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Explain with diagram the technique of pulse dispersion measurement in time domain.

Marks: 10M

Year: May2015, Dec2013

• It measures the relative contribution from absorption, bending, scattering losses. The total attenuation is a function of the wavelength λ of the light.

• The focused light is chopped using chopper at low frequency. This chopped light is fed to monochromater which utilizes prism to select required wavelength at which attenuation is measured.

• The fiber is also put into mode scrambler to remove light launched in the fiber cladding.

• Cut back method means taking a set of optical output power measurements over a long length fiber and then fiber is cutback maintaining same launch conditions for next set of output.

• The optical attenuation/unit length is given by:

$$α_{dB}=\frac{10}{L_1-L_2 }.log_{10} \frac{ P_{02}}{P_{01}} $$

Where, $L_1$ and $L_2$ are original lengths and cutback lengths

$P_{01}$ and $P_{02}$ are output power from original and cutback lengths.

Fiber dispersion measurements

• Dispersion measurement gives an indication of distortion to optical signals as they propagate down the fiber.

• Dispersion effect may be characterized by taking measurements of impulse response of fiber in time domain or baseband frequency response in frequency domain.

• Short optical pulses are launched into the fiber from suitable source. The pulse travels down the length of fiber under test and are broadened due to various dispersion mechanisms.

• The pulses are received by a high speed photodetector and displayed on a fast sampling oscilloscope.

• Beam splitter triggers the oscilloscope for input pulse measurement.

• After initial measurement of output pulse width, fiber is cutback and pulse width is measured again and the pulse dispersion is then by comparing the measured pulses.