Download 1. Which of the following statement are true about "LED life" term?

Technical University
Optoelectronics
Direction
In this test are provided multiple choice answers for the
questions.
1. Which of the following statement are true about "LED
life" term?
LEDs lose brightness (degrade) over time (operating
hours) at the same Iforward
LED life is defined as the time, in operating hours,
for an LED to reach ½ of its original brightness
Is a parameter which reflect long term product
performance (operating hours)
pulse modulated lighting is the reduction in the LEDs
power dissipation, and will decrease the LED life
LED life refers to time duration of the charge carriers
in the active region (lifetime of the carriers)
LED life increase if Iforward increase
2. Which are the main parameters of an LED that are affected
by temperature?
Luminous intensity (brightness)
LEDs are typically suitable for systems using monomode fibers
Can couple a more power in a fiber than any other
optical transmitter because of relatively small spectral
width and is considered a free-noise source
Can couple a small power in a fiber because of wide
beam divergence
Has a higher coupling efficiency to a mono-mode
fibers than lasers, which must improvement the
mechanism of coupling with complicated lens
arrangement
Lasers holds significant operational advantages over
the LED except for cost and temperature sensitivity
6. What can we do to modify the wavelength of the emitted
light for an optical transmitter?
we modify the structure of a given
semiconductor, or mix different ones
Using a fabry-perrot laser
We can use an external cavity laser (DBR)
We can use an distributed feedback laser (DFB)
power dissipation in p-n junction
Using a WDM (wavelength division
multiplexing)
We can use an VCSEL laser (vertical cavity
surface emitting lasers)
LED’s Material dispersion
In-fibre laser (with bragg gratings)
width of p-n junction
Double pumping laser
wavelength (hue or color)
With selective couplers, splitters and grattings
forward voltage
3. Numerical aperture is:
“cone” of acceptance of a ray enters the fiber
proportional with Sin of the largest angle of a ray
enters that will be captured and propagate as a bound
mode
proportional with Cos of the critical angle of a ray
enters that will be captured and propagate as a bound
mode
A measure related to the refractive index of the core
and cladding
Related to half angle of a ray’s “cone” of acceptance
4. Because of the operating properties of the LED:
LEDs are typically suitable for systems using
multimode fibers
LEDs are typically suitable for systems using monomode fibers
It can couples a more power in a fiber than any other
optical transmitter because of relatively small spectral
width and it is considered a free-noise source
It can couples a small power in a fiber because of
wide beam divergence
It has a higher coupling efficiency to a mono-mode
fiber than lasers. Laser must improve the mechanism
of coupling with complicated lens arrangement.
Lasers holds significant operational advantages over
the LED except for cost and temperature sensitivity
5. Which of the following statement are true for LEDs:
LEDs are typically suitable for systems using
multimode fibers
7. An optocoupler has to fulfill following essential
requirements:
Good isolation
High current transfer ratio (CTR)
Low degradation
Low coupling capacitance
No interference by field strength influences
Small commune mode rejection (CMR)
High speed which indicate small commutations time
and big slew-rate, LED prepolarisation
8. photodiode’s principle:
The principle of LED in reverse biased p-n junction
Emitted light directly supply from a electric current
In fotovoltaic mode, an external reverse bias is
applied to the photodiode and in fotoconductive
mode is un-biased
Has a fastest switching speed when operates in
photoconductive mode
9. An optical amplifiers is a device that:
Amplifies the optical signal based on some particular
amplification principle (SOA, REDFA, Raman etc.)
Used the principle of the laser in their applications,
using a pump for the input signal to the fiber
containing erbium/neodymium/praseodymium dopant
ASE is the single noise factor, than to decrease it, can
be used bi-directional pump, counter propagation,
different pump wavelength, place isolators to prevent
backward ASE propagation
It can be used as preamplifier, power amplifier, line
amplifier in an optical system
SRS (stimulated raman scattering) cause a stroke
wave, phenomena used in REDFA type amplification
10. the key principle of an circulator is like:
That of the isolator
Has no relation with the dispersion phenomena
16. A laser diode works like a LED:
for I applied on a p-n junction > Threshold
for I applied on a p-n junction < Threshold
Just for I applied on a p-n junction ~ Threshold
16. When the matter is perturbed by a photon, it loses energy
and the photon emitted has the same phase, frequency and
direction of travel as the original photon. This quantum
mechanical phenomenon is known as:
splitter
Y coupler
None of above
11. diffraction grating is a device that:
Reflects the light by an amount varying according to
the wavelength
Refracts the light by an amount varying according to
the wavelength
Has the main function of polarization of the light
Wavelength selection
Pulse compression with minimal chromatic dispersion
Has a birefrigerent material
12. Bragg diffraction grating is used in:
WDM systems
DBR laser
distributed feedback laser (DFB)
fabry-perrot laser
As a component of some specific optical amplifier
In a fiber section
As a coupler
as a filter
As a modulator
Spontaneous emission
Stimulated emission
Optical amplification
17. 3dB coupler is known as the:
4-port resonant coupler where input signal is
split in half at the outputs
4-port resonant coupler where light entering a
particular port on one side of the coupler exits
from each
of the two ports on the other side of the coupler,
attenuated with 3dB
4-port resonant coupler with an Optical
amplification of 3dB
18. the responsivity formula for a
photodiode is:
I  A
 P  
 I W 
I e
 P
 I EG
I C
e
e
 DM  DM
I C eCM
eCM
Mach-Zehnder interferometer
19. The eye-diagram indicates:
13. Light is confined within the core of a simple optical fiber
by
indicate the level difference between one-bits
and zero-bits
refraction
total internal reflection at the outer edge of the
cladding
Total internal reflection at the core cladding
boundary.
Reflection from the fiber's plastic coating.
the amount of jitter present
the lower will be the error rate at the receiver
20. Chromatic dispersion:
14. Graded index profile is related to:
Monomod optical fiber
Multimode optical fiber
Has no relation with the mode of ray propagation
15.
Multimode propagation of the ray along the fiber
Graded index profile
Reduce the modal dispersion
referred to the phenomena that
wavelength that will travel at a different
speed in the fiber
Is caused by the distance traveled by light
in each mode that is different from the
distance traveled in other modes and
some pulses arrived before the others
Is caused by the shape and index profile
of the fiber core.
21. The following picture referred to the:
Reduce chromatic dispersion
Page 2
continuous constant-amplitude output
26. Factors that need to be considered before lasers
are integrate in different types
of systems:
Step index multimode fiber
Graded index multimode fiber
Required wavelength
Graded index monomode fiber
Required wavelength stability (how
important are wander and chirp etc.)
Step index monomode fiber
Spectral width and linewidth
22. Modal noise is produced in the following stages:
Mode coupling
Required power output
Modulation rate required
Mode selective loss
when using lasers with multimode fiber
Puls shape
Data transfer protocol
Misalignment of two fiber tronson
Because of ASE
23. What is the Cutoff Wavelength?
Wavelengths shorter than the cutoff will travel in
single mode
whereas wavelengths longer than the cutoff will
travel in multiple modes
Wavelengths shorter than the cutoff will travel in
multiple modes
whereas wavelengths longer than the cutoff will
travel in a single mode
Wavelength for which the chromatic dispersion is
null
27. In a WDM system, demultiplexing the Light can be made
by using:
Bragg gratings
3 dB Splitter Array
Optical Switching nodes
Cascading filter
Optical splitter
Only Circulators
28. The following formula referred to
Wavelength for which loss is null
24. Dispersion Shifted Fiber
is a kind of fiber that operate in the 1550
nm band is done by manipulating the core
profile to introduce dispersion in
the opposite direction (with the opposite
sign) from the direction in which
chromatic dispersion operates
is a kind of fiber that operate in the 1370
nm band is done by manipulating the core
profile to introduce dispersion in
the opposite direction (with the opposite
sign) from the direction in which
chromatic dispersion operates
is a kind of fiber that operate in the 1370
nm band is done by manipulating the core
profile to introduce dispersion in
the opposite direction (with the opposite
sign) from the direction in which modal
dispersion operates
25. q-switching laser:
produces very short high-(gigawatt)
power pulses
very fast sequence of irregularly spaced
separate pulses
Is named in this manner because the
quality factor of the laser cavity is
changed to initiate the pulse.
very fast sequence of regularly spaced
separate pulses
Page 3
Optical attenuation
Electrical attenuation
it’s a mistake formula for dB conversion
29. Lasing threshold means:
The minimum pump power needed to begin
laser action
The value of pump power where gain is
saturated
The value of pump power at we can’t obtain
coherent beam
30. Vertical low cavity surface-emitting lasers
(VLCSELs) are:
semiconductor lasers whose emission
direction is perpendicular to the
surface of the wafer
gas lasers whose emission direction is
perpendicular to the surface of the
wafer
chemical lasers whose emission
direction is perpendicular to the
surface of the wafer
Don’t exist