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. Lasers based on Ti:sapphire were first constructed and invented in June 1982 by Peter Moulton at the MIT Lincoln Laboratory. [1]

Contents

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. Ti:sapphire lasers operate most efficiently at wavelengths near 800 nm.

Types of Ti:sapphire lasers

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. The pulse repetition 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 0.4 to 2.5 watts.

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, 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 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.

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. [2] When focused by a lens, these laser pulses will ionise any material placed in the focus, including air molecules.[ citation needed ]

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

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. [3] Strickland and Mourou, in addition to others, working at the University of Rochester, showed chirped pulse amplification of this laser within a few years [4] , for which these two shared in the 2018 Nobel Prize in physics [5] (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. [6]

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. [7] [8] [9] 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. [10] 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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Optical amplifier 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 fiberoptic 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'.

Kerr-lens modelocking 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.

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 of light 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. They are commonly referred to as ultrafast events. 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.

Terahertz time-domain spectroscopy

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. By measuring in the time-domain, the technique can provide more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude.

Chirped pulse amplification

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.

Here, is a list of initialisms and acronyms used in laser physics, applications and technology.

Chirped 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).

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Frequency comb

In optics, a frequency comb is a laser source whose spectrum consists of a series of discrete, equally spaced frequency lines. Frequency combs can be generated by a number of mechanisms, including periodic modulation of a continuous-wave laser, four-wave mixing in nonlinear media, or stabilization of the pulse train generated by a mode-locked laser. Much work has been devoted to the latter mechanism, which was developed around the turn of the 21st century and ultimately led to one half of the Nobel Prize in Physics being shared by John L. Hall and Theodor W. Hänsch in 2005.

Prism compressor optical device used to shorten the duration of a positively chirped ultrashort laser pulse

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.

In optics, femtosecond pulse shaping refers to manipulations with temporal profile of an ultrashort laser pulse. Pulse shaping can be used to shorten/elongate the duration of optical pulse, or to generate complex pulses.

Supercontinuum 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

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 ten picoseconds. 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.

Polarization ripples

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.

Donna Strickland 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.

References

  1. Moulton, P. F. (1986). "Spectroscopic and laser characteristics of Ti:Al_2O_3". Journal of the Optical Society of America B. 3 (1): 125–133. Bibcode:1986JOSAB...3..125M. doi:10.1364/JOSAB.3.000125.
  2. Erny, Christian; Hauri, Christoph P. (2013). "Design of efficient single stage chirped pulse difference frequency generation at 7 μm driven by a dual wavelength Ti:sapphire laser". Applied Physics B. 117 (1): 379–387. arXiv: 1311.0610 . Bibcode:2014ApPhB.117..379E. doi:10.1007/s00340-014-5846-6.
  3. Spence, D. E.; Kean, P. N.; Sibbett, W. (1991-01-01). "60-fsec pulse generation from a self-mode-locked Ti:sapphire laser". Optics Letters. 16 (1): 42–44. Bibcode:1991OptL...16...42S. CiteSeerX   10.1.1.463.8656 . doi:10.1364/OL.16.000042. ISSN   1539-4794.
  4. Strickland, Donna; Mourou, Gerard (1985-10-15). "Compression of amplified chirped optical pulses". Optics Communications. 55 (6): 447–449. Bibcode:1985OptCo..55..447S. doi:10.1016/0030-4018(85)90151-8.
  5. "The Nobel Prize in Physics 2018". www.nobelprize.org. Retrieved 2018-10-02.
  6. "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.
  7. Hänsch, Theodor W. (2006). "Nobel Lecture: Passion for precision". Reviews of Modern Physics. 78 (4): 1297–1309. Bibcode:2006RvMP...78.1297H. doi: 10.1103/RevModPhys.78.1297 .
  8. Hall, John L. (2006). "Nobel Lecture: Defining and measuring optical frequencies". Reviews of Modern Physics. 78 (4): 1279–1295. Bibcode:2006RvMP...78.1279H. doi: 10.1103/RevModPhys.78.1279 .
  9. "The Nobel Prize in Physics 2005". www.nobelprize.org. Retrieved 2017-11-02.
  10. Medeiros de Araújo, R. (2014). "Full characterization of a highly multimode entangled state embedded in an optical frequency comb using pulse shaping". Physical Review A. 89 (5): 053828. arXiv: 1401.4867 . Bibcode:2014PhRvA..89e3828M. doi:10.1103/PhysRevA.89.053828.