Spatially resolved detection and equalization of modal dispersion limited multimode fiber links

The authors present theoretical and experimental results of an optoelectronic-equalization technique that mitigates intersymbol interference caused by differential modal delay in multimode fiber. By exploiting the spatial diversity of the transverse optical-fiber modes, the authors are able to provide a sufficient additional information in the form of mode-dependent photocurrents to enhance total signal integrity. A fabricated two-segment photodetector is demonstrated with a routinely achievable two-times improvement in bandwidth-distance product. They also show the robustness of the technique to the expected variations in graded-index profiles.     

Exploiting diversity in multimode fiber communications links via multisegment detectors and equalization

we propose diversity combination via optical multisegment detectors and electrical equalization techniques to mitigate the effects of intermodal dispersion in multimode fiber (MMF). With no equalization in MMF links, intermodal dispersion produces intersymbol interference (ISI) in the received optical signal that severely limits the achievable data transmission rates and fiber link lengths. Our quasi-simulated performance results (obtained with measured impulse responses) demonstrate that multisegment detection and equalization provide a low-cost and efficient solution to combat ISI and hence, to enhance the performance of MMF communications links.     

Capacity enhancement in coherent optical MIMO (COMIMO) multimode fiber links

In this paper, we study a coherent optical MIMO (COMIEMO) multi-mode fiber link proposed for enhancing the fiber information capacity. We examine the statistical characterization of the equivalent MIMO channel and the improvement in the fiber capacity due to MIMO transmission. It is shown that the equivalent channel behaves similarly to a complex Gaussian MIMO channel, suggesting that the available results on wireless MIMO communication systems can be applied to optical fiber links for capacity enhancement.     

An experimental and theoretical study of the offset launchtechnique for the enhancement of the bandwidth of multimode fiber links

This paper reports an experimental and theoretical study of bandwidth enhancement in multimode fiber links achieved by using offset launch techniques. It is found, both from theory and experiment, that an up to four-fold bandwidth enhancement can be obtained compared to standard overfilled launch techniques despite exciting over 50% of the fiber modes, thus allowing good stability. The enhancement technique is found to be achieved for a range of fibers with different core diameters and core refractive index profiles. The level of bandwidth enhancement depends upon the inherent bandwidth of the fiber, being particularly effective in improving the performance of low bandwidth “worst case” fibers. The launch is found to be stable with respect to environmental variations, showing bandwidth enhancement for injection positions or connector offsets of up to 5 μm from optimum. In addition, unlike many restricted launch techniques, it is found to be tolerant to angular fiber misalignment for angles up to 6° from normal     

An erbium-doped multimode optical fiber amplifier

The authors describe the first experimental study of an erbium-doped multimode fiber amplifier. The focus has been to characterize an intermediate core erbium-doped optical fiber, a fiber that is capable of propagating many guided modes at both the signal and pump wavelengths, and to determine the feasibility of using such an active fiber as a multimode fiber amplifier, by measuring its gain, noise, and pump power requirements. For a 2-m length of a 13-μm-core erbium-doped fiber, the authors measured gain as high as 16 dB at a signal wavelength of 1543 nm, with approximately 100 mW pump power (980 nm). For these same test conditions, the smallest excess noise factor β was 42     

The bandwidth of a multimode fiber chain

We propose a new method for evaluating the baseband transmission in a multimode fiber chain. Carnevale and Paek (Bell Syst. Tech. J., vol. 62, pp. 1415-1431, 1983) stated that errors in the fiber manufacturing process will randomly distort a desired index profile, presumably of power-law type. We extend their discussion to the bandwidths of concatenated fibers, by considering Gaussian approximations to actual transfer functions. The bandwidth can thus be separated into two parts, one of which is due to the over- and undercompensation of individual, idealized power-law profiles and the other of which refers to random profile distortions as well as possible mode coupling within mode groups. The former part should normally dominate the length dependence of longer chains. The latter part may be replaced by an expectation value, typical for the actual manufacturing process. A remarkably good agreement is achieved between experimental and predicted bandwidths for various chain configurations.     

基于掺铒光纤环形滤波器和多模光纤光栅的双波长激光器

提出了基于掺铒光纤环形滤波器和多模光纤光栅的双波长激光器。在单波长光纤激光器的基础上,增加了多模光纤布拉格光栅(MM-FBG)和高精细度的光纤滤波器。其中多模光纤布拉格光栅作为激光器的波长选择元件,可产生两个波长的激光输出。高精细度的光纤滤波器由两个光耦合器和一段弱泵浦的掺铒光纤构成,掺铒光纤产生的增益和光纤时延使滤波器具有高精细度的梳状谱响应,从而抑制了激光器产生的不需要模式,保证了输出的激光具有窄线宽特性。以980 nm的激光二极管(LD)作为泵源,得到了线宽为0.07 nm或0.08 nm的双波长输出,表明滤波器具有良好的滤波效果。     

Three-dimensional method for simulation of multimode interferencecouplers

A three-dimensional (3-D) method for modeling multimode interference devices based on a finite element formulation is presented as an alternative to models having one-dimensional cross-sections only. The method is tested for couplers with two different strongly confined waveguides structures. The results show that full treatment of two-dimensional cross-section is of special importance for design and simulation of waveguide devices for which the effective index approximation is no longer valid. For deep rib waveguide geometries, excess loss greater than 15 dB can be obtained if the 3-D method is not used in the design of the couplers     

Performance of telecommunication-grade multimode fiber at 780 nm

Presents the physics of graded-index multimode fiber and test results that demonstrate 780-nm compact disc (CD) lasers operating at the high speeds (266 Mb/s) and moderate distances (~2 km) for data communications. The test results include fiber characterizations of bandwidth and attenuation, and system performance testing of bit error rate measurements and eye closure characterization. The predictability of 780-nm performance from existing characterization at the traditional wavelengths of 850 and 1300 nm is sufficiently precise to permit the conservativeness necessary for standardization. Telecommunication-grade 50-μm fiber is ideally suited to this application because commercial pressures have caused 62.5-μm fiber to be optimized for 1300-nm light emitting diode (LED) operation     

光缆线路的防护及其故障的检测

论述了光缆线路故障的现象、原因及防护方法，结合OTDR(光时域反射计)的使用经验，总结了光缆线路故障的定位方法和排除技巧。     

Frequency-modulated multimode laser diode for fiber Bragg gratingsensors

A demodulation scheme for fiber Bragg grating (FBG) sensors is presented. It is based on the generation of an electrical carrier by using a modulated multimode laser diode to illuminate the fiber grating. The change in Bragg wavelength is measured by tracking the phase of the carrier at the detector output in either an open- or a closed-loop scheme. A theoretical analysis of the interrogation technique in terms of linearity and dynamic range is presented. Experimental data were obtained for both strain and temperature measurements. Sensitivities of 0.7 μϵ/√Hz and 0.05°C/√Hz were obtained over a dynamic range of ≈60 dB. The application of this demodulation scheme to a multiplexed sensing system is also demonstrated     

Wavelength tunable fiber lens based on multimode interference

A new concept for a fiber-based wavelength-tunable condensing lens is theoretically and experimentally investigated in terms of its ability to shift the longitudinal focus position as a function of wavelength. By exploiting multimode interference effects with reimaging theory, a compact and robust device, completely contained within a standard FC connector, can be constructed by simply splicing a single-mode fiber (SMF) to a finite length section of multimode fiber (MMF). This paper summarizes the theoretical and experimental results of such a device, with additional results for wavelength sensitivity.     

多模光纤布里渊散射效应研究进展

面对人们日益增长的光纤传感需求,多模光纤的容量优势为解决单模光纤的容量壁垒、实现高精度的多参量光纤传感测量提供了新思路。理论分析了多模光纤的模式特性,综述了多模光纤模式耦合及偏振的研究进展,重点介绍了多模光纤中布里渊散射的阈值、增益谱和布里渊频移的国内外发展动态,分析了未来光纤传感的发展方向。     

Launching condition dependence of graded-index multimode fiber lossand bandwidth

Theoretical and experimental studies of the launching-condition dependence of graded-index 50-μm core, 0.2-numerical aperture multimode fiber transmission loss and baseband bandwidth are discussed. A combination of a SGS mode scrambler and a mandrel wrap mode filter was used as the launching system. It was found that steady-state fiber loss measurements should be carried out at an approximate mandrel curvature of ρ=0.07 mm^-1. This curvature is much smaller than the curvature ρ=0.15 mm^-1 which produces an equilibrium-mode distribution as defined by the International Telegraph and Telephone Consultative Committee (CCITT). Measurement at ρ=0.15 mm^-1 is shown to underestimate steady-state loss. It was also found that reproducible and reasonable bandwidth measurement can be obtained by choosing ρ=0 to 0.07 mm^-1     

Computer simulation of digital lightwave links

A hybrid approach is presented for evaluating the performance of single-mode digital lightwave communication links efficiently. The hybrid approach combines simulation with analysis to provide greater accuracy and flexibility than that available with analytical calculations alone. Because simulation is used, models can be made as detailed as necessary and computed waveforms can be compared directly to measured waveforms. Measured data can be used to characterize functional components, such as sources, fibers, and detectors, accurately. Simulation models and techniques for evaluating system degradations caused by the optical-source waveforms, fiber attenuation and dispersion, and receiver noise are provided. The end-to-end performance of typical digital lightwave links with broad- and narrow-spectrum sources (light-emitting diodes and laser), using both p-i-n and avalanche photodiode detectors, is evaluated and compared to experimental results     

Modeling and analysis of systems with multimode components anddependent failures

A model for the reliability and performance analysis of systems where components can degrade in a statistically dependent manner is presented. This cause-based multimode model is based on the idea that deviations of components from the up state have underlying physical causes which can be explicitly identified and are statistically independent. The effects of several causes can be combined in a flexible manner. System reliability and performance measures can be computed approximately by considering the most probable states. Such states can be efficiently generated by algorithms developed for the earlier multimode, statistically independent failure model     

New calculations for coupling laser diode to multimode fiber

A simple and useful method is presented for calculating characteristics of a laser diode to a multimode fiber coupling. Attenuation of higher order modes in multimode fiber is introduced into ray tracing. Maximum coupling efficiencies and misalignment tolerances of optical circuit components are computed for utilization of a relatively large spherical lens and combination of two relatively large spherical lenses. It is clarified that attenuation of higher order modes strongly affects both maximum coupling efficiency and misalignment tolerances. Agreement between theoretical and experimental coupling characteristics is greatly improved compared with the case for conventional ray tracing.     

Efficient Jacobian evaluation method in multimode hierarchicalcircuit simulation

An efficient method for Jacobian evaluations in multimode hierarchical circuit simulation is presented. The method evaluates the Jacobian matrix based on the block Jacobian contributions which can be evaluated by various methods for different types of blocks to achieve both accuracy and efficiency. Comparison of the method with the forward-difference method shows considerable improvement in simulation efficiency and convergence     

Fabrication of OH-free multimode fiber by vapor phase axial deposition

An ultimately OH-free multimode optical fiber was fabricated by vapor phase axial deposition (VAD). Dehydration conditions were investigated using a newly designed electric furnace. To exclude the OH ion contamination during a fiber-drawing process, simultaneous deposition of core and cladding was carried out. The loss mechanism for the fabricated OH-free fiber was analyzed. Transmission loss for this fiber approached the theoretically estimated value. The low-loss window, with less than 0.5 dB/km attenuation, ranged from 1.2 to 1.7 μm. Realization of the OH-free VAD fiber promises a very useful application in wavelength division multiplexing optical transmission systems, and widens the choice of wavelengths for optical sources.