ELG5381 Photonics Networks

Question:

1. Calculate the acceptance angle for single-mode fibres at wavelength 1.55 um. Use the parameter 2, the core radius 4. um, the fractional indice contrast 0.003, the refractive indice of the cladding 1.45.

2.In older-generation optical communication networks it was common to use LEDs as light sources.

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Please explain why LEDs may not be the best choice to power fibre optics networks.

What improvements have the laser diodes made to the network’s performance?

3.Calculate the responsivity at 1.3 or 1.55 um for a photodiode with p-i-2n quantum efficiency of 80%.

Why is 1.55 um more responsive for the photodiode?

4.How can 4 bits be encoded per symbol?

4.How can you encode 4 bits per symbol?

5. Depending on the direction light propagation is going, one port could function as an input port, or an output port.

What feature can such a device support if multiple input ports allow the same wavelength of light to be directed to one output port?

Answer:

Question 1

Question 2

LEDs are used for fiber optic communication networks.

But, LEDs are not the best choice. They emit incoherent and broad spectrum light.

This means that the signal will be bound to chromatic scattersion which limits the distance LEDs-based transmitters are able to transmit data.

LED transmitters are not able to be used in multimode fiber coupling because the LED emits non-directional light.

Additionally, LEDs are less efficient since only a small amount of light can be useful for the fiber optic cable.

Laser diode transmitters were developed to overcome the limitations in LED transmitters.

It is important to note that Laser diode transmitters have a higher output and therefore higher efficiency than LED transmitters.

Laser transmitters have a more directional output, which allows the use of single-mode fibers. This permits for higher coupling efficiency and a longer transmission distance.

Also, laser diode transmitters are coherent in light output. This ensures that light energy is only transmitted at one frequency. This reduces modal dispersion.

While both laser diodes as well as LEDs can be modulated, the latter have higher data rates.

Question 3

The responsivity for a p–i-n photodiode is the ratio between the electrical output and the optical power input.

Where, is the wavelength (in micrometers) and the quantum efficiency.

Responsivity is 1.3 micrometers

Responsivity is 1.55 micrometers

Higher responsivity results when the wavelength is 1.55 millimeters. This is because responsivity depends on the wavelength.

As such, larger wavelengths will have a higher responsivity than shorter wavelengths.

Question 4

To encode multilevel modulation, quadrature amplifier modulation (QAM), can be used.

As shown in the transmitter diagram, a 16-QAM is possible to encode 4 bits per symbols.

The transmitter can combine the functionality of both phase and amplitude modulation schemes.

The 16-QAM transmitter uses 16 constellations points on the constellation quad.

Figure 1: Block diagram of a 16-QAM (Readinggrat 2017)

The input data is split into four channels: I, I’ and Q.

The bit rate of each channel is equal to one-quarter of the input rate.

The bit rate is equal to f/4.

“The 2-4-level converters then produce a 4-level Pulse amplified modulation (PAM), signal, which modulates both quadrature carriers as well as in-phase carrier in the product modulators.

Linear summer combines outputs from both channels (I/Q channels), producing 16 desired output conditions.

Question 5

Multicasting is possible with systems that have multiple input ports. They can direct a wavelength from one port to another.

If both the output port and input ports are used as the outgoing ports, multicasting is possible.

References

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