Optical Communications Multiple Choice Questions on “LED Structures”.
1. The amount of radiance in planer type of LED structures is ____________
a) Low
b) High
c) Zero
d) Negligible
Answer: a
Explanation: Planer LEDs are fabricated using liquid or vapor phase epitaxial processes. Here p-type is diffused into n-type substrate which creates junction. Forward current flow through junction provides Lambertian spontaneous emission. Thus, device emits light from all surfaces. However a limited amount of light escapes the structure due to total internal reflection thus providing low radiance.
2. In optical fiber communication _____________ major types of LED structures are used.
a) 2
b) 4
c) 6
d) 3
Answer: c
Explanation: Optical fiber communication involves the use of 6 different major LED structure. These are the surface emitter, edge emitter, the super luminescent, the resonant cavity LED, planar LEDs and Dome LEDs.
3. As compared to planar LED structure, Dome LEDs have ______________ External power efficiency ___________ effective emission area and _____________ radiance.
a) Greater, lesser, reduced
b) Higher, greater, reduced
c) Higher, lesser, increased
d) Greater, greater, increased
Answer: b
Explanation: In Dome LEDs, the diameter of dome is selected so as to maximum the internal emission reaching surface within critical angle of GaAs. Thus, dome LEDs have high external power efficiency. The geometry of Dome LEDs is such that dome is much larger than active recombination area, so it has greater emission era and reduced of radiance.
4. The techniques by Burros and Dawson in reference to homo structure device is to use an etched well in GaAs structure.
a) True
b) False
Answer: a
Explanation: Burros and Dawson provided a technique to restrict emission to small active region within device thus providing high radiance. Etched well in a GaAs substrate is used to prevent heavy absorption of emitted region and physically accommodating the fiber. These structures provide low thermal impedance allowing high current densities of high radiance.
5. In surface emitter LEDs, more advantage can be obtained by using ____________
a) BH structures
b) QC structures
c) DH structures
d) Gain-guided structure
Answer: c
Explanation: DH structures provide high efficiency from electrical and optical confinement. Along with efficiency, they provide less absorption of emitted radiation.
6. Internal absorption in DH surface emitter Burros type LEDs is ____________
a) Cannot be determined
b) Negligible
c) High
d) Very low
Answer: d
Explanation: The larger band gap confining layers and the reflection coefficient at the back crystal space is high in DH surface emitter Burros type LEDs. This provides good forward radiance. Thus these structure LEDs have very less internal absorption.
7. DH surface emitter generally give ____________
a) More coupled optical power
b) Less coupled optical power
c) Low current densities
d) Low radiance emission into-fiber
Answer: a
Explanation: The optical power coupled into a fiber depends on distance, alignment between emission area and fiber, SLED emission pattern and medium between emitting area and fiber. All these parameters if considered, reduces refractive index mismatch and increases external power efficiency thus providing more coupled optical power.
8. A DH surface emitter LED has an emission area diameter of 60μm. Determine emission area of source.
a) 1.534*10-6
b) 5.423*10-3
c) 3.564*10-2
d) 2.826*10-9