This document discusses fiber optic communication links and components. It covers regenerative repeaters for digital links, including signal processing to amplify, reshape, retime and retransmit signals. It also discusses link power and rise time budget analysis for digital links, and C/N ratio analysis for analog links. Additionally, it covers wavelength division multiplexing (WDM) and dense WDM (DWDM), explaining their operational principles and how they expand network capacity. Coherent detection schemes like homodyne and heterodyne are also summarized.

1. Part B | Chapter 4 | Fiber Optic links :
Regenerative regenerators, Digital Links-link power and rise time budget analysis.
Analog links, C/N ration analysis. Coherent link – Homodyne and heterodyne
detection schemes. WDM operational principle of WDM. DWDM. Concepts and
components used for all optical for all optical Networks.
Regenerative Repeaters:
A repeater is a device in which signal processing is performed, i.e., received digital
signals are amplified, reshaped, retimed, and retransmitted. In other words, these are
used over digital links (transmission systems). The block diagram for the same is as
below:
The attenuated and dispersed optical pulse train is detected and amplified in the
receiver unit. This consists of a photodiode followed by a low-noise preamplifier. The
electrical signal thus acquired is given a further increase in power level in a main
amplifier prior to reshaping in order to compensate for the transfer characteristic of the
optical fiber (and the amplifier) using an equalizer.
Accurate timing (clock) information is then obtained from the amplified and equalized
waveform using a timing extraction circuit such as a phase-locked loop (PLL). This
enables precise operation of the following regenerator circuit within the bit intervals of
the original pulse train. The function of the regenerator circuit is to reconstitute the
originally transmitted pulse train, ideally without error. The pulse train is sampled at a
regular frequency equal to the bit rate, and at each sample instant a decision is made of
the most probable symbol being transmitted. The signal at the moment is in electrical
form and needs to be converted to optical form before it is fed back into the optical
fiber. This is taken care of using the drive circuit for the optical source which is either an
LED or a LASER depending on the kind of input that was present in the optical fiber.

2. This is an advantage over the analog transmission systems where the concept of
regenerative repeaters doesn’t exist.
Digital Links – link power and rise time budget analysis :
Solution is available in page no 323 to 331
Analog link
Solution is available in page no 359
C/N Ration analysis
Solution is available in page no 360
WDM optional principle is available in 4 sheet notes
In fiber-optic communications, wavelength-division multiplexing (WDM) is a
technology which multiplexes a number of optical carrier signals onto a single optical
fiber by using different wavelengths (i.e., colors) of laser light. This technique
enables bidirectionalcommunications over one strand of fiber, as well as multiplication
of capacity.
The term wavelength-division multiplexing is commonly applied to an optical carrier
(which is typically described by its wavelength), whereas frequency-division
multiplexing typically applies to a radio carrier (which is more often described
by frequency). Sincewavelength and frequency are tied together through a simple directly

3. inverse relationship, in which the product of frequency and wavelength equals c (the
propagation speed of light), the two terms actually describe the same concept.
A WDM system uses a multiplexer at the transmitter to join the signals together, and
a demultiplexer at thereceiver to split them apart. With the right type of fiber it is
possible to have a device that does both simultaneously, and can function as an optical
add-drop multiplexer. The optical filtering devices used have conventionally
been etalons (stable solid-state single-frequency Fabry–Pérot interferometers in the form
of thin-film-coated optical glass).
The concept was first published in 1978, and by 1980 WDM systems were being realized
in the laboratory. The first WDM systems combined only two signals. Modern systems
can handle up to 160 signals and can thus expand a basic 10 Gbit/s system over a single
fiber pair to over 1.6 Tbit/s.
WDM systems are popular with telecommunications companies because they allow
them to expand the capacity of the network without laying more fiber. By using WDM
and optical amplifiers, they can accommodate several generations of technology
development in their optical infrastructure without having to overhaul the backbone
network. Capacity of a given link can be expanded simply by upgrading the
multiplexers and demultiplexers at each end.
DWDM :

4. Dense wavelength division multiplexing (DWDM) is a fiber-optic transmission
technique that employs light wavelengths to transmit data parallel-by-bit or serial-by-
character
How does DWDM fair better :
► No O-E-O required
► Protocol & Bit Rate independence
► Increased overall capacity at much lower cost
 Current fiber plant investment can be optimized by a factor of at least 32
► Transparency
 Physical layer architecture  supports both TDM and data formats such
as ATM, Gigabit Ethernet, etc.
► Scalability
 Utilize abundance of dark fibers in metropolitan areas and enterprise
networks
Coherent link-Homodyne and Heterodyne detection schemes:
Coherent detection schemes are used in optical fiber communication as the direct
detection schemes employed (PIN Photodetector) varies only with the intensity (Power)
of the optical signal thus hindering the sensitivity and bandwidth capabilities of the
detector. The onus in direct detection scheme was to consider only the power level of
the optical signal ignoring the phase and frequency parameters by modulating the light
intensity with the input electrical signal voltage.
However in the coherent detection mode, the optical signal can be modulated based
frequency, amplitude or phase improving the sensitivity of the detector and enhancing
the bandwidth capabilities of the optical signal. This method along with the advantages
brings to the fore the need to have tuned semiconductor laser to produce coherent
output with as less spectral width as possible capable of being used in a single mode
optical fiber, a proper coding (NRZ, RZ, Manchester) method for the data to be
impressed upon the optical signal.

5. Block of optical coherent system of modulation and detection scheme:
In case of the modulation of the transmitting optical signal, the optical signal is
modulated either by ASK, PSK or FSK. In direct detection scheme, the signal is
amplitude modulated based on the electrical signal input which carries the information
and hence the detection is proportional to the intensity of the optical signal. However in
case of the coherent detection scheme, the incoming optical signal at the receiver end is
mixed with a wave generated by a local oscillator and this is done before the detection
by the photo-detectors take place and on the surface of the photo-detector.
If the frequency of the signal carrier and that of the local oscillator is same, then the
system is said to be a homodyne coherent detection scheme. Thus either the PSK or
ASK can be used while transmission modulation is being taken care as FSK is ruled out
here. Thus the output is a function of the intensities of the local oscillator wave and the
source optical signal with the output taking the form, where the
phase difference between source optical signal and that of the local oscillator is given by
and Ps, Plo are the intensities of the signals.
A sample block diagram for the homodyne coherent detection scheme is as below:

6. The advantage of the homodyne scheme is that the output frequency is brought to the
baseband level and no further electrical demodulation is necessary. Thus it is a highly
sensitive detection system along with providing high bandwidth capabilities. However
to manage the same frequency between the source and the local oscillator waves, it
becomes a difficult requirement. Also since the phase of the two waves is locked, a
requirement for the PLL is also necessary.
If the frequency of the signal carrier and that of the local oscillator is different, then the
system is said to be a heterodyne coherent detection scheme. Thus the phase
difference between the source and local oscillator signal becomes important and the
final output looks like this: .
A sample block diagram of the heterodyne coherent detection scheme is as below:

7. Though it doesn’t introduce the difficulty of having the systems to produce the same
frequency for source and local oscillator signals, the sensitivity decreases and a -3Db
degradation is introduced.
Digital Links-link power and rise time budget analysis :

8. Link Power Budgeted :

10. Rise Time Budgeted :

13. Analog links:

14. C/N ration analysis :

15. Concepts and components used for all optical for all optical Networks.
Available in page no 383
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