Construction of Lasers

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

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Construction of Lasers