Coaching Materials in Fiber Optics Part 2 | ECE Board Exam

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(Last Updated On: February 10, 2020)

Coaching Materials in Fiber Optics

This is the Coaching materials in Fiber Optics Part 2 as one topic in ECE Board Exam taken from various sources including but not limited to past Board Examination Questions in Electronic System and Technologies (EST), Communications Books, Journals and other Communications References. This particular Coaching Notes in Communications Engineering has random Questions and Answers in random topics. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.

Fiber Optics Coaching Materials Part 2


73. Semiconductor LEDs emit incoherent light. Define incoherent light.

Light waves that lack a fixed-phase relationship

74. What are the two most common semiconductor materials used in electronic and electro-optic devices?

Silicon and gallium arsenide

75. Describe the stimulated emission.

A photon initially produced by a spontaneous emission in the active region interacts with the laser material to produce additional photons

76. What are the three basic LED types?

Surface-emitting LEDs (SLEDs) edge-emitting LEDs (ELEDs) and super luminescent diodes (SLDs)

77. Which types of LEDs are the preferred optical sources for short-distance, low-data-rate fiber optic systems?


78. What are facets?

Cut or polished surfaces at each end of the narrow active region of an ELED

79. What is the lowest current at which stimulated emission exceeds spontaneous emission in a semiconductor laser called?

Threshold current

80. Which type of optical source usually lacks reflective facets and in some cases is designed to suppress reflections back into the active region?


81. How does the source drive circuit intensity modulate the source?

By varying the current through the source

82. What is a prebias?

A current applied in the laser off state just less than the threshold current

83. What are the two types of output interfaces for fiber optic transmitters?

Optical connectors and optical fiber pigtails

84. What type of source is typically used in low-data-rate digital applications?


85. Why would a laser diode be used in a low-data-rate digital application?

When extremely high transmitter output powers are required

86. What type of source is generally used in high-data-rate digital applications?

Laser diode


87. Which performance parameter is the minimum amount of optical power required to achieve a specific bit-error rate (BER) in digital systems or a given signal-to-noise ratio (SNR) in analog systems?

Receiver sensitivity

88. List the two principal optical detectors used in fiber optic systems.

The semiconductor positive-intrinsic-negative (PIN) photodiode and avalanche photodiode (APD)

89. What are the four most common materials used in semiconductor detector fabrication?

Silicon, gallium arsenide, germanium and indium phosphide

90. What is a photocurrent?

The current produced when photons are incident on the detector active area

91. Define responsivity.

The ratio of the optical detector’s output photocurrent in amperes to the incident optical power in watts

92. How are PIN photodiodes usually biased?


93. What is the dark current?

The leakage current that continues to flow through a photodetector when there is no incident light

94. Will dark current increase or decrease as the temperature of the photodiode increases?


95. Should the capacitance of the photodetector be kept small or large to prevent the RC time constant from limiting the response time?


96. Trade-offs between competing effects are necessary for high speed response. Which competing effect (fast transit time, low capacitance, or high quantum efficiency) requires a thin active area?

Fast Transit time

97. Why is detector saturation not generally a problem in fiber optic communications systems?

Because fiber optic communications systems operate at low optical power levels

98. How can the gain of an APD be increased?

By increasing the reverse-bias voltage

99. List the key operational parameters used to define receiver performance.

Receiver sensitivity, bandwidth, and dynamic range

100. List the main types of receiver noise.

Thermal noise, dark current noise, and quantum noise

101. What is the main factor that determines receiver sensitivity?


102. For a reduction in thermal noise, should the value of the detector’s load resistor be increased or decreased?


103. What are two types of noise that manifest themselves as shot noise?

Dark current and quantum noise

104. What are the two basic types of preamplifiers used in fiber optic receivers?

The high-impedance amplifier and the transimpedance amplifier

105. Which preamplifier design (high-impedance or transimpedance) provides improvements in bandwidth and greater dynamic range with some degradation in sensitivity from an increase in noise?


106. For what types of applications are APDs generally used?

For high-data-rate applications and for low- or moderate-data-rate applications where receivers with extremely low sensitivities are required

107. What type of modulation do most analog fiber optic communications systems use?

Intensity modulation

108. What two analyses are performed to determine if a link design is viable?

Power budget and rise time budget

109. Optical fibers or cables should never be bent at a radius of curvature smaller than a certain value. Identify this radius of curvature.

Minimum bend radius

110. In fiber optics, the main disadvantage of plastic over glass fiber is

high attenuation

111. The wavelength of visible spectrum is within the range of

0.4 to 0.8 microns

112. Range of frequency used for fiber optic (FO) system is

100 – 1000 THz

113. The bending of light rays due to change in velocity as a result of traveling from one medium to the other.


114. What is the infrared range used for fiber optics in Angstrom?

7,000 to 12,000

115. In fiber optic system, the core of PCs fiber is


116. What is the primary specification of a fiber cable usually expressed as the loss in dB/km?


117. What is the most widely used light generator in fiber optic system?

Injection laser diode

118. A more widely used and most sensitive photosensor is _____.

Avalanche photodiode

119. Attenuation null is fiber optic occurs at what wavelength?

1.3 microns

120. 1 angstrom is equal to how many microns?

0.0001 microns

121. 1 angstrom is equal to how many nanometers?

10 ns

122. A measure of quality of a fiber optic system.

Maximum distance between repeaters

123. The average maximum distance between repeaters in a fiber optic system.

10 – 30 km

124. Fiber optics performance is usually indicated by

product of bit rate and distance

125. The external incident angle for which light will propagate in the fiber is known as _____.

Acceptance angle

126. In fiber optics, the dominant loss mechanisms in silica fiber are _____.

Absorption and Rayleigh Scattering Loss

127. Rayleigh Scattering Loss at 8020 nm has a typical value of _____.

2.5 dB

128. Loss due to valence electron:

UV absorption

129. Laser used in fiber optic communications.

Semiconductor laser

130. What is the maximum data rate for fiber optics?

10 Gbps

131. What is the typical bandwidth of the single-mode step-index fiber?

50 to 100 GHz/km

132. What is the achievable rate of single-mode step-index fiber as used in digital communication?

2 Gigabytes

133. What is the typical margin of safety in dB used in preparing the power budget for fiber-optic system?

5 – 10 dB


134. A fiber has an index of refraction of 1.6 for the core and 1.4 for the cladding. Calculate: (a) critical angle (b) angle of refraction for angle of incidence of 30 degrees (c) angle of refraction for angle of incidence of 70 degrees.

(a) 61 degrees (b) 34.8 degrees (c) 70 degrees

Coaching Materials in Fiber Optics Part 2 | ECE Board Exam

135. Calculate the numerical aperture and the maximum angle of acceptance for the fiber that has an index of refraction of 1.6 for the core and 1.4 for the cladding.

N.A. = 0.775 / 50.8 degrees

Coaching Materials in Fiber Optics Part 2 | ECE Board Exam

136. A single-mode fiber has a numerical aperture of 0.15. what is the maximum core diameter it could have for use with infrared light with a wavelength of 820 nm?

4.2 um

Coaching Materials in Fiber Optics Part 2 | ECE Board Exam

137. An optical fiber has a bandwidth-distance product of 500 MHz-km. if a bandwidth of 85 MHz is required for a particular mode of transmission, what is the maximum distance that can be used between repeaters?

5.88 km

Coaching Materials in Fiber Optics Part 2 | ECE Board Exam

138. The fiber has zero dispersion at a wavelength of 1310 nm and has a zero-dispersion slope of 0.05 ps (nm2-km). Calculate the total dispersion of 50 km of this fiber when it is used with a source having a line width of 2 nm at a wavelength of 1550 nm.

949 ps

Coaching Materials in Fiber Optics Part 2 | ECE Board Exam

139. Find the bandwidth and bandwidth distance product for the fiber with total dispersion of 949 ps and a total length of 50 km.

B = 526.8 MHz, Bandwidth-Distance product = 26.3 GHz-km

Coaching Materials in Fiber Optics Part 2 | ECE Board Exam

140. Find the energy in electronvolts, in one photon at a wavelength of 1 um.

E = 1.24 eV

Coaching Materials in Fiber Optics Part 2 | ECE Board Exam

141. A typical photodiode has an input optical power of 500 nW. Calculate the diode current.

150 nA

Coaching Materials in Fiber Optics Part 2 | ECE Board Exam

142. A fiber optic link extends for 40 km. The laser diode emitter has an output power of 1.5 mW, and the receiver requires a signal strength of -25 dBm for a satisfactory signal-to-noise ratio. The fiber is available in lengths of 2.5 km and can be spliced with a loss of 0.25 dB per slice. The fiber has a loss of 0.3 dB/km. The total of all the connector losses at the two ends is 4 dB. Calculate the available system margin.

7.01 dB

143. A 45 km length of fiber must not lengthen pulses by more than 100 ns. Find the maximum permissible value for the pulse spreading contrast.

2.22 ns/km

144. A fiber is rated as having a bandwidth-distance product of 500 MHz-km. Find its dispersion in ns/km, and the rise time of a pulse in a 5 km length of this cable.

5 ns

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