Download Helium Neon Laser - Abbe School of Photonics

Module Labworks Optics
Abbe School of Photonics, Friedrich-Schiller-Universität,
Physikalisch-Astronomische-Fakultät,
Max-Wien-Platz 1, 07743 Jena, Germany
Phone: +49 3641 947 960
Fax: +49 3641 947 962
E-mail: [email protected]
Web : www.asp.uni-jena.de
Contact person: Dr. Roland Ackermann
Phone : +49 3641 947 821
E-mail : [email protected]
Supervisor: Christoph Stihler ([email protected]),
Thorsten Goebel ([email protected])
Helium Neon Laser
[ version of February 22, 2017]
Contents
1
Safety issues
1.1 Eye hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Chemical hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
Theoretical basics
2.1 Helium Neon Laser . . . . . . . . . . . . . . . . . .
2.2 Basics of resonator modes . . . . . . . . . . . . . .
2.3 Transversal modes in a laser resonator . . . . . . . .
2.4 Optical elements for wavelength selection . . . . . .
2.4.1 Brewster’s angle . . . . . . . . . . . . . . .
2.4.2 Fabry Perot Etalon . . . . . . . . . . . . . .
2.4.3 Littrow prism . . . . . . . . . . . . . . . . .
2.4.4 Birefringent filter . . . . . . . . . . . . . . .
2.4.5 Transmission grating . . . . . . . . . . . . .
2.5 Measurement of the beam quality factor M 2 (Task 8)
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3
Setup and equipment
3.1 Setup alignment procedure . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 Further adjustment of the optical elements . . . . . . . . . . . . . . .
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15
4
Goals of the experimental work
17
A Preliminary questions
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18
B Final questions (to be answered in the introduction/discussion section of the report!)
18
He Ne Laser
1 Safety issues
1.1 Eye hazard
The laser system used is classified according to DIN IEC 60825-1 as a Class 3B Laser.
This means the visible, continuous wave laser radiation emitted during laser operation has an
average power of less than 5 mW. Therefore the laser radiation itself and also the stray light is
potentially dangerous to the eye. It is recommended to use an appropriate laser safety goggles
in addition with protective sides against laser stray light caused by additional optics during
the measurements. Since some measurements and the alignment procedure may require to
take off the protective goggles temporarily, it is very important to remove all reflecting objects
attached to your hands/wrist (e.g. rings, watches etc.).
1.2 Chemical hazard
Acetone and its vapors are toxic. Use the minimal required quantity of acetone while cleaning
the optical elements. Do not sniff the vapors of the acetone for prolonged periods. Avoid contact with skin or eyes. If accidental contact happens, wash the interested area with abundant
cold water. Do not hesitate to ask for assistance if pain persists.
2 Theoretical basics
2.1 Helium Neon Laser
A helium-neon laser is a gas laser, consisting of a mixture of helium and neon gas in a ratio
between 5:1 and 20:1 bound in a glass tube. The pump energy of the laser is provided by an
electrical discharge of several hundred Volts between an anode and cathode at each end of the
glass tube. A current of 5 to 100 mA is typical for cw operation. The used HeNe tube has
Brewster’s angle windows at both ends. The HeNe Laser can work at different wavelengths.
There are infrared emissions at 3.39 µm and 1.15 µm and different emissions in the visible
spectrum. Normally a HeNe Laser is working at the red 632.816 nm wavelength with a very
narrow gain bandwidth of a few GHz, which is dominated by Doppler broadening. The laser
process in a HeNe laser starts with collision of electrons from the electrical discharge with
the helium atoms in the gas, which excites helium from the ground state to the 23 S1 and 21 S0
metastable excited states. Collision of the excited helium atoms with the ground-state neon
atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 3S2
level. The difference between the energy states of the two atoms is in the order of 0.05 eV,
which is supplied by kinetic energy. The number of neon atoms in the excited states builds up
as further collisions between helium and neon atoms occur, causing a population inversion.
Spontaneous and stimulated emission between the 3s2 and 2p4 states results in emission of
632.82 nm wavelength light. After this, fast radiative decay occurs from the 2p to the 1s
3
He Ne Laser
Figure 1: Energy level diagram of a He Ne system (origin: http://en.wikipedia.org/wiki/
File:Hene-2.png)
ground state. For more details we recommend to read [6]. Also more basics about laser
principles may be found in [7], specially about rate equations.
2.2 Basics of resonator modes
Laser light usually is assumed to have a Gaussian intensity distribution in the transverse plane.
Details of the theory of Gaussian beams can be found in [2]. Here only results are presented.
The intensity distribution of the laser spot in the beam waist plane for the fundamental TEM00
mode is described by a gaussian profile in the following way
I(r, z) = I0 exp −
2r2
,
w(z)2
(1)
with r being the distance from the beam center. Higher modes are characterized by so called
Hermit or Laguerre polynomials.
The laser mode stays in the gaussian distribution along the resonator but the beam width (the
distance from the beam axis to the point where the intensity drops to 1/e2 , see Fig. 2) increases
with increasing distance from the beam waist. In a certain distance z the beam width w(z) is
given by
v
t


 λz 2
(2)
w(z) = w0 1 +  2  .
πw0
4
Abbildung A.1 zeigt den Längsschnitt eines solchen Strahl mit Ta
Abbildung
A.1: Längsschnitt
eines Gaussschen
Strahls. Strahls.
Abbildung
A.1: Längsschnitt
eines Gaussschen
verdeutlicht
die
Bedeutung
oben eingeführten
Größen.Strahls.
Zusätzlich
Abbildung
A.1:der
Längsschnitt
eines Gaussschen
Abbildung A.1: Längsschnitt eines Gaussschen
kel θ0 eingezeichnet, welcher die lineare Aufweitung des Strahl im
Abbildung
A.1 zeigtA.1
denzeigt
Längsschnitt
eines solchen
Strahl mit
Taille
bei z0w0=bei
0u
Abbildung
den den
Längsschnitt
eineseines
solchen
Strahl
mitw0Taille
Abbildung
zeigt
Längsschnitt
solchen
Strahl
mit Taille
w0
schreibt.A.1
Abbildung
A.1
zeigt
den
Längsschnitt
eines
m
verdeutlicht
die Bedeutung
der obender
eingeführten
Größen.
Zusätzlich
istsolchen
der Divergenzw
verdeutlicht
die Bedeutung
oben
eingeführten
Größen.
Zusätzlich
istStrahl
der
He
Ne
Laser
verdeutlicht
die Bedeutung
der oben
eingeführten
Größen.
Zusätzlich
ist Div
der
verdeutlicht
diedie
Bedeutung
der oben
eingeführten
Größen.
kel θ0 eingezeichnet,
welcher
die lineare
Aufweitung
des Strahl
Fernfeld
z Zusä
zR z
kel θkel
welcher
lineare
Aufweitung
desim
Strahl
im Fernfeld
0 eingezeichnet,
θ0 eingezeichnet,
welcher
die lineare
Aufweitung
des
Strahl
im Fernf
√
kel θ0 eingezeichnet, welcher die lineare Aufweitung des Stra
schreibt.schreibt.
w(z)
w0
θ0
z
zr
2 w0
schreibt.
schreibt.
w(z)
w(z)
√ A Gausssche
√ √
Anhang
Strahlen und Strahlqualitä
w
θ
z
z
2r w0
(A
0
00
rz
w(z)
w
θ
z
2
w
√
0
0
w(z)
w0
θ0
z
zr
2 w0
w(z)
w0
θ0
z
zr
2 w0
w0
θ0
z
zR
√
2 w0
(A.9
Figure 2: Gaussian beam width w(z) as a function of the axial distance z. w0 : beam waist; zR : Rayleigh
range; θ: total angular spread (origin: http://en.wikipedia.org/wiki/Gaussian_beam)
The radiation converges towards the beam waist and diverges with increasing distance from
the center of the resonator, having a plane wavefront in the waist. In distance z from the waist
the radius of the wavefront curvature R(z) is

 2 2 

 πw  
R(z) = z 1 +  0   .
(3)
λz
In a confocal resonator (the focal points of both mirrors are at the same point, see Fig. 3) the
beam waist is in the middle of the resonators with the distance d between the mirrors and the
beam waist is given by
r
λd
w0 =
.
(4)
2π
In a non-confocal resonator the stability parameters g1 and g2 have to be defined
g1 = 1 − (d/R1 )
g2 = 1 − (d/R2 ),
(5)
whereas d is the resonator length and R1 and R2 are the curvatures of the mirrors. To reach
a stable resonator mode the wavefront curvature has to be equal to the curvature of the used
resonators. By this request it is possible to evaluate the position and size of the beam waist,
and the spot size on the the mirrors can be estimated. Hence, we know the curvature of the
beam front in two planes, which we name z1 (distance of the beam waist to mirror 1) and z2
(distance of the beam waist to mirror 2). If one claims R(z1 ) = R1 and R(z2 ) = R2 the position
of the beam waist can be evaluated
z1 =
g2 (1 − g1 )
d,
g1 + g2 − 2g1 g2
(6)
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He Ne Laser
and the radius of the beam waist is given by
λd
w0 =
π
!1/2
g1 g2 (1 − g1 g2 )
(g1 + g2 − 2g1 g2 )2
!1/4
.
(7)
The so called stability area is defined by
0 < g1 · g2 < 1.
(8)
Within this region of resonator parameters stable mode structure is guaranteed. If g1 · g2 is >1
the spot size is imaginary or infinity. If g1 · g2 '1 the spot size may be greater than the
resonators, resulting in significant losses, what means mode instability.
Figure 3: Stability diagram for a two-mirror cavity. Blue-shaded areas correspond to stable configurations. (origin: http://en.wikipedia.org/wiki/Optical_cavity)
2.3 Transversal modes in a laser resonator
In a laser with cylindrical symmetry, the transverse mode patterns are described by a combination of Gaussian beam profile and Laguerre polynomials. The modes are denoted TEM pl
where p and l are integers labeling the radial and angular mode orders, respectively. The
intensity at a point (r, φ) (in polar coordinates) from the centre of the mode is given by
I pl (ρϕ) = I0 ρl [Llp (ρ)]2 cos2 (lϕ)e−ρ ,
(9)
6
He Ne Laser
where ρ = 2r2 /w2 , and Llp is the associate Laguerre polynomial of order p and index l, w is the
spot size of the mode corresponding to the Gaussian beam radius. With p = l = 0, the TEM00
mode is the lowest order, or fundamental transverse mode of the laser resonator and has a form
of a Gaussian beam. The pattern has a single maximum and a constant phase across the mode.
Modes with increasing p show concentric rings of intensity, and modes with increasing l show
angularly distributed maxima. In general there are 2l(p + 1) spots in the mode pattern (except
for l = 0). The overall size of the mode is determined by the Gaussian beam radius w. This
size increases or decreases for different distances form the beam waist, however, the modes
preserve their general shape. Higher order modes are relatively larger compared to the TEM00
mode, and thus, the fundamental Gaussian mode of a laser may be selected by placing an
appropriately sized aperture in the laser cavity.
Figure 4: Rectangular transverse mode patterns TEMmn (origin: http://en.wikipedia.org/
wiki/Transverse_mode)
In many lasers, the symmetry of the optical resonator is restricted by polarizing elements
such as Brewster’s angle windows. In these lasers, transverse modes with rectangular symmetry are formed (Fig. 4). These modes are designated TEMmn with m and n being the horizontal
and vertical orders of the pattern. The intensity at point (x, y) is given by
 √ 
!2   √ 
!2
  2x 
−x2    2y 
−y2 







 ,
Imn (x, y) = I0 Hm 
 Hn 
 exp
 exp
w
w
w2
w2
(10)
7
He Ne Laser
where Hm (x) is the m-th order Hermite polynomial. The TEM00 mode corresponds to exactly the same fundamental mode as in the cylindrical geometry. Modes with increasing m and
n show maxima appearing in the horizontal and vertical directions, with in general (m+1)(n+1)
maxima present in the pattern. As before, higher-order modes have a larger spatial extent than
the 00 mode. The overall intensity profile of the laser output is a superposition of all transverse
modes allowed in the laser cavity, though often it is desirable to operate only on fundamental
mode.
2.4 Optical elements for wavelength selection
2.4.1 Brewster’s angle
Let us consider an electromagnetic wave with its polarization oriented parallel to the plane of
incidence. There exists an incidence angle, called Brewster’s angle, for which no reflection
occurs, considering this particular polarization state.
(a)
(b)
Figure 5: (a) Illustration of the polarization states of light which incides on an interface at Brewster’s angle. (origin: http://en.wikipedia.org/wiki/Brewster_angle) (b) Reflection coefficient for different angles of incidence (origin: http://en.wikipedia.org/wiki/Fresnel_
equation)
If the incident angle is equal to Brewster’s angle the reflected and transmitted beams are
perpendicular to each other (Fig. 5). Hence, by using the law of refraction n1 ·sin θB = n2 ·sin θ
one may easily calculate the Brewster’s angle by
θB = arctan
n2
.
n1
(11)
8
He Ne Laser
2.4.2 Fabry Perot Etalon
A Fabry Perot Etalon consists of two exactly parallel high reflective mirrors. Transmission of
light by a given wavelength λ occurs only if
mλ = 2nl cos θ,
(12)
with l being the distance between the two mirrors and θ being the angle between the optical
axis and the surface normal of the mirrors (Fig. 6). Hence an Etalon is capable to separate
different wavelengths by either tilting the etalon or increasing the distance of the mirrors. The
etalon used in this lab has a aperture of 12.7 mm and a thickness of l = 10 mm.
Figure 6: Light enters the etalon and undergoes multiple internal reflections. (origin: http://en.
wikipedia.org/wiki/Fabry_Perot)
2.4.3 Littrow prism
A Littrow prism is a prism with a mirror at one surface. Due to the wavelength depending
refraction of the prism each laser wavelength will propagate at a certain direction trough the
prism and hit the mirror at a different angle. So only one wavelength will be back reflected
into the incidence beam. Therefore one may separate wavelengths by tilting the Littrow prism
in the light path (Fig. 7).
2.4.4 Birefringent filter
Here another effect is used to select different wavelengths. In a birefringent crystal the optical properties, as the index of refraction, vary with the direction of the incoming beam. In
the simplest case an anisotropic crystal has one axis of symmetry, called the optic axis. For
propagation parallel to the optic axis the index of refraction is independent of the oscillation
9
He Ne Laser
Figure 7: If a shaft of light entering a prism is sufficiently narrow, a spectrum results. (origin: http:
//en.wikipedia.org/wiki/Prism_(optics))
direction of the E-field vector. If the propagation has another direction the incident light is
split into the ordinary and extraordinary beam. Each of them experiences another index of
refraction (no and ne ) depending on the angle of incidence. Also the two beams propagate in
different directions, according to their index of refraction and the direction of the optic axis.
In this lab, the optic axis is parallel to the surface of the quartz crystal in use. Hence, the
ordinary and extraordinary beam have a different index of refraction but no walk off occur.
Thus the quartz crystal may be used as phase plate.
Here we may separate the incident E-field vector into one component parallel and one perpendicular polarized to the optic axis. The component Ee , which is parallel to the optic axis, will
propagate as extraordinary wave with the index of refraction ne and the other one as ordinary
wave Eo with no . If no > ne the two beams will have, after a certain propagation distance d,
a phase difference of (no − ne ) · d. If this phase difference is equal π/2 and the amplitude of
the ordinary and extraordinary beam are equal the phase plate converts linear polarized light
into circular polarized light. Such a phase plate is called λ/4-plate. After another transition
the light is again linear polarized. Since each wavelength has a different index of refraction
only a certain wavelength will be linear polarized after two transitions through a λ/4-plate.
2.4.5 Transmission grating
When a wave propagates, each point on the wavefront can be considered to act as a point
source, and the wavefront at any subsequent point can be found by adding up the contributions
from each of these individual point sources. Here, an idealized grating is considered, which
is made up of a set of long and infinitely narrow slits with the spacing g. When a plane wave
with a wavelength λ incides normally on the grating, each slit acts as a line of point sources.
The light in a particular direction, ϕ, is made up of the interfering components from each
slit (Fig. 8). Due to the difference in phase the waves from different slits mainly cancel one
10
He Ne Laser
another partially or completely. However, when the path difference between the light from
adjacent slits is equal to the wavelength, λ, the waves are all in phase. Thus, the diffracted
light will have maxima at angles ϕm given by the grating equation
g sin ϕm = mλ ,
(13)
with m as an integer and g being the separation of the slits. The light that corresponds to
direct transmission is called the zero order, and is denoted m = 0. The other maxima occur at
angles which are represented by non-zero integers m. Note that m can be positive or negative,
resulting in diffracted orders on both sides of the zero order beam.
Figure 8: Diffraction of light by a transmission grating, with g being the slit separation and ϕ the
diffraction angle. (origin: http://de.wikipedia.org/wiki/Optisches_Gitter)
11
He Ne Laser
2.5 Measurement of the beam quality factor M 2 (Task 8)
Determine the quality parameter M 2 with the help of a convex lens with a focal length
f = 125 mm and a camera.
Basic equations:
w0 2
r2
exp
−2
2
w(z)
w(z)
1)
I(r, z) = I0
2)
q
2
w(z) = w0 1 + zzR
3)
zR =
4)
θ0 =
5)
BPPfund = θ0 w0 =
6)
BPP = M 2 λπ = θmeasured wmeasured
πw02
λ
w(z)
z
w0 →
1
e2
beam radius
zR → Rayleigh length, z → axial position
λ → wavelength
=
w0
zR
θ0 → divergence angle (half angle) in far field
λ
π
Beam parameter product for fundamental mode
General beam parameter product
M 2 → unitless value concerning the second moment
width of the beam
Procedure of evaluation:
a) Focus laser beam with 125 mm lens!
b) Capture images with CMOS-camera (pixel pitch distance 5.2 µm) at 35 different positions (Avoid overexposure by using gray filters!):
i. 20 points with increment of 5 mm
ii. 10 points with increment of 10 mm
iii. 5 points with increment of 50 mm
c) Find center of gravity concerning gray values in captured images.
d) Make line scan in x-direction crossing the center of gravity.
e) Fit a Gaussian
distribution
with regards to formula 1) with parameters A, B, and C:
(x−B)2
A · exp −2 C 2 to estimate e12 spot radius (C) for every single image.
f) Reconstruct the caustic (see figure below) of the beam by means of formula 2) to obtain
the waist radius (w0 ) and the beam quality factor M 2 :
w2 (z) = w02 + (M 2 )2 πwλ 0 (z − z0 )2 .
g) Estimate M 2 and compare it with other laser sources! What is influencing the beam
quality?
12
He Ne Laser
Figure 9: Gaussian beam and its fundamental parameters (origin: https://en.wikipedia.org/
wiki/Gaussian_beam)
3 Setup and equipment
The setup consists of the following components shown in Fig. 10:
• Profile Rail (1)
• HeNe Laser Tube with power supply (2)
• Laser mirror adjustment (3 + 4)
• Photo detector in holder (5)
• Alignment laser with power supply (6)
• Birefringent tuner (7)
• Littrow prism tuner (8)
• Single mode etalon (9)
• Set of laser mirrors in holder (10)
– PLAN - plane mirror
– R = 1000 - mirror curvature 1000 mm
– R = 700 - mirror curvature 700 mm
– OC24 - plane mirror output 2.4 %
• Grating (600 lines / mm)
• Translation stage with thin filament
For the whole setup it is very important that all optic elements are well cleaned and that
there is no staining on the optics.
13
He Ne Laser
Figure 10: Setup of the used elements (origin: micos He-Ne Laser manual)
3.1 Setup alignment procedure
3.1.1 Basics
The first task in this lab work is the definition of the optical axis for the laser system. For this
purpose an alignment laser should be used. The next step is to adjust the laser mirrors perpendicular to the optical axis so that the back reflected beam hits itself exactly at the beam output
aperture. The perfect alignment to the optical axis can be recognized by observing a flickering
laser beam caused by interference effects with the plane mirror in the holder. Afterwards, the
main laser tube has to be centered in the adjustment beam. Be sure that the Brewster’s angle
windows of the main laser (2) are well cleaned and its power supply is switched off. The goal
is to adjust the main lasers capillary around the optical axis of the adjustment laser. The actual
position can be observed at a reflective screen like a piece of white paper. Finally, to get the
laser to work arrange the pre-adjusted components 2, 3 and 4 like in Fig. 10. Switch off the
adjustment laser 6 and switch on the main laser tube 2. If no laser light can be observed, a
gentle twisting of max. ±45◦ of one of the adjustment screws shown in the draft above cause
the oscillation flicker up. If you get the laser oscillation the output power can be optimized
with the position of the laser mirrors and supplementary with the x/y-adjustments of the main
tube. Afterwards the laser output power has to be optimized.
Now you can start with the measurement of the laser output power depending on different laser geometries (task number 4 to 7) For measuring the wavelength the grating and a
screen should be used.
14
He Ne Laser
3.1.2 Further adjustment of the optical elements
Littrow prism
The adjustment of the Littrow prism can be done with the same procedure like the laser mirrors were adjusted. Afterwards, the parts 2 and 3 can be assembled again and the main laser
can be switched on. The cleaning of the optics is tremendously important. If the laser starts
oscillating, optimize it for best intensity. The laser line tuning can be done with the adjustment
screws of the Littrow prism mount.
Birefringent filter (BFP)
Rotate the birefringent filter (8) in its mount to Brewster’s angle. Insert the birefringent filter
(8) between the adjustment laser (6) and one plane mirror (4). Repeat the alignment of the
laser mirror to eliminate the beam displacement caused by the inserted optics. After that the
set-up can be completed again. Another way would be to directly place the BFP in the adjusted laser. With a gentle twist of one of the adjustment screws, the laser may start again.
However, this procedure is very sensitive and only possible, when the laser runs at the highest
output power. If the BFP is rotated around it optical axis you can see the main line several
times. If the birefringent filter is rotated in small angles around one main line you can get
additional wavelengths with different gains as well.
Etalon
The Etalon (9) will be placed on the rail instead of the BFP (8). If put into the resonator the
laser normally keeps its oscillation more or less strong. By observing the back reflections at
the mirror surface (3) you can identify a non perpendicular adjustment when several spots are
visible. The zero order adjustment is achieved if all reflections fall into the beam of the main
laser. Other orders can be adjusted by tilting the etalon holder (9).
The realization of another laser wavelength than the main wavelength is very difficult and
sensitive. At first try to maximize the output power. By using the 700 mm curvature mirror
and the 2.4 % transmission mirror, an output of 3.5 mW is possible. Also by using the BFP
an output power of 3 mW is possible. At such a high output power it is easy to get laser oscillation at different wavelengths. At an output power which is lower than 3 mW the gain of a
wavelength different from the main wavelength may be to low to compensate the losses within
the resonator.
The birefringent filter should be used at first. After the BFP is build into the resonator and
the laser is running, one may see the main line up to four times. The strongest one should be
chosen and the laser should be optimized again. If the birefringent filter is rotated in small
angles around the chosen main line one can get additional wavelength with different gains.
One may switch between different laser wavelengths by using the grating, as the position of
the diffracted beam depends on the wavelength used.
With the Littrow prism one can see the change of wavelength easily because the color changes
from red to orange. Here the Littrow prism should be used instead of the output coupler to-
15
He Ne Laser
gether with the 700 mm curvature mirror. Keep the distance between the prism and the laser
tube as short as possible and minimize the resonator length. Afterwards you can place the
grating behind the curved mirror and use this faint laser output for the wavelength selection.
The Etalon will be places on the rail instead of the BFP. When inserted into the resonator the
laser normally keeps its oscillation more or less strong. By using the Etalon one may find another wavelength only if the Etalon is placed exactly perpendicular into the beam. That is the
case if no back-reflection occurs. The zero order adjustment is achieved when all reflections
fall into the beam of the main laser. Afterwards one may switch to different wavelengths by
slightly tilting the etalon.
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He Ne Laser
4 Goals of the experimental work
In this lab different combinations of mirrors (M1 and M2) are used as resonator. These are:
a) M1: plan, M2: R = 1000 mm
b) M1: plan, M2: R = 700 mm
c) M1: plan, M2: plan
d) M1: R = 700 mm, M2: R = 1000 mm
1. (at home, before the lab!) Evaluate the beam width inside the resonator for a resonator
length of d = 50 cm. Use all four mirror combinations.
2. (at home, before the lab!) Evaluate the optical stability area with respect to the resonator length d for all given mirror combinations. Therefore, plot the corresponding
stability area (g1 · g2 ) over the distance d for all four mirror combination.
3. Build up and align a stable running HeNe-Laser out of the given components.
4. Measure the dependence of the laser output power on the tube current for d = 50 cm for
the mirror combination a) and b).
5. Measure the dependence of the laser output power on the resonator length for I =
6, 5 mA and the mirror combination a) and b).
6. Measure the output power dependence on the laser tube position while using the mirror
combination a) and a resonator length of d = 50 cm.
7. Measure the laser wavelength by using the grating.
8. Measure the laser beam quality factor M 2 by using a convex lens (focal length f =
125 mm) and a camera. Reduce the output power with gray filters to avoid overexposure
of the camera.
9. Additional: Change the setup by using a thin filament in order to achieve higher TEM
modes.
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He Ne Laser
A Preliminary questions
• What kind of Laser is used for this experiment?
• What are the main conditions for stable laser operation?
• Is the used laser radiation polarized and if so in which direction? Why?
• Determine Brewster’s angle for air - fused silica transition?
• What is the meaning of the stability area of two mirrors?
• Which parameters are important to achieve high laser output?
• How can you achieve different output wavelengths of the laser? How can you measure
this?
Keywords
resonator, polarization, wavelength, frequency, laser, gauss beam
B Final questions (to be answered in the introduction/discussion
section of the report!)
• What is the used setup?
• How does a HeNe-Laser works?
• How can you achieve a stable running HeNe-Laser?
• What can you say about the polarization?
• Which parameters of the laser geometry can you change? What consequences have
these changes? What can you say about the laser wavelength?
• What is the content of gaussian optics?
• Which optical elements have you used and how did they work?
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He Ne Laser
References
[1] Young, M.: Optics and lasers: including and optical waveguides. 4th edition. Springer,
Berlin, 1993
[2] Saleh, B. E. A. ; Teich, M. C.: Fundamentals of Photonics (Wiley Series in Pure and
Applied Optics). 2nd edition. JohnWiley & Sons, Hoboken, New Jersey, 2007
[3] Homepage of wikipedia
URL=<http://www.wikipedia.org/>, 2007-10-06
[4] Träger, F.: Springer handbook of lasers and optics, (Springer). 1st Edition 2007.
[5] Homepage and encyclopedia of the RP Photonic Consulting GmbH
URL=<http://www.rp-photonics.com/encyclopedia.html>
[6] Svelto, O.: Principles of Lasers
[7] Siegman, E.: Lasers, University Science Books (January 1986)
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