06-11-2012, 12:09 PM
Optical Communications Unit Question & Answer
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1. Discuss about the choice of different components in designing an optical
fiber link. Discuss the shortcomings of decency of each component.
Ans:
The system designer has many choices when selecting components for an optical fiber
system. The major components choices are,
(i) Optical Fiber Type and Parameters
Multimode or single mode, size, refractive index, attenuation, dispersion,
mode coupling, strength, joints etc.
(ii) Source Type
Laser or LED, optical power launched into the fiber, rise and fall time,
stability etc.,
(iii) Transmitter Configuration
Design for digital or analog, input impedance, supply voltage, dynamic
range, feedback etc.
(iv) Detector Type and Characteristics
p-n, p-i-n or avalanche photodiode, response time, active diameter, bias
voltage, dark current etc.
(v) Receiver Configuration
Preamplifier design, BER, SNR, range etc.
(vi) Modulation and Coding
Source intensity modulation, pulse frequency modulation, PWM and PPM
transmission.
(vii) Digital transmission or analog transmission such as biphase scheme and FM
respectively. These decisions will be taken depending on the system performance,
ready availability of suitable components and cost.
2. Discuss system considerations in point-to-point optical link.
Ans:
Point-to-Point Link
It has transmitter at one end and the receiver at the other end. The components must be
carefully chosen to ensure the desired performance level and can be maintained for the expected
system life time.
Figure represents the block diagram of a simplex point-to-point link. The three major
optical links building blocks are,
(i) Transmitter
(ii) Receiver and
(iii) Optical fiber.
The characteristics of transmitter and receiver are given the first priority and then the
characteristic of optical fiber is considered to meet the system performance requirements.
Optical sources (such as LED or LASER) are used based on the following characteristics.
(i) Signal dispersion
(ii) Data rate
(iii) Transmission distance and cost.
The characteristics of photo detector such as,
(i) Responsivity
(ii) Operating wavelength
(iii) Speed and
(iv) Sensitivity.
3. Give an account of fiber optic link power budget with an example.
Ans:
The power budget determines, whether the fiber optic link meets the attenuation
requirements or amplifiers are needed to boost the power level. It also determines the power
margin between the optical transmitter output and the minimum receiver sensitivity needed to
establish a specific Bit Error Rate (BER).
Figure shows the power loss model for a point-to-point link. In addition to losses, a link
power margin is provided in the analysis to allow for component aging, temperature fluctuation
and losses arising from components that might be added in future.