Photonics - Topic 8

Question 1

Eq: Signal-to-noise ratio

Answer 1

Note that SNR is usually given in db

Question 2

Eq: SNR of photonic systems

Answer 2

Question 3

What is the Quantum limit to detection?

Answer 3

• Min power for signal detection
• Thermal noise can be reduced by cooling and thus made negligible
• Shot noise however cannot be removed
• system will be limited by the shot noise → Quantum limit

Question 4

BER (bit error ratio) for quantum-limited OOK modulation

Answer 4

• For OOK, in principle we can distinguish between a ‘1’ and a ‘0’ provided we detect at least a single photon within a bit period
• probability of incorrectly identifying a ‘1’ as a ‘0’ is the probability of there being ZERO photons detected in the bit time of a transmitted ‘1’
• The probability of an error in a ‘1’ bit is therefore 𝑃( |𝑁) = 𝑒^–N
• If there are an equal number of 1’s and 0’s, the overall BER is given by 𝑃_𝑒 = ½𝑒^–N
• N = avg number of photons (average number of events)

Question 5

What does the eye diagram provide?

Answer 5

Convenient way of assessing the quality of a signal.

Question 6

Eq: Probability of error (or BER)

Answer 6

Pe = [(probability of receiving a 1 when a 0 is sent) x (probability of sending a 0)] + [(probability of receiving a 0 when a 1 is sent)x(probability of sending a 1)]

𝑃𝑒 is also known as the Bit Error Ratio (BER)

Question 7

Gaussian noise limited error rate

Answer 7

• In most practical cases however the receiver is not operating in the shot noise limit as other noise sources dominate
  
  • Thermal noise
  • Optical intensity noise
• To simplify the analysis we assume that the these noise sources follow a Gaussian Distribution

Question 8

Eq: Probability of error with Gaussian noise

Answer 8

Question 9

Eq: Q-parameter

Answer 9

The Q parameter is used as a measure of the signal quality  It is a measure of signal-to-noise ratio