High-power radially polarized Yb:YAG thin-disk laser with high efficiency

Compact nanosecond Yb:YAG/V:YAG solid-state laser generating switchable radially and azimuthally beams

We demonstrated a compact self-starting nanosecond Yb:YAG/V:YAG solid-state laser with cylindrical vector beams output modulated by the intracavity mode converter S-waveplate experimentally. We can deliver the stable Q-switched pulse with the highest repetition rate 3.61 kHz and minimum pulse width 26 ns at the wavelength of 1030.07 nm with the help of the V:YAG crystal. In addition, the switchable radially and azimuthally polarized beams can be realized with polarization extinction ratios of 92.3% and 89.6%, respectively. The compact laser configuration can provide solutions for generating stable nanosecond structured light, and may benefit the applications like micro/nano material processing.

>30 MW peak power from distributed face cooling tiny integrated laser

The Distributed Face Cooling (DFC) chip was fabricated from four pieces of 1 mm-length Nd:YAG plate sandwiched in four pieces of sapphire heat spreaders through advanced surface activated bonding (SAB) at room temperature. A sub-nanosecond (665.7ps) pulsed DFC-chip tiny integrated laser was achieved with output energy of 21.5 mJ and peak power of 32.3 MW with saturable absorber Cr⁴⁺:YAG. By finite element analysis, we confirmed the advantages of heat dissipation from DFC-chip compared with conventional bulk-chip. The SAB-DFC-chip based ubiquitous high peak power tiny integrated laser was experimentally within reach for laser-armed robot.

Radially and azimuthally polarized nanosecond Yb-doped fiber MOPA system incorporating temporal shaping

We report an Yb-doped fiber master-oscillator power-amplifier (MOPA) system with the capability of selectively generating doughnut-shaped radially and azimuthally polarized beams with user-defined temporal pulse shapes. The desired output polarization was generated with the aid of a nanograting spatially variant half-waveplate (S-waveplate). The latter was used to convert the linearly polarized fundamental (LP₀₁) mode output from the preamplification stages to a doughnut-shaped radially polarized beam prior to the power amplifier stage. A maximum output pulse energy of ∼860  μJ was achieved for ∼100  ns pulses at 25 kHz with user-defined pulse shape for both radial and azimuthal polarization states. The polarization purity and beam propagation factor (M²) were measured to be >12  dB and 2.2, respectively.

Wavelength-locked vectorial fiber laser manipulated by Pancharatnam-Berry phase

We report a wavelength-locked cladding-pumped ytterbium-doped fiber laser that can simultaneously emit radially and azimuthally polarized beams based on Pancharatnam-Berry phase optical elements. Multi-wavelength free running operation of the radially and azimuthally polarized laser beams can be switched to a single-wavelength one assisted by volume Bragg grating, with wavelength locked at around 1053.4 nm and spectral linewidth of 0.06 nm (FWHW). By rotating the glan-taylor polarizer, we can obtain switchable radially and azimuthally polarized beams output. The radially and azimuthally polarized beams mode purity can maintain 97.3% and 96.3% at maximum output power, and the polarization extinction ratio (PER) can reach 97.8% and 95.9% for the radially and azimuthally polarized laser, respectively.

Radially and azimuthally polarized laser beams by thin-disk laser

The generation of radially and azimuthally polarized beams is theoretically investigated in thin-disk laser configurations by writing Jones matrices for optical elements. Higher modes are omitted by aperture and the mode-selection operation is done by discontinuous phase elements. Two modes, TEM_01x and TEM_01y, are combined to generate the radially and azimuthally polarized laser beam. The polarization of the output beams is studied by the extended Jones matrices. In addition, the output power of the thin-disk laser is numerically estimated by solving the rate equations in ytterbium-doped materials.

Novel thin-disk oscillator concept for the generation of radially polarized femtosecond laser pulses

We report on the first demonstration of a radially polarized passively mode-locked thin-disk oscillator. Radial polarization was achieved by the use of a novel circular grating waveguide output coupler. We showed mode-locked operation up to a maximum average output power of 13.3 W with an optical efficiency of 21.8%. The degree of radial polarization of the emitted beam was measured to be 97±1%. The laser system generated pulses with a duration of 907 fs and an energy of 316 nJ corresponding to a peak power of 0.35 MW. To the best of our knowledge, these values exceed the performance of previously reported radially polarized mode-locked oscillator systems.

Radially polarized emission with 635 W of average power and 2.1 mJ of pulse energy generated by an ultrafast thin-disk multipass amplifier

We report on a thin-disk multipass amplifier delivering radially polarized laser pulses with an average power of 635 W and 2.1 mJ of pulse energy. To the best of our knowledge, this is the highest average output power and pulse energy reported so far for radially polarized ultrafast lasers. The amplifier is seeded by a TruMicro5050 with 115 W of average output power, 6.5 ps pulse duration, and a repetition rate of 300 kHz. A segmented half-waveplate was used for converting the linearly polarized beam into radial polarization in front of the amplifier. We present a scheme for direct amplification of such doughnut-shaped radially-polarized beams, the results obtained, and a solution to compensate for the depolarizing phase shift introduced by the tilted mirrors in the amplifier.

Ultra-sparse dielectric nanowire grids as wideband reflectors and polarizers

Engaging both theory and experiment, we investigate resonant photonic lattices in which the duty cycle tends to zero. Corresponding dielectric nanowire grids are mostly empty space if operated as membranes in vacuum or air. These grids are shown to be effective wideband reflectors with impressive polarizing properties. We provide computed results predicting nearly complete reflection and attendant polarization extinction in multiple spectral regions. Experimental results with Si nanowire arrays with 10% duty cycle show ~200-nm-wide band of high reflection for one polarization state and free transmission for the orthogonal state. These results agree quantitatively with theoretical predictions. It is fundamentally extremely significant that the wideband spectral expressions presented can be generated in these minimal systems.

Investigations on ring-shaped pumping distributions for the generation of beams with radial polarization in an Yb:YAG thin-disk laser

We present experimental investigations on the generation of radially polarized laser beams excited by a ring-shaped pump intensity distribution in combination with polarizing grating waveguide mirrors in an Yb:YAG thin-disk laser resonator. Hollow optical fiber components were implemented in the pump beam path to transform the commonly used flattop pumping distribution into a ring-shaped distribution. The investigation was focused on finding the optimum mode overlap between the ring-shaped pump spot and the excited first order Laguerre-Gaussian (LG(01)) doughnut mode. The power, efficiency and polarization state of the emitted laser beam as well as the thermal behavior of the disk was compared to that obtained with a standard flattop pumping distribution. A maximum output power of 107 W with a high optical efficiency of 41.2% was achieved by implementing a 300 mm long specially manufactured hollow fiber into the pump beam path. Additionally it was found that at a pump power of 280 W the maximum temperature increase is about 21% below the one observed with standard homogeneous pumping.

High-power Yb:YAG single-crystal fiber amplifiers for femtosecond lasers in cylindrical polarization

We demonstrate a three-stage diode-pumped Yb:YAG single-crystal-fiber amplifier to generate femtosecond pulses at high average powers with linear or cylindrical (i.e., radial or azimuthal) polarization. At a repetition rate of 20 MHz, 750-fs pulses were obtained at an average power of 85 W in cylindrical polarization and at 100 W in linear polarization. The report includes investigations on the use of Yb:YAG single-crystal fibers with different length/doping ratio and the zero-phonon pumping at a wavelength of 969 nm in order to optimize the performance.

Amplification of cylindrically polarized laser beams in single crystal fiber amplifiers

Yb:YAG single crystal fiber (SCF) amplifiers have recently drawn much attention in the field of amplification of ultra-short pulses. In this paper, we report on the use of SCF amplifiers for the amplification of cylindrically polarized laser beams, as such beams offer promising properties for numerous applications. While the amplification of cylindrically polarized beams is challenging with other amplifier designs due to thermally induced depolarization, we demonstrate the amplification of 32 W cylindrically polarized beams to an output power of 100 W. A measured degree of radial polarization after the SCF of about 95% indicates an excellent conservation of polarization.

Linearly polarized, narrow-linewidth, and tunable Yb:YAG thin-disk laser

We report on the design, fabrication, and implementation of grating-waveguide structures (GWS) for intracavity polarization and wavelength selection as well as wavelength tuning. The GWS discussed in this Letter is a combination of a low-index leaky-mode waveguide, a subwavelength diffraction grating, and a highly reflective mirror that was designed to operate in Littrow configuration. Using our device as the end mirror of an Yb:YAG thin-disk laser allowed the extraction of beams that exhibit an extremely narrow laser emission bandwidth of ≈25 pm FWHM and a high degree of linear polarization of 99±0.5%. Moreover, the GWS allows a wavelength tuning range from 1007 to 1053 nm. The high-power suitability and the low loss of the GWS was demonstrated by the intracavity use in an Yb:YAG thin-disk laser with an output power of 325 W in multimode operation (M(2)=6) and with 110 W in fundamental-mode operation (M(2)≈1.2), exhibiting optical efficiencies of 53.2% and 36.2%, respectively. An output power of 1.8 kW, corresponding to a power density of 125 kW/cm(2) on the grating, was achieved in further higher-power experiments.

Circular grating waveguide structures for intracavity generation of azimuthal polarization in a thin-disk laser

We report on the generation of beams with azimuthal polarization using resonant grating waveguide structures (GWSs) inside an Yb:YAG thin-disk laser (TDL) oscillator. Two different GWS concepts were used to select the polarization of the emitted beam. The first uses the resonant reflection principle, and the second is based on the leaky-mode approach already reported in our previous work. Up to 93 W and 103 W of output power were extracted from a TDL with an optical efficiency, η(oo), of 36.2% and 40.1% using the first and the second approaches, respectively. In both cases, a pure azimuthal polarization and a beam quality factor, M2, of about 2.2 were measured. The design, fabrication, and different experimental results, as well as the laser performances for both GWSs, are discussed in the present Letter.

Single-layer resonant-waveguide grating for polarization and wavelength selection in Yb:YAG thin-disk lasers

A single-layer resonant-waveguide grating consisting of a sub-wavelength grating coupler etched into a waveguide is proposed in order to achieve high polarization and high spectral selectivity inside an Yb:YAG thin-disk laser resonator. The designed structure was fabricated with the help of a Lloyd's-mirror interference lithography setup followed by reactive ion beam etching down to the desired grating groove depth. The wavelength and polarization dependent reflectivity is measured and compared to the design results. The behaviour of the device at higher temperatures is also investigated in the present work. The device is introduced as the end mirror of an Yb:YAG thin-disk laser cavity. Output powers of up to 123 W with a spectral bandwidth of about 0.5 nm (FWHM) is demonstrated in a multimode configuration (M2~6). In fundamental-mode operation (TEM00 with M2~1.1) 70 W of power with a spectral bandwidth of about 20 pm have been obtained. Moreover, the degree of linear polarization was measured to be higher than 99% for both multimode and fundamental mode operation.

Laser direct writing of rotationally symmetric high-resolution structures

We present a laser direct writing system for the efficient fabrication of high-resolution axicon structures. The setup makes use of scanning beam interference lithography incorporated with a fringe locking scheme for tight fringe phase control and allows us to fabricate large area structures with a period down to 450 nm.