Fractional optical Hilbert transform using phase shifted fiber Bragg gratings

In this paper, we demonstrate that an arbitrary order (including both integer and fractional orders) Hilbert transform (HT) of an input optical waveform can be implemented by a simple and practical phase-shifted fiber Bragg grating (PSFBG) operated in reflection mode. The PSFBG consists of two concatenated identical uniform FBGs with a proper phase shift between them. It is proved that both the phase shift of the FBGs and the apodizing profile of the refractive index modulation determine the order of the transform. The simulation results show that the device can perform arbitrary fractional Hilbert transform (FHT) with excellent accuracy for input signals with up to hundreds of GHz bandwidth using feasible FBG structures.     

Optimizing two-photon fluorescence of Coumarin dye by combined temporal–spatial pulse shaping

Two-photon fluorescence (TPF) process is an important research subject and can be optimized by pulse shaping techniques. In addition to temporal femtosecond pulse shaping by spectral phase modulation, we take into account of spatial configuration in the shaping process. The TPF of Coumarin 500 increases 40% with this additional modulation step. This spatial modulation results in not only transverse spatial profile variation but also effect of temporal redistribution at focus. We show that this spatial modulation is an important dimension for pulse shaping and optimization for TPF.     

Optimization of reflection spectra for phase-only sampled fiber Bragg gratings

In this paper, the reflection spectra of a phase-only sampled fiber Bragg grating is obtained by optimizing its phase profile. The standard simulated annealing method is improved for optimization, and superior phase profiles which contain as few segments as possible per sampling period are obtained for 8, 16 and 32 identical channels. As very well known, increased number of WDM channels results in higher grating length and refractive index modulation amplitude. It is observed that application of these phase profiles limits the increases in both of these parameters considerably.     

Communication techniques and coding for atmospheric turbulence channels

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. In this paper, we describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of the fading, and the observation interval is shorter than the correlation time of the fading. We assume that the receiver has no knowledge of the instantaneous fading state. The techniques we consider are based on the statistical properties of fading, as functions of both temporal and spatial coordinates. Our approaches can be divided into two categories: temporal domain techniques and spatial domain techniques. In the spatial domain techniques, one must employ at least two receivers to collect the signal light at different positions or from different spatial angles. Spatial diversity reception with multiple receivers can be used to overcome turbulence-induced fading. When it is not possible to place the receivers sufficiently far apart, the fading at different receivers is correlated, reducing the diversity gain. We describe a ML detection technique to reduce the diversity gain penalty caused by such fading correlation. In the temporal domain techniques, one employs a single receiver. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to optimize performance. When the receiver also knows the temporal correlation of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. We describe two reduced-complexity implementations of the MLSD, which make use of a single-step Markov chain model for the fading correlation in conjunction with per-survivor processing. Next, we also investigate the performance of using error-control coding and pilot symbol-assisted detection schemes through atmospheric turbulence channels.     

Temporally Modulated Phase Retrieval Method for Weak Temporal Phase Measurement of Laser Pulses

We propose a simple iterative algorithm based on a temporally movable phase modulation process to retrieve the weak temporal phase of laser pulses.This unambiguous method can be used to achieve a high accuracy and to simultaneously measure the weak temporal phase and temporal profile of pulses,which are almost transform-limited.A detailed analysis shows that this iterative method has valuable potential applications in the characterization of pulses with weak temporal phase.     

Simultaneous temporal characterization of telecommunication optical pulses and modulators by use of spectrograms

We demonstrate a simple technique for simultaneous and complete characterization of the optical pulses and temporal modulators commonly used in telecommunication. The electric field of a pulse and the response of a modulator are obtained from the analysis of the two-dimensional spectrogram of the pulse gated by the modulator. The measurement sensitivity is greatly improved compared with the conventional nonlinear optical techniques. Trains of picosecond pulses as weak as 10(-17)J are accurately characterized with an electroabsorption modulator as the temporal gate. The time-resolved transmission and phase of the modulator are also presented.     

All-fiber coherent beam combining with phase stabilization via differential pump power control

Coherent beam combining enables power scaling beyond the limits of single amplifiers. Therefore, improving the performance and simplicity of coherent combination techniques is of great interest for many high power applications. Here, we show all-fiber coherent beam combining of two ytterbium doped amplifiers with and without a dedicated phase actuator and a total output power up to 25 W. Instead of a dedicated phase actuator, we directly controlled the two ytterbium amplifiers to also stabilize their relative phase. We compared the performance of this method with phase stabilization using two piezo driven fiber stretchers. In both cases, power noise was dominated by the single amplifier.     

Storage and retrieval of coherent optical information in atomic populations

We propose a simple and powerful protocol to map an arbitrary atomic coherence between two quantum states into a population distribution of three metastable states, and later to retrieve the atomic coherence from the population distribution. The protocol applies simple sequences of radiation pulses with arbitrary temporal profile, either as coincident or as consecutive pulses. Mapping of rather short-lived atomic coherences into very long-lived atomic populations permits the prolongation of storage times (e.g. of optical information encoded in atomic coherences) by many orders of magnitude — without the need for complicated techniques to reduce homogeneous broadenings.     

An introduction to the statistical physics of active matter: motility-induced phase separation and the “generic instability” of active gels

In this work we review some statistical physics techniques to coarse grain active matter systems, writing down a set of continuum fields which track the evolution of macroscopic fields such as density, momentum, etc. While the method can be applied in general, we will focus here on two simple and by now well-studied, active matter examples. First, we will consider motility-induced phase separation, the phenomenon by which a concentrated suspension of self-propelled particles spontaneously separates into a dense and a dilute phase. Second, we will review the so-called “generic instability” of active gels, which refers to the nonequilibrium phase transition between a quiescent and a spontaneously flowing phase in a concentrated suspension of rodlike active particles. For both these cases, we also outline recent developments in the literature.     

Lossless equalization of frequency combs

Frequency combs obtained by sinusoidal phase modulation of narrowband cw lasers are widely used in the field of optical communications. However, the resulting spectral envelope of the comb is not flat. We propose a general and efficient approach to achieve flat frequency combs with tunable bandwidth. The idea is based on a two-step process. First, efficient generation of a train with a temporal flat-top-pulse profile is required. Second, we use large parabolic phase modulation in every train period to map the temporal intensity shape into the spectral domain. In this way the resulting spectral envelope is flat, and the size is tunable with the chirping rate. Two different schemes are proposed and verified through numerical simulations.     

Performance evaluation of fiber Bragg grating sensors by digital holographic technique, strain gauge measurement

In view of the growing interest for non-destructive tests of materials, geodynamical monitoring and in general remote sensing, there is a great effort to bring practical optical sensors from research labs to industrial and environmental applications. In this paper, we employ digital holographic technique as an efficient tool for evaluating the strain measurement capability of fiber Bragg gratings (FBG). A cantilever beam has been employed as a test structure under loading test. The strain measurements results obtained by fiber-based sensors have been compared to those obtained by using full-field digital holographic technique and point-wise strain gauge sensors glued on the same cantilever beam. A simple theoretical model is also presented to interpret and compare the experimental results coming from different techniques.     

Self-referenced temporal phase reconstruction from intensity measurements using causality arguments in linear optical filters

We introduce and numerically demonstrate a simple and general concept for direct reconstruction of the temporal phase profile of an optical signal from temporal intensity measurements at the input and output of an arbitrary linear optical filter. The concept is based on exploiting the linearity and causality properties of any physical filter. Very few restrictions need to be imposed on the optical filter response to ensure an unambiguous phase reconstruction. The filter can be specifically designed to minimize the noise influence on the measurement process.     

用SPIDER法测量飞秒激光脉冲的光谱相位

介绍了用SPIDER测量光谱相位的实验装置和模拟计算飞秒激光特性参数的原理；提出了Ω 和τ等重要参数的确定方法.实验上用自建的SPIDER进行了掺钛蓝宝石飞秒振荡器输出脉 冲的相位测量，并以此为基础还原出了原输入脉冲的时域形式，模合计算所得的脉宽为107 fs，与利用二次相关法直接测量的结果十分一致. 关键词： 飞秒激光 SPIDER 光谱相位 光谱相干     

Scanning acoustic Doppler microscopy and scanning acoustic correlation microscopy

Acoustic microscopy with vector contrast at 100 MHz in a fluid with immersed particles is used to detect the flow profile in front of a microscopic orifice. The velocity profile concerning the component in axial direction of the focused beam is derived from the phase contrast. Possibilities to resolve the flow profile also for the components in normal direction with respect to the axis are demonstrated. The methods concerning measurement techniques and data evaluation for scanning acoustic Doppler microscopy are presented. For scanning acoustic correlation microscopy the time dependent phase and amplitude signals resulting from sound waves scattered by the immersed particles (aluminium flakes with a typical diameter of 10 microm) have been analysed by correlation procedures. From the obtained autocorrelation functions the velocity distribution can be derived. Both methods can be applied simultaneously. Data analysis is based on the information contained in the originally obtained images in vector contrast derived from temporal and spatial resolved analogue and digital processing of the acoustic signals.     

Diffusion pore imaging with generalized temporal gradient profiles

In porous material research, one main interest of nuclear magnetic resonance diffusion (NMR) experiments is the determination of the shape of pores. While it has been a longstanding question if this is in principle achievable, it has been shown recently that it is indeed possible to perform NMR-based diffusion pore imaging. In this work we present a generalization of these previous results. We show that the specific temporal gradient profiles that were used so far are not unique as more general temporal diffusion gradient profiles may be used. These temporal gradient profiles may consist of any number of “short” gradient pulses, which fulfil the short-gradient approximation. Additionally, “long” gradient pulses of small amplitude may be present, which can be used to fulfil the rephasing condition for the complete profile. Some exceptions exist. For example, classical q-space gradients consisting of two short gradient pulses of opposite sign cannot be used as the phase information is lost due to the temporal antisymmetry of this profile.     

DNA denaturation for a tangled chain

Summary We have considered a model of a lattice gas defined on a periodic tangled chain to study the DNA denaturation by a modified transfer matrix method. By using an iterative algorithm we have obtained numerically different kinds of melting curves for different configurations of the tangled chain and different types of interactions. In some special cases of configurations and interactions we have found the same melting curves, which we have obtained before studying some simple lattice gas models, using different techniques. This more generalized model and the new results could be useful for the experimental investigations.     

60 GHz Radio over Fiber Technology for Wireless Access by employing Narrow-Angle PSK Modulation

This paper investigates two key techniques used in 60 GHz Radio over Fiber (RoF) technology for wireless access, namely, the generation of 60 GHz signals and the distribution of 60 GHz local oscillator (LO). In the proposed model, a 60 GHz PSK signal is generated by a heterodyne of two subcarriers with narrow-angle PSK (NA-PSK) modulation, whose phase shift is equal to one half that of a normal PSK signal. Then we use a 60 GHz mixer to frequency mix two PSK signals in two different bands, 60 GHz and baseband. By doing this, the modulation information can be fully eliminated, resulting in a 60 GHz LO. In the wireless terminal, coherent demodulation is realized by a self-mixing of the transmitted 60 GHz PSK signal and LO. Thus no millimeter-wave (mm-wave) band oscillator is needed in the wireless terminal.     

Complete temporal pulse characterization based on phase reconstruction using optical ultrafast differentiation (PROUD)

A simple and general technique for recovering the phase profile of a given optical waveform from temporal intensity measurements is introduced and experimentally demonstrated. The proposed method involves the measurement of the temporal intensity profiles at the input and output of a linear optical time differentiator. The signal phase profile can be unambiguously recovered from these intensity measurements using a direct and noniterative algorithm. Given that ultrafast optical differentiators can be readily implemented in all-fiber or free-space platforms, the proposed technique could be applied over time waveforms with durations ranging from the subpicosecond to the nanosecond regime.     

Spectral and temporal holograms with nonlinear optics

In this Letter we show how encoding techniques for computer-generated holograms may be used to arbitrarily shape a nonlinearly generated spectrum and consequently the temporal shape by modulating the quadratic nonlinear coefficient. We give examples of a modulation pattern and a simple setup that can generate high-order Hermite-Gauss and Airy functions through difference-frequency generation from a transform-limited Gaussian pulse, under practical fabrication considerations.     

Measurement of subpicosecond optical waveforms using a resonator-based phase modulator

We propose a method for the measurement of periodic optical waveforms based on the use of an electrooptic phase modulator placed in an optical Fabry-Perot or ring resonator. Significant broadening of the modulation spectrum extends the recently developed method of periodic modulation for pulse characterization into femtosecond scales. We numerically demonstrate the characterization of a 300-fs optical pulse. We also present a technique based on the temporal fractional Talbot effect for restoration of the pulse phase profile. After fast linear processing, subpicosecond pulses will be observed on the screen of a real-time oscilloscope. This complete characterization of optical pulses is entirely linear and therefore highly sensitive and simple in implementation.