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8/13/2019 Construction of Lasers
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Lasers Construction
Here we briefly describe how to construct a laser.
I. Laser Cavity
A schematic diagram of a typical laser cavity is shown in Fig. 6.
Gain medium
R1 ~ .!" R# ~ $ !"
%1 %# or &'(aser output
%1) %# * mirrors) intensity reflectivity of %1 ~ .!" or higher. %# is also called output coupler +&',) intensity reflectivity typically $" -".
Characteristics:+1, %1) %# very flat ~ #- + / laser wavelength,.+#, %1) %# are aligned such that they are 0very parallel. +see e2ercise,.
Remarks:+1, %1) %# / multi/layer dielectric coating mirrors.+#, %irror in your bedroom) reflectivity3+4, 5ometimes concave mirrors are used. + hy3,
II. Gain (Amplified)
%1 &'
(aser output
7op. 8nv.
Represents an atom with an e2cited electron in level A. (ight inside the cavity has gain +i.e.number of photons increases in time, if there is population inversion and the reflectivity of&' is high enough. 5everal round trips after photon p1 is emitted) this process results in a lotof photons +see e2ercise,.
Remark: 5timulated emissions shown in Fig. $ will be lost through the walls of cavity.
1
Fig. 1
Fig. 2
p1
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%1 &'
III. C aracteristics of Laser Lig t
0 Single fre9uency +or wavelength,) also:nown as monochromatic .
'oherent; photons are 0vibrating in phase . is
## #.,+ = z o e I z I ) where I - is a constant)
and is the width of the profile.
Remark: (aser beam intensity could have other distributions) an e2ample is depicted in Fig. 1#.
Again the electric field of the light wave +at a certain instant, is shown as a function of z .
M2=OCM1
Z
8 (z)
0 Z
?+=,
#
Fig. !
Fig. "
Fig. #
Fig. $
Another way to visuali>e this property is depicted inFig. 11 showing the electric field of light wave as afunction of z .
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%mall &eam divergence@ypical value of is ~ -.! 1- 4 rad.
Example ;%ean earth/moon distance 4.$ 1- ! :m
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(C By pumping andlaser levels
4/level laser /level laser
fast
pumplaser
transition
pump
gr. stategr. state
F
4
#laser
transition
4/level F/level
'I. *+amples
8n this 5ection) we give e2amples of specific lasers and briefly describe their characteristics.
!"#$ %e&'e laser
,e- e
/C
,ig voltage 0 1 '
glass tu&e
(1
Laser output
He pressure ~ 1 @orr +1 @orr is a pressure of 1 mm Hg,. Ce pressure ~ -.1 @orr wavelength 64#.$ nm +red, is the most common output.&utput power -.! 1- 4 to #! 1- 4 .
Remark: Recently) orange) yellow) and green outputs of wavelengths 61#.-) ! .-) and ! 4.!nm) respectively) are also available) typical output power ~ -.# 1- 4 .
!"# )ulsed C* laser +,ransverse Excited Atmospheric (,EA C* laser-
(1
/C
spar plugs
02 '
(aser cavity filled with '& # gas at atmosp eric pressure . %par plugs create charges to facilitate discharge between the two flat electrodes. @he electrodes have to be very smoot / avoid local discharge. 7ulse width ~1 ns 1- sec.
fast
transition
Ground state Ground state
Fig. 3
Fig. 4
Fig. 1
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avelength 1-.6 m) that is far in the infrared.
Remark ; similarly) a pulsed C # laser +9uite common, can be constructed. A C # laser lases at44 .1 nm +in the utra/violet spectrum,.
Exercise: %o. long does it take for an electron to travel a distance of /$ cm under 01000 !2
!"#3 4ye laser
laser output@unning plate
+wedge,
Eye cell
Eye out>le
focused pumping light
dye Iet. ~ 1-- m thic:
Jet surface should be very smoothB
Eetails of no>>le
@he reason for using a dye Iet is as follows;A short pulse becomes boarder after passing a piece of thic: +~ cm, material +say glass) water)
etc,.
!"#6 ,i:sapphire laser
@i;sapphire means sapphire crystal doped with @i D impurities. A @i;sapphire laser is a dyelaser with the dye cell replaced by a piece of @i;sapphire.
!
Fig. 11
Fig. 12
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@i; sapphire crystal +red in color,focused pumping light@he reason for choosing sapphire is that its thermal conductivity is good compared to other non/
metallic solids. Advantage / active medium suffers no degradation . 'an be pumped 0 harder ) and hence higher output.
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ound in C4 players . Recent years #- output at ~$-- nm wavelength are available. ed for structural construction.
indows / transparent to OK. Active medium should be extremely pure . &therwise) if the laser cavity is contaminated
by a small amount of impurities) the laser cannot laser anymore.
Fig. 1$ R A< // distance between the two atomic nuclei.
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Applications of Excimer lasers:
?tching %edical) e.g. corneal sculpting
(aser deposition of thin films
!"#= >&ray laser 8n recent year) there is intense interest in 2/ray lasers. However) it is not easy of ma:e a 2/ray
laser. &ne of the :ey problems is that there is no mirror that can reflect 2/ray efficiently.
ote< 8re:ster=s Angle (>indo:)
B
n1n#
8nterface
Fig. 1 n 1 and n # are refractive indices of media 1 and #) respectively. Eouble/headed arrows
indicate direction of the electric field +?, of the optical radiation. indicates that ? is pointinginto the paper.
/C1
active medium
polari?edoutput
Reasons for using ed) which is convenient for many optical applications.
A&out t e aut or