对称结构光纤光栅耦合器及其应用

光纤光栅耦合器(FGC)具有光纤光栅良好的波长选择特性和光纤耦合器的多端口特点，易于实现全光纤的光波分插复用。光纤光栅耦合器主要有4种结构：基于M Z干涉仪的分离型、基于100％耦合器的非对称型、基于0耦合器的非对称型和基于100％耦合器的对称型。着重介绍对称结构光纤光栅耦合器的结构、工作原理和研究现状。提出了测试方案，并探讨了这种器件在大规模波分复用光纤传感器阵列中的应用。     

Soliton switching and propagation in two-core nonlinear fiber coupler with high order coupling coefficient

We use the variational approach (VA) and the split-step Fourier transforms (SSFT) to study the transmission and switching characteristics inside the fiber nonlinear directional coupler (NDLC). The results, based on the VA, indicate that the second-order coupling coefficient dispersion and initiative chirp all reduce the coupling length, and the second-order coupling coefficient dispersion makes the switching characteristics become sharper and threshold power become bigger under the case of not having initiative chirp. The outcomes, based on the SSFT, indicate that the first order intermodal dispersion coefficient make optical pulses splitting in the propagation of fundamental solitons, and the second-order coupling coefficient dispersion reduces the coupling length, sharpens the switching characteristics and increases the switching threshold power, the results agrees well with those from the VA.     

All optical self signal processing using chalcogenide nonlinear fiber Bragg grating

In this study, we simulate numerically quasi pulse propagation in a uniform nonlinear fiber Bragg grating (FBG) made of chalcogenide glasses. Because of bistability and nonlinearity behavior of FBG, the shapes of output pulse vary abruptly and strongly according to the input peak intensity. We take Gaussian pulse as input and produce saw-tooth wave and square wave in the output by suitable input amplitude and FBG length also we could reach pulse chopping and pulse compression. These all optical signal processing achieved for a few length, i.e. 6.6 mm and few intensity, i.e. 35 W/m² because of high nonlinearity of chalcogenide glasses.     

Vector soliton switching in a fiber nonlinear directional coupler

This paper reports a detailed numerical study of soliton switching in a high as well as low birefringent nonlinear coupler. It is shown that by controlling the polarization angle one can have nearly 100% transmission with excellent switching characteristics. It is shown that soliton remains stable during its propagation inside the coupler. However it is observed that high birefringent coupler exhibits relatively better soliton stability. We show that the coupler could be used as a soliton switch even at an input peak power less than the critical power, the power at which 50-50 power sharing takes place between the two cores, just by a judicious choice of the polarization angle.     

High-contrast all-optical switching with Pt:ethynyl complex

We have theoretically analyzed all-optical switching in Pt:ethynyl complex based on nonlinear excited-state absorption. A detailed analysis for Pt:ethynyl complex has been presented based on rate equation approach. It is shown that a pulsed pump laser beam at 355 nm switches the transmission of a cw probe laser beam at 633 nm through a Pt:ethynyl sample. The effect of various parameters, such as pump pulse width, peak pumping intensity, normalized parameter , transition times of S₁→S₀ and S₁→T₁ states and lifetime of triplet state, on switching characteristics has been analyzed in detail. It has been shown that the probe beam can be completely switched off (i.e. 100% modulation) by a pulsed pump laser beam at 50 kW/cm². These results have been used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses.     

Design and simulation of a switch based on nonlinear directional coupler

In this paper, we have analyzed a nonlinear photonic crystal directional coupler by the finite difference time domain method. We have tried to increase the coupling efficiency and also reduce the coupling length in linear and nonlinear states in this device. In this coupler, refractive index of the rods of the central row is tuned by input signal power due to nonlinear Kerr effect; therefore, input signal beam can be controlled so as to be exchanged between two output ports. Physical length is chosen to be 24a so as to have the highest output power ratio and also the smallest amount of power required for nonlinear performance.     

MZ-MMI-based all-optical switch using nonlinear coupled waveguides

We propose an all-optical switch (AOS) based on Mach-Zehnder (MZ) and Multi-mode interference (MMI) using nonlinear closely coupled waveguides. The device operates by switching between two states of coupled waveguides. In first state the refractive index of waveguides are same and light field will completely couple to nonlinear waveguide in half length of coupler and will back in the second half. We will have π phase difference in this procedure and the input field will appear in Bar-state output. In the second state the refractive index of nonlinear waveguide increase with high intensity control field. In this case, we have lower coupling and change in phase. But, we choose the best refractive index change to obtain the phase change of multiple of 2π necessary for Cross-state in output. The beam propagation method is used to simulate the device operation.     

Experimental demonstration of single-mode fiber coupling using adaptive fiber coupler

Coupling plane wave into a single-mode fiber(SMF) with high and steady coupling efficiency is crucial for fiberbased free-space laser systems, where random angular jitters are the main influencing factors of fiber coupling. In this paper, we verified a new adaptive-optic device named adaptive fiber coupler(AFC) which could compensate angular jitters and improve the SMF coupling efficiency in some degree. Experiments of SMF coupling under the angular jitter situation using AFC have been achieved. Stochastic parallel gradient descent(SPGD) algorithm is employed as the control strategy, of which the iteration rate is 625 Hz. In closed loop, the coupling efficiency keeps above 65% when angular errors are below 80 μrad. The compensation bandwidth is 35 Hz at sine-jitter of 15 μrad amplitude with average coupling efficiency of above 60%. Also, experiments with simulated turbulence have been studied. The average coupling efficiency increases from 31.97% in open loop to 61.33% in closed loop, and mean square error(MSE) of coupling efficiency drops from 7.43% to 1.75%.     

Remote all-optical nanosecond gain switching of an erbium-doped fiber amplifier

All-optical gain switching of an erbium-doped fiber amplifier located up to 50 km away from the signal and control lasers is demonstrated. The amplifier is deactivated by optical control pulses within the amplifier bandwidth that strongly saturate the gain at the signal Wavelength. Fall times are approximately equal to the control pulse duration of 21 ns. From 25 km, a maximum extinction ratio of 16 dB is demonstrated with the signal at 1527 nm and the control pulse at 1554 nm. The dynamic range for remote switching is limited by a stimulated Raman scattering of the control pulses.     

UV-written long-period waveguide grating coupler for broadband add/drop multiplexing

We demonstrate a UV-written polymer long-period waveguide grating (LPWG) coupler, which offers a bandwidth of ∼20 nm, a maximum coupling efficiency of ∼80% and ∼60% for the TE and TM polarizations, respectively, and a wavelength-tuning range over the (S + C + L)-band (∼140 nm) with a temperature control of ∼25 °C. The LPWG coupler has the potential to be developed into a practical broadband add/drop multiplexer for coarse wavelength-division-multiplexing applications.     

两平行长周期光栅耦合器型上下话路滤波器的研究

以两平行长周期光栅耦合器的通用模型为依据，全面分析了两光栅完全对准和存在纵向偏离时器件性能的稳定性. 在此基础上深入研究光栅调制强度、环境折射率以及光纤包层厚度等因素的影响，确定出实现100%上下话路滤波效率的条件. 初步的实验结果表明，不需要对光纤包层进行处理，利用两平行长周期光栅耦合器完全可以实现有效的上下话路.这为进一步简化制作工艺提供了依据. 关键词： 长周期光纤光栅 光纤耦合器 上下话路滤波器     

Femtosecond pulse switching in a fiber coupler with third order dispersion and self-steepening effects

A numerical study of femtosecond pulse propagation and switching in a dual-core nonlinear directional coupler with the consideration of third order dispersion and self-steepening effects is reported. The Split Step Fourier Method (SSFM) is used to investigate the switching characteristics of nonlinear directional couplers. It is observed that the energy transfer from core to core is not affected by changing the input pulse shapes except super-Gaussian. While the normalized coupling co-efficient and the input peak power dominate the coupling characteristics, the effects of third order dispersion (TOD) and self-steepening (SS) are also reported.     

Bistable soliton states and switching in doubly inhomogeneously doped fiber couplers

Switching between the bistable soliton states in a doubly and inhomogeneously doped fiber system is studied numerically. Both the cases of lossless as well as lossy couplers are considered. It is shown that both up-switching (from the low state to the high state) and down-switching (from the high state to the low state) of solitons between bistable states are realizable, if the amplification of the input soliton for up-switching and the extraction of energy from it for down-switching are suitably adjusted.     

All optical method of developing OR and NAND logic system based on nonlinear optical fiber couplers

Logic gates are the fundamental building blocks in any digital data processing system. Several all optical logic operations are already proposed by scientists and technologists. Here the authors proposed a new method of all optical logic operation based on nonlinear directional coupler. Effective optical switching is achieved by modifying the coupling length between the coupler waveguides by means of an optical signal.     

A comparative numerical study of soliton switching in a nonlinear directional coupler with saturating nonlinearity

A detailed numerical study of soliton switching in a nonlinear directional coupler with saturating nonlinearity is carried out. All commonly used models have been studied. Relevant physical characteristics for switching have been determined, compared and discussed.     

Effects of third-order dispersion on soliton switching in fiber nonlinear directional couplers

The split-step Fourier method is used to study the energy switching characteristics of fiber nonlinear directional couplers with the third-order dispersion. The effects of the third-order dispersion increases with the third-order dispersion coefficient and input power and result in pulse shift and energy decreases. Adding high-order nonlinear can partly overcomes these effects.     

Multimode optical fiber demultiplexer-star coupler using a single gradient-index-rod lens and a multi-facet grating

An optical fiber multi-function device consisting of a single gradient-index-rod lens and a multi-facet blazed reflection grating is proposed to simultaneously realize functions of wavelength demultiplexing and optical signal distribution in a multimode optical fiber transmission system. We analyzed the demultiplexing characteristics and the tolerance of optical components using the ray trace method. This device can realize not only low loss optical signal distribution but also offers improved demultiplexing characteristics in comparison with the previously proposed demultiplexer-multiposition switch. The following characteristics are expected from the design using commercially available optical components: a working band of 0.64–0.88 μm, channel separation of 34–36 nm, 3 dB bandwidth of 27–28 nm, channel cross-talk of less than - 40 dB and minimum excess insertion loss of 0.9–2.1 dB.     

Analysis and experiment of all-optical time-interleaved multi-channel regeneration based on higher-order four-wave mixing in a fiber

Simultaneous all-optical multi-channel regeneration based on second-order four-wave mixing (FWM) in a single highly nonlinear fiber (HNLF) is studied. Interchannel crosstalk, especially cross gain saturation and generation of interchannel FWM, is avoided by properly time-interleaved channels. Preliminary experiment of 2 ch × 10 Gbit/s operation shows simultaneous noise reduction of the two channels and good agreement with numerical analysis. Numerical investigation on 2 ch × 40 Gbit/s operation using the same parameters as the experiment shows that the regeneration can improve qualities of both channels. For more number of channels and higher speed of operation, several parameters need to be adjusted to avoid interchannel crosstalks. Time synchronization techniques for the input channels are also discussed.     

Static and dynamic analysis of an all-optical inverter based on a Vertical Cavity Semiconductor Optical Amplifier (VCSOA)

We report the static and dynamic properties of an all-optical inverter based on an 850 nm Vertical Cavity Semiconductor Optical Amplifier (VCSOA). The inverter exhibits low switching power requirements (~ 15 μW), large on/off contrast ratio (> 11 dB), and high speed operation (~ 1.4 GHz). Large and small signal measurements show that the speed of operation and the on/off contrast ratio improve with increased bias current. This holds important prospects for the development of VCSOA-inverters for high-speed, low-power optical logic applications. Finally, a theoretical model of the VCSOA-inverter has been employed giving good agreement with experiments.     

Thermal characteristics analysis of an IGBT using a fiber Bragg grating

This paper proposes a new method to develop a thermal model of an insulated gate bipolar transistor (IGBT) employing an optical fiber sensor mounted on the chip structure. Some features of the sensor such as electromagnetic immunity, small size and fast response time, allow the identification of temperature changes generated by the energy loss during device operation through direct measurement. In fact, this measurement method is considered impossible with conventional sensors. The online monitoring of the junction temperature enables identify the thermal characteristics of the IGBT. The results are used to develop an accurate model to simulate the heat generated during the device conduction and switching processes. The model showed a difference of only 0.3% between the measured and simulated results, besides allowing evaluate separately the heat generated by each turn-ON/OFF process.