Lasers Flashcards

1
Q

What are the key properties of laser light

A

collimation
monochromaticity
coherence

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2
Q

How can a laser beam be considered collimated

A

despite natural divergence due to diffraction, they can be considered well collimated

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3
Q

what is the far field diffraction angle for a laser

A

wavelength/beam diameter

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4
Q

How can a laser beam be considered monochromatic

A

spectrally pure would comprise of a single optical frequency

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5
Q

what is choherence length of a laser

A

the distance over which interference fringes are still visible lc=tc*c

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6
Q

what relates the coherence time to the coherence length

A

the speed of light

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7
Q

how can coherence time be related to optical bandwidth of a laser

A

spread of optical frequencies is Ξ”πœˆ = 1/𝑑𝑐

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8
Q

what is spatial coherence for a laser and how can it be measured

A

points across the beam are coherent, measured using young’s slits

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9
Q

what are the three basic elements needed to make a laser

A

gain medium
resonator
pump

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10
Q

what is the role of the gain medium in a laser

A

to amplify the light

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11
Q

what is the role of the resonator in a laser

A

to provide optical feedback

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12
Q

what is the role of the pump in a laser

A

to provide the energy for the amplification

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13
Q

what are the three mechanisms by which the population (N) of electrons may move between states

A

spontaneous emission
stimulated absorption
stimulated emission

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14
Q

describe spontaneous emission

A

electron in upper state spontaneously relaxes down to lower state and emits a photon
the emission of the photon is an incoherent process

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15
Q

what is the state lifetime

A

the average lifetime an electron will remain in one state before relaxing

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16
Q

describe stimulated absorption

A

an incoming photon absorbed and excites the electron from a lower to an upper state

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17
Q

describe stimulated emission

A

an incoming photon causes an electron in an upper state to relax to the lower state, emitting a second photon which has exactly the same frequency as the fist and the same phase
this is the key to laser action

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18
Q

what is the energy difference between states

19
Q

what are the einstein relations

A

g1B12=g2B21 (g is degeneracy)
A21/B21 = (8πœ‹π‘›3β„Žπœˆ)/𝑐^3

20
Q

what is beers law which relates the intensity at a frequency v to the absorption coefficient 𝛼

A

Iv(z) = Iv(z=0)exp (βˆ’π›Όπ‘§ )

21
Q

what happens if the absorption coefficient is negative

A

amplification

22
Q

what happens if the absorption coefficient is positive

A

incoming light absorbed

23
Q

what happens to the beam as it masses through the gain medium

A

it loses energy through absorption to level 1 atoms but gains energy through stimulated emission from level 2

24
Q

what is a general requirement for lasing

A

a population inversion g2/g1(N1)<N2 which gives a negative 𝛼

25
what is the small signal gain coefficient
𝛾 = (𝑁2 βˆ’ (𝑁1)𝑔2/𝑔1)(𝐡21β„Žπœˆπ‘›/𝑐)
26
What happens once a population inversion is established
gain medium will amplify the input signal
27
what is optical feedback
by reflecting the light back and forth an intense beam of radiation can be generated this creates a laser oscillator
28
why would a very small gain not be enough to establish a laser
gain needs to overcome the stimulated emission loss and the optical system losses
29
what is the threshold inversion
the necessary population inversion needed to satisfy the threshold gain
30
How can we create a population inversion
pumping
31
why can we not achieve inversion in a 2 state model
illuminating the system with light of a frequency to take electrons from E1 to E2 has an equal probability of stimulating the population down from 2 to 1 so at best the 2 level system is transparent to incoming photons - which happens at very strong pumping (bleached)
32
What is the process of achieving optical gain in a three level laser
- pump from 1 to 3 - decay from 3 to 2 is non radiative. In a good laser medium, the lifetime in 3 is short and all population rapidly decays to 2 - stimulated emission from 2 to 1. Ideally lifetime of 2 is long to allow the population to grow to create inversion wrt 1 - once this happens stimulated emission will give optical gain
33
why do we need to actively pump atoms into 3
in thermal equilibrium the majority of the population will stay in 1
34
what is the population inversion condition (3-level)
N2>=Ntotal/2
35
What do you multiply the threshold inversion condition by to find the pump rate (3-level)
1/the life time of 2 because 2 also decays to 1 by spontaneous emission
36
What is the process of achieving optical gain for a 4 level laser
- pump from 1 to 4 - non radiative decay from 4 to 3 (lifetime of 4 should be short) - stimulated emission from 3 to 2 (lifetime of 3 should be long enough to build up population inversion wrt 2) - non radiative decay from 2 to 1
37
why is a population inversion easier to maintain in a 4-level laser
the lower laser state is not the ground state in the 4 level laser
38
what happens under thermal eq in the 4 level laser
most of the population in 1, the lower laser level 2 is empty even for small populations in 3 there can be a population inversion
39
what is the pump rate for a 4 level laser
R=Nthresh(1/𝜏3)
40
What are some system losses that must be overcome to achieve laser action
transmission of mirrors absorption and scattering by mirrors unwanted absorption by the laser gain medium scattering by optical imperfections diffraction losses
41
what is the effective gain coefficient
𝛾𝑒𝑓𝑓 = 𝛾 βˆ’ π›Ύπ‘™π‘œπ‘ π‘  where π›Ύπ‘™π‘œπ‘ π‘  encompasses all system losses except mirror loss
42
What is the threshold inversion
Ξ”π‘π‘‘β„Ž = (8πœ‹π‘›^3𝜈^2𝜏2)/𝑐^3𝑑𝑐
43
what are pulsed lasers
the pump power needed to attain threshold can be achieved for a short time
44
what are continuous wave lasers
the inversion can be maintained indefinitely and the output is continuous