The modal noise occurs when uncorrected source frequency is?a.δf>>1/δTb.δf=1/δTc.Negligibled.δf<<1/δT

To determine the condition under which modal noise occurs related to the source frequency, we need to analyze the options provided:

The frequency variation (represented as $\delta f$) and the time variation $\delta T$ (or $\delta T_b$ and $\delta T_c$) play significant roles in the analysis of modal noise, particularly in communication systems.

1. Understand Modal Noise:

   • Modal noise is particularly prevalent in multimode optical fibers and arises from the interference of different modes of light. It often has a direct relationship with the frequency of the light source and the bandwidth of the system.

2. Analyze the Options: a. $\delta f >> \frac{1}{\delta T_b}$ – This suggests that the frequency variation is much greater than the inverse of the bit time. This could lead to significant distortion and thus modal noise. b. $\delta f = \frac{1}{\delta T_c}$ – Indicates that the frequency variation equals the inverse of the correlation time, which might not necessarily lead to modal noise as it suggests stable operation around this frequency. c. Negligible. $\delta f$ – Implies that frequency changes are insignificant, which typically means less chance of modal noise. d. $\delta f << \frac{1}{\delta T}$ – This states that the frequency variation is much less than the inverse of the time variation, implying stable operation and typically low noise.

3. Conclusion: Among the options, option a ($\delta f >> \frac{1}{\delta T_b}$) best describes the scenario wherein uncorrected source frequency variations can lead to modal noise, as large variations would signify potential distortion and introduces uncertainty in the signal reception.

Final Answer

The modal noise occurs when uncorrected source frequency is $a. \ \delta f >> \frac{1}{\delta T_b}$.