laser = active medium + cavity 2010-11/lasers ii.pdf · laser = active medium + cavity •gain -...

J-D Ganiere MEP/2010-2011

Laser = active medium + cavity • gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

L

active medium

R1 R2

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Net gain per round trip

Steady state operation : g = 1the optical gain is exactly balanced by the sum of all the losses

gain (intrinsic)losses

expgI

Intensity after one round tripR R l2

01 2$ $ $b a= = -^ h6 @

• gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

L

active medium

R1 R2

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Cavity

The optical cavity is nothing else than a Perot-Fabry interferometer ... !

Only frequences, νmnq , corresponding to the optical

modes of cavities are authorized:

with:

g1 = 1 - r1l

g2 = 1 - r2l


o

o

o

o

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Cavity ... longitudinal modes

For a perfect cavity (100% reflectivity for the mirrors, g1=g2=1).

The longitudinal modes (m=n=0) are separated by c/2L


o

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Resonator ... optical modes

spatial distribution of the intensity for different radial and angular modes


The modes are denoted TEMpl where p and l are integers labeling the radial and angular mode orders

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Unstable / stable resonators

g1 = 1 - r1l

g2 = 1 - r2l

0 # g1 $ g2 # 1stability condition


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Laser ... all-lines

laser He-Ne

To get only one single line, one must introduce in the cavity an element which induces losses for the different wavelengths except one ...

A dispersive element inside the cavity can force the laser to work on a single line

A laser can work in a multi-line mode !


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0

collisions

**

*

fast decay onstrong visible transitions(0.54 - 0.73 !m)En

ergy

[eV]

3.39 !m

632.8 nm

1.15 !m

Helium Neon

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Laser ... single line operation

Argon laser

This configuration is known as the Littrow configuration ... (i.e. the wavelength λ2 is refected back on the same axis ...)

Single line operation

Idea: Introduce some losses at the different wavelengths except at λ2


mm m

m

m

m

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Laser ... multimode operation

gain curve (closely related to the line width profile)

optical cavity modes

We observe experimentally that the laser tends to work on several wavelengths ... This is a bit paradoxical, because of the gain saturation ...


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Why does the laser work on several modes ?

There is a saturation of the gain above the threshold ... only one mode should lase !

Hole burning (spatial and spectral)

Laser ... multimode operation• gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

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spatial hole burning

The different modes use different parts of the active material

Laser ... spatial hole burning• gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

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spectral hole burning In the case of an inhomogeneous broadening

Laser ... spectral hole burning • gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

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Laser ... single-frequency mode • gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

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To force the laser to work in a single-frequency mode, we can choose two different options:

• Ultra-short cavity • Conventionnal cavity + etalon

Laser ... single-frequency operation• gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

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Ring lasers

A ring laser is a laser, with a resonator which has the form of a ring. In contrast to a standing-wave laser resonator, such a ring resonator a l l ows for two d i fferent propagation directions of the intracavity light.

In many cases, unidirectional operation (where light propagates only in one of the two possible directions) is enforced by introducing an element into the resonator which leads to different losses for the propagation directions; this can be, e.g., a Faraday rotator combined with a polarizing element (e.g. a Brewster surface of the laser crystal ). If unidirectional operation is achieved, there is no standing-wave interference pattern in the laser gain medium (except near reflection points), and consequently no spatial hole burning. Therefore, single-frequency-operation, is easily achieved. Particularly for solid-state bulk lasers, unidirectional ring laser designs can be considered as a standard approach to obtain stable single-frequency emission. from RP photonics


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Dye lasers

A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. A dye laser consists of an organic dye mixed with a solvent, which may be circulated through a dye cell, or streamed through open air using a dye jet. A high energy source of light is needed to "pump" the liquid beyond its lasing threshold.The dyes used in these lasers contain rather large organic molecules which fluoresce when exposed to the proper frequency of light. Dyes will emit stimulated radiation when the molecules are in their singlet state. In this state, the molecules emit light via fluorescence, and the dye is quite clear to the lasing wavelength. Within a microsecond, or less, the molecules will change to their triplet state . In the triplet state, light is emitted via phoosphorescence , and the molecules begin to absorb the lasing wavelength, making the dye opaque,the dye must be circulated at high speeds to keep the triplet molecules out of the beam path.


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Dye lasers

due to the lifetime of the triplet states, the laser does not work in CW regime ....


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Les lasers accordables (dye lasers)• gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

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Q-switched laser

Q-switching, sometimes known as giant pulse formation, is a technique used to produce a pulsed output beam. Compared to modelocking, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations.Q-switching is achieved by putting some type of variable attenuator, called "Q-switched" inside the optical resonator . When the attenuator is functioning, that's correspond to a decrease in the Q factor of the optical resonator .

Active Q-switchingHere, the Q-switch is an externally-controlled variable attenuator. This may be a mechanical device such as a shutter, chopper wheel, or spinning mirror/prism placed inside the cavity, or (more commonly) it may be some form of modulator such as an acousto-optic device or an electro-optic device — a Pockels cell or Kerr cell.

Passive Q-switchingIn this case, the Q-switch is a saturable absorber, a material whose transmission increases when the intensity of light exceeds some threshold. The material may be an ion-doped crystal like Cr:YAG, which is used for Q-switching of Nd:YAG lasers, a bleachable dye, or a passive semiconductor device.

from Wikipedia


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Q-switched lasers

end mirror (100%) Flash lamp

(pumping)

laser used to probe the angular position of the output coupler

detector

Output coupler

Active medium

time

time

time

time

time

time

Output power

Gain

Q cavity

pumping intensity

mirror angle

detector

0°

The output coupler is mounted on a rotating holder. Lasing is obtained only when there is no loss, e.g. when the output coupler is perfectly aligned with the end mirror.


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Q-switched lasers

laser crystal cut at bewster angle

end mirror(100 %)

polarzation selectorElectro-optic crystal

output coupler

The laser crystal is cut at the brewster angle and only one polarization can propagate without loss.The Q-switching is simply achieved using a Pockels cell (electro-optic crystal which can rotate the polarization ...).


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Mode-locked lasers

Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10!12s) or femtoseconds (10!15s).The basis of the technique is to induce a fixed phase relationship between the modes of the laser's resonant cavity. The laser is then said to be phase-locked or mode-locked. Interference between these modes causes the laser light to be produced as a train of pulses. Depending on the properties of the laser, these pulses may be of extremely brief duration, as short as a few femtoseconds.In a simple laser, each of these modes will oscillate independently, with no fixed relationship between each other, in essence like a set of independent lasers all emitting light at slightly different frequencies. The individual phase of the light waves in each mode is not fixed, and may vary randomly due to such things as thermal changes in materials of the laser. In lasers with only a few oscillating modes, interference between the modes can cause brating effects in the laser output, leading to random fluctuations in intensity; in lasers with many thousands of modes, these interference effects tend to average to a near-constant output intensity, and the laser operation is known as a c.w. or continuous wave.


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Mode-locked lasers

If instead of oscillating independently, each mode operates with a fixed phase between it and the other modes, the laser output behaves quite differently. Instead of a random or constant output intensity, the modes of the laser will periodically all constructively interfere with one another, producing an intense burst or pulse of light. Such a laser is said to be mode-locked orphase-locked. These pulses occur separated in time by " = 2L/c, where " is the time taken for the light to make exactly one round trip of the laser cavity. This time corresponds to a frequency exactly equal to the mode spacing of the laser, #$ = 1/". The duration of each pulse of light is determined by the number of modes which are oscillating in phase (in a real laser, it is not necessarily true that all of the laser's modes will be phase-locked). If there are N modes locked with a frequency separation #$, the overall mode-locked bandwidth is N#$, and the wider this bandwidth, the shorter the pulse duration from the laser. In practice, the actual pulse duration is determined by the shape of each pulse, which is in turn determined by the exact amplitude and phase relationship of each longitudinal mode.


time bandwidth product:

.tN0 44$ o

DD

=

.tN0 44$ o

DD

=

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Mode-locked lasers• gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

( )I t E Nmax 02 2$=

tN1

p $yD

D=

laser gainbandwidth

cavity longitudinal modes

laser output spectrum

frequency

inte

nsit

y

c2L

=

N modes

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Mode-locked lasers• gain - absorption, linewidth • 3-4 levels models• Optical feedback, threshold conditions• Single line, single frequency operating modes• pulsed lasers (Q-switched, mode-locked lasers)

!0 + #!

!0 - #!

!0

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Laser safety

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Wavelengths of common lasers used at IPEQ

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Argon laser• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser, chemical laser• Nd:YAG laser, Ti:saph laser

3p44p

3p44s

3p5

lasing488 nmA=7.8 107/sec

73.1 nmA=4.5 108/sec

72.3 nmA=23 108/sec

3P01/23P03/2

levels 4p

levels 4s

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Argon laser

The upper laser levels (there are several, clumped tightly together) are about 20 eV above the ground state of the argon ion or nearly 36 eV above the ground state of the argon atom. Obviously, this is a very high energy which will require a large pump energy to build-up a high population of ions in that high-energy state. The dynamics of the argon ion are good for CW laser action in that the lifetime of the lower level is very short compared to the upper level. This allows population inversion to be maintained so long as a large pump energy is available (and all argon lasers need large pump energies - most have between 10 to 70A continuously through the discharge!). The short lifetime of the lower lasing level lead to another problem though in that ions in that energy state (i.e. having just emmitted a coherent photon) drop rapidly to the ground state of argon ion. This is a large jump and results in the spontaneous emission of a 74-nm extreme-ultraviolet photon (the energy had to go somewhere, right?). From an efficiency standpoint, this means an excited ion at the upper lasing level loses about 2 eV of energy in producing the coherent photon then loses 18 eV in spontaneous emission of that UV photon. These dynamics limit efficiency severely. As well the extreme UV light from that emission can damage many optical materials so mirrors and windows in an argon laser must be built to withstand such punishment!

There are three major mechanisms which raise the argon ion to the upper laser levels. One is energy transfer from an electron to a ground-state argon ion. Another is collisional transfer of energy from an electron to the argon ion in an excited metastable state similar to the way in which helium pumps neon's levels in the HeNe laser. The third is decay of higher levels produced by electron excitation. All three effects combine to pump the argon ions to the upper levels for lasing.

• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser, chemical laser• Nd:YAG laser, Ti:saph laser

from Pofessor Mark Csele's Homebuilt Lasers Page

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Argon laser • Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser• Nd:YAG laser, Ti:saph laser

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Argon laser ... single mode • Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser• Nd:YAG laser, Ti:saph laser

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He-Ne laser

A 1400 V high voltage, DC power supply maintains a glow discharge or plasma in a glass tube containing an optimal mixture (typically 5:1 to 7:1) of helium and neon gas. The discharge current is limited to about 5 mA by a 91 kW ballast resistor. Energetic electrons accelerating from the cathode to the anode collide with He and Ne atoms in the laser tube, producing a large number of neutral He and Ne atoms in excited states. He and Ne atoms in excited states can deexcite and return to their ground states by spontaneously emitting light. This light makes up the bright pink-red glow of the plasma that is seen even in the absence of laser action.

• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser• Nd:YAG laser, Ti:saph laser

http://community.middlebury.edu/~PHManual/heliumneon.html

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0

collisions

**

*

fast decay onstrong visible transitions(0.54 - 0.73 !m)En

ergy

[eV]

3.39 !m

632.8 nm

1.15 !m

Helium Neon

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Helium-Neon laser• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser• Nd:YAG laser, Ti:saph laser

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He-Ne laser• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser• Nd:YAG laser, Ti:saph laser

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Excimer laser

The term excimer is short for 'excited dimer', while exciplex is short for 'excited complex'. An excimer laser typically uses a combination of an inert gas (Ar, Kr, or Xe) and a reactive gas (fluorine or chlorine). Under the appropriate conditions of electrical stimulation, a pseudo-molecule called an excimer (or in case of noble gas halides, exciplex) is created, which can only exist in an energized state and can give rise to laser light in the UV range.Laser action in an excimer molecule occurs because it has a bound (associative) excited state, but a repulsive (disassociative) ground state. This is because noble gases such as Xe and Kr are highly inert and do not usually form chemical compound. However, when in an excited state (induced by an electrical discharge or high-energy electron beams, which produce high energy pulses), they can form temporarily-bound molecules with themselves (dimers) or with halogens (complexes) such as Fl and Cl. The excited compound can give up its excess energy by undergoing spontaneous or stimulated emission, resulting in a strongly-repulsive ground state molecule which very quickly (on the order of a picosecond) dissociates back into two unbound atoms. This forms a population inversion between the two states.


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Le laser excimer ArF, KrF, ...• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser, chemical laser• Nd:YAG laser, Ti:saph laser

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Excimer laser

The wavelength of an excimer laser depends on the molecules used, and is usually in the ultraviolet


Excimer Wavelength Relative Power (mW)

Ar2 126 nm

Kr2 146 nm

F2 157 nm 10

Xe2 172 and 175 nm

ArF 193 nm 60

KrF 248 nm 100

XeBr 282 nm

XeCl 308 nm 50

XeF 351 nm 45

KrCl 222 nm 25

Cl2 259 nm

N2 337 nm 5

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Chemical laser

A chemical laser is a laser that obtains its energy from a chemical reaction Chemical lasers can achieve CW output with power reaching to MW levels. They are used in industry for cutting and drilling.Common examples of chemical lasers are the chemical oxygen iodine laser (COIL), all gas-phase iodine laser (AGIL), and the HF laser and deuterium fluoride laser, both operating in the mid-infrared region.

Origin of the CW chemical HF/DF laserVery quickly, deuterium was dropped in favor of hydrogen, since it is far less costly and more readily available. However, later it was realized that HF produces infrared radiation in the 2.6 to 3.1 µm waveband, a region of the spectrum absorbed by water vapor in the atmosphere. Interest was renewed in DF, which produces radiation in the 3.7 to 4.2 µm band, which passes easily through the atmosphere.


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Les lasers chimiques• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser, chemical laser• Nd:YAG laser, Ti:saph laser

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Chemical laser• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser, chemical laser• Nd:YAG laser, Ti:saph laser

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CO2 laser

The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed (invented by Kumar Patel of Bell labs in 1964), and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available.CO2 lasers are also quite efficient: the ratio of output power to pump power can be as large as 20%.The CO2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.4 and 10.6 micrometers%.


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CO2 Lasers• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser, chemical laser• Nd:YAG laser, Ti:saph laser

collisions

Pumping

Fast decay

Fast decay

Ground level

Lasertransitions

[ R10 ]9.6 !m

[ P10 ]10.6 !m **

collisions

Nitrogen N2 Carbon Dioxyde CO2

Asymmetricstrech

Symmetricstrech

Bending

0.0

0.1

0.2

0.3

C OO

C OO

COO

C OO

CO2 molecule

asymmetric stretch mode

bending mode

symmetric stretch mode

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CO2 laser• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser, chemical laser• Nd:YAG laser, Ti:saph laser

Because of the high power levels available (combined with reasonable cost for the laser), CO2 lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving. They are also very useful in surgical procedures because water (which makes up most biological tissue) absorbs this frequency of light very well. Some examples of medical uses are laser surgery, skin resurfacing ("laser facelifts") (which essentially consist of burning the skin to promote collagen formation), and dermabrasion. Also, it could be used to treat certa i n sk i n cond i t i ons by remov ing embarrassing or annoying bumps, podules, etc. Researchers are experimenting with using CO2 lasers to weld human tissue, as an alternative to traditional sutures.

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YAG lasers

Nd:YAG (neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12) is a crystal that is used as a lasing medium for solid state laser [DPSS - Diode-Pumped Solid-State]. The dopant, triply ionized neodynium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet (YAG), since they are of similar size. Generally the crystalline host is doped with around 1% neodymium by atomic percent.Laser operation of Nd:YAG was first demonstrated by Geusic et al. at Bell Laboratories in 1964.

Nd:YAG lasers are optically pumped using a flashlamp or laser diodes . They are one of the most common types of laser, and are used for many different applications. Nd:YAG lasers typically emit light with a wavelength of 1064 nm. Nd:YAG lasers operate in both pulsed and continuous mode. Pulsed Nd:YAG lasers are typically operated in the so called Q-switching mode.

Ref.: Wikipedia

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YAG laser

Pumping bands

Pum

ping

ground state

nonradiative decay

nonradiative decay

1.06 !mlaser transition

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Ti:sapphire laser

Ti:sapphire lasers (also known as Ti:Al2O3 lasers, titanium-sapphire lasers, or simply Ti:sapphs) are tunable lasers which emit red and near-infrared light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulses . Lasers based on Ti:sapphire were first constructed in 1982.

Titanium-sapphire refers to the lasing medium , a crystal of sapphire (Al2O3) that is doped with titanium ions . A Ti:sapphire laser is usually pumped with another laser with a wavelength of 514 to 532 nm, for which argon-ion lasers (514.5 nm) and frequency-doubled Nd:Yag, Nd:YLF, and Nd:YVO lasers (527-532 nm) are used. Ti:sapphire lasers operate most efficiently at wavelengths near 800 nm.

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Ti:sapphire laser

opti

cal p

umpi

ng

collisonal relaxation

collisonal relaxation

***tunablelaser

output

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Semiconductor laser• Argon laser, CO2 laser, He-Ne laser• Semiconductor laser• Excimer laser, chemical laser• Nd:YAG laser, Ti:saph laser

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Temporal coherence

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Spatial coherence

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Cavité P-F

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Cavité P-F

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Cavité P-F

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Lamb dip

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laser = active medium + cavity 2010-11/lasers ii.pdf · laser = active medium + cavity •gain -...

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