I-Jen Hsu

Myocardial tissue characterization based on a polarization-sensitive optical coherence tomography system with an ultrashort pulsed laser

A polarization-sensitive optical coherence tomography (PSOCT) system using a femtosecond-laser as the broadband light source is implemented with the axial resolution of 5 microm in free space. Through the design of path-length difference between the two polarization inputs and the modulation of one of the polarization inputs, the PSOCT images of various input and output polarization combinations can be distinguished and simultaneously collected. The PSOCT system is then used for in vitro scanning of the myocardium tissues of normal and infarcted rat hearts. The destruction of the birefringence nature of the fiber muscle in the infarcted heart can be clearly observed.

Resolution improvement with dispersion manipulation and a retrieval algorithm in optical coherence tomography

We propose and demonstrate what is to our knowledge a novel technique of improving the spatial resolution of an optical coherence tomography (OCT) system given a non-Gaussian light source spectrum. By using dispersive materials in the reference arm of the OCT system, the resultant dispersion variation led to a full-width at half maximum (FWHM) of the interference fringe envelope smaller than the Fourier transform-limited value of a Gaussian spectral shape with the same spectral FWHM, at the expense of significant tails. The effects of the tails, which would blur the OCT images, were tremendously reduced with a retrieval algorithm. Simulation results and processed OCT scanning images have shown the capability of the proposed technique.

Polarization gating in ultrafast-optics imaging of skeletal muscle tissues.

By comparing the results of polarization-dependent, time-resolved intensity profiles of photons transmitted through diluted milk, chicken breast tissue, and chopped chicken breast tissue, we found that the inherent anisotropic optical property of skeletal muscle tissue resulted in coherent coupling between two mutually perpendicular polarization directions. This coupling process led to difficulty in using the conventional polarization gating method for imaging unless the anisotropy characteristics were well understood. However, imaging based on polarization gating in diluted milk and chopped chicken breast tissue, which had an isotropic random-scattering nature, was quite effective.

Optical coherence tomography using nonlinear optics in fiber for broadband source generation

An optical coherence tomography (OCT) system of sub-10-μm resolution, with its broadband source generated through nonlinear optics processes of a femtosecond Ti:sapphire laser in a single-mode fiber, was built for biological tissue scanning. With 400 mW power coupled into the fiber, a spectral full-width-at-half-maximum (FWHM) of 46 nm, centered at 840 nm, could be obtained. The available power for sample illumination could be as high as 100 mW. This condition was used to achieve the OCT depth resolution of 8.6 μm in tissues. The OCT system was used for scanning a human gingival mucosa sample. Cavity structures were observed that were quite consistent with the results of histological examination.

Polarization-dependent characteristics and polarization gating in time-resolved optical imaging of skeletal muscle tissues

The comparisons of the time-resolved transmitted intensity profiles and imaging results based on the polarization gating method between the samples of diluted milk, chicken breast tissue, and chopped chicken breast tissue revealed that the anisotropic structure of chicken breast tissue resulted in coherent coupling between the two inherent polarization directions and difficulty of using the polarization gating method for optical imaging through skeletal muscle tissues. To explore the polarization-dependent optical properties of chicken breast tissues, we calibrated the extinction coefficients of the polarization components parallel with and perpendicular to tissue filaments and the cross-polarized intensity-coupling coefficients between the two polarization components, based on the measured snake-photon intensity data. The calibrated values of these coefficients were quite consistent with previously reported. The extinction coefficient in the polarization along tissue filaments was significantly higher than that of the other polarization. Also, the cross-polarized coupling coefficient of the coupling from the polarization of tissue filaments into the other was stronger than that of inverse coupling.

Pump-tuning optical parametric oscillation and sum-frequency mixing with KTP pumped by a Ti:sapphire laser

Abstract We report the implementation of a KTP optical parametric oscillator pumped by a pulsed Ti:sapphire laser. Including signal and idler, it could be continuously tuned from 1.261 through 2.532 μm by varying the pump wavelength. Two major improvements were achieved, including the connection of the signal and idler tuning ranges and the high output pulse energy through the signal and idler tuning ranges. The first improvement was achieved by discovering two particular sets of phase-matching angles. The second improvement was realized by using five sets of resonator mirrors for oscillating either signal or idler. Theoretical calculations were also conducted for the effective nonlinear coefficient, walk-off angle, phase-matching acceptance angle, and output spectral widths for the concerned phase-matching angles. The second part of this paper presents the experimental and theoretical results of sum-frequency mixing (SFM) of a Ti:sapphire laser and a 1.064 μm Nd:YAG laser. By using two KTP crystals cut at θ=76° and 85° (ϕ=90° in both crystals), respectively, we have experimentally achieved SFM tuning range from 459.3 through 472.9 nm. The energy conversion efficiencies were reasonably high.

Application of optical coherence tomography to monitoring the subsurface morphology of archaic jades

We demonstrate the use of optical coherence tomography for scanning archaic jades from the Liangzhu and the Qijia cultures in ancient China for understanding the subsurface structures of the whitening process or the secondary alteration.

Software dispersion compensation in optical coherence tomography

We demonstrate a new method for dynamically and efficiently compensating the fundamental and high-order dispersion mismatches in an optical coherence tomography system based on multiple scans with split spectral bands and software process.

High-resolution optical coherence tomography with fiber-induced broadband source and process algorithm for oral cancer study

A polarization controllable optical coherence tomography (OCT) system was built with the broadband source generated with femtosecond Ti:sapphire laser pulses in fiber. Spectral broadening in such fiber originated from self-phase modulation, four-wave mixing, Raman scattering, and other nonlinear-optics effects. Two different mode-locked Ti:sapphire lasers with 100 fsec and 12 fsec pulses were used. The generated spectral shape and width were compared in terms of the application to the OCT system. The relationship between the OCT resolution and the source spectrum shape was studied. Also, an algorithm was built for increasing the effective longitudinal resolution in data processing. The scheme of this algorithm meant to separate the contribution of the central portion from those of the tails in the interference fringe envelope. By removing the tail contribution to the scanning results, the effective longitudinal resolution was improved. Such a procedure is particularly important when the light source spectrum is not a well-defined shape. This procedure involved in the computation of a matrix inversion. The OCT system and the process algorithm were used for oral cancer study. Features of oral cancer were well identified. A probe was also fabricated for in vivo scan of oral tissues.

Time-independent dispersion compensation in rapid-scanning optical coherence tomography

We demonstrate a new method for simultaneous compensation of time-dependent and time-independent second- and third-order dispersion mismatches in an optical coherence tomography (OCT) system. There have been several methods for dispersion compensation in scanning interferometry by translating or tilting the diffraction grating in a rapid-scanning optical delay line (RSODL). Although these methods can provide a time-independent or time-dependent compensation of the second- or third-order dispersion, they cannot compensate the dispersion mismatch at both orders at the same time. However, the effects of both orders of dispersion mismatch caused by different lengths of fibers in the sample and reference arms in a fiber-based OCT system are significant and cannot be neglected. In this paper, we propose a new method for simultaneous compensation of time-dependent and time-independent second- and third-order dispersions by placing and adjusting a prism between the grating and the lens of the RSODL. The resulting dispersion can be calculated from the phase response of the RSODL with ray tracing analysis. The dependence of each order of dispersion on the configuration of the system is discussed with numerical analysis. The effects of the dispersion compensation are demonstrated with experiments.