Modal noise

Epworth, R. E. Sept. 1978. The Phenomenon of Modal Noise in Analogue and Digital Optical Fiber Communication. Proc. of 4th European Conf. on opt. comm., pp. 492-501. Genoa, Italy. 

Y., Inoue, A., and Koike, Y. (2013) Evaluation of modal noise in graded-index silica and plastic optical fiber links for radio-over-multimode fiber systems. 

The information-carrying capacity of an optical fiber is determined by its impulse response. The impulse response and hence the bandwidth are largely determined by the modal properties of the fiber. As single-mode transmissions avoid modal dispersion, modal noise, and other effects that occur with multimode transmissions, single-mode fibers can carry... 

Chemical, Physical, and Mechanical Tests. Manufactured friction materials are characterized by various chemical, physical, and mechanical tests in addition to friction and wear testing. The chemical tests include thermogravimetric analysis (tga), differential thermal analysis (dta), pyrolysis gas chromatography (pgc), acetone extraction, liquid chromatography (lc), infrared analysis (ir), and x-ray or scanning electron microscope (sem) analysis. Physical and mechanical tests determine properties such as thermal conductivity, specific heat, tensile or flexural strength, and hardness. Much attention has been placed on noise /vibration characterization. The use of modal analysis and damping measurements has increased (see Noise POLLUTION AND ABATEMENT). 

Moreover, in recent years broad band lasers have appeared which lack any frequency modal structure, at the same time retaining such common properties of lasers as directivity and spatial coherence of the light beam at sufficiently high spectral power density. The advantages of such a laser consist of fairly well defined statistical properties and a low noise level. In particular, the authors of [245] report on a tunable modeless direct current laser with a generation contour width of 12 GHz, and with a spectral power density of 50 /xW/MHz. The constructive interference which produces mode structure in a Fabry-Perot-type resonator is eliminated by phase shift, introduced by an acoustic modulator inserted into the resonator. 

Although the above formulation is presented for displacement time histories, it can be modified easily for velocity or acceleration measurements by using the corresponding modal impulse response functions for velocity or acceleration in Equation (4.24). Of course, the case of relative acceleration with white noise excitation is not realistic since the response variance is infinity. However, absolute acceleration measurements can be considered for ground excitation or relative acceleration measurements with non-white excitation. 

The nice feature of this idea is that the order of the measured mode m is not necessarily known since there is no matching between the measured and model modes in contrast to Equation (5.5). This is important for practical situations where it is difficult to judge whether the Nm measured modes are the lowest Nm modes. This is because some of the modes may not be excited in the modal testing and they are missing in modal identification. However, this formulation requires complete mode shape measurements, which are not available in practice. Furthermore, the measurements are contaminated by measurement noise but this formulation does not have an explicit treatment on it. 

In this simulated-data example, a twelve-story shear building is considered. It is assumed that this building has uniformly distributed floor mass and uniform stiffness across the height. The mass per floor is taken to be 100 metric tons, while the interstory stiffness is chosen to be k = 202.767 MN/m so that the first five modal frequencies are 0.900,2.686,4.429,6.103 and 7.680 Hz. The covariance matrix is diagonal with the variances corresponding to a 1.0% coefficient of variation of the measurement error of the squared modal frequencies and mode shapes for all modes, a reasonable value based on typical modal test results. For the simulated modal data, a sample of zero-mean Gaussian noise with covariance matrix was added to the exact modal frequencies and mode shapes. 

It is assumed that only the first three x-directional and y-directional modes are measured but not any of the torsional modes. This is done deliberately to simulate a common situation where some of the modes are not excited sufficiently to be able to observe. In the identification process, it is unknown that there are some missing modes. The six measured modes correspond to the 1st (3.432 Hz), 2nd (3.837 Hz), 4th (10.10 Hz), 5th (11.29 Hz), 7th (18.08 Hz) and 9th (21.31 Hz) modes. Sensors are placed on the +y and —y faces of the 1st, 2nd, 5 th and 6th floors, and the -x face of all floors to measure the modal frequencies and mode shape components. The covariance matrix Te is diagonal with 0.5% COV for the modal data. For the simulated modal data, a sample of zero-mean Gaussian noise with covariance matrix Ze was added to the exact modal frequencies and mode shapes. Initial values for all stiffness parameters are taken to be 100 MN/m, which overestimates the values by 100% and 150% for the x and y faces, respectively. 

Thanks to the definitions and evaluation procedures provided by the assessors, we could distinguish between eight types of attributes depending on the aspects they referred to (Force, Vibrations, Precision, Travel, Noise, Pattern, Reverse gear and Others). We observe that consumers and test drivers did not give the same importance to each modality, as they did not generate the same numbers of terms (Fig. 20.2). However, even if test drivers seemed to speak more about force, vibrations and... 

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