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Titanium–sapphire Lasers

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Definition: lasers based on a Ti:sapphire gain medium

German: Titan-Saphir-Laser

Category: lasers

How to cite the article

Titanium-doped sapphire (Ti3+:sapphire) is a widely used transition-metal-doped gain medium for tunable lasers and femtosecond solid-state lasers. It was introduced in 1986 [1], and thereafter Ti:sapphire lasers quickly replaced most dye lasers, which had previously dominated the fields of ultrashort pulse generation and widely wavelength-tunable lasers. Ti:sapphire lasers are also very convenient e.g. for pumping test setups of new solid-state lasers (e.g. based on neodymium- or ytterbium-doped gain media), since they can easily be tuned to the required pump wavelength and allow one to work with very high pump brightness due to their good beam quality and high output power of typically several watts.

Properties of Ti:sapphire

Special properties of the Ti:sapphire gain medium (see also Table 1) are:

Table 1: Properties of Ti3+:sapphire crystals.

PropertyValue
chemical formula Ti3+:Al2O3
crystal structurehexagonal
mass density 3.98 g/cm3
Moh hardness9
Young's modulus335 GPa
tensile strength400 MPa
melting point2040 °C
thermal conductivity33 W / (m K)
thermal expansion coefficient ≈ 5 × 10−6 K−1
thermal shock resistance parameter790 W/m
birefringencenegative uniaxial
refractive index at 633 nm1.76
temperature dependence of refractive index 13 × 10−6 K−1
Ti density for 0.1% at. doping 4.56 × 1019 cm−3
fluorescence lifetime3.2 μs
emission cross section at 790 nm 41 × 10−20 cm2

Ti:sapphire may contain some amount of unwanted Ti4+ ions, leading to parasitic absorption and thus to a loss of laser efficiency. It is important to optimize the fabrication technique such that the Ti4+ content is minimized.

Pulse Generation

Ultrashort pulses from Ti:sapphire lasers can be generated with passive mode locking, usually in the form of Kerr lens mode locking (KLM). The combination with a SESAM allows for reliable self-starting of the pulse generation process. A pulse duration around 100 fs is easily achieved and is typical for commercial devices. However, even pulse durations around 10 fs are possible for commercial devices, and the shortest pulses obtained in research laboratories have durations around 5.5 fs [8, 9]. For such high performance, it is essential to introduce very precise dispersion compensation e.g. with double-chirped mirrors.

Typical output powers of mode-locked Ti:sapphire lasers are of the order of 0.3–1 W, whereas continuous-wave versions sometimes generate several watts. A typical pulse repetition rate is 80 MHz, but devices with multi-gigahertz repetition rates are also commercially available, which can be used e.g. as frequency comb sources. For optical frequency metrology, Ti:sapphire lasers with ultrabroad (octave-spanning) optical spectra [11, 12] are very important.

If the requirements in terms of pulse duration and output power are less stringent, Ti:sapphire lasers may be replaced with Cr:LiSAF or Cr:LiCAF lasers, which can be pumped at longer (red) wavelengths, where laser diodes are available. In other cases, fiber lasers may be used.

Ti:sapphire is also often used for multi-pass amplifiers and regenerative amplifiers. Particularly with chirped-pulse amplification, such devices can reach enormous output peak powers of several terawatts, or in large facilities even petawatts. Such huge powers are interesting for nonlinear optics in an extreme regime, e.g. for high harmonic generation, but also for nuclear fusion research.

Frequency Conversion

Nonlinear frequency conversion can be used to extend further the range of emission wavelengths of a Ti:sapphire laser system. The simplest possibility is frequency doubling to access the blue, ultraviolet and green spectral region. Another approach is to pump an optical parametric oscillator, offering a wide tuning range in the near- or mid-infrared spectral region. For tuning the OPO, it is often sufficient to tune the Ti:sapphire wavelength, rather than e.g. tuning the OPO itself, e.g. by actively affecting the phase-matching conditions.

Bibliography

[1]P. F. Moulton, “Spectroscopic and laser characteristics of Ti:Al2O3”, J. Opt. Soc. Am. B 3 (1), 125 (1986)
[2]P. Albers et al., “Continuous-wave laser operation and quantum efficiency of titanium-doped sapphire”, J. Opt. Soc. Am. B 3 (1), 134 (1986)
[3]A. Sanchez et al., “Room-temperature continuous-wave operation of a Ti:Al2O3 laser”, Opt. Lett. 11 (6), 363 (1986)
[4]E. Gulevich et al., “Current state and prospects for tunable titanium–sapphire lasers”, Proc. SPIE 2095, 102 (1994)
[5]J. F. Pinto et al., “Improved Ti:sapphire laser performance with new high figure of merit crystals”, IEEE J. Quantum Electron. 30 (11), 2612 (1994)
[6]A. Stingl et al., “Sub-10-fs mirror-dispersion-controlled Ti:sapphire laser”, Opt. Lett. 20 (6), 602 (1995)
[7]G. N. Gibson et al., “Electro-optically cavity-dumped ultrashort-pulse Ti:sapphire oscillator”, Opt. Lett. 21 (14), 1055 (1996)
[8]D. H. Sutter et al., “Semiconductor saturable-absorber mirror-assisted Kerr lens modelocked Ti:sapphire laser producing pulses in the two-cycle regime”, Opt. Lett. 24 (9), 631 (1999)
[9]U. Morgner et al., “Sub-two cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser”, Opt. Lett. 24 (6), 411 (1999)
[10]S. H. Cho et al., “Low-repetition-rate high-peak-power Kerr-lens mode-locked TiAl2O3 laser with a multiple-pass cavity”, Opt. Lett. 24 (6), 417 (1999)
[11]R. Ell et al., “Generation of 5-fs pulses and octave-spanning spectra directly from a Ti:sapphire laser”, Opt. Lett. 26 (6), 373 (2001)
[12]L. Matos et al., “Direct frequency comb generation from an octave-spanning, prismless Ti:sapphire laser”, Opt. Lett. 29 (14), 1683 (2004)
[13]T. M. Fortier et al., “Octave-spanning Ti:sapphire laser with a repetition rate > 1 GHz for optical frequency measurements and comparisons”, Opt. Lett. 31 (7), 1011 (2006)
[14]I. Matsushima et al., “10 kHz 40 W Ti:sapphire regenerative ring amplifier”, Opt. Lett. 31 (13), 2066 (2006)
[15]G. T. Nogueira et al., “Broadband 2.12 GHz Ti:sapphire laser compressed to 5.9 femtoseconds using MIIPS”, Opt. Express 16 (14), 10033 (2008)
[16]A. Bartels et al., “Passively mode-locked 10 GHz femtosecond Ti:sapphire laser”, Opt. Lett. 33 (16), 1905 (2008)
[17]P. W. Roth et al., “Directly diode-laser-pumped Ti:sapphire laser”, Opt. Lett. 34 (21), 3334 (2009)
[18]P. W. Roth et al., “Direct diode-laser pumping of a mode-locked Ti:sapphire laser”, Opt. Lett. 36 (2), 304 (2011)

See also: solid-state lasers, transition-metal-doped gain media, femtosecond lasers, vibronic lasers, dye lasers, tunable lasers, Kerr lens mode locking, ultrashort pulses, ultrafast lasers, regenerative amplifiers, chirped-pulse amplification, frequency combs


Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) and software could become very valuable for your business!

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