Ti-sapphire laser

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The correct title of this article is Ti:sapphire laser. The substitution of the colon is due to technical restrictions.
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

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

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]

Types

The inner optical setup of a femtosecond Ti-sapphire pulsed laser

Mode-locked oscillators

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

Main article: Chirped pulse amplification

These devices generate

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

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

Tunable continuous wave lasers

Titanium-sapphire is especially suitable for pulsed lasers since an

linewidths
tunable over a wide range.

History and applications

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.

References

  1. doi:10.1364/JOSAB.3.000125
    .
  2. PMID 19773946
    .
  3. ISBN 978-0-12-369395-2
    . Retrieved 2021-10-02.
  4. S2CID 119237744
    .
  5. PMID 19773831
    .
  6. doi:10.1016/0030-4018(85)90151-8
    .
  7. ^ "The Nobel Prize in Physics 2018". www.nobelprize.org. Retrieved 2018-10-02.
  8. ^ "Peter Moulton on the Ti:Sapphire laser. The Ti:sapphire laser has gained broad usage and new applications in biological research and other areas since its inception in 1982". spie.org. Retrieved 2017-11-02.
  9. ^ "Titanium–sapphire Lasers".
  10. doi:10.1103/RevModPhys.78.1297
    .
  11. doi:10.1103/RevModPhys.78.1279
    .
  12. ^ "The Nobel Prize in Physics 2005". www.nobelprize.org. Retrieved 2017-11-02.
  13. S2CID 32829164
    .

External links
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