Encai Ji

4.0 μm, high repetition rate periodically poled magnesium-oxide-doped lithium niobate mid-infrared optical parametric oscillator pumped by steep leading edge pulsed fiber laser

A high repetition rate optical parametric oscillator (OPO) generating an idler laser with a wavelength as long as 4.0 μm at 200 and 400 kHz was demonstrated in this paper. The OPO was pumped by a master oscillator power amplifier structure fiber laser with excellent characteristics. The pump pulse from the fiber laser had a steep leading edge, which was theoretically proved to improve the OPOs performance, compared with the Gaussian pump pulse. A homemade periodically poled magnesium-oxide-doped lithium niobate crystal with a grating period of 29 μm was employed in our experiment. By optimizing the resonator, 2.75 and 1.67 W idler lasers were finally achieved at repetition rates as high as 200 and 400 kHz, respectively, with a wavelength as long as 4.0 μm. The conversion efficiencies were 12.03% and 7.31%, respectively.

High-slope-efficiency 2.06 μm Ho: YLF laser in-band pumped by a fiber-coupled broadband diode

We first demonstrate the laser performance of a compact 2.06 μm Ho: YLF laser resonantly pumped by a broadband fiber-coupled diode. In continuous-wave (CW) operation, maximum output power of 1.63 W, corresponding to a slope efficiency of 89.2%, was obtained with a near diffraction-limited beam quality. In actively Q-switched operation, maximum pulse energy of 1.1 mJ was achieved at the repetition frequency of 100 Hz. The minimum pulse duration was 43 ns. The performance in both the CW and Q-switched regimes indicates that the current fiber-coupled diode in-band pumped Ho: YLF laser has great potential in certain conditions that require several watts of output power or several millijoules of short pulse energy.

End-pumped temperature-dependent passively Q-switched lasers.

In this paper, we compare pulse parameters in two end-pumped passively Q-switched lasers by increasing the boundary temperature of the laser crystal. For the Nd:YAG/Cr:YAG laser, the pulse energy at 1 kHz increased from 12.4 to 21 μJ when the boundary temperature increased from 20°C to 120°C, while the pulse width decreased by 1.2 times. For the Nd:YVO4 passively Q-switched laser using Cr:YAG as the saturable absorber, the pulse energy at 100 Hz increased from 5.45 to 24.5 μJ when the boundary temperature increased from 26°C to 113°C, while the pulse width decreased by 2.4 times. An improved model based on the rate equations was introduced to analyze the large variation of pulse energy when heating the Nd:YVO4 crystal.

Design of High-Gain Single-Stage and Single-Pass Nd:YVO 4 Amplifier Pumped by Fiber-Coupled Laser Diodes: Simulation and Experiment

In this paper, factors influencing the performance of high-gain Nd:YVO4 amplifiers are analyzed and a schematic map of the relationships between different factors is given. A model, including the pump absorption saturation effect and overlap efficiency along the beam propagation direction, as well as non-radiative decays due to high doping concentration, is developed. The performance of laser amplifier is studied by changing doping concentration, pump beam quality, and focal size of pump laser. To get high gain, samples with low doping concentration should cooperate with long crystals and laser diodes with good beam quality. Based on the model, the focal radius of pump laser could be optimized for different doping concentrations and beam qualities of pump laser. Experiments were conducted and the results agreed well with the simulated ones. Moreover, a single-stage and single-pass amplifier was realized, achieving a small-signal gain of 51 dB.

Resonantly Fiber-Coupled Diode-Pumped Ho 3+ : YLiF 4 Laser in Continuous-Wave and Q-Switched Operation

In this paper, the theoretical quantitative energy-level character and the experimental laser performance of resonantly fiber-coupled diode-pumped Ho3+: YLiF4 (Ho: YLF) laser are first presented. More precise 2.7-level character of diode-pumped Ho: YLF laser can be acquired with our theoretical analysis. With the experimental study, a maximum slope efficiency of 88.5%, corresponding to an output power of 1.85 W, was obtained. In actively Q-switched operation, a maximum pulse energy of 0.3 mJ was achieved at the repetition frequency of 5 kHz with a minimum pulse duration of 160 ns. At a maximum repetition frequency of 100 kHz, a pulse energy of 16 μJ was achieved with a pulse duration of 816 ns. Both the performance in continuous-wave and Q-switched regimes indicates that the current resonantly fiber-coupled diode-pumped Ho: YLF laser has great potential in certain conditions that require several watts of average output power.

Gain change by adjusting the pumping wavelength in an end-pumped Nd:YVO 4 amplifier.

In this paper, the performance of an end-pumped laser amplifier was experimentally studied by adjusting the pumping wavelength of a laser diode. An interesting phenomenon was observed: that the gain would decrease with an increase in absorbed pump power. The scaled output power of 0.3 at. % doped 20 mm long Nd:YVO4 crystal decreased from 0.48 to 0.2 W when the absorbed pump power increased from 17.5 to 17.8 W. Theoretical analysis was demonstrated according to our previous model. The long crystal length is the main reason for the observed phenomenon. This phenomenon could be utilized to enhance amplifier gain and improve thermal performance, including the maximal temperature and thermal stress of the crystal.

Theoretical and experimental analysis of high-power frequency-stabilized semiconductor master oscillator power-amplifier system.

We present a compact high-power 780 nm frequency-stabilized diode laser with a power of as high as 2.825 W, corresponding to an estimated overall efficiency of 38.5%. The tapered amplifier (TPA) gain was about 24.5 dB, which was basically consistent with the simulation results. The beam quality factor was M2<1.72. The core feature of the system was stabilizing the frequency of the narrowband semiconductor TPA system with the matured saturated absorption spectrum technique. The laser frequency was stabilized against mode hops for a period of >4200  s with a frequency fluctuation around 6.7×10-10 within 1 s of the observation period, and the linewidth was no more than 0.95 MHz. The laser performance indicates that the current frequency-stabilized semiconductor laser has great potential in certain conditions that require several watts of output power.

880-nm-Diode-Laser-End-Pumped Electrooptically Q-Switched Nd:YLF Laser With High Energy and Good Beam Quality at 1047 nm and 1 kHz

An electrooptically Q-switched Nd:YLF laser end-pumped by an 880-nm laser diode with high energy and short pulsewidth at 1 kHz is demonstrated. The asymmetry of the focal lengths of thermal lens along the a-axis and c-axis is observed, and the cylindrical lens is used to compensate for the thermal lens effect along the a-axis. We experimentally studied the performance of the Nd:YLF laser with different combinations of the output coupler transmission T and the focal length f of cylindrical lens. With T = 40% and f = 300 mm, 9.5-mJ pulse energy, 9.5-ns pulsewidth, and beam quality factors of M2x = 1.74 andM2y = 1.97 are obtained. Moreover, the relationships between the pulse width and pump power as well as beam transverse mode were experimentally studied and discussed.

Transverse mode propagation studies in a thulium-doped fiber laser

In this paper, we apply a transverse mode propagation model of thulium-doped fiber laser based on propagation equations. Based on this model the behaviors of transverse mode propagation are demonstrated and individual transverse mode power distributions of output are simulated numerically under various conditions including bending radius, fiber length, and pump power. Furthermore, the output power and beam quality characteristic for the thulium-doped fiber laser are investigated experimentally, and the experimental results agree well with the numerical results.