Ti-sapphire laser

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Part of a Ti:sapphire oscillator. The Ti:sapphire crystal is the bright red light source on the left. The green light is from the pump diode Titanium sapphire oscillator.jpg
Part of a Ti:sapphire oscillator. The Ti:sapphire crystal is the bright red light source on the left. The green light is from the pump diode

Ti:sapphire lasers (also known as Ti:Al2O3 lasers, titanium-sapphire lasers, or Ti:sapphs) are tunable lasers which emit red and near-infrared 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 pulses 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. [1]


Titanium-sapphire refers to the lasing medium, a crystal of sapphire (Al2O3) that is doped with Ti3+ ions. A Ti:sapphire laser is usually pumped with another laser with a wavelength of 514 to 532 nm, for which argon-ion lasers (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 1,100 nm wavelength. [2] Ti:sapphire lasers operate most efficiently at wavelengths near 800 nm. [3]


The inner optical setup of a femtosecond Ti-sapphire pulsed laser The inner optical setup of a femtosecond Titanium-Sapphire Pulsed laser.jpg
The inner optical setup of a femtosecond Ti-sapphire pulsed laser

Mode-locked oscillators

Mode-locked oscillators generate ultrashort pulses with a typical duration between a few picoseconds and 10 femtoseconds, in special cases even around 5 femtoseconds (few carrier wave cycles in each laser pulses). The pulse repetition frequency 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 watts (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, while a larger, multistage facility can produce pulses up to several joules, 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 'chirped-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.

A Ti:Sapphire crystal in the centre of a multipass amplifier Quantronix Odin is pumped by 5W green beam (faintly visible coming from right), amplifies femtosecond pulses that pass it several times under different angles (invisible on the photo) and loses part of energy as red fluorescence light ODIN Ti-Sapphire laser in operation.jpg
A Ti:Sapphire crystal in the centre of a multipass amplifier Quantronix Odin is pumped by 5W green beam (faintly visible coming from right), amplifies femtosecond pulses that pass it several times under different angles (invisible on the photo) and loses part of energy as red fluorescence light

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

At 5 mJ in 100 femtoseconds, the peak power of such a laser is 50 gigawatts. [4] 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 and strong nonlinear optics effects that generate a wide spectrum of wavelengths.

Femtosecond pulses generate multiple angle-resolved colour patterns when focused; note their fan-out angle is even higher than that of the focused laser beam Femtosecond laser spark.jpg
Femtosecond pulses generate multiple angle-resolved colour patterns when focused; note their fan-out angle is even higher than that of the focused laser beam

Tunable continuous wave lasers

Titanium-sapphire is especially suitable for pulsed lasers since an ultrashort pulse 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. However, with an appropriate design, titanium-sapphire can also be used in continuous wave lasers with extremely narrow linewidths tunable over a wide range.

History and applications

CW single-frequency ring Ti:Sapphire laser in operation at Novosibirsk State University Ti Sapphire laser TIS-SF-07.jpg
CW single-frequency ring Ti:Sapphire laser in operation at Novosibirsk State University

The Ti:sapphire laser was invented by Peter Moulton in June 1982 at MIT Lincoln Laboratory in its continuous wave version. Subsequently, these lasers were shown to generate ultrashort pulses through Kerr-lens modelocking. [5] Strickland and Mourou, in addition to others, working at the University of Rochester, showed chirped pulse amplification of this laser within a few years, [6] for which these two shared in the 2018 Nobel Prize in physics [7] (along with Arthur Ashkin 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. [8] [9]

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 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. [10] [11] [12] 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 experiments. [13] 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 research.

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<span class="mw-page-title-main">Laser</span> Device which emits light via optical amplification

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 anacronym that originated as an acronym for light amplification by stimulated emission of radiation. The first laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories, based on theoretical work by Charles H. Townes and Arthur Leonard Schawlow.

<span class="mw-page-title-main">Optical amplifier</span> Device that amplifies an optical signal

An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Optical amplifiers are important in optical communication and laser physics. They are used as optical repeaters in the long distance fiber-optic cables which carry much of the world's telecommunication links.

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 (10−12 s) or femtoseconds (10−15 s). A laser operated in this way is sometimes referred to as a femtosecond laser, for example, in modern refractive surgery. The basis of the technique is to induce a fixed phase relationship between the longitudinal modes of the laser's resonant cavity. Constructive interference between these modes can cause the laser light to be produced as a train of pulses. The laser is then said to be "phase-locked" or "mode-locked".

<span class="mw-page-title-main">Kerr-lens modelocking</span> Laser mode-locking method

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.

<span class="mw-page-title-main">Optical parametric amplifier</span>

An optical parametric amplifier, abbreviated OPA, is a laser light source that emits light of variable wavelengths by an optical parametric amplification process. It is essentially the same as an optical parametric oscillator, but without the optical cavity.

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 or less. Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators. Amplification of ultrashort pulses almost always requires the technique of chirped pulse amplification, in order to avoid damage to the gain medium of the amplifier.

<span class="mw-page-title-main">Terahertz time-domain spectroscopy</span>

In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique in which the properties of matter are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample's effect on both the amplitude and the phase of the terahertz radiation.

Chirped pulse amplification (CPA) is a technique for amplifying an ultrashort laser pulse up to the petawatt level, with the laser pulse being stretched out temporally and spectrally, then amplified, and then compressed again. The stretching and compression uses devices that ensure that the different color components of the pulse travel different distances.

Raman amplification is based on the stimulated Raman scattering (SRS) phenomenon, when a lower frequency 'signal' photon induces the inelastic scattering of a higher-frequency 'pump' photon in an optical medium in the nonlinear regime. As a result of this, another 'signal' photon is produced, with the surplus energy resonantly passed to the vibrational states of the medium. This process, as with other stimulated emission processes, allows all-optical amplification. Optical fiber is today most used as the nonlinear medium for SRS for telecom purposes; in this case it is characterized by a resonance frequency downshift of ~11 THz. The SRS amplification process can be readily cascaded, thus accessing essentially any wavelength in the fiber low-loss guiding windows. In addition to applications in nonlinear and ultrafast optics, Raman amplification is used in optical telecommunications, allowing all-band wavelength coverage and in-line distributed signal amplification.

This is a list of acronyms and other initialisms used in laser physics and laser applications.

<span class="mw-page-title-main">Chirped mirror</span> Dielectric mirror

A chirped mirror is a dielectric mirror with chirped spaces—spaces of varying depth designed to reflect varying wavelengths of lights—between the dielectric layers (stack).

Ultrafast laser spectroscopy is a category of spectroscopic techniques using ultrashort pulse lasers for the study of dynamics on extremely short time scales. Different methods are used to examine the dynamics of charge carriers, atoms, and molecules. Many different procedures have been developed spanning different time scales and photon energy ranges; some common methods are listed below.

<span class="mw-page-title-main">Frequency comb</span> Laser source with equal intervals of spectral energies

A frequency comb or spectral comb is a spectrum made of discrete and regularly spaced spectral lines. In optics, a frequency comb can be generated by certain laser sources.

<span class="mw-page-title-main">Prism compressor</span> Optical device

A prism compressor is an optical device used to shorten the duration of a positively chirped ultrashort laser pulse by giving different wavelength components a different time delay. It typically consists of two prisms and a mirror. Figure 1 shows the construction of such a compressor. Although the dispersion of the prism material causes different wavelength components to travel along different paths, the compressor is built such that all wavelength components leave the compressor at different times, but in the same direction. If the different wavelength components of a laser pulse were already separated in time, the prism compressor can make them overlap with each other, thus causing a shorter pulse.

<span class="mw-page-title-main">Supercontinuum</span>

In optics, a supercontinuum is formed when a collection of nonlinear processes act together upon a pump beam in order to cause severe spectral broadening of the original pump beam, for example using a microstructured optical fiber. The result is a smooth spectral continuum. There is no consensus on how much broadening constitutes a supercontinuum; however researchers have published work claiming as little as 60 nm of broadening as a supercontinuum. There is also no agreement on the spectral flatness required to define the bandwidth of the source, with authors using anything from 5 dB to 40 dB or more. In addition the term supercontinuum itself did not gain widespread acceptance until this century, with many authors using alternative phrases to describe their continua during the 1970s, 1980s and 1990s.

An ultrashort pulse laser is a laser that emits ultrashort pulses of light, generally of the order of femtoseconds to one picosecond. They are also known as ultrafast lasers owing to the speed at which pulses "turn on" and "off"—not to be confused with the speed at which light propagates, which is determined by the properties of the medium, particularly its index of refraction, and can vary as a function of field intensity and wavelength.

<span class="mw-page-title-main">Polarization ripples</span>

Polarization ripples are parallel oscillations which have been observed since the 1960s on the bottom of pulsed laser irradiation of semiconductors. They have the property to be very dependent to the orientation of the laser electric field.

Pulsed operation of lasers refers to any laser not classified as continuous wave, so that the optical power appears in pulses of some duration at some repetition rate. This encompasses a wide range of technologies addressing a number of different motivations. Some lasers are pulsed simply because they cannot be run in continuous mode.

<span class="mw-page-title-main">Donna Strickland</span> Canadian physicist, engineer, and Nobel laureate

Donna Theo Strickland is a Canadian optical physicist and pioneer in the field of pulsed lasers. She was awarded the Nobel Prize in Physics in 2018, together with Gérard Mourou, for the practical implementation of chirped pulse amplification. She is a professor at the University of Waterloo in Ontario, Canada.

<span class="mw-page-title-main">János Hebling</span> Hungarian Physicist

János Hebling is a Hungarian physicist, known for his preliminary works at Terahertz physics and spectroscopy. He was born at Zirc on 9 May 1954 and currently works as a professor at the Institute of Physics at University of Pécs and is an active researcher at the Hungarian Academy of Sciences and ELI.


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