Question

For a single mode optical cable with 0.25 dB/km loss, the optical power 100 km from a 0.1 mW source will be _______.
1. -30 dBm
2. -35 dBm
3. -40 dBm
4. -45 dBm

Answer 1

Correct Answer - Option 2 : -35 dBm

Concept:

For the optical power

\(\alpha = \frac{{10}}{L}\log \left( {\frac{{{P_{0}}}}{{{P_{L}}}}} \right)\)

Where α is the attenuation constant, P₀ is the input power and P_L is the power at length L.

Calculation:

\(0.25 = \left( {\frac{{10}}{{100}}} \right)\log \left( {\frac{0.1}{{P_L}}} \right)\)

\(2.5 = \log \left( {\frac{0.1}{{P_L}}} \right)\)

P_L (dBm) = 10 log(0.1/10^2.5)

P_L = -35 dBm