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Finger on the pulse

Matthew Dale finds that both the commercial and research sectors are pushing the limits of ultrafast laser technology, leading to advances such as a new ultrashort pulse power record

Ultrafast lasers produce picosecond, femtosecond and attosecond pulse durations, and feature heavily in applications where extreme precision is required. With recent breakthroughs in power made by Austrian [1], German [2] and Chinese researchers, ultrafast lasers are being increasingly considered for strong field applications, such as high resolution X-ray production, cancer treatment and dark matter detection.

The market for ultrafast lasers is currently seeing a growing demand for devices that provide both high peak power and high repetition rates. Laser manufacturers are faced with the challenge of achieving both, while still maintaining short pulse duration. Short laser pulses grant high temporal resolution, high repetition rates enable rapid data collection, and high peak powers allow access to the strong field and relativistic regimes.

The strong field regime – where an electric field imparts enough energy to the electrons in a material for them to depart their original atom via ‘tunnelling ionisation’ – occurs at intensities of 1,014-1,015W/cm2. Strong field applications include high harmonic generation, which leads to the production of extreme UV radiation (XUV) and attosecond pulse duration.

The relativistic regime occurs at even higher intensities – above 1,019W/cm2. At this level, both the electrons and protons of atoms can be accelerated to release high-energy radiation, which has applications in medicine and scientific research.

The titanium-sapphire (TiS) laser is considered to be the workhorse of ultrafast lasers and amplifiers, according to Marco Arrigoni, director of marketing at Coherent. Their ability to produce sub-10fs pulses and up to petawatt (PW) peak powers when amplified puts them at the centre of a broad range of ultrafast applications, with close to 10,000 units installed worldwide. According to Arrigoni, however, TiS amplifiers are difficult to scale up in terms of repetition rate and average power, or to offer with flexible repetition rates.

Overcoming these limitations will be a crucial development in ultrafast lasers, as ‘an ever-growing number of applications benefit from higher average power and repetition rate, especially if there is still sufficient peak power to reach the strong field regime,’ Arrigoni said.

Ytterbium (Yb) doped active fibre lasers are able to offer these sought-after higher average powers, and flexibility in repetition rate adjustable in the tens of kilohertz and megahertz ranges. However, this can only be offered at longer pulse durations, which subtracts from the laser’s precision. In response to this, recent developments in non-linear microstructured fibres are being directed towards reducing the pulse duration of the amplifiers containing them. ‘So far, sub-10fs pulses have been demonstrated using two cascaded hollow fibre compressors,’ explained Arrigoni. These results therefore signal that Yb laser systems could eventually be designed to match the pulse duration, or possibly even the energy, of TiS systems.

‘Considering all these developments, it is likely that proportionally larger performance gains should be expected from ytterbium media rather than from titanium-sapphire, simply because of the different degree of technology maturity,’ Arrigoni concluded.

 

Laser material processing

Pushing boundaries

The upper limit of peak power in ultrafast lasers is currently in the order of 1,015 watts (PW), and is accessed commonly for high-energy applications, such as high resolution X-ray imaging or cancer treatment. With demand increasing for these types of application, developments at this level of peak power become ever more crucial.

Amplitude Technologies’ ultrafast lasers are currently able to reach 1.5PW of peak power at repetition rates of 1Hz and below, with the company working to increase this to 2PW at a 10Hz repetition rate. These new parameters are intended for the European Extreme Light Infrastructure (ELI) project, which aims to create user facilities featuring the latest 
laser technologies in the world.

Petawatt peak powers are accessed through chirped pulse amplification (CPA), a technique that features heavily in the ultrafast field. It works by first stretching an ultrashort pulse to decrease its peak power, and then amplifying this pulse with huge amounts of energy. The pulse is then re-compressed to ultrashort pulse widths while maintaining the high energies gained through amplification. Amplitude uses 20-30fs pulses to access petawatt peak-powers through CPA.

When lasers of such high peak power are focused on a target, energy intensities reach extreme levels of up to 1,021-1,023W/cm2– well within the relativistic regime – as explained by Gilles Riboulet, CEO of Amplitude Technologies. At these extreme intensities, strong nuclear effects take place in the atoms hit by the laser beam. The electrons and protons of the atom can be accelerated, and strong XUV/X-ray emissions are released, which have benefits in many applications.

‘The purpose of these lasers is to create a secondary source for the emission of electrons and protons that usually big accelerators like those at CERN provide, at far less cost,’ commented Riboulet. ‘We are already engaged in a race as a community for demonstrating the capabilities of this technique for applications in society such as high resolution X-ray imaging and cancer treatment.’

According to Riboulet, lasers in the 2PW, 10Hz range currently under development are highly sought after by customers, and are expected to be able to address proton therapy applications within the next 10 years. Ultimately, the objectives and expectations of ultrafast laser systems are to go beyond the capabilities of particle accelerators while being only a fraction of the price, Riboulet commented.

In addition to laser companies, researchers around the world and pushing ultrafast laser technology to its limits.

A team based in Jena, Germany has recently broken the record for the average power output of an ultrafast laser using 6fs pulse durations. The team, with members belonging to the Friedrich Schiller University, the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF), and start-up company Active Fiber Systems, has built a femtosecond system delivering an average power of 216W.

The new laser source is also being developed for the ELI project, and was recently described in a paper published in September’s issue of Optics Letters. The paper explains how the system’s femtosecond fibre laser uses CPA in two non-linear compression stages to produce a maximum of 216W average power, a new record. The target for the ELI project is to be able to produce 1mJ of energy at a 100kHz repetition rate, with the Jena team believing it will deliver these specifications in early 2017.

Meanwhile, researchers at ShanghaiTech have recently demonstrated the first ever ultrafast laser capable of delivering a peak power of more than 5PW, using chirped pulse amplification techniques to do so. The laser is part of the Shanghai Superintense-Ultrafast Lasers Facility being developed by the Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences.

‘This very high power would lead to a super high intensity after the laser beam is focused in a space domain, reaching an unprecedentedly high electric field and super high-energy density conditions…’ said Ruxin Li, director of SIOM. These extreme conditions are promising for compact high-energy electron and ion accelerators, as well as bright X-ray and gamma-ray sources. Li predicts that this could also be used for applications such as finding new materials under extreme conditions, or even detecting dark matter.

The researchers plan to increase the peak-power capabilities of the laser even further in the coming years. ‘This is a first step of the project to realise a 10PW output in 2018,’ stated Li. The laser is intended to be part of a user facility similar to those created through the ELI project.

Progression through compression

A separate group of researchers at the Vienna University of Technology are taking a different approach to increasing the peak powers of ultrafast lasers. By inducing a phenomenon known as ‘self-compression’ in femtosecond pulses, the group are able to maintain the energy of each pulse while increasing the peak power of their laser to up to half a terawatt. Through compression, the researchers maintain the number of photons being delivered by each pulse, but deliver them over a shorter period of time, increasing the power of the laser.

The researchers induced the self-compression by passing the pulses of infrared laser light through an yttrium aluminium garnet (YAG) crystal. Because of non-linear interaction in YAG, laser pulses spectrally broaden, leading to a production of additional spectral components. At the same time, the material causes certain spectral components of the laser pulse to move faster than others, shortening the pulse in the medium. Although this phenomenon is well known, it had not previously been achieved using very high pulse energies.

Dr Audrius Pugzlys, a co-author of a recent Nature publication that features the research, said: ‘Up to now people thought that this type of compression was impossible because the energies and peak powers at the energy level we are working with are so high, that on a very short length scale this usually leads to the beam breaking up into multiple filaments.’

If a pulsed laser beam of very high intensity is sent through a material, the beams tends to collapse chaotically into many separate filaments, similar to a bolt of lightning that spontaneously breaks up into various branches. ‘Each of these branches only carries a small part of the energy of the original beam, therefore the resulting beam cannot be used for advanced strong-field laser applications,’ said Pugzlys.

The researchers identified the parameters that enable self-compression without causing the beam to collapse into filaments. ‘As it turns out, we are dealing with two different length scales,’ said Valentina Shumakova, lead author of the paper. ‘The length scale of the unwanted filamentation is longer than the length on which self-compression occurs. Therefore, it is possible to find a parameter regime in which the pulse is compressed, but filamentation does not yet set in.’

According to Pugzlys, in principle it’s possible to separate these length scales where pulses get shorter before they break up into chaotic uncontrollable behaviour. In doing this, the researchers were able to achieve a power 10,000 times higher than the filamentation threshold of the laser pulse in YAG without experiencing beam collapse.

The researchers used a home-built femtosecond laser with an infrared optical parametric pulse amplifier. In combination with this laser, the YAG crystal provided the conditions required to self-compress ultrashort pulses. ‘It’s very important to choose the length of the crystal in a careful way,’ said Pugzlys, ‘if the crystal’s too long the beam will break up, if it’s too short you have only partial or no self-compression at all.’

 

A laser filament travelling through air (first image) and crystal (second and third images). (Credit: Vienna University of Technology) 

By sending each laser pulse through the crystal, their duration decreased from 94 to 30 femtoseconds. The energy of the pulse stayed almost the same, but the power increased by a factor of three. ‘We started from less than 200 gigawatts of peak power and we ended up with half a terawatt or even more.’

The combination of the high peak power, long infrared wavelength and short pulse duration that this laser provides is sought after in a variety of applications. ‘The main application one can imagine is strong-field physics,’ said Pugzlys. He proposed that the laser could be used to drive particle acceleration in both scientific and medical applications, and to generate coherent soft X-ray radiation, meaning that the laser could become a valuable research tool in high-energy sectors.

Another potential application field for the device is laser filamentation, according to Pugzlys. The process can be used to perform remote atmospheric sensing by firing the laser into the sky, where it becomes self-guided after forming a channel.

The ability to self-guide could enable further applications in meteorology. ‘People are trying to see how it’s possible to guide discharges with those laser filaments,’ Pugzlys continued. An application could be discharge guiding, or even lightning guiding, for example. According to Pugzlys, research is also being conducted in the community into exciting applications such as laser induced condensation, the ability to produce rain or snow by laser filaments.

The researchers have shown that they can increase the energy of the laser by up to 10 times while still being able to induce self-compression. With plans to increase the energy and repetition rates of the laser further, the researchers look to develop a separate, more suitable laser for material processing applications in industry.

References

[1] V. Shumakova, P. Malevich, S. Ališauskas, A. Voronin, A. M. Zheltikov, D. Faccio, D. Kartashov, A. Baltuška & A. Pugzlys, ‘Multi-millijoule few-cycle mid-infrared pulses through nonlinear self-compression in bulk’, Nature Communications 7, Article number: 12877 (2016) doi:10.1038/ncomms12877
[2] Steffen Hädrich, Marco Kienel, Michael Müller, Arno Klenke, Jan Rothhardt, Robert Klas, Thomas Gottschall, Tino Eidam, András Drozdy, Péter Jójárt, Zoltán Várallyay, Eric Cormier, Károly Osvay, Andreas Tünnermann, and Jens Limpert, ‘Energetic sub-2-cycle laser with 216  W average power,’ Opt. Lett. 41, 4332-4335 (2016)

A glimpse inside

Producing high peak powers with an ultrafast laser leads to extreme conditions internally that must be fully endured by the laser’s components, which presents certain difficulties for manufacturers. ‘The AR coatings and the optical materials [in ultrafast lasers] need to have a damage threshold which is sufficient for the intended use,’ commented Markus Fegelein, product specialist of Qioptiq Photonics. ‘To avoid thermal lensing… one needs to select ultra-low absorbing materials like fused silica glasses, as opposed to standard glasses.’

 

Qioptiq's low-outgassing Faraday Isolator helps to increase the live time of encapsulated, high power and UV short pulse laser systems. (Credit: Qioptiq)

According to Fegelein, another challenge is that both peak power and repetition rate are limited by the gain mediums of ultrafast lasers. ‘The laser pulse is trapped for a certain number of round trips in the amplifier cavity, with each round trip, the pulse gains energy,’ he explained. The high repetition rates sought by customers, however, reduce the time that the pulse is trapped in the amplifier, limiting the amount energy that can be gained. ‘Strategies to overcome this contradiction are to either pump the laser media harder or to make the laser rods longer,’ Fegelein said.

Dr Thomas Binhammer, managing director at Laser Quantum, outlined that other ‘principle challenges [in ultrafast lasers] include non-linearity in the crystal, heat dissipation, and managing intra-cavity dispersion that broadens pulses and reduces peak powers.’ Laser Quantum uses ion beam sputtered, matched pair chirped mirrors, to ensure phase control and minimise dispersion in its ultrafast lasers. The company’s component research and development concentrates on ensuring phase control and maintaining the broad spectrum available from ultrafast lasers.

 

Laser Quantum's femtosecond Venteon OPCPA laser. (Credit: Laser Quantum)