Mid-infrared (MIR) laser sources operating in 2.7-3 µm spectral region have attracted extensive attention for many applications due to the unique features of locating at the atmospheric transparency window, corresponding to the "characteristic fingerprint" spectra of several gas molecules, and strong absorption of water. Up to now, the most advanced MIR solid-state laser technologies that used for 2.7-3 µm laser generation including but not limited the rare-earth doped crystalline and fiber lasers, the semiconductor laser diodes (LDs), the quantum cascade lasers (QCLs), and nonlinear optical frequency conversion {optical difference frequency generation (DFG), optical parametric sources [OPSs, including optical parametric oscillators (OPOs) , optical parametric generators (OPGs)], etc.}, and so on. Over the past two decades, significant developments have been achieved in 2.7-3 µm MIR lasers benefiting from the sustainable innovations in laser technology and the great progress in material science. Laser transition of Er³⁺: ⁴I_11/2→⁴I_13/2, Ho³⁺: ⁵I₆→⁵I₇ and Dy³⁺: ⁶H_13/2 → ⁶H_15/2 can emit wavelength at 2.7-3 µm spectral region ^[1], and thus are the mostly studied MIR bulk lasers.

Fig. 1 The simplified energy-level diagram of Er³⁺-, Ho³⁺- and Dy³⁺-doped crystals and typical emission spectrum ^[1]

Prof. Baitao Zhang and Jingliang He's group from Shandong University summarize and review the recent progress of MIR bulk laser sources based on the rare-earth ions-doped crystals at 2.7-3 µm spectral region, including Er³⁺, Ho³⁺ and Dy³⁺ doped crystalline lasers. The outlooks and challenges for future development of rare-earth doped MIR bulk lasers are also discussed. Related content was published in Chinese Optics Letters Volume 19, Issue 9 (Hongkun Nie, Baitao Zhang, and Jingliang He et al, Rare-earth ions-doped mid-infrared (2.7-3 µm) bulk lasers: a review [Invited]).

Easy growing and the pumping wavelength around ~970 nm (commercial LD operation wavelength) are the two fundamental merits that make Er³⁺-doped crystals to be much more extensively studied for 2.7-3 µm laser generation compared to that of Ho³⁺-and Dy³⁺-doped crystals. However, for Er³⁺: ⁴I_11/2→⁴I_13/2 transition, there is a so-called self-terminated effect leading to population bottleneck issue caused by the life time of upper laser level (⁴I_11/2) much shorter than that of lower level (⁴I_13/2), which is the main obstacle that prevents the development of Er³⁺doped 2.7-3 µm crystalline lasers. Increasing the doping concentration of Er³⁺ ion to generate "quenching effect" and co-doping with sensitized ions (Yb³⁺) with efficient population of Er³⁺:⁴I_11/2 or deactivated ions (Pr³⁺) with efficient depopulation of Er³⁺:⁴I_13/2 are the two main approaches to solve this detrimental feature ^{[2, 3]}. Fig. 2 shows the room temperature CW output power and the corresponding slope efficiency ever obtained with Er³⁺ ions-doped crystalline lasers in 2.7-3 µm region. The highest CW output power generated from Er³⁺-doped solid-state lasers at room temperature is 6.9 W ^[4], while the largest slope efficiency is 41.4% ^[5]. In the pulsed regime, flash-side-pumping is an effective and commonly used architecture to produce high energy 2.7-3 μm laser pulses at low repetition rate. Combined with LGS EO modulator, the highest pulse energy of 216 mJ and pulse width of 14.36 ns was obtained ^[6]. By using SESAM, Fe:ZnSe and novel 2D materials as saturable absorber, the passively Q-switched Er³⁺-doped crystalline lasers was realized ^[7-10].

Fig. 2 The summarization of the room temperature CW output power and slope efficiency of Er-doped crystalline lasers at 2.7-3 µm

Ho³⁺ ion is another promising candidate for generating 2.7-3 μm lasers related to ⁵I₆→⁵I₇ transition. However, Ho-doped crystalline lasers emitting in 2.7-3 μm spectral region are much less studied compared to that of emitting around ~2.1 μm and Er³⁺-doped lasers. The main limiting issues are the pumping wavelength around ~1150 nm and the same self-terminated effect occurring with Ho³⁺:⁵I₆→⁵I₇ transition (Ho³⁺:⁵I₇ has a longer lifetime than Ho³⁺:⁵I₆ resulting in the lower laser level during the oscillation). The same as Er³⁺:⁴I_11/2→⁴I_13/2 transition, the saturation of Ho³⁺:⁵I₆→⁵I₇ transition can also be suppressed by cascade lasing (Ho³⁺:⁵I₆→⁵I₇ and Ho³⁺:⁵I₇→⁵I₈ transitions) or co-doping with sensitized (Yb³⁺) or deactivated ions (Nd³⁺ and Pr³⁺). In 2019, by optimizing the doping concentration of Ho³⁺ and Pr³⁺ ions in Ho,Pr:YLF crystal, the lifetime of Ho³⁺:⁵I₆ was designed to be larger than that of Ho³⁺:⁵I₇, as shown in Fig. 3(a) ^[11]. By using the dual-end-pumping configuration, a maximum output power of 1.46 W was obtained with a slope efficiency of 7.7%, which is the largest CW output power ever obtained with Ho-doped crystalline lasers ^{[12, 13]}. A pulse energy as high as 41 mJ was obtained with flash lamp pumped (Ho,Nd): Y₃Al₅O₁₂ laser ^[14]. Besides, the passive Q-switching laser operation with Ho³⁺ ion-doped crystals have been demonstrated with 2D materials as saturable absorber.

Fig. 3 (a)The fluorescence life time "reversion" of Ho:⁵I₆ and Ho:⁵I₇ in Ho,Pr:YLF crystal with doping concentrations of 0.498 and 0.115 at. % for Ho³⁺ and Pr³⁺ ions; (b) the experimental setup and laser output power of dual-end pumped EO Q-switched Ho,Pr:YLF laser^[11]

Dy³⁺ ion is also a most promising and efficient candidate for emitting MIR laser wavelength around ~3 μm based on its energy level structure with Dy³⁺:⁶H_13/2 → ⁶H_15/2 transition. The study of Dy³⁺-doped MIR lasers is much less than that of Er³⁺ and Ho³⁺ ions, basically because of lack of high-quality crystal and pump source. It is excited that an efficiency 3.04 μm Dy:ZBLAN fiber laser was realized with a record slope efficiency of 51% pumped by a 2.8 μm Er:ZBLAN laser ^[15]. However, there is no reports about Dy³⁺ ion-doped MIR crystalline lasers.

In this review, we mainly summarize the state-of-art development of all solid-state MIR crystalline lasers in 2.7-3 μm spectral region based on Er³⁺, Ho³⁺ and Dy³⁺ doped crystals. However, there are still several challenges and a series of potential studies need to be further pursued in future. First, the host material selection and the preparation of the high-quality crystals are the basis for high power and high-efficient solid-state MIR crystalline lasers in 2.7-3 μm. Second, the selection of sensitized and deactivated ions and the doping concentration are also important for rear-earth doped crystalline lasers in 2.7-3 μm. Third, cascade laser operation is very attractive for multi-wavelength MIR laser generation. Fourth, the mode-locked laser operation is another challenge for rear-earth doped crystalline lasers in 2.7-3 μm.