Analysis of non-linear refractive index influences on four-wave mixing conversion efficiency in semiconductor optical amplifiers

In this paper, we have analyzed the influences of non-linear refractive index on the four-wave mixing (FWM) characteristics in semiconductor optical amplifiers (SOAs). It has been shown that the generated FWM signal characteristics can be modified due to the variation of non-linear refractive index of the SOA's medium. The wave propagation in the SOA has been modeled using the nonlinear propagation equation taking into account gain spectrum dynamics, gain saturation, which depends on carrier depletion, carrier heating, spectral hole-burning, group velocity dispersion, self-phase modulation and two photon absorption. Simulation of optical wave evolution in the SOA has been carried out using the finite-difference beam propagation method (FD-BPM) both in time and spectral domains. Our simulation results confirm that higher FWM conversion efficiency and lower time bandwidth product are achieved for higher absolute values of non-linear refractive index. Moreover, non-linear refractive index is more efficacious for high power propagated waves in SOAs. Finally, we have studied the modification of waveguide refractive index due to the propagation of optical pulses. We have also shown that when |n₂|=1 cm²/TW, refractive index variation is in the order of 10^?4 to 10^?7 for high and low power input pulses, respectively.     

Fluorescence Modulation and Photochromism in Azobismaleimide Derivatives

A fluorophore-photochrome system incorporating an aryleneimine type fluorophore and an azobismaleimide photochrome was developed and the photochemical properties of this system were investigated. The photoisomerization of trans-azoaromatic chromophore leads to the increase of the fluorescence intensity of fluorophore. The cis azobismaleimide isomers revert photochemically to the trans form and the emission intensity decreases. The fluorescence intensity of the imine fluorophore can be modulated under irradiation with UV and visible (436 nm) light due to reversible trans-cis-trans photoisomerization of azobismaleimide partner. The photoisomerization kinetics was obeyed a first-order relationship with a rate constant of 1.95?×?10^?2 s^?1 for azobismaleimide/imine fluorophore system and for polyazothioetherimide/imine derivative the kinetics was described by a biexponential equation.     

Proposal for nonlinear refractive index measurement using spectral ratio in modulated OFRR dynamics

A novel method for measuring the nonlinear refractive index of an optical fiber using a spectral ratio between the modulation frequency and a harmonic component in a modulated optical fiber ring resonator (OFRR) is proposed. The spectral ratio between the modulation frequency and the 2nd-harmonics generated by phase-modulation through the OFRR is increased with increasing the input light power and has peaks above 5 W input power, however, the peaks was shifted to the lower input power below 1 W by averaging taken into account of the phase distribution. A experimental setup consisted of an OFRR system and an Ar-laser as a pump light source was used to determine the nonlinear refractive index of an optical fiber. In the experimental results, the peaks of the spectral ratio as a function of the input power was found out at 0.8 W and 0.45 W of the input power corresponding to the input source line at 488.0 nm and 514.5 nm, respectively. The profile was similar to that obtained by the simulation and the nonlinear refractive index of a optical fiber was determined as 1.0 × 10⁻²² m²/V² by a relationship between the input power giving the peak and the nonlinear refractive index.     

Photorefractive polymer composites for the IR region based on carbon nanotubes

The photorefractive properties of polymer composites based on aromatic polyimide and single-wall carbon nanotubes are studied using radiation at a wavelength of 1064 nm. It is found that the nanotubes possess photoelectric sensitivity in this spectral region and that the kinetic photorefractive characteristics of the polymer composites are entirely determined by the photogeneration and charge transport characteristics of the layers. The two-beam gain coefficient of the signal beam measured for a composite consisting of aromatic polyimide and 0.25 wt % of single-wall carbon nanotubes in a constant electric field E₀ = 79 V/μm is equal to 84 cm^?1 and exceeds the optical absorption coefficient by 59 cm^?1. The refractive index modulation is equal to Δn = 0.004 at E₀ = 54 V/μm.     

Miniature intra-cavity modulated solid state lasers and their use in optical communication systems

Based on a model for the calculation of the variance of relaxation noise we show that it is possible in principle to modulate aminiature solid state laser at data rates (random) of 50 Mbit s^?1 and higher by modulating the cavity loss, while keeping the relaxation oscillation noise small enough to permit a 10^?9 error rate. Because of the small transit time through the miniature cavity a large modulation index (M ～ 45) can be achieved, and although the laser power output suffers this can be compensated for in data transmission by the narrow output pulse shape that characterizes our intra-cavity modulation scheme. We discuss the design of a proposed miniature laser based on an acousto-optic intracavity modulator and compare its predicted performance with that of a modulated laser source based on an external modulator. Its major advantages are a somewhat lower electrical operating power and the simple construction which is based on an all-planar technology.     

Measurement of refractive index variation by differential surface plasmon resonance technique

A differential detection technique combined with a surface plasmon resonance technique is proposed for refractive index variation measurement. The experimental results demonstrate that our system can attain a resolution of 1.2 × 10^?5 RIU (refractive index unit). In addition, a method for two-dimensional detection of the refractive index variation is demonstrated. The system is free from the influence of optical source fluctuation, and can improve the measurement sensitivity.     

An Analysis of Parameters of an Oscillatory Chemical Reaction by Holography Methods Involving a Transient Reference Wave

The results of an experimental study of an oscillatory chemical reaction (the Belousov-Zhabotinskii reaction) are reported. The measurements were performed for both the reaction proceeding in the regime of synchronization in the volume and the reaction proceeding without synchronization when the oscillations are less pronounced. Radiation passed through the sample and detected by the holography method with a transient reference wave was used in the analysis. This made it possible to obtain data on the time dependence of amplitudes and phases of the transmitted wave. The signal spectrum was reconstructed from the hologram. In this case, a spectrum rich in features was observed even when there was no synchronization. The shift of the zero-order component in the spectrum of oscillatory variations in the refractive index was measured and was found to be constant in the course of the reaction. The oscillation frequency decreased during the reaction. Special measurements with the use of interference were performed at the stage when the hologram was reconstructed and the signal-spectrum pattern was obtained; the time dependence of the signal in the course of the reaction was evaluated. Nonperiodic and periodic phase variations in the transmitted wave were studied; the parameters characterizing the variation of refractive index in the course of the reaction were determined. Variations in the refractive index of no less than 5 · 10^?6 were measured to an accuracy of no lower than 0.2·10⁶ and the rate of the refractive-index variation was measured to an accuracy of no lower than 10^?8 s^?1.     

Simultaneous pulse carving misalignment and pi-phase-shift monitoring of a return-to-zero differential phase-shift keying system using a coherent hybrid

A technique for monitoring both the pulse carving misalignment and pi phase shift in a return-to-zero (RZ) differential phase-shift keying (DPSK) system based on a phase modulator is proposed and demonstrated. By monitoring the mean-power variation of a hybrid local oscillation lightwave with the modulated signal lightwave without any deliberated control, the technique can provide the information on pulse carving misalignment and phase modulation index simultaneously, as the power variation reflects the asymmetric distribution of RZ-DPSK in a signal constellation diagram. It is characterized as transparent to signal bit rate, central wavelength, and absolute power level. Both the simulation and experiment results show that more than +/- 15% bit duration misalignment and +/- 5% phase shift deviated from pi can be detected.     

Choice of optimum modulation frequency when determining the scattering index of a medium by the phasemeter method

The results of calculations on the variation of the depth of modulation and the phase shift of a signal propagating through a scattering medium in the “backward” direction as a function of the scattering index and the thickness of the scattering layer are presented. The flux is modulated sinusoidally in the frequency range from 1 to 100 MHz.     

Inhibition of light tunneling in chirped and longitudinally modulated semi-infinite waveguide arrays

The inhibition of light tunneling in chirped and longitudinally modulated semi-infinite waveguide arrays where the refractive index is linearly modulated in the transverse direction and harmonically modulated along the light propagation direction is considered. We report on the effect of the refractive index transverse amplitude modulation rate, longitudinal modulation frequency and depth on tunneling inhibition in both linear and nonlinear regimes. We show that in the linear regime an optimal value for the transverse amplitude modulation rate of refractive index exists and can determine the optimal longitudinal modulation frequency or depth leading to a maximum of distance-averaged power fraction. In the nonlinear regime the tunneling inhibition dynamics is affected dramatically by the transverse amplitude modulation rate and the associated electric field amplitude of the input beam.     

Characterization of ROF signal based on cascaded optical carrier suppression modulation technique

The paper experimentally demonstrated the optical frequency quadrupling microwave signal generation, a 4 GHz radio frequency (RF) signals up-conversion to 16 GHz in a radio over fiber (ROF) link, using twice optical carrier suppression modulation. The RF signal was mixed with 1.25 Gbps NRZ-OOK data firstly and then modulated by two cascaded single-electrode optical intensity modulators. The obtained 1.25 Gbps/16 GHz ROF signal was transmitted and characterized in the optical fiber link. At BER of 10^?9, low power penalty of 1.0 and 1.4 dB were obtained over a fiber link with a transmission distance of 25 and 50 km.     

利用散射系数变化测量血糖浓度的研究

论述了血糖浓度变化与溶液的散射系数的关系。在浑浊介质中散射特性取决于散射粒子和溶剂的折射率的相对大小,在血液中葡萄糖浓度的改变会引起血液折射率的变化,从而使血液的散射系数发生改变,从理论上给出了血糖浓度变化时计算散射系数变化的公式,得出了散射系数和血糖浓度成正比的结论。进一步讨论了满足临床应用目标下,散射系数的相对变化的测量精度要达到4.8×10-4,而对于中国人的血液绝对测量精度要达到6.82×10-3mm-1。用双积分球实验验证了理论计算的正确性,使用牛奶作为浑浊介质测量其散射系数与糖浓度的关系,在不同波长下散射系数都随葡萄糖浓度改变而改变,且线性度可以达到0.95以上。由此可以得出结论,血糖浓度可以通过测量散射系数来得到。     

一种提高迈克尔逊干涉系统抗干扰能力的新方法

为了提高迈克尔逊干涉系统的抗干扰能力,取代传统的动镜扫描结构,设计了基于电光调制晶体折射率实现光程扫描的干涉系统。通过加载在可变折射率晶体上的调制电信号,使晶体折射率产生周期性变化,从而在原有系统光路中调制折射率改变光程差。通过理论计算获得了电光调制过程中系统可以产生的最大光程差,并仿真分析了晶体厚度及晶体衍射效率对调制过程的影响。经仿真分析可知,随着调制电压范围的增大,可获得的光程变化范围也增大,从而系统光谱分辨能力也相应增大。同时,在调制过程中设置调制范围使衍射损失的能量小于总能量的10%,从而保证较好信噪比。实验结果显示,随着调制电压的变化,干涉条纹产生周期性移动,但超过一定范围时会产生非线性误差。通过修正算法后系统光谱分辨率可达7.2 cm-1。在无抗震实验平台的条件下,传统干涉系统的相对误差超过20%,而本系统的相对误差低于3%,证明了系统采用静态电光调制后抗干扰能力显著增强。     

Annealed Silver-Island Films for Applications in Metal-Enhanced Fluorescence: Interpretation in Terms of Radiating Plasmons

The effects of thermally annealed silver island films have been studied with regard to their potential applicability in applications of metal-enhanced fluorescence, an emerging tool in nano-biotechnology. Silver island films were thermally annealed between 75 and 250^∘C for several hours. As a function of both time and annealing temperature, the surface plasmon band at ≈420 nm both diminished and was blue shifted. These changes in plasmon resonance have been characterized using both absorption measurements, as well as topographically using Atomic Force Microscopy. Subsequently, the net changes in plasmon absorption are interpreted as the silver island films becoming spherical and growing in height, as well as an increased spacing between the particles. Interestingly, when the annealed surfaces are coated with a fluorescein-labeled protein, significant enhancements in fluorescence are osbserved, scaling with annealing temperature and time. These observations strongly support our recent hypothesis that the extent of metal-enhanced fluorescence is due to the ability of surface plasmons to radiate coupled fluorophore fluorescence. Given that the extinction spectrum of the silvered films is comprised of both an absorption and scattering component, and that these components are proportional to the diameter cubed and to the sixth power, respectively, then larger structures are expected to have a greater scattering contribution to their extinction spectrum and, therefore, more efficiently radiate coupled fluorophore emission. Subsequently, we have been able to correlate our increases in fluorescence emission with an increased particle size, providing strong experiment evidence for our recently reported metal-enhanced fluorescence, facilitated by radiating plasmons hypothesis.     

A Streak Camera System for Time Resolved Fluorimetry

Presently, there are several techniques for measurement of fluorescence lifetimes of organic molecules. These techniques, reviewed by Ware,¹ can be divided into two basic groups, those based upon pulsed sources and those based on modulated sources and phase shift measurement. In the pulsed methods, repetitive, short pulse width, intense excitation pulses excite the fluorophor and the fluorescence decay is measured; the source temporal response must be deconvoluced from the fluorescence decay in order to evaluate the fluorescence signal and lifetime. Typical sources of excitation include nanosecond flashlamps and more recently nanosecond N₂-laser (with or without a dye laser) and mode-locked lasers with picosecond pulse widths^2-6. The modulated source phase shift methods,¹ involve sinusoidal excitation of the fluorophor and measurement of the phase shift between the modulated excitation source and the modulated flourescence. Because modulation frequencies are limited to approximately 20 MHz, fluorescence lifetimes are limited to ～.1 ns and above. In addition, in the phase shift methods, only “one point” lifetimes are obtained, i.e., the entire fluorescence decay curve is not obtained.     

Microstructured fiber based plasmonic index sensor with optimized accuracy and calibration relation in large dynamic range

A novel surface plasmon resonance photonic sensor is proposed using an index-guided microstructured fiber with an analyte channel introduced into the central core. Compared with the previous designs of porous fiber core, variation of the signal amplitude with exterior refractive index is demonstrated to be contrary to that of the sensitivity in the proposed fiber, contributing to optimized detecting accuracy over a large refractive index range of 1.33 to 1.42. By carefully choosing the central channel size, the analyte-filled core can achieve narrower resonance spectral width and higher signal to noise ratio (SNR) than the air-filled core. Sensor responses are also studied in this paper based on two spectral interrogation methods, including monitoring single resonance shift and measuring change in the resonance separation. For both methods, response linearity has been improved considerably through partially filling the core with analyte. The maximal sensitivity reaches 10^− 6 refractive index unit (RIU). The linear sensing performance along with the broad measurement range is very promising in the application of the proposed sensor as sensitive refractometer.     

Chaos in Fiber-Optic Stimulated Brillouin Scattering without External Feedback in Large Nonlinear Refractive Index Regime

Stokes dynamics in stimulated Brillouin scattering generated in optical fibers is analyzed over a wide range of nonlinear refractive indices under a no feedback condition. The Stokes fluctuation becomes complicated as the nonlinear refractive index increases. Chaotic behavior appears without external feedback at a large nonlinear refractive index of 1×10^-19 m²/V² with pump power larger than 0.3 W.     

Measuring magnetic fields in plasmas by means of light scattering

The theory of light scattering in plasmas containing a magnetic field yields the special case of modulated scattering spectra. The modulation frequency is governed by the field in the plasma and is equal to the electron cyclotron frequency. In this investigation magnetic fields in a plasma were determined by a laser scattering experiment. The experimental data were: electron densityn _e=10¹⁶cm^?3, electron temperatureT _e=3.2 eV, scattering angle θ=90 °, scattering parameter α=0.6, and a maximum field in the plasma of 125 kG. The spectrum measured at the maximum magnetic field was modulated with 3.6 × 10¹¹ Hz. In scattering experiments with a field reduced by about 20% the observed modulation frequency was 2.8 × 10¹¹ Hz. A thermal spectrum with a smooth profile was found when no field was present in the plasma. Applying the theory of cyclotron modulated spectra one obtains from the scattering experiment magnetic fields of 128, 100, and 0 kG. Within the experimental accuracy these values agree well with the fields determined by means of magnetic probes. Other possible interpretations of the measured deviations from thermal spectra (modulation with the plasma frequency or additional cold electron components in the plasma) are discussed, but they afford no explanation. This experiment has domonstrated that magnetic fields in plasmas can be measured locally and almost without disturbance by means of light scattering.     

Nonlinear diffraction in gratings based on polymer–dispersed TiO 2 nanoparticles

In this letter we report a simple technique to produce volume holographic gratings based on photopolymerizable composites containing TiO₂ nanoparticles. Diffraction gratings with high refractive index modulation amplitude (up to 1.25 × 10⁻²) have been formed due to the periodic distribution of high refractive index nanoparticles in a low refractive index polymer matrix. The diffraction efficiency increases strongly on increasing the nanoparticle concentration. Taking the mixture with 10 wt.% TiO₂ nanoparticles, gratings with high diffraction efficiency, low level of scattering and high transparency in the visible-wavelength range have been obtained. This will ultimately lead to different applications of diffractive optical elements based on nanocomposites. The dependence of the gratings’ diffraction efficiency on the intensity of probe laser pulses at 1064 nm has been explored. It is shown that the nonlinear response of the gratings is attributed mainly to the nonlinear properties of the TiO₂ nanoparticles embedded in the polymer matrix. The mechanism of the grating formation and the reasons for the nonlinear behavior of the diffraction efficiency are discussed.     

Nonlinear refractive index measurements and self-action effects in Roselle-Hibiscus Sabdariffa solutions

We report the observation of self-action phenomena, such as self-focusing, self-defocusing, self-phase modulation and beam fanning in Roselle-Hibiscus Sabdariffa solutions. This material is found to be a new type of natural nonlinear media, and the nonlinear reflective index coefficient has been determined using a Z-scan technique and by measuring the critical power for the self-trapping effect. Z-scan measurements show that this material has a large negative nonlinear refractive index, n ₂ = 1 × 10^?4 esu. A comparison between the experimental n ₂ values and the calculated thermal value for n ₂ suggests that the major contribution to nonlinear response is of thermal origin.