2 光源 (Optical Sources) 與檢光器 (Photodetectors) 重要參數量測. LED’s and lasers are important optical sources for the electro-optical transformation in fiber optic communications. They are essential for optical transmitters .
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LED sources are low cost.
LED sources can be used for short-distance fiber-optic systems.
Type LED’s are:
2.1.1 Surface-Emitting LED (SLED)
2.1.2 Edge-Emitting LED (EELED)
Fig 2.1. Side view of a surface-emitting LED
: AR(Antireflection) Coating
: Optical gain
: Optical absorption
Fig 2.2. Side-view of an edge-emitting LED (EELED)
: Current Input & Output Layer
: Gain Region & Optical Waveguide
Fig 2.3 Cross-section of a
Fabry-Perot laser diode
Fig 2.4 Light vs. Current characteristic
Fig 2.5 The gain, mirror loss,and longitudinal mode location.
Fig 2.6 Cross-section of a
Amplifier Gain (dB)
Fig 2.7 Bragg grating reflectivity,
amplifier gain,and longitudinal
Fig 2.8 Cross-section view of a
Amplifier Gain (dB)
Fig 2.9 Mirror loss,amplifier
gain,and longitudinal mode
[from “Fiber Optic Test and Measurement”, edited by D. Derickson]
Fig 2.10 p-i-n photodetector diagram
Fig 2.11 APD diagram with
separate absorption and
The APD use a low-bandgap semiconductor absorption region.
The photo-generated electrons are accelerated to high velocities and lead to avalanche process.
The high energy electrons collide with the lattice to generate new free carriers in avalanche processes.
The ideal low-noise APD multiplies the number of electrons but does not multiply holes in the avalanche process.
The APD require a high-bias voltage to produce avalanche conditions.
The avalanche process is also temperature-dependent.
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