We review progress in quasi-phase-matched optical parametric oscillators in bulk periodically poled LiNbO3. Using the electric-field poling process, we can reliably fabricate 0.5-mm-thick crystals with uniform domain structures over a 15-mm length. Periodically poled material retains the low-loss and bulk power handling properties of single-domain LiNbO3, and quasi phase matching permits noncritical phase matching with d33, the highest-valued nonlinear coefficient. Optical parametric oscillators pumped by 1.064-μm pulsed Nd:YAG lasers have been operated over the wavelength range 1.4–4 μm with tuning by temperature or by quasi-phase-matched period. We have shown an oscillation threshold as low as 0.012 mJ with a Q-switched pump laser and pumping at greater than ten times threshold without damage. We have also demonstrated a cw doubly resonant oscillator near 1.96 μm pumped directly with a commercial cw diode laser at 978 nm.

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Quasi-phase-matched optical parametric oscillators in bulk periodically poled LiNbO 3

BACKGROUNDAn improved intravascular ultrasonic Doppler device could aid the clinical assessment of coronary hemodynamics. We evaluated a new device consisting of a 12-MHz piezoelectric transducer integrated onto the tip of a 0.018-in. flexible, steerable angioplasty guide wire.METHODS AND RESULTSDoppler spectra were recorded in model tubes with pulsatile blood flow and in-line electromagnetic flowmeter. In four straight tubes (i.d., 0.79-4.76 mm), the time average of spectral peak velocity (APV) was linearly related to blood flow (QEMF) (r2 greater than or equal to 0.98 for each tube). A Doppler-derived quantitative flow estimate (QD) was calculated as the product of vessel cross-sectional area and mean velocity, with mean velocity estimated as 0.5 x APV. The slope of QD versus QEMF for the four tubes was near unity. APV was less accurate in a 7.94-mm straight tube and in tortuous segments. In four dogs, the left circumflex coronary artery (LCx) was perfused from the femoral artery via a cannula with in-l...

Validation of a Doppler guide wire for intravascular measurement of coronary artery flow velocity.

Flat optical devices thinner than a wavelength promise to replace conventional free-space components for wavefront and polarization control. Transmissive flat lenses are particularly interesting for applications in imaging and on-chip optoelectronic integration. Several designs based on plasmonic metasurfaces, high-contrast transmitarrays and gratings have been recently implemented but have not provided a performance comparable to conventional curved lenses. Here we report polarization-insensitive, micron-thick, high-contrast transmitarray micro-lenses with focal spots as small as 0.57 λ. The measured focusing efficiency is up to 82%. A rigorous method for ultrathin lens design, and the trade-off between high efficiency and small spot size (or large numerical aperture) are discussed. The micro-lenses, composed of silicon nano-posts on glass, are fabricated in one lithographic step that could be performed with high-throughput photo or nanoimprint lithography, thus enabling widespread adoption.

Subwavelength-thick Lenses with High Numerical Apertures and Large Efficiency Based on High Contrast Transmitarrays

The localization of atherosclerosis in the coronary arteries may be governed by local hemodynamic features. In this study, the pulsatile hemodynamics of the left coronary artery bifurcation was numerically simulated using the spectral element method for realistic in vivo anatomic and physiologic conditions. The velocity profiles were found to be skewed in both the left anterior descending and the circumflex coronary arteries. Velocity skewing arose from the bifurcation as well as from the curvature of the artery over the myocardial surface. Arterial wall shear stress was significantly lower in the bifurcation region, including the side walls. The greatest oscillatory behavior was localized to the outer wall of the circumflex artery. The time-averaged mean wall shear stress varied from about 3 to 98 dynes/cm 2 in the left coronary artery system. The highly localized distribution of low and oscillatory shear stress along the walls strongly correlates with the focal locations of atheroma in the human left coronary artery.

Pulsatile flow in the human left coronary artery bifurcation: average conditions.

Polymer microring resonators were demonstrated for sensing biomolecules without using fluorescent labels. The microring devices, fabricated by a direct imprinting technique, possess high Q factors of /spl sim/20000. This feature provides high sensitivity and a low detection limit for biochemical sensing applications. With these properties, the devices were used to detect and quantify the biomolecules present either in a homogeneous solution that surrounds the microring waveguide (homogeneous sensing) or specifically bound on the waveguide surface (surface sensing). In the former sensing mechanism, the current devices can detect an effective index change of 10/sup -7/ refractive index units (RIU); in the latter, they can reach a detection limit of /spl sim/250 pg/mm/sup 2/ of biomolecular coverage on the microring surface. In addition, the experiments show that the devices can detect both small and large biomolecules.

Polymer microring resonators for biochemical sensing applications

Optical integrated circuits

以上, Ti拡散LiNbO3を中心として, 光集積化の特長について述べ, 代表的な導波路材料を含めて具体的な光ICを紹介したこれによって, 光ICの基礎的な概念と最近の研究動向の一端がご理解いただければ幸いである現状は, ようやく実用的なニーズにこたえる光ICが試作される段階に達したところであり, 実際のシステムサイドの要求を満足するには, なお光ICの実装技術を含めて検討すべき問題が残されているしかし, 今や薄膜光導波を基礎とする光IC技術は光エレクトロニクスの分野において不可欠なキーテクノロジーである今後は, 非線形光学効果を用いた光-光制御素子を始めとして, 新しい機能をもつ導波形素子, 光ICの開発に向けてのなお一層の研究の進展が期待される

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Optical integrated circuits.

A monolithic-integrated optical-disk pickup device capable of detecting readout and focus/tracking error signals is proposed, designed, and fabricated. The integrated-optic disk pickup device (IODPU) is constructed by integrating a focusing grating coupler, a grating beam splitter, and photodiodes in a film waveguide on a Si substrate. The function of detecting readout and error signals is demonstrated by butt-coupling a laser diode to the fabricated IODPU.

An integrated-optic disk pickup device

A blazing technique using electron-beam lithography to achieve higher efficiency of gratings and Fresnel lenses is described. Transmission-type blazed gratings have been formed in polymethyl methacrylate films. As a result of measurement, we found that their diffraction efficiency of the first order in these gratings amounts to as much as 60 to 70% at 0.633 microm. Fresnel lenses of 1-mm diameter and 5-mm focal length, which have a sawtooth relief profile, have been also fabricated, and the experimental results showed high-efficiency performance (about 50%) and nearly diffraction-limited focusing.

Blazed gratings and Fresnel lenses fabricated by electron-beam lithography

Micro lenses are basic components of micro optics. We have proposed a new fabrication technique for micro lenses that uses electron-beam lithography and have developed an electron-beam lithography system that is specially designed for this purpose. To demonstrate the feasibility of this technique, ordinary-type Fresnel (zone-plate) lenses and a special-type lens for converting an incident Gaussian intensity distribution into a uniform one were designed and fabricated. It was found that these Fresnel (zone-plate) lenses have near-diffraction-limited performance. The fabrication technique and experimental results are discussed.

Hiroshi Nishihara

Papers

Optical integrated circuits

Optical integrated circuits.

An integrated-optic disk pickup device

Blazed gratings and Fresnel lenses fabricated by electron-beam lithography

Fabrication of micro lenses using electron-beam lithography