Download The Nd:YAG

The Nd:YAG
&
The Dye Lasers
THEORY ................................................................................................................................................. 1
THE ND:YAG LASER ........................................................................................................................ 1
Crystal ............................................................................................................................................. 1
Pumping .......................................................................................................................................... 1
Resonator ........................................................................................................................................ 2
Q-switching and frequency-doubling .............................................................................................. 3
THE DYE LASER ............................................................................................................................ 5
The dye ............................................................................................................................................ 5
Laser pumped dye lasers ................................................................................................................. 7
EQUIPMENT ......................................................................................................................................... 9
Time resolved electronics................................................................................................................ 9
The power supply for the Nd:YAG laser ......................................................................................... 9
The cooling system of the Nd:YAG laser......................................................................................... 9
The optics of the Nd:YAG laser..................................................................................................... 10
SAFETY ................................................................................................................................................ 11
PREPARATORY ASSIGNMENTS .................................................................................................... 13
LAB EXERCISES ................................................................................................................................ 14
THE ND:YAG LASER ...................................................................................................................... 14
Start the laser ................................................................................................................................ 14
Energy and efficiency .................................................................................................................... 14
The Q-switch. ................................................................................................................................ 14
Frequency-doubled light ............................................................................................................... 14
THE DYE LASER ............................................................................................................................. 15
Theory
The Nd:YAG laser
Crystal
In the laser world community Yttrium Aluminium Garnet is a frequently used
crystalline material that can be manufactured with high quality. If it is doped with a
small amount of neodymium the crystal becomes slightly red and achieves good laserqualities. The red colour is a result of the absorption bands of the neodymium ion
between 430 och 590 nm (figure 1). The absorption bands are shifted and broadened
according to the host-crystal. Nd:Glass and Nd:YLF are other frequently used
neodymium doped laser crystals.
Figure 1. Absorption spectrum of Nd:YAG from Goodwin (1970). The wavenumber is
the inverse of the wavelength.
Pumping
To have an idea about which light sources that could be used to pump a Nd:YAG laser
we have to study the absorption-profile of the crystal. Figure 1 shows a strong
absorption band around 800 nm (12500 cm-1). If we take a closer look at this part we
notice that the absorption profile consists of a number of peaks. In the early days of
the Nd:YAG laser, halogen lamps were used for pumping because of their broad
spectral profile.
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Figure 2. Absorption spectrum for Nd(0.5%):YAG at room temperature (Kaminskij
1967).
However this way of pumping turned out to be inefficient and a lot of energy simply
had to be cooled away. A better solution is to use rare gas discharge lamps. These are
a tubes filled with a rare gas emitting an intense linespectrum with a continuous
background. Often used lamps of this type are krypton- and xenon-lamps. Both have
strong emission lines in the near infra-red part of the spectrum but the emission lines
from the krypton-lamp agrees better with the absorption lines in Nd:YAG, e.g. a
partial overlap is found at 810 nm. Krypton-lamps turn out to be efficient pump
sources for continuous Nd:YAG lasers.
The pump source in a pulsed laser is demanding a much higher current compared to a
continuous laser. During these circumstances the xenon-lamp is the most efficient
choice. This is because the continuous background from the xenon-lamp is stronger
and that there is a strong re-absorption at 810 in krypton. The lamps that are used
during the lab-exercise are xenon-lamps with a maximum in-energy of 80 J and a life
span of some ten million discharges.
In recent years diode laser pumping of Nd:YAG lasers has developed in a
tremendously fast way. It is possible to make a diode laser narrow banded and
tuneable to an appropriate absorption band in Nd:YAG hence the pumping efficiency
will be high with an efficiency of the laser of 20-30 % which is a magnitude higher
compared to high quality rare gas discharge lamps. In particular for continuous
Nd:YAG lasers at low power, diode laser pumping by far turn out to be the best
solution. The use of diode laser pumping have made it possible to make the lasers
more compact and today there exists portable, battery driven, diode laser pumped
Nd:YAG lasers. Another large application of continuous diode lasers is injection
seeding where a pulsed laser is forced to run in single mode by feeding the cavity with
light from a continuous laser. Normally the laser emission is triggered by quantum
electrical noise and the resulting mode is randomly chosen. By having laser light
corresponding to one of the mode of the Nd:YAG laser inside the cavity during
pumping this mode will dominate and repress every other mode. For a discussion
about the mode-structure in resonators see the instructions for the HeNe-laser.
Resonator
There are a number of different types of resonators. Basically they can be divided into
two categories; stable (0<g1g2<1) and unstable. A continuous laser must have a stable
resonator to be able to emit laser light. In a pulsed laser with a pulse length of about
10 ns and a resonator length of 60 cm the pulse length corresponds to two or three
round-trips in the resonator and from that follows that the resonator don’t have to be
stable anymore. It is possible to make both the end mirrors totally reflective and the
output coupling is done at the sides of the small mirror (figure 3).
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Figure 3. Unstable resonators (a) the positive branch (b) the negative branch.
There are several technical benefits with unstable resonators. The most important one
is that it is easy to make the mode volume bigger. This makes it possible to achieve
higher energies in the laser pulse without the risk of burning the laser crystal. The
resonator is unstable in the negative branch if g1g2<0 and in the positive branch if
g1g2>1 (see figure 3).
Q-switching and frequency-doubling
For many applications a high peak power is desired e.g. to pump dye lasers. Qswitching is used to achieve this (figure 4). The principle is simple: If the cavity is
blocked the oscillation is prevented and the laser pumping builds up a larger than
usual population inversion. When the cavity feedback is restored the inverted
population is depleted in a very short time and the result is a short laser pulse (~10ns)
with high peak power (0.1GW).
Figure 4. In the left part of the figure high voltage is applied to the Pockels cell. The
spontaneous emission from the Nd:YAG is non-polarised. The Brewster plate makes
the light linear-polarised. After one pass through the Pockels cell the light is circularpolarised. It is then reflected in the end mirror and makes a second pass through the
Pockets cell. The light is now linear-polarised again and the polarisation is rotated
90o compared to the polarisation before passing the Pockels cell. The light is now
partly out-coupled by the Brewster plate and doesn’t reach the threshold for laser
oscillation. The lower part of the figure shows a simplified atomic energy-level
diagram. When the cavity losses are big no laser oscillation occurs and a large
population inversion is build-up. In the right part of the figure the high voltage is
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turned off. The Brewster plate still makes the light linear-polarised but now the
Pockels cell don’t have any influence on the polarisation so the initial polarisation is
kept and it is possible for the light to pass the Brewster plate on its way back. This
light act as feedback and we have laser emission. The large population inversion we
have achieved gives as a result a high peak power for the laser pulse. (The rays in the
forward and the backward direction are spatially separated to clearly show the
polarisation.)
To be able to quickly change the cavity losses a Pockels cell is used. When a high
voltage is applied to the Pockels crystal it acts like a quarter-wave plate and energy
circulating once around the cavity has the polarisation rotated 90o. A polarising
element e.g. a glass plate in Brewster angle inside the cavity is then used to couple out
the energy after just one round-trip. By turning the voltage off it is possible for the
circulating energy to pass the polarising element and hence for laser oscillation to
build-up.
One of the most common applications for the Nd:YAG laser is to pump dye lasers.
The light from a dye laser always have a longer wavelength compared to the pumping
light, as a consequence of this visible light cannot be achieved if the dye is pumped
with the fundamental wavelength (1064 nm) from the Nd:YAG laser. To get tuneable
light in the visible part of the spectrum from a Nd:YAG pumped dye laser the
pumping light has to be frequency-doubled. Let us look at a simplified description of
an electromagnetic wave propagating in a medium. The medium consists of atoms
with bound electrons that are brought to oscillation by the electromagnetic wave. The
oscillating bound electrons and the positively charged nucleus can be regarded as
oscillating dipoles that, according to Huygen’s principle, generate electromagnetic
wavelets. These electromagnetic wavelets bring electrons further into the medium to
oscillate and the electromagnetic wave propagates in the medium.
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Figure 5. An initial wave causes the electron in the material to oscillate. The
generated wave (a) is a consequence of the electron oscillation. This wave can be
expressed as (a)=(b)+(c). (b) has the same frequency as the initial wave and (c) has
the double frequency. Phase matching condition is fulfilled if the zero-points of (b)
agree with the zero-points of (c). (Picture from Yariv: Quantum Electronics).
Now, let’s see what happens if the amplitude is so large that it is impossible for the
electrons to follow the rapid oscillation of the electromagnetic wave (figure 5). The
motion of the electrons will no longer be a perfect sinus oscillation and this
discrepancy is transferred to the electromagnetic wavelets. If the generated
electromagnetic wave is divided into its Fourier components we will find the second
harmonic, i.e. light with twice the fundamental frequency. The intensity of the higher
order harmonics is several magnitudes lower and may be neglected. If we use a birefrignent doubling medium and the condition for phase matching is fulfilled the
frequency doubled wave and the fundamental wave will propagate with the same
velocity in the material and we can reach a frequency conversion of 40-60%.
Explanation of phase matching is given in the literature for the course.
The dye laser
The dye
The amplifying medium in a dye laser is organic dye molecules in a solvent. A
schematic energy level diagram for a dye molecule is shown in figure 6. When the
molecules are excited by visible or ultraviolet radiation they emit an intense and
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spectrally broad fluorescens. Higher located vibrational levels in the first excited
singlet state S1 get their population from thermally populated rotational-vibrational
levels in the groundstate S0. The excited dye molecules are rapidly relaxing down to
the lowest located vibrational level in S1 by collisions with the molecules of the
solvent. From this level the dye molecules are de-excited by spontaneous emission to
rotational-vibrational levels in S0 or by radiative free transitions to the triplet state T1.
Due to collisions between molecules of
the solvent and of the dye the closely
lying rotational-vibrational levels are
broadened so that they are overlapping.
The result is that the absorption and
fluorescens spectra are broad continuum,
see figure 7. In the same figure it is
noticed that the fluorescens is shifted
towards the red part of the spectrum as
expected. With efficient pumping it is
possible to get population inversion
between the vibration states in S1 and the
higher located rotational-vibrational
levels in S0, which have small thermal
population. If the dye is placed inside a
resonator it is possible to have stimulated
emission, but then the amplification has
to be bigger than the losses in the
resonator and the dye.
Figure 6. Schematic energy level diagram
for a dye molecule.
The losses in the dye are of two kinds, singlet and triplet losses. Singlet losses are due
to reabsorption and occur if some transitions S1  S0 are overlapping with the
flourescens spectrum. Singlet losses cannot be avoided and often they restrict the
spectral region for the stimulated emission. Triplet losses occur because molecules
ending up in the triplet sequence (mainly by the S1  T1 transition) cannot be
pumped for a long time (some milliseconds). The only way for the triplet molecules to
relax down to S0 is by phosphorescens or by collisions.
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Figure 7. Absorption and fluorescens spectra for Rhodamine 6G solved in methanol.
Another type of triplet losses occur if the absorption spectrum for the transition T1 
Tm have a partial overlap with the fluorescens spectrum between S1 and S0, the result
is absorption losses (see figure 6). A way to reduce the S1  T1 losses is to have
triplet-quenching additives, which increase the transition probability T1  S0, in the
dye solution. An example of a triplet-quencher is cyclo-octatetraene (COT). Another
way to avoid triplet losses is to transport away the triplet molecules from the
excitation region by circulating the dye. This method is used in continuous wave dye
lasers.
Figure 8. Laser emission curves for a number of dyes.
Figure 8 shows laser emission curves for a number of dyes. The emission curves are
affected of the dye concentration, the solvent and the pump source. If a wavelength
selective (dispersive) optical element is placed in the resonator a small region of the
laser emission curve can be selected for stimulated emission. An often used dispersive
element is the grating. The spectral resolution of a grating, R, is defined as
R=/=Nm, where N is the number of grooves covered and m is the order of
diffraction. The wavelength is selected by rotating the grating round an axis parallel to
the grooves.
Laser pumped dye lasers
A condition that has to be fulfilled (if the dye isn’t circulating) is that the excitation
pulse has to be shorter than the inverse of the transition probability S1  T1. This
condition is easily fulfilled with a Nd:YAG laser, but excimer, nitrogen or ruby lasers
might also be used. There are two ways of pumping: longitudinal and transversal
pumping. When longitudinal pumping is used the pump beam enters the resonator
through one of the mirrors, see figure 9. Most of the commercially available
continuous wave dye lasers are of this type despite the fact that geometrical design is
complicated.
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Figure 9. Two examples of a longitudinally pumped resonator. AR stands for antireflection coated.
Most pulsed dye lasers are transversely pumped. When the pump beam is focused in
the dye cell the stimulated emission is emitted perpendicular to the pump beam.
Figure 10 shows two examples of transversely pumped, tuneable dye lasers. The one
to the left, the Hänsch cavity [3], has a telescope between the dye and the grating to
increase the spectral resolution and in addition you avoid local heating of the grating
that might damage the surface.
Figure 10a. Hänsch cavity.
Figure 10b. Littman cavity.
The Littman cavity uses another solution. A grating at grazing incidence is used and
the wavelength tuning is done by rotating the mirror marked M2. The laser radiation is
coupled out in the zeroth order diffraction as in figure 10b or through the end mirror
marked M1, this method gives a narrower spectral bandwidth. The advantage with the
Littman cavity is that it is possible to make it shorter than the Hänsch cavity, it is
important when the pump pulse only is a couple of nanoseconds long and you want to
have the highest possible intensity in the cavity.
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Equipment
Time resolved electronics
During the lab exercise we will record the light intensity as a function of time. The
detector is a photosensitive diode with the rising time tr ~ 1 ns. The diode is
connected to a fast digital memory oscilloscope and a printer.
The power supply for the Nd:YAG laser
A schematic drawing of the electrical circuit is shown in figure 11. When the
discharge lamp is switched off, the capacitor (C ~ 10 F) is charged to the same
voltage as the DC power supply. When the capacitor is charged a trigger pulse is sent
from the trigger unit to the trigger transformer. This makes the voltage over the
discharge lamp high enough during a short time to ignite it and this lowers the
resistance. The capacitor is discharged through the inductor (L ~ 10 H) and the
discharge lamp. The inductor reduces the current and makes the discharge lamp pulse
appropriately long. The resistance of the discharge lamp is reduced when the current is
increasing. The capacitor, the inductor and the discharge lamp form a strongly damped
electrical system (RCL-circuit). The exact oscillation time is hard to calculate due to
the varying resistance. An approximation of the duration of the discharge is given by
the time for a half period for the undamped system (LC-circuit).
t  12  2 L  C  0.3 ms
The resistor R is chosen so that the charging time for the capacitor tc=RC is much
longer than the oscillation time for the discharge.
Figure 11. A schematic drawing of the triggering of the discharge lamps.
The cooling system of the Nd:YAG laser
A big amount of the electrical energy that is used for the pumping process is
transformed to heat and have to be cooled away. It is particularly important that the
discharge lamps and the YAG rod are cooled. In a primary cooling circuit di-ionised
water is circulating. The primary cooling circuit is cooled in a heat exchanger with a
secondary cooling circuit. This cooling circuit is made of copper tubes and it is
ordinary tap water circulating (figure 12).
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Figure 12. A schematic drawing of the cooling system of the Nd:YAG laser.
The optics of the Nd:YAG laser
The laser used during the lab exercise is a commercially available DCR-1 Quanta-Ray
laser. The pumping is done by two discharge lamps each placed in the focal spot of a
cut ellipse, which is coated with a highly reflective gold layer. The other focal spot of
those ellipses are common and the Nd:YAG rod is placed here (figure 13). The
Nd:YAG rod is approximately 10 cm long and has a diameter of 8 mm. The resonator
is unstable in the positive branch. A drawing of the optical layout of the resonator is
found in figure 14. The Q-switch consists
of a Pockels cell and a glass plate in
Brewster angle. Normally the voltage over
the Pockels cell is 3.1 kV. When the high
voltage is turned off the cavity becomes
transparent for light having such a
polarisation that it may pass the Brewster
plate. A second discharge lamp pumped
Nd:YAG rod is placed outside the cavity
and works as an amplifier. The frequency
doubling is done in a KD*P-crystal.
Figure 13. Geometrical arrangement for
the pumping of the Nd:YAG rod.
Figure 14. Optical layout of the Nd:YAG laser being used during the lab exercise.
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Safety
Several parts of the Nd:YAG laser (e.g. the Pockels cells and the discharge lamps) are
charged with high voltage. To prevent accidents due to electrical shock, NEVER
TOUCH THE INTERIOR OF THE LASER.
The direct laser beam or its reflection can cause severe eye damage. To prevent
accidents, all objects with shiny reflective surfaces (rings, watches, metal pencils...)
must be kept away from the work area. Since the laser is radiating in the infrared part
of the spectrum, which is not visible for the human eye, we wear LASER
PROTECTIVE EYEWEAR that is protecting the eye from the infrared light.
During the second part of the lab exercise, you will build a dye laser. Since the dye
laser is radiating in the visible part of the spectrum we can see all the reflexes. The
residual infrared light will be led into a beamdump.
Below is a list of precautions that will minimise risques in the laboratory:
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Use laser protective eyewear when working with the Nd:YAG laser.
Keep your head well above the optical table. Don’t bend down to pick up
pieces of dropped equipment.
Put away rings and watches before starting the laser.
NEVER attempt to insert new optical components into the beam without
blocking it first.
After inserting new components, think about where the new reflections will hit
BEFORE you let the beam through.
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Preparatory Assignments
1. Which wavelengths should be used to pump a Nd:YAG laser? Why?
2. A f=+200 mm lens is used to focus a gaussian beam with a radius of 3 mm. What
is the diffraction limited focal spot radius? (The wavelength of the light is
1.06 m.)
3. Would you say that most unstable lasers are unstable in the positive or negative
branch? Why?
4. In Star Wars, a planet is evaporated with a single shot from the laser on the Death
Star. How much Energy was required for that? (Suppose that the radius of the
planet was 6000 km, the density 10000 kg/m3 and the total heat required for
evaporation 1 MJ/Kg.)
5. How does the Q-switch work?
6. What is the frequency difference between two consecutive longitudinal modes if
the cavity length is 75 cm?
7. Suppose the laser fires 10 shots per second, and we measure the mean power of
the beam to be 0.75 W. What is then the pulse energy?
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Lab Exercises
The Nd:YAG laser
Start the laser
 Turn on cooling water.
 Turn on the main power on the voltage supply unit.
 Put on laser protective eyewear.
 Switch on the power switch on the laser.
 When the LED on the laser is lit, the discharge lamps are charged and ready to
be triggered.
Energy and efficiency
 Start by running the laser with no Q-switch.
 Increase energy to 40-50 J/pulse and examine the output with an infrared
viewer. Explain the spatial mode of the beam.
 Find the threshold for laser oscillation by decreasing the pump energy. Measure
the output energy. Increase the energy and draw Eout(Ein). Calculate the
efficiency of the laser.
The Q-switch.
The following steps should be done with and without the Q-switch. Compare and
explain the results.
 Use the oscilloscope and a photosensitive diode to record the time resolved
reflex from a black screen. Explain the pulseshape. What is the pulselength
(FWHM)?
 Zoom in on the pulse curve. A structure appears. What is the origin of this
structure? Calculate the frequency of the structure. Is the value of the frequency
what you expected?
 Calculate the peak power of the pulse.
Hole shooting in brass.
 Focus the infrared light onto a brass plate. The energy should be about 5
mJ/pulse. Fire single shots until the laser burns a small hole.
 Try to shot hole in the brass plate with one single pulse with an energy
corresponding to the total energy of the shots fired with 5 mJ/pulse.
 Compare the energy needed with a theoretical calculation and try to explain the
difference.
Frequency-doubled light
 Turn off the laser, put the frequency-doubling crystal in the active position and
start the laser. Optimise the green light!
 Use diachronic mirrors to isolate the green light and measure the energy.
Calculate the doubling efficiency with and without Q-switch
 With the Q-switch enabled, measure pulse length and peak power.
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The Dye Laser
You will now build and examine some properties of a Littman-type dye laser (see fig
10b).
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
Turn on the dye pump.
Block the exit windows of the dye cell with black screens.
By focusing the green light near the cell wall, it will be absorbed by the dye and strong
flourescence should be seen at both sides of the cell. To obtain a good beam quality, it
is important that the illuminated dye volume has a uniform cross section.
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Use the cylindrical lens to focus the green light into the dye cell. There will
probably be a strong back reflection from the lens. Be careful when you let the
beam through and block all reflexes before you continue.
Use the spectrometer to examine the flourescence from the dye. Determine
which dye you are using by comparing to the dye curves in figure 8.
Reflect the flourescence back into the cell with one of the mirrors.
Insert the holographic grating. Find the strongest reflection order, and use a
mirror to send the beam back.
Optimise!
Turn the mirror to find how large the wavelength interval is that you can have
laser emission in.
Measure wavelength, pulse energy, pulse duration and peak power at the
optimum wavelength.
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