Question

A.

A 1550nm single mode
digital fiber optic link needs to
operate at 622Mb/s over 60 km
without amplifiers. A single mode
InGaAsP laser launches an
average power of -11dBm into the
fiber. The fiber has loss of
0.1dB/km. The coupling loss at
the receiver is 0.5dB and the
receiver uses an APD
photodetector with sensitivity of
-39dBm.
Set up an optical power budget for
this link and find the system
margin.
* 7 points

Answer 1

The optical power budget for a 1550nm single mode digital fiber optic link with an initial power of -11dBm and a receiver sensitivity of -39dBm over 60km results in total losses of 6.5dB, yielding a system margin of 21.5dB.

Step-by-step explanation:

To set up an optical power budget for a 1550nm single mode digital fiber optic link operating at 622Mb/s over 60 km without amplifiers, we need to consider the initial power, the losses in the fiber, coupling loss, and the sensitivity of the receiver.

Initial (launched) optical power (Pin): -11dBm

• Fiber loss: 0.1dB/km × 60km = 6dB
• Coupling Loss at receiver: 0.5dB

Receiver sensitivity (Pmin): -39dBm

Total losses: Fiber loss + Coupling loss = 6dB + 0.5dB = 6.5dB

The power at the receiver (Pout) without accounting for margin is the initial power minus the total losses: Pout = Pin - Total losses = -11dBm - 6.5dB = -17.5dBm

Finally, the system margin is the difference between the receiver's sensitivity and the actual power at the receiver: System Margin = Pmin - Pout = -39dBm - (-17.5dBm) = 21.5dB

The system margin of 21.5dB indicates the link has a good buffer before signal degradation would be expected, which is essential for a reliable communication system.