30 Questions
What is the critical angle defined as?
A minimum value
Which term is commonly used to measure the magnitude of the acceptance angle?
Numerical aperture
What does numerical aperture describe in an optical fiber?
Ability to couple light into the cable from an external source
What does a larger magnitude of numerical aperture indicate about an optical fiber?
It will accept more external light
What do absorption losses in optical fibers result from?
Impurities absorbing light and converting it to heat
What are the factors that contribute to absorption losses in optical fibers?
Ultraviolet absorption, infrared absorption, and ion resonance absorption
What is the main cause of Rayleigh scattering loss in optical fibers?
Escape of light rays
How can chromatic distortion be eliminated in optical fibers?
Opting for an injection laser diode (ILD)
What is a common characteristic of microbends in optical fibers?
They contribute to Rayleigh scattering loss
Which type of bends in optical fibers are caused by differences in thermal contraction rates?
Microbends
Why do light rays emitted from an LED and propagated through an optical fiber experience chromatic distortion?
Different velocities while propagating in glass
In optical fibers, what contributes less than 20% to the total attenuation?
Microbending losses
What phenomenon occurs when each ray of light in a fiber reaches the far end at a different time?
Modal dispersion
In a single-mode step-index fiber, why is modal dispersion virtually eliminated?
Each ray of light travels the same distance in a given period of time
What type of fibers show multiple modes of light propagation?
Multimode graded-index fibers
Which device is commonly used as a light detector in fiber-optic communications systems?
PIN diodes
What is the basic construction of a PIN diode?
Depletion-layer photodiode
How does the refractive index affect the velocity of light rays in a fiber?
Inversely proportional
What is the Acceptance Angle or Acceptance Cone Half Angle?
The maximum angle at which external light rays may strike the air/fiber interface and still propagate down the fiber
What is Numerical Aperture (NA) used to measure?
The light gathering or light collecting ability of the optical fiber
Which of the following is a requirement for light sources used in optical transmission?
Produce nearly monochromatic light
What type of light sources are Light Emitting Diodes (LEDs)?
Low-cost, low-heat, non-coherent injection light sources
What makes Injection Laser Diodes (ILDs) stand out among light sources?
They make the most efficient use of electric energy
What is a key advantage of LEDs in terms of coupling power?
They can couple about 100uw of power with a 2% coupling efficiency
What can be a consequence of the imperfections in the mechanical interface used to house the light detector and attach it to the cable?
Introduction of losses to the system
What type of loss can be introduced due to splicing multiple sections of optical cable together?
Losses ranging from a couple tenths of a dB to several dB
In fiber optics, what can happen if an optical cable is bent at too large an angle?
Introduction of losses ranging from a few tenths of a dB to several dB
How do splices in optical cables impact the signal according to the text?
Signal loss introduction
What is the primary concern when an optical cable undergoes large angle bending?
Loss of total reflections leading to signal loss
What is the impact of imperfections in the mechanical interface on the light entering the detector according to the text?
Prevention of some power from entering the light detector
Study Notes
Acceptance Angle and Numerical Aperture
- Acceptance angle (θin) is the maximum angle at which external light rays can strike the air/fiber interface and still propagate down the fiber.
- Numerical aperture (NA) is a figure of merit that measures the light-gathering or light-collecting ability of an optical fiber.
- NA = sin θin = √(N1² - N2²)
Light Sources
- Light sources must have a wavelength between 300 microns and 0.5 microns.
- Requirements for light sources:
- Monochromatic light (single frequency)
- Capable of being easily modulated (PCM) for better noise immunity
- High intensity light output
- Small, compact, and easily coupled to fibers
- Inexpensive to manufacture
- Light Emitting Diodes (LEDs) are low-cost, low-heat, and reliable light sources.
- Injection Laser Diodes (ILDs) make efficient use of electric energy.
Optical Fiber Losses
- Absorption losses: impurities in the fiber absorb light and convert it to heat.
- Factors contributing to absorption losses:
- Ultraviolet absorption
- Infrared absorption
- Ion resonance absorption
- Rayleigh scattering loss: light rays that escape due to impurities in the fiber.
- Chromatic dispersion: different wavelengths of light travel at different velocities, causing chromatic distortion.
Radiation Losses
- Caused by small bends and kinks in the fiber.
- Types of bends:
- Microbends: miniature bends or geometric imperfections along the axis of the fiber.
- Constant-radius bends: bends with a constant radius.
Modal Dispersion
- Occurs in multimode fibers, where different rays of light take different paths.
- Results in a stretched pulse that is reduced in amplitude at the output of the fiber.
Light Detectors
- PIN diodes: depletion-layer photodiodes used to detect light energy in fiber-optic communications receivers.
- APDs: used to detect light energy in fiber-optic communications receivers.
Test your knowledge about the different types of losses that occur in optical fiber systems, such as absorption, scattering, connector loss, and splicing loss.
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