Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 10

Years: MAY 2015

Fiber attenuation measurements:

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

$\Rightarrow $ 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.

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

$\Rightarrow $ 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.

$\Rightarrow $ The optical attenuation/unit length is given by:

$α_{dB} = \dfrac {10}{L_1-L_2}∙\log_{10} \dfrac {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

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

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

$\Rightarrow $ 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.

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

$\Rightarrow $ Beam splitter triggers the oscilloscope for input pulse measurement.

$\Rightarrow $ 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.