Parviz Elahi

Ablation-cooled material removal with ultrafast bursts of pulses

The use of femtosecond laser pulses allows precise and thermal-damage-free removal of material (ablation) with wide-ranging scientific, medical and industrial applications. However, its potential is limited by the low speeds at which material can be removed and the complexity of the associated laser technology. The complexity of the laser design arises from the need to overcome the high pulse energy threshold for efficient ablation. However, the use of more powerful lasers to increase the ablation rate results in unwanted effects such as shielding, saturation and collateral damage from heat accumulation at higher laser powers. Here we circumvent this limitation by exploiting ablation cooling, in analogy to a technique routinely used in aerospace engineering. We apply ultrafast successions (bursts) of laser pulses to ablate the target material before the residual heat deposited by previous pulses diffuses away from the processing region. Proof-of-principle experiments on various substrates demonstrate that extremely high repetition rates, which make ablation cooling possible, reduce the laser pulse energies needed for ablation and increase the efficiency of the removal process by an order of magnitude over previously used laser parameters. We also demonstrate the removal of brain tissue at two cubic millimetres per minute and dentine at three cubic millimetres per minute without any thermal damage to the bulk.

Generation of picosecond pulses directly from a 100 W, burst-mode, doping-managed Yb-doped fiber amplifier

Burst-mode laser systems offer increased effectiveness in material processing while requiring lower individual pulse energies. Fiber amplifiers operating in this regime generate low powers in the order of 1 W. We present a Yb-doped fiber amplifier, utilizing doping management, that scales the average power up to 100 W. The laser system produces bursts at 1 MHz, where each burst comprises 10 pulses with 10 μJ energy per pulse and is separated in time by 10 ns. The high-burst repetition rate allows substantial simplification of the setup over previous demonstrations of burst-mode operation in fiber lasers. The total energy in each burst is 100 μJ and the average power achieved within the burst is 1 kW. The pulse evolution in the final stage of amplification is initiated as self-similar amplification, which is quickly altered as the pulse spectrum exceeds the gain bandwidth. By prechirping the pulses launched into the amplifier, 17 ps long pulses are generated without using external pulse compression. The peak power of the pulses is ∼0.6 MW.

Doping management for high-power fiber lasers: 100 W, few-picosecond pulse generation from an all-fiber-integrated amplifier

Thermal effects, which limit the average power, can be minimized by using low-doped, longer gain fibers, whereas the presence of nonlinear effects requires use of high-doped, shorter fibers to maximize the peak power. We propose the use of varying doping levels along the gain fiber to circumvent these opposing requirements. By analogy to dispersion management and nonlinearity management, we refer to this scheme as doping management. As a practical first implementation, we report on the development of a fiber laser-amplifier system, the last stage of which has a hybrid gain fiber composed of high-doped and low-doped Yb fibers. The amplifier generates 100 W at 100 MHz with pulse energy of 1 μJ. The seed source is a passively mode-locked fiber oscillator operating in the all-normal-dispersion regime. The amplifier comprises three stages, which are all-fiber-integrated, delivering 13 ps pulses at full power. By optionally placing a grating compressor after the first stage amplifier, chirp of the seed pulses can be controlled, which allows an extra degree of freedom in the interplay between dispersion and self-phase modulation. This way, the laser delivers 4.5 ps pulses with ~200 kW peak power directly from fiber, without using external pulse compression.

Disorder-mediated crowd control in an active matter system

Living active matter systems such as bacterial colonies, schools of fish and human crowds, display a wealth of emerging collective and dynamic behaviours as a result of far-from-equilibrium interactions. The dynamics of these systems are better understood and controlled considering their interaction with the environment, which for realistic systems is often highly heterogeneous and disordered. Here, we demonstrate that the presence of spatial disorder can alter the long-term dynamics in a colloidal active matter system, making it switch between gathering and dispersal of individuals. At equilibrium, colloidal particles always gather at the bottom of any attractive potential; however, under non-equilibrium driving forces in a bacterial bath, the colloids disperse if disorder is added to the potential. The depth of the local roughness in the environment regulates the transition between gathering and dispersal of individuals in the active matter system, thus inspiring novel routes for controlling emerging behaviours far from equilibrium.

In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon

Silicon is an excellent material for microelectronics and integrated photonics1–3, with untapped potential for mid-infrared optics4. Despite broad recognition of the importance of the third dimension5,6, current lithography methods do not allow the fabrication of photonic devices and functional microelements directly inside silicon chips. Even relatively simple curved geometries cannot be realized with techniques like reactive ion etching. Embedded optical elements7, electronic devices and better electronic–photonic integration are lacking8. Here, we demonstrate laser-based fabrication of complex 3D structures deep inside silicon using 1-µm-sized dots and rod-like structures of adjustable length as basic building blocks. The laser-modified Si has an optical index different to that in unmodified parts, enabling the creation of numerous photonic devices. Optionally, these parts can be chemically etched to produce desired 3D shapes. We exemplify a plethora of subsurface—that is, ‘in-chip’—microstructures for microfluidic cooling of chips, vias, micro-electro-mechanical systems, photovoltaic applications and photonic devices that match or surpass corresponding state-of-the-art device performances.By exploiting dynamics arising from nonlinear laser–material interactions, functional microelements and arbitrarily complex 3D architectures deep inside silicon are fabricated with 1 μm resolution, without damaging the silicon above or below.

Prediction of pulse-to-pulse intensity fluctuation characteristics of high power ultrafast fiber amplifiers

We report on the experimental characterization and theoretical prediction of pulse-to-pulse intensity fluctuations, namely, intensity noise, for ultrafast fiber amplifiers. We present a theoretical model with which the intensity noise of a Yb-doped fiber amplifier can be predicted with high accuracy, taking into account seed and pump noise, as well as generation of amplified spontaneous emission. Transfer of pump and seed signal modulations to the amplified output during fiber amplification are investigated thoroughly. Practically, our model enables design and optimization of fiber amplifiers with regards to their intensity noise performance. As a route to reducing noise imparted by pump diodes in a double-clad amplifier, we show the use of multiple, low-power diodes is more beneficial compared to a single, high-power diode due to the largely uncorrelated nature of their individual noise contributions.

Theoretical analysis of doping management

Two opposing requirements, such as thermal load and nonlinear effects are important limitations in rapid progress of high-power fiber laser technologies. Thermal effects, which limit the average power, can be minimized by using low-doped, longer gain fibers, whereas presence of nonlinear effects requires use of high-doped, shorter fibers to maximize the peak power. Proposed solution of the problem is the use of varying doping levels along the gain fiber to circumvent to mitigate the trade-off between thermal load and nonlinear effects.

Ultrafast micromachining of Cu and Si at ultra-high repetition rates with pulse bursts

We report a novel ultrafast burst mode fiber laser system, which can deliver pulses at ultra-high repetition rates in order to systematically investigate micromachining efficiency on copper and silicon samples.

Ablation-cooled material removal at high speed with femtosecond pulse bursts

We report exploitation of ablation cooling, well-known in rocket design, to remove materials, including metals, silicon, hard and soft tissue. Exciting possibilities include ablation using sub-microjoule pulses with efficiencies of 100-μJ pulses.

3.5-W, 42-MHz, single-mode chirped pulse amplification fiber laser system at 1560 nm

There is much interest in the development of high power ultra-short fiber laser systems due to their significant properties and applications. Among them, Er-doped fiber lasers are showing more attention, especially in silicon processing and photovoltaic industries [1]. Chirp pulse amplification (CPA) is the most common approach to stablish high-power/high energy fiber lasers [2].