飞秒激光直写Tm∶YAP波导脉冲激光器

Tm∶YAP Waveguide Pulsed Lasers of Femtosecond Laser Direct Writing

利用飞秒激光直写技术制备Tm∶YAP光波导并实现1.9μm波导调Q锁模激光输出。结合具有金属特性的NbSe2薄膜作为可饱和吸收元件进行光学调制,波导激光输出的激光脉冲重复频率为7.8 GHz,最短脉宽为62 ps。这也是已报道的从Tm∶YAP波导中获得的最短激光脉宽。通过调整泵浦光的偏振,可以获得1855.87/1892.54 nm的双波长激光输出。结果表明,具有金属特性的NbSe2薄膜在调制中红外超快脉冲激光器方面具有较大的应用价值。此外,双波长输出的紧凑型Tm∶YAP波导脉冲激光器在多功能集成光子学研究方面具有较好的应用前景。

Objective The eye-safe nature of 2μm 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 2μm 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 2μm 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 Qswitching 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 2μm waveguide lasers operating at pulsed regimes.Methods 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 0.13μJ 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 10μm is produced,and a cladding waveguide with a diameter of around 50μm 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 NbSe_(2)thin film SA element is prepared by chemical vapor deposition(CVD)method.Results and Discussions 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 10μm by a microscope objective,and the emitted signal is detected via a spectrometer.Theμ-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 ofμ-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 NbSe_(2)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 NbSe_(2)thin film SA element has a higher threshold,lower laser slope efficiency,and maximum output power.This is mainly because the insertion of the NbSe_(2)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 NbSe_(2)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.Conclusions The demonstration of a 1.9μm Q-switched mode-locked Tm∶YAP cladding waveguide laser fabricated by FsLDW is reported.Modulated by metallic NbSe_(2)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 NbSe_(2)thin films for modulating mid-infrared ultra-fast pulsed lasers and compact Tm∶YAP waveguide lasers for multi-functional integrated photonics.