Titanium–sapphire Lasers

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

• Sapphire (monocrystalline Al₂O₃) has an excellent thermal conductivity, alleviating thermal effects even for high laser powers and intensities.
• The Ti³⁺ ion has a very large gain bandwidth (much larger than that of rare-earth-doped gain media), allowing the generation of very short pulses and also wide wavelength tunability (typically using a birefringent tuner). The maximum gain and laser efficiency are obtained around 800 nm, and many Ti:sapphire lasers operate with emission wavelengths between about 700 nm and 900 nm. The possible tuning range is ≈ 650 nm to 1100 nm, but different mirror sets are normally required for covering this huge range, and exchanging mirror sets is a tedious task. (The number of mirror sets required can be reduced by using ultrabroadband chirped mirrors.)
• There is also a wide range of possible pump wavelengths, which however are located in the green spectral region (with the absorption peak at ≈490 nm), where powerful laser diodes are not available. In most cases, several watts of pump power are used, sometimes even 20 W. Originally, Ti:sapphire lasers were in most cases pumped with 514-nm argon ion lasers, which are powerful, but very inefficient, expensive to operate, and bulky. Other kinds of green lasers are now available, and frequency-doubled solid-state lasers based on neodymium-doped gain media are widely used. The pump wavelength is then typically 532 nm, with a slightly reduced pump absorption efficiency compared with 514 nm. Direct diode pumping at shorter wavelengths, e.g. at 455 nm with GaN-based laser diodes, is also possible, but here one does not only have substantially reduced pump absorption but also a detrimental induced loss which substantially further degrades the performance [17].
• The Ti³⁺ doping concentration has to be kept fairly low (e.g. 0.15% or 0.25%) because otherwise no good crystal quality is possible. The therefore limited pump absorption usually enforces the use of a crystal length of several millimeters, which in combination with the small pump spot size (for high pump intensity) means that a rather high pump brightness is required.
• The upper-state lifetime of Ti:sapphire is short (3.2 μs), and the saturation power is very high. This means that the pump intensity needs to be high, so that a strongly focused pump beam and thus a pump source with high beam quality is required.
• Despite the huge emission bandwidth, Ti:sapphire has relatively high laser cross sections, which reduces the tendency of Ti:sapphire lasers for Q-switching instabilities.

Table 1: Properties of Ti³⁺:sapphire crystals.

Property	Value
chemical formula	Ti3+:Al2O3
crystal structure	hexagonal
mass density	3.98 g/cm3
Moh hardness	9
Young's modulus	335 GPa
tensile strength	400 MPa
melting point	2040 °C
thermal conductivity	33 W / (m K)
thermal expansion coefficient	≈ 5 × 10−6 K−1
thermal shock resistance parameter	790 W/m
birefringence	negative uniaxial
refractive index at 633 nm	1.76
temperature dependence of refractive index	13 × 10−6 K−1
Ti density for 0.1% at. doping	4.56 × 1019 cm−3
fluorescence lifetime	3.2 μs
emission cross section at 790 nm (polarization parallel to the c axis)	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.

In principle, Ti:sapphire lasers are built in similar ways as other types of solid-state lasers: with a Ti:sapphire crystal, typically between two curved mirrors for forming a tight focus in the crystal, with pump light injected through one or two of those dichroic mirrors, and some additional components such as mirrors and possibly optical elements for wavelength tuning and/or ultrashort pulse generation (see below). The laser crystal is usually quite small, typically with an optical path length of only a few millimeters for pump and laser radiation.

As explained above, diode pumping is usually still difficult to realize because of the high power and high beam quality required from the pump source at a somewhat inconvenient wavelength. Therefore, one often requires frequency-doubled solid-state lasers as pump sources.

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.

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.

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