Pulsed ultrasound-modulated light tomography.

Ultrasound-modulated light tomography is a new technique that combines laser light and ultrasound to provide a representation of the light density inside turbid media. We present a method that can produce two- or three-dimensional light density representations with standard ultrasonic pulses. This technique should allow simple, direct fusion of ultrasonic images with optical tomography.

Direct, noninvasive detection of photon density in turbid media.

Ultrasound tagging of light provides a unique way to probe photon density inside turbid media. We show that this technique allows one to probe the well-known banana-shaped photon density noninvasively, giving rise to a new tool for modeling diffusive photon propagation. Moreover, we show that this technique is quantitative and allows one to get a precise determination of the absorbing constituents inside the turbid medium.

Detection of inhomogeneities with ultrasound tagging of light.

Ultrasound modulated light for optical tomography is very useful, since it can provide three-dimensional data with minimal mathematical processing. Although several experimental studies have shown the potential of this method, the link between the ultrasound location and the modulated signal intensity at the detector is not yet fully understood. We derive an analytical formula relating the position of the ultrasound transducer and the optical signal at the detector. We also derive an expression for the signal-to-shot-noise ratio as a function of the transducer position. We show that in certain conditions this ratio is only slowly decreasing as a function of the light penetration depth, which makes this technique attractive for optical tomography.

Ultrasound tagged light imaging in turbid media in a reflectance geometry.

A combination of light and focused ultrasound waves provides a unique way to obtain directly three-dimensional absorption data in a turbid medium. We present the combination of an ultrasound wave and light in which both the input and the output optodes are on the same side of the sample (reflectance geometry). This technique permits local detection in depth of the presence of a purely absorbing object, without further mathematical processing. It is a promising technique for medical imaging and monitoring of tissues.

Measurement of the nonlinear refractive index of semiconductors included in monolithic étalons.

We describe a measurement technique for the determination of the nonlinear refractive index of semiconductors embedded in a Fabry-Perot cavity. This method relates the intensity-dependent spectral shift of the resonance peak to the nonlinear index variations. The use of square pulses allows the decoupling of electronic and thermal nonlinearities. Our experimental results show that this method is particularly suitable for measuring the nonlinear-optical properties of monolithic semiconductor étalons with integrated Bragg reflectors.

Femotosecond switching in a dual-core-fiber nonlinear coupler.

We report all-optical switching of 100-fsec pulses in a fused-quartz dual-core-fiber directional coupler. The length of the device is 0.5 cm, and the switching power is 32 kW. Pulses are routed to either of two separate fiber guides, depending on the input power. Measurements of pulse reshaping by the nonlinear coupler provide compelling evidence of the device's ability to response on a femotosecond time scale.

All-optical phase- and power-controlled switching in nonlinear waveguide junctions.

We propose and study a novel all-optical waveguide switching element made from a nonlinear symmetric junction. Nonlinear index changes break the symmetry of the structure and induce routing of the signal to one of two output ports. Power- and phase-controlled switching with a weak control signal are possible. The switching is insensitive to the precise value of either power or phase and to the structure parameters.