
A titanium-sapphire laser (also known as a Ti:sapphire laser, Ti:Al
2O
3 laser or Ti:sapph) is a
tunable laser
A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all active laser medium, laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a sign ...
which emits
red and
near-infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...
light in the range from 650 to 1100 nanometers. This type of
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'' originated as an acronym for light amplification by stimulated emission of radi ...
is mainly used in scientific research because of its tunability and its 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, thanks to its broad light emission spectrum. 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 a 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 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, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name ''sapphire ...
(Al
2O
3) 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 convent ...
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; it has 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, th ...
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. The crystal is often made using the heat exchanger method of production.
Types
Mode-locked oscillators
Mode-locked oscillators 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 with a typical duration between a few
picoseconds 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.
A femtosecond is to a second, as a second is to approximately 31.6 ...
s, in special cases even around 5 femtoseconds (few carrier wave cycles in each laser pulses). The pulse repetition
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
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 (physics), 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 quantification (science), quantify the rate of Work ...
s (5.7 to 35 nJ in each laser pulse for the 70 MHz repetition rate).
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), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...
, while a larger, multistage facility can produce pulses up to several
joule
The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...
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 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 syste ...
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.

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 typicall ...
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.
...
and strong
nonlinear optics
Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in Nonlinearity, 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 ...
effects that generate a
wide spectrum of wavelengths.
Tunable continuous wave lasers
Titanium-sapphire is especially suitable for pulsed lasers since an
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 ...
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 lasers with extremely narrow
linewidth
A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...
s tunable over a wide range.
History and applications

The Ti:sapphire laser was invented by Peter Moulton in June 1982 at
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 ...
in its continuous wave version. Subsequently, these lasers were shown to generate ultrashort pulses through
Kerr-lens modelocking.
Strickland and
Mourou, in addition to others, working at the
University of Rochester
The University of Rochester is a private university, private research university in Rochester, New York, United States. It was founded in 1850 and moved into its current campus, next to the Genesee River in 1930. With approximately 30,000 full ...
, 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 Labs. Ashkin has been considered by many as the father of optical tweezers, "LaserFest – the 50th anniversary of the firs ...
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
The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prize, Nobel Prizes establi ...
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 and quantum chemistry that studies the behavior of photons (individual quanta of light). It includes the study of the particle-like properties of photons and their interaction ...
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 nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...
research.
References
External links
Encyclopedia of laser physics and technology on Ti:sapphire lasers
{{DEFAULTSORT:Ti-Sapphire Laser
Solid-state lasers
Titanium