Ti:sapphire lasers (also known as Ti:Al₂O₃ lasers, titanium-sapphire lasers, or Ti:sapphs) are

tunable laser A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength ran ...

s which emit

red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...

and

near-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

light in the range from 650 to 1100 nanometers. These

lasers A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

are mainly used in scientific research because of their tunability and their ability to generate

ultrashort pulse In optics, an ultrashort pulse, also known as an ultrafast event, is an electromagnetic pulse whose time duration is of the order of a picosecond (10−12 second) or less. Such pulses have a broadband optical spectrum, and can be created by ...

s. Lasers based on Ti:sapphire were first constructed and invented in June 1982 by Peter Moulton at the

MIT Lincoln Laboratory The MIT Lincoln Laboratory, located in Lexington, Massachusetts, is a United States Department of Defense federally funded research and development center chartered to apply advanced technology to problems of national security. Research and dev ...

. Titanium-sapphire refers to the

lasing medium The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a ...

, a crystal of

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, chromium, vanadium, or magnesium. The name sapphire is derived via the Latin "sapphir ...

(Al₂O₃) that is doped with Ti³⁺

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s. A Ti:sapphire laser is usually pumped with another laser with a wavelength of 514 to 532 nm, for which

argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

ion laser An ion laser is a gas laser that uses an ionized gas as its lasing medium. Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium and mirrors forming a Fabry–Pérot resonator. Unlike helium–neon lasers, the ...

s (514.5 nm) and frequency-doubled Nd:YAG, Nd:YLF, and Nd:YVO lasers (527-532 nm) are used. They are capable of laser operation from 670 nm to nm wavelength. Ti:sapphire lasers operate most efficiently at wavelengths near 800 nm.

Types of Ti:sapphire lasers

The inner optical setup of a femtosecond Titanium-Sapphire Pulsed laser

Mode-locked oscillators

Mode-locked oscillators generate

s with a typical duration between a few

picosecond A picosecond (abbreviated as ps) is a unit of time in the International System of Units (SI) equal to 10−12 or (one trillionth) of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 ...

s and 10

femtosecond A femtosecond is a unit of time in the International System of Units (SI) equal to 10 or of a second; that is, one quadrillionth, or one millionth of one billionth, of a second. For context, a femtosecond is to a second as a second is to about 31. ...

s, in special cases even around 5 femtoseconds. The pulse repetition

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

is in most cases around 70 to 90 MHz, as given by the oscillator's round-trip optical path, typically a few meters. Ti:sapphire oscillators are normally pumped with a continuous-wave laser beam from an argon or frequency-doubled Nd:YVO4 laser. Typically, such an oscillator has an average output power of 0.4 to 2.5

watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...

Chirped-pulse amplifiers

These devices generate ultrashort, ultra-high-intensity pulses with a duration of 20 to 100 femtoseconds. A typical one stage amplifier can produce pulses of up to 5 millijoules in energy at a repetition frequency of 1000

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...

, while a larger, multistage facility can produce pulses up to several

joule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied ...

s, with a repetition rate of up to 10 Hz. Usually, amplifier crystals are pumped with a pulsed frequency-doubled Nd:YLF laser at 527 nm and operate at 800 nm. Two different designs exist for the amplifier: regenerative amplifier and multi-pass amplifier. Regenerative amplifiers operate by amplifying single pulses from an oscillator (see above). Instead of a normal

cavity Cavity may refer to: Biology and healthcare *Body cavity, a fluid-filled space in many animals where organs typically develop **Gastrovascular cavity, the primary organ of digestion and circulation in cnidarians and flatworms *Dental cavity or too ...

with a partially reflective mirror, they contain high-speed optical switches that insert a pulse into a cavity and take the pulse out of the cavity exactly at the right moment when it has been amplified to a high intensity. The term '

chirp A chirp is a signal in which the frequency increases (''up-chirp'') or decreases (''down-chirp'') with time. In some sources, the term ''chirp'' is used interchangeably with sweep signal. It is commonly applied to sonar, radar, and laser system ...

ed-pulse' refers to a special construction that is necessary to prevent the pulse from damaging the components in the laser. The pulse is stretched in time so that the energy is not all located at the same point in time and space. This prevents damage to the optics in the amplifier. Then the pulse is optically amplified and recompressed in time to form a short, localized pulse. All optics after this point should be chosen to take the high energy density into consideration. In a ''multi-pass amplifier'', there are no optical switches. Instead, mirrors guide the beam a fixed number of times (two or more) through the Ti:sapphire crystal with slightly different directions. A pulsed pump beam can also be multi-passed through the crystal, so that more and more passes pump the crystal. First the pump beam pumps a spot in the gain medium. Then the signal beam first passes through the center for maximal amplification, but in later passes the diameter is increased to stay below the damage-threshold, to avoid amplification the outer parts of the beam, thus increasing beam quality and cutting off some amplified spontaneous emission and to completely deplete the inversion in the gain medium. ODIN Ti-Sapphire laser in operation

The pulses from chirped-pulse amplifiers are often converted to other wavelengths by means of various

nonlinear optical Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typic ...

processes. At 5 mJ in 100 femtoseconds, the peak power of such a laser is 50 gigawatts. When focused by a lens, these laser pulses will ionise any material placed in the focus, including air molecules, and lead to short

filament propagation In nonlinear optics, filament propagation is propagation of a beam of light through a medium without diffraction. This is possible because the Kerr effect causes an index of refraction change in the medium, resulting in self-focusing of the beam. F ...

and strong

nonlinear optics Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typica ...

effects that generate a wide spectrum of wavelengths. Femtosecond_laser_spark

Tunable continuous wave lasers

Titanium-sapphire is especially suitable for pulsed lasers since an

inherently contains a wide spectrum of frequency components. This is due to the inverse relationship between the frequency bandwidth of a pulse and its time duration, due to their being

conjugate variables Conjugate variables are pairs of variables mathematically defined in such a way that they become Fourier transform duals, or more generally are related through Pontryagin duality. The duality relations lead naturally to an uncertainty relation— ...

. However, with an appropriate design, titanium-sapphire can also be used in

continuous wave laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

s with extremely narrow

linewidth A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s tunable over a wide range.

History and applications

The Ti:sapphire laser was invented by Peter Moulton in June 1982 at

in its continuous wave version. Subsequently, these lasers were shown to generate ultrashort pulses through

Kerr-lens modelocking Kerr-lens mode-locking (KLM) is a method of mode-locking lasers via the nonlinear optical Kerr effect. This method allows the generation of pulses of light with a duration as short as a few femtoseconds. The optical Kerr effect is a process whi ...

. Strickland and Mourou, in addition to others, working at the

University of Rochester The University of Rochester (U of R, UR, or U of Rochester) is a private research university in Rochester, New York. The university grants undergraduate and graduate degrees, including doctoral and professional degrees. The University of Roc ...

, showed chirped pulse amplification of this laser within a few years, for which these two shared in the 2018 Nobel Prize in physics (along with

Arthur Ashkin Arthur Ashkin (September 2, 1922 – September 21, 2020) was an American scientist and Nobel laureate who worked at Bell Laboratories and Lucent Technologies. Ashkin has been considered by many as the father of optical tweezers, "LaserFest – th ...

for optical tweezers). The cumulative product sales of the Ti:sapphire laser has amounted to more than $600 million, making it a big commercial success that has sustained the solid state laser industry for more than three decades. The ultrashort pulses generated by Ti:sapphire lasers in the time domain corresponds to mode-locked optical frequency combs in the spectral domain. Both the temporal and spectral properties of these lasers make them highly desirable for frequency metrology, spectroscopy, or for pumping nonlinear optical processes. One half of the

Nobel prize for physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...

in 2005 was awarded to the development of the optical frequency comb technique, which heavily relied on the Ti:sapphire laser and its self-modelocking properties. The continuous wave versions of these lasers can be designed to have nearly quantum limited performance, resulting in a low noise and a narrow linewidth, making them attractive for

quantum optics Quantum optics is a branch of atomic, molecular, and optical physics dealing with how individual quanta of light, known as photons, interact with atoms and molecules. It includes the study of the particle-like properties of photons. Photons have b ...

experiments. The reduced amplified spontaneous emission noise in the radiation of Ti:sapphire lasers lends great strength in their application as optical lattices for the operation of state-of-the-art atomic clocks. Apart from fundamental science applications in the laboratory, this laser has found biological applications such as deep-tissue multiphoton imaging and industrial applications cold micromachining. When operated in the chirped pulse amplification mode, they can be used to generate extremely high peak powers in the terawatt range, which finds use in

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manifest ...

research.

References

External links

Encyclopedia of laser physics and technology on Ti:sapphire lasers
{{DEFAULTSORT:Ti-Sapphire Laser Solid-state lasers Titanium