Compensating for pulsed lasers

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Greg Blackman looks at the optics for ultrashort pulsed lasers, including those planned for the Extreme Light Infrastructure Beamlines facility, expected to become operational at the end of 2017

Optics for ultrafast lasers operating in the picosecond or femtosecond regime must be relatively specialised in order to control such short pulses. The Extreme Light Infrastructure (ELI) Beamlines facility that’s currently under construction in the Czech Republic will deliver 10PW of peak power and pulse durations of around 15fs when it is completed in 2017. The optics employed at the facility will – unlike single-shot projects like those run at the National Ignition Facility (NIF), for example – operate below the damage threshold of the coatings with some safety margins, but there are still reasonably high levels of power passing across the mirrors and gratings – the 15fs beam line optics will have to withstand 200W of average power, while the optical parametric chirped-pulse amplifiers (OPCPAs) generating the pulses will be subjected to higher average powers. ‘We expect up to 800W of average power in the picosecond regime across the [OPCPA] coating, which is extremely challenging,’ noted Dr Daniel Kramer, chief optical designer for laser systems at ELI Beamlines. ‘The absorption levels are in the order of tens to hundreds of parts per million in the high reflection coatings and play a critical role in the thermal management.’

The facility now has its own laser-induced damage threshold station to test some of the coatings in-house. ‘We know what the damage threshold of a coating is at the test pulse duration and, using some scaling laws, we can estimate the threshold for the working pulse durations. This gives us an idea of how much fluence we can put across a given coating,’ explained Kramer.

The highest damage threshold coatings are usually made from stacks of hafnia and silica – these are the most resilient coatings that are at the same time broadband enough for ultrashort pulses, said Kramer. The mirrors employ multi-layer dielectric (MLD) coatings to maintain high efficiency.

As with other ultrafast optics, the optics used in ELI Beamlines also have to compensate for beam dispersion. ‘One consideration is to avoid damaging the optic. The inverse problem is to make sure the optic is not damaging the beam, i.e. the optic is not extending the pulse duration,’ Kramer remarked.

The reflective index of all materials, including air, changes the travel time for different wavelengths, which is especially apparent with broadband pulses. Thomas Binhammer, co-founder of ultrafast laser provider, Venteon, which is now owned by Laser Quantum, commented: ‘It’s all about managing the arrival times of the pulse and dispersion management in general. That’s the key challenge we face for our oscillators.’

Laser Quantum’s ultrashort pulsed lasers have an output spectrum covering 600nm to 1,200nm, and the company is also now considering different wavelength regimes, including 400nm to 700nm. The challenge is to control the dispersion of these broadband pulses very precisely.

Inside the laser cavity there are several optical components that introduce dispersion, making the pulse longer, the main one in Laser Quantum’s ultrashort pulsed lasers being the Ti:sapphire gain material. The short wavelengths, around 600nm, travel slower through the gain material than the infrared wavelengths at 1,200nm, so specially designed mirrors are used to compensate for these effects. The lasers use pairs of chirped mirrors with a sophisticated coating layer structure – around 80 layers – which have to be manufactured with nanometre precision using ion beam sputtering. The mirrors delay the long wavelengths in respect to the short wavelengths to compensate for dispersion.

‘A pulse moving out of a laser through 1 to 2mm of glass would broaden from 5fs to 25fs,’ commented Binhammer. The simplest dispersion compensation method is achieved by an arrangement of gratings or prisms, where different path lengths are introduced geometrically, according to Binhammer. So, the wavelengths are split apart, travel different distances, and then recombined. ‘This doesn’t really need complicated optics, but the drawback is it can’t be done for a large bandwidth or for very short (sub-8fs) pulses, since here also higher orders of dispersion have to be considered,’ he said.

The only way to compensate for dispersion for sub-8fs systems is to use specially designed optics. There are also approaches with adaptive optics which use pulse shapers like liquid crystal displays, but these are much more complicated and incur losses.

Optics for ultrafast lasers are generally coated with ion beam sputtering to get the accuracy needed in the deposition process. ‘What you need is to control the reflectivity in depth, so you have a few micrometres of coating thickness on these mirrors with interchanging high and low refractive index materials,’ explained Binhammer. Laser Quantum’s mirrors for its ultrafast systems consist of around 80 coating stacks.

Dr Wolfgang Ebert, CEO of coating company Laseroptik, commented: ‘Especially the topmost layers of such dispersive coatings are terribly sensitive to manufacture errors. Thus, the coating method, and the film thickness control need to be as precise as can be.’

Laseroptik’s femtosecond optics are coated with ion beam sputtering, on machines that can directly feed in the theoretical designs and control the machine online, in situ. The company also measures the group delayed dispersion (GDD) in-house after the coating has been applied.

The group delayed dispersion properties of the optical coatings used in the ELI Beamlines facility are an integral part of the laser designs, said Kramer. The beam delivering 15fs pulses is compressed using a chirped mirror compressor. The deposition process for a chirped mirror coating has to be controlled extremely precisely – ‘there is a tight tolerancing of the thickness of the layers’, Kramer noted. All the ELI Beamlines lasers are in full development. The 15fs laser is almost entirely developed at the facility apart from the pump lasers, and the scientists already have mirrors for the low-energy part, according to Kramer. For the large optics, the tendering process will start next year. The main beam line will operate at more than 1PW at 10Hz; its transport optics will have a clear aperture of 250 x 370mm. The largest beam will have a peak power of 10PW, the optics for which will have a clear aperture of around 500 x 700mm. Both beams are intended to be operational by the end of 2017.

‘The advantage of the 10PW optics is that while the beam line is high energy, the spectral bandwidth is narrower – pulse duration will be somewhere around 150fs,’ Kramer said. In order to reach 10PW, the system will use less than 1.5kJ of energy per pulse.

‘Usually the weak point for the laser chain is not the mirror but the compressor grating,’ Kramer continued. ‘These gratings have coatings which limit the fluence of the beam. For one of the lasers these are gold gratings – gold because the metal has a very large bandwidth.’ All the mirrors and the 10PW grating will have multi-layer dielectric (MLD) coatings. However, it is very difficult to make a sufficiently large bandwidth grating with an MLD coating. ‘The gratings we hope will last; they are very expensive and very sensitive to contamination,’ he added.

Coating gratings for ultrashort pulsed lasers operating at high average powers is one topic of investigation, but, according to Binhammer, there is also work around the optics for attosecond lasers operating in the EUV range at a wavelength of a few nanometres. ‘There are no [EUV] optics at the moment for chirped control,’ he said. ‘Here, glass materials can’t be used, so the coating layers are more complicated. The challenge of dispersion compensation is the same though. That [attosecond optics] is an ongoing hot topic.’

Scanning the pulse

Ultrashort pulsed (USP) lasers are no longer confined to the laboratory, and significant advances have been made for continuous use in industrial micromachining. Short pulse widths and high repetition rates enable new methods of processing material in the medical, solar, and automotive industries, to name a few. Predominantly, the USP lasers are in use where the demand for short processing times is strong.

Traditionally, galvanometer-based scanners have been used to steer the laser beam, but while fairly fast on their own they fail to take advantage of the high repetition rates that USP laser can offer. When such scanners are combined with a polygon, then the fast deflection speeds needed for USP applications can be achieved.

Raylase, a company based near Munich in Germany manufacturing a variety of scan heads, has developed a UHSS polygon scanner with a scanning speed of up to 200m/s over a 160 x 160mm working field. The scan head integrates a polygon scanner, which deflects the beam in the x-axis, with two galvos – one to take care of the y-axis and the other to correct for field distortion from the lens in the x-axis.

‘Our customers are happy that there are now scanning options available to steer the beam at faster rates than what can be achieved with traditional 2D scan heads,’ said Markus Weber, R&D manager at Raylase.

One of the applications the scan head has been used for is marking the passivation layer in solar wafers. In this application, the entire wafer is covered with a grid of dots spaced 80µm apart. Roughly around 200 lines per wafer are laid down and Weber noted that the entire wafer takes 0.3 seconds to process, compared to two seconds with a traditional scan head.

One limitation of the scanners for ultrafast laser processing is that they can only scan in straight lines. ‘The UHSS scan head can move very fast, but it can’t do a random pattern,’ stated Weber ‘If we wanted to do a circle, we would have to raster it.’

It then becomes a design decision as to which scan head to choose – it might be better to choose a traditional scan head that might move slower for curves and circles.

At the moment, Raylase is offering the UHSS scanner at 532nm and 1,064nm, and is evaluating 355nm. The scanner has an entrance aperture of 15mm, with a spot size that can be focused down to 25 to 30µm. The company could also offer the system at different scan speeds and field sizes upon request.

‘Both industries – ultrafast laser and scanner industries – still have to grow a little bit,’ concluded Weber. ‘We’re starting to deliver a fast deflecting system and the laser manufacturers are developing lasers with high repetition rates that maintain the power.’