization of absorbers in scattering media by use of frequency-domain measurements of time-dependent photon migration

Frequency-domain studies of time-dependent light propagation in tissuelike phantoms that contain optical heterogeneities are described. Specifically the phase shift and amplitude modulation of reemergent light were measured when illuminated by an amplitude-modulated light source. Changes in the phase angle and the extent of modulation revealed the presence of a light-absorbing object. Furthermore the magnitude and direction of these changes were sensitive to the absorber depth and the light modulation frequency in a manner that could be used to infer the location of the heterogeneity. These data suggest the feasibility of optical imaging by frequency-domain methods.

Time-gated imaging through dense scatterers with a Raman amplifier

A time-gated Raman amplifier has been used to detect a bar chart hidden by a strongly scattering material. The time gating was provided by a frequency-doubled Nd:YAG pump laser having a pulse duration of 30 ps. We have amplified and detected images with resolved structures smaller than 125 µm through suspensions of polystyrene spheres and nondairy creamer for light extinction factors of up to e(33). The Raman amplifier system has been shown to produce images under conditions in which the scattering medium was sufficiently dense that an image could not be detected on either a streak camera or by integration on a sensitive, low-noise camera.

Use of Fourier synthesis holography to image through inhomogeneities

A method for image formation through inhomogeneities is demonstrated. A broad spectral source is decomposed into its Fourier components, and a hologram is recorded at each wavelength through a diffusing medium. When the holograms are synthesized in a computer, a clear image can be formed of the obscured object.