Di Lin

Controlling the handedness of directly excited Laguerre–Gaussian modes in a solid-state laser

The first higher order Laguerre-Gaussian (LG01) beam is characterised by a donut-shaped intensity profile and has a phase front that advances with a helical trajectory described by exp(±iφ-ikz), where φ is the azimuthal angle and z is the axial position. As a consequence, the LG01 beam has an orbital angular momentum of ±ħ for each photon. These unique properties of LG01 beams have attracted growing interest due to a wealth of promising applications in areas such as optical manipulation of particles, trapping and guiding of atoms and laser processing of materials. A number of different techniques for generating LG01 beams have been devised over the years. The most popular techniques exploit external beam-shaping arrangements. However, these techniques are generally not suitable for high power operation due to power handling limitations.

Cladding-pumped ytterbium-doped fiber laser with radially polarized output.

A simple technique for directly generating a radially polarized output beam from a cladding-pumped ytterbium-doped fiber laser is reported. Our approach is based on the use of a nanograting spatially variant waveplate as an intracavity polarization-controlling element. The laser yielded ~32 W of output power (limited by available pump power) with a radially polarized TM (01)-mode output beam at 1040 nm with a corresponding slope efficiency of 66% and a polarization purity of 95%. The beam-propagation factor (M(2)) was measured to be ~1.9-2.1.

Polarization-dependent transverse mode selection in an Yb-doped fiber laser

A simple technique for selectively generating the fundamental LP01 and the next higher order donut-shaped LP11 mode with vortex phase front in a cladding-pumped few-moded ytterbium-doped fiber laser is reported. Our approach exploits the difference in polarization behavior of individual transverse modes due to transverse variation of birefringence in a few-mode fiber allowing robust mode discrimination through the use of an appropriately aligned intracavity polarizing element. The laser yielded ∼36  W of output power for both transverse modes with corresponding slope efficiency of 74% with respect to absorbed pump power and a mode purity of 98% for the donut-shaped LP11 mode.

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.

High power fiber lasers with radially polarized output beams

A scheme for directly exciting the radially-polarized TM01 mode in a fiber laser is reported. Preliminary results for cladding-pumped ytterbium-doped and thulium-doped fiber lasers are discussed along with prospects for scaling to high power levels.

Demonstration of arbitrary temporal shaping of picosecond pulses in a radially polarized Yb-fiber MOPA with > 10 W average power

High precision surface processing has an unmet demand for picosecond pulses with arbitrary temporal profiles in radial polarization states and at high average powers. Here, simultaneous spatial and arbitrary temporal shaping of chirped 10 - 100 picoseconds pulses is demonstrated with an Yb-doped fiber laser system generating an output power of more than 10 W at 40 MHz repetition frequency. The closed-loop control algorithm carves the pulses using a commercial, rugged, and fiberized optical pulse shaper placed at the front end of the system and uses feedback from the output pulse shapes for optimization. Arbitrary complex temporal profiles were demonstrated using a dispersive Fourier transform based technique and limits set by the system were investigated. Pulse shaping in the spatial domain was accomplished using an S-waveplate, fabricated in-house, to change the linearly polarized fundamental mode into a doughnut mode with radial polarization. This was amplified in a final-stage few-mode large-mode area fiber amplifier. Placing both temporal and spatial shaping elements before the power-amplifier avoids complex and potentially lossy conversion of the spatial mode profile at the output and provides an efficient route for power-scaling. The use of properly oriented quarter- and half-wave plates, which have both low loss and high power handling capability, enabled the output to be set to pure radial or azimuthal polarization states. Using commercial off-the-shelf components, our technique is able to immediately enhance the versatility of ultrashort fiber laser systems for high precision material processing and other industrial applications.

Data for '106W, picosecond Yb-doped fiber MOPA system with a radially polarized output beam'

This is original data for the paper: 106W, picosecond Yb-doped fiber MOPA system with a radially polarized output beam, published in the journal Optics Letters by the authors Di Lin, Neda Baktash, Shaif-ul Alam and David J. Richardson.

Data for Radially- and azimuthally- polarized nanosecond Yb-doped MOPA system incoporating temporal shaping

Abstract: 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 pre-amplification 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 nanosecond 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 >12dB and 2.2 respectively.The data relates to the paper: Lin, Di, Baktash, Neda and Berendt, Martin et al. (2017) Radially- and azimuthally- polarized nanosecond Yb-doped MOPA system incoporating temporal shaping. (doi:10.1364/OL.99.099999).

Reduced thermal lensing in an end-pumped Nd:YVO 4 laser using a ring-shaped pump beam

We report a simple technique for reducing thermal lensing in end-pumped solid-state lasers using a pump with a ring-shaped intensity distribution to decrease the radial temperature gradient. Preliminary results for a Nd:YVO4 laser are presented.

Cladding-pumped radially-polarized fiber lasers

Direct excitation of the radially-polarized TM01 mode in cladding-pumped ytterbium-doped and thulium-doped fiber lasers using a novel mode selection scheme is reported. The prospects for scaling to very high power levels are discussed.