Optical Link Design

Question 1

When designing an optical link, I usually have a BER requirement needs to be satisfied.

What following constraints need to be considered:

Answer 1

1. Power budget
2. OSNR
3. Other impairments

Question 2

IPAD Q: Draw a typical optical link

Answer 2

X

Question 3

What is the objective of the power budget for an amplified system?

Answer 3

The received power is >= receiver sensitivity

Question 4

Give the equation for received power

Answer 4

Precv=Plaunch-L1+G1-L2+G2…-Ln+Gn - M> receiver_sensitivity

Question 5

What happens for long distances in terms of amplifiers

Answer 5

For long distances, multiple optical amplifiers are used in cascade

Question 6

Name the three types of noise in an optical system and describe them

Answer 6

1. Amplified Spontaneous Emission; optical amplifiers -> amplifies signal = noise
2. Thermal noise; random thermal-driven motion of electrons at the receiver -> sums up to signal carrying information
3. Shot noise; photons carrying the information arrive at the detector with random arrival times

Question 7

What is OSNR and describe its relation with BER

Answer 7

1. Optical signal-to-noise-ratio is the relation bet. signal power and noise power @ receiver

Reminder: BER is a measure of the accuracy of data transmission in a digital communication system -> we want a low BER

inc. OSNR LOWER BER -> easier to determine whether a received bit has a 0 or 1 value

Question 8

Which error correction codes are typically used in optical communications

Answer 8

Turbo codes

Question 9

What is the Forward Error Correction threshold

Answer 9

The maximum BER a code can correct

Question 10

Explain what the noise figure is

Answer 10

A measure of the noise introduced by the amplifier; measured in dB

Question 11

Explain how changing the spacing affects the OSNR

Answer 11

Amplifiers typically spaced 80km apart

Reducing spacing -> inc. OSNR

Question 12

In a chain of amplifiers, each additional amplifier amplifies both incoming signal and incoming noise, plus it will add its own noise.

Explain what happens of the OSNR

Answer 12

After each amplifier the OSNR decreases and this is permanent, it cannot be recovered.

Question 13

Explain the concept of margins

Answer 13

Margin = safety distance (SNR) you decide to keep, not being sure where you stand w.r.t the edge

Optical communication margin -> distance to the FEC threshold

Question 14

Explain the concept of signal regeneration

Answer 14

Before the OSNR drops below the target BER, we need to regenerate the signal:

1. terminate signal
2. calculate BER
3. re-modulate the signal for next fibre span
4. for new span, calculate new OSNR as for previous span

Question 15

What do we know about regenerating signals at multiple points?

Answer 15

Regenerating the signal at multiple points, the total BER will be the sum of the BER of the individual spans.

Question 16

What is dispersion compensation used for?

Answer 16

Non-coherent system

Question 17

How do we compensate chromatic dispersion?

Answer 17

Adding a length of fiber with opposite direction characteristics, called Dispersion compensating Fibre

DCF -> done at the end of the link. Usually distributed over multiple stages and co-located with the optical amplifiers, to reduce nonlinear effects

Question 18

How can you compensate for dispersion with coherent systems

Answer 18

Electronic equalisation can be used at the receiver to compensate for dispersion BUT

1. Higher data rate, more transmission rate -> inc. processing power and memory
2. Coherent receivers are more expensive than direct-detector ones