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Laser Flashcards

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1
Q

The following is not a candidate material for Laser source in Fiber Optics

a. Nd-YAG
b. He-Ne
c. Argon
d. Phosphorous

A

Phosphorous

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2
Q

The total efficiency of an injection laser with a GaAs active region is 18%. The voltage applied to the device is 2.5 V and the bandgap energy for GaAs is 1.43 eV. The external power efficiency of the device is

A

10 %
External power efficiency is given by
Efficiency_ext = efficiency_total*(Eg/V)

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3
Q

Population inversion in semiconductor laser diode is achieved by:

a. Lightly doping p and n sides
b. Introducing trap centres on p and n sides
c. Heavily doping p and n sides
d. Reverse biasing the junction

A

Introducing trap centres on p and n sides

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4
Q

What is a DISADVANTAGE of LED lights over LASER lights?

a. Non-coherent light source
b. Hardly available
c. Consuming more power
d. Costlier than LASER

A

Costlier Than Laser

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5
Q

Laser beam is highly intense, monochromatic and coherent, and can be used in monochromatic light and the experiments for the phenomenon of _________.

a. polarization of light
b. interference of light
c. diffraction of light
d. scattering of light

A

interference of light

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6
Q

What is the full form of LASER?

A

Light amplification by stimulated emission of radiation

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7
Q

In Stimulated Absorption, what is the lifetime of atoms ground state?
a) 1 second
b) 1 minute
c) 1 hour
d) Infinity

A

Infinity

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8
Q

Which of the following is not a characteristic of LASERS?
a) Monochromatic
b) Coherent
c) Divergent
d) Intense

A

Divergent

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9
Q

Laser is used in LIDAR for what purpose?
a) High-Speed Photography
b) Range finder
c) Optical Carrier signal
d) Drilling

A

Range Finder
LIDAR : Light detection and Ranging

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10
Q

The first laser was invented by an American engineer named as

A

Sir Theodore Maiman

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11
Q

In which one of the following year the first laser was discovered?

A

1960

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12
Q

In X-ray emission tubes, X-ray is emitted by the acceleration of _____
a) Atoms
b) Protons
c) Electrons
d) Neutrons

A

Electrons

In X-ray emission tubes, the electrons are accelerated by a very high voltage and then decelerated very fast by introducing a metal plate. In this process, the Kinetic energy of electrons is converted into light energy and, thus, X-ray is emitted.

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13
Q

In holographic data storage, the information is stored in ______________
a) Pendrives
b) Cells
c) Crystals
d) Diode

A

Crystals

Holography is used in holographic data storage. In this method, all the data is stored in crystals or photopolymers. It is mostly used in electronic data storage devices.

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14
Q

The technique by which image is obtained from a hologram is called as ____________
a) Formation
b) Construction
c) Reconstruction
d) Projection

A

Reconstruction

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15
Q

Which of the following is used for the formation of holograms?
a) X-ray
b) Visible Light
c) Infrared
d) Lasers

A

Lasers

Laser is highly coherent. Due to this, they are widely used in the reconstruction process. In a hologram, each point contains light from the whole of the original scene.

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16
Q

Which of the following is an example of optical pumping?
a) Ruby laser
b) Helium-Neon laser
c) Semiconductor laser
d) Dye laser

A

Ruby laser

The atoms of Ruby are excited with the help of photons emitted by an external optical source. The atoms absorb energy from photos and raises to excited state. Therefore Ruby laser is an example of optical pumping.

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17
Q

When laser light is focussed on a particular area for a long time, then that particular area alone will be heated.
a) True
b) False

A

Trues

Laser beam has very high intensity, directional properties and coherence. When it is focussed on a particular area for a long time, then the area alone will be heated and the other area will remain as such. This is called thermal effect.

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18
Q

For an ordinary light source, the coherence time t = 10^-10 s. The degree of Monochromaticity for a wavelength of 6000 Å is ___________
a) 0.1 X 10-4
b) 0.2 X 10-4
c) 0.3 X 10-4
d) 0.4 X 10-4

A

0.2 X 10^-4

δv = 1/t
Monochromaticity = δv/v0
v : frequency

δv = 10^10 Hz
v0 = C/lambda = 510^14
Monochromaticity = 210^-5

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19
Q

Lasers are used for welding of wires because they can be focused onto a fine spot.
a) True
b) False

A

True

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20
Q

Where is ND: YAG most commonly used?
a) Cosmetic Surgery
b) Welding
c) Photography
d) Optical Communications

A

Cosmetic Surgery

ND: YAG is most commonly used for cosmetic energy because it has the property of maximum energy absorption by the target (hair or lesion) with minimum absorption by the surrounding skin structures.

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21
Q

The information carrying capacity of laser is enormous due its large _________
a) Coherence
b) Bandwidth
c) Directionality
d) Intensity

A

Bandwidth

Laser has a large bandwidth. The rate at which the information can be transmitted is proportional to bandwidth and the bandwidth is proportional to carrier frequency. Because of these properties, Laser is widely used as optical carrier signal.

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22
Q

Which characteristic of LASER allows it to be used in holography?
a) Coherency
b) Directionality
c) Intensity
d) Monochromaticity

A

Coherency

The production of an image in a hologram takes place via a process called reconstruction. In this process, the image is “reconstructed” in the form of a hologram. This reconstruction if possible, via LASER as they are highly coherent.

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23
Q

What is the region enclosed by the optical cavity called?
a) Optical Region
b) Optical System
c) Optical box
d) Optical Resonator

A

Optical Resonator

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24
Q

Photons emitted by spontaneous emission are __________
a) Coherent and Monochromatic
b) Non-coherent and monochromatic
c) Coherent and Non-Monochromatic
d) Non-Coherent and Non-monochromatic

A

Non-Coherent and Non-monochromatic

Spontaneous emission is the one when an atom undergoes transition to a lower energy state emitting a photon, without external stimulation. In this process, the emitted photon is non-coherent and non-monochromatic.

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25
Q

The frequency of incident photon so that the atom makes a transition from E1 to E2 should be __________
a) E2 – E1
b) E2 – E1/c
c) E2 – E1/h
d) E2 – E1/λ

A

E2 – E1/h

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26
Q

What is the general lifetime of an atom in an exited state?
a) 10-10
b) 10-8
c) 10-6
d) 10-4

A

b) 10-8

The excited state is an unstable state. The exited atom tends to jump back to the ground state, where it will be stable. Therefore, it’s lifetime in the exited state is 10-8s.

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27
Q

In Stimulated emission, the emitted photons are _____________
a) Coherent and Monochromatic
b) Non-coherent and monochromatic
c) Coherent and Non-Monochromatic
d) Non-Coherent and Non-monochromatic

A

Coherent and Monochromatic

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28
Q

Which of the following is the correct expression for the relation between Einstein’s coefficients A and B?

a) 8πv^3h/c^3
b) 8πv^2h/c^3
c) 8πv^2h/c^2
d) 8πh^v/c^3

A

a) 8πv^3h/c^3

The expression 8πv^3h/c^3 is the correct expression for the relation between the two Einstein’s coefficients. This expression is known as the Einstein’s relation.

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29
Q
  1. What is the relationship between B21 and B12?
    a) B12 > B21
    b) B12 < B21
    c) B12 = B21
    d) No specific relation
A

B12 = B21

B21 is the coefficient for the stimulated emission while B12 is the coefficient for stimulated absorption. Both the processes are mutually reverse processes and their probabilities are equal. Therefore, B12 = B21.

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30
Q

Which of the following Einstein’s coefficient represents spontaneous emission?
a) A12
b) A21
c) B12
d) B21

A

A21

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31
Q

The correct expression for the rate of stimulated emission is _______________
a) Rse = A21N2
b) Rse = A21uN2
c) Rse = B21N2
d) Rse = B21uN2

A

Rse = B21uN2

The stimulates emission is directly proportional to the energy density u, of the external radiation field. Also, stimulated emission rate increases with the increase in number N2 of exited atoms.

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32
Q

Which law is used for achieving the relation between the Einstein’s coefficients?
a) Heisenberg’s Uncertainty Principle
b) Planck’s radiation law
c) Einstein’s equation
d) Quantum law

A

Planck’s radiation law

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33
Q

The probability of spontaneous emission increases rapidly with the energy difference between the two states.
a) True
b) False

A

True

the ratio of Einstein’s coefficients is proportional to the cube of the frequency. Hence, the probability of spontaneous emission increases rapidly with the energy difference between the two states.

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34
Q

f the frequency of emitted photon is 10 Hz, the ratio of Einstein’s coefficient is _____________
a) 2.177 X 10-51
b) 3.177 X 10-51
c) 5.177 X 10-51
d) 6.177 X 10-51

A

6.177 X 10-51

Einstein’s relation = 8πv3h/c3

35
Q

What is the unit of the coefficient of spontaneous emission?
a) s-1
b) s
c) J-1
d) J

A

a) s^-1

For spontaneous emission, the expression for the rate is = A21N2, where N2 is the number of particles in exited state. As the unit of rate is Number of particles per second, the unit of A21 is s-1.

36
Q

What is the unit for the coefficient of stimulated emission?
a) s-2
b) m3 s-2
c) J−1 m3
d) J−1 m3 s-2

A

d) J−1 m3 s-2

For stimulated emission, the expression for the rate is B21uN2 where u stands for the energy density and N is the number of exited atoms. Therefore, the unit of B turns out to be J−1 m3 s-2
P : No. of atoms/s
u : Energy density - defined as the incident energy on an atom as per unit volume in a state. unit : Js/m^3
N : No. of atoms
B21 : J−1 m3 s-2

36
Q

Unit of A21 (coefficient of Spontaneous emission)

A

1/s

36
Q

Unit of B21 (coefficient of stimulated emission)

A

J−1 m3 s-2

37
Q

Unit of energy density (u)

A

Js/m^3

energy per unit volume per unit frequency of radiation

37
Q

What is spectral energy density

A

energy per unit volume per unit frequency

38
Q
  1. During Population inversion, which of the following process is dominant?
    a) Stimulated Absorption
    b) Stimulated Emission
    c) Spontaneous Emission
    d) Spontaneous Absorption
A

Stimulated Emission

39
Q

The relationship between N1 and N2 for stimulated emission to be dominant is ___________
a) N1 = N2
b) N1 > N2
c) N2 > N1
d) No such relationship

A

N2 > N1

40
Q

The ratio of N2 and N1 is given by ___________
a) e^−(hv/kT)
b) e^(hv/kT)
c) e^−(hv/T)
d) e^(hv/T)

A

e^−(hv/kT)

from Maxwell Boltzmann law
N = N0e^-(E/kT)

41
Q

During pumping, the atoms are exited to ___________
a) Higher Exited States
b) Lower Energy states
c) Meta Stable states
d) Not Excited

A

Meta Stable states

42
Q

At the state when N1 > N2, the intensity of light wave _____________
a) Increases linearly
b) Increases exponentially
c) Decreases linearly
d) Decreases exponentially

A

Decreases exponentially

When N1 > N2, the population of atoms in the ground state is higher than that in the exited state. In this case, absorption is dominant and hence, the intensity of light wave decreases exponentially.

43
Q

To achieve optical amplifications, it is essential to create a non-equilibrium distribution of atoms.
a) True
b) False

A

True

to achieve lasing action, non-equilibrium distribution of atoms in such a way that the population of the upper energy is greater than that of the lower energy level.

44
Q

What will be the relative population of atoms in a ruby layer that produces a light beam of wavelength 6943 Å at 300 K.
a) 5 X 10-31
b) 6 X 10-31
c) 7 X 10-31
d) 8 X 10-31

A

d) 8 X 10-31

The relative population of atoms in two states with energy E1 and E2 is given by
N2/N1 = e−{(E2−E1)/kT}

45
Q

Which of the following is not a four-level laser?
a) CO2
b) Ruby
c) He-Ne
d) Er:YAG

A

Ruby

except ruby All others are 4 level system

46
Q

Value of Planck’s constant

A

h = 6.62610^-34 JS
h = 4.13510^-15 ev.s

47
Q

1 ev =

A

1 ev = 1.602*10^-19 J

48
Q

Boltzmann Constant

A

K = 1.38 ×10−23 J/K
K = 8.617* 10-5 ev/K

49
Q

Optical pumping happens via ___________
a) Spontaneous emission
b) Spontaneous Absorption
c) Stimulated emission
d) Stimulated Absorption

A

Stimulated Absorption

50
Q

Pumping is done in order to achieve __________
a) Steady state
b) Population inversion
c) Equilibrium
d) Photon emission

A

Population inversion

51
Q

Which pumping method is used in He-Ne laser?
a) Optical Pumping
b) Electrical Excitation
c) Chemical Pumping
d) Direct Conversion

A

Electrical Excitation

Explanation: Generally, in He-Ne laser, an electric discharge is used to excite the atoms of the active medium. This process is known as Electrical Excitation and is normally used in gas lasers.

52
Q

Direct conversion of electrical energy into radiation takes place in ___________
a) Ruby laser
b) ND: YAG
c) LED
d) He-Ne laser

A

LED

In Light Emitting Diodes (LED) and semiconductor lasers, direct conversion of electrical energy into radiation takes place. This process of pumping is called as Direct conversion.

53
Q

At least 2 levels are required in a crystal to achieve population inversion.
a) True
b) False

A

False

Explanation: Population inversion takes place in crystals with a minimum three-level system. Two level system is not suitable for achieving population inversion because in that case the population of both the levels would remain same as B12 = B21.

54
Q

Which of the following is a four-level laser?
a) ND: YAG
b) Ruby
c) He-Ne
d) Semiconductor laser

A

ND: YAG

55
Q

The meta-stable state have a larger lifetime than lower energy state.
a) True
b) False

A

False

meta-stable states generally have a longer lifetime than ordinary excited states, but a shorter lifetime than the lowest, stable energy state (ground state).

In 4 level laser system, metastable state have larger life time than the lower laser energy level

56
Q
  1. Which of the following is a three-level laser?
    a) ND: YAG
    b) Ruby
    c) He-Ne
    d) Semiconductor laser
A

Ruby

Ruby laser is a three-level laser. Ruby crystal is a crystal of Al2O3 with some Al3+ ions replaced by Cr3+ ions. The energy levels of Cr3+ are responsible for lasing action.

57
Q

The lifetime of meta-stable state in a Ruby laser is ___________
a) 10-8s
b) 10-6s
c) 10-3s
d) 10-2s

A

10-3s

58
Q

The ends of the ruby rod works as __________
a) Pumping source
b) Active medium
c) Cavity mirrors
d) Energy levels

A

Cavity mirrors

59
Q

The pumping mechanism used in Ruby rod is __________
a) Optical Pumping
b) Electrical Excitation
c) Chemical pumping
d) Thermal pumping

A

Optical Pumping

The Ruby rod is placed inside a xenon flash lamp so as to provide sufficient amount of light to cause excitation of atoms. This kind of pumping is called optical pumping.

60
Q

Which material is used for cooling of the ruby rod for efficient continuous operation?
a) CFC
b) Liquid helium
c) Liquid oxygen
d) Liquid Nitrogen

A

Liquid Nitrogen

A large amount of energy is dissipated in the ruby rod. Thus, it has to be cooled for efficient continuous operation. For this purpose, liquid nitrogen is used.

61
Q

The laser beam is emitted in the form of __________ in Ruby Laser
a) Fluctuating radiations
b) Continuous spectrum
c) Pulsed output
d) Exponentially decreasing intensity

A

Pulsed output

Since the Ruby laser work on 3 level system, it can not continuously sustain population inversion

62
Q

For a Ruby laser, the coherence time t = 10-10 s. The degree of Monochromaticity is __________

A

dv = 1/t = 10^10 s-1
v = c/lambda =310^8/694010^-10
= 4.322 X 10^14 Hz
degree of Monochromaticity = dv/v
= 2.313*10^-5

63
Q

Wavelength of Ruby Laser

A

6940 A°

64
Q

What will be the relative population of atoms in a ruby layer that produces a light beam of wavelength 6943 Å at 300 K.
a) 5 X 10-31
b) 6 X 10-31
c) 7 X 10-31
d) 8 X 10-31

A

8 X 10-31.

65
Q

What is the wavelength of the emitted laser in a Ruby laser?
a) 694 nm
b) 650 nm
c) 780 nm
d) 754 nm

A

694 nm

Ruby laser emits visible light. It is a three-level laser. The emitted radiations have a wavelength of 694.3 nm, which represents the red color. The output observed in a Ruby laser is spiked.

66
Q

he energy levels of which ion/atom/molecule are responsible for lasing action?
a) Al3+
b) Cr3+
c) Al2O3
d) Cr2O3

A

b) Cr3+

It is the three energy levels of Cr3+ ion, that the population inversion takes place and light is emitted via stimulated emission.

67
Q

He-Ne laser is a type of ____________
a) Solid laser
b) Liquid laser
c) Gas laser
d) Diode laser

A

Gas laser

68
Q

The He-Ne laser operates at a wavelength of ____________
a) 540 nm
b) 632 nm
c) 690 nm
d) 717 nm

A

632 nm

The helium-neon laser operates at a wavelength of 632.8 nanometers (nm), in the red portion of the visible spectrum. It is the most widely used gas laser.

69
Q

The number of photons emitted for a 2.5 mW He-Ne laser is __________
a) 4.9 X 10^15
b) 5.9 X 10^15
c) 6.9 X 10^15
d) 7.9 X 10^15

A

7.9 X 10^15

E = n * hv
n = E/hv = E*Lambda/hc

70
Q

in He-Ne Laser, When the transition takes place from En6 -> En5, what is the wavelength of produced beam?
a) 6328 Å
b) 33913 Å
c) 11523 Å
d) 7550 Å

A

33913 Å

71
Q

He-Ne laser is used in Holography.
a) True
b) False

A

True

He-Ne laser is highly coherent and monochromatic. Due to this, it is used in holography, spectrometers, prints, scanners, etc. It is widely used in Laboratories.

72
Q

What is the wavelength of the emitted laser by a carbon dioxide?
a) 9.4 μm
b) 10.6 μm
c) 11.4 μm
d) 12.5 μm

A

10.6 μm

The radiations emitted in a CO2 laser has a wavelength of 10.6 μm. It is a four-level laser. The transition takes place between the different vibrational states of the molecule.

73
Q

Which of the following is a characteristic of semiconductor lasers?
a) Output in Visible region
b) High Efficiency
c) Output in UV region
d) Pulsed output

A

High Efficiency

Semiconductor lasers such as GaAs, InP, InSb etc. are used extensively because of their high efficiency. Also, they can be employed in optical communications with ease.

74
Q

In the CO2 molecular gas laser, transition takes place between the ______________
a) Molecular states
b) Atomic states
c) Vibrational states
d) Energy states

A

Vibrational states

75
Q

Which of the following gas is not a part of the active medium in a CO2 laser?
a) CO2
b) N2
c) He
d) O2

A

O2

The active medium of a CO2 laser consists of a mixture of CO2, N2 and He. It is the vibrational transition in the CO2 that results in the lasing action.

76
Q

The highest powered CO2 laser had a power of _________
a) 1 W
b) 10 W
c) 1000 W
d) 10000 W

A

1000 W

Light from a CO2 laser is powerful enough to cut many materials, including cloth, wood and paper; the most powerful CO2 lasers are used for machining steel and other metals. The highest-powered CO2 lasers run over 1,000 W.

77
Q

The active medium of a semiconductor diode is the junction of the forward biased P-N diode.
a) True
b) False

A

True

A semiconductor laser is essentially a semiconductor diode, where the active medium is the forward biased p-n junction.

78
Q

Semiconductors lasers do not need mirrors to form optical cavity.
a) True
b) False

A

True

Semiconductor lasers do not require two external mirrors to form an optical cavity. Reflection from the cleaved ends of the semiconductor is enough to produce lasing.

79
Q

Where is ND: YAG most commonly used?
a) Cosmetic Surgery
b) Welding
c) Photography
d) Optical Communications

A

Cosmetic Surgery

ND: YAG is most commonly used for cosmetic energy because it has the property of maximum energy absorption by the target (hair or lesion) with minimum absorption by the surrounding skin structures.

80
Q

In which region of the electromagnetic spectrum, does the semiconductor laser lies?
a) Visible Region
b) UV Region
c) Microwave Region
d) Infrared Region

A

Infrared Region

Physics (5 decks)

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