Tuan-Shu Ho

Non-invasive characterization of the domain boundary and structure properties of periodically poled ferroelectrics

Shaping the ferroelectric domains as waveguide, grating, lens, and prism are key to the successful penetration of periodically-poled ferroelectrics on various wavelength conversion applications. The complicated structures are, however, difficult to be fully characterized, especially the unexpected index contrast at the anti-parallel domain boundaries are typical in the order of 10(-4) or less. An ultrahigh resolution optical coherence tomography was employed to fully characterize the domain boundary and structure properties of a periodically-poled lithium niobate (PPLN) waveguide with an axial resolution of 0.68 μm, an transversal resolution of 3.2 μm, and an index contrast sensitivity of 4x10(-7). The anti-parallel domain uniformity can clearly be seen non-invasively. Dispersion of the ferroelectric material was also obtained from 500 to 750 nm.

Full-depth epidermis tomography using a Mirau-based full-field optical coherence tomography

With a Gaussian-like broadband light source from high brightness Ce(3+):YAG single-clad crystal fiber, a full-field optical coherence tomography using a home-designed Mirau objective realized high quality images of in vivo and excised skin tissues. With a 40 × silicone-oil-immersion Mirau objective, the achieved spatial resolutions in axial and lateral directions were 0.9 and 0.51 μm, respectively. Such a high spatial resolution enables the separation of lamellar structure of the full epidermis in both the cross-sectional and en face planes. The number of layers of stratum corneum and its thickness were quantitatively measured. This label free and non-invasive optical probe could be useful for evaluating the water barrier of skin tissue in clinics. As a preliminary in vivo experiment, the blood vessel in dermis was also observed, and the flowing of the red blood cells and location of the melanocyte were traced.

Diode-Laser-Pumped Glass-Clad Ti:Sapphire Crystal-Fiber-Based Broadband Light Source

Glass-clad crystal fibers were grown using the codrawing laser-heated pedestal growth method. Both single-clad and double-clad structures were attempted. Using a 446-nm laser diode as the pump, 2.45 mW of collimated broadband emission was generated with a 3-dB bandwidth of 180 nm. As much as 7.7 mW of output power with a 3-dB bandwidth of 182 nm was generated using a 532-nm solid-state pump laser. The broadband light source is advantageous to use in optical coherence tomography with an axial resolution of 1.5 in air and a low image pixel crosstalk.

Broadband and high-brightness light source: glass-clad Ti:sapphire crystal fiber.

High-brightness near-infrared broadband amplified spontaneous emission (ASE) was generated by glass-clad Ti:sapphire crystal fibers, which were developed using the co-drawing laser-heated pedestal growth method. As much as 29.2 mW of ASE power was generated using 520 nm laser diodes as the excitation source on an a-cut, 18 μm core-diameter Ti:sapphire crystal fiber (CF). The 3 dB bandwidth was 163.8 nm, and the radiance was 53.94  W·mm(-2) sr(-1). The propagation loss of the glass-clad sapphire CF measured using the cutback method was 0.017  cm(-1) at 780 nm. For single-mode applications, more than 100 μW of power was coupled into a SM600 single-mode fiber.

Spectroscopic measurement of absorptive thin films by spectral-domain optical coherence tomography.

A non-invasive method for measuring the refractive index, extinction coefficient and film thickness of absorptive thin films using spectral-domain optical coherent tomography is proposed, analyzed and experimentally demonstrated. Such an optical system employing a normal-incident beam of light exhibits a high spatial resolution. There are no mechanical moving parts involved for the measurement except the transversal scanning module for the measurement at various transversal locations. The method was experimentally demonstrated on two absorptive thin-film samples coated on transparent glass substrates. The refractive index and extinction coefficient spectra from 510 to 580 nm wavelength range and film thickness were simultaneously measured. The results are presented and discussed.

Drawing of single-crystal and glass-clad lithium tantalate fibers by the laser-heated pedestal growth method

Lithium tantalate (LT) single-crystalline fibers with various growth conditions were obtained using the laser-heated pedestal growth (LHPG) method. The fibers can be produced with a diameter down to 70 µm and tens of centimetres in length by using different pulling rates. Preliminary experiments also showed that the LT fibers can be electrically poled for quasi-phase matching. To facilitate wave propagation with low loss, glass-clad LT fibers were fabricated by a co-drawing LHPG method for the first time. A vacuum apparatus was developed to eliminate the bubbles from incongruent evaporation of lithium oxide during the drawing of the glass-clad LT fibers. The glass-clad fibers are classified into two different categories by the measurements of refractive index profiles. One is a step-index fiber and the other is a graded-index fiber. Both fiber types are multimode at present, but single-mode LT fibers could be fabricated with the high-precision control of the LHPG systems.

Mirau-based full-field time-domain optical coherence tomography using Ce3+:YAG crystal fiber light source

Based on single-objective construction utilizing high brightness Ce3+:YAG single-clad crystal fiber light source, this Mirau-based full-field time-domain optical coherence tomography with circular polarization incident light represents deeper penetration in scattering medium. Using objective-changeable ability of home-designed Mirau objective, this system provides different applications, like biological tissue and single cells, by different spatial resolution with corresponding dynamics. High quality image relying on less ghost image and near common-path interference was demonstrated under this compact and power-stable system.

Laser-diode pumped glass-clad Ti:sapphire crystal fiber laser.

Efficient glass-clad crystal fiber (CF) lasers were demonstrated using a Ti:sapphire crystalline core as the gain medium. With a core diameter of 18 μm, the laser diode (LD) pump source can be effectively coupled and guided throughout the crystal fiber for a low threshold and high slope efficiency laser operation. The advantage of high heat dissipation efficiency of the fiber structure can be derived from the low core temperature rising measurement (i.e., 17 K/W) with passive cooling. At an output transmittance of 23%, the lowest absorbed threshold of 118.2 mW and highest slope efficiency of 29.6% were achieved, with linear laser polarization.

In vivo 3-D cellular level imaging using Mirau-based full-field optical coherence tomography on skin tissue

With silicone-oil-immersion Mirau objective utilizing high brightness Ce3+:YAG crystal fiber light source through multi-mode fiber, this system respectively images in vivo tomographic cellular structure of tissue and flowing pathway of red blood cell in dynamics.

Absorptive thin film characterization with spectroscopic full-field optical coherence tomography

We have developed a spectroscopic full-field optical coherence tomography system, which can provide an ultrahigh isotropic spatial resolution. The complex refractive index and thickness of an embedded absorptive thin film was simultaneously measured.