Jianlang Li

Generation of radially polarized mode in Yb fiber laser by using a dual conical prism

For the first time to our knowledge, a radially polarized beam is generated in an Yb-doped multimode double-clad fiber laser by using an intracavity dual conical prism. Up to 6.2 mW of output power is obtained from a 2 m long gain fiber with 7.4% slope efficiency. This research opens a new window to obtaining a radially polarized beam directly from an active fiber.

Radially polarized ring and arc beams of a neodymium laser with an intra-cavity axicon

Placing a Brewster-angle axicon inside a laser resonator makes it possible to produce a radially-polarized (RP) oscillation pattern distributed on a thin ring or a portion of a ring. Laser-diode end-pumped, Nd:Y(3)Al(5)O(12) and Nd:YVO(4) lasers were studied. Spatially coherent RP beams distributed on circular arcs were obtained with a polarization contrast ratio up to 80:1. Incoherent RP outputs on a full ring were also produced with a polarization contrast ratio of about 5:1. Applications of these beams to increase absorption efficiency in laser-matter interaction are discussed.

Efficient, high-power, and radially polarized fiber laser

We demonstrate an ytterbium-doped fiber laser that emits high-power radially polarized light efficiently. In this study, a photonic crystal grating (PCG) was used as a polarization-selective output coupler, and the power of the radially polarized laser reached 2.42 W with a slope efficiency of 45.9% and a polarization purity of 96%. The results reveal that the inclusion of the PCG mirror into the fiber laser are particularly promising for generating high-power radially polarized light efficiently in view of its many important applications.

Efficient excitations of radially and azimuthally polarized Nd 3+ :YAG ceramic microchip laser by use of subwavelength multilayer concentric gratings composed of Nb 2 O 5 /SiO 2

Cylindrical vector beams were produced from laser diode end-pumped Nd:YAG ceramic microchip laser by use of two types of subwavelength multilayer gratings as the axisymmetric-polarization output couplers respectively. The grating mirrors are composed of high- and low-refractive- index (Nb(2)O(5)/SiO(2)) layers alternately while each layer is shaped into triangle and concentric corrugations. For radially polarized laser output, the beam power reached 610mW with a polarization extinction ratio (PER) of 61:1 and a slope efficiency of 68.2%; for azimuthally polarized laser output, the beam power reached 626mW with a PER of 58:1 and a slope efficiency of 47.6%. In both cases, the laser beams had near-diffraction limited quality. Small differences of beam power, PER and slope efficiency between radially and azimuthally polarized laser outputs were not critical, and could be minimized by further optimized adjustment to laser cavity and the reflectances of respective grating mirrors. The results manifested, by use of the photonic crystal gratings mirrors and end-pumped microchip laser configuration, CVBs can be generated efficiently with high modal symmetry and polarization purity.

Radially polarized and passively Q-switched fiber laser

We report, for the first time to our knowledge, a radially polarized and passively Q-switched Yb-doped fiber laser. By using a Cr(4+):YAG crystal as a saturable absorber and a photonic crystal grating as a polarization mirror, a radially polarized pulse is produced, which has 202 W of peak power, 75 ns duration, and ~92% polarization purity at a 56.6 kHz repetition rate. The Q-switched pulse with radial polarization from the fiber laser would facilitate numerous applications.

Converging-axicon-based radially polarized ytterbium fiber laser and evidence on the mode profile inside the gain fiber

The radially polarized mode is achieved from an active Yb fiber by utilizing of an intracavity converging axicon, where the axicon acts as a TM(01) mode selector based on not only its Brewster convex surface but also the distance between its vertex and plane output coupler. The polarization state of the annular laser beam is checked by using a home-made eight-hole aperture. Furthermore, an uncoated plane glass plate is inserted into the cavity, and the reflected beam points to the existence of an annular lasing mode inside the gain fiber. The issues for developing high-power radially polarized fiber lasers also are considered.

Radially polarized and pulsed output from passively Q-switched Nd:YAG ceramic microchip laser

For the first time, to the best of our knowledge, a radially polarized laser pulse was produced from a passively Q-switched Nd:YAG ceramic microchip laser with a piece of Cr4+:YAG crystal as the saturable absorber and multilayer concentric subwavelength grating as the polarization-selective output coupler. The averaged laser power reached 450 mW with a slope efficiency of 30.2%. The laser pulse had a maximum peak power of 759 W, a minimum pulse duration of 86 ns, and a 6.7 kHz repetition rate at 3.7 W absorbed pump power. The polarization degree of the radially polarized pulse was measured to be as high as 97.4%. Such a radially polarized laser pulse with a high peak power and a short width is important to numerous applications such as metal cutting.

Residual pump light as a probe of self-pulsing instability in an ytterbium-doped fiber laser

Self-pulsing instability in the ytterbium-doped fiber (YDF) laser is investigated with the pump-bypassed cavity configuration. The residual pump light acts as a probe of the intracavity dynamics, and the temporal behavior of the light shows correlations with the self-mode locking instability in the original cavity and the sustained self-pulsing instability in the modified cavity of the YDF laser. The results suggest that the interactions among stimulated emission, pump absorption and/or reabsorption could account for self-pulsing instability in the YDF laser. The pump-bypassed laser cavity configuration can be used to investigate the instabilities of various kinds of fiber lasers.

Radially Polarized and Passively Q-Switched Nd:YAG Laser Under Annular-Shaped Pumping

We reported, for the first time, a radially polarized and passively Q-switched Nd:YAG laser with annular-shaped pumping. The annular intensity profile of the pump light was originated from the mode conversion in a conventional multi-mode fiber under off-focus coupling. Radially polarized pulse from this laser had 3.3 kW peak power, 39 ns duration at 2.5-kHz repetition rate with ~90% degree of polarization. When the laser crystal was slightly tilted, the polarization state of laser beam was switched to azimuthal polarization at high pump power, while remained radial polarization at lower pump power.

Self-pulsing laser as oscillator Toda: approximations through elementary functions

A simple model of self-pulsation in lasers is considered. The laser is described by the system of two ordinary differential equations for the number of photons in the cavity and the number of excitations in the active medium, leading to the equation for the oscillator Toda with damping. For the case of strong spiking, the damping is considered as perturbation; the estimates in terms of elementary functions are suggested for the period of pulsation, damping rate, amplitude and phase of pulsation, quasi-energy and the output power. These estimates are compared to the numerical solution and to the experimental data.