Measurement of the magnetic field-dependent refractive index of magnetic fluids in bulk

An optical alignment-free and highly accurate method is employed to measure the magnetic field-dependent refractive index of magnetic fluid(MF) in bulk.The measured refractive index decreases significantly with the increasing magnetic strength and then tends to saturate in the high intensity range.By applying a tunable magnetic field ranging between 0 and 1661 Oe,the maximum shift of the refractive index of MF in bulk iS found to be 0.0231.     

Optical sensing characteristics of the photonic crystal fiber filled with magnetic fluid

A novel photonic crystal fiber sensing theory filled with magnetic fluid is proposed based on the change of the MF refractive index under varied magnetic field. The magnetically induced tuning of the magnetic fluid filled PCF propagation properties were investigated by the full-vector finite element method with a perfectly matched layer. Theoretical calculations show that both the effective refractive index and the effective mode area increase vs. the increased magnetic field, and the PCF filled MF with larger d/Λ is more sensitive to magnetic field. When the wavelength λ = 1550 nm, the duty ratio d/Λ = 0.9, d/Λ = 0.6, the effective refractive indexes increase respectively from 1.598279 to 1.617572, from 1.61948 to 1.632484, and the effective mode areas increase respectively from 3.561115 μm² to 7.052360 μm², from 6.167494 μm² to 37.221998 μm² as the magnetic field changes from 25 Oe to 175 Oe. This scheme provides theoretical foundation to use magnetic field to control light in photonic crystal fiber and also offers a potential method for magnetic field sensing based on the TIR-PCF.     

Influence of switchable magnetic field on the modulation property of nanostructured magnetic fluids

Magnetic fluid is a kind of colloidal material with tunable microstructure and unique optical properties. The tunable magneto-optical modulation property of magnetic fluid under externally switchable magnetic field with various modulation periods is investigated theoretically and experimentally. The transitional modulation period (lower limit of the working frequency) between the square-like and oscillation-like modulation is achieved and found to be magnetic-field- and sample-concentration-dependent. The modulation mechanism is analyzed and ascribed to the dynamic microstructure of magnetic fluid under different modulation periods of external magnetic fields. The result of this work may be helpful for the pragmatic applications of magnetic fluid based on the square-like modulation.     

Design of thermo-optic tunable optical filter based on Si/Air DBR and polymer Fabry–Perot microcavity in SOI

An integrated tunable optical filter (TOF) based on thermo-optic effect in silicon on insulator (SOI) rib waveguide is designed and simulated. The device is comprised of two high refractivity contrast Si/Air stacks, functioning as high reflectivity of DBRs (distributed Bragg reflectors) and separating by a variable refractive index polymer Fabry–Perot (F–P) cavity. The designed device exhibits Q = 24077, FWHM = 0.065 nm and finesse = 566. Wavelength tuning is achieved through thermal modulation of refractive variation of the cavity. As the cavity polymer is heated, the refractive index of the cavity decreases. When the temperature of cavity polymer changes within 105, the central wavelength gets a continuous 35 nm shift from 1530 nm to 1565 nm, which can operate the whole C-band in the WDM (wavelength division multiplexing) networks. Moreover, by calculating, the tuning sensitivity is about 0.33 nm/°C. Owing to the compact size and excellent characteristics of integration, the proposed component has a promising utilization in spectroscopy and optical communication.     

Mach–Zehnder interferometer by utilizing phase modulation of transmitted light through magnetic fluid films possessing tunable refractive index

Due to its diverse applications in photonics, bio-sensors, mechanics, etc., Mach–Zehnder interferometer becomes one of important devices. Hence, lots of efforts have been paid to develop advanced Mach–Zehnder interferometers. In this work, we explore new-model Mach–Zehnder interferometer, in which one of arm is consisted of magnetic fluid films. By utilizing the tunable refractive index of magnetic fluid films under external magnetic fields, the traveling phase of a propagating light through the magnetic fluid film is changed. This could lead to a variation in the interfered intensity of the Mach–Zehnder interferometer when an external magnetic field is applied. The modulation in the interfered intensity by the external magnetic field is demonstrated experimentally, and the relevant physical origin is also discussed.     

Single-step writing of Bragg grating waveguides in fused silica with an externally modulated femtosecond fiber laser

For the first time to our knowledge, high-strength (>30 dB) first-order Bragg grating waveguides were fabricated in bulk fused silica glass in a single-scanning step by modulating a high-repetition-rate femtosecond fiber laser with an external acousto-optic modulator. The modulation induced a waveguide segmentation by delivering controlled bursts of laser pulses to define an array of partially overlapped refractive index voxels. With appropriate choice of modulation frequency and sample scanning speed, low loss waveguides could be formed at high writing speeds to yield sharp Bragg spectral resonances tunable over the 1300 to 1550 nm telecom band. Effects of acousto-optic modulation duty cycle on propagation loss and grating strength are characterized. This modulation method offers facile control and integration of multiwavelength Bragg grating devices to enhance overall functionality of optical circuits in three-dimensional geometries.     

光子晶体线缺陷波导中的折射率相位移调制增强效应

在传统的基于全内反射原理的低折射率比介质波导所构建的相位移调制型光学器件中,调制区域的长度通常在毫米到厘米量级.由于器件横向尺寸保持在微米量级,因此狭长结构成为了传统光波导器件的典型特征,这限制了光学器件集成度的提高,严重制约了集成光路的进一步发展.光子晶体的出现为高密集成光路的发展提供了一条新的途径.本文使用平面波展开方法计算了光子晶体线缺陷波导中的色散曲线.研究发现:在色散曲线下边缘处,材料折射率的一个微小变化可以引起传输常数的较大变化,如果工作频率点选择在带下边缘附近,则可以大幅度减小相位移调制型器件调制区域的长度.本文使用时域有限差分方法进一步验证这种增强效应,计算结果表明,对于0.46%的折射率变化,光子晶体线缺陷波导中的相位调制长度仅为均匀媒质中相位移调制长度的11.7%.通过以进一步研究,这种增强效应有望应用与高密度集成光路.     

Tunable magnetoben-optic modulation based on magnetically responsive nanostructured magnetic fluid

Magnetic fluid is a kind of functional composite material with nanosized structure and unique optical properties. The tunable magneto-optic modulation of magnetic fluid under external magnetic field, achieved by adjusting the polarization direction of incident light, is investigated theoretically and experimentally in this work. The corresponding modulation depth and response time are obtained. The accompanying mechanisms are clarified by using the theory of dichroism of magnetic fluid and the aggregation/disintegration processes of magnetic particles within magnetic fluid when the external magnetic field turns on/off.     

基于宽光谱应用的波长解复用多重体光栅的光写入特性

针对光通信中波长覆盖范围日益增宽的特点,从理论上研究了用于宽光谱范围波长解复用器件的多重体光栅的布喇格匹配和各波长对应光栅衍射效率的均匀性.计算及实验结果均表明,由于记录介质的色散效应将导致多重体全息光栅读出过程中明显的布喇格失配,并且在写入过程中获得的均匀折射率调制度的多重体光栅,在读出过程中各通信波长对应光栅的衍射效率将不均匀为此,提出了改进多重体光栅的光写入方法,给出了利用角度复用法写入透射型多重体光栅时解复用波长与写入角度的关系,以及写入体光栅过程中记录介质的折射率调制度修正公式.     

Numerical analysis of plasmon polarition refractive index fiber sensors with hollow core and a long period grating

The main principle of this design is based on the efficient energy transfer between the waveguide mode (WM) and the co-directional SPP provided by a properly designed fiber long period grating (LPG). This LPG is imprinted into a waveguide fiber layer of a specially designed hollow core optical fiber. The simulations are based on the finite element method (FEM) algorithm in electromagnetics and coupled mode theory for gratings. Compared to the previous proposed structure using a fiber Bragg grating (FBG), this novel kind of sensor can greatly enhance the refractive index sensitivity, e.g., from 5.93 nm/RIU (with FBG) to 817 nm/RIU (with LPG) at the sensing refractive index of 1.40. The other advantage is that the working conditions can be performed for the well-developed telecom wavelength windows 1500-1600 nm.     

Tunable plasmon-induced transparency based on asymmetric H-shaped graphene metamaterials

We propose and numerically demonstrate a tunable plasmon-induced transparency(PIT) phenomenon based on asymmetric H-shaped graphene metamaterials. The tunable PIT effect is realized through varying the applied polarization angles rather than changing the structure geometry. By simply adjusting the polarization angle, the transmission spectra can be controlled between the switch-on state and switch-off state. The physical mechanism of the induced transparency is revealed from magnetic dipole inductive coupling and phase coupling. Importantly, by varying the Fermi energy of the graphene or the refractive index of the substrate, the resonant position of the PIT can be dynamically controlled and the maximum modulation depths can reach up to 60.7%. The sensitivity(nm/RIU) of the graphene structure, which is the shift of resonance wavelength per unit change of refractive index, is 5619.56 nm/RIU. Moreover, we also extend our research to the x-axis symmetric H-shaped structure, and the tunable PIT transmission window can also be realized. The physical mechanism of the induced transparency is revealed from the electric dipole hybridization coupling. Our designed H-shaped graphene-based structures is a promising candidate for compact elements such as tunable sensors, switches and slow-light devices.     

Parametrically shielding electromagnetic fields by nonlinear metamaterials

An analytical theory is developed for parametric interactions in metamaterial multilayer structures with simultaneous nonlinear electronic and magnetic responses and with a near-zero refractive index. We demonstrate theoretically that electromagnetic fields of certain frequencies can be parametrically shielded by a nonlinear left-handed material slab, where the permittivity and permeability are both negative. The skin depth is tunable, and even in the absence of material absorption, can be much less than the wavelength of the electromagnetic field being shielded. This exotic behavior is a consequence of the intricate nonlinear response in the left-handed materials and vanishing optical refractive index at the pump frequency.     

Optimization of photonic bandgap fiber long period grating refractive-index sensors

Long period gratings in low-index contrast solid-core photonic bandgap fibers are a promising platform for fiber-based fluid refractive index sensing with very low detection limits. We provide a comprehensive investigation of the possibilities for refractive index sensing using that principle in a commercial photonic crystal fiber filled with a fluid: using an acoustic grating, we map out the cladding bands, and use this data to optimize a long period grating’s sensitivity. We then implement the optimized long period grating, again using an acoustic grating, and directly measure its sensitivity to refractive index. We demonstrate a sensitivity of 17,900 nm/RIU (6.94 nm/°C) which corresponds to a smallest detectable index change of the fluid of 8.4 × 10⁻⁶.     

Temperature effect on the tenability of an eight-channel demultiplexer

A new tunable wavelength division multiplexing (WDM) with a two-dimensional photonic crystal structure using (Multi-layer on InP substrate) is proposed. By tuning the temperature of the photonic crystal, the refractive index of the InP as well as the selected wavelengths can be changed. We show that the designed WDM has the ability to tune eight wavelengths by different values of temperature. The proposed filter has a cross section equal to 16.5 μm × 6.5 μm. The Results of the tenability has been done numerically by using the finite difference time domain (FDTD) method. We can use the proposed structure as temperature sensing device, and in many optical systems.     

Refractive index sensor using microfiber-based Mach-Zehnder interferometer

A simple and robust refractive index (RI) sensor based on a Mach-Zehnder interferometer has been demonstrated. A section of optical microfiber drawn from silica fiber is employed as the sensing arm. Because of the evanescent field, a slight change of the ambient RI will lead to the variation of the microfiber propagation constant, which will further change the optical length. In order to compensate the variation of the optical length difference, a tunable optical delay line (ODL) is inserted into the other arm. By measuring the delay of the ODL, the ambient RI can be simply demodulated. A high RI sensitivity of about 7159 μm/refractive index unit is achieved at microfiber diameter of 2.0 μm.     

石墨烯太赫兹波动态调制的研究进展

石墨烯是一种有着独特电学和光学性质的二维材料,近年来在太赫兹波动态调制的研究中有着广泛的应用。本文主要对基于石墨烯的太赫兹波动态调制器件进行了综述,分析了电调制、光调制和光电混合调制3种调制方法的原理和优缺点,介绍了近几年来将石墨烯应用于太赫兹波动态调制所取得的一系列科研成果,着重对不同器件的调制性能进行了对比,分析了优势和不足。石墨烯可调超材料为实现更快速、高效的太赫兹调制器件提供了新的思路。     

The measurement of sucrose concentration by two-tapered all-fiber Mach–Zehnder interferometer employing different coupling structures and manufacture processes

The sucrose concentration measurement and characteristics of light coupling taper structure on sensitivity with various fabrication processes of taper structure for all-fiber Mach–Zehnder interferometer (AFMZI) are presented. Using fusion splicer with electrical discharge, the standard single-mode fiber is employed to be fabricated as conical coupling/decoupling taper structure. The basic two fabrication processes are designed as single fusion-stretching (SFS), multiple fusions without stretching (MF). The third advanced process is composed of SFS and multiple fusions without stretching processes, and called multiple fusions with single stretching (MFSS). Various types of coupling/decoupling taper structures were fabricated based on the three kinds of fabrication processes. The effects of geometry shape including taper waist, taper angle, and sensing length on sensing sensitivity of AFMZIs are estimated. The modifications of fiber core and cladding induced by thermal effect affect the refractive index distributions and shapes of taper structure. The effects of refractive index changes of fiber core and cladding on sensing sensitivity are also discussed. The AFMZI was tested by measuring aqueous sucrose solution of refractive index unit (RIU) from 1.333 to 1.420 RIU. The optical spectrums are measured by a spectrometer. The spectrum dip shifts and sensing sensitivity was measured and calculated, respectively. As shown in results, sensing sensitivities of AFMZIs of taper structure fabricated by MFSS and multiple fusions without stretching processing are generally higher than SFS. The reasons could be aimed on materials modification through thermal effect on blurring fiber core-cladding interface and proper taper angle of taper structure. The more homogeneous refractive index distribution on fiber core-cladding interface, the more detecting light power decoupled through core-cladding interface to interact with exterior environment and enhance the sensing sensitivity. Similarly, an appropriate taper angle also provides a better coupling/decoupling performance. The optimal sensitivities relative to low refractive index, high refractive index, and full refractive index range are 87.62, 133.55, and 104.20 nm/RIU, respectively, and the corresponding sensing length are 30, 50, and 30 mm, respectively, with taper angle of 25° and taper waist of 40 μm.     

Study on magnetic fluid optical fiber devices for optical logic operations by characteristics of superparamagnetic nanoparticles and magnetic fluids

We propose two optical fiber-based schemes using two magnetic fluid optical fiber modulators in series or in parallel for optical logic signal processing and operation. Here, each magnetic fluid optical fiber modulator consists of a bare multimode fiber surrounded by magnetic fluid in which the refractive index is adjustable by applying external magnetic fields amplifying the input electrical signal to vary the transmission intensity of the optical fiber-based scheme. The physical mechanisms for the performances of the magnetic fluid optical fiber devices, such as the transmission loss related to Boolean number of the logic operation as well as the dynamic response, are studied by the characteristics of superparamagnetic nanoparticles and magnetic fluids. For example, in the dynamic response composed of the retarding and response sub-procedures except the response times of the actuation coil, the theoretical evaluation of the retarding time variation with cladding magnetic fluids length has good agreement with the experimental results.     

Tunable highly birefringent photonic bandgap fibers

A novel tunable highly birefringent photonic bandgap fiber (PBGF) is designed theoretically by filling its air holes with high-index material. The transmission band can be continuously tuned by changing the refractive index of the filling material. Accordingly, the tunable modal birefringence and polarization mode dispersion of the PBGFs are investigated by adjusting the refractive index of the filling material. Furthermore, we have also analyzed the effect of surface modes in the photonic bandgap on the characteristics of the tunable PBGFs. The simulation results show the feasibility of constructing birefringence-tunable photonic crystal fibers and related fiber devices in practical applications.     

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.