Martin Richardson

Diode Side Pumping of a Gain Guided, Index Anti-Guided Large Mode Area Neodymium Fiber Laser

Diode side pumping of a gain guided, index anti-guided neodymium doped fiber laser is demonstrated. This method of pumping may lead to a scalable approach to create high power, extremely large mode area fiber lasers.

Numerical Investigation of Beam Propagation inside an index antiguided fiber laser

We numerically investigate the beam propagation inside an index antiguided fiber laser. The results show that reflection on the air-cladding boundary of a fiber has strong influence on the guided beam profile along the core.

Thulium Fiber Lasers Stabilized by a Volume Bragg Grating in High Power, Tunable and Q-Switched Configurations

Thulium fiber lasers are spectrally stabilized using a volume Bragg grating. 159 W power and >100 nm tuning range are achieved in CW configurations, and pulses 350 ?J are generated when Q-switched.

700 μJ, 100 ns, 20 kHz pulses from a 1.5 m Thulium-doped fiber amplifier

We report on a 2 μm master oscillator power amplifier (MOPA) fiber laser system capable of producing 700 μJ pulse energies from a single 1.5 m long amplifier. The oscillator is a single-mode, thulium-doped fiber that is Q-switched by an acousto-optic modulator. The oscillator seeds the amplifier with 1 W average power at 20 kHz repetition rate. The power amplifier is a polarization-maintaining, large mode area thulium-doped fiber cladding pumped by a 793 nm fiber-coupled diode. The fiber length is minimized to avoid nonlinearities during amplification while simultaneously enabling high energy extraction. The system delivers 700 μJ pulse energies with 114 ns pulse duration and 14 W average power at 1977 nm center wavelength.

Chalcogenide fibers for improved reliability of active infrared sensing systems (Conference Presentation)

Defense sensing systems must be both productive and robust to accomplish their mission. Active infrared sensing devices consist of many components such as the active medium, mirrors, beam-splitters, modulators, gratings, detectors, etc. Each of these components is subject to damage by the laser beam itself or environmental factors. Misalignment of these components due to vibration and temperatures changes can also reduce performance. The result is a complex and expensive system subject to multiple points of degradation or complete failure. However, beam confinement or “no free-space optics” via fiber transmission and even component assembly within the fiber itself can achieve reliability and low cost for sensing systems with reduced component count and less susceptibility to misalignment. We present measurements of high-power infrared laser beam transmission in chalcogenide fibers. The fiber compositions were As39S61 for the core and As38.5S61:5 for the cladding, resulting in a numerical aperture of 0.2. A polyetherimide jacket provided structural support. Multiwatt CW transmission was demonstrated in near single-mode 12 micron core fiber. Efficient coupling of quantum cascade lasing into anti-reflection coated chalcogenide fiber was also demonstrated. Efficient beam transport without damage to the fiber required careful coupling only into core modes. Beams with M2 ≥ 1.4 and powers higher than 1 W produced damage at the fiber entrance face. This was most likely due to heating of the highly absorptive polymer jacket by power not coupled into core modes. We will discuss current power limitations of chalcogenide fiber and schemes for significantly increasing power handling capabilities.

Enhanced ablation with a femtosecond-nanosecond dual-pulse

Single-shot ablation of GaAs samples by a collinear femtosecond-nanosecond (fs-ns) dual-pulse is investigated. Significantly enhanced material removal is achieved by optimally combining a single 8 ns pulse at 1064 nm and a single 50 fs pulse at 800 nm in time. The resulting ablation craters are examined for inter-pulse delays ranging from -50 ns (ns first) to +1 μs (fs first) as well as very long delays of ±30 s. Crater profilometry is conducted with white light interferometry and optical microscopy to determine the volume of ablated material and identify surface features that reveal information about the physical mechanism of material removal during fs-ns dual-pulse ablation.

Experimental investigation on varying spectral bandwidth when amplifying a pulsed superfluorescent 2-μm source in Tm:fiber

Delivering high peak powers from fiber lasers is limited by the accumulation of nonlinear effects due to the high optical intensities and the long interaction lengths of fibers. Peak power scaling at 2 μm is limited by modulation instability (MI), which is not found for 1 μm sources. This work investigates the performance of a spectrally broadband, nanosecond pulsed thulium-doped fiber laser. The average power and pulse energy performance of the single-mode amplifier delivers >20 W and ~280 μJ. A variable spectral filter is incorporated to study the onset of MI and subsequent spectral broadening as a function of seed linewidth. It is observed that MI-induced spectral broadening is enhanced for larger linewidths. However, when the seed linewidth is increased beyond >10 nm, this trend is reversed. A fiber amplifier model including MI (treated as degenerate four-wave mixing) simulates a parametric gain bandwidth of ~900 GHz for this amplifier configuration, which is equivalent to ~11.5 nm at the 1960 nm center wavelength. Therefore, the decrease in spectral broadening for seed linewidths <10 nm is due to a reduced overlap with the MI gain bandwidth. The capability to scale 2 μm fiber lasers to high powers is strongly dependent on the spectral quality of the seed. Any power initially located within the MI gain bandwidth will degrade performance, and must be considered for power scaling.

Comparison of different ultra-short pulsed laser sources used for semiconductor substrate processing

Singulation of semiconductor substrates was carried out with ultra-short pulsed laser radiation. Experimental studies were aimed to improve the resulting edge quality by maintaining minimal cut width and sufficient cut depth, in addition to the minimizing of the heat-affected zone and debris generation. Single-pulse ablation thresholds were determined for both laser systems adopted. The influence of various process parameters on wafer processing was studied, and the resulting kerf geometries were investigated by determining the width and the depth obtained in different process conditions. Silicon wafers were processed using two different ultra-short pulsed laser sources with pulse durations 100 fs and 350 fs. Laser radiation was focused onto the wafer surface using three different microscope objectives with numerical apertures in the range NA=0.1–0.4. Irradiation was performed with varying process parameters such as the laser power and the translation speed, as well as by adopting different laser beam scanning strategies. A set of process parameters resulting in a combination of optimal edge quality, kerf geometry and overall ablation rate was identified for each laser system.Singulation of semiconductor substrates was carried out with ultra-short pulsed laser radiation. Experimental studies were aimed to improve the resulting edge quality by maintaining minimal cut width and sufficient cut depth, in addition to the minimizing of the heat-affected zone and debris generation. Single-pulse ablation thresholds were determined for both laser systems adopted. The influence of various process parameters on wafer processing was studied, and the resulting kerf geometries were investigated by determining the width and the depth obtained in different process conditions. Silicon wafers were processed using two different ultra-short pulsed laser sources with pulse durations 100 fs and 350 fs. Laser radiation was focused onto the wafer surface using three different microscope objectives with numerical apertures in the range NA=0.1–0.4. Irradiation was performed with varying process parameters such as the laser power and the translation speed, as well as by adopting different laser beam sca...

Ultrafast laser-based post-processing of parts produced by additive manufacturing

Laser Additive Manufacturing (LAM) is a near-net shape manufacturing approach, meaning that the resulting part geometry can be considerably affected by heat-induced distortions, solidified melt droplets, partially fused powders, and surface modifications induced by the laser tool motion and processing strategy. High-repetition rate femtosecond laser radiation was utilized to improve surface quality of metal parts manufactured by laser additive techniques. Different laser scanning approaches were utilized to increase the ablation efficiency and to improve the surface finish. Processing of 3D-shaped parts made of Ni- and Ti-base superalloys resulted in the reduction of the average surface roughness to a few microns. This approach can be used to post-process parts made of thermally and mechanically sensitive materials, and to attain complex designed shapes with micrometer precision.Laser Additive Manufacturing (LAM) is a near-net shape manufacturing approach, meaning that the resulting part geometry can be considerably affected by heat-induced distortions, solidified melt droplets, partially fused powders, and surface modifications induced by the laser tool motion and processing strategy. High-repetition rate femtosecond laser radiation was utilized to improve surface quality of metal parts manufactured by laser additive techniques. Different laser scanning approaches were utilized to increase the ablation efficiency and to improve the surface finish. Processing of 3D-shaped parts made of Ni- and Ti-base superalloys resulted in the reduction of the average surface roughness to a few microns. This approach can be used to post-process parts made of thermally and mechanically sensitive materials, and to attain complex designed shapes with micrometer precision.

Trans-wafer processing of semiconductors with nanosecond mid-IR laser radiation

In recent years, a major push was made for the use of novel laser sources in the processing of semiconductors and other materials used in photovoltaic and IC applications. In addition to a large number of highly automated laser processes already adopted by the industry, more laser-based processing approaches are being developed to improve performance and reduce manufacturing costs. Common semiconductors are transparent in the infrared spectral region. Therefore laser sources operating at mid-IR wavelengths can be successfully utilized to induce material modifications in semiconductor wafers even beyond the laser-incident surface. We present our initial studies of this processing regime utilizing a self-developed nanosecond-pulsed thulium fiber laser emitting at the wavelength 2 µm. Our experimental approach confirmed that morphology changes could be induced not only at the front (laser-incident) surface of the wafer, but also independently at the back surface. We investigated the influence of process parameters, such as the incident pulse energy, duration and focusing conditions, on the induced surface morphology. In addition, we studied experimental routes to a number of potential applications of this processing regime, such as the PV cell edge isolation and the wafer scribing.In recent years, a major push was made for the use of novel laser sources in the processing of semiconductors and other materials used in photovoltaic and IC applications. In addition to a large number of highly automated laser processes already adopted by the industry, more laser-based processing approaches are being developed to improve performance and reduce manufacturing costs. Common semiconductors are transparent in the infrared spectral region. Therefore laser sources operating at mid-IR wavelengths can be successfully utilized to induce material modifications in semiconductor wafers even beyond the laser-incident surface. We present our initial studies of this processing regime utilizing a self-developed nanosecond-pulsed thulium fiber laser emitting at the wavelength 2 µm. Our experimental approach confirmed that morphology changes could be induced not only at the front (laser-incident) surface of the wafer, but also independently at the back surface. We investigated the influence of process para...