Ti:sapphire: Material, Lasers and Amplifiers

The Ti:Al₂O₃ (Ti:sapphire) laser has several ancestors, including the first laser [1], based on the crystal ruby, formally Cr³⁺ ions doped into sapphire (Al₂O₃). Ti:sapphire employs the same robust host crystal as in ruby, which is crucial for the ruby laser to operate under the intense optical pumping from flashlamps, required by the three-level nature of the ruby laser transition. Another class of ancestors were the first broadly tunable solid-state lasers, based on divalent transition-metal ions, such as Ni²⁺ and Co²⁺, doped into mainly fluoride host crystals, such as MgF₂ [2–5]. While these types of lasers could be tuned over an impressive range of wavelengths (1600–2500 nm in the case of Co:MgF₂ [6]), the low gain and the general need to operate under cryogenic cooling were drawbacks that limited their utility. Finally, another Cr³⁺-doped crystal, alexandrite (BeAl₂O₄), developed in the late 1970s [7,8] showed that tunable solid-state lasers could be made with robust host crystals and operate at room temperature. Later variations of the alexandrite laser, providing broader tuning ranges, included Cr³⁺-doped fluoride materials [9,10], such as LiCaAlF₆ (LiCAF) and LiSrAIF₆ (LiSAF).