2
Solid-state Crystalline Mid-IR Lasers

Crystalline mid-IR lasers are direct sources of coherent light in the sense that they require a minimal number of energy conversion steps. When combined with laser diode pumping, these lasers are efficient, simple, and compact. The gain medium of a crystalline laser is a host crystal doped with active ions. These active ions (also referred to as impurity ions) doped into a crystalline matrix acquire, due to energy-level splitting, characteristic set of energy levels, not present in free ions. For rare earths, the primary cause of energy-level splitting is the interaction of electron spins of the dopant ion with the orbital angular momentum of electrons (spin-orbit interactions), while in transition metal ions, it is mostly due to the interaction of the optically active electron with the crystalline electric field of the host (the Stark effect).

The most common active media for mid-IR crystalline lasers are based on triply ionized rare-earth thulium (Tm), holmium (Ho), and erbium (Er) ions in yttrium aluminum garnet (Y3Al5O12 or YAG), yttrium lithium fluoride (LiYF4 or YLF), yttrium-scandium-gallium garnet (Y3Sc2Ga3O12 or YSGG), or other crystalline hosts. Alternatively, transition-metal-doped (Cr2+, Fe2+) II-VI zinc chalcogenide crystals (ZnSe, ZnS) or other chalcogenides (CdSe, CdS, ZnTe, and CdMnTe) can serve as active media for mid-IR lasers with an extremely broad gain bandwidth.

2.1 Rare-Earth-based Tm3+, Ho3+, and Er3+ Lasers

2.1.1 Tm3+ Lasers

The energy-level diagram for the trivalent thulium ion is shown in Figure 2.1a. Tm-doped crystalline lasers can provide tunable operation in two spectral regions: 1.8-2.2?µm using the 3F4-3H6 transition and 2.2-2.4?µm using the 3H4-3H5 transition.

Figure 2.1 (a) Energy-level diagram for the trivalent thulium. Wavy arrows indicate nonradiative phonon-assisted decay. Laser upper-state lifetimes are also indicated. (b) Spectrum of fluorescence from the 3F4 level of Tm3+ in YAG.

Source: reproduced from figure 1 of [1], with permission of OSA, The Optical Society.

At first glance, a laser that uses 3F4-3H6 transition appears as a three-level system. In such a system the lower laser level is the ground state. Nevertheless, it should be mentioned that both 3F4 and 3H6 levels consist of manifolds of energy levels split due to the Stark effect in the electric field of the crystalline lattice, so that the lower laser level is not necessarily the lowest energy state. Due to the fast energy relaxation between these sublevels, the system becomes virtually a four-level system. An additional effect is that the presence of partially overlapping manifolds of Stark levels within the upper and lower laser state broadens the bandwidth of fluorescence, resulting in broad emission linewidths. Furthermore, thulium lasers (e.g. Tm:YAG and Tm:YSGG) are characterized by large phonon broadening. The phonons (vibrations of the ions of the host crystal) modulate the crystal field at the site of the "lasing" dopants (thulium ions in this case), which in turn broadens the energy levels [1]. Both of these effects allow laser tunability over several hundreds of nanometers. The room-temperature (RT) fluorescence spectrum from the 3F4 state to the 3H6 ground state of Tm3+ in YAG is shown in Figure 2.1b.

YAG crystal is one of the most commonly used host materials for thulium because of its unique thermal-mechanical and optical properties. Typically, Tm3+-doped solid-state lasers are pumped (3H6 3H4) by commercially available high-power AlGaAs diode bars at ~800?nm. There is a "two-for-one" cross-relaxation process that can lead to pumping quantum efficiencies approaching a factor of two. (The pumping quantum efficiency indicates how many laser photons are emitted per one absorbed pump photon.) The essence of this effect (Figure 2.2) is that because of the fortuitous proximity (resonance) of the energy spacing 3H4-3F4 and 3F4-3H6, the 3F4 upper laser level is populated through the cross-relaxation process 3H4?+?3H6 3F4?+?3F4 [1]. The effectiveness of this two-body cross-relaxation process increases with Tm3+ doping concentrations; it becomes significant, typically at >3?at.% concentration. For example, it was shown that at Tm3+ concentration of 12% for Tm:YAG and Tm:YSGG, the cross-relaxation totally dominates the decay of the 3H4 state. As a result, a slope efficiency of as high as 59% has been demonstrated in Tm:YAG, considerably larger than the 39% maximum expected from the quantum defect alone [1]. (The quantum defect is defined as the ratio of the energy of the lasing photon to that of the pump.)

Figure 2.2 Resonant pumping diagram for the 2-µm Tm3+ laser. The "two-for-one" cross-relaxation process leads to the efficient transfer of all absorbed pump energy to the excitation of Tm ions to the 3F4 level.

With a 785-nm laser pumping, broadly tunable continuous wave (CW) laser emission over the ranges 1.87-2.16?µm in Tm:YAG and 1.85-2.14?µm in Tm:YSGG has been reported [1]. Similarly, tunable Tm laser emission using a different host crystal - Tm:YALO (Tm:YAlO3) - was observed over the range 1.93-2.0?µm [2].

The 3F4-3H6 transition in Tm is especially attractive for high-power applications because of the ability to use highly efficient AlGaAs diodes or diode stacks operating around 800?nm as a pump source. A compact diode-pumped Tm:YAG laser capable of generating 115?W of CW power at 2.01?µm has been demonstrated at the 805-nm pumping power of 360?W [3]. Another high-power CW Tm:YAG laser used a linear laser cavity with three laser rods, each side-pumped by arrays of laser diode bars with central wavelength of 785?nm, arranged in fivefold symmetry around each laser crystal (Figure 2.3). The laser was water-cooled at 8?°C and yielded a maximum output CW power of 267?W at 2.07?µm, with the total laser-diode pump power of 1.3?kW. The corresponding optical-to-optical conversion efficiency was 20.7%, with slope efficiency of 29.8% [4].

The Q-switching performance of Tm:YAG near 2?µm is facilitated by a long fluorescence lifetime of 11?ms, which leads to a high energy storage capability [5]. For example, Q-switched pulses at 2.016?µm with 20.4?mJ energy and 69?ns duration were demonstrated at 500?Hz pulse repetition rate; a Tm:YAG ceramic slab laser was end-pumped by a diode laser (the absorbed pump power 53?W) [6]. In general, because of the low gain due to low stimulated emission cross section of Tm ions, Q-switched Tm lasers operate by necessity at high intracavity energy density (fluence), close to the material damage threshold [5]. The reason is that in order to achieve the laser threshold in this low-gain medium, one needs high population inversion; this in turn results in a high amount of energy stored in the medium that is eventually released as an energetic Q-switched laser pulse that may cause material damage.

Figure 2.3 High-power (267?W) Tm:YAG laser system at 2.07?µm based on a linear laser cavity containing three laser rods. The laser cavity was formed by two plane mirrors: M1 with reflectivity R?>?99.5%, and the outcoupler mirror M2 with transmission T = 5% around 2?µm. Each rod (Tm concentration of 3.5?at.%, 4?mm in diameter, and 69?mm in length) was side-pumped by an array of laser diode bars at 785?nm. Undoped YAG end caps were bonded to end faces of the rods to reduce thermal effects as well as reabsorption losses in the unpumped regions.

Source: reproduced from figure 1 of [4], with permission of OSA, The Optical Society.

2.1.2 Ho3+ Lasers

Figure 2.4a represents the energy-level diagram for trivalent holmium. With Ho3+ as a laser ion, one can obtain oscillation using the 5I7-5I8 transition in the 1.95-2.15?µm range (the corresponding emission spectrum for this transition is shown in Figure 2.4b), as well as 5I6-5I7 transition at 2.85-3.05?µm, 5I5-5I6 transition at 3.94?µm, and 5S2-5F5 transition near 3.2?µm. Holmium lasers are more favorable (as compared to thulium lasers) to operation in the Q-switched mode due to high stimulated emission cross section (high gain); also, they have long (~10?ms, similar to thulium) fluorescence lifetime for the upper laser energy manifold 5I7.

Since there are no convenient schemes for direct diode pumping of Ho3+ lasers (at least with available high-efficiency AlGaAs or InGaAs laser diodes), holmium laser crystals are often codoped (sensitized) with other ions. For example, crystals doped with a combination of Tm3+ and...