The course will cover all the active optical building blocks of optical communication systems. A physical description of semiconductor lasers, optical modulators, photodetectors and receivers is discussed. A basic theory of noise in optical communication systems is also presented.

1. Examination Questions Photonics Technologies
© Haris Hassan, Sant ’Anna School of Advanced Studies Page 2 of 5
1) The relation between the amplitude coefficients of the incident, transmitted, and reflected waves at an
interface between two transparent homogeneous media are provided by:
a) The Fresnel coefficients.
b) The Einstein relations.
c) The Plank's law of black body
2) Which of these recombination processes require the presence of a photon?
a) Spontaneous Recombination
b) Auger Recombination
c) Stimulated Recombination
3) Which of these recombination process require the presence of a phonon (2)
a) Spontaneous Recombination
b) Auger Recombination
c) Stimulated Recombination
4) An electrically-pumped semiconductor laser is equivalent to:
a) A four-level system with transition energy exactly matching-the bandgap energy.
b) A four-level system with transition energy lower than the energy difference between quasi-Fermi levels in
the conduction and valence bands, and greater than the bandgap energy.
c) A two-level system with transition energy equal to the energy difference between the quasi-Fermi levels in
the conduction and valence bands.
5) Indirect-bandgap semiconductor
a) Are more effective than direct-bandgap materials for realizing photodetectors when the absorbed photons
energy is close to the bandgap energy.
b) Are less effective than direct bandgap-materials for-realizing photodetectors at any energy value of the
absorbed photons.
c) Are as effective as direct bandgap materials for-realizing photodetectors at energy values of absorbed
photons sufficiently larger than the bandgap energy.
6) The maximum available optical gain in an electrically-pumped direct bandgap semiconductor
a) Increases as temperature of the active medium is increased.
b) Decreases as temperature of the active medium is increased
c) Does not depend on as temperature of the active medium.
Note that the active medium is the direct bandgap semiconductor pumped above transparency
7) The optical gain bandwidth of a semiconductor medium becomes larger when
a) The difference between the·quasi-Fermi level of conduction & valence bands gets closer to the energy gap
value.
b) The difference between the quasi-Fermi levels of conduction & valence bands gets larger than the energy-
gap value.
c) The optical gain bandwidth of a semiconductor medium is a constant quantity that does not depend on the
quasi-Fermi levels
8) The relationship between the energy & wave-vector of free electrons & boles in a semiconductor:
a) Shows a parabolic behavior over any considered range of energies, but requires using an effective-mass
approximation for the free carriers in the semiconductor.
b) Can be approximated by a parabola only for the energetic levels at the edges of conduction and valence band
using an effective--mass approximation for the free carriers.
c) Has in general a very complex behavior and affects the optical properties of the material like the probability
of radiative recombination.

2. Examination Questions Photonics Technologies
© Haris Hassan, Sant ’Anna School of Advanced Studies Page 2 of 5
9) Non-radiative recombination process (2)
a) Are dominant in indirect-bandgap. semiconductor materials
b) Are dominant in direct-bandgap. semiconductor materials
c) Do not depend on the bandgap properties of the semiconductor material
10) The relationship between various generation and loss terms for carriers and pbotons within a laser diode
cavity can be modeled by:
a) The Fresnel' s coefficients.
b) The coupled carriers and photons-rate equations.
c) The small-signal frequency-response analysis.
11) Which of these noise sources in optical receivers depends on the incident optical power
a) Thermal noise
b) Shot noise
c) Both noises are independent of incident optical power
12) Optical gain in a pumped semiconductor medium
a) Increases with temperature.
b) Decreases with temperature.
c) Does not depend on temperature.
Laser
13) The threshold condition in a semiconductor laser cavity is attained when (3)
a) The injected current is increased up to the point that cavity gain compensates for cavity losses
b) The injected current is increased up to the point that the circulating field within the cavity replicates itself
after each roundtrip
c) The injected current is increased up to the point that the carrier density changes to above threshold value
14) Which of these laser structure offers the lowest threshold current? (3)
a) The homojunction laser
b) Index-guided double heterostructure laser
c) Gain-guided double heterostructure laser
15) The basic structure of a semiconductor laser is formed ·by
a) Atleast one has direct bandgap and both have same energy gap and lattice constant
b) The one with smaller energy gap has direct bandgap and they have similar lattice constant
c) Both have direct bandgap with different energy gap and lattice constant.
16) The basic structure of a semiconductor laser is formed ·by
a) Two direct-bandgap semiconductor compounds having different energy gap and different lattice constant
b) Two direct bandgap semiconductor compounds having different energy gap and the same lattice constant
c) Two direct bandgap semiconductor compounds having same energy gap and the same lattice constant
17) The optical receiver power sensitivity is
a) the value of the optical power for which the-receiver makes same number of errors on "ones" and "zeros"
b) the value of the optical power for which the receiver makes a certain number of errors
c) the value of the optical power for which the receiver can be used in power limited transmission systems
18) Electro-optic anisotropic crystals
a) can have more than a birefringence axis
b) can be used to make waveguides by ion implantation
c) are very good high frequency conductive materials

3. Examination Questions Photonics Technologies
© Haris Hassan, Sant ’Anna School of Advanced Studies Page 2 of 5
19) Electro-optic anisotropic crystals
a) are used for the fabrication of electro-absorption modulators
b) show a strong Kerr effect for modulating the phase of the optical carrier
c) may show different light propagation speed depending on their orientation
MODULATORS
20) The Mach-Zebnder modulators
a) are used to obtain intensity modulation from phase modulators
b) can be biased to obtain a minimum or maximum optical power transfer
c) require lower peak-to-peak driving voltage when used in push-pull configuration
21) The Mach-Zehnder modulators
a) exploit interference to convert phase modulation into in tensity modulation
b) have a periodic transfer function if made in LiNb03
c) require lower peak-to-peak driving voltage when used in push-pull configuration
22) Electro--absorption modulators
a) may show a biastuneable chirp
b) are the best choice for coherent optical systems
c) can potentially be photonic integrated to lasers and other components
23) Electro--absorption modulators
a) Are promising for modern coherent optical systems
b) Can generate logical inverted or not inverted signals
c) Their chirp can be adjusted by changing their electrical bias
24) Direct modulation of semiconductor lasers
a) is cost-effective but has the limitation of output frequency chirp
b) is cost-effective but speed is limited by bandwidth
c) can be used to modulate pulses at very high bit rates
25) A phase modulator:
a) can be realized only using anisotropic electro-optic crystal
b) can be based also on semiconductor materials but always shows a related intensity modulation
c) are used in photonic integrated circuit to generate multilevel phase and amplitude signals
Photodetectors
26) In order to maximize both the responsivity and the-bandwidth of a p-i-n diode:
a) The intrinsic region width must be as small as possible
b) The intrinsic region width must be as large as +possible
c) The intrinsic region should be sufficiently large
27) APD photodetector
a) are devices in which sensitivity increases because of the photocurrent multiplication
b) sensitivity is so good that can be used in noise limited transmission systems
c) Require much large electrical bias in respect of pin detectors
28) In an APD photodetector
a) Sensitivity increases because of the very high mobility of the electron
b) Sensitivity is generally higher (in works with lower Input power} than in a PIN with the same bandwidth
c) There is a region for photocurrent multiplication

4. Examination Questions Photonics Technologies
© Haris Hassan, Sant ’Anna School of Advanced Studies Page 2 of 5
29) Photodiode responsivity
a) is larger in longer devices
b) is reduced in-large bandwidth devices
c) does not depend on the device geometry
30) The pbotodetector responsivity is
a) the value of the optical power for which the receiver makes same number of errors on "ones" and "z.eros"
b) a parameter accounting for the opto-electronic conversion efficiency
c) never larger than 1
31) Avalanche pbotodetectors
a) exploit a photon multiplication stage to improve the sensitivity
b) are used in optical transmission links which are noise limited
c) are used in optical transmission links which are power limited
32) Dark current in pbotodetectors
a) is unavoidable and depends on the photodetector materials
b) is typically smaller in III-V photodetectors than in group IV ones
c) can be almost cancelled cooling the photodetector close to 0 Kelvin
33) What is the quantum limit in optical detectors?
a) Number of photons (received power) for which thermal noise balances shot noise
b) The shot noise dominated regime for which you can calculate how many photons you need to obtain a
certain BER value
c) A detector regime at low optical power which is dominated by thermal effects
PART II - OPEN ANSWER QUESTION (upto score 7.5 each):
1) Describe the benefits of the double hetero-structure for-realizing semiconductor-lasers and discuss some
advanced design aimed at improving the current, carriers, and photons confinement along the plane of the
junction-(lateral confit1ement) for reducing-threshold current of practical devices.
2) Describe the generic operation of a semiconductor laser and discuss some techniques for single-mode
operation
3) Describe the generic operation of a semiconductor laser & discuss some techniques for single-mode
operation
4) Illustrate the output-light versus input-current characteristic of a semiconductor laser, describing its main
operating regimes and the corresponding relevant parameters and governing physical effects.
5) Describe structure, working principle and main parameters of p-i-n photodetectors.
6) Describe the operation and the physical effects of the franz-keldysh electro-absorption modulators (EAMs)
7) Describe the operation and the physical effects at the basis of different kinds of optical modulators
8) Describe electro-absorption modulators in terms of their main parameters and their working principle
according to the two possible physical mechanism for the electro-absorption effect