Two-dimensional ultrashort echo time imaging using a spiral trajectory

Tissues with very short transverse relaxation time (T2) cannot be detected using conventional magnetic resonance (MR) sequences due to the rapid decay of excited MR signals. In this work, a multiecho sequence employing half-pulse excitation and spiral sampling was developed for ultrashort echo time (UTE) imaging of tissues with short T2. Spiral readout gradients were measured and precompensated to reduce gradient distortions due to eddy currents and gradient anisotropy. The effects of spatial blurring due to fast signal decay were investigated experimentally through spiral UTE (SUTE) imaging of rubber bands with different spiral sampling duration. The unwanted long T2 signals were suppressed through the use of an inversion pulse and nulling, and/or subtraction of a later echo image from the initial one. This technique has been applied to imaging of the short T2 components in brain white matter, knee cartilage, bone and carotid vessel wall of normal volunteers at 1.5 T. Preliminary results show high spatial resolution and excellent image contrast for a variety of short T2 tissues in the human body under a relatively short scan time. A quantitative comparison was also made between radial UTE and SUTE in terms of signal-to-noise ratio efficiency.     

Phase modulation of an electro-optic polymer cladded polarization-maintaining optic fiber

We present, for the first time based on our knowledge, a novel in-fiber optical phase modulator that is inherently broadband, efficient, and polarization-maintaining (PM). The modulator is a 25 mm long section of a D-shaped cladding PM fiber with half of its silica cladding replaced with an electro-optic (EO) silicone gel. A phase modulation of more than pi rad has been demonstrated. We describe techniques that enable the long-distance exposure of the fiber core, microfabrication of on-fiber electrodes, a low refractive index EO cladding material for replacing silica fiber cladding, and accurate phase measurement utilizing a Sagnac interferometer.     

High power single transverse mode operation of a tapered large-mode-area fiber laser

To get high output power with good beam quality, a tapered section is introduced to large-mode-area (LMA) Yb-doped fiber laser. Output characteristics of the fiber laser without tapered section and with tapered section are compared experimentally. When the launched pump power is 119.1 W, 77.9 W with M² 3.08 and 56.4 W with M² 1.14 can be obtained, respectively. The corresponding slope efficiencies are 71.8% and 54.1%, respectively. Although output power of the tapered fiber laser has 30.6% penalty, brightness of it is as much as 5.28 times of the fiber laser without tapered section. Moreover, spectra of them are measured. It is found that tapered section makes lasing wavelength of the fiber laser shorter.     

Detection of scattered light pulses at femto-Joule level by using a picosecond BBO optical parametric amplifier

We report a technique for detecting weak scattered light pulses based on a 532 nm pumped picosecond β-barium-borate collinear optical parametric amplifier. The measured maximum slope gain factor of the amplifier was found to be around 10⁷, and the energy detection limit was of the order of fJ/pulse for the signal of 730 nm and the idler of 1.5 μm at a pumping intensity of 2.83 GW/cm². The linearity of the gain for this amplifier was found to be excellent for a seeding level of lower than 420 fJ per pulse. The maximum gains and the energy detection limits for the scattered light pulses from various scattering targets were found to be poorer than that from the reflective mirror, owing to the degraded beam quality and the depolarization of the scattered light. A reduction of the maximum gain for the scattered light with the increase of the angle of incidence, which causes pulse broadening and reduces the photon flux of the signal, was investigated. The feasibility of detecting weak scattered light in the infrared by using idler-to-signal frequency up-conversion was also demonstrated, in which the infrared seeder located in the idler branch of the amplifier was injected as the seeding beam and was then parametrically up-converted into the visible signal branch, with an even higher gain.     

Time-domain investigation of the tunneling modes in photonic heterostructure containing single-negative materials

We present a theoretical investigation into the energy transport and transient wave propagation in metamaterial tunneling structures consisting of ??-negative (ENG) and ??-negative (MNG) materials. It is proved that a conjugated matched ENG/MNG bilayer and a (zero-index-material doped) photonic crystal heterostructure can work as a sub-wavelength resonator at tunneling frequency. The tunneling modes need a certain time to achieve the steady state and the charge up characteristic time increases (nearly) exponentially with the thickness of the structures. Under the steady state, the wave in the single-negative-material structures is not evanescent, but a hybrid of a traveling wave and a reactive standing wave. The phase difference between the electric field and the magnetic field varies with the position and time. The investigation of transient wave propagation in the metamaterial tunneling structures will help us to understand the interaction process between wave and metamaterial and to design special functional apparatus.     

宽谱调谐二极管激光吸收光谱的数据预处理研究

二极管激光吸收光谱以温度扫描实现快速宽谱调谐,其原始数据不便于传统的处理及运算,适当的预处理是进行后续数据处理的基础.在分析了温度调谐数据特征的基础上,利用Savitzky-Golay滤波器去除了波长序列中温度检测噪声的干扰,选择单调的波长区间对信号幅值进行三次样条插值,以固定的波长点对应采样点,得到一组位置固定的幅值序列用于后期处理.此方法将两列检测数据合并为一列,相同的起始点和固定的波长位置不仅将多周期平均后的信噪比提升为原始数据的6倍,也使多组数据的比较及评价成为可能,一致的波长分辨率保证了线形检测的精确度,减小了吸收峰的频带宽度,有助于后续的处理、运算以及评价.     

Luminescence properties and energy transfer investigations of BaAl2B2O7:Ce3+,Tb3+ phosphors

Ce³⁺ and Tb³⁺ co-doped BaAl₂B₂O₇ phosphors were synthesized by the solid-state method. X-ray diffraction (XRD) was used to characterize the phase structure. The photoluminescent properties of Ce³⁺ and Tb³⁺ co-doped BaAl₂B₂O₇ phosphors were investigated by using the photoluminescence emission and excitation spectra. Under the excitation of near ultraviolet (n-UV) light, BaAl₂B₂O₇:Ce³⁺,Tb³⁺ phosphors exhibited blue emission corresponding to the f–d transition of Ce³⁺ ions and green emission bands corresponding to the f–f transition of Tb³⁺ ions, respectively. Effective energy transfer occurred from Ce³⁺ to Tb³⁺ in BaAl₂B₂O₇ host due to the observed spectra overlap between the emission spectrum of Ce³⁺ ion and the excitation spectrum of Tb³⁺ ion. The energy transfer efficiency from Ce³⁺ ion to Tb³⁺ ion was also calculated to be 71%. Furthermore, the concentration quenching and critical distance of BaAl₂B₂O₇:Ce³⁺,Tb³⁺ phosphors were also discussed. The energy transfer from Ce³⁺ to Tb³⁺ in BaAl₂B₂O₇ host was demonstrated to be resonant type via a dipole–dipole interaction mechanism with the energy transfer critical distance of 16.13 Å.     

High-order harmonic generation with a two-color laser pulse

We theoretically investigate the electron dynamics of the high-order harmonics generation process by combining a near-infrared 800 nm driving pulse with a mid-infrared 2000 nm control field.We also investigate the emission time of harmonics using time-frequency analysis to illustrate the physical mechanisms of high-order harmonic generation.We calculate the ionization rate using the Ammosov-Delone-Krainov model and interpret the variations in harmonic intensity for different control field strengths and delays.We find that the width of the harmonic plateau can be extended when the control electric field is added,and a supercontinuum from 198 to 435 eV is generated,from which an isolated 61-as pulse can be directly obtained.