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Fiber Optic Fundamentals
Published in David R. Goff, Kimberly Hansen, Michelle K. Stull, Fiber Optic Reference Guide, 2002
David R. Goff, Kimberly Hansen, Michelle K. Stull
Many characteristics are considered when making the decision to use LED's or LD's in fiber optic systems. Some basic considerations include center wavelength, spectral width (the full range of wavelengths around the center wavelength, often called the FWHM, Full Width Half Maximum), and optical output power of the light source. The spectral width is important as larger values bring with them an increased possibility of dispersion problems, limiting bandwidth. The optical output power of the light source is also important in that it must be neither too weak nor too strong. A weak source will not provide enough power to transmit a light signal through a usable length of optical fiber, while a source that is too strong could cause distortion of the signal by overloading the receiver. These and other light source characteristics will be explored at length in Chapter 5.
Electro-optic and Opto-electronic Devices
Published in David R. Goff, Kimberly Hansen, Michelle K. Stull, Fiber Optic Video Transmission, 2013
David R. Goff, Kimberly Hansen, Michelle K. Stull
Several key characteristics of LEDs and lasers determine their usefulness in a given application. These are: Peak Wavelength: The source emits the most power at its peak wavelength. It equates to the wavelengths transmitted with the least attenuation through optical fiber. Common peak wavelengths occur near 780 nm, 850 nm, 1310 nm, 1550 nm, and 1625 nm.Spectral Width: The emitted light spans a range of wavelengths, known as the spectral width, centered at the emitter's peak wavelength.Emission Pattern: A source's emission pattern directly influences the amount of light that can be coupled into the optical fiber. The size of the emitting region should correspond to the diameter of the fiber core.Power: The output power of the source must provide sufficient power to the detector at the receiving end after fiber attenuation, coupling losses and other system constraints have been taken into consideration. Lasers offer a higher output power than LEDs.Speed: A light source must turn on and off fast enough to meet the bandwidth limits of the system. The source's rise time or fall time, the time required to go from 10% to 90% of peak power, governs the speed of a system. Lasers have faster rise and fall times than LEDs.
Transmitters
Published in Lynne D. Green, Fiber Optic COMMUNICATIONS, 2019
Another key characteristic of the optical output is the wavelength spread over which the power is distributed. The spectral width, σλ, is the 3 dB optical power width (usually measured in nm or µm). The spectral width impacts the system bandwidth; a larger spectral width decreases the system bandwidth. In Figure 4-7, the optical power is the area under the curve and the spectral width is the half–power spread. A laser diode always has a smaller spectral width than an LED. The value of the spectral width depends on both device structure and semiconductor material. Typical values are around 40 nm for an LED operating at 850 nm and 80 nm at 1300 nm, and 1 nm for an LD at 850 nm and 3 nm at 1300 nm.
Element differentiation with a Hartmann- based X-ray phase imaging system
Published in Nondestructive Testing and Evaluation, 2022
Ombeline de La Rochefoucauld, Ginevra Begani Provinciali, Alessia Cedola, Philip K. Cook, Francesca Di Lillo, Guillaume Dovillaire, Fabrice Harms, Mourad Idir, Xavier Levecq, Laura Oudjedi, Tan-Binh Phan, Martin Piponnier, Giuliana Tromba, Philippe Zeitoun
The X-ray spectrum available at SYRMEP beamline expends over a very large set of energies from a few keV up to 50 keV. In order to reduce the spectral width, we placed a set of specially designed filters before the sample. A 2 mm beryllium filter is permanently installed on the beamline as a window between vacuum and air and had to be taken into account in the filter calculation. We present results obtained with four different homemade filters defined by their central energy (and bandwidth): ~16 keV +/1 keV, 17.6 keV ± 0.6 keV, 20.5 keV ± 3.2 keV and 22.6 keV ± 3.2 keV.