Dispersion Measurements Lecture-3

Dispersion Measurements Measurement of Intermodal Dispersion The most common method for measuring multimode fiber bandwidth is based on measurement of the impulse response. A pulsed laser source is coupled through a mode scrambler to the input of the test fiber. The source spectrum must be narrow enough that the results are not significantly influenced by chromatic dispersion.

Dispersion Measurements Measurement of Intermodal Dispersion Multimode fiber bandwidth measurements are sensitive to optical launch conditions and the deployment of the test sample. For stable repeatable measurement, a mode-scramble device should be inserted ahead of the test device to assure excitation of a large number of modes. In addition cladding light should be removed.

Dispersion Measurements Measurement of Chromatic Dispersion Chromatic Dispersion is simply a variation in the speed of propagation of a lightwave signal with wavelength. The optical source in a high-speed communication system is typically a single line diode laser with nonzero spectral width. Pulse modulation increases the spectral width. Each wavelength component of the signal travels at a slightly different speed, resulting in the pulse broadening.

Dispersion Measurements Measurement of Chromatic Dispersion In singlemode fiber, chromatic dispersion results from the interplay of two underlying effects. Material dispersion results from the nonlinear dependence upon wavelength of the refractive index, and the corresponding group velocity, of doped silica. Waveguide dispersion is rooted in the wavelength dependent relationships of the group velocity to the core diameter and the difference in index between the core and the clading.

Dispersion Measurements Measurement of Chromatic Dispersion Control of total chromatic dispersion of transmission paths is critical to the design and construction of long haul, high speed telecommunications system. The first objective is to reduce the total dispersion to the point where its contribution to the error rate of the system is acceptable. The dispersion of a single channel system can be controlled by concatenating fibers of differing dispersion such that the total dispersion is near zero.

Dispersion Measurements Measurement of Chromatic Dispersion The precision and accuracy of chromatic dispersion measurements is dependent upon test equipment design. The impact of phase instability and phase measurement resolution depends upon the modulation frequency. A higher modulation frequency will produce a phase change that is larger in comparison with a given phase measurement uncertainty.

Measurement of Chromatic Dispersion Wavelength accuracy is important because the actual phase shift is proportional to the wavelength step. Depending upon measurement objectives, the inherent wavelength accuracy of a tunable laser or filtered broadband optical source may be sufficient. Thermal transients in the measurement setup and the fiber under test can contribute significant measurement error. Dispersion Measurements