Photonics - Topic 7 Flashcards
Optical communications link: Transmitter
- semiconductor laser as the optical source
- efficient
- low power
- high optical output
- high quality single transverse mode output
- monochromatic
- low loss region wavelengths
- efficient
Modulation
- 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
Direct and Indirect Modulation
- 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)
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Transmission
- Optical signal can be transmitted over free space or in optical fibres
- The optical fibre has low loss and high b.w.
Eq: Attenuation in silica fibre
Optical amplifiers, what do they do ?
- 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
What is chromatic dispersion?
- 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.
Eq: Material Dispersion Coefficient
Eq: Bit-rate distance product
B·L = P/(Δλ|D|) ≈ 1/Δλ|D|
What does the Bit-rate distance product tell us?
Describes max bt-rate that can be transmitted over an optical fibre with a given dispersion parameter
Receiver for digital signals: Diagram
- 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
What is Shot noise?
- 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
Eq: Shot Noise
- Poisson Process
- 𝑃(𝑘 | 𝑁) = 𝑁𝑘 𝑒−𝑁 / 𝑘!
- Consider I(t) = I– + is(t)
- where I– is the average current arising from photo-generated carriers
- is(t) is the current due to shot noise
- Shot noise variance
- 〈𝑖𝑠ℎ2 〉 = 2𝑒I–B
What is thermal noise?
- 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
Eq: Thermal Noise
- I(t) = I– + is(t) + ith(t)
- Noise Variance
- 〈𝑖<em>t</em>ℎ2〉 = 4𝑘𝑇𝐵 / 𝑅
Eq: System Noise
Eq: Photoreceiver circuit: Low impedance front end
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Eq: Photoreceiver circuit: High impedance front end
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Eq: Photoreceiver circuit: Transimpedance front end
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