titanium-doped laser gain media (original) (raw)

Definition: laser gain media doped with titanium ions

Category: laser devices and laser physics

• laser gain media
  • transition-metal-doped laser gain media
    * chromium-doped laser gain media
    * titanium-doped laser gain media
    * iron-doped laser gain media

Related: titanium-sapphire laserstransition-metal-doped laser gain media

DOI: 10.61835/r42 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

Titanium (chemical symbol: Ti) is a chemical element belonging to the group of transition metals. Trivalent titanium ions (Ti3+) can be used as laser-active dopants of laser gain media. They have very broad vibronic absorption and emission bands, which enables wide wavelength tunability and the generation of ultrashort pulses.

To date, only Ti:sapphire has achieved broad practical success; a few other Ti-doped hosts have been investigated and are briefly noted below.

Titanium-doped Sapphire

Titanium-doped sapphire (Ti3+:sapphire, sometimes Ti:Sa) is used mainly for tunable lasers and for femtosecond solid-state lasers. Introduced in 1986 [1], Ti:sapphire rapidly displaced most dye lasers in ultrashort-pulse generation and widely wavelength-tunable lasers. Ti:sapphire lasers are also convenient pump lasers for testing new solid-state media (e.g., Nd- or Yb-based), because they tune to the required pump line and deliver high brightness thanks to excellent beam quality and multi-watt power.

Ti:sapphire is predominantly used as bulk laser crystals, but engineered waveguides also exist (see below).

Because pump requirements dominate the cost, Ti:sapphire is typically chosen when its exceptional tunability or ultrashort-pulse performance is essential.

In the following sections, the key properties of Ti:sapphire are explained:

Excellent Thermal Properties

Sapphire (single-crystal Al2O3) has high thermal conductivity and good thermal shock resistance. It minimizes beam distortions by thermal lensing at high power.

Large Gain Bandwidth

With its vibronic emission band, the Ti3+ ion has a very large gain bandwidth (much larger than that of rare-earth-doped laser gain media), allowing the generation of extremely short laser pulses and also wide wavelength tunability (typically using a birefringent tuner).

Small-signal gain and efficiency peak near 800 nm; practical emission typically spans roughly 700–900 nm. With suitable optics, tuning from ≈650 nm up to ≈1100 nm is possible, though covering that full span in one resonator usually requires swapping mirror sets (or using ultrabroadband chirped mirrors).

Pumping of Ti:sapphire

Ti:sapphire offers a wide pump band, strongest in the blue-green with an absorption maximum near 490 nm. Common pump sources are 514 nm argon ion lasers, 527/532 nm frequency-doubled diode-pumped solid-state lasers, and high-power 515–525 nm laser diodes. The gain efficiency (in dB/W) is modest because the σ–τ product is small — an inherent consequence of the broad emission band. Consequently, high pump radiance is needed; multi-watt pump powers are typical for bulk lasers (often 3–10 W and up to 20 W for demanding setups), while waveguides can work with much lower pump powers.

Ti3+ Doping Concentration

The Ti3+ doping concentration has to be kept low (typically 0.05% to 0.25%) because otherwise no good crystal quality is achievable. 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 radiance (brightness) is required.

Ti4+ Content

Ideally, a Ti:sapphire crystal would contain only Ti3+ ions and no Ti4+, but some small amount of Ti4+ is unfortunately hard to avoid (especially in the presence of impurities like Fe2+). This causes parasitic absorption at the laser wavelength, which deteriorates the laser performance. Details of fabrication processes often need to be optimized such that primarily Ti3+ is obtained.

In order to quantify the quality of Ti:sapphire crystals in that respect, one often uses a figure of merit (FOM) which is defined as the ratio of absorption coefficients at the pump and laser wavelengths — typically, with 514 nm or 532 nm as the pump wavelength and something around 800 nm as the laser wavelength.

Upper-state Lifetime and Transition Cross-sections

The upper-state lifetime of Ti:sapphire is short (3.2 μs) because of relatively high emission cross-sections and the broad emission bandwidth. The saturation power is very high (far higher, for example, than for Yb:YAG) and the gain efficiency relatively low. 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.

Figure 1: Transition cross-sections of Ti3+:sapphire for ($\pi$) and ($\sigma$) polarization. Source: Evgeni Sorokin, TU Wien.

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.

Ti:sapphire Waveguides

Bulk Ti:sapphire demands substantial pump power because of its low gain efficiency. Waveguides mitigate this by maintaining a small effective mode area over significant length. Various technical approaches for waveguide fabrication have been demonstrated:

• Ti:sapphire crystal fibers with a fiber cladding of fused silica have enabled tunable lasers [10, 20, 22].
• Planar and channel waveguides can be fabricated with pulsed laser deposition of Ti:sapphire films [13, 17].
• High-temperature titanium indiffusion providing both waveguiding and active centers [12, 16]. A thin Ti stripe is deposited lithographically on sapphire and thermally indiffused at ≈1700–2000 °C for one or several hours. Rapid cooling has been found to improve the Ti3+ fluorescence yield.
• Implantation of Ti3+ and oxygen with ion beams, followed by thermal annealing, is another possibility [15].
• Waveguides can be directly written into Ti:sapphire using intense femtosecond laser pulses [18].
• The silicon-nitride-on-sapphire integrated photonics platform allows one to realize Ti:sapphire waveguides forming microring resonators in photonic integrated circuits [23, 24]. Here, a Si3N4 film is grown on a sapphire wafer and structured with masked etching. Later, a thinned-down Ti:sapphire crystal (typically 350–450 nm thick) is flip-chip bonded to this photonic chip. The bonded chip is then coated with plasma-enhanced chemical vapor deposition silicon oxynitride (SiON) with a refractive index close to that of sapphire to obtain symmetric waveguiding in the vertical direction, minimizing the radiation loss. Laser thresholds below 10 mW have been reported, and sub-milliwatt thresholds appear to be feasible due small mode areas on the order of 1 μm2 [23].

Table 1: Optical, mechanical and other properties of Ti3+:sapphire crystals as used for lasers.

Besides Ti:sapphire, some other Ti-doped laser media have been investigated, but with limited success:

Titanium-doped Chrysoberyll

In the same year as the first demonstration of Ti:sapphire (1986), a tunable laser based on Ti3+:BeAl2O4 was reported [2]. However, that material has found very little attention later on, as its properties are less favorable than those of Ti:sapphire.

Titanium-doped YAG

YAG is yttrium aluminum garnet (Y3Al5O12), a common crystal material with cubic symmetry. YAG lasers are mostly based on laser crystals doped with rare earth ions such as neodymium, ytterbium, erbium or thulium-doped laser gain media, sometimes with the transition metal chromium (Cr4+). A rare case is doping with Ti3+. Ti:YAG crystals show broad absorption and fluorescence [4] for green pumping, but the quantum efficiency and achievable gain are well below Ti:sapphire.

Titanium-doped Lithium Aluminate (Ti:LiAlO2)

Ti:LiAlO₂ single crystals have been grown as large high-purity crystals [21]. They display thermoluminescence and photo-stimulated luminescence, suggesting that Ti3+ creates optically active centers. However, there appears to be no report of successful lasing.

Titanium-doped forsterite (Ti:Mg2SiO4)

Crystals of Ti-doped forsterite grown by the Czochralski technique have been studied for potential tunable laser applications [7]. Spectroscopic studies confirm Ti3+ substitution in the host lattice, with visible and near-infrared absorption. Experimental data show fluorescence but no reported laser emission, likely due to low emission cross sections and a short excited-state lifetime.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 22 suppliers for titanium-doped laser gain media. Among them:

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Shalom EO offers high quality Ti:sapphire crystals grown using Temperature Gradient Technique (TGT) with absorption from 1.0 — 4.0 cm–1 @ 490 nm. Titanium-doped sapphire is one of the most prevalent laser crystals for tunable and ultrashort pulsed lasers with high gain and high power outputs.

Shalom EO’s TGT-grown Ti:sapphire crystal is characterized by the (0001) oriented growth, featuring large figure of merit, high gain, broad gain bandwidth, while also exhibiting excellent thermal conduction, large laser cross-section, and exceptional laser damage threshold. Dislocation densities less than 102 cm–2 contributing to minimized light scatter can be obtained. The tunable wavelength range that covers a broad range from 700 nm to 1000 nm makes Ti:sapphire an excellent alternative for dye lasers in many applications such as ultrafast lasers.

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Titanium-doped sapphire (Ti3+:Al2O3) is widely used for ultrashort pulse generation and broadly tunable laser systems due to its exceptionally wide gain bandwidth of 670–1070 nm and excellent thermal conductivity.

Optogama offers Ti:sapphire crystals with absorption values ranging from 0.2 cm−1 to 7.5 cm−1 at 532 nm, optimized for both oscillator and amplifier applications. High optical quality is ensured, with a Figure of Merit (FOM) > 250 for low-doped material. Crystals are available in large sizes up to 110 × 110 × 40 mm.

These crystals are ideal for high-performance femtosecond lasers, tunable systems, and demanding scientific and industrial applications.

⚙ hardware

Titanium-doped sapphire crystals combine outstanding physical and optical properties with the broadest laser wavelength range. EKSMA Optics offers Ti:sapphire crystals according to your specific requirements.

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