Q-switched ceramic lasers for remote sensing

Solid-state lasers that operate in the so-called eye-safe region of the spectrum (∼2μm) are particularly well suited for applications in medicine and remote sensing.^1–3 These lasers enable the detection of strong water absorption bands and are highly transmissible in air. Among laser sources, resonantly pumped holmium-doped crystals have undergone wide study due to their minimal quantum defects, a property which leads to reduced heat generation in the pumping process and therefore high lasing efficiencies.⁴Recently, holmium-doped yttrium aluminum garnet (Ho:YAG) ceramics^5–7 have also become attractive due to the many advantages that they have over Ho:YAG crystals, including their ease and speed of fabrication, low cost, and the feasibility of using large ceramic elements, which are particularly important for high-energy lasers.^{5, 6}

We have successfully demonstrated a resonantly pumped, Q-switched Ho:YAG ceramic laser⁸ for the first time: see Figure 1. Q-switching is a technique in which the quality factor (2π multiplied by the ratio of energy stored and lost in one oscillation cycle) of the emission is inhibited during pumping by using an optical switch. The built-up energy is then released, resulting in the delivery of a short, high-energy pulse. For the pump source, we fabricated a thulium-doped yttrium lithium fluoride (Tm:YLF) laser with a wavelength of 1908nm. We investigated Ho:YAG ceramics with two different holmium-doping concentrations (0.8 and 1.0at%, Ho:YAG-8 and Ho:YAG-10). To tune the wavelength, we used a 0.1mm-thick uncoated etalon inside the resonator. The Ho:YAG ceramic laser, which has a U-shaped resonator, was Q-switched using an acousto-optic modulator (AOM). The input mirror has an anti-reflection coating at 1908nm and a high-reflection coating at 2097nm. The output mirror has a radius of curvature of 750mm, with a transmissivity of 30% at 2097nm.

Figure 1. The experimental setup of the holmium-doped yttrium aluminum garnet (Ho:YAG) ceramic laser pumped by a thulium-doped yttrium lithium fluoride (Tm:YLF) laser. LD: Laser diode. DM: Dichroic mirror. AO: Acousto-optic.

Figure 2(a) shows the output energies of Q-switched Ho:YAG ceramic lasers at varying pump powers and pulse repetition frequencies (PRFs). At PRFs of 500Hz and 1kHz, the output energy of the Ho:YAG-8 laser is higher than that of the Ho:YAG-10, but the energy extraction efficiencies of the two are comparable. When the PRF is decreased from 500 to 300Hz, the maximum output energy of the Ho:YAG-10 laser is close to that of the Ho:YAG-8. However, the slope efficiency of the Ho:YAG-8 laser shows a little saturation as pump power is increased.

Figure 2. (a) Output energies of the Ho:YAG ceramic lasers with two different doping concentrations (0.8 and 1.0at%) at different pump powers and pulse repetition frequencies (PRFs). (b) Output energies and pulse widths of the lasers at 200Hz PRF at different pump powers. (c) Beam propagation and intensity profile of Q-switched Ho:YAG ceramic laser in x and y directions. Inset: The 2D beam profile of the laser. M²_x, M²_y: Beam quality factor in the x and y directions, respectively.

Figure 2(b) shows the output energies and pulse widths of the Ho:YAG ceramic lasers at a PRF of 200Hz. For the Ho:YAG-8 laser, we obtained an output energy of 9.6mJ and a pulse width of 83ns, compared with an output energy of 9.4mJ and a pulse width of 85ns using the Ho:YAG-10 laser. The energy extraction efficiency of the Ho:YAG-10 laser was 45.1%, higher than that of the Ho:YAG-8 (42.9%). At 100Hz, output energies of 10.2mJ and 9.7mJ were obtained using the Ho:YAG-8 and Ho:YAG-10 ceramic lasers, respectively. These results are comparable to those that we obtained in Ho:YAG crystal lasers⁹ and suggest that a lower doping concentration of holmium is more effective in the development of Ho:YAG ceramic lasers.

By altering the angle of the intracavity etalon, we were also able to investigate the wavelength tuning of the two ceramic lasers. We measured the wavelengths using an EXFO WA-650 spectrum analyzer combined with a WA-1000 wavemeter with a resolution of 10pm. The tuning ranges of the Ho:YAG-8 and Ho:YAG-10 lasers were found to be 2091.90–2098.19nm, with a maximum power of 4.51W at a wavelength of 2097.56nm and 1.19W at 2097.61nm, respectively. These results show once more that a lower doping concentration leads to higher output power and increased efficiency.

The beam quality of the Ho:YAG-8 laser output at 9mJ was determined by measuring the evolution of its beam radius along the propagation direction. The measured radii and 2D beam profile can be seen in Figure 2(c). By fitting the measured data with a hyperbolic curve, the beam quality (M²) was found to be very high (1.085 and 1.052 in the x and y directions, respectively).

In summary, the Q-switched Ho:YAG ceramic lasers provide an output energy comparable to their crystal counterparts. The development of these lasers may enable simpler manufacturing at a lower cost. In future work, we intend to use injection seeding technology to obtain a single-frequency Ho:YAG ceramic laser with high energy. Such a device could serve as a light source for coherent lidar.