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9-Laser
Laser is the acronym for Light Amplification by Stimulated Emission of Radiation.
A laser consists of two fundamental elements:
• An amplifying or gain medium (this can be a solid, a liquid or a gas). This medium is composed of atoms, molecules, ions or electrons whose energy levels are used to increase the power of a light wave during its propagation. The physical principle involved is called stimulated emission.
• A system to excite the amplifying medium (also called a pumping system). This creates the conditions for light amplification by supplying the necessary energy. There are different kinds of pumping system: optical (the sun, flash lamps, continuous arc lamps, diode or other lasers), electrical (gas discharge tubes, electric current in semiconductors) or even chemical.
These two components are sufficient to amplify an existing light source. This is known as a laser amplifier. However, most lasers also incorporate a cavity in order to produce a very special radiation. Technically, the whole device is known as a laser oscillator, but this term is often shortened to simply laser. The laser oscillator uses reflecting mirrors to amplify the light source considerably by bouncing it back and forth within the cavity. It also has an output beam mirror that enables part of the light wave in the cavity to be removed and its radiation used.
The different components that make up a basic laser are illustrated in the diagram below (Figure 1).

Figure 1
The emission-absorption principle
The three different mechanisms are shown below (Figure 2):

1. Absorption: An atom in a lower level absorbs a photon of frequency hν and moves to an upper level.


2. Spontaneous emission: An atom in an upper level can decay spontaneously to the lower level and emit a photon of frequency hν if the transition between E2 and E1 is radiative.


3. Stimulated emission: An incident photon causes an upper level atom to decay, emitting a “stimulated” photon whose properties are identical to those of the incident photon. The term “stimulated” underlines the fact that this kind of radiation only occurs if an incident photon is present. The amplification arises due to the similarities between the incident and emitted photons.

Figure 2
Competition between the three mechanisms:
For a radiative transition, these three mechanisms are always present at the same time. To make a laser medium, conditions have to be found that favour stimulated emission over absorption and spontaneous emission.

• An incident photon of energy hѵ has an equal chance of being absorbed by a ground-state atom as being duplicated (or amplified) by interacting with an excited-state atom. Absorption and stimulated emission are really two reciprocal processes subject to the same probability. To favour stimulated emission over absorption, there need to be more excited-state atoms than ground-state atoms.
• Spontaneous emission naturally tends to empty the upper level. It has been proved that stimulated emission is much more likely to happen if the medium used have a large number of photons. A good way to do this is to confine the photons in an optical cavity.

Principles of laser action:
1-Population inversion and pumping:
If there are more atoms in the upper level (N2) than in the lower level (N1), the system is not at equilibrium. In fact, at thermodynamic equilibrium, the distribution of the atoms between the levels is given by Boltzmann's Law:

  In this case, N2 is always less than N1. A situation not at equilibrium must be created by adding energy via a process known as pumping in order to raise enough atoms to the upper level. 

This is known as population inversion. Light is amplified when the population inversion is positive. Pumping may be electrical, optical or chemical.
Spectroscopic systems used to create a laser:
Not all atoms, ions and molecules, with their different energy levels, are capable of creating a population inversion and a laser effect. Only radiative transitions (where the atoms are excited due to light absorption) should be used and nonradiative transitions should be avoided. Some transitions have both a radiative and a non-radiative part. In this case, the upper level empties as a result of a nonradiative effect as well as spontaneous emission. This leads to additional problems for achieving a population inversion because it is difficult to store atoms in the upper level under these conditions. Thus, this type of transition should also be avoided.
Next, the relative energy levels specific to each type of atom must be considered. For example, choosing a lower level with more energy than the ground state will greatly limit the population N1, which may even be zero (Figure 3). This means that only one atom would have to be excited to achieve population inversion.

   In addition, pumping must be able to move atoms to a higher level. Every pumping system (particularly optical or electrical) corresponds to a certain energy, which must be transferable to the atoms of the medium. In optical pumping, there must be at least three different energy levels to create a population inversion. Figure 4 illustrates such a system. It shows the pumping transition (between E1 and E3) and the laser transition (between E2 and E1). The objective is to store atoms in level E2 by absorbing pumping radiation whose wavelength is shorter than that of the laser transition. This means that the excited atoms must quickly decay from level 3 to level 2 only, a condition that limits the choice of systems that will work.  

Figure 4: Three-level system
Figure 4 also shows an ideal cycle for an atom: it rises into level 3 by absorbing a photon from the pumping light. It then falls very rapidly into level 2. Finally, it decays by stimulated emission to level 1. Despite its simplicity, this is not a very easy system to implement as the ground state of the laser transition has a large population at thermodynamic equilibrium and at least half of this population must be excited to level 2 to obtain population inversion. Moreover, level 2 must be able to store these atoms so spontaneous emission must be very unlikely. This affects the choice of the system. A large pumping energy is also needed.

Another example of a spectroscopic system is the four-level laser (Figure 5). Here, the pumping transition (optical pumping) and the laser transition occur over a pair of distinct levels (E0 to E3 for the pump and E1 to E2 for the laser). E1 is chosen to be sufficiently far from the ground state E0 so that the thermal population at thermodynamic equilibrium is negligible. Similarly, atoms do not stay in level 3 or level 1.

Figure 5: The four level system
Figure 5 represents an ideal four-level system. Unlike the three-level system, as soon as one atom moves to level 2, a population inversion occurs and the medium becomes amplifying. To maintain the population inversion, atoms must not accumulate in level 1 but must rapidly decay to level 0.

A final example of a spectroscopic system providing a laser effect is the helium-neon gas system (Figure 6). In this case the pumping method is electrical. Neon transitions are used for the laser transitions: there are several but the most well-known is the coloured one at 632.8 nm. Helium is used as an intermediary gas, capable of transferring energy from the electrons to the neon particles via collisions. Helium is also unique in having two excited states said to be metastable i.e. atoms can stay there a long time before falling to the ground state. Helium atoms are carried into the excited state by collisions with electrons. Energy is easily transferred to neon when the atoms collide because these metastable levels coincide with the excited states of neon. This process is given by the equation:
He* + Ne  He +Ne*
An excited helium atom meets a ground-state neon atom and transfers its energy while decaying. Figure 6 also shows that the lower levels of the laser transitions are far from the ground state, which favours population inversion.

Figure 6: A Helium-Neon System
2- The role of optical cavity:
Population inversion is not enough to generate a laser effect. As stated previously, stimulated and spontaneous emissions are competing with each other. Thus, before becoming an amplifying medium, a laser medium pumped by an external energy source is first a lamp (spontaneous emission). It is the optical cavity that creates the conditions necessary for stimulated emission to become predominant over spontaneous emission. The cavity is composed of several mirrors that bounce the beam back and forth through the amplifying medium. There are two different types (Figure 7): linear cavities (light is reflected back and forced) and ring cavities (light circulates round and round).

Figure 7: The two types of optical cavity
When the laser starts up, the lamp-amplifying medium emits spontaneously in all directions. However, a small part of the emission occurs along the axis of the laser cavity. These spontaneous photons can travel backwards and forwards. Thus, over time, thanks to the amplifying medium, the amount of light in the cavity increases considerably. The confinement of the light increases the probability of stimulated emission rather than spontaneous emission occurring. At the same time, as explained in figure 8, the cavity acts as a filter due to the numerous round trips: only the wave perfectly perpendicular to the axis of the cavity will be propagated and certain frequencies will be favoured (the resonance frequencies of the cavity). In this way, the cavity produces a specific radiation.

Figure 8: Behaviour of a non-perpendicular light beam in an optical cavity
A laser operating in a steady state produces a light wave whose spatial structure does not change despite numerous round trips inside the cavity. In this case, the laser cavity must contain a light wave able to propagate in the cavity and remain constant after each round trip. This is known as a Gaussian wave whose light distribution is Gaussian in shape in the plane perpendicular to the axis of propagation. Physically, a Gaussian wave concentrates the light along the axis of the cavity.

For a linear cavity, only waves of a certain frequency can be successfully
propagated. This frequency is defined by where k is an integer, c the speed of light in a vacuum and L the optical length of the (linear) cavity. In the case of optical frequencies, k is very large and may reach tens of thousands for a cavity of a few centimetres. Waves that propagate with these frequencies in the cavity are known as longitudinal modes.
In the case of a ring cavity, the frequency is defined by where L is the optical length of the cavity circumference.
This filter is directly applied to the spectrum spontaneously emitted when the laser starts up. Progressively, the frequencies that cannot exist in the cavity disappear leaving only those that verify the equation above.

Figure 9: Appearance of the emitted spectrum of a laser compared to the spontaneous emission of a laser transition.

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    A laser is often described as monochromatic (for example, the helium-neon laser), a definition that must be well understood. In fact, broadly speaking, the spectral bandwidth of a laser is given by the width of the spontaneous emission: if the transition between the upper and lower levels is narrow, then the spontaneous emission will be fractions of a nanometre (this is the case for the red line in neon, which has a width equal to 1/1000th of a nanometre and a frequency of 1 GHz). The spectrum of a helium-neon laser is therefore “monochromatic” in the sense that only one colour is visible to the naked eye as the line is very narrow. Other types of laser have a much wider transition (for example, several hundreds of nanometres for the titanium-doped sapphire, which has a spontaneous emission spectrum ranging from 700 to more than 1000 nm) and consequently emit a spectrum that cannot be defined as monochromatic.  

3- Q-switching
In order to store many atoms in an upper level, the flow to a lower level must first be limited. Thus, stimulated emission must be prevented by placing an attenuator in the cavity to stop light from travelling back and forced (note: this attenuator is usually a light modulator, rather than a mechanical shutter, which reduces the amplitude or power of the light beam). In this case, for a radiative transition, the only decay to a lower level is due to spontaneous emission. When the pumping system supplies more atoms per second than lose energy by spontaneous emission, the population in the upper level can become very large (Figure 10).

Figure 10: Accumulation of atoms in the upper level when the optical cavity is blocked.
This operating condition is much easier to achieve with media that have a low rate of spontaneous emission. This is true for solid state ion-doped lasers but not for gas (neon or argon) or semiconductor lasers.

After a certain time, the energy losses in the cavity are suddenly reduced so that laser oscillation becomes possible. As there is a very large population in the upper level, stimulated emission becomes very probable and the laser is suddenly triggered. The flow due to stimulated emission is much greater than the other flows (filling by pumping and emptying by spontaneous emission): all the atoms stored in the upper level fall sharply, emitting stimulated photons (starting with the spontaneous emission trapped in the cavity). Thus, the laser cavity fills with stimulated photons at the same time as the upper level empties (Figure 11).

Figure 11: Laser effect once the optical cavity is suddenly opened.
Eventually, the upper level is completely empty. There is no further stimulated emission and the cavity will also empty due to the losses created by the output mirror (in general, the cavity empties after only a few round trips) (Figure 12).

Figure 12: Depletion of the optical cavity once all the atoms have returned to the ground state.
This process gives rise to a dramatic variation in the number of photons in the cavity (first by a significant amplification due to stimulated emission then by the complete emptying of the cavity at the end). The net result is the emission of a short pulse of light via the output mirror.
Generally, several round trips are needed to completely depopulate the upper energy level and several more round trips to empty the optical cavity so the duration of the pulse is greater than one round trip.
It should be noted that Q-switched lasers never reach a steady state as they stop functioning after several round trips of the light in the cavity.

4- Mode-locking
The second operating technique is completely different. This time, the laser oscillator is left to reach a steady state and the oscillation in the cavity is not blocked. However, the cavity is prevented from filling with photons everywhere at the same time: only a packet of photons is allowed to propagate in the cavity. This pulse lasts for a shorter time than a round trip in the cavity.

The method used to obtain these operating conditions consists in using a rapid light modulator that can chop the light in the cavity into periods of exactly the same length as a round trip. Thus, only those photons allowed to pass through the modulator in its on-state will be amplified and will always find the modulator in this state after each round trip. The other photons elsewhere in the cavity will be subject to losses when they travel through the modulator (Figure 13).

Figure 13: A pulse propagating in the optical cavity of a mode-locked laser.
Generally, the pulses last for a much shorter time than a round trip in the cavity. They are limited by the Fourier transform of the spectrum emitted by the laser: the wider the spectrum, the shorter the pulse. This means that if the amplifying medium is exceptionally wide (for example the titanium-doped sapphire has a spectral width greater than 300 nm), then the pulse generated will be only several femtoseconds long. The term mode-locking comes from the analysis of the various frequencies.

Different types of laser:
The different types of laser can be classified according to the nature of the amplifying medium: gas, liquid (dye) or solid state. The types of laser are:
1- Gas laser.
2- Chemical laser.
3- Dye laser.
4- Metal – vapour laser.
5- Solid state laser.
6- Photonic crystal lasers.
7- Semiconductor lasers.
8- Other types of lasers.
1- Gas laser:

Gas lasers all have in common the same pump source: electricity. The gaseous species enter the excited state either directly, by collision with electrons, or indirectly, by collision with other gases, themselves electrically excited.
Gas lasers cover the whole optical spectrum, from the ultraviolet to the far infrared. However, the spectrum is not continuously covered: gas lasers emit very narrow spectral lines. The most common gas lasers (from the UV to the far IR) include:
a) excimer lasers (ArF:193 nm, KrF:249 nm, XeCl:308 nm)
b) argon-ion lasers (blue and green wavelengths)
c) helium-neon lasers (the neon is used for the laser effect) 632.8 nm, 543.3 nm,
1.15 μm, 3.39 μm
d) CO2 lasers: a large number of wavelengths around 9.6 μm and 10.6 μm.
Only CO2 lasers are really efficient (15 to 20%). They are used in industry for processing materials. The efficiency of the others is mostly less than 1%. Gas lasers are often bulky and need a great deal of water-cooling (almost all the energy provided by the pump is lost as heat). Even though those operating in the visible (Argon, Helium, Neon) are tending to be replaced by solid state lasers, excimer lasers and CO2 lasers are still very frequently used.
• A helium–neon laser or He Ne laser shown in figure 14, is a type of gas laser whose gain medium consists of a mixture of helium and neon (10:1) inside of a small bore capillary tube, usually excited by a DC electrical discharge. The pressure inside the tube is 1 mm of Hg. The best-known and most widely used He Ne laser operates at a wavelength of 632.8 nm in the red part of the visible spectrum.

Figure 14 Energy level diagram of a He Ne laser.

The mechanism producing population inversion and light amplification in a HeNe laser plasma originates with inelastic collision of energetic electrons with ground state helium atoms in the gas mixture. As shown in the accompanying energy level diagram, these collisions excite helium atoms from the ground state to higher energy excited states, among them the 23S1 and 21S0 long-lived metastable states. Because of a fortuitous near coincidence between the energy levels of the two He metastable states, and the 3s2 and 2s2 (Paschen notation) levels of neon, collisions between these helium metastable atoms and ground state neon atoms results in a selective and efficient transfer of excitation energy from the helium to neon. This excitation energy transfer process is given by the reaction equations:
He*(23S1) + Ne1S0 → He(1S0) + Ne2s2 + ΔE and
He(21S) + Ne1S0 + ΔE → He(1S0) + Ne3s2
where () represents an excited state, and ΔE is the small energy difference between the energy states of the two atoms, of the order of 0.05 eV or 387 cm−1, which is supplied by kinetic energy. Excitation energy transfer increases the population of the neon 2s2 and 3s2 levels manyfold. When the population of these two upper levels exceeds that of the corresponding lower level neon state, 2p4 to which they are optically connected, population inversion is present. The medium becomes capable of amplifying light in a narrow band at 1.15 μm (corresponding to the 2s2 to 2p4 transition) and in a narrow band at 632.8 nm (corresponding to the 3s2 to 2p4 transition at 632.8 nm). The 2p4 level is efficiently emptied by fast radiative decay to the 1s state, eventually reaching the ground state.
The remaining step in utilizing optical amplification to create an optical oscillator is to place highly reflecting mirrors at each end of the amplifying medium so that a wave in a particular spatial mode will reflect back upon itself, gaining more power in each pass than is lost due to transmission through the mirrors and diffraction. When these conditions are met for one or more longitudinal modes then radiation in those modes will rapidly build up until gain saturation occurs, resulting in a stable continuous laser beam output through the front (typically 99% reflecting) mirror.
• Nitrogen laser:

A nitrogen laser is a gas laser operating in the ultraviolet range (typically 337.1 nm) using molecular nitrogen as its gain medium, pumped by an electrical discharge.
The population inversion in the laser is achieved by the following sequence:

1. Electron impact excites vibrational motion of the nitrogen. Because nitrogen is a homonuclear molecule, it cannot lose this energy by photon emission, and its excited vibrational levels are therefore metastable and live for a long time.


2. Collisional energy transfer between the nitrogen and the carbon dioxide molecule causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to the desired population inversion necessary for laser operation.


3. The nitrogen molecules are left in a lower excited state. Their transition to ground state takes place by collision with cold helium atoms. The resulting hot helium atoms must be cooled in order to sustain the ability to produce a population inversion in the carbon dioxide molecules. In sealed lasers, this takes place as the helium atoms strike the walls of the container. In flow-through lasers, a continuous stream of CO2 and nitrogen is excited by the plasma discharge and the hot gas mixture is exhausted from the resonator by pumps.
   • Krypton laser
   A krypton laser is an ion laser, a type of gas laser using krypton ions as a gain medium, pumped by electric discharge. Krypton lasers are used for scientific research, or when krypton is mixed with argon, for creation of "white-light" lasers, useful for laser light shows. Krypton lasers are also used in medicine (e.g. for coagulation of retina), for manufacture of security holograms, and numerous other purposes.
   Krypton lasers emit at several wavelengths through the visible spectrum: at 406.7 nm, 413.1 nm, 415.4 nm, 468.0 nm, 476.2 nm, 482.5 nm, 520.8 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm.
   • Argon laser
   This argon-ion laser emits blue-green light at 488/514 nm.
   The Argon Ion laser is one of a family of Ion lasers that use a noble gas as the active medium.
   Argon ion lasers are used for retinal phototherapy (for diabetes), lithography, and pumping other lasers. Argon ion lasers emit at 13 wavelengths through the visible, ultraviolet, and near-visible spectrum, including: 351.1 nm, 363.8 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm, 528.7 nm, 1092.3 nm.
   
   An argon laser beam consisting of multiple colors (wavelengths) strikes a silicon diffraction mirror grating and is separated into several beams, one for each wavelength. The wavelengths are (left to right) 458nm, 476nm, 488nm, 497nm, 502nm, 515nm.
   Laser gain medium and type Operation wavelength(s) Pump source Applications and notes
   Helium–
   neon laser
   632.8 nm (543.5 nm, 593.9 nm, 611.8 nm, 1.1523 μm, 1.52 μm,
   3.3913 μm) Electrical discharge B scanning, alignment, optical demonstrations.
   Argon laser
   454.6 nm, 488.0 nm, 514.5 nm
   (351 nm, 363.8, 457.9 nm,
   465.8 nm, 476.5 nm, 472.7 nm, 528.7 nm, also frequency doubled to provide 244 nm, 257 nm)
   Electrical discharge Retinal phototherapy (for diabetes), lithography, confocal microscopy, spectroscopy pumping other lasers.
   Krypton
   laser
   416 nm, 530.9 nm, 568.2 nm,
   647.1 nm, 676.4 nm, 752.5 nm,
   799.3 nm Electrical discharge Scientific research, mixed with argon to create "whitelight" lasers, light shows.
   Xenon ion
   laser
   Many lines throughout visible spectrum extending into the UV and IR.
   Electrical discharge Scientific research.
   Nitrogen
   laser
   337.1 nm Electrical discharge Pumping of dye lasers, measuring air pollution, scientific research.
   
   Carbon
   dioxide laser
   10.6 μm, (9.4 μm) Transverse (high power) or longitudinal (low power) electrical discharge Material processing (cutting, welding, etc.), surgery, dental laser, military lasers.

Carbon monoxide
laser
2.6 to 4 μm, 4.8 to 8.3 μm Electrical discharge Material processing.
Excimer
laser
193 nm (ArF), 248 nm (KrF),
308 nm (XeCl), 353 nm (XeF) Excimer recombination via electrical discharge Ultraviolet lithography for semiconductor
manufacturing, laser surgery, LASIK.

2- Chemical lasers:
Chemical lasers are powered by a chemical reaction permitting a large amount of energy to be released quickly. Such very high power lasers are especially of

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interest to the military, however continuous wave chemical lasers at very high power levels, fed by streams of gasses, have been developed and have some industrial applications. As examples, in the hydrogen fluoride laser (2700–2900 nm) and the deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride.
Laser gain medium and type Operation wavelength(s) Pump source Applications and notes
Hydrogen
fluoride laser
2.7 to 2.9 μm for
Hydrogen fluoride (