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Optics and Photonics > Cavities, Beams and Modes > Flashcards

Cavities, Beams and Modes Flashcards

1
Q

what are superfluorescent lasers

A

lasers which can achieve laser action without feedback as the gain is so high

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2
Q

what do most lasers require

A

a feedback mechanism usually achieved by a resonator

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3
Q

what does the output coupler do

A

allows photons to escape the laser cavity to give output

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4
Q

what kind of emission initiates laser action

A

spontaneous

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5
Q

what kind of laser emission gives gain

A

stimulated

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6
Q

what doe the high reflectivity mirror in the laser do

A

gives feedback and more stimulated emission

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7
Q

how do we extract the energy from the cavity to form a useful laser beam

A

one of the cavity mirrors will be highly reflective while the second will be slightly lower to allow some of the light to leak out.
this is the output coupler

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8
Q

when is the maximum value of the cavity lifetime (and therefore the lowest possible threshold)

A

when both mirrors are 100% reflective

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9
Q

what is beam walk off

A

in a two plane mirror cavity, if one of the mirrors is slightly misaligned then a ray propagating will eventually escape
it is a source of cavity loss

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10
Q

with the mirrors perfectly aligned what losses still need to be considered

A

diffraction losses due to the finite aperture of the mirror

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11
Q

what is the transverse mode pattern of the light

A

the intensity distribution of light across the width of the cavity

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12
Q

what is a confocal cavity

A

a laser cavity where the radius of curvature of the mirror equals the length of the cavity
their focal points are coincident with the centre of the resonator

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13
Q

what can an optical cavity made from two mirrors be considered equivalent to

A

a series of mirrors, hence a series of lenses

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14
Q

what is the ABCD matrix for a simple lens

A

(1 0)
(-1/f 1)

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15
Q

what is the ABCD matrix for free space propagation

A

(1 d)
(0 1)

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16
Q

for a free space propagation followed by a lens, what is the ABCD matrix

A

(1 d)
(-1/f 1-d/f)

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17
Q

What is one round trip of a laser

A

the free space propagation followed by the lens, followed by free space propagation, followed by the lens

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18
Q

what is the stability criteria for a two mirror cavity

A

0<= (1-d/R1)(1-d/R2)<=1 where d is the mirror separation and R is the mirror radius of curvature
often written 0<=g1g2<=1

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19
Q

what type of waves propagate in a laser cavity

A

Gaussian

20
Q

what is the beam waist

A

the position at z=0 where w=wmin=w0

21
Q

what is the rayleigh range

A

zR is the position where the beam area doubles and the wavefront curvature is at a maximum

22
Q

what is the beam divergence angle

A

sin(theta)= 𝜆/𝜋𝑤0𝑛 ≈ 𝜆/beam dia

23
Q

in a stable cavity, what is the relationship between the radius of curvature and the curvature of the mirrors

A

they match

24
Q

what does the transverse mode determine

A

the spot pattern seen as the laser output

25
Q

what type of beams are normal laser modes

A

Hermite Gaussian Beams

26
Q

What is the general form for transverse electric modes

A

TEMmn where m is the number of modes in the x direction and n is the number of modes in the y direction

27
Q

what are hybrid transverse modes

A

a superposition of two transverse modes

28
Q

what are the determining factors in which mode oscillates

A

the aperture of the gain medium and the radial dependence of the gain

29
Q

what is the preferred and most efficient mode

A

TEM00

30
Q

how can we supress higher order modes

A

using an inactivity aperture

31
Q

what is the intermode spacing (FSR)

A

c/2d

32
Q

what is the condition for light to form standing waves within the cavity

A

m 𝜆/2 = L

33
Q

why is the output spectrum a combination of the gain spectrum and the cavity modes

A

because of broadening, the gain spectrum will give optical gain over a continuous set of wavelengths/frequencies but the resonator only supports feedback at the cavity mode frequencies

34
Q

what is a laser with only one longitudinal mode called and where would it be used

A

a single longitudinal mode laser
high resolution spectroscopy and laser cooling

35
Q

what does the number of cavity modes depend on

A

the broadening mechanism and the cavity configuration

36
Q

In CW lasers what two things balances the pumping rate

A

the circulating power and the associated stimulated emission

37
Q

at what condition is the population inversion of a CW laser maintained

A

threshold conditions with a gain of unity

38
Q

What are the CW conditions

A

round trip gain is unity
CW inversion = threshold inversion

39
Q

in a homogeneously broadened laser, what happens as the population inversion is reduced as radiation intensity builds and stimulated emission reduces

A

gain reduces at all frequencies and the laser only oscillates at the frequency corresponding to the longitudinal mode closest to the line centre with maximum gain

40
Q

In a steady state homogeneously broadened laser why can it be considered single frequency

A

in steady state only one longitudinal mode oscillates as no ther modes have enough gain to reach threshold

41
Q

in a standing wave cavity, what are nodes

A

where the e-field = zero

42
Q

what effect happens at nodes

A

no gain
no photon density
no stimulated emission
called spatial hole burning
laser gain wasted and multiple longitudinal mode oscillation more likely

43
Q

what does spatial hole burning mean for the modes accessing gain

A

pop inversion untouched by first mode can be accessed by a second with different node positions - both modes access gain and oscillate

44
Q

how to circumnavigate spectral hole burning

A

travelling wave cavity - ring cavity

45
Q

what is the travelling wave condition

A

light only goes one way round the cavity using an optical diode

46
Q

how do we create an optical diode

A

using a Faraday rotator, the faraday effect uses a strong magnetic field to change the polarisation of light passing through a magneto optically active material and prevents unwanted reflections back into laser cavity

47
Q

what is the longitudinal mode spacing

A

FSR = c/L(roundtrip)

Optics and Photonics (20 decks)

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