Dependence of THz Time-Domain Spectroscopy on the Crystal Thickness and Optical Width in Electro-Optic Sampling

The processes of electro-optical (EO) sampling of terahertz (THz) pulse were studied theoretically by considering the finite optical probe width and the double-sampling within the same THz pulse. The dependence of THz time-domain waveforms on the crystal thickness and optical width in EO sampling was revealed. Furthermore, the optimal thickness was analyzed, which shows whether or not an antireflection film coated for the optical probe beam is needed.     

A Theoretical and Experimental Research on Terahertz Electro-Optic Sampling at Near-Zero Optical Transmission Point

We report an improved theoretical model to characterize terahertz electro-optic （F,O） sampling detection operating at near crossed-polarizer design by considering the nonzero minimal transmission of polarizers and the polarization purity of laser beam. The proof-of-concept experiments show that the model can give quiet reasonable explanations on many experimental phenomena in THz EO detection, e.g. the modulation depth vs the different optical biases, the noise level from the balance detector vs the input probe beam power, the main origin of the background light and the incomplete cancellation of noise from the laser intensity fluctuation in balanced detection. The terahertz signals and the noises are also measured based on two different EO sampling schemes for comparison. The measured results support our model very well.     

Acceleration of multi-dimensional propagator measurements with compressed sensing

NMR can probe the microstructures of anisotropic materials such as liquid crystals, stretched polymers and biological tissues through measurement of the diffusion propagator, where internal structures are indicated by restricted diffusion. Multi-dimensional measurements can probe the microscopic anisotropy, but full sampling can then quickly become prohibitively time consuming. However, for incompletely sampled data, compressed sensing is an effective reconstruction technique to enable accelerated acquisition. We demonstrate that with a compressed sensing scheme, one can greatly reduce the sampling and the experimental time with minimal effect on the reconstruction of the diffusion propagator with an example of anisotropic diffusion. We compare full sampling down to 64× sub-sampling for the 2D propagator measurement and reduce the acquisition time for the 3D experiment by a factor of 32 from ～80 days to ～2.5 days.     

Ultrafast optical signal measurementusing optoelectronic techniques

Optical and Quantum Electronics - A novel ultrafast optical signal measurement technique is proposed that uses a fast photodetector and electrooptic (EO) sampling. The technique can satisfy three...     

Jet-entrainment sampling: A new method for extracting particles from flames

We have developed a new method for extracting particulates and gas-phase species from flames. This technique involves directing a small jet of inert gas through the flame to entrain the sample, which is then collected by a probe on the other side of the flame. This sampling technique does not require inserting a probe or sampling surface into the flame and thus avoids effects on the flame due to conductive cooling by the probe and recombination, quenching, and deposition reactions at the sampling surface in contact with the flame. This approach thus allows for quenching and diluting the sample during extraction while minimizing the perturbations to the flame that have a substantial impact on flame chemistry. It also circumvents clogging of the probe with soot, a problem that commonly occurs when a probe is inserted into a hydrocarbon-rich premixed or diffusion flame. In this paper, we present experimental results demonstrating the application of this technique to the extraction of soot particles from a co-flow ethylene/air diffusion flame. The extracted samples were analyzed using transmission electron microscopy (TEM), and the results are compared with measurements using in situ diagnostics, i.e., laser-induced incandescence and small-angle X-ray scattering. We also compare TEM images of particles sampled using this approach with those sampled using rapid-insertion thermophoretic sampling, a common technique for extracting particles from flames. In addition, we have performed detailed numerical simulations of the flow field associated with this new sampling approach to assess the impact it has on the flame structure and sample following extraction. The results presented in this paper demonstrate that this jet-entrainment sampling technique has significant advantages over other common sample-extraction methods.     

Benchmarking of electro-optic monitors for femtosecond electron bunches

The longitudinal profiles of ultrashort relativistic electron bunches at the soft x-ray free-electron laser FLASH have been investigated using two single-shot detection schemes: an electro-optic (EO) detector measuring the Coulomb field of the bunch and a radio-frequency structure transforming the charge distribution into a transverse streak. A comparison permits an absolute calibration of the EO technique. EO signals as short as 60 fs (rms) have been observed, which is a new record in the EO detection of single electron bunches and close to the limit given by the EO material properties.     

Design of a spectrum-expanded polymer Mach�CZehnder interferometer electro-optic switch using two-phase generating couplers

A novel structure, thorough analysis and design technique are presented for a kind of polymer Mach?CZehnder Interferometer (MZI) electro-optic (EO) switch consisting of two-phase generating couplers (PGCs). By optimizing the PGCs structure, the output spectrum of the two PGCs based MZI EO switch is greatly expanded to almost six times of that of the traditional MZI EO switch with two 3?dB directional couplers. Under a central wavelength of 1550 nm, the driving voltages of the designed switch are as low as about 0 (ON state) and ±0.925?V (OFF state) with an EO region length of 5000???m. A wide spectrum more than 350 nm (1380??1730?nm) is achieved, and within this wavelength range, the crosstalk is less than ?30?dB, and the insertion loss is less than 5.91?dB which is larger than that of the traditional MZI EO switch due to the longer waveguide length by introducing two PGCs. The design technique on the two PGCs based MZI EO switch is supported to be reasonable compared with the beam propagation method (BPM). The proposed structure shows potential wide applications especially in optical cross-connect (OXC) and optical add-drop multiplexing (OADM) systems.     

一种基于电光调制光频梳光谱干涉的绝对测距方法

提出了一种基于电光调制光学频率梳的光谱干涉测距方法.理论分析了电光调制光学频率梳的数学模型和光谱扩展原理,并分析得出了光谱干涉测距方法的非模糊范围和分辨力的影响因素.在实验中,使用三只级联的电光相位调制器调制单频连续波激光生成了40多阶高功率梳齿状边带,并通过单模光纤和高非线性光纤对电光调制器输出的激光进行光谱扩展,得到重复频率为10 GHz,光谱宽度达30 nm的光学频率梳.将该光频梳作为光谱干涉测距装置的光源,可以实现无"死区"的绝对距离测量.另外,使用等频率间隔重采样和二次方程脉冲峰值拟合算法对测量结果进行数据处理,可以修正系统误差,提升测距精度.实验结果表明,在1 m的测量范围内,使用该装置可以在任意位置达到±15μm以内的绝对测距精度.     

Beam Probe Space-Potential Measurement in a Minimum-B Field

A measurement of plasma space potential in a minimum-B magnetic field has been made with a particle beam probe. The diagnostic technique is an extension of conventional heavy ion beam probing in that, among other features, the probing beam is neutral rather than ionized cesium and the parallel-plate electrostatic energy analyzer is fully characterized in a new manner which permits its use in complex magnetic confinement geometries.     

Switchable optical element with Bragg mode diffraction

A theoretical model of a new electronically switchable grating design that uses a multilayer structure of an electro-optic (EO) material with an interdigitated-electrode type of array is proposed as an original technique for calculating the induced refractive index. It is shown that asymmetrical distribution of the electric field induces a slanted Bragg grating, which allows the slant angle to be switched electronically among more than two switching states. Parameters of the suggested design are calculated for a number of EO materials. A special case of frequency-based switching is anticipated for some polymer-dispersed liquid-crystal materials.     

Comparison of spatial resolution in internal and external electro-optic probing

A new method, based on zero point of longitudinal electric field, was used to determine the spatial resolution of internal and external electro-optic (EO) probing systems. An interdigital structure electrode was fabricated on GaAs substrate to examine the spatial resolution of internal and external EO probing system. Considering the air gap between the electrode under test and the probe tip, the result of EO probing was simulated which is well accorded with experiment. The result indicates that spatial resolution of less than 0.5 and 1 m are obtained in internal and external EO probing system respectively.     

Particle Sampling and Real Time Size Distribution Measurement in H2/O2/TEOS Diffusion Flame

Growth characteristics of silica particles have been studied experimentally using in situ particle sampling technique from H₂/O₂/Tetraethylorthosilicate (TEOS) diffusion flame with carefully devised sampling probe. The particle morphology and the size comparisons are made between the particles sampled by the local thermophoretic method from the inside of the flame and by the electrostatic collector sampling method after the dilution sampling probe. The Transmission Electron Microscope (TEM) image processed data of these two sampling techniques are compared with Scanning Mobility Particle Sizer (SMPS) measurement. TEM image analysis of two sampling methods showed a good agreement with SMPS measurement. The effects of flame conditions and TEOS flow rates on silica particle size distributions are also investigated using the new particle dilution sampling probe. It is found that the particle size distribution characteristics and morphology are mostly governed by the coagulation process and sintering process in the flame. As the flame temperature increases, the effect of coalescence or sintering becomes an important particle growth mechanism which reduces the coagulation process. However, if the flame temperature is not high enough to sinter the aggregated particles then the coagulation process is a dominant particle growth mechanism. In a certain flame condition a secondary particle formation is observed which results in a bimodal particle size distribution.     

Spectral narrowing and manipulation in an optical parametric oscillator using periodically poled lithium niobate electro-optic polarization-mode converters

We report a unique spectral narrowing and manipulation technique in an optical parametric oscillator (OPO) realized by an integrated periodically poled lithium niobate comprising an optical parametric gain medium sandwiched by two electro-optic polarization-mode converters (EO PMCs). We achieved a manipulation of the gain spectrum of the OPO via EO and/or temperature control of the EO PMCs, in which we obtained single to multiple signal spectral peaks from the OPO with a spectral width reduced by up to 10 times and peak intensity increased by up to 6 times in comparison with the original signal. Fast EO tuning of the narrowed signal spectral peak has also been demonstrated.     

Research of spatial resolution in external electro-optic probing

A new method, based on zero point of longitudinal electric field, was used to determine the spatial resolution of external electro-optic (EO) probing equipment. Considering the diffraction of Gauss beam, the result of external EO probing was simulated which was in accordance with the experiment. A spatial resolution <1 μm was demonstrated initially in our equipment using 650 nm laser diode as probe beam and semi-insulating GaP as probe tip.     

Direct time-response measurement of high-speed optical modulators based on stretched-pulse interferometry

A simple technique for the direct measurement of the complex temporal response of a high-speed electro-optic (EO) modulator is proposed. This technique recovers the amplitude and phase temporal profiles of an instantaneous modulation over the duration of a chirped pulse (obtained by linear dispersion) using Fourier-transform interferometry, and it exploits the time-to-frequency mapping induced by the pulse dispersion process. The method can be implemented by using either time- or frequency-domain interferometric detection, allowing the characterization of modulation bandwidths in the tens-of-gigahertz range. The concept is demonstrated by accurately measuring the complex temporal response of a 2.5 Gbps intensity EO modulator.     

Three-dimensional electric-field vector measurement with an electro-optic sensing technique

A new technique for three-dimensional (3D) electric-field (e-field) vector measurement is presented. Three laser beams with different propagation paths in an electro-optic (EO) crystal were used to resolve 3D components of e-field vectors. We adopted a special geometric shape of bismuth silicon oxide EO crystal so that the three beams would propagate within it. A sensitivity of 0.6 V/cm radicalHz was achieved. A commercial Ansoft Maxwell 3D field simulator was also used to verify our measurements.     

Reconfigurable generation of high-repetition-rate optical pulse sequences based on time-domain phase-only filtering

We propose and demonstrate a fiber-based phase-only filtering technique for programmable optical pulse shaping, in which the filtering operation is implemented in the time domain by means of an electro-optical (EO) phase modulator. The technique has been applied for generating customized ultrahigh-repetition-rate optical pulse sequences (>40 GHz) from single input pulses by driving the EO phase modulator with a periodic electronic waveform (RF tone). The generated output pulses are replicas of the input pulse and both the repetition rate and the envelope profile of the generated sequences can be controlled and tuned electronically using this approach.     

Echo-planar rotating-frame imaging

A new rotating-frame imaging method that produces a complete cross section of an object in a single experiment is reported. The echo planar rotating frame imaging (EPROFI) technique uses two perpendicular RF gradients for two-dimensional spatial encoding and fully exploits the formation of rotary echoes for fast sampling of spatial frequencies. The acquisition scheme yields the Fourier transform of the spin distribution on Cartesian coordinates for straightforward image reconstruction. Implementation of the technique on a low-field portable NMR probe is described and results are presented for test objects with different geometries.     

Electron distribution function measurement in irregular ionization waves

The influence of ionization waves irregularity on electron distribution function (EDF) measurement has been investigated. The probe method with sampling technique has been applied. The results show that the time irregularity of high amplitude ionization waves may cause an error in interpretation of the EDF measurement.     

Electrooptic sensor module fabrication for near-field intrabody communication

In this paper, we describe how to obtain a low cost electrooptic (EO) sensor module for the mass production of near-field intrabody communication devices. In our previous study, we used a bulk cleavage technique to fabricate EO modulators without the need for any optical polishing or washing processes. In this study, we fabricated EO modulators as a miniaturized chip sensor without a base portion, and clarified the feasibility of assembling optical components by only a passive alignment technique with a compact housing.