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Ti-sapphire laser

Ti-sapphire laser

Overview
Ti:sapphire lasers are tunable laser
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 range....

s which emit red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

 and near-infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulse
Ultrashort pulse
In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a femtosecond . Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators...

s. Lasers based on Ti:sapphire were first constructed in 1982.

Titanium-sapphire refers to the lasing medium, a crystal of sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...

 (Al2O3) that is doped with titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s.
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Encyclopedia
Ti:sapphire lasers are tunable laser
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 range....

s which emit red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

 and near-infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulse
Ultrashort pulse
In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a femtosecond . Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators...

s. Lasers based on Ti:sapphire were first constructed in 1982.

Titanium-sapphire refers to the lasing medium, a crystal of sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...

 (Al2O3) that is doped with titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s. A Ti:sapphire laser is usually pumped
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...

 with another laser with a wavelength of 514 to 532 nm, for which argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

-ion laser
Ion laser
An ion laser is a gas laser which 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 HeNe lasers, the energy level transitions that contribute to laser action come from ions...

s (514.5 nm) and frequency-doubled
Second harmonic generation
An optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency...

 Nd:YAG, Nd:YLF, and Nd:YVO lasers (527-532 nm) are used. Ti:sapphire lasers operate most efficiently at wavelengths near 800 nm.

Mode-locked oscillators


Mode-locked
Modelocking
Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds or femtoseconds ....

 oscillators generate ultrashort pulse
Ultrashort pulse
In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a femtosecond . Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators...

s with a typical duration between 10 femtoseconds and a few picosecond
Picosecond
A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....

s, in special cases even around 5 femtoseconds. The pulse repetition frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 is in most cases around 70 to 90 MHz. 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 0.5 to 1.5 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

.

Chirped-pulse amplifiers


These devices generate ultrashort
Ultrashort pulse
In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a femtosecond . Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators...

, 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
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

 in energy at a repetition frequency of 1000 hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

, while a larger, multistage facility can produce pulses up to several joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

s, with a repetition rate of up to 10 Hz. Usually, amplifiers 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
Laser construction
A laser is constructed from three principal parts:*An energy source ,*A gain medium or laser medium, and*Two or more mirrors that form an optical resonator.-Pump source:...

 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
Chirp
A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...

ed-pulse' refers to a special construction that is necessary to prevent the pulse from damaging the components in the laser.

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 optics
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...

 processes.

At 5 mJ in 100 femtoseconds, the peak power of such a laser is 50 gigawatts, which is many times more than what a large electrical power plant delivers (about 1 GW). When focused by a lens, these laser pulses will destroy any material placed in the focus, including air molecules.

Tunable continuous wave lasers


Titanium-sapphire is especially suitable for pulsed lasers since an ultrashort pulse
Ultrashort pulse
In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a femtosecond . Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators...

 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
Conjugate variables are pairs of variables mathematically defined in such a way that they become Fourier transform duals of one-another, or more generally are related through Pontryagin duality. The duality relations lead naturally to an uncertainty in physics called the Heisenberg uncertainty...

. However, with an appropriate design, titanium-sapphire can also be used in continuous wave
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...

 lasers with extremely narrow linewidths tunable over a wide range.

Application to generation of pulsed hard radiation


When a laser pulse passes an electron the electron is shaken heavily, but afterwards it flies on as if nothing has happened, though a little bit of Compton scattering
Compton scattering
In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...

 has taken place. Additionally an electron can either enter or leave an atom and in this process the electron can either emit an X-ray photon or absorb an X-ray photon. In a complex situation with an atom, an electron, and a laser pulse, either the energy of the X-ray photon depends on the electric field of the laser pulse at the time of creation or the energy of the electron depends on the electric field of the laser pulse at the time of leaving the atom. This is called either pulsed X-ray generation or attosecond transient recorder. Though the atom and the laser pulse interact in various ways, this is ignored here (see high harmonic generation
High Harmonic Generation
- Perturbative Harmonic Generation :Perturbative Harmonic Generation is a process whereby laser light of frequency ω and photon energy ħω can be used to generate new frequencies of light. The newly generated frequencies are integer multiples nħω of the original light's frequency...

instead).

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