Optical Link Design Flashcards

1
Q

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

What following constraints need to be considered:

A
  1. Power budget
  2. OSNR
  3. Other impairments
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2
Q

IPAD Q: Draw a typical optical link

A

X

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3
Q

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

A

The received power is >= receiver sensitivity

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4
Q

Give the equation for received power

A

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

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5
Q

What happens for long distances in terms of amplifiers

A

For long distances, multiple optical amplifiers are used in cascade

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6
Q

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

A
  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
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7
Q

What is OSNR and describe its relation with BER

A
  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

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8
Q

Which error correction codes are typically used in optical communications

A

Turbo codes

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9
Q

What is the Forward Error Correction threshold

A

The maximum BER a code can correct

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10
Q

Explain what the noise figure is

A

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

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11
Q

Explain how changing the spacing affects the OSNR

A

Amplifiers typically spaced 80km apart

Reducing spacing -> inc. OSNR

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12
Q

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

A

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

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13
Q

Explain the concept of margins

A

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

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14
Q

Explain the concept of signal regeneration

A

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
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15
Q

What do we know about regenerating signals at multiple points?

A

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

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16
Q

What is dispersion compensation used for?

A

Non-coherent system

17
Q

How do we compensate chromatic dispersion?

A

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

18
Q

How can you compensate for dispersion with coherent systems

A

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
19
Q
A