the ndyag and dye lasers

8/8/2019 The NdYAG and Dye Lasers

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The Nd:YAG&

The Dye Lasers

THEORY..................................................................................................................................................2

THE ND:YAG LASER........................................................................................................................2Crystal..............................................................................................................................................2Pumping...........................................................................................................................................2Resonator.........................................................................................................................................3Q-switching and frequency-doubling..............................................................................................4

THEDYELASER.................................................................................................................................6The dye.............................................................................................................................................6Laser pumped dye lasers.................................................................................................................9

EQUIPMENT.........................................................................................................................................11

Time resolved electronics..............................................................................................................11The power supply for the Nd:YAG laser.......................................................................................11The cooling system of the Nd:YAG laser......................................................................................11

The optics of the Nd:YAG laser.....................................................................................................12

SAFETY..................................................................................................................................................13

PREPARATORY ASSIGNMENTS.....................................................................................................15

LAB EXERCISES..................................................................................................................................16

THE ND:YAG LASER......................................................................................................................16Start the laser.................................................................................................................................16Energy and efficiency....................................................................................................................16The Q-switch. ................................................................................................................................16Hole shooting in brass. .................................................................................................................16 Frequency-doubled light...............................................................................................................16

THE DYE LASER.............................................................................................................................17


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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 laser-qualities. 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

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the Nd:YAG laser, halogen lamps were used for pumping because of their broad

spectral profile.

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 simplyhad 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 efficientchoice. 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 continuousNd: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


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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 g1g21 (see figure 3).

Q-switching and frequency-doubling

For many applications a high peak power is desired e.g. to pump dye lasers. Q-switching 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).

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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 circular-polarised. 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 doesnt 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 turned off. The Brewster plate still makes the

light linear-polarised but now the Pockels cell dont 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. Thislight 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 thecirculating 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 Huygens 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, lets 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 bi-refrignent 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. Aschematic energy level diagram for a dye molecule is shown in figure 6. When the

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molecules are excited by visible or ultraviolet radiation they emit an intense and

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 thesolvent. 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 isnoticed 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 hasto 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 moleculesending 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 T1S0, 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 dyelasers.

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

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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 isnt circulating) is that the excitationpulse 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.

Figure 9. Two examples of a longitudinally pumped resonator. AR stands for anti-

reflection 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 Hnsch 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. Hnsch cavity. Figure 10b. Littman cavity.

The Littman cavity uses another solution. A grating at grazing incidence is used andthe wavelength tuning is done by rotating the mirror marked M2. The laser radiationis coupled out in the zeroth order diffraction as in figure 10b or through the endmirror marked M1, this method gives a narrower spectral bandwidth. The advantagewith the Littman cavity is that it is possible to make it shorter than the Hnsch cavity,

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it is important when the pump pulse only is a couple of nanoseconds long and youwant 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. Thedetector 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).

3.022

1 CLt msThe 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-ionisedwater is circulating. The primary cooling circuit is cooled in a heat exchanger with a

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secondary cooling circuit. This cooling circuit is made of copper tubes and it is

ordinary tap water circulating (figure 12).

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.

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Figure 14. Optical layout of the Nd:YAG laser being used during the lab exercise.

Safety

Several parts of the Nd:YAG laser (e.g. the Pockels cells and the discharge lamps) arecharged with high voltage. To prevent accidents due to electrical shock, NEVERTOUCH THE INTERIOR OF THE LASER.

The direct laser beam or its reflection can cause severe eye damage. To preventaccidents, 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 partof the spectrum, which is not visible for the human eye, we wear LASER

PROTECTIVE EYEWEARthat is protecting the eye from the infrared light.

During the second part of the lab exercise, you will build a dye laser. Since the dyelaser 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:

Use laser protective eyewear when working with the Nd:YAG laser.

Keep your head well above the optical table. Dont 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. Whatis the diffraction limited focal spot radius? (The wavelength of the light is1.06 m.)

3. Would you say that most unstable lasers are unstable in the positive or negativebranch? Why?

4. In Star Wars, a planet is evaporated with a single shot from the laser on the DeathStar. 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 forevaporation 1 MJ/Kg.)

5. How does the Q-switch work?

6. What is the frequency difference between two consecutive longitudinal modes ifthe cavity length is 75 cm?

7. Suppose the laser fires 10 shots per second, and we measure the mean power ofthe 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 fig10b).

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.

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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the ndyag and dye lasers

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