8th Sem Subject Ofc 8th chapter notes by Lohith kumar 11GUEE6018

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.

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

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 :

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.

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:

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:

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 :

Link Power Budgeted :

Rise Time Budgeted :

Analog links:

C/N ration analysis :

Concepts and components used for all optical for all optical Networks.

Available in page no 383

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