Ping Pei Ho

Second-harmonic tomography of tissues

A novel noninvasive second-harmonic-generation tomographic method of mapping the structure of animal tissues by use of 100-fs laser pulses at 625nm is described. Subsurface structures were measured with this approach, which is potentially a symmetry-sensitive tool for optical histological reconstruction.

Induced spectral broadening of a weak picosecond pulse in glass produced by an intense picosecond pulse

Spectral broadening of a weak 80-microJ picosecond 530-nm laser pulse in a BK-7 glass has been enhanced over the entire spectral band by the presence of an intense millijoule picosecond 1060-nm laser pulse. The spectral distributions of the self-phase modulation and the induced-phase modulation signals are similar. The dominant enhancement mechanism for the induced supercontinuum was determined to be caused by an induced-phase modulation process, not by stimulated four-photon scattering.

Generation of infrared supercontinuum covering 3–14 μm in dielectrics and semiconductors

Infrared supercontinua spanning the range 3-14 microm were observed when an intense pulse generated from a CO(2) laser was passed into GaAs, AgBr, ZnSe, and CdS crystals. These supercontinua have been qualitatively compared with theoretical predictions.

ULTRAFAST OPTICAL PULSE DIGITIZATION WITH UNARY SPECTRALLY ENCODED CROSS-PHASE MODULATION

A method to digitize the intensity of ultrashort laser pulses for high-speed optical signal processing is described. This digitization was based on the spectral broadening of a weak probe (carrier) pulse by a more intense pump (signal) pulse through the nonlinear optical process of cross-phase modulation (XPM). The signal pulse intensity was varied to generate different spectral widths that can be encoded into digital form. Using a 50-ps time-divided multiplexing pulse train with a waveguide splitter, combiner, and an array of fibers with variable lengths, a unary XPM encoding approach is demonstrated. The spectral encoding scheme can be used to achieve a 5-GHz sampling rate at a 16-level accuracy.

Dependence of specklon size on the laser beam size via photo-induced light scattering in LiNbO3:Fe.

The dependence of the scattering intensity patterns on the illumination laser beam spot size in LiNbO3:Fe is described. These observations provide direct evidence of the origin of photo-induced light scattering resulting from amplification of scattered radiation caused by imperfections in the crystal.

Femtosecond optical Kerr shutter using lead-bismuth-gallium oxide glass

Abstract A femtosecond Kerr shutter was demonstrated using a lead–bismuth–gallate oxide glass. The optical gating time was measured to be less than 350 fs and the peak Kerr transmittance was found to be 2.3 times larger than that of the standard Kerr CS 2 liquid medium, giving nonlinear Kerr coefficient n 2 =4.4×10 −11 esu.

Time dynamics of photon migration in semiopaque random media

The temporal behavior of femtosecond laser pulses propagating and scattering in random media was studied using an optical fiber probe and a streak camera. Information on the temporal profile of laser pulse scattering at 90 degrees in semiopaque media was analyzed. Apparent ballistic transport was observed for both single and multiple scattering regimes. A phantom (hidden foreign object) was identified in a semiopaque environment using time resolved techniques with a total optical density of ~3.

Noninvasive two-photon-excitation imaging of tryptophan distribution in highly scattering biological tissues

Abstract We report on noninvasive two-photon fluorescence (TPF) image mapping of tryptophan distribution in highly-scattering native biological tissues. Fluorescence from tryptophan molecules in tissue samples was excited by two-photon absorption of 625 nm, 100 fs light pulses from a colliding-pulse-modelocked dye laser. A two-dimensional map of the tryptophan distribution in a chicken-tissue sample was obtained by scanning the laser beam over the tissue surface and accumulating the fluorescence signal point-by-point with a photomultiplier tube and a lock-in amplifier system. Images of a tissue at an axial position up to 250 μm from the surface were recorded with a lateral resolution of ∼10 μm. The technique is wavelength selective, morphology dependent, and molecule specific. This method offers potential for a less invasive method by giving the analysis of certain molecules in tissue structures.

Photons for prompt tumour detection

Aeroplanes in clouds, submarines in murky water and cancerous tumours in breast tissue all have one thing in common – they are objects hidden in highly scattering media. Imaging through such media remains one of the most challenging problems in science and engineering but the advent of ultrafast lasers and detectors, coupled with a range of time-resolved techniques, has led to breakthroughs. Interest is currently focused on developing a time-resolved imaging technique to detect breast cancer tumours as early as possible.

Temporal behavior of cross-phase-modulated second-harmonic generation of ultrashort laser pulses in nonlinear-optical media

Pulse propagation and phase modulation of second-harmonic generation of ultrafast pulses in nonlinear-optical media have been modeled and analyzed based on experimental work. In order to describe the temporal pulse profile, both cross-phase-modulation and induced-absorption processes have been introduced. The pulse profiles of cross-phase-modulated second-harmonic pulses for non-phase-matched nonlinear media with short interaction length are shown to depend on group-velocity dispersion, absorption, and induced phase matching.