Photonics - Topic 7

Question 1

Optical communications link: Transmitter

Answer 1

• semiconductor laser as the optical source
  
  • efficient
    
    • low power
    • high optical output
  • high quality single transverse mode output
  • monochromatic
  • low loss region wavelengths

Question 2

Modulation

Answer 2

• the process of encoding the data onto the optical carrier
• on-off keying (OOK)most commonly used method of encoding the data onto the optical carrier
• preferred format, NRZ (non-return-to-zero), minimises the required bandwidth

Question 3

Direct and Indirect Modulation

Answer 3

• Direct
  
  • laser drive current varies in response to data signal → modulate optical power
  • suitable for short links, limited by quality of signal produced, low cost and bandwidth
• Indirect
  
  • external modulator
  • for long links
  • high b.w.
  • low chirp (signal quality)
    *

Question 4

Transmission

Answer 4

• Optical signal can be transmitted over free space or in optical fibres
• The optical fibre has low loss and high b.w.

Question 5

Eq: Attenuation in silica fibre

Answer 5

Question 6

Optical amplifiers, what do they do ?

Answer 6

• same principles as lasers to boost the optical power by stimulated emission
• Erbium-doped fibre amplifiers are the most common form of optical amplifier
• Like any amplifier, optical amplifiers add noise
  
  • Despite the added noise, by placing amplifiers at regular intervals (40 – 100 km), the maximum system reach can exceed 10,000 km

Question 7

What is chromatic dispersion?

Answer 7

• As pulse broaens in time, it leads to ISI
• In uniform fibres chromatic dispersion depends on:
  
  • The wavelength dependence of the propagation velocity in the fibre material – material dispersion.
  • The guiding properties of the optical fibre – waveguide dispersion.

Question 8

Eq: Material Dispersion Coefficient

Answer 8

Question 9

Eq: Bit-rate distance product

Answer 9

B·L = P/(Δλ|D|) ≈ 1/Δλ|D|

Question 10

What does the Bit-rate distance product tell us?

Answer 10

Describes max bt-rate that can be transmitted over an optical fibre with a given dispersion parameter

Question 11

Receiver for digital signals: Diagram

Answer 11

• Detects the optical signal and converts it into an electrical signal
• Filtering is needed to reduce the noise bandwidth
• A decision device is used to convert the analogue signal into a digital signal

Question 12

What is Shot noise?

Answer 12

• Arises because electric current consists of a stream of discrete electrons that are generated at random times
• The rate of photon arrival (another discrete process) at the detector is also a random process that exhibits shot noise

Question 13

Eq: Shot Noise

Answer 13

• Poisson Process
  
  • 𝑃(𝑘 | 𝑁) = 𝑁^𝑘 𝑒^−𝑁 / 𝑘!
• Consider I(t) = I^– + i_s(t)
  
  • where I^– is the average current arising from photo-generated carriers
  • i_s(t) is the current due to shot noise
• Shot noise variance
  
  • 〈𝑖_𝑠ℎ² 〉 = 2𝑒I^–B

Question 14

What is thermal noise?

Answer 14

• Due to the random motion of carriers within the resistive material due to their kinetic energy
• Leads to a random current variation about a mean zero value
• Variables affecting it
  
  • Increases with temperature
  • Decreases with resistance

Question 15

Eq: Thermal Noise

Answer 15

• I(t) = I^– + i_s(t) + i_th(t)
• Noise Variance
  
  • 〈𝑖_{<em>t</em>ℎ}²〉 = 4𝑘𝑇𝐵 / 𝑅

Question 16

Eq: System Noise

Answer 16

Question 17

Eq: Photoreceiver circuit: Low impedance front end

Remember this!

Answer 17

Question 18

Eq: Photoreceiver circuit: High impedance front end

Remember this!

Answer 18

Question 19

Eq: Photoreceiver circuit: Transimpedance front end

Remember this!

Answer 19