Optical Propogation

Question 1

Explain the concept of the twisted coaxial cable and why this is beneficial.

Answer 1

• Pair of copper wires closed into a circuit both emit and receive radiation
• Going and returning current -> cancel interference

Question 2

When does coaxial loss occur?

Answer 2

1. Dissipation in the conductor
2. Dissipation in the dielectric
   .3. Radiation

Question 3

The amount of power lost during propogation depends on what?

Answer 3

Frequency (or wavelength) used for transmission

Question 4

What are the three types of dispersion?

Answer 4

1. Modal dispersion; different modes propogate at different speeds
2. Chromatic dispersion; different frequencies within a single mode propagate at different speeds
3. Polarisation mode dispersion; different spatial

Question 5

What’s the difference between single-mode and multi-mode fibre?

Answer 5

1. Multi-mode fibre; larger core, multiple modes to coexist @ freq
2. Single-mode fibre; smaller core, allow one propagation mode

Question 6

IPAD FC: Explain the modal dispersion in an optical fibre

Answer 6

red and green travelling in the fibre

Question 7

Describe the impairments caused by modal dispersion

Answer 7

1. Transmit pulse -> composed of no. of different modes
2. Red slower than green -> pulse spreads and peak power decreases
3. Communication channel (pulses of light) overlap -> ISI

Question 8

Modal dispersion only occurs in what type of fibres?

Answer 8

Multi-mode fibres

Question 9

What does the amount of dispersion depend on?

Answer 9

1. Data rate
2. Length of fibre where the signal propogates

Question 10

Rate at in decreasing order which dispersion has the most to least effect

Answer 10

1. Modal dispersion
2. Chromatic dispersion
3. Polarisation Mode Dispersion

Question 11

The dispersion in the fibre is due to:

Answer 11

1. The material (can’t be changed)
2. Shape of waveguide i.e. shape of cross-section

Question 12

Explain polarisation-mode dispersion

Answer 12

1. Electric field in a SMF propogates with two orthogonal polarisation.
2. Imperfect production techniques -> fibre not completely symmetrical.

Question 13

What does non-linearity imply?

Answer 13

The transfer function of the fiber depends on the signal being transmitted.

Effects increase with transmitted power.

Question 14

Name the different type of non-linear effects

Answer 14

1. Raman Scattering
2. Self-phase modulation
3. Cross-phase modulation
4. Four-wave mixing

Question 15

Explain Raman Scattering

Answer 15

1. Light interacts with phonons, generating phonons with different frequency from incoming signal
2. Spontaneous -> random frequency and phase == noise
3. Stimulated -> can produce same frequency and phase == amplification

Question 16

Explain Self-Phase modulation

Answer 16

1. Refractive index depends on the intensity of travelling light.
2. Amplitude modulated signal sees different values of n -> subject to a diff propagation constant.

Question 17

Explain Cross-Phase modulation

Answer 17

If there are multiple WL in a fiber (Wave Division Multiplexing)

Modulation of other wavelengths produce changes in the refractive index that affects signal

Question 18

Explain Four-wave mixing

Answer 18

1. Refractive index nonlinearity generates new frequencies
2. Based on 3rd order nonlinearity, produces signals whose frequency is the sum or difference of input signals
3. Generate noise in WDM systems. Worse when the wavelength is small.

Question 19

Give the equation for the shannon capacity and its relationship with non-linearities.

Answer 19

C = B*log2(1+S/N)

• Capacity is in relationship with bandwidth and the log of the SNR

Question 20

What does SPM and CPM affect the pulse

Answer 20

It broadens the pulse