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
OPTICAL SOURCES AND FIBER OPTIC TRANSMITTERS
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?
SLEDs and ELEDs
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?
LED
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?
LED
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
OPTICAL DETECTORS AND FIBE OPTIC RECEIVERS
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?
Reverse-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?
Increase
95. Should the capacitance of the photodetector be kept small or large to prevent the RC time constant from limiting the response time?
Small
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?
Noise
102. For a reduction in thermal noise, should the value of the detector’s load resistor be increased or decreased?
Increased
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?
Transimpedance
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.
Refraction
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
Glass
116. What is the primary specification of a fiber cable usually expressed as the loss in dB/km?
Attenuation
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
FIBER OPTIC PROBLEMS
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
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
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
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
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
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
140. Find the energy in electronvolts, in one photon at a wavelength of 1 um.
E = 1.24 eV
141. A typical photodiode has an input optical power of 500 nW. Calculate the diode current.
150 nA
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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