Ultrafast laser micromachining has been extensively researched for its “clean, cold” cutting potential in fields from microelectronics to dentistry. It is clear that the mechanism of laser ablation with pulses shorter than about 500 fs differs significantly different from the light-to-heat dominated processes with longer pulsed (ns, ps) and CW laser machining. However, the details of the femtosecond laser ablation mechanism remain incompletely understood.
The ablation threshold (J/cm^2) is widely used for characterizing laser machining efficiency. Unfortunately, it is not entirely clear what the ablation threshold means in the ultrashort pulse regime. For example, our diameter regression measurements of the ablation thresholds of several materials using 800 nm, 120 fs laser pulses reveal multiple distinct ablation regimes, each characterized by a different effective beam waist. Evidence of similar behavior can be found in the literature, however it is often unremarked upon.
In this paper, we present thorough characterization of the ultrafast laser ablation for a diverse collection of materials (undoped silicon, sapphire, stainless steel and cortical bone). For example, for undoped silicon we find three ablation regimes each characterized by a different ablation threshold and apparent beam waist: (1) 1.56 J/cm^2, 11.8 µm; (2) 1.21 J/cm^2, 51.9 µm; and (3) 0.85 J/cm^2, 159.9 µm. We show the presence of up to three different ablation regimes that vary depending on the type of material. Using computational modeling, we address the mechanistic underpinnings of these observations, particularly the dependence upon pulse energy and spatial beam shape.
Ultrafast laser micromachining that utilises pulses on a femtosecond timescale is a rapidly growing area of research with applications in a wide variety of fields, from microelectronics to microsurgery. Femtosecond pulses are often praised for their ability to perform precise cutting of materials through a ‘cold-cutting’ mechanism which avoids mechanical and thermal collateral damage to the surrounding area. However, the high precision and clean ablation features associated with ultrafast laser micromachining can be counteracted through the intense plasma in air that is generated at high pulse energies. The highly reflective plasma generated above the sample surface can result in a distorted beam profile at the target machining plane, producing machined features with reduced edge quality and accuracy. In addition, the highly reflective plasma results in underutilised portions of the incident pulse energy, therefore decreasing machining efficiency.
We present the ablation threshold data and trends for a variety of materials including undoped silicon, stainless steel and sapphire laser machined under vacuum and other ambient conditions. Ablation thresholds were determined using the diameter regression technique with 130 fs, 800 nm laser pulses at a repetition rate of 500 Hz. Ablation features are analysed extensively to observe the impact of the ambient conditions on the resulting feature quality.
We demonstrated herein a new type of cladding light strippers suitable for high power systems. By precisely micro-machining the surface of the fiber we create CLS with efficiencies as high as 97 % for large NA, multi-mode, cladding light (NA = 0.3), and 70 % for single-mode, low NA, light. The NA of the cladding light is reduced from 0.3 down to 0.08. The CLS exhibit a 1°C/stripped-Watt temperature elevation making them very suitable for high power applications. This fabrication method is simple and reliable. We have tested different texturization geometries on several different fibers: 20/400 from Nufern, KAGOME, and LMA 10 and LMA 15 fibers (results not shown herein) and we observed good efficiencies and temperature elevation behavior for all of them. Finally, large scale production of CLS with this method is possible since the time necessary to prepare on CLS is very small, in the order of few seconds.
We demonstrate herein a PM-fiber based cavity design capable of supporting many different pulse dynamics, such as soliton propagation or dissipative solitons in a dispersion managed cavity. By changing the dispersion of the fiber Bragg grating of the cavity we modify the net cavity dispersion, and thus stimulate various pulse dynamics. In particular we demonstrate the first net normal cavity, all-PM, all-fiber, dipersion managed cavity operating the in the 2μm range. Furthermore, we also demonstrate an all-fiber all-PM MOPA system capable of delivering up to 6 W of average power at 16 MHz by direct amplification of 70 ps long narrowband pulses. The amplifier stages are not fully saturated and are currently limited by the pump power available.
We report on a new design for a passively mode locked bre laser employing all normal dispersion polarisation
maintaining bres operating at 1 μm. The laser produces linearly polarized, linearly chirped pulses that can be
recompressed down to 344 fs. Compared to previous laser designs the cavity is mode-locked using a nonlinear
amplifying bre loop mirror that provides an additional degree of freedom allowing easy control over the pulse
parameters. This is a robust laser design with excellent reliability and lifetime.
In this contribution we report a high repetition rate optical parametric amplifier (OPA) pumped by a chirped pulse fiber amplifier system. Fiber CPA systems have demonstrated power scaling and open the route to OPAs at repetition rates in the 100 kHz-10MHz range. The OPA stage is seeded by a continuum generated in a Sapphire plate and more than 50 nm bandwidth is efficiently amplified, resulting in 3 &mgr;J, 29 fs pulses.
We report on an optical parametric amplification system which is pumped and seeded by fiber generated laser radiation.
Due to its low broadening threshold, high spatial beam quality and high stability, the fiber based broad bandwidth signal
generation is a promising alternative to white light generation in bulky glass or sapphire plates. As pump source we
propose the use of a high repetition rate ytterbium-doped fiber chirped pulse amplification system.
We report on an optical parametric amplification system which is pumped and seeded by fiber generated laser
radiation. Due to its low broadening threshold, high spatial beam quality and high stability, the fiber based broad
bandwidth signal generation is a promising alternative to white light generation in bulky glass or sapphire plates.
We demonstrate a novel and successful signal engineering implemented in a setup for parametric amplification
and subsequent recompression of resonant linear waves resulting from soliton fission in a highly nonlinear
photonic crystal fiber. The applied pump source is a high repetition rate ytterbium-doped fiber chirped pulse
amplification system. The presented approach results in the generation of ~50 fs pulses at MHz repetition rate.
The potential of generating even shorter pulse duration and higher pulse energies will be discussed.
The generation of high energy femtosecond pulses in Optical Parametric Amplifier (OPA) pumped by fiber laser at a repetition rate of 1MHz is reported. Highly nonlinear fibers are used to create an intrinsically synchronized signal for the parametric amplifier. Seeding the OPA by a supercontinuum generated in a photonic crystal fiber, large tunability extending from 700 nm to 1500 nm of femtosecond pulses is demonstrated, with pulse energies as high as 1.2 μJ. Generating the seed using only SPM in a standard fiber, broadband amplification over more than 85 nm and subsequent compression down to 46 fs in a prism sequence are achieved. Pulse peak powers pulses above 10 MW together with 0.5 W of average power is achieved. This system appears to be very interesting due to scalability of pulse energy and average power of both involved concepts: fiber laser and parametric amplifier.