Pulse Delay Method 1.The OTDR sends 4 (or more) wavelengths to the fiber. The first laser wavelength is used as the reference time, the other transmission of wavelengths times are compared to this reference 2.The OTDR measures then the group delay for each injected light signal. OTDR FUT End reflection 1 2 3 4 Distance (km) RelativeIntensity(dB)
Pulse Delay Method The acquired time-delay points are plotted on a graph, and a curve is then fitted to these points thanks to the right nonlinear regression : the 3-Term or 5-Term Sellmeier Equation (according to the fiber type under test) The dispersion is the first derivated ( d/d) of the delay versus the wavelength response. ( ) 22 11 33 44 Wavelength (nm) Dispersion(ps/nm.km) 0
Pulse Delay Method Advantages Easy to use Field Convenient Possibility of detecting dispersion within a link as it can analyze reflections at intermediate splices (automatic measurement of distance, as CD given per km) Access to only one end of a link (as it analyses backscattered light) Large band coverage (1255-1650nm) Excellent Price/Performance ratio Provides three functions in one: sources, CD and OTDR Drawbacks The distance range is limited by the end reflection : the higher the reflectance, the longer the distance. The use of a reflective terminator is needed to reach distances longer than 80km Not suitable for long haul testing (> 30 dB) Cannot measure over non-bi-directional components (e.g. EDFA…)
Phase Shift Method The modulated laser light is then sent over the Fiber Under Test. An optical to electrical converter converts to an electric signal the light received. The phase of the electrical signal is compared to the phase of the reference signal used to modulate the input signal. The phase measurement is repeated at intervals across the wavelength range of interest. FUT Variable Wavelength Light Source Phase Meter Converter Intensity Modulator (Reference signal) Electrical Source
Phase Shift Method The measured value is the group delay corresponding to a wavelength interval. The group delay is calculated with an equation. The chromatic dispersion is then calculated by taking the derivated of the group delay with respect to wavelength. Remarque: Reference signal could travel with test signal; avoid 2nd fiber to be used. Phase Shift ref
Phase Shift Method Drawbacks The phase of the pulse is sensitive to temperature, dispersion value, modulation frequency and wavelength step. The combination of the modulation frequency, dispersion and the wavelength step can result in a phase change that exceeds the capability of the phase detection equipment. Measurement time highly depends on the number of data points. Requires two units, one at each end Average chromatic dispersion value for the fiber length – not possible to sectionalize Fiber length must be known or measured separately Low band coverage (one source per band) Expensive (at least two times compared with time domain) Advantages Established in the market (approved method) Accurate measurements High dynamic range (40 dB) : about 15 dB higher than the Pulse Delay method. High repeatability Data extrapolation possible (calculated with fitting equations thanks to the group delay) Can measure over non-bi-directional components (EDFA, filters…)
Differential Phase Shift Method The differential phase method differs from the phase shift method in that it measures the dispersion directly instead of the group delay. Two modulated signals with two different wavelengths are sent over the Fiber Under Test. The phase of the two signals are compared to each other. FUT Variable Wavelength Light Source Phase Meter Converter Intensity Modulator ( Reference signal) Electrical Source Wavelength selector
Differential Phase Shift Method Average chromatic dispersion over the interval between the two wavelengths is determined from the phase-shift and wavelength interval. The process is then repeated with several wavelengths pairs in the test range which produces a CD curve. n+1 Phase Shift n
Differential Phase Shift Method Advantages Established in the market (approved method) Provides direct measurement of Chromatic Dispersion Accurate measurements Higher dynamic range: 55 dB (with 0dB laser power) High repeatability Can measure over non-bi-directional components (EDFA, filters…)
Differential Phase Shift Method Drawbacks The phase of the pulse is sensitive to temperature, dispersion value, modulation frequency and wavelength step. The combination of the modulation frequency, dispersion and the wavelength step can result in a phase change that exceeds the capability of the phase detection equipment. Measurement time highly depends on the number of data points. Requires two units, one at each end Average chromatic dispersion value for the fiber length Fiber length must be known or measured separately Low band coverage (one source per band) Expensive (two times compared with time domain) No delay measurements Resolution is limited by the wavelength step: as the step get smaller, the error grows larger Extrapolation outside the tested band is not possible. No fitting equations.
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