Spontaneous chaos and extreme events in a solid-state laser with the transverse-mode-degenerate cavity configuration.

In this Letter, we theoretically analyze cavity beam propagation in a gain medium and cavity using the rate equation and generalized Huygens integral, respectively. Spontaneous chaos and extreme events (EEs) occurred around the transverse-mode-degenerate cavity configuration because of transverse-mode competition. A closed occurrence region relating to varying pump power and cavity length was observed. The experimental results in a continuous-wave Nd:YVO4 laser agreed with the aforementioned numerical results. Because gain effect is essential to a laser, we assume that EEs can be intrinsic and universal in a well-aligned laser system if it satisfies the specific transverse-mode competition.

Selection rule for cavity configurations to generate cylindrical vector beams with low beam quality factor.

Considering the birefringence of the gain medium, we propose a selection rule for designing cavity configurations that enable the generation of radially and azimuthally polarized laser beams with a low beam quality factor, i.e., high beam quality. Using this rule, all stable regions can support cylindrical vector (CV) beams by using the position of the end mirror as the tuning parameter to vary the cavity configuration. Such cavity configurations tend to sustain the fundamental or lowest-order CV beam, and radially or azimuthally polarized beams can be obtained simply by varying the tuning parameter. Based on experimental measurements of the beam quality factor and polarization characteristics, we verified our analyses and simulations for a four-element laser system.

Chaos and extreme events in an azimuthally polarized Nd:GdVO4 laser with pump modulation.

Nonlinear dynamics and extreme events were explored in a spatially distributed polarization laser based on an azimuthally polarized Nd:GdVO4 laser with pump modulation. When the modulation frequency approached the relaxation oscillation frequency, a period-doubling route to chaos was observed with an increasing modulation depth, and spatiotemporal extreme events occurred with the generation of the chaos. An efficient azimuthal polarization was kept, even though the intensity fluctuated in the chaotic region.

Simultaneous dual-wavelength pulses achieved by mixing spiking and passive Q-switching in a pulsed Nd:GdVO4 laser with a Cr4+:YAG saturable absorber.

A method for generating a dual-wavelength, dual pulse Nd:GdVO4 laser at 1.06 µm (F3/24→I11/24) and 1.34 µm (F3/24→I13/24) is proposed. When the 1.34-µm spiking threshold is less than the 1.06-µm Q-switched threshold, the generation of a 1.34-µm spiking pulse leads to a 1.06 µm Q-switched pulse, resulting in a dual-wavelength laser. With a pump power of 11 W, the pulse widths are 89.5 and 427.8 ns for 1.06 and 1.34 µm, respectively, with a 36.5-KHz repetition frequency.

Periodic and chaotic dynamics in a passively Q-switched Nd:GdVO4 laser with azimuthal polarization.

A period-doubling route to chaos was observed in a passively Q-switched Nd:GdVO4 laser beam with azimuthal polarization. At a period of two, the peak intensities gradually varied with the azimuthal angle. When the pump power was increased, the chaotic behavior was spatiotemporal, but the polarization was still azimuthal. To the best of our knowledge, this is the first report of spatiotemporal chaos under a stationary spatial distribution of the polarization.

Polarization bistability associated with 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions in Nd:YVO4 laser with intra-cavity periodically poled lithium niobate Bragg modulator.

We experimentally demonstrate polarization bistability in a dual-wavelength Nd:YVO4 laser at 1064 nm (4F3/2→4I11/2) and 1342 nm (4F3/2→4I13/2) by using an intra-cavity electro-optic periodically poled lithium niobate (PPLN) Bragg modulator to control the loss at 1064 nm. An inverse hysteresis switch was observed between 1064 nm and 1342 nm lasers with orthogonal polarizations by increasing and reducing the loss induced by the PPLN. The size of the hysteresis increased with increasing pump power. This paper provides an explanation based on cross-gain saturation for this bistable behavior of polarization.

Role of modulation frequency in a hybrid Q-switched Nd:LuVO4 laser with an acousto-optic modulator and a Cr4⁺:YAG saturable absorber.

This study presents a hybrid Q-switched Nd:LuVO4 laser with an acousto-optic modulator (AOM) and a Cr4⁺:YAG saturable absorber. The importance of the modulation frequency of the AOM in this dual-loss modulation system is examined with two distinct observations: (1) the periodic-ramp pulse characteristics with the modulation frequency, and (2) the existence of the intrinsic modulation frequency for minimizing the pulsewidth. The shortest pulsewidth and the highest peak power were 10.6 ns and 13.9 kW, respectively, at a pump power of 10 W. Compared with the passively Q-switched laser, which has a pulsewidth of 18.3 ns, the ratio of pulse compression reaches 42%. Finally, numerical simulations based on the rate equations are used to verify the experimental results.

Generation of a radially polarized laser beam in a single microchip Nd:YVO4 laser.

A radially polarized beam was generated in a microchip Nd:YVO4 laser by shaping the pump profile to be a group of focused plane waves, the directions of which are located on a conical surface. The oblique incident pump beam induces thermal lensing combined with birefringence to distinguish the radial ray from the azimuthal ray to achieve radial polarization. The average contrast ratio was 23.6 when the pump power was between 3.06 and 4.27 W.

Generation of azimuthally and radially polarized off-axis beams with an intracavity large-apex-angle axicon.

Depending on cavity configuration, a c-cut Nd:YVO4 laser by using a large-apex-angle axicon can execute azimuthally and radially polarized operations. A large-apex-angle axicon dominates and stabilizes the generation of the pattern, and expands the difference between the ordinary and extraordinary rays to generate off-axis cylindrical vector beams. When the cavity length is properly adjusted, the polarization of off-axis laser beams can exhibit a transition from azimuthal to radial polarization. The degree of polarizations can be up to 95.4% ± 2.6% and 94% ± 3.7% for azimuthally and radially polarized beams, respectively; and the slope efficiencies are approximately 20.5% for both polarized operations. Using two-pass-mode ray tracing, the ray generating mechanisms and divergent angles of their patterns were analyzed.

Chaos suppression in a Nd:YVO4 laser by biharmonical pump modulation.

This work demonstrates the suppression of chaos in a Nd:YVO(4) laser by biharmonical pump modulation (the first for chaos-inducing and the second for chaos-suppressing). The laser exhibits chaotic behavior when only the first signal is applied for pump modulation and its frequency is adjusted close to the relaxation-oscillation frequency. Adding the second signal with subhamonic and a specific phase difference to the first modulation signal will reshape the modulated waveform of the pump beam to suppress the aforementioned chaotic behavior. The initial phase of the second harmonic perturbation plays an important role in the suppression of chaos. This result is confirmed by numerical simulation.

Stable Q-switched mode-locked Nd3+:LuVO4 laser by Cr4+:YAG crystal.

In use of saturable Cr(4+):YAG crystals, we have demonstrated the stable Q-switched and mode-locked (QML) operation of the Nd:LuVO(4) laser. The operation range of QML in use of the Cr(4+):YAG is larger than that use of SESAM and NLM. The obtained shape of Q-switched envelope, repetition rate and pulse energy are demonstrated to depend on the initial transmittance of the Cr(4+):YAG and reflectivity of the output coupler. Using R = 60% and T(0) = 40%, the highest pulse energy of 77 microJ of each Q-switched envelope, and the highest peak power about 200 kW of Q-switched mode-locked pulses can be obtained at 15 W pump power. It demonstrate the superior characteristic of the Nd:LuVO(4) crystal.

Generation of supercontinuum bottle beam using an axicon.

A supercontinuum bottle beam was successfully produced by focusing a supercontinuum laser beam after passing through an axicon. The supercontinuum radiation was generated from a microstructured fiber pumped by a self-kerr-lens mode-locked femtosecond Ti:sapphire laser. The cross-section intensity distributions of the generated bottle were recorded by a beam profiler. Using the line filters to select different central wavelengths, the different colors of bottle beams show slightly different bottle ranges and diameters due to the dispersion of axicon and focusing lens. The results consist with the theoretical prediction using the Fresnel-Kirchhoff's formula in considering an incident Gaussian beam.

Spatial and temporal instabilities in a passively Q-switched Nd:YAG laser with a Cr(4+):YAG saturable absorber.

A period-doubling route to chaos with nonstationary transverse pattern is observed in a diode-pumped Nd:YAG laser with a Cr(4+):YAG saturable absorber. The nonlinear behavior results from the multitransversemode competition with gradually adding the number of the transverse modes. By analyzing the transverse pattern, we find that the bifurcation accompanies with the increase of the higher-order family of transverse modes in sequence. Moreover, a series of period-doubling route to chaos beginning from the low period is attained by varying the pump power at specific cavity configurations.