23 laser system-ll

1. CO2 laser

CO2 is a linear and symmetric molecule and its ground electronic state is a 1∑. As CO2 is a linear molecule, therefore it has 3N – 5 =4 internal vibrational degree of freedom where N = 3 is the number of nuclei in the molecule.

These four vibrations correspond to (i) symmetric stretching (ii) doubly degenerate bending mode and (iii) asymmetric stretching mode and have been shown in the figure.

In the symmetric stretching mode, the oxygen atoms oscillate along the axis of the molecule simultaneously departing or approaching the carbon atom that is stationary.

In the bending mode, that is doubly degenerate, the molecule ceases to be exactly linear. It occurs both in the plane of the figure and the plane perpendicular to it. In this mode all the three atoms in the molecule undergo vibrational motion perpendicular to the molecular axis.

In antisymmetric stretching all the three atoms oscillate but both the oxygen atoms move in one direction and carbon atom moves in the opposite direction.

(a) symmetric stretching (b) bending mode (c) antisymmetric stretching

In the first approximation these three modes are described as independent harmonic oscillators, and hence the excited vibrational states are completely described by the number of excited quanta in each mode. These states are represented by a set of three vibrational quantum number v1, v21 , v3 where subscript 1, 2 and 3 refer to the symmetric bending and antisymmetric modes, respectively. The superscript 1 gives the angular momentum of this vibration about the axis of molecule in units of h/2π. For example, 02⁰0 indicates that the two vibrations combine to give an angular momentum l= 0.

The Fig depicts the energy level diagram for the lowest levels. Each of the vibrational levels has superimposed upon it a set of rotational levels. The rotational levels are spaced by energies that are more than three orders of magnitude smaller than the vibrational energies.

The CO₂ laser uses a mixture of CO₂, N₂ and He or water vapor. The active centres are the CO₂ molecules lasing on the transition between the rotational levels of the vibrational bands of the electronic ground state. The figure also shows the vibrational energy level for the ground state of N2 molecule.

Nitrogen being a homonuclear diatomic molecule does not posses a permanent electric dipole moment and hence radiation decay from v’’ = 1 to v’’=    0 is forbidden. The lifetime of the level v’’ = 1is therefore quite long and ~ 0.1 s at one torr. The vibrational level 00⁰1 of CO₂ is in near coincidence (∆E = 18 cm^-1) with v’’= 1 level of nitrogen.

The broken lines in the figure show non – radiative transitions.

When a discharge is passed in a tube containing mixture of CO₂, N₂ and He, electron collisions excite the molecules to higher electronic vibrational rotational states. The electronic collision cross section for the excitation to the level 00⁰1 is very large. The electron collision excite the N₂ molecules by

The cross section for this process is quite large and produce significant amount of metastable N2 in the discharge. These nitrogen molecules transfer their internal energy to CO2 anymmetric stretching modes through near resonance collision.

The cross section for this process is also large therefore, the excitation of CO₂00⁰1 level is unusually efficient. Further, the discharge containing the CO₂ produces dissociation of CO₂ into CO and oxygen. As Co vibrational frequency is not grossly different from that of the CO₂ asymmetric mode, it will rapidly transfer its internal energy to the CO₂system through near resonant collision in the same manner as N₂.

The high density of population arises principally from three factors

(i) the long vibrational lifetime of the asymmetric mode of CO₂

(ii) the near resonant N₂– CO₂ energy transfer,

(iii)  the large cross section for the production of the vibrationally excited nitrogen through inelastic

collision with electrons

(iv)  population gain from other levels of the asymmetric mode.

The 10⁰0 and 02⁰0 levels are in resonance. The coupling between these levels is sufficiently strong so that 10⁰0 and 02⁰0 level system behaves like a single level. The separation between these levels ~100cm-1that is significantly less than the thermal energy of motion kBT ~ 210 cm^-1 at room temperature. The similarity of vibrational motion in combination with the near degeneracy of the levels causes the rate of transfer between them by collision process, corresponding to

10⁰0 + 00⁰0 →02⁰0 + 00⁰0 + ∆E

to be extremely rapid. Thus 10⁰0 and 02⁰0 levels of CO₂ reachs thermal equilibrium in a very short time. For population inversion, between upper 00⁰1 level and lower 10⁰0 and 02⁰0 levels, it is necessary that population of lower levels should decay very fast. The presence of He has a considerable influence on the population decrease of lower levels. The He atoms collide with CO₂and increase the rate of relaxation of 01⁰0 levels.

The rate is approximately twenty times in collision of the type

01⁰0 + He → 00⁰0 + He + ∆E that for the corresponding

01⁰0 + 00⁰0 → 00⁰0 +00⁰0+ ∆E

process where two CO₂ molecules collide. This reduces the bending mode population, that in turn reflects itself in reduction of lower level population, and hence increases the gain.

Another function of He is to keep CO₂ cold. The excited CO₂ can decay spontaneously and directly to the ground state, liberating energy as heat. It is necessary to keep temperature of CO₂ as low to avoid population of lower level by thermal excitation. Helium has high thermal conductivity and hence helps to conduct heat away to the walls keeping CO₂ cold. Thus while N₂ helps to increase the population of the upper level, helium helps to depopulate the lower level.

The laser oscillation is at a wavelength corresponding to the transition from the upper vibrational level 00⁰1 to the lower vibrational level 10⁰0. With each vibrational level, rotational levels are also associated. Thus a number of laser lines are possible. The population of J’= 21 of 00⁰1 has the maximum population and transition corresponding to J’= 21 to J’’ = 22 or P(22) is strongest and this corresponds to the wavelength of 10.6µm. Another laser line of = 9.6 µm from transition between 00⁰1 and 02⁰0 is also prominent.

A Schematic of CO₂ laser is shown in the Figure. The discharge is produced in tube having diameter of 2.5 cm and length 5m. Brewster windows are used at the end. These windows are made of NaC1, CaC1, BaFC1, ZnSe or KC1 as these materials are transparent to 10.6µm radiation. As glass strongly absorbs most of the frequencies in infrared region therefore, near confocal Si mirrors coated with A1 are used.

He, N2 and CO2are filled in the ratio 8:1:1 with pressure of He, N2 and CO2 as 0.7 torr, 1.2 torr and 0.33 torr, respectively. To remove the dissociation products such as Co and O, that would contaminate the laser, the continuous flow of the gas mixture is maintained in the tube.

Grating can be used to separate out different laser lines. CO2 laser can be made to operate in pulsed or CW mode. Pulse energy ~ 2KJ and pulse width is of few µs. For CW operation, power > 50 W is easily available.

Applications

The advantage of using this laser is based on the fact that it provides very intense heating source over a very small area. It has applications in

1. Material processing: cutting, drilling, material removal, welding, cladding, alloying, hardening, melting etc.
2. In medicine it is used for cutting and cauterizing.

2.    Nitrogen Laser

The values of r_e indicate that the minimum of potential for C state lies almost vertically above that of the X state. On the other hand the minima of A and B shifted to high r. The electron collides with the molecules and excites them to various levels, when the electric discharge is passed through the gas, According to Franck Condon principle the probability of transition from X to C is much larger than that of X to A or X to B. Thus, population inversion occurs at level C (v = 0 level) relative to level B (v = 0 level). A lasing action is observed between C and B levels (corresponding to 0-0 and 0-1 vibrational levels). As the lifetime of state C is about 40 ns and that of B is about 100µs, therefore, the molecule remains in C state for a short duration and after transition accumulates in level B. Hence the population difference between B and C decreases very rapidly and this renders laser action impossible. The population inversion can therefore be achieved only for a short time and after this the laser action is self-terminating. Due to high gain of this self-terminating transition, oscillations take place in the form of amplified spontaneous emissions. Thus the laser can be operated without mirror. The mirror placed at one end of the tube reduces the threshold power and also provides unidirectional output. This also reduces the beam divergence. The pulse width for nitrogen laser is normally 10ns and emission wavelength is at 337.1 nm. Peak power up to 1MW can be achieve.

A pulsed high voltage of about 20kV, triggered by a thyratron or spark gap is applied across the tube through which N2 at a pressure of about 100 torr, is flowing. On one end of the tube a mirror is used while a window used on the other end has a very high transmission. The length of the tube is between 0.1 – 1.0m.

3. Titanium Sapphire Laser

Introduction: In 1960 T. H. Maiman demonstrated the operation of the first optical maser, or laser, a crystal of synthetically grown ruby, or sapphire (Al₂O₃) doped with a small amount of chromium was used in the experiments. P. F. Moulton demonstrated a widely tunable laser at Lincoln Laboratory, twenty-two years later, where titanium instead of chromium was incorporated as an impurity into sapphire. Titanium-doped sapphire or Ti:Al₂O₃, has the largest tuning range of any laser that is, from 660 to 1180 nm.

The Ti:Al₂O₃ crystals used in the initial experiments exhibited significant scattering and an absorption at the laser wavelength and these losses affected the efficiency of the laser, and only pulsed operation was possible. Further with the advances in the development of higher-quality laser crystals of Ti:Al₂O₃, (with significantly smaller losses).room-temperature continuous-wave operation became areality that was first reported in 1986.

Sapphire is an ideal host crystal in both the ruby and Ti:Al₂O₃ laser. It is transparent from ultraviolet to infrared and also, it is nonhygroscopic and very hard, that is required for producing good optical-quality surfaces..

The most common tunable lasers are the organic dye lasers. A dye laser consists of an organic dye (such as polymethine, xanthenes, or coumarin dye) in a liquid solvent or host. A typical dye laser such as Rhodamine 6G can be tuned from 570 to 610 nm; other dyes have laser bandwidths that cover portions of the spectrum from the ultraviolet to the infrared.

In a Ti:Al₂O₃ laser the laser transition is between two electronic levels of a single Ti³⁺ ion, and in dye lasers the laser transition is from one molecular electronic level to another. The broadening of the absorption and luminescence bands is caused by the multitude of rotational levels associated with each molecular electronic level. However there are undesirable characteristics of dye lasers as toxicity of the dyes and solvents, degradation of the dyes with time, and amplitude noise in the laser output (because the dyes are flowed to reduce thermal loads).

Another class of tunable solid state lasers is the alkali-halide color-center lasers. A color center is a crystal defect in which an electron becomes trapped. For example, an F color center consists of an electron trapped at an anion vacancy of the crystal lattice. The excitations of the trapped electron strongly couple to the phonons of the lattice and give rise to broad absorption and emission bands. Color-center lasers can span the wavelength range from 800 nm to 4 μm where organic dye lasers are of limited usefulness. Color-center lasers have many drawbacks;

• in many alkali-halide crystals the color-centers are not stable at room temperature and
• degrade on the time scale of a day.

In a tunable solid state laser,the interaction between the ion and host crystal is such that lattice vibrations or phonons usually accompany the emission or absorption of photons. As a result the absorption and emission spectra become broadened in each laser. These vibronic transitions can provide gain over the large bandwidth required for tunable lasers.

Spectroscopy:

In titanium-doped sapphire the titanium ions substitute for the aluminum ions and exist in only 3+ charge state. The energy levels of the titanium ions are simple where a single d electron is in the outermost shell while the remaining 18 electrons have a filled-shell configuration of a neutral argon atom. When the titanium ions are placed in a host crystal, the electrostatic field of neighbouring atoms, or the crystal field, remove the five-fold angular momentum degeneracy of the single d electron.

In Ti:Al₂O₃ the 3d electron electrostatically interacts with the electronic charges of six surrounding oxygen ions that are positioned at the corners of an octahedron.

The figure shows a simplified energy-level diagram of Ti³⁺.

The electronic configuration of the free ion is that of an argon shell plus a single 3d electron.

The electronic energy levels of the Ti³⁺ ions in Ti:Al₂O₃ are further perturbed by the sapphire host lattice. When the Ti³⁺ ion is in the excited state, the overall energy of the system gets lowered if the position of the Ti3+ ion displaces itself with respect to the surrounding oxygen atoms understood to be due to the Jahn-Teller effect. This displacement removes the degeneracy of the two excited angular momentum states that leads to a splitting of the green absorption band. Also, as the Ti³⁺ ion moves to its new equilibrium position, it kicks the surrounding lattice and excites vibrations (or phonons); that is why the Ti:Al₂O₃ laser is called a vibronic laser.

The coupling of the electronic energy levels of the Ti³⁺ ions with the vibrational energy levels of the surrounding sapphire lattice is essential for Ti:Al₂O₃ to operate as a laser. The energy-level diagram resembles that of a large polyatomic molecule such as that of an organic dye molecule. When the Ti³⁺ ion either absorbs or emits a photon, the 3d electron rearranges its orbital more quickly than the heavier Ti³⁺ nucleus can move understood due to the Franck-Condon principle.

The Gaussian-shaped curves at points A and C in the figure represent the probability of finding the Ti³⁺ at a particular position in the lowest vibrational state of the T and E levels, respectively.

Figure shows absorption of light as the transition from point A to B. The transition is to either of the Jahn-Teller split upper states, and it results in a broad blue-green absorption. The Jahn-Teller splitting is not totally resolved and is manifested as a main peak with a shoulder. At point B, the ion displaces itself and lowers its energy by emitting phonons.

The transition from point C to point D shows the emission of light and the ion relaxes quickly to the ground state by emitting phonons.

Two important results are to be noted; First, the emission of light following the absorption of green light is at a longer wavelength (Stokes (red) shifted). A population inversion in the red emission band, necessary for amplification, is more easily achieved because the emission terminates on high vibrational levels of the ground state that are unpopulated because of the fast vibrational relaxation rate. Second, a large emission bandwidth required for broad tenability results because the spread in probability of the Ti³⁺ ion position at the bottom of the E potential can connect via vertical transitions to a large spread of vibrational levels of the T potential.

Further excited levels of the 3d electron lie far above the E levels and hence the Ti³⁺ exhibits no excited state absorption (ESA).

Laser Structure

•  Continuous wave Titanium Sapphire laser
•  Mode Locked Titanium Sapphire laser

Pumping is done either by argon ion laser (for cw operation) or a doubled Nd:YAG or Nd:YLF lasers for pulsed operations is used as pumping sources.

Applications

• Titanium Sapphire Lasers are used in Infrared spectroscopy of semiconductors and in laser radar,
• rangefinders, and remote sensing.
• They are used in medical applications such as photodynamic therapy.
• They are used to produce short pulses of X-rays by focusing the mode locked pulses onto solid target from
• which high density and high temperature radiating plasmas are produced ,plasmas that in turn emit
• large fluxes of X-rays

4. Free Electron Laser(FEL)

More than 50 years ago the first coherent red light has been generated by a ruby laser and thereafter more than 20 years ago the first coherent infrared light has been generated at the Stanford University (USA), and this laser was named FREE ELECTRON LASER (FEL).

PRINCIPLE:

This new type of coherent radiation source appeared from the very beginning to be quite promising. The reason lies on the peculiarity of its operating principle. In a FEL the active medium is not made of atoms or molecules, it consists of a beam of free electrons, propagating at relativistic velocities in a spatially periodic magnet (undulator), where the electrons experience the Lorentz force, execute transverse oscillations and emit synchrotron radiation in the forward direction as has been shown in figure. The wavelength λ of the emitted radiation depends on the electron energy E, on the period of the undulator magnet λ u and on its magnetic field B,

The figure depicts the Synchrotron emission in Undulator Magnet.

The above relationship can be easily derived by taking into account the synchronism condition, i.e. considering that the electron, that is oscillating with a spatial period λ u and it is moving with at a longitudinal speed v, maintains, oscillation after oscillation, the same phase relation with respect to the emitted electromagnetic wave of wavelength λ, running along the same direction at the speed c.

Taking into account the relationship between electron energy and relativistic velocities, the process described so far can be viewed as a kind of spontaneous emission by electrons propagating in an undulator magnet. By spontaneous mean that the emission occurs in absence of a field quasi resonant with the emitted radiation and co-propagating with the electrons. The possibility of realizing a stimulated process is quite natural if the photons emitted along the passage through the undulator are trapped in an optical cavity and then interact again with a fresh electron beam as is shown in the figure. These electrons are forced to emit stimulated synchrotron radiation.

One can associate to this process a gain and if the gain is larger than the cavity losses, the onset of the laser action is achieved thjat is in full analogy with conventional lasers, when saturation is reached, highly coherent (transverse and longitudinal) radiation can be extracted from the optical cavity. A quantity of much importance is the small signal gain coefficient that characterize the gain of the device for its operation in low or high gain regime

This suggests that the gain decreases drastically at short wavelength. The gain decreases indeed with the wavelength and, in the meantime, with the increase of the electron energy, that is required in order to go to shorter wavelengths. In this region (~ up to the VUV-10 to 200 nm) the operation of FEL oscillator requires very high brilliant electron beams (high j) that can be obtained in high quality linac accelerators or in Storage Rings.

The figure gives the layout of FEL (single passage). In the spectral region where the mirror reflectivity is too small (typically in the VUV region) a different operation scheme, based on single passage self amplified spontaneous emission (SASE), is possible if the electron beam brightness is good enough to have very high gain. In this operation mode, the electron beam interacts with its own spontaneous radiation and an unstable wave is excited. The radiation intensity I grows exponentially along the undulator

The most interesting features of FELs are:

λ can be tuned (tenability) by varying electron energy, period and amplitude of magnetic field; in addition

harmonic generation can provide even shorter λ

Time structure: depending on the e.beam time structure it ranges from c.w. to ultrashort pulsed regime (fraction of ps) ·

Coherence: transverse and longitudinal for oscillators and coherent seed amplifiers, only transverse for

SASE ·

Brilliance: the FEL brilliance can be larger, in some spectral regions (in particular in VUV-X), by many order

of magnitude than the brilliance of the existing sources (lasers and synchrotron radiation).

There are many FEL devices in operation around the world ranging from millimeter waves to UV.

 5. X-ray Laser

After T. Maiman demonstrated the first laser in 1960, the optical community immediately began seeking shorter and shorter wavelengths, through the visible and into the ultraviolet. By the early 1970s, lasers had reached the vacuum ultraviolet. However, the push toward shorter wavelengths stalled around 110 nm as problems emerged with short-wavelength laser physics.

As the energy of a laser transition increases, the excited state lifetime decreases, and the pump energy required to produce a population inversion increases. In fact, the two effects combine, so pump energy must be concentrated in a pump pulse that can be deposited very quickly in the laser material. A further complication is the lack of materials able to reflect or transmit light efficiently in the deep or extreme ultraviolet, making conventional mirrors and windows impractical.

With these problems in mind, scientists shifted their approach and instead of trying to make a laser oscillator, they sought to highly excite cylindrical plasmas that could generate intense X-ray pulses by amplifying stimulated emission along their lengths. The obvious way to excite the plasma was by firing intense optical laser pulses onto a target, and fortunately, high-power lasers became possible.

Shorter wavelengths and smaller lasers

The selenium experiments marked a major milestone for Livermore, but had two important limitations. The 20-nm wavelength is relatively long. Biological imaging researchers wanted wavelengths shorter than 4.5 nm, and some definitions of X-rays include only wavelengths shorter than 10 nm.

The logical way to shorter wavelengths was by using neon-like ions of heavier elements. Molybdenum brought Livermore to 13.3 nm, but Nova didn’t have the power to reach 4.5 nm with heavier neon-like ions. Reaching that target required shifting to nickel-like ions, heavy elements stripped of all but their 28 innermost electrons. Europium was first, at 7.1 nm, and Livermore later reached 4.316 nm in tungsten.

Hagelstein, in 1986, devised a dual-pulse approach, with the first pulse ionizing the plasma and the second exciting the ions to produce a population inversion in nickel-like niobium

Lacking mirrors suitable for X-ray oscillators, all those X-ray laser demonstrations generated only amplified spontaneous emission with limited coherence. Early this year, Rocca’s group reported an important new twist— aiming high harmonics from a titanium-sapphire laser into the excited plasma. Where plasma gain matched a harmonic frequency, the plasma amplified the harmonic seed pulse, generating gain-saturated output at 18.9 nm from nickellike molybdenum and 13.9 nm from nickel-like silver. Those experiments are a big step toward a practical coherent tabletop X-ray source, and the shorter wavelength is in a band that is attractive for future generations of semiconductor photolithography.