Splice-free interfacing of photonic crystal fibers

We report a new method for making low-loss interfaces between conventional single-mode fibers and photonic crystal fibers (PCFs). Adapted from the fabrication of PCF preforms from stacked tubes and rods, this method avoids the need for splicing and is versatile enough to interface to virtually any type of index-guiding silica PCF. We illustrate the method by forming interfaces to two problematic types of PCF, highly nonlinear and multicore. In particular, we believe this to be the first method capable of individually coupling light into and out of all the cores of a fiber with multiple closely spaced cores, without input or output cross talk.     

Highly birefringent photonic crystal fibers

We report a strongly anisotropic photonic crystal fiber. Twofold rotational symmetry was introduced into a single-mode fiber structure by creation of a regular array of airholes of two sizes disposed about a pure-silica core. Based on spectral measurements of the polarization mode beating, we estimate that the fiber has a beat length of approximately 0.4 mm at a wavelength of 1540 nm, in good agreement with the results of modeling.     

Supercontinuum generation in tapered fibers

Supercontinuum light with a spectrum more than two octaves broad (370-1545 nm at the 20-dB level) was generated in a standard telecommunications fiber by femtosecond pulses from an unamplified Ti:sapphire laser. The fiber had been tapered to a diameter of :2mum over a 90-mm length. The pulse energy was 3.9 nJ (average power, 300 mW). This source of high-intensity single-mode white light should find widespread applications in frequency metrology and spectroscopy, especially since no unconventional fibers are needed.     

SINGLET AND TRIPLET MECHANISMS IN PHOTOCHEMISTRY

A perturbation method is proposed for determining the photochemical quantum yields of the singlet and triplet mechanisms for an organic compound in solution. Some common fallacies about singlet and triplet mechanisms in the photochemical literature are discussed.     

Dual Fluorescence of isolated aromatic molecules

Expressions are derived for the S^*₂ and S^*₁ fluorescence quantum yields and response functions of isolated excited aromatic molecules in the statistical limit. The S^*₂ and S^*₁ fluorescences have a common decay time, a property previously considered characteristic of intermediate strong coupling. Data on pyrene, naphthalene, and 3:4-benzopyrene vapours are discussed in relation to the two models.     

The preparation of powder specimens for shear cell testing

Conclusion The main aim of the present work was to provide a reliable method for preparing powder specimens for testing in a shear cell, and the method described above is recommended for use in cases where the simpler method described byJenike is found not to work.The discussion of what happens during the compaction process, and whether or not it is necessary to shear the specimen almost to the point of failure during preparation, must be regarded as tentative. More exact information depends on a knowledge of the shape of the failure surfaces for industrial powders; work along these lines is at present being planned.Paper presented at a meeting of the British Society of Rheology, University of Nottingham, April 6–8, 1965.     

The fluorescence of diphenyl- and retinol-polyenes

The unusual fluorescence properties of the polyenes are listed. The fluorescence lifetimes and/or quantum yields of the first four all-trans diphenylpolyenes have been determined in seven solvents between ?50 and 50°C. A model is proposed to account for the unusual fluorescence behaviour in terms of a low-lying ¹Ag state, coupled to ¹B_u, and which shifts to lower energies on intramolecular rotation.     

时频方法分析长骨中的超声导波及皮质骨厚度

超声导波在长骨中传播时,接收信号中含有相互叠加的多个导波模式。本文提出将希尔伯特-黄变换(HHT)用于分析叠加的多模式导波信号,将其分解成许多单个独立的模式,然后对分解出的模式求得其对应频率下的群速度,并与短时傅里叶变换所得的结果进行比较。通过与理论计算结果比较,可得到长骨皮质骨的厚度。研究结果表明,实验得到的厚度与实际厚度一致。说明通过测量导波模式的速度可以评价皮质骨的厚度,也说明HHT方法是一种识别叠加多模式导波信号的有效方法。      

Detection of long bones based on Barker code excited ultrasonic guided waves

Single pulse excited ultrasonic guided wave surfers high attenuation during the propagation in long bones.This results in small amplitude and low signal-to-noise ratio(SNR)of measured signals.Thus,the Barker code excitation is introduced into long bone detection to improve the quality of received signals,due to its efficiency in increasing amplitude and SNR.Both simulation and in vitro experiment were performed,and the results were decoded by the weighted match filter(WMF) and the finite impulse response- least squares inverse filter(FIRLSIF),respectively.The comparison between the results of Barker code excitation and sine pulse excitation was presented.For 13-bit Barker code excitation,WMF produced 13 times larger amplitude than sine pulse excitation,while FIR-LSIF achieved higher peak-sidelobe-level(PSL) of —63.59 dB and better performance in noise suppression.The results show that the Barker code excited guided waves have the potential to be applied to the long bone detection.     

Optofluidic microchannels in aerogel

We report optofluidic waveguides made by filling microchannels in aerogel with water. The aerogel cladding is a nanoporous material with an extremely low refractive index of ~1.05, giving a large index step from the water core. Channels were formed by removing embedded optical fibers, which could be nonuniform or multiple. The porosity of the aerogel allowed air to be displaced from the channel, preventing the trapping of bubbles. The attenuation of red light in the highly multimode water core waveguide was no greater than 1.5 dB/cm.