Photonics Networks Assignment - Module Name: Photonics Networks

This assignment solution addresses key concepts in photonics networks, including the calculation of acceptance angles, the comparison of LEDs and laser diodes in fiber optic communication, and the concept of responsivity in photodiodes. It also explores quadrature amplitude modulation (QAM) and its application in encoding data, with a focus on a 16-QAM scheme. The solution further discusses the concept of multicasting in systems with multiple input ports. The assignment covers topics relevant to electrical engineering, particularly in the domain of optical fiber communication and signal modulation techniques. The solution provides detailed explanations, and includes relevant figures and references to support the answers, offering a comprehensive understanding of the subject matter.

Running head: PHOTONIC NETWORKS 1
Photonic Networks
Name
Institution

PHOTONICS NETWORKS 2
Photonic networks
Question 1
λ0=1.55 μm , v=2, Core radius a=4 μm, Index contrast ∆=0.003, and refractive of cladding
n2 =1.45, Parameter v=2
V = 2 π
λ0
a √n1
2−n2
2
But we know that sin ( θa ) = √ n1
2−n2
2
Therefore, V = 2 π
λ0
a sin ( θa )
sin (θa )= V λ0
2 πa = 2×1.55
2 π × 4 =0.12334 5
Thus, the acceptance angle, θa=sin−1 0.123345=7.085 °
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.

PHOTONIC NETWORKS 3
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. Responsivity , ρ= λ0
1.24 ɳ
Where, λ0 is the wavelength in micrometers and ɳ the quantum efficiency.
Responsivity ρ at 1.3 micrometers is ρ=1.3 ×0.8
1.24 =0.8387 A
W
Responsivity ρ at 1.55 micrometers is ρ=1.55 ×0.8
1.24 =1 A
W
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

PHOTONIC NETWORKS 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

PHOTONIC NETWORKS 5
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
http://readingrat.net/qam-modulator-block-diagram/quadrature-amplitude-modulation/