Superchirped moiré grating based on an acousto-optic superlattice with a chirped fiber Bragg grating

A superchirped moiré grating has multiple superimposed chirped Bragg gratings. We show that a superchirped moiré grating can be formed in an acousto-optic superlattice with a chirped fiber Bragg grating. There are multiple transmission peaks related to Fabry-Perot resonance in the transmission spectrum. The peak spacing is varied by the acoustic frequency. The proposed devices can be used as tunable multiple-passband transmission filters and switches.     

Synchronous, dual-wavelength, injection-seeded amplification of 5-ns pulses in a flash-lamp-pumped Ti:sapphire laser

The outputs of two cw diode lasers are coupled into a flash-lamp-pumped Ti:sapphire laser cavity for regenerative amplification. Slices of each seed beam are simultaneously trapped in the cavity and amplified for approximately 33 round trips. The output is a 4.7-ns pulse, the total pulse energy is 110 mJ, and linewidths are only 50% greater than the Fourier-transform limit. The timing jitter between the pulses at the two wavelengths is less than +/-75 ps , and their relative energy is controlled by adjustment of the seed power of the cw diode lasers. Gain competition between the two frequencies does not lead to instabilities.     

In vivo ultrahigh-resolution optical coherence tomography

Ultrahigh-resolution optical coherence tomography (OCT) by use of state of the art broad-bandwidth femtosecond laser technology is demonstrated and applied to in vivo subcellular imaging. Imaging is performed with a Kerr-lens mode-locked Ti:sapphire laser with double-chirped mirrors that emits sub-two-cycle pulses with bandwidths of up to 350 nm, centered at 800 nm. Longitudinal resolutions of ~1mum and transverse resolution of 3mum, with a 110-dB dynamic range, are achieved in biological tissue. To overcome depth-of-field limitations we perform zone focusing and image fusion to construct a tomogram with high transverse resolution throughout the image depth. To our knowledge this is the highest longitudinal resolution demonstrated to date for in vivo OCT imaging.     

Frequency-upconverted organic crystal

The fluorescent characteristics of Troger's base have been investigated experimentally. A one-photon-induced fluorescent spectrum was observed, and the peak wavelength was at 337 nm with a full width at half-maximum of ~85nm . The violet radiation of the Troger's base crystal could result from the pi(*) -pi transition of the benzenoid pi bond. Two-photon-pumped frequency-upconverted fluorescence was obtained, which is a wideband emission with the main peak wavelength at 337 nm and a subordinate peak. Donor groups and acceptor groups of electrons in the aromatic rings strengthen the two-photon-pumped frequency-upconverted fluorescence.     

Weak Copositive and Intertwining Approximation

It is known that shape preserving approximation has lower rates than unconstrained approximation. This is especially true for copositive and intertwining approximations. ForfL_p, 1p<∞, the former only has rateω(f, n⁻¹)_p, and the latter cannot even be bounded byC f_p. In this paper, we discuss various ways to relax the restrictions in these approximations and conclude that the most sensible way is the so-calledalmostcopositive/intertwining approximation in which one relaxes the restriction on the approximants in a neighborhood of radiusΔ_n(y_j) of each sign changey_j.     

The Existence of Positive Solutions to Neutral Differential Equations

In this paper, we shall consider a class of neutral differential equations of the form

Full-size image (1K)

where τ (0, ∞), σ [0, ∞), Q(t) C([t₀, ∞), R ⁺ ), r(t) C([t₀, ∞), (0, ∞)) with r(t) nondecreasing on [t₀ − τ, ∞). We shall show that all positive solutions of ( * ) can be classified into four types, A, B, C, and D, and we shall obtain sufficient and necessary conditions for the existence of A-type, B-type, and D-type positive solutions of ( * ), respectively. A sufficient condition for the existence of C-type positive solutions of ( * ) is also given. Finally, we shall offer a sharp oscillation result for all solutions of ( * ). Our results generalize and improve those established in B. Yang and B. G. Zhang (Funkcial. Ekvac.39 (1996), 347–362).     

Bi2212晶须中磁通线运动临界电流的反常现象

我们对同一Ｂｉ２２１２晶须样品加不同方向的磁场,比较了Ｈ∥ｂ和Ｈ∥ｃ两种情况下的Ｉ－Ｖ曲线。我们发现,当Ｈ∥ｃ时,驱动磁通线运动的临界电流Ｉｃ与温度的关系遵循一般的单调地随温度上升而下降的关系。但是,当Ｈ∥ｂ时,Ｉｃ在Ｔ＾＊＝２５．５Ｋ附近出现了一个峰值：当Ｔ〉Ｔ＾＊时,Ｉｃ温度的下降而升高;Ｔ〈Ｔ＾＊时,Ｉｃ随温度的下降而下降。我们在文章中对此现象进行了讨论。     

磁通钉扎液体的动力学特性

依据Ｍａｌｏｚｅｍｏｆｆ和Ｇｒｉｅｓｓｅｎ等建议的表征高温超导体有效钉扎势的唯象表式Ｕ（Ｔ,Ｂ,Ｊ）＝Ｕｏ（Ｔ,Ｂ）／μ（Ｔ,Ｂ）［Ｊｃ（Ｔ,Ｂ）／Ｊ）＾μ（Ｔ,Ｂ）－１］和热激活涡旋运动模型,并通过ＹＢａ２Ｃｕ３Ｏ７－δ薄膜直流传输特性的测量结果,利用“函数唯一性”数据处理方法,研究了钉扎液体的玻璃态指数μ（Ｔ,Ｂ）随温度与外磁场的负值函数形式。以及钉扎势垒Ｕｏ（Ｔ,Ｂ）和Ｊｃ（Ｔ,Ｂ）随外场与     

Design criteria of a chemical reactor based on a chaotic flow

We consider the design criteria of a chemical mixing device based on a chaotic flow, with an emphasis on the steady-state devices. The merit of a reactor, defined as the Q-factor, is related to the physical dimension of the device and the molecular diffusivity of the reactants through the local Lyapunov exponents of the flow. The local Lyapunov exponent can be calculated for any given flow field and it can also be measured in experimental situations. Easy-to-compute formulae are provided to estimate the Q-factor given either the exact spatial dependence of the local Lyapunov exponent or its probability distribution function. The requirements for optimization are made precise in the context of local Lyapunov exponents. (c) 1999 American Institute of Physics.