Thermal Emission and Temperature Measurement
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Questions and Answers

What is the purpose of controlling the indices of refraction in the core and cladding of an optical fiber?

To enable internal reflection and propagation of light along the length of the fiber.

What is the main difference between step-index and graded-index optical fibers?

The refractive index changes sharply in step-index fibers, whereas it varies gradually in graded-index fibers.

Why do light rays follow a helical path in graded-index fibers?

Due to the gradual variation in refractive index, which causes the light rays to bend and follow a curved path.

What is the advantage of using graded-index fibers over step-index fibers?

<p>Graded-index fibers reduce pulse broadening, while step-index fibers do not.</p> Signup and view all the answers

What type of fibers are both step-index and graded-index fibers classified as?

<p>Multi-mode fibers.</p> Signup and view all the answers

Study Notes

Thermal Emission

  • Thermal emission is a process where excited electrons release photons as they drop back to the ground state.
  • A continuous spectrum of radiation is emitted with a minimum wavelength, and the intensity distribution depends on the temperature.
  • Higher temperatures result in a wider range of emitted wavelengths.
  • By measuring the intensity of a narrow band of emitted wavelengths with a pyrometer, a material's temperature can be estimated.

Photoconductivity

  • Photoconductivity occurs when semiconductors are bombarded by photons with energy equal to or greater than the band gap, creating electron-hole pairs.
  • This process generates current, and the current produced is directly related to the incident light intensity.
  • Photoconductivity is utilized in photographic light meters, and cadmium sulfide (CdS) is commonly used for detecting visible light.
  • Photoconductivity is also the underlying principle of the photovoltaic cell, or solar cell, used for converting solar energy into electricity.

Optical Fibers

  • Optical fibers consist of a core, cladding, and coating, with the core transmitting signals and the cladding constraining the light beam to the core.
  • The outer coating protects the core and cladding from the external environment, and all reflection losses should be considered.
  • Optical fibers have revolutionized the communication industry.

Luminescence

  • Luminescence is the process where a material absorbs energy and then immediately emits visible or near-visible radiation.
  • It involves electron excitation and then dropping down to lower energy states.
  • If the emission of radiation occurs within 10^(-8) seconds after excitation, the luminescence is called fluorescence, and if it takes longer, it is known as phosphorescence.
  • Special materials called phosphors have the capability of absorbing high-energy radiation and spontaneously emitting lower-energy radiation.
  • The intensity of luminescence is given by the formula: I = I0 * e^(-t/τ), where I0 is the initial intensity of luminescence, I is the fraction of luminescence after time t, and τ is the relaxation time constant for a material.

Types of Optical Fibers

  • In step-index optical fibers, there is a sharp change in refractive index between the core and cladding, resulting in pulse broadening.
  • Graded-index fibers avoid pulse broadening by adding impurities such as boron oxide (B2O3) or germanium dioxide (GeO2) to the silica glass, varying the index of refraction in a parabolic manner across the cross-section.
  • Both step- and graded-index fibers are termed as multi-mode fibers.

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Description

This quiz covers the principles of thermal emission, including the continuous spectrum of radiation and how it relates to temperature. It also touches on measuring temperature using pyrometry.

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