MUX Flashcards
What is a multiplexer?
A multiplexer is best defined as a combinational logic circuit that acts as a switcher for multiple inputs to a single common output line.
What is a demultiplexer?
The demultiplexer is a combinational logic circuit designed to switch one common input line to one of several seperate output line. It is the opposite of a multiplexer
What is an optical multiplexer?
An optical multiplexer is a device that combines wavelengths travelling on separate channels into a single channel
What is WDM?
WDM or Wavelength Division Multiplexing is technology that uses a multiplexer to combine wavelengths
travelling on separate channels into a single channel. At the receiver end of the link, a demultiplexer separates the wavelengths and routes them into different channels.
Each channel of different wavelengths/frequencies carries its own signals without interfering with the other channels when the same waveguide/fibres are shared for transmission.
§ Transmission system’s capacitance can be significantly expanded depending on the number of channels in the WDM system.
What is CWDM?
CWDM or coarse wavelength is a WDM technology using wide channel spacing typically Δλ = 20nm.
CWDM systems can realise comparatively cost-effective applications with relaxed wavelength tolerances. A variation of one-third of Δλ is expected for the wavelength to be acceptable to be acceptable
This method is comparatively less sensitive than DWDM systems.
Used in transport networks for a wide range of services.
What is DWDM?
DWDM or dense wavelength division multiplexing refers to signals multiplexed within the EDFA effective band. Typically either in the C band of ( 1525-1565nm) or the L band (1570-1610nm). The channel spacing is typically around 0.8nm, corresponding with a frequency spacing of 100GHZ.
This allows a greater capacity compared to CWDM systems, however, imposes strict precision requirements on the laser transmitter and multiplexer.
What devices are used for WDM?
Arrayed Waveguide Grating(AWG), Planar Concave Grating(PCG), Ring Resonator(RR), Mach-Zender Interferometer, Angled Multimode Interferometer(AMMI)
PCG vs AWG
Both are based on interfering and focusing,
PCG are more compact, but AWG has more channel spacing.
PCGs experience more cross talk as input and output waveguides share the same interface.
The number of channels in a PCG is less than for AWG
• Difficult to obtain high order diffraction
devices in compact PCG
What is a Rowland circle?
A circle, which is virtually drawn tangent to the centre point of a spherical concave grating with a diameter equal to the radius of curvature of the grating. When a slit is placed at an arbitrary point on the “Rowland circle” and is illuminated with light.
Describe an echelle grating?
• Grating curve on a circle of radius 2R
• I/O waveguides on a circle of radius R
• I/O waveguides point to pole (tangent point
of the two circles)
Input angle, θi, and diffraction order, m, are properly chosen so that::
• Channel resolution is high enough.
• Channel uniformity is good.
• Device footprint is reasonably small.
n (sinθi + sinθd)d = mλ
n – effective refractive index
λ – wavelength in vacuum
d – the distance in the y-direction between
adjacent constructive interference points
m – diffraction order
More or less facets?
As more facets are added the insertion loss decreases as more light is deflected through the grating.
Grating Wall Vs Loss
The verticality of a grating wall has a significant effect on the loss, there are three common types of grating wall, as pictured in the figure.
DBR is the most efficient however, it is also the most difficult to fabricate.
How does temperature affect these devices?
The temperature causes these devices to experience a shift in refractive index in accordance with the thermo-optic effect.
Thermo-optic coefficient of Si:
dn/dT = 1.84X10-4 K-1
How does an AWG work?
AWG comprises the input waveguide, the input star coupler, the arrayed waveguides, the output star coupler, the output waveguide array. The star coupler is also called the free propagation region (FPR).
In an AWG, the light that carries multiwavelength signals is incident
from the input waveguide into the input star coupler. In the input star
coupler, each light beam expands and splits into the arrayed
waveguides. The arrayed waveguides are designed with a constant
path length difference of ΔL between the neighbouring waveguides.
Thus, the light wave in each of the arrayed waveguides is delayed by
different times but with a fixed differential time delay between the
neighbouring waveguides. At the output star coupler, multiple light
beams exiting from the arrayed waveguides at a certain wavelength
interfere with each other at the output waveguide interface, resulting
in light constructive or destructive interference. Light at different
wavelengths is focused on different locations at the output facet
connecting to the output waveguides. As the phase delay determines
the light interference and thus the focus point locations, the arrayed waveguides, and the star couplers require critical design to attain thegrating functions.
What is an AMMI?
The device consists of tilted input/output waveguides of width WIO, in conjunction with a multimode waveguide of width WAMMI. The input waveguide is tilted with respect to the multimode waveguide axis by an inclination angle θ, and is laterally offset from it such that the side wall of the input waveguide coincides with that of the multimode guide. An array of tilted output waveguides is arranged on the opposite side-wall of the multimode waveguide with the same inclination angles θ, to the multimode waveguide axis. The axial position of the output waveguides (Li) is designed to match the inverted self-imaging points for different wavelengths in the input field.
Comparison of WDM structures
