Laser sources possessing sub-nanosecond pulse widths, such as mode-locked lasers, have garnered significant attention owing to their wide-ranging applications in fields such as medical cosmetology, lidar, archaeological bone cleaning, engine ignition, and pumping optical parametric oscillators (OPOs) for mid-infrared lasers. The generation of laser pulses possessing sub-nanosecond widths is generally achieved through passively Q-switched microchip lasers or actively mode-locked lasers. However, in the case of the latter, a repetition rate of approximately 100 MHz-level is attained, necessitating the use of an acousto-optic or electro-optic modulator, which renders the system complicated. On the other hand, the former method achieves high peak power but only offers a repetition rate at the kilohertz level. Thus, there exists a need to explore the use of passively mode-locked lasers possessing narrow laser spectrums, which can generate laser pulses possessing sub-nanosecond widths and high repetition rates.

The experimental setup is shown in Fig.1. The pump source is a fiber-coupled laser diode (LD) at 793 nm, with the maximum output power of 30 W, the numerical aperture of 0.22, and the core diameter of 105 μm. The pump spot diameter is approximately 168 μm in the center of the Tm∶GdVO₄ crystal, which is transformed by lens f1 (focal length f =50 mm) and f2 (f =80 mm). A Tm³⁺∶GdVO₄ crystal with size of 3 mm×3 mm×4 mm is used as the gain medium, and both surfaces of this crystal are antireflection (AR) for the pump and laser wavelengths. The crystal is enclosed with indium foil and fixed onto a copper heat sink, which is kept at a temperature of 18 ℃ using a water chiller. To achieve mode-locking, a semiconductor saturable absorption mirror (SESAM), three plane mirrors, and three curved mirrors are employed to construct a resonator with a cavity length of 2487 mm, corresponding to a repetition rate of 60.3 MHz. M1 and M2 are coated with high reflectivity (HR) films at 1.8?2.0 μm and high transmission films at 793 nm pump light, respectively. M3, M4, and M5 are coated with the HR films at the laser wavelength. The radii of curvature (ROC) of M2, M4, and M5 are 500, 200, and 500 mm, respectively. Three output couplers (OCs) (ROC is ∞) with transmission of 10%, 20%, and 30% at 1850?1890 nm are separately used in the experiment. The bandwidth of the high-speed detector and the high-speed digital oscilloscope is 12.5 GHz, which is enough to diagnose an ultrafast pulse with a pulse width of approximately 100 ps or beyond. An optical spectrum analyzer with a resolution of 0.05 nm is used to record the laser spectrum.

For the continues-wave (CW) operation, three different OCs with transmissions of 10%, 20% and 30% are used, and the maximum output power exceeds 1 W. The wavelengths of the CW lasers are 1844, 1850, 1851, 1861, and 1865 nm, respectively (Fig.3). For the continuous-wave mode-locking (CWML) operation, a commercial SESAM is used. A maximum average output power of 320 mW is achieved using the OC with 30% transmission (Fig.4). When the absorbed pump power is 5.3 W, the signal-noise ratio (SNR) of the fundamental frequency signal is approximately 59 dBm in the radio frequency (RF) spectrum of the pulse trains at the OC with 30% transmission( Fig.5). Stable outputs are achieved for all the three different OCs. The operating wavelengths of the mode-locked lasers are around 1851.6 nm and their full width at half-maximum (FWHM) values are always below the resolution of 0.05 nm (Fig.6). The pulse durations for the three different OCs are 474, 752, and 651 ps (Fig.7).

This study presents the demonstration of a sub-nanosecond pulse width mode-locked Tm∶GdVO₄ laser at 1.85 μm. To narrow the spectral widths of the laser pulses, we employ output couplers with high transmission. By using different OCs with transmission of 10%, 20%, and 30%, pulse durations of 474, 752, and 651 ps are obtained, corresponding to maximum pulse energies of 1.0, 3.5, and 5.3 nJ, respectively. The repetition rate of the laser pulses is approximately 60.3 MHz.