AI-algorithm-assisted 895-nm praseodymium laser emitting sub-100-fs pulses.

Praseodymium (Pr) lasers have achieved outstanding pico- and sub-picosecond pulsations covering the near-infrared (NIR) and visible spectral range in recent years. However, it has been a stagnant task for more than two decades to leapfrog into the sub-100 femtosecond (fs) regime as the Pr gain bandwidths are too narrow for their major transition lines. Although the wide tunability at the NIR bands in the Pr:YLF crystals has been explored, the spectral tails in these transitions suffer severely from weak gains for mode locking, combined with the intricate dispersion control to achieve transform-limit formation. In this work, we target the Pr:YLF's 895-nm line with a specially designed edge-pass filter to balance the gain bandwidth and transitional strength. By deploying a symmetric dispersion scheme and tuning with the soft actor-critic artificial intelligence (AI) algorithm, we have achieved the pulse duration down to sub-100-fs in a Pr laser for the first time. This work also enriches the AI-assisted methodology for ultrafast solid-state laser realizations.

High-power yellow DSR pulses generated from a mode-locked Dy:ZBLAN fiber laser.

Ultrafast yellow lasers are in high demand in recent biomedical and medical applications; however, direct emission of mode-locked pulses in yellow at the high-power level still presents a huge technical challenge to date. By integrating the nonlinear polarization rotation (NPR) scheme into a Dy:ZBLAN fiber laser, dissipative soliton resonance pulses at ∼575 nm are demonstrated for the first time, to the best of our knowledge. The average output power reaches ∼240 mW at maximum, which is an improvement of almost two orders of magnitude over those reported from the latest mode-locked visible fiber lasers. The laser scheme combines a piece of large-core Dy:ZBLAN gain fiber and free-space NPR components designated at the yellow bandwidth. The maximal pulse energy is 2.4 nJ at the repetition rate of ∼100 MHz and the minimal pulse duration is 83 ps. The achieved wavelength of 575 nm is the shortest ever reached from a fiber-based mode-locked laser to date.

Ultrafast true-green Ho:ZBLAN fiber laser inspired by the TD3 AI algorithm.

Ultrafast lasers in the true-green spectrum, which are scarce due to the "green gap" in semiconductor materials, are in high demand for the surging field of biomedical photonics. One ideal candidate for efficient green lasing is Ho:ZBLAN fiber, as ZBLAN-hosted fibers have already reached picosecond dissipative soliton resonance (DSR) in the yellow. When attempting to push the DSR mode locking further into the green, traditional manual cavity tuning is faced with extreme difficulty, as the emission regime for these fiber lasers is so deeply concealed. Breakthroughs in artificial intelligence (AI), however, provide the opportunity to fulfill the task in a fully automated manner. This work, inspired by the emerging twin delayed deep deterministic policy gradient (TD3) algorithm, represents the first application, to the best of our knowledge, of the TD3 AI algorithm to generate picosecond emissions at the unprecedented true-green wavelength of ∼545 nm. The study thus extends the ongoing AI technique further into the ultrafast photonics region.

Optical manipulation of Rayleigh particles by metalenses-a numerical study.

Based on the focusing feature of a metalens, we numerically studied its application in optical manipulation of Rayleigh particles. Three types of metalenses-point focusing, line focusing, and line focusing with phase gradient-were designed. Simulation results using the finite-difference time-domain method showed that the incident optical beams could be focused into a spot or a line for stable particle trapping. Through engineering a gradient phase in the direction of the focal line, the proposed metalens can push the particles along the line. This provides a unique capability to move particles along a line without the need of any mechanical movement. Given its thin sheet structure and compactness, the proposed metalens can be easily integrated into microfluidic and optical tweezers systems, and it can find potential applications in optical sorting of biological cells.

Determining topological charge based on an improved Fizeau interferometer.

We propose a new method to determine topological charge by using an improved Fizeau interferometer. This interferometer is very easy to realize, as well as interference fringes are very distinct. Phases of vortex, Hermite-Gaussian, and elliptical vortex beams are experimentally verified using this method. It provides a convenient way to determine the sign and magnitude of topological charge. This method may have some potential applications in space optical communication.

Diode-pumped 915-nm Pr:YLF laser passively mode-locked with a SESAM.

A diode-pumped, passively mode-locked laser emitting at 915 nm with a praseodymium-doped yttrium lithium fluoride (Pr:YLF) crystal was demonstrated for the first time, to the best of our knowledge. Utilizing two polarization-combined blue pumping laser diodes (LDs) and a semiconductor saturable absorber mirror (SESAM), stable continuous-wave (CW) mode-locking operations were achieved with a maximum average output power of 408 mW and a slope efficiency of 10.8%. Laser pulse durations of 15 ps were obtained with a spectral full width at half maximum (FWHM) of 0.15 nm and a repetition rate of 1.53 GHz.

High-power self-mode-locked Pr:YLF visible lasers.

We demonstrate efficient self-mode-locked green and red lasers with repetition rates of tens and hundreds of megahertz in a Pr:YLF crystal. Using double-end blue-diode-pumped geometry, more than 0.68 W average output power at 522 nm and more than 1.44 W at 639 nm are obtained, which are believed to be the highest average output powers for mode-locked praseodymium lasers operating in the visible wavelength region.

Diode-pumped continuous-wave dual-wavelength c-cut Pr³âº:LiYF4 laser at 696 and 719 nm.

A continuous-wave, InGaN-LD-pumped dual-wavelength laser is demonstrated with simultaneous emission at 696 (3P03F(→)3) and 719 nm (3P03F(→)4) using a c-cut Pr(3+):LiYF4, for the first time to our knowledge. Maximum output power of 102 mW at these two wavelengths is achieved with slope efficiency of about 15.6% with respect to the absorbed pump power. The beam propagation factors in x and y directions are measured to be 1.50 and 1.32, respectively.

Comparative study on diode-pumped continuous wave laser at 607 nm using differently doped Pr(3+):LiYF(4) crystals and wavelength tuning to 604 nm.

We comparatively study an InGaN laser-diode-pumped continuous-wave laser at ∼607 nm (σ polarization) using differently doped Pr:LiYF4 single crystals. Maximum output power and slope efficiency at this wavelength were up to 209 mW and 47.1%, respectively, using a 0.2 at. % doped and 8 mm sample. Findlay-Clay analysis shows roundtrip losses, including reabsorption loss at this particular emission of about 1.2% using the 0.2 at. % doped sample, which is lower than that of samples with higher doping concentrations at 0.5 and 1 at. %. Using a 0.15 mm glass plate as a Fabry-Perot etalon, a maximum output power of 73 mW was achieved at ∼604 nm (π polarization) with slope efficiency of 17.2% for what is believed to be the highest result currently.