Charles Kerbage

Dielectrophoretic manipulation of drops for high-speed microfluidic sorting devices

We demonstrate a high-throughput drop sorter for microfluidic devices that uses dielectrophoretic forces. Microelectrodes underneath a polydimethylsiloxane channel produce forces of more than 10nN on a water drop in an inert oil, resulting in sorting rates greater than 1.6kHz. We investigate the dependence of such forces on drop size and flow. Alternate designs with electrodes on either side of a symmetric channel Y junction provide refined control over droplet selection.

High-speed imaging of human retina in vivo with swept-source optical coherence tomography

We present the first demonstration of human retinal imaging in vivo using optical frequency domain imaging (OFDI) in the 800-nm range. With 460-muW incident power on the eye, the sensitivity is 91 dB at maximum and >85 dB over 2-mm depth range. The axial resolution is 13 mum in air. We acquired images of retina at 43,200 depth profiles per second and a continuous acquisition speed of 84 frames/s (512 A-lines per frame) could be maintained over more than 2 seconds.

Highly nondegenerate femtosecond four-wave mixing in tapered microstructure fiber

We demonstrate efficient, highly nondegenerate four-wave mixing of femtosecond pulses, with a frequency shift of ∼6000 cm−1, in an 18 cm tapered microstructure fiber. Using a pump at 810 nm and a signal at 1540 nm, light is generated at wavelengths between 535 nm and 570 nm with 10% efficiency. Due to the walk-off between pump and signal pulses in the fiber, the interaction length in the tapered fiber is only 1.4 cm. Ten percent efficiency is achieved in this short length because of the enhanced nonlinearity of the tapered fiber and its unique dispersion characteristics.

Spectrally balanced detection for optical frequency domain imaging

In optical frequency domain imaging (OFDI) or swept-source optical coherence tomography, balanced detection is required to suppress relative intensity noise (RIN). A regular implementation of balanced detection by combining reference and sample arm signal in a 50/50 coupler and detecting the differential output with a balanced receiver is however, not perfect. Since the splitting ratio of the 50/50 coupler is wavelength dependent, RIN is not optimally canceled at the edges of the wavelength sweep. The splitting ratio has a nearly linear shift of 0.4% per nanometer. This brings as much as +/-12% deviation at the margins of wavelength-swept range centered at 1060nm. We demonstrate a RIN suppression of 33dB by spectrally corrected balanced detection, 11dB more that regular balanced detection.

Large depth-high resolution full 3D imaging of the anterior segments of the eye using high speed Optical Frequency Domain Imaging

Three dimensional rapid large depth range imaging of the anterior segments of the human eye by an optical frequency domain imaging system is presented. The tunable source spans from 1217 to 1356 nm with an average output power of 60 mW providing a measured axial resolution of 10 mum in air based on the coherence envelope. The effective depth range is 4 mm, defined as the distance over which the sensitivity drops by 6 dB, achieved by frequency shifting the optical signal using acousto-optic modulators. The measured maximum sensitivity is 109 dB at a sample arm power of 14.7mW and A-lines rate of 43,900 per second. Images consisting of 512 depth profiles are acquired at an acquisition rate of 85 frames per second. We demonstrate an optical frequency domain imaging system capable of mapping in vivo the entire area of the human anterior segment (13.4 x 12 x 4.2 mm) in 1.4 seconds.

Optical manipulation and rotation of liquid crystal drops using high-index fiber-optic tweezers

We report an optical fiber tweezer based on high-index material for trapping and optical manipulation of microscale particles in water. The use of a high-index material increases the trapping force with respect to the more common silica, through tighter focusing of light. We demonstrate the potential of this simple and versatile device by trapping and rotating nematic liquid crystal drops. We monitor the rotation of the drop by detecting light modulation observed with the same fiber using backscattered light, which exhibits modulation in intensity due to the rotation of the drop; this further extends the capabilities of the fiber tweezers.

Microstructured Optical Fibers

Microstructured optical fibers have unique properties that offer an opportunity to manipulate light in new ways, providing a platform for a novel class of all-fiber photonic devices. The authors review recent developments in the design and application of microstructured optical fibers for tuning and switching optical signals.

Influence of scattering on transmission through long-period fiber gratings and tunable microstructure fibers

Controlled optical scattering within or around an optical fiber provides a potentially useful mean for adjusting its transmission characteristic. This approach can complement conventional methods based on the establishment of well-defined variations in the index of refraction of the core or the cladding of the fiber. We describe the use of a highly scattering submonolayer of nanoparticles deposited onto the fiber surface for adjusting the resonance wavelength, depth, and width of an in-fiber long-period grating filter. We also introduce a polymer-dispersed liquid-crystal material that has a thermally tunable scattering cross section and can be incorporated into the channels of a microstructure optical fiber; this system may provide the means for a fiber-based scattering switch.

Optical manipulation of liquid crystal drops: Application towards all-optical tunable photonic devices

Trapping and switching of liquid crystal micron-size droplets using a lensed fiber with 1.55 mum cw light is demonstrated. Intensity modulation due to droplet rotation is achieved by controlling the power and polarization of pump light.

Spectrally balanced detection for OFDI

Balanced detection is required to suppress relative intensity noise (RIN) in optical frequency domain imaging (OFDI). Because a 50/50 fiber coupler does not provide a 50% splitting ratio over the full sweeping wavelength, balanced detection by the couplers differential output with a balanced receiver is not perfect. We have designed a new balancing scheme that detects two outputs of 50/50 coupler separately and corrects the spectral deviation in the digital domain. A better balanced detection scheme has been designed in this work. In stead of detecting the hardware balanced signal from the 50/50 fiber coupler, we digitize the two channel fringe signal independently and perform the signal balancing in the poset process. The new software based balancing significantly improves the RIN suppression. Afterward, a systematic noise analysis has been performed on the 1050nm OFDI system. The results demonstrate a RIN suppression of 33dB by spectrally corrected balanced detection, which is 11dB more that regular balanced detection.