sign in sign up
<<
Home , Laser, Ruby laser, Tunable laser

Dr Raj Kumar

CSIR - Central Scientific Instruments Organisation, Chandigarh Overview  Introduction to basics of • Working principle of a Laser • Main components of a Laser • based on number of levels • Lasers modes • Main properties of a Laser • Types of Lasers  Solid State Lasers • Laser: the first laser • Nd: YAG & Nd: Glass Lasers • Tunable Solid State Lasers • Alexandrite Laser • Ti: Laser • Colour Center Lasers • Fiber Lasers  Applications of Solid State Lasers What is a Laser ? Amplification by of Radiation

Spontaneous emission Stimulated emission Working principle of a Laser

E2 E2 h h h h =E2-E1 E1 E1 Absorption Spontaneous Stimulated emission emission Working principle of a Laser

Let n1 be number of in E1 state and n2 be number of atoms in E2 then If n > n 1 2 E2 • radiation is mostly absorbed • spontaneous radiation dominates E1 If n2 >> n1

• most atoms occupy level E2, weak absorption • stimulated emission dominates • light is amplified Necessary condition:

For stimulated emission to dominate, there must be more atoms in excited states than in ground state. Such a configuration of atoms is called a population inversion. Main components of a Laser

 All the lasers comprise of three basic components

• Active medium, • Excitation source/pump • Reflecting /

. Lasers differ only in terms of Active medium or Excitation process. Lasers based on number of energy levels

Three-level laser • No lasing action in two level system : no population inversion • Three level system: lasing possible but require high pump energy than four level system • Example: (three level) Lasers based on number of energy levels

Four-level laser • Number of thermally excited ions in the lower laser level is small • Easy to achieve population inversion even by pumping a relatively small number of ions into the upper laser level • Lower threshold compared to a three-level system • Example: Nd: YAG Laser Lasers modes

Longitudinal mode frequency separation

• Laser oscillates in a number of transverse and longitudinal modes • is selected by using mechanical apertures in the cavity to allow only selected mode and suppress other oscillating modes • is selected by using Fabry-Perot Etalon in the cavity

• TEM00 is preferred for most of the applications Main properties of a Laser

: from phase correlation  Directionality  High intensity: results from directionality Monochromaticity: results in high temporal coherence Short pulse duration Types of Lasers  Several ways to classify lasers . Mode of operation : (CW) or Pulsed . Active medium: - Solid lasers - Gas lasers - Liquid lasers - Semiconductor lasers

 Classification may be done on basis of other parameters . of the laser medium . Power delivered by laser . Efficiency or . Applications Solid State Laser • For historical reasons, solid-state lasers are lasers in which active ions in or glass host materials are optically pumped to create a population inversion

• Other types of lasers that employ solid-state gain media are semiconductor lasers and lasers and amplifiers. Since these lasers employ very specialized and design principles, they are usually treated separately from conventional bulk solid-state lasers

• Semiconductor or lasers are mostly electrically pumped (though in principle, may be possible with some) Solid State Laser

Are versatile and provide a large range of average and peak power, pulse width, pulse repetition rate, and

The flexibility of solid-state lasers stems from the fact that: • The size and shape of the active material can be chosen to achieve a particular performance • Different active materials can be selected with different gain, energy storage, and wavelength properties • Output energy can be increased by adding amplifiers • A large number of passive and active components are available to shape the spectral, temporal and spatial profile of the output beam Solid State Laser: basics

Active centers are fixed /doped (~ 1%) in a dielectric crystal or glassy material Electrically non-conducting  also called Doped-insulator lasers.

• Crystal atoms act as host lattice to active centers • Crystal usually shaped as rod • Pumping: Flash lamp or diode laser • Active centers are from the rare earth, transition metals, or actinides • Water cooled Solid State Laser: schematic

Mirrors on both sides of laser rod form a resonant cavity Solid State Laser: requirements  Requirements for Host material : • Should not absorb light at laser wavelength • Must possess sharp fluorescent lines, strong absorption bands, and high quantum efficiency • Crystal should have good thermal conductivity  Problems with Host material : o Most of excitation energy ends up as heat rather than light o Excess heat damages the laser crystal

. Active centres are ions from: (Cr), (Nd), (Ti), Cerium (Ce), (Er), (Ho) and Cobalt (Co)  Chromium is active centre in Ruby and Alexandrite lasers  Neodymium is active centre in commonly used Nd: YAG laser Representative Solid State Laser

• Ruby Laser • Nd:YAG Laser • Nd:Glass Laser

. Tunable Solid State Lasers • Alexandrite Laser • Titanium-Sapphire Laser • Colour-Centre Laser

. Fiber Lasers • Erbium in a Glass host Ruby Laser: the first laser . First Laser developed in 1960 (TH Maiman) Ruby laser rod:

3+ . A synthetic pink Ruby crystal (Al2O3 doped with Cr ions) . Cr3+ ions concentration: 0.05%, Approx 1.61025 ions per cubic meter. • Ruby crystal as cylindrical • Active Centres (Cr3+ ions) have a set of three energy rod (4cm length 0.5 cm in diameter) • Aluminum & Oxygen • Helical photographic flash ions are inert lamp filled with .

The Al2O3 (sapphire) host is hard, with high thermal conductivity, and transition metals can readily be incorporated substitutionally for the Al Ruby Laser: the first laser

A typical Ruby laser (a) with internal mirrors (b) with external mirrors Ruby Laser: commercial

. End faces grounded and polished . Mostly silvered faces (100% & 90 % reflection) Febry-Perot Resonator

• System is cooled with the help of a coolant circulating around the ruby rod

. In practical lasers flash lamps of helical design no longer used . Most commonly used are linear lamps Ruby Laser : energy levels

Energy levels of chromium ions is Ruby laser Ruby Laser : working principle . A Three level laser system

. E2 - metastable state (3ms) • Ruby rod pumped with an intense Xenon flash lamp • Ground state of Cr3+ ions absorb light at pump bands 550nm 400nm

• Non-radiative transitions to E2

• Population Inversion at E2

 Radiative transitions from E2 to E1  wavelength at 694.3 nm Under intense excitation: Pumping > Critical threshold A spontaneous fluorescent (red) acts as input and trigger  Stimulated emission; SYSTEM LASES Ruby Laser: output Laser Output: Pulsed with low repetition rate (1 to 2 per sec) Ruby laser light pulses

• Series of irregular spikes stretching over the duration of pump pulse • Q-switching concentrates output into a single pulse Ruby Laser: output • Stimulated transitions faster than rate at which population inversion is maintained

• Once stimulated emission commence, the metastable state E2, depopulate very rapidly

• At the end of each pulse, population at E2 falls below the threshold value required for sustaining emission of light • Lasing ceases & Laser becomes inactive

Next pulse will arrive only after P.I. is restored

 High energy storage capability due to long upper laser level lifetime  Pulse energy upto 100J  Relatively inefficient;  0.1 to 1%  Variety of applications: diagnostics; . Nd: YAG Laser

• Aluminum Garnet (YAG) Y3Al5O12 best choice of a host for neodymium ions (Nd) • YAG offers low threshold and high gain • YAG is a very hard, isotropic crystal • good thermal and mechanical properties • can be grown and fabricated in rods of high optical quality • Operation: CW and pulsed mode (high repetition rate)

• Efficiency about 10 times as compared to ruby • Drastic weight reduction • Replaced ruby in military Rangefinders, other applications • Used in the semiconductor industry for resistor trimming, scribing, and marking For continuous or very high repetition-rate operation, crystalline materials provide higher gain and greater thermal conductivity Nd: YAG Laser . Active center: Neodymium (Nd) ion- a rare earth metallic ion . Host: YAG . Emission at  1.064m

• In Nd:YAG laser, Nd 3+ ions take place of yttrium ions • Doping conc. ;  0.72% by weight corresponds to 1.41026 atoms/m3 • Rod: 10cm in length, 12mm in diameter

. Nd: YAG rod & a linear flash lamp housed in an elliptical cavity . In practice, external mirrors (100% , 99% reflectivity) used . System cooled by water circulation Nd: YAG Laser Nd: YAG Laser

lifetime 230 μs

Energy levels of Nd –ions in a crystal Nd: YAG Laser . A Four level laser system: Require lower pump energy • Terminal laser level sufficiently far from ground state

• E3 – metastable level (lifetime 230 μs)

• Two pump bands: 700 nm & 800nm • Pump: intense Xenon flash lamp 3+ • Nd ions level E4, decays to upper laser level at E3

• Population inversion easily achieved between E3 and E2 levels. • Stimulated to emit 1064 nm laser transition.

3+ From E2 level, Nd ions quickly drop to E1 by transferring energy to crystal Nd: YAG Laser . Many other transitions in near IR region; all weaker than 1064 nm • Only 1318 nm transition produces 20% power as that of 1064 nm Useful in Fiber Optic Transmission.

Laser Output: • In the form of pulses of variable repetition high rate • Overall efficiency  0.1 to 1% range • Xenon flash lamps : Pulsed output • Tungsten halide incandescent lamps ; CW output CW output power of over 1 kW obtainable.

Can be pumped by a diode laser (GaAs) for high efficiency

2nd results in half the wavelength (532 nm) Nd: Glass Laser

• are more suitable for high-energy pulsed operation because of their large size, flexibility in their physical parameters, and the broadened fluorescent line • Can deliver much higher • Can be doped at very high concentrations with excellent uniformity • Practical doping limit is determined by the fact that the lifetime and therefore the efficiency of stimulated emission, decreases with higher concentrations • Can be made in a variety of shapes and sizes, from fibers a few micrometers in diameter to rods 2m long and 7.5 cm in diameter and disks up to 90 cm in diameter and 5 cm thick

The major disadvantage of glass is a low thermal conductivity Nd: Glass Laser  Glass: An excellent host material for Nd

 Attraction for Glass: well developed for making large size glass (laser) with good optical quality  While Nd: YAG laser can be operated in CW mode; Nd: glass laser only operate in pulsed mode because of low thermal conductivity of glass

 Nd:glass laser very high output energy per unit volume of material

• High energy in short pulses can heat matter to thermonuclear temperatures, thus generating energy in small controlled explosions (inertial fusion)

• lasers developed for by Lawrence Livermore National Lab. (USA) – employed a large number of Nd: glass amplifiers to produce 100 kJ of energy in a 2.5 ns pulse. Nd: Glass Laser

. An inertial confinement fusion implosion on the NOVA laser creates "microsun" conditions of tremendously high density and temperature rivaling even those found at the core of our Sun. Tunable Solid State Lasers  Produce output over a range of tunable . Tunability: existence of a cluster of vibrationally excited terminal levels near the ground state – Vibronic states . laser transitions take place between coupled vibrational and electronic states

 Dye lasers, though tunable, but suffer from dye degradation and other limitations  Solid state tunable lasers have long self and operational life

Applications: Remote sensing, space, Tunable Solid State Lasers: Alexandrite Laser 3+ • Alexandrite (BeAl2O4 : Cr ) is the common name for chromium-doped chrysoberyl • Tunability is due to band of vibrational levels which are a result of strong coupling between Cr3+ ion and the lattice vibrations • Doping ~ about 0.1% (density~31025 ions /m3); Rod shaped ; 10cm long, 6mm in diameter • Pump levels at 380 nm & 630 nm; flash lamp pumped • Cr3+ levels in Alexandrite form upper and lower vibronic bands Electronic levels of Cr3+ and vibrational levels of crystal lattice . Vibronic transitions can occur over a range of energies; excited ion can drop from upper level to anywhere in lower vibronic band – Gain Bandwidth Tunable to any desired wavelength within its . Can operates in a pulsed or CW mode . Widely used in , kidney stone removal and pollution detection Tunable Solid State Lasers: Alexandrite Laser Can lase both as a four-level vibronic laser and as a three-level

Absorption bands are very similar to those of ruby

Energy level diagram for chromium ions in alexandrite Tunable Solid State Lasers: Alexandrite Laser

• In three level mode laser transition is from 2E state, which is coupled 4 4 to T2, down to ground state A2. • High threshold, fixed output wavelength (680.4nm at room temperature) and relatively low efficiency

4 • In four level mode T2 is the absorption state continuum 4 4 • Lasing occurs between T2 state to excited vibronic states within A2 (ground state) • Laser wavelength depends on vibrationally excited terminal • Any energy not released as laser photon will be carried off by a vibrational , leaving the chromium ion at its ground state (system comes in equilibrium) Tunable Solid State Lasers: Ti: Sapphire Laser

• Titanium-Sapphire (Ti : Al2O3) laser is widely used tunable • Broad vibronic fluorescence band allows output between 670–1070 nm, with the peak of the gain curve around 800 nm • Relatively large gain cross section (half of Nd :YAG at the peak of its tuning range) • The energy level structure of the Ti3+ ion is unique among transition- metal laser ions in that there are no d state energy levels above the upper laser level  Ti3+ ions replace some of Al3+ ions  Doping concentration  0.1% by weight  Operation: Pulsed or CW modes Tunable Solid State Lasers: Ti: Sapphire Laser

The broad, widely separated absorption and fluorescence bands are caused by the strong coupling between the ion and host lattice and are the key to broadly tunable laser operation Tunable Solid State Lasers: Ti: Sapphire Laser

• Pumping with other lasers like argon and copper vapor lasers, frequency doubled Nd :YAG and Nd : YLF lasers due to short lifetime of upper laser level (3.8s) • Flash lamp pumping is inefficient and requires very high pump flux is required.

Energy level scheme Most widely used in laser (), range finders, remote sensing and spectroscopy Colour Center Lasers . Broadly tunable SSLs – operates in wavelength range of 800-4000nm . Tuning achieved using different colour-centre in sequence.

. Typical CCL consist of an alkali halide crystal that contains point defects known as F-centre  Colour Centres Usually produced when crystal irradiated with X-rays. . Colour centres remain in crystals for duration ranging few days to many years. . Absorb and emit light as the atoms at the defect site change position.

CCLs must be pumped with other laser & maintained at very low temperatures.  Need for a pump lasers & Cryogenic cooling limits the use of CCLs in practical application. Colour Center Lasers: Energy Levels

CCLs must be pumped with other laser & maintained at very low temperatures Fiber Lasers Erbium in a glass host – forms a three level laser with wavelength centered around 1550nm (range: 1520-1560nm). 1550 nm is important operational window in OFC technology

EDFA is used as an in DWDM technology

Highly useful in undersea and long haul OFC links Fiber Lasers: Energy Levels

• Needs lasers for pumping to get desired output. • Output transitions in the range from 1520-1560nm Optical • Parametric oscillators based on introduced in 1971 • Discovery of damage-resistant nonlinear crystals with large nonlinear coefficients in the early 1990s revived interest in OPOs • OPO can provide tunable range through UV-visible-IR • OPO works on the principle of non-linear harmonic generation • In the parametric process, a nonlinear medium (usually a crystal) converts the high energy photon (the pump wave) into two lower energy (the and idler waves) • Wavelengths of signal and idler beams are determined by the angle that pump wave-vector makes with crystal axis • Energy can be transferred efficiently to the parametric waves if all three waves are traveling at the same velocity (phase matching condition) • Variation in index of refraction with crystal angle and wavelength allows "phase matching“ condition to be met only for a single set of wavelengths for a given crystal angle and pump wavelength • Thus as the crystal rotates, different wavelengths of light are produced Optical Parametric Oscillator

pump energy = signal energy + idler energy

Signal and Idler beam generated in a non-linear crystal Optical Parametric Oscillator

Variation of OPO output energy (signal and idler) with wavelength Diode Laser as optical pumping source

• As diode lasers became less expensive, these are being used as optical pump in solid-state lasers • Diode pumping offers significant improvements in overall system efficiency, reliability, and compactness • Radiation from laser can be collimated providing great flexibility of designing solid-state lasers with regard to shape of laser medium and orientation of pump beam

• In end-pumped lasers, pump beam and resonator axis are collinear which led to highly efficient lasers with excellent beam quality • A number of solid-state lasers with outputs up to 20 W are pumped with diode arrays • Lasers at multi-hundred level are pumped by arc lamps because of high cost of arrays Applications of Solid State Lasers Solid State Lasers have a wide spectrum of applications • Materials processing (cutting, drilling, , marking, heat treating, etc.), • Semiconductor fabrication (wafer cutting, IC trimming), • Graphic arts (high-end and copying), • Medical and surgical (Welding of detached , correction of vision defects, , treatment of skin cancer) • Defence (ranging, anti-missile shield, laser detonators, instruments, spying and in war time) • A high energy pulsed YAG laser has even been used in rocket propulsion experiments • The largest lasers (with the highest peak power) in the world are solid state lasers • Space, remote sensing, spectroscopy, holography LASER APPLICATION EXAMPLES

Laser for Cutting Fabric in a Clothing Factory

Laser in Material Processing INDUSTRIAL APPLICATIONS LASER APPLICATION EXAMPLES

Laser at War time LASER APPLICATION EXAMPLES

Laser fusion LASER APPLICATION EXAMPLES

HOLOGRAPHY References / suggested books

 Solid State Laser Engineering , W. Koechner  Principles of Lasers, O. Svelto  Lasers and Non-linear , B. B. Laud  Laser Fundamentals, W. T. Silfvast Thank you Why Alexandrite is tunable and Ruby not?

4 4 • Equilibrium coordinate for both the T2 and T1 states, due to their 4 2 symmetry, is shifted to a larger value than that of A2 and E states 3+ 4 2 • As in other Cr -doped hosts, the decay between the T2 and E states is via a fast internal conversion (decay-time of less than 1 ps) probably due to the level-crossing which occurs between the two states. • These two states can be considered to be in thermal equilibrium at all times, and, since the energy difference between the bottom vibrational 4 2 levels of T2 and E states in alexandrite is only a few kT, an 4 appreciable population will be present in vibrational manifold of T2 state when 2E state has been populated. • Invoking the Franck-Condon principle, one sees that the vibronic 4 4 transitions from the T2 state end in empty vibrational levels of the A2 state, thus becoming the preferred laser transition. •Because there is a very large number of vibrational levels involved, the resulting emission is in the form of a broad continuous band