Popular Pumping Mechanisms
Requirements of a LASER (i)
Active Medium :- Gain per unit length is an inherent property. property.
(ii) (i i) Op Opti tica call Re Reso sona nato torr :- Length (L) dependence of gain ( but L can not be very very high without limit, rather it should be as low as possible). (iii)) Pumpi (iii Pumping ng Mechanism Mechanism (Method (Method of obtaining obtaining Populati Population on Inversion) Inversion) :Has to be very effective in order to ensure high enough “ Population Difference”. NOTE :- All three of the above are equally important. However, having chosen an active medium and optimized the resonator parameters, the factor most potentially tailored is the Pumping mechanism.
Requirements of a LASER (i)
Active Medium :- Gain per unit length is an inherent property. property.
(ii) (i i) Op Opti tica call Re Reso sona nato torr :- Length (L) dependence of gain ( but L can not be very very high without limit, rather it should be as low as possible). (iii)) Pumpi (iii Pumping ng Mechanism Mechanism (Method (Method of obtaining obtaining Populati Population on Inversion) Inversion) :Has to be very effective in order to ensure high enough “ Population Difference”. NOTE :- All three of the above are equally important. However, having chosen an active medium and optimized the resonator parameters, the factor most potentially tailored is the Pumping mechanism.
Population Inversion Methods / Pumping Mechanisms (i)
Optical Pu Pumping
(ii) (i i) El Elec ectr tric ical al Pu Pump mpin ing g
}
Most common
Other Methods (iii) Chemical Pumping (iv) Gas dynamic Pumping (v) Laser Pu Pumping (vi) (v i) Nu Nucl clea earr Pu Pump mpin ing g (vii)) Parti (vii Particle-k cle-kinet inetic ic energy Pumping
Optically Pumped Lasers :Ruby (Al2O3 – Cr3+) Nd:YAG (Nd – Y2Al5O12), Yb:YAG, Er: YAG Nd:Glass, Er:Glass Nd:KGW Nd:YVO4 Nd:GSGG Nd:YLF Ti:Sapphire, Cr:LiSAlF, Cr:LiCaAlF Alexandrite (Cr doped chrysoberyl = BeAl 2O4 – Cr 3+) Dye Lasers (Liquid Laser) Cs Vapor (Gas Laser) Fiber Lasers
Solid State Lasers
Electrically Pumped Lasers :Gas Lasers :(i)
Atomic :- He-Ne, He-Cd, He-Zn, He-Hg, Cu Vapor, Au Vapor, Pb Vapor, Water Vapor (Far IR – 30 µm to 1.8 mm)
(ii) Ionic :- Ar, Kr, Ne, Xe (iii) Molecular :- CO2, CO, N2, Excimer, CH3OH, C2H2F2, CH3F, HCN (Far IR) Semiconductor Lasers :(i) Binary :- Ga As, In P, Zn Se ( II-VI), Ga N (Blue-Green) (ii) Ternary :- In Ga As, Ga Al As (iii) Quaternary :- In Al Ga As, In Al Ga P, Ga In As Sb, Al Ga As Sb
Chemical Pumping :HF, DF, HCl, HBr, COIL, I-Photodissociation
Gas Dynamic Pumping :- CO2 GDL
Laser Pumping :CH3 OH (by CO2 Laser), Nd:YAG (by Diode Lasers), Dye Laser (by doubled & tripled Nd:YAG, N 2, Ar+, Excimer - KrF, XeF, XeCl, Q-switched Ruby, Copper vapor, Krypton Laser)
Nuclear Pumping :- X-ray Laser Other Pumping methods:Free Electron Laser (Particle-kinetic energy Pumping), X-ray Laser, Gamma Ray Laser
NOTE :- Gas lasers do not lend themselves so readily to optical pumping because of the small widths of their absorption lines and usually broad emission of the pumping lamps. However, He lamp (~ 390 nm) and Cs vapor absorption lines match and hence optical pumping of the gas laser is possible.
OPTICAL PUMPING
Optical Pumping
Many optically pumped lasers have a gain medium
consisting
transition
metal
of
ions
rare
earth
or
doped
into
an
insulating dielectric solid.
E
ULL
In a laser that is optically pumped, the upper
laser
level
is
populated
by
absorption of a photon from some optical source.
That is the laser material is illuminated with light at the right wavelength to excite the lasing species.
LLL
The light source can be a high-intensity lamp (lamp pumping) or another laser (laser pumping).
G
The upper levels of the pump transition usually span a range of energies. In fact, there are typically multiple upper levels, which all decay to the metastable ULL.
This means the laser can be excited at many wavelengths corresponding to any transition between G and those many upper levels. Thus, many (solid state) lasers are optically pumped with light sources emitting a broad range of wavelengths.
The early lasers were mostly lamp-pumped.
But the trend in recent years has been toward laser-pumped lasers.
Pumping Process :
Light from a powerful source (Flash lamp or Arc lamp or incandescent lamp) is conveyed to the active material which is usually in the form of a cylindrical rod (diameter of few mms to few cms and length of few cms to few 10s of cms).
The laser can be operated in pulsed or CW mode depending on whether the pump source is pulsed or continuous.
NOTE :“Optical pumping is a process in which light is used to raise (or ‘pump’)
electrons from a lower energy level in an atom or molecule to a higher one”.
The technique was developed by 1966 Nobel Prize winner Alfred Kastler in the early 1950s.
Schematic of Optical pumping of a laser rod (bottom) with an arc lamp (top). [Red : hot. Blue : cold. Green : light. Non-green arrows: water flow. Solid colors: metal. Light colors: fused quartz].
Commonly used Optical Pumping Configurations 1. Helical :-
Lamp
Rod Rod
Lamp Lamp is a long Quartz tube coiled into a helix. Diameter of the helix is small and helix is wound tightly. Light reaches directly or after reflection at the specular cylindrical surface.
2. Elliptical / Cylindrical :Lamp is in the form of a cylinder (Linear Lamp) and the length is ~ that of the active rod. Lamp Lamp is placed along one of the focal axes Rod
F1 of the elliptical cylinder and the rod is placed along the second focal axis F 2.
Cylindrical reflector
The distance between the electrodes, referred to as the “arc length” of this lamp, is generally chosen to be about the same as the laser rod length.
The “bore” of the flashlamp (the inside diameter of the quartz tubing or "envelope") is usually the same as the diameter of the laser rod. The lamp and rod are placed inside a reflecting housing with their axes parallel.
Elliptical reflector
Gas arc lamp with water jacket for cooling
Example :-
Linear single and double lamp pumping of Nd:YAG Laser
Single lamp elliptical reflector cavity
3. Close Coupled Configuration :The rod and the lamp are placed as close as possible and are surrounded by a close coupled cylindrical reflector. Cylinders made of diffusely reflecting material (Eg : Compressed MgO or BaSO 4 powder or
Cylindrical close-coupled
white ceramic) are often used.
4. Multiple Configurations :Multiple configurations using more than one elliptical cylinder or several lamps are used.
Double- Ellipse
Efficiency of multiple designs is lower than the corresponding single configurations, but are used in High Power systems. Close-coupled (Double)
Close-coupled configurations (a) Circular cylinder; (b) Single-lamp closewrap; (c) Double-lamp close-wrap (d) Four-lamp close-wrap (e) Closecoupled multiple coaxial design
Four-lobe Elliptical
Spherical
NOTE :
Helical pumping is simple but efficiency is poor.
A reasonably efficient pumping geometry is an elliptical cylinder reflector.
Greater pumping efficiency is achieved with the rod and lamp as near one another as possible with an ellipse of low eccentricity.
The most common pumping configurations are single ellipses with one lamp and double ellipses with two lamps.
Arrangement of Pump and Laser Rod
A ruby laser head
Laser pumping lamps. The top three are xenon flashlamps while the bottom one is a krypton arc lamp
These gas discharge lamps show the spectral line outputs of the various noble gases.
NOTE :
Three sources (lamps) for Optical Pumping :(a) Flash lamps (Pulsed pumping) (b) Arc lamps (CW pumping) (c) Incandescent lamps (Cheaper CW pumping with a Tungsten wire)
Flash/Arc Lamps generally use Xenon or Krypton gas inside a Quartz tube.
The Krypton lamp produces most of its output light in the infrared region of the absorption bands of Nd:YAG and Nd:Glass. Thus, it is the best spectral match for these laser materials.
Krypton lamps are not widely used because of their cost (Far more expensive than xenon lamps).
Xenon lamps usually have lower efficiency. But they have sufficient output in the desired spectral region & their lower efficiency is usually acceptable.
Xenon flash lamps have greater emission in the blue-green region of ruby laser absorption. Thus, they are used with all ruby lasers.
In spite of being expensive, high efficiency requirements and high power Nd:YAG and Nd:Glass systems demand the use of Krypton lamps.
Diode Laser Pumping
End pumping
Side pumping
Pumping Efficiency :To calculate or estimate the pumping efficiency, the pump process can be divided into four distinct steps : 1.
Emission of radiation by the lamp
2.
Transfer of this radiation to the active medium
3.
Absorption in the medium
4.
Transfer of the absorbed power to the upper laser level
Thus, the pumping efficiency η P can be written as the product of four terms as follows –
P
r t a pq
Where, ηr = Lamp radiative efficiency ηt = Transfer efficiency ηa = Absorption efficiency ηpq = Power quantum efficiency
. . . . . . . . . . . (1)
Lamp radiative efficiency (ηr) = The efficiency of conversion from electrical input to light output in the wavelength range corresponding to the pump bands of the laser medium. Transfer efficiency (ηt) = The ratio of the pump power actually entering the rod to that emitted by the lamp in the useful pump range. Absorption efficiency (ηa) = The fraction of the light entering the rod that is actually absorbed by the material. Power quantum efficiency ( ηpq) = The fraction of the absorbed power that leads to the population of the ULL. Typical Values :ηr = 0.43
= 0.36 ηt = 0.9 – 0.8
= 0.62
for flash lamp pumped Nd:YAG laser for flash lamp pumped Alexandrite laser for elliptical pump cavity for helical lamp
Comparison of computed ηP values : [ 6.3 mm diameter rod ; Elliptical pump chamber, lamp current density 2000 3000 A/cm2 ; Lamp diameter = 5 mm ]
Material
ηr (%)
ηt (%)
ηa (%)
ηpq (%)
ηP (%)
Ruby
27
78
31
46
3.0
Alexandrite
36
65
52
66
8.0
Nd:YAG
43
82
17
59
3.5
Nd:Glass
43
82
28
59
5.8
Nd:Cr:GSGG
43
82
54
48
9.1
Summary :1.
Radiative efficiency is < 50 % in each case.
2.
Absorption efficiency for Nd;Cr:GSGG is about 3 times that of Nd:YAG (because of Cr doping).
3.
Absorption efficiency for Alexandrite is quite high.
4.
Nd:Cr:GSGG and Alexandrite show high overall efficiency.
Comparison between Lamp pumped and Diode pumped Nd:YAG laser
Nd:YAG pumped by GaAlAs QW laser at 808nm with emission Bandwidth 1-2 nm. It can be seen that radiative and transfer efficiency is almost same but there is very large increase in absorption efficiency which leads to higher overall pump efficiency.
Example of Dye Laser
Dye Laser “Stimulated emission observed from an organic dye, chloroaluminum phthalocyanine” P. P. Sorokin and J. R.Lankard IBM J. Res.Dev. 10, 162 (1966).
755 µm
Dye Laser A dye (Liquid) laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths. The wide bandwidth makes them particularly suitable for tunable lasers and pulsed lasers. Moreover, the dye can be replaced by another type in order to generate different wavelengths with the same laser, although this usually requires replacing other optical components in the laser as well. Some of the dyes are Rhodamine 6G, fluorescein, coumarin, stilbene, umbelliferone, tetracene, malachite green.
Setup of a Tunable Dye Laser
Attractions :1. Unusual flexibility and Tunability Near UV to Visible and Near IR 2. Extremely narrow Spectral Bandwidth (Ultrapure light) 3. Ultrashort pulses (ps to ~ 25 fs)
Disadvantages :
Rapid degradation during operation
Very Complex liquid handling requirement
Limited output power
Need for pumping with green or blue laser, making the pump sources expensive
Handling of poisonous, often even carcinogenic and dirty material
Dyes themselves as well as the used solvents are sometimes
highly toxic (A particularly hazardous solvent, sometimes used for cyanide dyes, is dimethylsulfoxide (DMSO), which greatly accelerates the transport of dyes into the skin)
Construction
Since organic dyes tend to degrade under the influence of light, the dye solution is normally circulated from a large reservoir. The dye solution can be flowing through a cuvette, i.e., a glass container, or be as a dye jet, i.e., as a sheet-like stream in open air from a specially-shaped nozzle.
With a dye jet, reflection losses from the glass surfaces and contamination of the walls of the cuvette are avoided. These advantages come at the cost of a complicated alignment.
Dye lasers emission is inherently broad. In order to produce narrow bandwidth tuning there are many types of cavities and resonators which include gratings, prisms and etalons.
Rhodamine 6G, emitting at 580 nm (yellow-orange).
The most popular dye used for the dye laser is Rhodamine 6G. The reasons for its popularity :
Its low cost
Effectiveness
Easy availability
Low toxicity
Using Rhodamine 6G as the dye enables tuning of the output laser beam’s wavelength between 540 nm to 640 nm, (peak energy at 590 nm) depending on other factors in the laser.
Dye Laser Nd:YAG Laser
Doubled Nd at 532 nm
Dye laser pumped by 532 nm doubled Nd:YAG
Lasers suitable as pump source for Dye Lasers
Nitrogen (N2)
Argon Ion
Q-switched Ruby
Copper vapor
KrF
XeF
Frequency doubled Nd:YAG
Frequency tripled Nd:YAG
XeCl
Krypton
Working of Dye Laser The laser cycle begins with the dye molecule in the S 0
Excited state (S2) Triplet state (T2)
level. A photon is absorbed, raising the molecule to some
Triplet state (T1)
vibrational and rotational energy in the S 1 level.
(Intersystem crossing)
The molecule very quickly undergoes
an
electronic
transition (in a few femtoseconds), so that the molecule settles into the S1,G level. This transition does not produce laser radiation.
There are five paths by which the dye molecule may leave the S1,G state. These are :
Spontaneous emission
Stimulated emission
Excited-state absorption of a pump photon
Excited-state absorption of a laser photon
Decay into a triplet band
Only stimulated emission produces a usable laser beam. The other processes generally only reduce the amount of usable energy for the output beam and increase the heating of the solvent.
states from which laser emission takes place are called Singlets (S). The Triplet states (T) do not contribute to lasing process. Absorption of radiation takes the molecule from the
bottom level of S0 to one of the S1 levels, where non-radiative decay quickly brings it to the bottom level of S 1. The later serves as the ULL, the LLL being one of the Vibrational-Rotational levels of S0. Although S
T transitions are radiatively forbidden,
they may occur non-radiatively in collisions between molecules. Eg :- S1
T1 , T1
S0 . (Intersystem crossing)
Lasing begins when incident energy is absorbed by the dye, exciting it from the lowest singlet state to a high-energy level within the upper singlet band.
From the high-energy level the dye falls to a slightly lower state within the same singlet band, which serves as an upper lasing level.
A laser transition can then occur between the upper lasing level and the lower singlet state, which serves as a lower lasing level.
NOTE :An alternative pathway exists to destroy laser action in the triplet states of the dye.
NOTE :
Triplet states originate when excited electrons in the dye molecule spin in the same direction as that of the remaining electrons in the dye molecule.
The singlet states result when the excited electron spins in the direction opposite to the lower-energy-state valence electrons still in the dye molecule.
Because triplet states have lower energies
than
corresponding
singlet states, dye molecules can
easily migrate to those states and in doing so depopulate the upper lasing level.
Thus, Triplet quenching is required for efficient operation of Dye lasers. This is done by either (a) Rapidly flowing the dye (b) Using a pump source with a short pump pulse (e.g., N 2 laser
with 10 ns pulses) (c) Adding triplet quenching additives like cyclooctatetraene. They provide deexcitation pathway; dye molecules re-enter. NOTE :
Triplet states are metastable and have much longer lifetimes
than the singlet levels.
When a flashlamp is used (generally have pulse widths of over 1 ms), triplet states can form. For this reason, flashlamps
must be designed to discharge as quickly as possible.
Pumping Configurations :1. Longitudinal/End Pumping Dye laser cavity is collinear with the pump laser cavity.
Output
Pump Laser
Dye Cuvette Mirror
Mirror
Longitudinal Pumping Configuration
2. Transverse Pumping
The axis of the dye cavity is perpendicular to the axis of the pump laser cavity. Mirror
Cylindrical Lens Pump Laser
Dye Cuvette
Mirror
Output
Transverse Pumping Configuration
Schematic of Laser-pumped (transverse) dye laser
590 nm
Problems :-
The biggest problems in Dye lasers are – (a) The heat management (b) Degradation of the dye itself Solution :Both problems are alleviated by forming the dye into a
continually flowing sheet of liquid called a laminar flow. Flowing dye is pumped through a nozzle to create a broad, flat stream onto which pump laser light is focused by a lens. NOTE :- Dye flow helps suppress the effects of triplet absorption in the dye by ensuring a fresh supply of dye .
ELECTRICAL PUMPING
Electrical Pumping Achieved by allowing a current to pass through the gas mixture. Generally, the current through is passed either along the laser axis direction (Longitudinal discharge) or transversely to it (Transverse discharge).
An electric discharge may be produced in a gas contained inside a glass tube by applying a high voltage to the electrodes on either side of the tube.
Electrons are ejected from the cathode and drift towards the anode. When an electron collides with an atom (or molecule), there is a probability of raising it to some higher energy state.
Discharge Process :- In an electrical discharge, ions and free electrons acquire additional KE from the applied electric field and are able to excite a neutral atom by collision.
NOTE :- The positive ions, owing to their much heavier mass, are accelerated to lower velocities and thus do not play any significant part in the excitation process. Electrical pumping occurs via one or both of the following processes – (i)
Electron Impact (Direct) Eg: N2 Laser
(ii) Resonant Energy Transfer (Indirect) Eg: He-Ne Laser, CO 2 Laser
Electron Impact *
e X X e Here, the gas consists of only one species. X is the atom in the ground state and X * is in excited state. This is called “Collision of the 1 st kind”
The electron loses KE. Energy lost by the electron is converted to internal excitation energy of the atom.
Total energy (Internal + KE) before and after the collision are the same. The internal energy added to the molecule may be in the form of vibrational and rotational energy, as well as electronic energy.
Resonant Energy Transfer
For a gas consisting of two species (A & B), the excitation can occur as a result of collisions between atoms of different species.
Let B be in the ground state and A be in the excited state brought about by electron impact. After the collision, species A will be in ground state and B in excited state *
*
A B A B E
The energy difference Δ E will be added to or subtracted from the translational energy.
This is called “Collision of the 2 nd kind”.
This is an attractive way of pumping B, if the upper state of A is metastable forbidden transition.
Hence, once A is excited to its upper level, it will remain there for a long time, thus constituting an energy reservoir for excitation of the species B.
Resonant Energy Transfer Process
An excited species can transfer energy to another by photon transfer. That is, the photon spontaneously emitted by one species is absorbed by the other.
Here, the 1st species drops to a lower level and the 2 nd species is raised to a higher level. This means there is an excitation transfer. transfer.
Hence, one requirement is : The photon emitted by the donor species must be within the absorption linewidth of the acceptor species, i.e., there must be a resonance (or near-resonance near-resonance)) of the atomic transitions.
The transfer cross-section is large, when the corresponding atomic or molecular transition frequencies are approximately equal. However, excitation transfer can occur between species A and B even if the transitions are not precisely resonant.
The energy defect (ΔE) can be made up from translational degrees of freedom, thus in accordance with the Law of Conservation of Energy.
Based on whether the temperature of the system is raised or lowered, the energy defect can be of two types ty pes :(i) Positive energy defect (Exothermic) (ii) Negative energy defect (Endothermic)
Positive energy defect :
A* + B = A + B* + ΔE
A*
B*
ΔE
This process raises the temperature of the system
A
B
There is Exothermic excitation transfer. The extra energy ΔE after the excitation transfer appears as additional KE of A & B.
Negative energy defect :
A* + B = A + B* - ΔE B*
A*
A
ΔE
B
This endothermic process lowers the temperature of the system AB. The defect in energy is made up for at the expense of the collision partners. The KE of AB after the excitation transfer is less than that before the transfer.
Example of CO2 Laser
CO2 Laser
Lasing in a CO2 molecule was first demonstrated by CKN Patel in 1964.
CO2 is the active gas in which the lasing process occurs.
The standard CO 2 laser includes in the active medium a
mixture of CO2 with N2 and He. The optimal proportion of these three gases in the mixture depends on the laser system and the excitation mechanism. Generally, for a continuous wave laser the typical proportions are: CO2:N2:He - 1:1:8
CO2 is a linear tri-atomic molecule, and the three atoms are situated on a straight line with the Carbon atom in the middle. Three vibrational modes of CO 2 molecule are illustrated :
Symmetric stretch mode
Normal mode frequency 1388 cm -1
Bending mode
Normal mode frequency 667 cm -1
Asymmetric stretch mode
Normal mode frequency 2349 cm -1
N2 (V = 0)
Lasing in CO2 laser occur when there is a transition from higher energy level of the asymmetric mode into one of the other two. The transition to the symmetric stretching mode correspond to the wavelength of 10.6 μm (The most powerful and popular line). The transition to the bending mode correspond to ~ 9.6 μm
V1 001
100
020 010
V0
