Analysis of models for extracellular fiber stimulation

This paper presents the mathematical basis for analysis as well as for the computer simulation of the stimulus/response characteristics of nerve or muscle fibers. The results follow from the extracellular potential along the fiber as a function of electrode geometry. The theory is of a general nature but special investigations are made on monopolar, bipolar, and ring electrodes. Stimulations with monopolar electrodes show better recruitment characteristics than ring electrodes.     

On relations between covering radius and dual distance

The covering radius of a code tells us how far in the sense of Hamming distance an arbitrary word of the ambient space can be from the code. For a few decades this parameter has been widely studied. We estimate the covering ratios of a code when the dual distance is known. We derive a new bound on covering radii of linear codes. It improves essentially on the previously known estimates in a certain wide range. We also study asymptotic bounds on the cardinality of constant weight codes     

Effects of electrode-to-fiber distance on temporal neural response with electrical stimulation

This paper presents an analysis of the effects of the electrode-to-fiber distance on the temporal response properties of an auditory nerve fiber stimulated by electric current pulses. This analysis was based upon results from a computational model of a mammalian auditory nerve fiber axon having 50 nodes of Ranvier, each consisting of 130 stochastic sodium channels and 50 stochastic potassium channels, making it possible to represent the temporal fluctuations of action potential initiation and conduction. A monopolar stimulus electrode was located above a central (26th) node at electrode-to-fiber distances of 1, 4, and 7 mm, while the recording electrode was located at the 36th node. Action potentials (spikes) were generated by the biophysical model using the Crank-Nicholson method to solve a diffusive partial differential equation. By observing the occurrence times of spikes in response to 2000 cathodic monophasic stimulus pulses, temporal jitter (i.e., the standard deviation of spike times) was calculated and the poststimulus time (PST) histogram was generated as well. Furthermore, by computing the PST histogram for each initiation node as functions of space (node number) and time (PST), it was shown that spike initiation was distributed not only spatially but also temporally for stimulus levels producing firing efficiencies (FEs) near 0.5. However, at levels producing FEs near 0.99, while temporal variations approached zero, the spatial distribution of initiating nodes was comparable to that observed for the FE near 0.5. As temporal fluctuations are important for speech coding in cochlear implants, we conclude that spatial characteristics of the electrode-auditory nerve fiber interface may play a significant role in influencing these stochastic temporal processes.     

A fiber lens with a long working distance for integrated couplingbetween laser diodes and single-mode fibers

A new scheme is proposed for lensed fibers with a long working distance, as used in the coupling between laser diodes and single-mode fibers. The scheme consists of the combination of a hemispherically-ended coreless-fiber tip and an expanded-core fiber. The working distance is measured as 169 μm with a relatively low coupling-loss of 4.2 dB at a wavelength of 1.49 μm. Tolerances for a 1-dB loss increment for axial-, lateral-, and angular-misalignment are 35, 2.6 μm, and 0.8 degrees, respectively. The new configuration is suitable for alignment-free coupling between arrayed laser diodes and an array of single-mode fibers     

Current/voltage relations in punchthrough transistors

Observations of punchthrough in narrow-base (?0.1 ?m) n-p-n transistors are reported that do not conform to conventional theory. A general theory of punchthrough current/voltage relations is developed that explains the experimental results.     

Ultra-long distance wavelength-division-multiplexed solitontransmission using inhomogeneously broadened fiber amplifiers

The use of inhomogeneously broadened amplifiers for wavelength-division-multiplexed (WDM) optical soliton transmission is proposed. Dynamic gain equalization can be achieved through saturation, by adjustment of the time-averaged intensity of control lasers tuned between the WDM channels. The authors investigate numerically an example in which seven WDM channels at 2 Gb/s each are equalized using parameter values reasonable for germanosilicate core erbium-doped fibers     

Lens-free optical fiber connector having a long working distance assisted by matched long-period fiber gratings

Optical fiber connectors that had a long working distance and wide alignment tolerance were implemented by utilizing long-period fiber gratings (LPGs) written in double-cladding fiber. The power coupling between two separated pieces of fiber was accomplished through the inner cladding modes of both fibers. Assisted by the LPGs, approximately 450 /spl mu/m was achieved for a 1-dB working distance, as well as about 3 mm of 3-dB working distance. When the separation between two fiber tips was 1.5 mm, the 3-dB lateral tolerance was measured to be about 30 /spl mu/m, which was five times better than was the case when not utilizing the LPGs. By using the inner cladding mode of a dispersion-compensating fiber, it was possible to implement a fiber connector that was insensitive to the perturbations at the cladding surface of the fiber. The total insertion loss of the device at the maximum coupling was 2.06 dB, from which the loss purely induced by inserting the fiber gratings was analyzed to be smaller than 0.9 dB. The experimental results show strong feasibility for implementing a simple and cost-effective fiber connector that does not require any bulk optic elements.     

高精度远程光纤传输延时测量系统研究

介绍了当前的光纤传输延时测量方法和发展现状,针对远程光纤传输延时测量设计了一种高精度的测量系统,详细地阐述了系统的总体设计、工作机理及关键技术的解决等方面问题,最后介绍了系统的验证实验方法和系统所能达到的测量精度.     

Sliding-block line codes to increase dispersion-limited distance ofoptical fiber channels

We investigate the use of a run-length-limited (RLL) sliding-block line code to reduce the effects of intersymbol interference in high-speed (multi-Gb/s) data transmission over the direct-detection single-mode optical fiber channel. In addition to their well-known use for synchronization purposes, line codes can be designed to eliminate certain worst-case patterns that would otherwise preclude reliable data transmission at high rates and very long fiber lengths. The code considered here prohibits isolated “one's” (the 010 pattern) from appearing in the transmitted sequence. Simulation results indicate that this code, coupled with a simple compensation scheme at the receiver, can significantly increase dispersion-limited data rates and/or fiber lengths for both externally modulated and directly modulated systems. While our emphasis is on one particular line code, we discuss sliding block coding for the optical fiber channel in a general context     

光纤传输在脱靶量测量系统中的应用

在某脱靶量测量系统中,在测量雷达和测量数据处理、存储模块之间必须实现50m距离分离,根据使用要求,利用一个光收发器HFBR5208和一个串行/解串行芯片TLK1501设计实现了一种光纤传输摸块,详细介绍了DSP硬件实现和传输差错控制方法,在实际应用中达到了预期目的.     

大功率双包层光纤准直器的建模与仿真分析

研究了一种适用于大功率激光应用的双包层光纤准直器。考虑在制作准直器时,双包层光纤尾纤和C-lens 透镜端面8倾斜角度的影响,从高斯光束通过复杂光学系统的一般模型出发,利用矩阵光学的理论,在光线传输的子午面和弧矢面上分别进行建模计算,推导光线的传输变换矩阵。结合实际应用中光纤及C-lens 透镜的相关参数,仿真分析了尾纤与C-lens 透镜间距及C-lens 透镜参数对准直器工作距离和出射光束腰直径的影响。研究结果对进行大功率准直器的设计具有指导意义。     

Evaluation of 4-Gbit/s optical fiber transmission distance with direct and external modulation

Transmission characteristics for a recently modulated measured distributed-feedbacked (DFB) laser and an externally modulated DFB laser using a Ti:LiNbO/sub 3/, Mach-Zehnder modulator at 4 Gb/s are discussed. The transmission characteristics are estimated by an advanced eye-pattern analysis method. The maximum measured fiber dispersion with a directly modulated laser is 100 to 140 ps/nm when the chirp power penalty is 1 dB. However, for external modulation, there is no power penalty after transmission over a 2220-ps/nm dispersive fiber. This confirms that external modulation has superior transmission characteristics. The modulation scheme for 4-Gb/s systems in terms of these results is discussed.<>     

基于合成光学相干函数的长距离分布式传感技术

研究了基于合成光学相干函数的长距离分布式光纤传感系统,阐述了系统工作原理,测量了10.225km光纤末端6个连续反射点,空间分辨率达到了2.20m,并验证了反射光解调信号强度与回波损耗的对应关系.     

基准信号的低相噪声长距离光纤传输设计

介绍了基准光传输设备的基本组成和原理,指出低相噪长距离光纤传输中的问题所在,并提供了解决问题的方法,即数模混合器件的选择及改进滤波器设计.     

GPS系统在长途干线光缆设计中的应用

随着科技的发展,GPS系统在各个领域已经得到了越来越广泛的应用。本文阐述GPS系统的基本原理,并着重介绍了GPS系统在长途干线光缆工程中的应用,与传统勘察设计方法的比较,并总结出GPS系统应用在干线光缆勘察设计中的优势。     

Extending optical transmission distance in fiber wireless links using passive filtering in conjunction with optimized modulation

The authors demonstrate a simple passive technique for increasing the transmission distance in fiber wireless links through the application of a narrowband fiber Bragg grating (FBG) with high reflectivity (90-99%). They are applied in a conventional downstream link and also for the upstream in a wavelength-reused scheme. In the conventional nonwavelength-reused system, the grating is used to optimize the optical modulation depth and, hence, the receiver sensitivity in the downstream transmitted signal by reducing the power of the optical carrier before fiber transmission. In the wavelength-reused systems, the highly reflective FBGs can be effectively used at the base stations to recover a major portion of the optical carrier (as high as 99% of the downlink carrier) for uplink transmission from a weakly modulated downstream signal. In the latter case, the penalty in the downstream signal due to the large extraction of carrier power is partially offset by the sensitivity enhancement obtained in the filtering process. The authors present experimental results for the increased transmission performance for both the nonwavelength-reused and wavelength-reused scenarios. The increase in the sensitivity (at biterrorrate=10/sup -9/) in the nonwavelength-reused scheme for a given launch channel power that facilitates the increased transmission distance can be as large as 7 dB. The relative increase in the power margin for the upstream signal can be up to 4 dB in the wavelength-reused scheme. This would more than double the currently demonstrated transmission distances in such wavelength-reused systems without any optical amplification. The scheme is applicable to a wide range of radio frequencies and modulation depths.     

Simulation of multipolar fiber selective neural stimulation usingintrafascicular electrodes

A realistic, quantitative model is presented for the excitation of myelinated nerve fibers by intrafascicular electrodes. It predicts the stimulatory regions of any configuration of any number of electrodes, positioned anywhere inside the fascicle. The model has two parts. First, the nerve fiber is represented by a lumped electrical network and its response to an arbitrary extracellular potential field is calculated. Second, assuming a cylindrical geometry of the nerve bundle and its surroundings, an analytical expression for this field is derived. With realistic parameters, the model is applied to two cases: monopolar stimulation by a single cathode and stimulation by a specific tripolar configuration. It is shown that tripolar stimulation has the better spatial selectivity. Also tripolar stimulation is less sensitive to the conductivity of the medium surrounding the nerve and yields a more natural recruitment order.     

Extending transmission distance of high-density WDM systems usingpost transmitter fiber Raman amplifiers

The authors investigate the behavior of multichannel signals in a high-density wavelength division multiplexing (WDM) system with the consideration of stimulated Raman scattering. A parameter to evaluate the depth of Raman crosstalk is provided, and the transmission distance limitation imposed by Raman crosstalk is analyzed. To extend the system transmission distance, a post-transmitter fiber Raman amplifier is used. The allocation of signal channels on the pump gain profile is discussed. It is found that there exists an optimum pump power which results in a maximum amplifier gain. The example shows that the transmission distance limitation imposed by Raman crosstalk can be extended by 20 km for a 30-channel system with 0.25-dB/km fiber loss     

短距离通信中的塑料光纤无源器件技术

阐述了塑料光纤的真正优势在于其器件和连接的低价与便捷.着重介绍应用在短距离通信中的塑料光纤无源器件的研究进展,包括塑料光纤连接器、耦合器、塑料光纤放大器、塑料光纤光栅等.分析比较各种器件的技术特点,并指出今后面临的问题与挑战.     

Analysis of tunable single-mode fiber directional couplers using simple and accurate relations

We present simple and accurate relations for the study of parallel and curved tunable single-mode fiber directional couplers employing step-index or graded-index fibers. Analytical results for the various coupler parameters such as coupling coefficient, effective interaction length, tunability, and channel wavelength separation in directional coupler wavelength filters are presented. These results must prove very useful for an easy, quick, and accurate prediction or design of tunable single-mode fiber directional couplers.