The eye-safe nature of 2 μm lasers endow them with great market potential, especially in free space applications, such as light detection and ranging (LIDAR), remote chemical sensing, and direct optical communication. Accordingly, there is an increasing research interest in 2 μm lasers operating in both continuous-wave (CW) and pulsed regimes. Fiber lasers and bulk lasers are the most commonly employed Tm-laser systems, while neither of them is compatible with integrated photonics, which hinders the development of Tm lasers in practical applications. A compact and robust solution to the minimization of solid-state lasers is a waveguide laser, in which a well-structured optical waveguide is utilized as the gain medium and the key component of the laser cavity. Due to their compact geometries, waveguide structures offer strong confinement of light propagation over a relatively long interaction length, effectively removing the beam divergence and in turn enhancing the optical gain.

As one of the most attractive gain media for 2 μm solid-state lasers, Tm∶YAP has relatively higher absorption and emission cross sections and a relatively broad absorption band. However, Tm∶YAP waveguide lasers operating either in CW or pulsed regimes have not been demonstrated till now. Waveguide lasers operating in pulsed regimes, typically by Q-switching or mode locking techniques, can be realized by both active and passive methods. In contrast to active methods that require external signals for optical modulation, passive methods typically rely on the saturable absorber (SA) elements placed inside the laser cavity for light self-modulation, allowing for robust laser designs with compact packages. We aim to fabricate high-quality Tm∶YAP waveguides and experimentally demonstrate 2 μm waveguide lasers operating at pulsed regimes.

We adopt the femtosecond laser direct writing (FsLDW) technique for fabricating cladding waveguides in a Tm:YAP crystal wafer with doping concentration of 3% (atomic number fraction) Tm³⁺ ions and dimensions of 12 mm (a)×10 mm (b) × 2 mm (c), and the surface of the crystal wafer is polished to the optical-grade quality. The crystal wafer is placed on a PC-controlled XYZ micro-position stage for precise translation with a constant velocity of 0.5 mm/s. The 800 nm laser is delivered by a femtosecond laser system with a pulse width of 38 fs and repetition rate of 1 kHz, and focused by a microscope objective lens beneath the largest crystal surface. A pulsed energy of 0.13 μJ is set for producing laser-damage tracks and avoiding crystal cracking. For each scanning, a damage track induced by an fs-laser with a vertical length of 10 μm is produced, and a cladding waveguide with a diameter of around 50 μm in the crystal is obtained after multiple scannings. The main intention of choosing this FsLDW parameter combination is to realize low-loss waveguides with optimized guiding and lasing performances. The employed NbSe₂ thin film SA element is prepared by chemical vapor deposition (CVD) method.

To study the FsLDW-induced crystalline lattice changes and the preservation of fluorescence properties within waveguide volume, we conduct micro-photoluminescence (μ-PL) analysis by employing a fiber confocal microscope at room temperature. In the experiment, a CW 488 nm laser source for luminescence excitation is focused through the cladding waveguide cross-section with a depth of 10 μm by a microscope objective, and the emitted signal is detected via a spectrometer. The μ-PL intensity collected from the waveguide and the bulk material has slight differences, which results in very slight fluorescence quenching in the guiding area due to the lattice damage caused by fs-laser pulses. The nearly identical intensity indicates sound preservation of the original luminescence properties of Tm∶YAP crystal in the guiding area. There is a noticeable intensity reduction of μ-PL emission in the filament region compared with the bulk region, which indicates the partial lattice distortion and damage in the laser-modified region.

By inserting the NbSe₂ thin film as an SA element between the end face of the waveguide and the laser cavity mirror, we successfully obtain a waveguide pulsed laser with a repetition rate of 7.8 GHz under optical pumping at 799.3 nm. The lasing threshold of the prepared Tm∶YAP waveguide is about 45 mW (43 mW) under the optical pumping with TE (TM) polarization. Correspondingly, the maximum output power is 65 mW (34 mW) and the slope efficiency is 11.86% (6.02%). The superior lasing performance under TE polarization is mainly due to the higher lasing gain of the bulk material along this crystal direction. In the experiments, by adjusting the polarization of the pump light, the wavelength of the output waveguide pulse laser can be adjusted, and dual-wavelength laser output of 1855.87 nm and 1892.54 nm can be obtained. Compared with Tm∶YAP continuous waveguide laser, the waveguide pulsed laser modulated by NbSe₂ thin film SA element has a higher threshold, lower laser slope efficiency, and maximum output power. This is mainly because the insertion of the NbSe₂ thin film introduces a certain optical absorption loss, reducing the optical gain of the waveguide laser cavity to a certain extent. In terms of output waveguide laser mode, laser wavelength, and polarization dependence characteristics, the waveguide laser in pulsed mode is similar to that in the continuous wave mode. This shows that the insertion of NbSe₂ thin films exerts little influence on the waveguide wave characteristics of the optical waveguide. The experimentally determined mode-locked pulse width is about 62 ps and the repetition frequency of the outgoing laser is 7.8 GHz. The obtained Tm∶YAP waveguide pulsed laser has the narrowest pulse till now.

The demonstration of a 1.9 μm Q-switched mode-locked Tm∶YAP cladding waveguide laser fabricated by FsLDW is reported. Modulated by metallic NbSe₂ thin films as an SA element, the fabricated waveguide laser delivers laser pulses has a pulse duration of as short as 62 ps at a fundamental repetition rate of up to 7.8 GHz. This is up-to-date with the shortest laser pulses that are achieved from Tm∶YAP waveguides. By adjusting the polarizations of the optical pumping, a dual-wavelength laser operating at 1855.87/1892.54 nm is obtained. The results indicate promising applications of metallic NbSe₂ thin films for modulating mid-infrared ultra-fast pulsed lasers and compact Tm∶YAP waveguide lasers for multi-functional integrated photonics.