Module 19 : WDM Components
Lecture     : WDM Components - II
 


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Consider a strong signal at a wavelength $ \lambda_s$and a weak continuous probe at a wavelength $ \lambda_p$incident on an SOA. When the signal is in logic state 1, the power is high, carrier depletion occurs and the probe is blocked. However, when the signal goes to logic zero, there is no depletion and the probe signal passes at full power so that probe is in logic 1 state. Thus the probe signal has been modulated same way as the signal, implying that a wavelength conversion of the signal has taken place. It may be noted that the output signal is inverted in the sense that when the signal should have been in logic state 1, the output is in logic state zero and vice versa. This may be easily set right. Cross-Phase Modulation (XPM)
Non-linear properties of some semi conducting material can be used to convert wavelength. As the refractive index of the active medium depends on the carrier density, an incoming signal which depletes carrier density will modulate the refractive index. The change in refractive index, in turn, will phase modulate the continuous wave probe signal. The signal itself is filtered out at the output state, passing the modulated probe signal at a wavelength of $ \lambda_p$carrying the same information as the original signal with wavelength $ \lambda_p$was carrying.

Four Wave Mixing (FWM)

Four wave mixing, which is an undesirable feature in fiber propagation can be exploited to convert wavelength. When a high powered optical signal is launched into a fiber, the linearity of the medium for optical response is lost. Four wave mixing occurs due to a third order nonlinearity in silica fibers, known as optical Kerr effect . When three frequencies $ f_1, f_2$and $ f_3 (\ne f_1,f_2)$are launched into the fiber, it results in a fourth wave of frequency

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