S. K. Gayen

Laser action in chromium‐doped forsterite

Room‐temperature vibronic pulsed laser action in trivalent chromium‐activated forsterite (Cr3+:Mg2SiO4) is reported for the first time. The free‐running laser emission is centered at 1235 nm of the broad 4T2→4A2 fluorescence band, and has a bandwidth of ∼22 nm.

Laser action in chromium‐activated forsterite for near‐infrared excitation: Is Cr4+ the lasing ion?

Room‐temperature pulsed laser action has been obtained in chromium‐activated forsterite (Cr:Mg2SiO4) for excitation of the near‐infrared absorption band of the system by the 1064 nm radiation from a Nd:YAG laser. The characteristics of laser emission are similar to those observed for 532 nm pumping. It is suggested that the laser action is due to a ‘‘center’’ other than the trivalent chromium (Cr3+), presumably the tetravalent chromium (Cr4+).

Continuous-wave laser operation of chromium-doped forsterite

Room-temperature continuous-wave laser action in chromium-activated forsterite (Cr:Mg(2)SiO(4)) has been achieved for longitudinal pumping in a nearly concentric cavity by the 1064-nm radiation from a cw Nd:YAG laser. The laser emission is centered at 1244 nm and has a spectral bandwidth of 12 nm. An output-power slope efficiency of 6.8% is measured. The effective emission cross section is estimated to be ~1.1 x 10(-19) cm(2).

Emerging Optical Biomedical Imaging Techniques

Since Victorian times, the search has been on for a noninvasive way to peer inside the human body using light. Today’s researchers don’t have the perfect solution yet. This article explores the optical imaging techniques that look most promising.

Advances in optical imaging of biomedical media.

In this article, we have presented an overview of fundamental issues involved in mediphotonic imaging, and reviewed some of the emerging techniques for early-light transillumination imaging of body organs. The results on human breast tissues presented here, together with the data accumulated and advances made by researchers around the globe, not only demonstrate the feasibility of optical imaging as a clinical procedure but indicate a road map to reach that goal. The milestones include evaluation of relative merits of available approaches for a particular imaging application; selection of diagnostic wavelengths, as well as sources to generate and detectors to monitor light at those wavelengths; accumulation of data on optical, spectroscopic, and transport properties of tissues and organs; in vivo testing; prototype instrumentation development; clinical trials; governmental approval; cost analysis and marketing; and finally system improvement based on feedback from end users. A new era of optical clinical imaging is at the door.

Optical tomographic image reconstruction from ultrafast time-sliced transmission measurements

Optical imaging and localization of objects inside a highly scattering medium, such as a tumor in the breast, is a challenging problem with many practical applications. Conventional imaging methods generally provide only two-dimensional (2-D) images of limited spatial resolution with little diagnostic ability. Here we present an inversion algorithm that uses time-resolved transillumination measurements in the form of a sequence of picosecond-duration intensity patterns of transmitted ultrashort light pulses to reconstruct three-dimensional (3-D) images of an absorbing object located inside a slab of a highly scattering medium. The experimental arrangement used a 3-mm-diameter collimated beam of 800-nm, 150-fs, 1-kHz repetition rate light pulses from a Ti:sapphire laser and amplifier system to illuminate one side of the slab sample. An ultrafast gated intensified camera system that provides a minimum FWHM gate width of 80 ps recorded the 2-D intensity patterns of the light transmitted through the opposite side of the slab. The gate position was varied in steps of 100 ps over a 5-ns range to obtain a sequence of 2-D transmitted light intensity patterns of both less-scattered and multiple-scattered light for image reconstruction. The inversion algorithm is based on the diffusion approximation of the radiative transfer theory for photon transport in a turbid medium. It uses a Green s function perturbative approach under the Rytov approximation and combines a 2-D matrix inversion with a one-dimensional Fourier-transform inversion to achieve speedy 3-D image reconstruction. In addition to the lateral position, the method provides information about the axial position of the object as well, whereas the 2-D reconstruction methods yield only lateral position.

Experimental demonstration of an excited-state Faraday filter operating at 532 nm.

We outline what is to our knowledge the first experimental demonstration of an excited-state Faraday filter. The filter consists of potassium vapor between crossed polarizers in a dc magnetic field and operates on the 4P((1/2)) ? 8S((1/2)) transition in potassium. The 4P((1/2)) state is populated by a linearly polarized, 10-ns light pulse from a dye laser operating at 769.9 nm. Another linearly polarized, 10-nsec pulse at 532.33 nm traverses the pumped volume of the K cell and is absorbed from the 4P((1/2)) state to the 8S((1/2)) state. The transmission of the filter is approximately 3.5% at 532.33 nm with a bandwidth of less than 10 GHz.

Time-gated backscattered ballistic light imaging of objects in turbid water

Time-gated optical imaging of objects in turbid water was carried out in a backscattering geometry using light pulses of different pulse widths and a time-gated detection scheme with variable gate widths. Experimental results demonstrate that ultrashort pulsed illumination with ultrashort gated detection significantly improve imagecontrast as compared to any other combinations. These results are important for imaging objects embedded in turbid media, such as cloud, fog, smoke, murky water, and biological tissues for military, civilian, and medical applications.

Near infrared tunable operation of chromium doped forsterite laser

Tunable, room temperature pulsed laser operation of a chromium-doped forsterite laser for 1064-nm pumping is reported. Using different sets of mirrors and a single birefringent plate as the intracavity wavelength selecting element, tunability over the 1167-1345-nm spectral range has been demonstrated.

Determination of Optical Coefficients and Fractal Dimensional Parameters of Cancerous and Normal Prostate Tissues

Optical extinction and diffuse reflection spectra of cancerous and normal prostate tissues in the 750 to 860 nm spectral range were measured. Optical extinction measurements using thin ex vivo prostate tissue samples were used to determine the scattering coefficient (μ′s), while diffuse reflection measurements using thick prostate tissue samples were used to extract the absorption coefficient (μa) and the reduced scattering coefficient (μ′s). The anisotropy factor (g) was obtained using the extracted values of μs and μ′s. The values of fractal dimension (Df) of cancerous and normal prostate tissues were obtained by fitting to the wavelength dependence of μ′s. The number of scattering particles contributing to μs as a function of particle size and the cutoff diameter dmax as a function of g were investigated using the fractal soft tissue model and Mie theory. Results show that dmax of the normal tissue is larger than that of the cancerous tissue. The cutoff diameter dmax is observed to agree with the nuclear size for the normal tissues and the nucleolar size for the cancerous tissues. Transmission spectral polarization imaging measurements were performed that could distinguish the cancerous prostate tissue samples from the normal tissue samples based on the differences between their absorption and scattering parameters.