Atitanium-sapphire laser (also known as aTi:sapphire laser,Ti:Al2O3 laser orTi:sapph) is atunable laser which emitsred andnear-infrared light in the range from 650 to 1100 nanometers. This type oflaser is mainly used in scientific research because of its tunability and its ability to generateultrashort 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 theMIT Lincoln Laboratory.[1]
Titanium-sapphire refers to thelasing medium, a crystal ofsapphire (Al2O3) that isdoped withTi3+ions. A Ti:sapphire laser is usuallypumped with another laser with a wavelength of 514 to 532 nm, for whichargon-ion lasers (514.5 nm) andfrequency-doubledNd:YAG,Nd:YLF, andNd: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 crystal is often made using the heat exchanger method of production.[4]
Mode-locked oscillators generateultrashort pulses with a typical duration between a fewpicoseconds and 10femtoseconds, in special cases even around 5 femtoseconds (few carrier wave cycles in each laser pulses). The pulse repetitionfrequency 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-doubledNd:YVO4 laser. Typically, such an oscillator has an average output power of 0.4 to 2.5watts (5.7 to 35 nJ in each laser pulse for the 70 MHz repetition rate).
These devices generateultrashort, ultra-high-intensity pulses with a duration of 20 to 100 femtoseconds. A typical one stage amplifier can produce pulses of up to 5millijoules in energy at a repetition frequency of 1000hertz, while a larger, multistage facility can produce pulses up to severaljoules, with a repetition rate of up to 10 Hz. Usually, amplifier crystals are pumped with a pulsed frequency-doubledNd: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 normalcavity 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 amulti-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 variousnonlinear optical processes.
At 5 mJ in 100 femtoseconds, the peak power of such a laser is 50 gigawatts.[5] When focused by a lens, these laser pulses will ionise any material placed in the focus, including air molecules, and lead to shortfilament propagation and strongnonlinear optics effects that generate awide spectrum of wavelengths.
Titanium-sapphire is especially suitable for pulsed lasers since anultrashort 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 beingconjugate variables. However, with an appropriate design, titanium-sapphire can also be used incontinuous wave lasers with extremely narrowlinewidths tunable over a wide range.
The Ti:sapphire laser was invented by Peter Moulton in June 1982 atMIT Lincoln Laboratory in its continuous wave version. Subsequently, these lasers were shown to generate ultrashort pulses throughKerr-lens modelocking.[6]Strickland andMourou, in addition to others, working at theUniversity of Rochester, showed chirped pulse amplification of this laser within a few years,[7] for which these two shared in the 2018 Nobel Prize in physics[8] (along withArthur 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.[9][10]
The ultrashort pulses generated by Ti:sapphire lasers in the time domain corresponds to mode-lockedoptical 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 pumpingnonlinear optical processes. One half of theNobel 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.[11][12][13] 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 forquantum optics experiments.[14] 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 coldmicromachining. 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 innuclear fusion research.
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