Differential Mode Attenuation

DMA is measured by the cutback technique under the DML (see Section 6.2). The output optical power from a long fiber is measured while shifting the position of the fiber probe over the input end face then the fiber is cut off a few meters from the input end, and the output power is again measured at this output end of the short fiber while shifting the position of the fiber probe. The attenuation of the individual mode is calculated using Equation 6.10. 

Bandwidth is one of the most important parameters of optical fibers, in addition to attenuation. Bandwidth determines the maximum transmission data rate or the maximum transmission distance. Most optical fiber communication systems adopt pulse modulation. If an input pulse waveform can be detected without distortion at the other end of the fiber, except for a decrease in the optical power, the maximum link length is limited by the fiber attenuation. However, in addition to the optical power attenuation, the output pulse will ako be generally broader in time than the input pulse. This pulse broadening restricts the transmission capacity, namely, the bandwidth of the fiber. The bandwidth is determined by the impulse response as follows [17] Optical fibers are usually considered quasi-linear systems, and therefore the output pulse is described by 

The output pulse from the fiber can be calculated in the time domain 

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Nihei, E., Ishigure, X, and Koike, Y. (1996) High-bandwidth, graded-index polymer optical fiber for near-infrared use. Appl Opt, 35 (36), 7085-7090. Makino, K., Kado, T., Inoue, A., and Koike, Y. (2012) Low loss graded index polymer optical fiber with high stability under damp heat conditions. Opt Express, 20 (12), 12893-12898. Olshansky, R. and Oaks, S.M. (1978) Differential mode attenuation measurements in graded-index fibers. Appl. Opt, 17 (11), 1830-1835. 

Noda, T. and Koike, Y. (2010) Bandwidth enhancement of graded index plastic optical fiber by control of differential mode attenuation. Opt Express, 18 (3), 3128-3136. 

Figure 15 Differential mode attenuation of Gl POFs based on the PMMA-DPS systems prepared by the rod-in-tube method and the interfacial-gel polymerization technique. Adapted with permission from Noda, T. Koike, Y. Opt Exp. 2010, 18 3128, 2010 OSA.

Sometimes the high-frequency attenuation is insufficient to meet the specifications and a third pole needs to be added to the EMI filter. This filter is typically a differential-mode filter and will share the Y capacitors from the common-mode filter. Its corner frequency is typically the same as the commonmode filter. This filter is made up of a separate choke on each power line, and is placed between the input rectifiers and the common-mode filter. 

The addition of this stage of filtering will bring the very high-frequeney attenuation under eontrol and further attenuate any differential-mode noise on the earth ground lead. It will also produee a eombined attenuation of -36 dB at the switehing frequency of the power supply. 

E. (2003) Quantitative estimates of mode coupling and differential modal attenuation in perfluorinated graded-index plastic optical fiber. J. Lightwave TechnoL, 21 (1), 111-121. 

Further accuracy improvement was experienced when two or more receivers were used simultaneously at two or more stationary points, and relative positions in terms of interstation vectors Ar were produced. The reason for the increased accuracy was the attenuation of the effect of conrmon errors/biases (atmospheric delays, orbital errors, etc.) through differencing. This relative or differential mode of using the system became very popular and remains the staple mode for geodetic positioning even with the more modem GPS used today. 

A physically separate and remote reference electrode is desirable becau.se the reference electrode is not electrically isolated from the inputs to the differential amplifier. The reference electrode is in contact with the person and consequently is electrically coupled to both inputs of the amplifier by the electrical impedance of the body. This impedance is a function of distance, i.e., the further away the reference electrode is, the greater is the attenuation. The reference electrode forms a floating ground signal rather than a true ground. Furthermore, the noise signal that can be introduced by the reference electrode is not necessarily a conunon-mode signal that will be removed by the differential amplification process. 

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