Real-time two-dimensional imaging in scattering media by use of a femtosecond Cr(4+):forsterite laser

An original femtosecond Cr(4+):forsterite laser source associated with a nonlinear optical correlator was used for imaging through scattering media with 1220-nm light. The system, which operates as an ultrafast optical gate by sum-frequency generation in a nonlinear crystal, was able to detect the light reflected from a resolution chart hidden in a turbid medium, at an attenuation of as much as 15 mean free paths. When the object was illuminated with a collimated beam, real-time two-dimensional images were obtained, with a maximum transverse resolution of ~20 microm.     

Motion imaging of objects in layers hidden by scattering media using low-coherence speckle interferometry

A novel method is proposed for motion imaging of objects in layers hidden by a scattering medium in a depth-resolved manner by means of the intensity difference in a temporal sequence of low-coherence speckle images. Depth-resolved motion imaging was demonstrated with a semi-transparent scattering plate and an aluminum diffusive plate behind Intralipid solution. Experimental results confirmed feasibility of the proposed method for imaging of moving objects by their relative velocity.     

Transillumination imaging through scattering media by use of photorefractive polymers

We demonstrate the use of a near-infrared-sensitive photorefractive polymer with high efficiency for imaging through scattering media, using an all-optical holographic time gate. Imaging through nine scattering mean free paths is performed at 800nm with a mode-locked continuous-wave Ti:sapphire laser.     

Pulsed acousto-optic imaging in dynamic scattering media with heterodyne parallel speckle detection

We present a new detection scheme for acousto-optic tomography based on pulsed-wave ultrasound and illumination combined with heterodyne parallel speckle detection. This setup can perform tomographies inside several-centimeter-thick scattering samples. Test experiments confirm the suitability of this method for performing tomographies inside various types of optically scattering media, including liquids.     

Blink speckle spectroscopy of scattering media

The speckle method of characterization of multiple scattering media by squared-pulse modulation of the probe light frequency is described. This method assumes the analysis of the decay in speckle modulation that is due to the superposition of two decorrelated blinking speckle patterns. Theoretical predictions based on the phenomenological model of formation of partially blurred speckles under the condition of frequency switching are in reasonable agreement with the corresponding experimental data.     

Polarized optical coherence imaging in turbid media by use of a Zeeman laser

A method that uses a Zeeman laser in conjunction with a Glan-Thompson analyzer to image an object in a turbid medium is proposed. A heterodyne signal is generated only when the scattering photons are partially polarized, and the spatial coherence is not seriously degraded after the signal propagates in the turbid medium. A system combining polarization discrimination with optical coherence detection to image the object in a scattering medium is successfully demonstrated. The medium is a solution of polystyrene microspheres measuring 1.072 mum in diameter suspended in distilled water contained in a 10-mm-thick quartz cuvette. The advantages of this optical system, including better selectivity of the weak partially polarized scattering photons and better imaging ability in higher-scattering media, are discussed.     

Coherent anti-Stokes Raman scattering imaging with a laser source delivered by a photonic crystal fiber

We demonstrate laser-scanning coherent anti-Stokes Raman scattering (CARS) imaging with two excitation laser beams delivered by a large-mode-area photonic crystal fiber. The group-velocity dispersion and self-phase modulation effects are largely suppressed due to the large mode area of the fiber and the use of picosecond pulses. The fiber delivery preserves the signal level and image spatial resolution well. High-quality images of live spinal cord tissues are acquired using the fiber-delivered laser source. Our method provides a basic platform for developing a flexible and compact CARS imaging system.     

Regaining profiles of extinction coefficients of optically dense scattering media in laser location measurements

We suggest a procedure for regaining spectral values of the extinction coefficients ε(r) of optically dense scattering media in laser location measurements. Allowance is made for the contribution of multiple scattering to a recorded signal and its correction for the degree of change of the qualitative composition of the scattering medium. The procedure can increase the accuracy of regaining ε(r) and eliminate the “edge effect” at the end of the probed path. The latter is achieved by determining a calibration constant from the transparency value of the whole probing range, which is calculated from back-scattered signals corrected based on the constancy of the lidar ratio. We present an algorithm for calculating the correction coefficient. The efficiency of regaining profiles of ε(r) is estimated using the atmospheric situation of a pure atmosphere and an extended smoke cloud arising from forest fires as an example. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 522–527, July–August, 2007.     

Coherent transverse speckle dynamics in the case of probing of stationary bulk scattering media by a laser with frequency deviation

A coherent transverse dynamics of scattered speckle fields that arises upon propagation of tunable laser radiation through stationary optically inhomogeneous bulk media has been discovered. The dynamic photochromic speckle effect can be observed experimentally if the deviation of the laser radiation frequency is comparable with or greater than the effective phase delays arising upon scattering of waves in a random or partly organized medium. It was found experimentally that, with an increase in the frequency tuning range of a probing laser diode (λ = 650 nm) or a compact YAG:Nd laser (λ = 532 μm), which can reach hundreds of gigahertz for multiply scattering bulk media from several millimeters to two centimeters thick (fluoroplastic phantoms), the two-dimensional correlation coefficient of speckle structures decreased monotonically, and the characteristic drop for thicker structures was observed at smaller frequency detunings.     

First order statistics of speckle produced by weak scattering media

A weak scattering medium produces random phases in a transmitted or reflected wavefront distributed over less than 2π. The far-field speckle pattern has a central maximum. The statistics of intensity in this maximum are calculated and the applications of the results to surface roughness measurement are indicated.     

Direct imaging through scattering media by use of efficient third-harmonic generation in organic materials

We report on real-time, time-gated, direct imaging through scattering media with an attenuation of 14 mean-free paths by use of third-harmonic generation in the eye-safe and telecommunication-compatible near-IR spectral region (1550 nm).     

Attenuation and speckle modulation of laser light in dispersive dye-doped media: Competition of absorption and scattering processes

The effect of competition of laser light absorption and scattering in dispersive media (1 μm polystyrene microspheres in ethylene glycol) with added dye (rhodamine 6G) was studied under the condition of near-resonant absorption of probe light in the host medium (solution of rhodamine 6G in ethylene glycol). The parameters of speckle modulation of transmitted light (the average speckle intensity and the oscillation index) were applied for characterization of light transport in the probed scattering systems as a function of dye concentration. The increase in dye concentration does not cause the expected decay in the transmittance of the examined dispersive systems for 532 nm laser light, but results in the decrease of their turbidity. This effect is accompanied by the rise of the average intensity and the oscillation index of speckle-modulated light, and can be rationalized by partial matching of the real parts of refractive indices for the host medium and embedded scatterers. This interpretation was supported by statistical modeling of light transport through the examined scattering systems.     

Quantitative modeling of laser speckle imaging

We have analyzed the image formation and dynamic properties in laser speckle imaging (LSI) both experimentally and with Monte Carlo simulation. We show for the case of a liquid inclusion that the spatial resolution and the signal itself are both significantly affected by scattering from the turbid environment. Multiple scattering leads to blurring of the dynamic inhomogeneity as detected by LSI. The presence of a nonfluctuating component of scattered light results in the significant increase in the measured image contrast and complicates the estimation of the relaxation time. We present a refined processing scheme that allows a correct estimation of the relaxation time from LSI data.     

Correlated imaging in scattering media

We demonstrate for the first time (to our knowledge) that an image with high quality can still be obtained in scattering media by applying the second-order correlation of illuminating light field. The effect of multiple scattering on the imaging quality is analyzed both theoretically and experimentally. Potential applications and methods to further improve the imaging quality in scattering media are also discussed.     

Co-registration method for photoacoustic imaging and laser speckle imaging

Combining photoacoustic (PA) imaging with laser speckle (LS) imaging (LSI) can simultaneously determine total hemoglobin concentration (HbT), hemoglobin oxygen saturation (SO2), and blood flow rates. Thus,the co-registration of PA and LS images is important in physiological studies and pathological diagnosis.This letter presents a co-registration algorithm combining mutual information with the maximum betweenclass variance segmentation method (Otsu method). The mutual information and Otsu method are used to provide the registration measure criterion and image feature recognition, respectively. The evaluation results show that the registration function possesses a single maximum peak and high smoothness across the global co-registration district, indicating a robust behavior. Moreover, this method has good registration accuracy, and the fusion result simultaneously visualizes the separate functional information of two kindsof images.     

Depth-resolved holographic imaging through scattering media by use of a photorefractive polymer composite device in the near infrared

We report full-field depth-resolved holographic imaging through scattering media by use of a photorefractive polymer device in the near infrared. Real-time images through seven scattering mean free paths are acquired in a few seconds with a broadband superluminescent diode at 794 nm. A depth resolution of 15 microm and a transverse resolution of better than 42 microm are achieved. The photorefractive devices used are based on a poly(N-vinylcarbazole):2,4,7-trinitro-9-fluorenone dimalenitrile charge-transport network doped with the electro-optic chromophore 1-(2'-ethylhexyloxy)-2,5-dimethyl-4-(4"-nitrophenylazo)benzene.     

A study of the use of laser speckle to measure small tilts of optically rough surfaces accurately

A new method is proposed where small tilts can be measured with high accuracy even in the presence of a lateral shift. For this purpose the speckling of the object, an optically rough surface, is recorded in the Fourier plane before and after a tilt. By illuminating the developed plate with a laser, fringeswith spacings inversely proportional to the tilt angle can be observed.     

Imaging and multiple scattering through media containing optically active particles

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Comparison of laser Doppler and laser speckle contrast imaging using a concurrent processing system

Full field laser Doppler imaging (LDI) and single exposure laser speckle contrast imaging (LSCI) are directly compared using a novel instrument which can concurrently image blood flow using both LDI and LSCI signal processing. Incorporating a commercial CMOS camera chip and a field programmable gate array (FPGA) the flow images of LDI and the contrast maps of LSCI are simultaneously processed by utilizing the same detected optical signals. The comparison was carried out by imaging a rotating diffuser. LDI has a linear response to the velocity. In contrast, LSCI is exposure time dependent and does not provide a linear response in the presence of static speckle. It is also demonstrated that the relationship between LDI and LSCI can be related through a power law which depends on the exposure time of LSCI.     

Imaging and multiple scattering through media containing optically active particles

Abstract

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.