Ch11Sec4_1

Ch11Sec4_1 - THE FIBER FORUM Fiber Optic Communications...

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Dr. Joseph C. Palais 11.4 1 THE FIBER FORUM Fiber Optic Communications Dr. JOSEPH C. PALAIS PRESENTED BY
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Dr. Joseph C. Palais 11.4 2 Sections 11.4 More Noise Factors
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Dr. Joseph C. Palais 11.4 3 11.4.1 Modal Noise Modal noise occurs only in multimode fibers which use coherent sources (such as the laser diode) and which contain imperfect connectors, splices, or other components. To understand this phenomenon consider a cross section of the fiber core as shown on the next slide:
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Dr. Joseph C. Palais 11.4 4 Light pattern A random speckle pattern of light and dark appears. This is the interference pattern caused by the many modes. It only appears for coherent sources. The speckle pattern changes if the source wavelength changes or if there is fiber motion.
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Dr. Joseph C. Palais 11.4 5 Now consider the changes in splice loss which occur if the speckle pattern varies and the splice has some lateral offset. Transmitting fiber core Receiving fiber core As the speckle pattern changes, the amount of power in the overlap region changes. Thus, the splice loss changes with time.
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Dr. Joseph C. Palais 11.4 6 The resulting power variation, as detected at the receiver, is called modal noise. To reduce modal noise, use low loss components (such as connectors and splices) and use low coherence light sources (such as LEDs).
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Dr. Joseph C. Palais 11.4 7 11.4.2 Mode-Partition Noise Consider the light from a multimode laser diode. The power associated with the different modes varies randomly. The total power is the sum of the mode powers and, thus, has a random component.
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11.4 8 11.4.3 Electronic-Amplifier Noise Determine the effect of the electronic amplifier which follows photodetection in the receiver. Consider an amplifier with no input signal:
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This note was uploaded on 05/11/2010 for the course EEE EEE-546 taught by Professor Palais during the Spring '10 term at ASU.

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Ch11Sec4_1 - THE FIBER FORUM Fiber Optic Communications...

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