1. 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
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.
2. 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
Explanation: The emission area A of source is given by
A = π(30*10-6) 2= 2.826*10-9cm2.
3. 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
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.
4. In a multimode fiber, much of light coupled in the fiber from an LED is ____________
a) Increased
b) Reduced
c) Lost
d) Unaffected
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.
5. 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
Explanation: he overall power conversion efficiency is determined by
η pc = Pc/P = 170*10-6/20*10-3*2
= 4.25*10-3.
6. 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
Explanation: Optical power launched can be computed by
η pc = Pc/P
Pc = η pc* P
= 0.8 * 0.0375
= 0.03.
7. Mesa structured SLEDs are used ____________
a) To reduce radiance
b) To increase radiance
c) To reduce current spreading
d) To increase current spreading
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
8. 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
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
9. The active layer of E-LED is heavily doped with ____________
a) Zn
b) Eu
c) Cu
d) Sn
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.
10. Intrinsically _________________ are a very linear device.
a) Injection lasers
b) DH lasers
c) Gain-guided
d) LEDs
Explanation: The ideal light output power against current characteristics for an LED linear. This tends to be more suitable for analog transmission where several constraints are put in linearity of optical source.