Lab 4 Laser - Institutt for elektronikk og telekommunikasjon - NTNU

From
pink light
to red LASER
Lab exercise TFE4160 Elektrooptikk og lasere
NTNU
Institutt for elektronikk og telekommunikasjon
September 2004

Contents
1 Lab exercise:
From pink light to red LASER 3
1.1 Laser safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1 Electrical hazard . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2 Light hazard . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.3 Rules of thumb when working with lasers . . . . . . . . . 5
1.2 Laser fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.1 Light amplification . . . . . . . . . . . . . . . . . . . . . . 7
1.2.2 Laser resonators and standing waves . . . . . . . . . . . . 8
1.3 Lab exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.1 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.2 HeNe-tube . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.3 HeNe-laser alignment . . . . . . . . . . . . . . . . . . . . 10
1.3.4 Transversal modes . . . . . . . . . . . . . . . . . . . . . . 10
1.3.5 Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A Output spectrum 12
2

Chapter 1
Lab exercise:
From pink light to red LASER
This lab exercise gives insight into the fundamentals of laser operation. A
HeNe-discharge tube is used and aligned with a mirror to construct a laser.
The fundamental properties of this laser are then investigated.
1.1 Laser safety
A laser can be a powerful and harmful device. It is therefore very important
that the laser is handled with outmost care and that your eyes always are
protected. Never look into the laser aperture nor into the reflected light from
the brewster window.
1.1.1 Electrical hazard
The HeNe laser is pumped with a very high voltage: 1900 Volts!! Therefore:
Do not touch any connectors on the HeNe discharge tube. The PSU (power
supply unit) feeds the tube with 7mA. This is enough to kill you if you touch
both wires!! Therefore, again: Do not tamper with any of the electrical wires
concerning the discharge tube!
1.1.2 Light hazard
Do NOT stare into laser light!!
This is the first and most important rule for operating lasers. A laser provides
high intensity light and may just burn a hole in your retina if you look into the
laser light.
Since the first laser was made in 1960 it has been clear that laser emissions
are a hazard to the eye and skin, more of a hazard than most other light
sources. Lasers are hazardous because they produce intense directed energy
that can damage living tissues. There are other common light sources strong
enough to cause permanent eye or skin damage, including: direct sunlight, arc
3

1.1 Laser safety 4
welders, movie projectors, searchlights and special tv photo lamps.
Living tissues can be damaged when they absorb light because of excessive
(even explosive!) heating or by photochemical changes. Examples of heat damage
include skin burned by concentrated sunlight or laser emission, and retina
lesions caused by laser beams. Examples of photochemical damage includes
sunburn and cataracts caused by excessive exposure to ultraviolet light from
the sun or from arc welders.
When you look at a strong halogen bulb you feel that it is strong light and
that you should not continue looking at it (you would see green spots for a couple
of days if you continued long enough). The laser light is more concentrated
and has a typical spot diameter of 300µm which, when it gets focused, makes
a spot that is 10µm on your retina. Let’s now compare the light intensity of a
5mW laser-pointer to the intensity from the image of the sun focused on your
retina.
Sun retina intensity =
Power
Spot size =
3 · 10−3 W
π(75 · 10 −6 = 180 kW/m2
) 2
Laser retina intensity =
Power
Spot size = 5 · 10−3 W
π(5 · 10 −6 = 63.6 MW/m2
) 2
In other words: the laser retina intensity is 350 times the sun retina intensity. It
is not recommended to stare at the sun, and with these calculations, I’d suggest
that you don’t even think of staring into the laser light. It will burn your retina
rather quickly.
During this lab exercise you will handle a laser tube which is capable of
delivering up to 5mW of coherent light at wavelengths around 632nm(red). It
is dangerous to look into the laser beam so do not do that. You shall also wear
welding glasses when working with the laser. This is to be sure that no one
gets black spots on their retina due to focused laser light.

1.1 Laser safety 5
Class Power Hazards Precautions Examples
I ≤ 4µW None none, ”exempt” UPC Scanners
II > 4µW
≤ 1mW
Retina damage
Laserpointers,
small HeNe lasers
IIIa
> 1mW
≤ 10mW
possible after 10
seconds or more
of steady direct
viewing
Retina damage
possible in 0.25
seconds
IIIb > 10mW Retina damage
≤ 0.5W likely in 0.1
seconds.
IV > 0.5W Severe eye or skin
damage from direct
exposure.
Eye damage likely
from indirect exposure.
Can ignite flammable
materials
Labelling and warnings
do not stare into
beam
Trained operators
attached beam stop
warninglamp
contain beams
Key switch
interlock connector
eye protection
trained personnel
only
emission delay
intracavity shutter
door interlock
contain all reflections
HeNe and semicounductor
laserpointers
Semiconductor
Dye
Cutters, welders
laser shows
pulsed lasers
argon lasers
Table 1.1: Summary of ANSI and CDRH Laser Classes (see Winburn 1990, Ch.
6)
1.1.3 Rules of thumb when working with lasers
The following rules of thumb will prevent accidents that can damage vision. If
in doubt of the safety of any situation, seek the advise of the lab instructor.
1. Know the appropriate laser Class(see table 1.1) and necessary precautions.
2. Do not look into a laser beam under any circumstances!
3. Control and confine all laser beams.
4. Use beam stops and carefully plan the placement and movement of optical
elements.
5. Beware of stray reflections from the many surfaces the beams might encounter.
6. Confine the beams to the horizontal plane just above the table.
7. Keep your eyes above the level of the laser beams. (see rule 7)

1.1 Laser safety 6
8. Close your eyes if you bend down to pick something up from the floor.
(see rule 7)
9. Do not wear a watch or jewelry with shiny flat surfaces.
10. Resist the impulse to look toward a flash or glare of light, it may be a
laser beam illuminating your face.
11. Turn room lights ON when possible. The eye’s pupils open wide in a
dark room and present a larger target for a stray laser beam.
12. Wear safety laser goggles when appropriate.
13. Follow laser operating instructions carefully.
14. Use common sense and be alert at all times.

1.2 Laser fundamentals 7
1.2 Laser fundamentals
1.2.1 Light amplification
Atoms and molecules interact with light in three different processes:
Absorption, spontaneous emission and stimulated emission.
- Absorption: An atom or molecule can capture, ”absorb”, a photon of
light and be excited, converted to a higher energy level. The photon
must have a specific frequency n to provide the proper amount of energy
E = hν to excite the atom or molecule. This process called ”stimulated
absorption” is the atom’s response to electromagnetic stimulation by the
incoming photon. Stimulated absorption is responsible for the colors of
dyes, which absorb light of specific frequencies, while a black or gray object
absorbs all frequencies equally..
- Spontaneous emission:An excited atom or molecule with excess stored
energy E can release that energy spontaneously, by emission of a photon
with frequency ν = E/h Spontaneous emission is responsible for the light
emission by fires, fireflies, fluorescent paint, sunlight, LEDs, hot pokers,
and most types of lamps.
- Stimulated emission: An excited atom or molecule can be stimulated
to release excess energy E in response to an incident photon of the exactly
the right frequency (ν = E/h) and simultaneously release, or emit, two
photons with the same frequency, using the energy of the incident photon
plus the stored energy of the excited atom. Stimulated emission is the
process that provides optical amplification in most lasers, since one photon
in produces two photons out, an amplification factor of exactly two per
event.
Real atomic and molecular systems absorb or emit light at many discrete
frequencies, each with a range, or bandwidth D ν centered around the main
frequency ν = E/h. Optical amplification occurs when the rate of stimulated
emission exceeds the rate of (stimulated) absorption, producing gain G where
G =
light out
light in . (1.1)
Amplification (G > 1) occurs in an atomic or molecular system with a population
inversion, which means that the number of excited atoms (or molecules)
exceeds the number of unexcited atoms or molecules. This means that there
will be more stimulated emission than stimulated absorption.

1.2 Laser fundamentals 8
1.2.2 Laser resonators and standing waves
The optical feedback and delay
functions are served by an optical
resonator, such as the one shown
in Fig. 1.1a constructed with two
mirrors that face each other. The
mirrors serve to recirculate, or feed
back, the light with efficiency R =
R1R2, the product of the reflectivities
R1 and R2 of the two mirrors.
The delay time t = 2L/c is
the time it takes light to complete
a round trip between the mirrors.
The net gain for a complete round
trip is GR and must be equal to
unity to sustain steady oscillation.
The condition GR = 1 means that
the gain compensates for the loss
and the oscillator will operate with
steady power. (What if GR < 1 or
GR > 1?).
There is an additional criterion
that after each round trip the light
wave crests line up with the crests
from the previous round trip. This
is called resonance and results in
a standing wave as shown in Fig.
1.1b. This condition is similar
to the resonances associated with
standing sound waves in an organ
pipe or on a guitar string. The condition
on the mirror separation is
that one round trip contains an integral
number of wavelengths 2L =
mλ, where m is an integer. The
corresponding resonant frequencies
are f m = c/λ = mf fsr , where the
free spectral range f fsr = c/2L is
the separation between resonance
frequencies. Fig. 1.1b has harmonic
number m = 4, but a typical
visible wavelength laser resonator
might have length L = 30 cm operating
at wavelength l = 600 nm
so that m = 1 million.
Figure 1.2a shows resonant fre-
Figure 1.1a: Mirror resonator
Figure 1.1b: Standing wave in a mirror
resonator
Figure 1.2: Typical laser output spectrum

1.2 Laser fundamentals 9
quencies of an optical resonator
near m = 1 million. For an L = 30 cm resonator, the spacing between successive
resonance frequencies is f fsr = c/2L = 500 MHz while the resonance
frequencies are near f m = c/l = 500 THz, one million times larger.
The spectrum, or collection of different frequencies, emitted by a laser is
a combination of the resonant frequencies f m of the many possible standing
optical waves in the resonator (fig. 1.2a) with the range of frequencies in the gain
spectrum (fig. 1.2b) produced by stimulated emission and population inversion.
A representative laser output spectrum is shown in Fig. 1.2c. Modified laser
resonators can suppress all but one of the resonant frequencies, permitting more
precise control of the frequency, a desirable feature in scientific and engineering
applications. The typical He-Ne laser illustrated in Fig. 1.3 consists of a sealed
tube containing a mixture of helium and neon gasses and an optical resonator.
An electrical discharge in the tube excites the helium atoms, which then collide
with the neon atoms, transferring excess energy to pump the neon atoms from
level 0 to level 2. The neon atoms develop a population inversion between
levels 2 and 1, providing gain at one of several different visible and infrared
wavelengths. The light circulating between the two mirrors is amplified by
the stimulated emission from the excited neon atoms on each round trip. One
mirror, called the back reflector, has high reflectivity(> 99%) while the other
mirror, called the output coupler, has lower reflectivity (typically 95 − 98%)
and transmits a fraction (2 − 5%) of the light to the outside this is the laser
output that we observe. The power inside the laser is many (20 − 50) times
higher than the power emitted.
Figure 1.3: Construction of HeNe laser tube

1.3 Lab exercises 10
1.3 Lab exercises
1.3.1 Equipment
Welding glasses for eye protection
HeNe one-brewster discharge tube
High Voltage(1900 V) power supply
Alden cable with ballast resistor
Output coupler-mirror (OC)99%@632.8nm
Power meter
Polarizer
Quarter wave plate
Chopper
Photo diode with amplifier
Oscilloscope
1.3.2 HeNe-tube
The HeNe-tube is mounted in a stand with the electrical wires attached. Do
NOT touch any of this. The tube is very fragile and easy to break!
Plug the 220V power supply unit into the wall socket and wait for the tube to
ignite.
Describe the color of the light that is emitted. And measure the output power
from the tube with the power meter.
1.3.3 HeNe-laser alignment
Put the mirror onto the rail and put it 1 cm from the brewster-window. Observe
the change of light color that is transmitted through the mirror. Why does the
light have this color? Measure the power emitted.
Now try to align the mirror so that the light gets reflected off the mirror and
back through the brewster mirror so that lasing is obtained. Measure the power
emitted from the laser at once when lasing is obtained. It is quite easy to see,
because a flash of red light will occur when lasing starts.
What is the µm readings when lasing starts? What is the effect when lasing
starts? What is the maximum power that you can get out of this tube? What
is the corresponding µm readings?
Try to slide the mirror away from the tube. Measure and see how far you can
slide while maintaining lasing. Plot the power of emitted radiation vs. length
of the tube. Be careful and don’t touch the anode of the tube (High voltage)!!
1.3.4 Transversal modes
Look at the mode picture projected at the wall (use a sheet of paper to get a
better mode picture). Which TEM mode is this? Use a lens to inflate the mode
picture if it is difficult to see. What happens when the mirror is or tilted back
and forth? Why?

1.3 Lab exercises 11
1.3.5 Polarization
Use the polarizers and the quarter wave plate to construct an analyzer. Determine
the state of polarization by using an oscilloscope, the photodiode with
amplifier and a chopper. What is the state of polarization? Measure also the
state of polarization on the light that is reflected from the brewster window.
What is the use of brewster windows? Why is the polarization state as it is?

Appendix A
Output spectrum
Figure A.1a shows the output spectrum of the HeNe laser. Figure A.1b shows
the spectrum of the gas glow. Notice the broad spectrum and the many
lines emitted. It is possible to get the the fluorescent tube lasing at both
green, yellow and orange wavelengths. A blue laser may also be possible!!
2500
2000
Effect [arb. units]
1500
1000
500
0
630 635 640 645 650
[nm]
Figure A.1a: Laser output
1200
1000
800
Effect [arb. units]
600
400
200
0
−200
100 200 300 400 500 600 700 800 900
[nm]
Figure A.1b: Output spectrum from the gas glow outside of tube
12

Bibliography
[Duc04] Stephen Ducharme. Laboratory manual for physics 343: Physics of
lasers and modern optics. Department of physics and astronomy, University
of Nebraska-Lincoln, 2004.
[ST91] Saleh and Teich. Fundamentals of photonics. Wiley, 1991.
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