Optical Communications Questions and Answers – LED Structures


This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “LED Structures”.

1. The amount of radiance in planer type of LED structures is ____________
a) Low
b) High
c) Zero
d) Negligible
View Answer

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
View Answer

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
View Answer

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
View Answer

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
View Answer

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
View Answer

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
View Answer

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
View Answer

Answer: d
Explanation: The emission area A of source is given by
A = π(30*10-6) 2= 2.826*10-9cm2.

9. Estimate optical power coupled into fiber of DH SLED having emission area of 1.96*10-5, radiance of 40 W/rcm2, numerical aperture of 0.2 and Fresnel reflection coefficient of 0.03 at index matched fiber surface.
a) 5.459*10-5
b) 1.784*10-3
c) 3.478*102
d) 9.551*10-5
View Answer

Answer: d
Explanation: The optical power coupler in the step index fiber of SLED is given by
Pc = π(1-r) A RD(NA) 2
= 3.14 (1-0.03)*1.96*10-5*40*(0.2) 2
= 9.551*10-5W.

10. In a multimode fiber, much of light coupled in the fiber from an LED is ____________
a) Increased
b) Reduced
c) Lost
d) Unaffected
View Answer

Answer: c
Explanation: Optical power from an incoherent source is initially coupled into large angle rays falling within acceptance angle of fiber but have more energy than Meridional rays. Energy from these rays goes into the cladding and thus may be lost.

11. Determine the overall power conversion efficiency of lens coupled SLED having forward current of 20 mA and forward voltage of 2 V with 170 μWof optical power launched into multimode step index fiber.
a) 1.256*10-5
b) 4.417*102
c) 4.25*10-3
d) 2.14*10-3
View Answer

Answer: c
Explanation: The overall power conversion efficiency is determined by
η pc = Pc/P = 170*10-6/20*10-3*2
= 4.25*10-3.

12. The overall power conversion efficiency of electrical lens coupled LED is 0.8% and power applied 0.0375 V. Determine optical power launched into fiber.
a) 0.03
b) 0.05
c) 0.3
d) 0.01
View Answer

Answer: a
Explanation: Optical power launched can be computed by
η pc = Pc/P
Pc = η pc* P
= 0.8 * 0.0375
= 0.03.

13. Mesa structured SLEDs are used ____________
a) To reduce radiance
b) To increase radiance
c) To reduce current spreading
d) To increase current spreading
View Answer

Answer: c
Explanation: The planar structures of Burros-type LED allow lateral current spreading specially for contact diameters less than 25 μm.This results in reduced current density and effective emission area greater than contact area. This technique to reduce current spreading in very small devices is Mesa structured SLEDs.

14. The InGaAsP is emitting LEDs are realized in terms of restricted are ____________
a) Length strip geometry
b) Radiance
c) Current spreading
d) Coupled optical power
View Answer

Answer: a
Explanation: The short striped structure of these LEDs around 100 μmimproves the external efficiency of LEDs by reducing internal absorption of carriers. These are also called truncated strip E-LEDs.

15. The active layer of E-LED is heavily doped with ____________
a) Zn
b) Eu
c) Cu
d) Sn
View Answer

Answer: a
Explanation: Zn doping reduces the minority carrier lifetime. Thus this improves the device modulation bandwidth hence active layer is doped in Zn in E-LEDs.

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