Yongguang Zhao

Stable passively harmonic mode-locking dissipative pulses in 2µm solid-state laser

We report on stable passively harmonic mode-locking dissipative pulses with high repetition rate and narrow bandwidth in 2µm Tm: CaYAlO4 laser. At the large intracavity intensity, the laser generated 1st-order to 5th-order passively harmonic solitons with fundamental repetition rate of ~198 MHz and 5th-order repetition rate up to 0.98 GHz, which was mainly caused by the peak power clamp effect. The solitons yielded a tunable central wavelength from 1940nm to 1950 nm, and a narrow optical spectrum bandwidth of 60 pm without any active optical filter. At low intracavity intensity, the laser operated on the typical SESAM-guided mode-locking mode, with the scaled output average power up to 1.15 W. To our knowledge, this is the first observation of passively harmonic mode locking in 2µm solid laser system, and the first Watt-level output average power in Tm: CYA mode locking laser.

~1 mJ pulsed vortex laser at 1645 nm with well-defined helicity.

We report on a millijoule level Er:YAG ceramic pulsed vortex laser resonantly pumped by an annular beam at 1532 nm. By means of an uncoated YAG crystal plate inserting into the cavity and a proper thermal gradient on the ceramic, 1.03 mJ LG0,-1 mode and 0.97 mJ LG0, + 1 mode were respectively produced, and no vortex mode instability problem was observed. We believe the directly generated millijoule level optical vortex with well-defined helicity will have potential applications in material modification and processing, high-field laser physics, and nonlinear optics, etc.

Novel Raman Fiber Lasers Emitting in the U-Band With Combined Volume Bragg Gratings

Narrow linewidth and widely tunable high-power Raman fiber lasers emitting in the U-band wavelength were demonstrated based on combinations of volume Bragg gratings with different center wavelengths. The narrowed linewidth was 65 pm at a center wavelength of 1657.86 nm with a maximum output power of 8.3 W, corresponding to a slope efficiency of 55.6%. For the tunable Raman fiber laser, continuously tunable dual-wavelength emission was obtained in a wide band (1621.3-1683.5 nm) with a wavelength difference over 60 nm (0.7-62.2 nm). The maximum output power was 11 W, corresponding to a slope efficiency of 76.6%.

Highly efficient continuous-wave and Q-switched Tm:CaGdAlO 4 laser at 2 µm

We report on a highly efficient continuous-wave (CW) and Q-switched Tm:CaGdAlO4 (Tm:CGA) laser at ~1.96 µm in-band pumped by a high power Raman fiber laser at ~1.7 µm. Over 7.2 W of CW output power with a polarization extinction ratio (PER) of 22 dB has been achieved, corresponding to a slope efficiency of up to 60%. For Q-switching, the shortest pulse duration of 75 ns was obtained at 10 kHz pulse repetition frequency (PRF) and over 5.2 W of average output power has been generated at 50 kHz PRF. The prospects for further improvement in continuous-wave and Q-switched performance are discussed.

Wavelength- and OAM-tunable vortex laser with a reflective volume Bragg grating

Vortex beams carrying orbital angular momentum (OAM) have been recently investigated intensely in optical communication systems, as using OAM mode multiplexing simultaneously with other conventional multiplexing techniques is the key to further expand data capacity. This article demonstrates a wavelength- and OAM-tunable vortex laser at 1.6 µm in an Er:YAG system. For the first time to the best of our knowledge, a reflective volume Bragg grating (VBG) was theoretically and experimentally proved to be an effective OAM-preserving wavelength selector inside the laser cavity. A z-shaped laser cavity employing a VBG as a folding mirror was constructed for the direct generation of vortex beams, and we finally obtained wavelength-tunable beams of five OAM states (0, ± ħ, and ± 2ħ) with a narrow bandwidth less than 0.04 nm. This laser supplies a new way for optical communication by combining the spatial degree of freedom for multiplexing information channels with the conventionally used wavelength domains in packable and robust resonant cavity.

Integration of helicity-control and pulse-modulation for vortex laser based on a black phosphorus plate.

Using a home-made black phosphorus plate (BPP) as handedness controller and Q-switch modulator synchronously, a ~1.6 µm pulsed vortex laser with well-determined handedness is demonstrated in this letter. Stable vortex pulses of LG0, + 1, LG0,-1, LG0, + 2 and LG0,-2 modes were respectively achieved from compact resonant cavities in this experiment. Such pulsed vortex laser should have promising applications in various fields based on its simple structure, controllable handedness, and carried orbital angular momentum.

High repetition rate gain-switched Er:YAG ceramic laser at 1645 nm

We report on a gain-switched Er:YAG ceramic laser resonantly pumped by an acousto-optically modulated Er, Yb co-doped fiber laser at 1532 nm. The laser produced stable pulse trains at 1645 nm with pulse repetition rate tunable between 13–100 kHz and corresponding pulse width of 480–450 ns under a pump power-level of 8.7 W. At the 100 kHz of repetition rate, over 2.2 W of average output power was generated for 6.5 W of incident pump power, corresponding to a slope efficiency of 40% with respect to incident pump power.

Control of Vortex Helicity With a Quarter-Wave Plate in an Er:YAG Ceramic Solid State Laser

In this paper, we research the characteristics of the Laguerre-Gaussian (LG01) beams generated in an Er:YAG ceramic solid state laser pumped by an annular beam. The generated two LG01 beams (LG0,+1 and LG0,-1 modes) were found to have different thermally induced polarization behaviors and alternately existed in the resonator; then, a novel method was proposed and experimentally demonstrated to control the vortex helicity using a simple quarter-wave plate.

High-average-power operation of a pulsed Raman fiber amplifier at 1686 nm

We report on high-average-power operation of a pulsed Raman fiber amplifier at ~1686 nm which cannot be covered by rare-earth-doped fiber lasers. The Raman fiber amplifier was pumped by a home-made 1565.2 nm Q-switched Er,Yb fiber laser and worked at a repetition frequency of 184 kHz. With 0.8 km Raman fiber, 4.4 W of average output power at the 1st order Stokes wavelength of 1686.5 nm was obtained for launched pump power of 16.2 W, corresponding to an optical-to-optical conversion efficiency of 27.2%. Further increasing the pump power, high-order Stokes waves grew gradually, resulting in a total output power of 6.7 W at the 19.2 W launched pump power.