ELG5381 Photonics Networks And Networking Management

1.Calculate the acceptance angle of a single-mode fibre at the wavelength 1.55 µm with the parameter 2, the core radius 4 µm, the fractional index contrast 0.003, and the refractive index of the cladding 1.45.

 

2.In earlier-generation optical communication networks, it was common to use LED as light sources. Give explanation why LEDs are not the best choice for fibre optics networks. What improvements did the laser diodes introduce to the network performance?

 

3.Calculate the responsivity of a p-i-n photodiode at 1.3 and 1.55 µm if the quantum efficiency is 80%. Why is the photodiode more responsive at 1.55 µm?

 

4.How can one encode 4 bits per symbol? Sketch the schematic of a transmitter for such multilevel modulation and explain what is the role of each component in the encoding process.

 

5.With many of the components, an individual port may function as an input port or an output port depending upon the direction of light propagation (e.g., consider passive combiners and passive splitters). If such a device allows multiple input ports to direct the same wavelength to an output port, then what feature does it also support?

Question 1

Question 2

LEDs are used in fiber optic communication networks. However, they are not the best choice because: LEDs emit incoherent light characterized by a broad spectrum since they generate light through spontaneous emission. As a result, the signal will be bound by chromatic dispersion thus limiting the distance over which LEDs based transmitters can transmit data.

Also, LED transmitters can only be coupled into multimode fiber because the light emitted by the LED is non-directional. Moreover, the LEDs are less efficient as a small percentage of the light can be useful to the fiber optic cable.

Due to the numerous limitations observed in LED transmitters, laser diode transmitters were introduced. It is worth noting that, unlike LED transmitters, Laser diode transmitters have higher output thus higher efficiency. Additionally, the output from a laser transmitter is more directional permitting the use of single mode fiber thus a higher level of coupling efficiency and longer transmission distance.

Furthermore, laser diode transmitters have coherent light output ensuring that light energy is transmitted in on a single frequency which limits modal dispersion. Although both LEDs and laser diodes can be modulated directly, the latter has higher data rates.

Question 3

The responsivity of a p-i-n photodiode refers to the ratio of the electrical output to the optical power input.

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

Responsivity  at 1.3 micrometers is

Responsivity  at 1.55 micrometers is

Higher responsivity is observed when the wavelength is 1.55 micrometers because responsivity is directly proportional to the wavelength. Therefore, we expect a higher responsivity at larger wavelengths as compared to smaller wavelengths.

Question 4

Quadrature amplitude modulation (QAM) can be used to encode multilevel modulation. In this case, a 16-QAM can be used to encode 4 bits per symbol as shown in the transmitter figure 1. The transmitter combines the functionality of both phase modulation and amplitude modulation schemes. The 16-QAM transmitter uses 16 constellation points on the constellation square.

Figure 1: The block diagram of a 16-QAM (Readinggrat, 2017)

The input data is divided into four channels as I, I’, Q, and Q’. In each of the channels, the bit rate is equal to a quarter of the input bit rate. That is, f/4.

“The 2-4 level converters then generate a 4-level Pulse amplitude modulation (PAM) signal which modulates the quadrature carriers and in-phase carriers in the product modulators.

The linear summer combines the outputs from the two channels (I and Q channels) producing the desired 16 output conditions.”

Question 5

Systems with multiple input ports that direct a wavelength to the output port can also permit multicasting. If the output port is used as the output port and the input ports used as the output ports, then the given light signal can be propagated to multiple destinations rather than one destination.

References

Readinggrat. (2017). Quadrature Amplitude Modulation [Photograph]. Retrieved from https://readingrat.net/qam-modulator-block-diagram/quadrature-amplitude-modulation/