Measurement of small chirp-parameter for Mach-Zehnder-type optical modulator

A new measurement method of chirp-parameters is proposed for electro-optic (EO) intensity modulators with the Mach-Zehnder (MZ) waveguide interferometer. This method is suitably applied for the measurement of the small chirp with operation at a specific RF-frequency. To determine the chirp-parameter, optical spectrum components of the modulated light are observed with varying the relative optical phase difference between the two arms of the MZ waveguide interferometer. The chirp-parameter of 0.17, which is a value small enough for EO intensity modulation, was successfully measured by the experiment.     

Measurement of the linear electro-optic tensor of cubic boron nitride single crystals

The transverse electro-optic(EO)modulation system is built based on cubic boron nitride(cBN)single crystals unintentionally doped and synthesized at a high pressure and high temperature.The photoelectric output of the system includes two parts that can be measured respectively and the value of elements in the linear EO tensor of the cBN crystal can be obtained.This method does not need to measure the absolute light intensity.All of the surfaces of the tiny cBN crystals whose hardness is next to the hardest diamonds are{111}planes.The rectangular parallelepiped cBN samples are obtained by cleaving along{110}planes and subsequently grinding and polishing{112}planes of the tiny octahedral cBN flakes.Three identical non-zero elements of the EO tensor of the cBN crystal are measured via two sample configurations,and the measured results are very close,about 3.68 and 3.95 pm/V,respectively,which are larger than the linear EO coefficients of the general III-V compounds.     

Polarisation-insensitive strip-loaded waveguide for electro-optic modulators and switches

A polarisation-insensitive electro-optic (EO) waveguide consisting of a dye-doped TiO₂/SiO₂ slab and a SU-8 strip-loaded rib is designed and fabricated. By optimizing the refractive index and size of waveguide, a trade-off between polarisation-insensitive condition and large EO efficiency (optical field interaction with the EO material) is obtained. The average transmission loss of the waveguide is less than 2.0 dB/cm. A Mach-Zehnder (M-Z) interferometer intensity modulator based on this waveguide with excellent poling stability is fabricated and measured, exhibiting 7 V half-wave voltage with 1.8 cm EO interaction length and 2.7 cm total length. This strip-loaded structure is proved to be a valuable application in EO modulators and switches.     

Gaussian-beam-induced photorefractive birefringence and its application to radio-frequency signal excision

For Gaussian-beam-induced optical limiting based on photoconductive field shielding of electro-optic (EO) birefringence, power-limiting notch widths may be accurately determined by considering the power-limiting threshold for the photorefractive crystal where excess charge accumulates. With sufficient optical intensity the space-charge field completely screens the externally applied electric field, and only a small diffusion field remains. The upper limit of light intensity attenuation is the extinction ratio for the combination of polarizers and EO crystal.     

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.     

Measurement of the electro-optic coefficients: description and comparison of the experimental techniques

This paper presents a review of experimental methods used to determine electro-optic (EO) coefficients in bulk materials. The techniques are based on the relations describing the electro-optic effect in one-beam-ellipsometric or two-beam-interferometric optical arrangements. The methods are classified according to the nature (dc or ac) of the electric field applied onto the sample (static or dynamic methods) and with respect of the measured physical quantity (voltage, phase retardation, light beam intensity, etc.). Theoretical analysis and considerations of practical interest are presented in the article with particular attention paid to the resulting error in the determination of the EO coefficient associated with each method. A complete analysis of advantages and drawbacks of methods is finally given as a function of different experimental parameters such as the accuracy or sensitivity, the complexity of the implementation, the signal and the physical quantity used to derive the EO coefficient. Measurements of the coefficient r _c as obtained by means of the various methods on the same congruent crystal of LiNbO₃, are used to support the comparison between techniques. Received: 14 October 1999 / Revised version: 4 January 2000 / Published online: 23 February 2000     

Linear and nonlinear optical properties of Au/SiO2 nanocomposite prepared by P123

Mesoporous silica thin films loaded with gold nanoparticles are synthesized in the presence of EO20PO70EO20（P123）.Transmission electron microscope images show that the matrix of the nanocomposite is an ordered porous structure with a two-dimensional hexagonal phase.The wide-angle X-ray diffraction pattern implies that the nanocomposite contains gold crystals.These metallic nanoparticleembedded solid thin films show some linear and nonlinear optical properties due to their special structure and composition.Gold nanoparticles bring about surface plasmon resonance,and an absorption peak stemming from this effect has been observed.The linear absorption property is analyzed by a quantum mechanism,and the results show that it is influenced by the size and volume fraction of gold nanoparticles. Furthermore,it shows an obviously clear nonlinear optical property measured by the z-scan technique.The magnitude of the nonlinear refractive index of the nanocomposite is estimated to be about 10？ 10 cm 2 /W.     

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.     

Modified total intensity ratio methods for measuring cell gap of twisted nematic liquid crystal cells

This paper modifies the total intensity ratio method (TIRM) used to measure the cell gap of twisted nematic liquid crystal (TNLC) cells. Compared to the conventional TIRM in which a mechanical mechanism is used to physically rotate the polarizer, the modified TIRM methods presented in this study measure the total intensity ratio using a polarization rotation modulator. Two modified measurement methods are introduced. In the first, the saw-tooth signal applied to the electro-optic (EO) modulator is used as a reference signal in order to determine the polarization state of the measured signal. In the second method, a beam splitter and an additional quarter wave plate are added to the optical configuration. The quarter wave plate is adjusted such that a phase-matching condition is obtained between the reference and measured signals. The experimental results confirm that the modified TIRM approaches yield comparable accuracy to the conventional TIRM. Furthermore, in the proposed approaches, sufficient intensity signals to determine the cell gap can be obtained in just 2 s.     

Theoretical investigation of nonlinear properties of electrooptical chromophores

Organic electrooptical (EO) chromophores are now gaining more attention because the property of organic photorefrative (PR) materials could be controlled by doped EO chromophores. In this paper, nonlinear optical (NLO) properties of a new group of organic electrooptical chromophores, synthesized recently in our laboratory, were elucidated theoretically with the quantum chemical density functional theory (DFT) and the intermediate neglect of differential overlap Hamilton and the configuration interaction (INDO/CI), as well as semiemperical Austin Model 1 (AM1) methods. The electronic transition intensity, dipole moment and the second- order polarizability were obtained. The results show this group of chrormophores possess appropriate optical absorption property and good electrooptical property and optical activity. The second-order polarizabilities βare as large as the order of 10-29 to 10-28 ESU, indicating the promising applications in the future. The physical mechanism of NLO is discussed by means of molecular orbital and electronic charge distribution.     

Design and fabrication of electro-optic waveguides with self-assembled superlattice films

This paper presents a strategy for fabricating low loss waveguide devices based on high electro-optic (EO) coefficient self-assembled superlattice (SAS) films, a new sort of polymeric films grown on SiO₂ film and coated with other polymeric films to form multi-layer EO waveguides structure without electric field poling. Firstly, the optical propagation loss induced by the absorption of electrodes is simulated and optimized to obtain both the low optical loss and the low drive voltage. Then this paper gives the scanned electron microscopic (SEM) images of the fabricated devices, the simulated and experimental images of the single guided mode, and the measured optical propagation loss of the EO waveguide devices of 1.0 dB/cm. Finally, the very great agreement between the simulated and measured results of propagation loss of devices is observed.     

Wave-coupling theory of linear electro-optic effect for ultrashort laser pulses

A wave-coupling theory of linear electro-optic (EO) effect for ultrashort laser pulses has been developed. As one of its applications, the linear EO effect of ultrashort Gaussian pulses propagating in LiNbO₃ crystal is demonstrated, with the intensity lower than the damage threshold. It is found that the EO effect is sensitive to both the azimuth angle and the polar angle of wave vector of light. And the larger the applied electric field, the shorter the crystal used for optimization of EO coupling is. The numerical results show that, with the duration of input pulse changing from 150 fs to 5 fs, the durations of output pulses are broadened due to the group-velocity dispersion, as well as the first- and second-order refractive dispersion. In addition, when input pulse duration is larger than 20 fs, in contrast with zero chirp, the output durations can be slightly compressed if a positive chirp input pulse is used. The validities of slowly varying amplitude approximation and continuous wave approximation are also discussed. The influence of self-phase modulation and cross-phase modulation on the EO effect is also discussed briefly.     

Direct Fabrication Methods of Surface Relief Electro-Optic Gratings in Azo-Polymer Films

Two fabrication methods of surface relief electro-optic (EO) gratings in azo-polymer films are described: surface deformation by a continuous wave laser and laser ablation by a pulse laser. In the fabrication process, the periodic intensity modulation of laser beams is formed by an interference fringe or a phase mask. The surface relief EO grating is fabricated by poling the surface relief grating (SRG) after the SRG fabrication process. The linear dependence of an applied voltage in the modulation efficiency of the first-order diffraction beam is observed. The proposed methods are effective for highly-efficient surface relief EO grating fabrication.     

ZnS/CdSe量子点中线性和三阶非线性光吸收系数研究

在有效质量近似下,利用量子力学的密度矩阵理论,采用无限深势阱模型,从理论上研究了ZnS/CdSe柱型核壳结构量子点中线性和三阶非线性光吸收系数。导出了柱型量子点中线性和三阶非线性光学吸收系数的解析表达式,分析了该系统在不同条件下线性和三阶非线性光吸收系数与入射光频率之间的关系。改变系统的参数,该系统的光吸收系数呈规律性变化。计算结果表明:弛豫时间τ、入射光强I和壳半径R2对系统的吸收系数α有很大的影响,从而为实验上研究核壳结构量子点的非线性光学效应提供了必要的理论依据。     

Optical modulator using metal-oxide-semiconductor type Si ring resonator

Electric-field drive optical modulators using a Si ring resonator were fabricated on silicon-on-insulator (SOI) wafers. The fabricated resonators consisted of Si waveguides with width and thickness of 1.0 and 0.3 μm, respectively. In order to induce the linear electro-optic (EO) effect in the Si core layer, the strain was applied by covering the layer with Si₃N₄ film (0.26 μm thick) deposited by low pressure chemical vapor deposition (LPCVD) at 750 °C. The vertical electric-field was applied to the Si waveguide through the top and bottom cladding layers, and the optical output from the drop port at the resonance wavelength was measured. At a wavelength of 1501.6 nm, the optical modulation of 33% was obtained at 200V (electric-field at the silicon surface ∼3 × 10⁵ V/cm, nearly the breakdown field). The resonance wavelength was shifted toward the short wavelength side by applying both positive and negative voltages, this shift was explained by carrier concentration modulation. The linear EO effect in the Si core layer was not observed, presumably because the strain in the Si core layer was too small.     

Phase transitions of new antiferroelectric bent-core thiobenzoate

This article reports the results on phase transitions, mesomorphic and electro-optical (EO) properties of new bis [4-(4′-alkylphenylthiocarbonyl)phenyl] isophtalates from a homologous series of banana-shaped thioesters, referred to as IFOSn, when n?=?8, synthesized by us. The IFOS8 compound has two enantiotropic phases: B2 with antiferroelectric order and B6. The mesomorphic properties were investigated by means of three complementary methods: differential scanning calorimetry, polarized light optical microscopy and transmitted light intensity. EO measurements were also carried out.     

相位连续可调诱导下双艾里光束形成的可控方形光瓶(英文)

提出一种利用相位连续可调诱导产生的双艾里光束形成方形光瓶的方法。该方法首先利用二值化后的相位调制出双艾里光束,为了能够实现光瓶能量分布,一个可调控的线性因子被引入到相位调制函数中形成新改进的相位,该相位能够灵活地调节光瓶的大小。数值模拟结果表明高斯光束通过该改进相位调制,能够形成光瓶能量分布的光束。该方形光瓶光束可应用于光镊、原子捕获与操纵。     

Thermo-optic effects in electro-optic crystals used in an intensity-modulation system. – Application in LiTaO3

Linear electro-optic (EO) and thermo-optic (TO) effects are jointly considered on the basis of field-induced and temperature-affected perturbations of the operating point along the optical transmission curve of an EO-modulation system. In this framework, methods of EO measurements based on the frequency doubling appearing at the minimum-transmission point are formulated. Moreover, TO coefficients for various optical quantities, such as the refractive indices, n₀ and n_e, the spontaneous birefringence Δn⁽⁰⁾, the EO coefficient r, the half-wave voltage V_π, the optical phase retardation Γ, and the intensity modulation (IM) depth, m, are introduced and useful relationships between them are extracted. Also, applications of the above in the experimental field, with determinations of EO parameters and TO coefficients, are presented for the case of a congruent LiTaO₃ crystal. A comparison of the so-obtained experimental results with those derived from literature when a LiNbO₃ crystal is used demonstrates a number of advantages for the LiTaO₃-based system. PACS 78.20.Jq; 65.40.De     

Study of broadband THz time-domain spectroscopy at different relative humidity levels

Two detection techniques of broadband terahertz(THz)time-domain spectroscopy-THz air-biased coherent detection(THz-ABCD;from 0.3 to 14 THz)and electro-optical(EO)detection(from 0.3 to 7 THz)-are both performed at several different relative humidity levels.The THz power exponentially decays with the increase in relative humidity.The dynamic range of the main pulse in the time domain linearly decreases as the relative humidity increases from 0%to 40%,and linear fittings show that the slopes are-0.017 and-0.019 for THz-ABCD and EO detection,respectively.Because of the multiple reflections caused by the crystal in the common EO detection,THz-ABCD has better spectral resolution(17 GHz)than that of EO detection(170 GHz).The spectrum of water vapor absorption measured by THz-ABCD is also compared with that measured by the Fourier transform infrared spectroscopy(FTIR).     

荧光光谱用于光动力疗法中光敏剂光漂白特性研究

应用荧光光谱技术研究溶液中血卟啉单甲醚(HMME)的光漂白与光产物生成。以532 nm倍频Nd∶YAG激光器照射样品,功率密度为100 mW·cm-2,以光学多通道分析仪(OMA)采集荧光光谱。照光过程与荧光光谱采集同步进行。通过构建基本光谱与最小二乘拟合,由单条实测光谱中分解求得HMME荧光(613 nm)、光产物荧光(639 nm)及自体荧光的强度。HMME初始浓度不超过10 μg·mL-1时符合荧光-浓度线性函数关系。对照光过程的荧光光谱监测同时观察到HMME漂白、光产物生成与漂白,以及样品光学特性变化引起的自体荧光强度起伏。光产物漂白后的二次产物引起样品光学特性显著改变。所建立的荧光光谱探测系统与光谱分析方法可满足光敏剂漂白特性体外研究的需要,并为光动力治疗的剂量学在体监测提供有效研究方法。