Group delay in multimode optical fibers

The group delay τ in multimode fibers was previously derived by Gloge and Marcatili under the assumption that τ be dependent on the propagation constant β only. This letter derives the group delay without the above assumption and clarifies when the approach used by Gloge and Marcatili fails.     

Bragg gratings in multimode and few-mode optical fibers

Bragg gratings in optical fibers in multimode propagation are investigated experimentally and theoretically. Bragg gratings formed in optical fibers in multimode propagation show multiple reflection peaks or multiple transmission dips in the reflection or transmission spectra, respectively. For standard graded-index multimode fiber, the number of reflection peaks of a Bragg grating depends on excitation condition of propagating modes. The number of reflection peaks of a Bragg grating at around 1.55 μm is 19 for highly multimode excitation and 3-4 for lower order mode excitation. We analyze the phase-matching conditions of the propagating modes and identify half of the reflection peaks as the reflection to the same mode and the rest as the reflection to the neighboring modes. In dispersion-shifted fiber, a Bragg grating at around 0.8 μm in three-mode propagation shows three reflection peaks in the reflection spectrum. The temperature dependence of each reflection peak is similar to that of a conventional Bragg grating in single-mode fiber. Polarization dependence measured on a Bragg grating in multimode graded-index fiber is negligible. An advantage of Bragg gratings in multimode fiber (MMF) and the applications are discussed     

Mode conversion at splices in multimode graded-index fibers

Mode conversions occurring at a splice in multimode graded-index fibers are investigated theoretically, and their effects on impulse responses are verified experimentally. The relation describing the mode behavior at a splice in existence of a geometrical offset and fiber parameter mismatches is derived by taking a skew ray as well as meridional ray into consideration. Then the mode transfer matrix is obtained to determine the variations in mode power distribution and impulse response due to the mode conversions occuring at a splice. The measured mean delay time difference between lower and higher mode pulse responses and baseband frequency response for spliced graded-index fibers are compared with the theory obtained from the transfer functions of individual fibers and the mode transfer matrix which describes the mode conversions at a splice due to a transverse displacement and fiber parameter mismatches.     

Group velocity matching of multimode microstructure optical fibers

Group velocity matching is a key to the performance of multimode microstructure fibers. Simulations show that only partial matching is achieved using an intuitive graded-hole strategy, but that the usual velocity-matching limit can be achieved by numerically optimizing designs. These results demonstrate the perturbative multipole numerical method and are a step toward improved strategies for achieving high-performance microstructure fibers for a multimode transmission.     

多模光纤带宽的计算与优化

文章对多模光纤的带宽进行了理论分析,并进行仿真计算和优化,找出了不同工作波长下多模光纤最优折射率指数(gopt),同时对50/125μm新一代多模光纤的带宽进行了分析.     

玻璃基多模波导光功分器的研制

利用Ag+-Na+电场辅助玻璃基离子交换技术制作了低损耗的掩埋多模波导,并在此基础上研制了多模波导光功分器.测试分析了多模波导和光功分器的损耗和偏心率特性.所研制的多模直波导传输损耗小于0.1dB/cm,所制作的1×2的多模波导光功分器附加损耗小于1.3 dB.     

GaAs双异质结激光器和多模光学纤维的耦合

本文仅就GaAs双异质结激光器和多模光学纤维的耦合问题作一些讨论,以带有微透镜型的光学纤维为例,对耦合效率进行了计算.     

Optical power distribution in multimode fibers with angular-dependent mode coupling

An expression for the power flow, in multimode optical fibers, is derived, which is mathematically simple and allows a clear physical interpretation of the effects of mode coupling. Coupling strength is assumed to decrease with increasing axial angle. The solution obtained under this assumption is supported by experimental observations. Following the analysis, an experimental procedure to determine the coupling constant is devised. The theoretical results enable the prediction of the loss due to the mode coupling and the transmission bandwidth of the angular multiplexed channels in step-index optical fibers.     

Mode conversion due to tilts in multimode optical fibre

Coupled-mode theory is used to study the mode-conversion phenomenon due to a tilt in a multimode fibre. It is shown that there exists a critical angle beyond which power tends to be uniformly distributed among a large number of modes. This critical angle decreases with the increase in the number of modes in the fibre.     

Mode excitation in graded-index optical fibers

We report on theoretical and experimental investigations of the excitation of graded-index fiber modes by a laser beam. The far-field irradiance distribution and the focusing qualities are measured and are related to the measured refractive-index profiles of some commercially available graded-index fibers. If the fundamental mode is the only one excited, fibers with smooth and homogeneous profiles give good output beam qualities for fiber lengths of several tens of meters.     

Efficient power coupling using taper-ended multimode optical fibres

Highly efficient power coupling of up to 97% between a d.h. laser and a multimode step-index fibre is demonstrated by employing a tapered fibre launcher. Also, it is shown theoretically that a tapered end increases the effective numerical aperture of a fibre. For a Lambertian emitter, the use of a taper launcher permits the maximum coupling efficiency allowed by the Liouville theorem.     

Pulse dispersion and refractive-index profiles of some low-noise multimode optical fibers

The refractive-index profile and impulse response of three low-loss multimode optical fibers have been measured. Pulse dispersion of about 1.5 ns or less was observed in two of the fibers, each 1 km long.     

Interferometric demodulation of optical FM signals and its relation to modal distortion in optical multimode fibers

A general theoretical analysis of two-beam interference between modulated light signals is presented. It is demonstrated that a Michelson interferometer (MI) with short path difference can be used for demodulation of optical frequency modulation (FM) signals. The demodulated signal is usually a wide-band FM signal charaeterized by an effective FM index βeffin the order of 10-100, and the "conversion efficiency" of the demodulation is given by the ratio between the actual βeff-value and its low-frequency limit. For incoherent applications interference must generally be avoided as the demodulated wide-band signal generates harmonic distortion of high order. The close connection between interferometric demodulation and modal distortion in multimode fibres is demonstrated. An improved theory of modal distortion is presented and numerical examples of modal distortion are calculated. In practical cases the amount of distortion can be estimated on the basis of the fiber dispersion, the frequency deviation of the laser and the amount of spatial filtering.     

RF transmission over multimode fibers using VCSELs-comparing standard and high-bandwidth multimode fibers

Fiber-optic radio-frequency links have been assembled using oxide-confined vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers. Links with single and multimode VCSELs and with standard and high-bandwidth fibers have been evaluated and compared in the frequency range of 0.1-10 GHz. The best results were obtained for links with a multimode VCSEL and a high-bandwidth fiber. For a 500-m-long link, a spurious free dynamic range of 104 dB/spl middot/Hz/sup 2/3/ at 2 GHz and 100 dB/spl middot/Hz/sup 2/3/ at 5 GHz were obtained while allowing for a VCSEL-fiber misalignment of /spl plusmn/12 /spl mu/m. Corresponding numbers for the intrinsic link gain and noise figure are -29 and -33 dB, and 39 and 42 dB at 2 and 5 GHz, respectively. Inferior performance was observed for the standard fiber link due to a larger variation in modal group velocities. This paper also presents a detailed link analysis to identify performance limitations and to suggest modifications for improved performance.     

New method to calculate mode conversion coefficients in SI multimode optical fibers

A simple method is proposed for the experimental calculation of the mode conversion coefficients in multimode optical fibers. It only requires observing the far-field output pattern from a fixed length of fiber as the launching angle changes, as well as the intersection point between two far-field output patterns corresponding to two different input angles. The results obtained with this method are quite insensitive to small variations of the experimental parameters. A good agreement between theoretical and experimental results is also found.     

多模光纤布里渊散射角及频移的分析与计算

为了确定多模光纤布里渊频移的取值范围,进而评价多模光纤布里渊分布式传感系统的性能,从多模光纤的结构出发,根据射线光学理论分析了多模光纤中的布里渊散射过程,提出了确定布里渊散射角取值范围的方法,得到了阶跃型和渐变型多模光纤布里渊散射角的取值范围为两倍的全反射临界角到π,进而得到布里渊频移的变化范围为10.978~11.083 GHz,该结论为多模光纤布里渊分布式传感系统的设计和优化提供了理论依据。     

Polarization preservation in circular multimode optical fibers and its measurement by a speckle method

The first-order statistics of the Stokes parameters (SP's) are used to investigate the polarization properties of a circular multimode optical fiber. Linearly polarized light launched into an optical fiber 3 m in length is found to be partially preserved. The degree of polarizationPwas a proper parameter to measure quantitatively the polarization preservation in our experiments.Pwas especially high, when the speckle field produced at the end of the fiber was nonuniform. In all experimental investigations, the probability density functions (PDF's) of the Stokes parameters, calculated theoretically, coincide well with the experimental results. The spatial stationarity of the fiber speckle pattern has been verified when all modes ate equally excited.     

Mode field diameter measurements in single-mode optical fibers

The role of the mode field diameter in the characterization of single-mode fibers is examined. The most relevant definitions of this parameter are reviewed, and a comparative analysis of methods for its measurement is performed. All the discussed measurement methods have reached a repeatability and reproducibility which are quite satisfying. Emphasis is given to the requirements posed by the new fiber designs, such as the polarization-maintaining structures. Most of the discussed techniques have been industrialized, and a number of instruments based on them are commercially available; however, it is predicted that the evolution of fiber design will impose new requirements on some of these instruments     

Parametric four-photon mixing followed by stimulated Ramanscattering with optical pulses in birefringent optical fibers

An analysis into the phase matching condition of four-photon mixing in birefringent optical fibers is presented. The analysis yields six independent parametric processes. Some of the associated frequency shifts show a dependence on the fiber birefringence, allowing the authors to obtain constraints under which the parametric Stokes pulse will fall within the Raman gain band to undergo subsequent Raman amplifications. Both polarizations of the pump contribute to the Stokes gain through the stimulated Raman effect. Emphasis is placed on short optical pulses with tradeoffs among frequency shift, pulse walk-off length, and fiber attenuation to yield maximum power conversion to the Stokes frequency. Numerical simulation with a highly birefringent germania-doped silica fiber is used to illustrate the conditions for maximum gain of the Stokes signal     

Profile inhomogeneity in multimode graded-index fibers

Profile parameters (g) of several multimode graded-index fibers have been measured. It was found that g may vary azimuthally by ±0.15 or more in fibers for which the average value is between 1.8 and 2.2