Cavities, Beams and Modes Flashcards
what are superfluorescent lasers
lasers which can achieve laser action without feedback as the gain is so high
what do most lasers require
a feedback mechanism usually achieved by a resonator
what does the output coupler do
allows photons to escape the laser cavity to give output
what kind of emission initiates laser action
spontaneous
what kind of laser emission gives gain
stimulated
what doe the high reflectivity mirror in the laser do
gives feedback and more stimulated emission
how do we extract the energy from the cavity to form a useful laser beam
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
when is the maximum value of the cavity lifetime (and therefore the lowest possible threshold)
when both mirrors are 100% reflective
what is beam walk off
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
with the mirrors perfectly aligned what losses still need to be considered
diffraction losses due to the finite aperture of the mirror
what is the transverse mode pattern of the light
the intensity distribution of light across the width of the cavity
what is a confocal cavity
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
what can an optical cavity made from two mirrors be considered equivalent to
a series of mirrors, hence a series of lenses
what is the ABCD matrix for a simple lens
(1 0)
(-1/f 1)
what is the ABCD matrix for free space propagation
(1 d)
(0 1)
for a free space propagation followed by a lens, what is the ABCD matrix
(1 d)
(-1/f 1-d/f)
What is one round trip of a laser
the free space propagation followed by the lens, followed by free space propagation, followed by the lens
what is the stability criteria for a two mirror cavity
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
what type of waves propagate in a laser cavity
Gaussian
what is the beam waist
the position at z=0 where w=wmin=w0
what is the rayleigh range
zR is the position where the beam area doubles and the wavefront curvature is at a maximum
what is the beam divergence angle
sin(theta)= 𝜆/𝜋𝑤0𝑛 ≈ 𝜆/beam dia
in a stable cavity, what is the relationship between the radius of curvature and the curvature of the mirrors
they match
what does the transverse mode determine
the spot pattern seen as the laser output
what type of beams are normal laser modes
Hermite Gaussian Beams
What is the general form for transverse electric modes
TEMmn where m is the number of modes in the x direction and n is the number of modes in the y direction
what are hybrid transverse modes
a superposition of two transverse modes
what are the determining factors in which mode oscillates
the aperture of the gain medium and the radial dependence of the gain
what is the preferred and most efficient mode
TEM00
how can we supress higher order modes
using an inactivity aperture
what is the intermode spacing (FSR)
c/2d
what is the condition for light to form standing waves within the cavity
m 𝜆/2 = L
why is the output spectrum a combination of the gain spectrum and the cavity modes
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
what is a laser with only one longitudinal mode called and where would it be used
a single longitudinal mode laser
high resolution spectroscopy and laser cooling
what does the number of cavity modes depend on
the broadening mechanism and the cavity configuration
In CW lasers what two things balances the pumping rate
the circulating power and the associated stimulated emission
at what condition is the population inversion of a CW laser maintained
threshold conditions with a gain of unity
What are the CW conditions
round trip gain is unity
CW inversion = threshold inversion
in a homogeneously broadened laser, what happens as the population inversion is reduced as radiation intensity builds and stimulated emission reduces
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
In a steady state homogeneously broadened laser why can it be considered single frequency
in steady state only one longitudinal mode oscillates as no ther modes have enough gain to reach threshold
in a standing wave cavity, what are nodes
where the e-field = zero
what effect happens at nodes
no gain
no photon density
no stimulated emission
called spatial hole burning
laser gain wasted and multiple longitudinal mode oscillation more likely
what does spatial hole burning mean for the modes accessing gain
pop inversion untouched by first mode can be accessed by a second with different node positions - both modes access gain and oscillate
how to circumnavigate spectral hole burning
travelling wave cavity - ring cavity
what is the travelling wave condition
light only goes one way round the cavity using an optical diode
how do we create an optical diode
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
what is the longitudinal mode spacing
FSR = c/L(roundtrip)
