Ytterbium challenges neodymium monopoly

17 Jun 2002

Since the birth of the laser, neodymium has dominated solid-state lasing media as the dopant of choice. Now, however, the superior properties of its chemical neighbour are making ytterbium an increasingly attractive alternative for high-power lasers. Rob van den Berg investigates.

From Opto & Laser Europe November 2001

The numbers 1064 and 532 need no introduction to a laser physicist. The fundamental and doubled wavelength of the Nd:YAG laser are as well known to them as the values of p and e are to a mathematician. But neodymium is no longer the top choice for all applications - fellow lanthanide ytterbium has much more favourable properties for use in a variety of laser systems.

Until recently, ytterbium could not compete with neodymium because it is less suited to pumping with flashlamps and, owing to its quasi-three-level laser transition, it requires high pump intensities. The advent of high-brightness diode lasers in the last 10 years, however, has removed these disadvantages, because ytterbium's absorption maximum is near 940 nm - perfect for pumping with InGaAs diodes. As a result, Yb:YAG laser output at its fundamental wavelength (near 1030 nm) has increased from a few hundred milliwatts in 1991 to more than 1 kW today. Novel laser designs, especially the thin-disc laser, were developed to exploit the properties that Yb:YAG offers: a simpler energy level scheme, higher doping levels - up to 25%, compared with 1.5% for neodymium - and much less heat generation when pumped. In addition, its wide emission bands allow ultrashort pulse generation.

Solid-state laser performance at high average powers is normally limited by the thermal behaviour of the laser crystal. Thermally induced refractive-index variations lead to beam-shape aberrations, and in extreme cases excessive heating may lead to crystal fracture due to stress build-up. There is, therefore, hardly any alternative to YAG, which is by far the most robust crystal available and has the highest thermal conductivity and fracture resistance.

The dominance of neodymium, however, is not so assured. Pump power from laser diode arrays can now be delivered at much higher irradiances than was once possible, and this has opened up a path to lasers that rely on ions other than neodymium.

Yb:YAG emerged as a practical laser gain medium from an idea that was developed by Adolf Giesen's group at the University of Stuttgart, Germany. Giesen said: "YAG can be more highly doped with ytterbium, so we can use much thinner laser crystals and heat dissipation is less of a problem." The 100-200 µm thin disc, which is fixed to a heat sink with a high-reflection coating in between, loses virtually all of its heat via its cooled end face. Another advantage of the thin disc is that higher pump intensities are possible because the way in which the crystal's fracture limit scales is inversely proportional to the thickness of the gain medium. "At first the thin disc may seem to be a problem, because of the low single pass absorption," explained Giesen. "However, re-absorption of the laser light is equally low, which implies that the system has a low lasing threshold, and it therefore has a high efficiency of up to 70%."

Giesen developed an intricate pumping scheme in which four mirrors repeatedly reflect the pumping beam onto the disc. More than 90% of the pumping light gets absorbed through this multiple pass design. "The system is ideal for high-power applications at more than 1 kW. The thin-disc laser based on Yb:YAG offers better beam quality, greater efficiency and a higher output power than conventional lasers, such as Nd:YAG and CO₂ lasers," he said.

The thin-disc laser is currently licensed to 15 firms around the world. Nanolase of Meylan, France, was the first to bring an ytterbium-based DiskLaser to the market in 1998. Today, several companies are developing high-power versions. Giesen is working closely with Haas-Laser of Schramberg in Germany, and has installed three of its kilowatt ytterbium systems at Stuttgart's Institute for Materials Processing.

Haas recently presented a 1.3 kW prototype with two discs. According to its head of innovation management, Kurt Mann, systems for welding and cutting are expected to appear on the market early next year: "We are working on a product with the highest reliability under tough industrial conditions - high power isn't everything."

Haas's competitor Rofin-Sinar holds the high-power record, with 1.5 kW from a single disc. Marketing manager Thorsten Frauenpreiss said: "Our patented disc-cooling system provides a higher output power per disc, giving us a competitive advantage. Ytterbium is the only material that makes sense to us in these kinds of lasers. We have a prototype running in the lab, but it will probably not be available for industrial use in micromachining until next year."

As far as more conventional applications, such as cutting and welding steel plates, film marking and thermal printing are concerned, Frauenpreiss sees too much competition from industrially proven rod-pumped systems and CO₂ lasers. Giesen is more optimistic. Theoretically, he says, it should be possible to extract 10 kW from a single disc: "The key problem is fixing the crystal to the heat sink. Different host crystals have a higher efficiency and lower losses, and we think that we have a much better alternative to indium, which is normally used." Ytterbium-based thin-disc geometry is also being studied at the Lawrence Livermore National Laboratory (LLNL) in the US. In LLNL's advanced lasers and components department, Luis Zapata is leading a project that is working on a source of high-brightness laser power for anti-aircraft weaponry. "One of the crucial advantages of thin-disc geometry is that the thermal gradients are aligned with the beam propagation direction, so that wavefront distortions onto the light field are minimized," said Zapata.

Undoped endcaps are applied to the disc, which prevent the input face from bulging out under the load due to thermal expansion. "These endcaps, which are diffusion-bonded, suppress amplified spontaneous emission and parasitic lasing," said Zapata. "They increase the disc's stiffness, keeping deformations to a minimum, and also allow side pumping." Zapata's laser has produced 260 W in quasi-continuous-wave operation, and he expects this to improve.

A more conventional Yb:YAG geometry studied at LLNL is based on twin end-pumped polarization-compensated rods. Light from more than 60 laser-diode packages is guided from two sides towards the rods via hollow lens ducts. Together with the application of undoped endcaps, this results in more than 80% pump-delivery efficiency in the rods. Recently, more than 1 kW of continuous-wave output was achieved with an optical conversion efficiency of 27.5% and an M² of 1.25.

One goal at LLNL is to build a laser to succeed the Nd:glass system that is currently used by its National Ignition Facility (NIF), where laser-induced nuclear fusion is studied. The material of choice at the NIF is strontium fluorapatite (Yb:S-FAP). "It can store four times as much energy as Nd:glass," said Zapata. "It is slightly softer than YAG, but it has the right optical properties and is also a very good host for ytterbium. We grow large crystalline slabs of it, and we are currently working on a diode-array-pumped, seven-slab amplifier." Ytterbium is also making its mark in pulsed lasers. Ursula Keller's group at the Swiss Federal Institute of Technology in Zurich is trying to increase the output power of passively mode-locked systems based on semiconductor saturable absorber mirrors.

Project leader Rüdiger Paschotta said: "Ytterbium has got a small quantum defect - the energy difference between emission and absorption bands - which means that little pump power is lost as heat. It has also got a broad amplification bandwidth [10 times as great as Nd:YAG], which implies much wider tunability and enables shorter pulse generation. Mode-locked Nd:YAG systems are limited to a duration of just a few picoseconds, but the record for Yb:YAG currently stands at 340 fs." Novel tungstate host crystals are expected to make pulses of less than 200 fs possible, and in a thin-disc laser head could generate many tens of watts.

Such lasers could find numerous applications, such as in ultraviolet generation for lithography and the pumping of optical parametric oscillators. "They can be used to make displays for cinema laser projection," said Paschotta. "You can generate 15-20 W in the red, green and blue spectral regions using non-linear crystals with 100 W of mode-locked power. With short-pulse lasers and regenerative amplifiers you can drill very small holes and get much cleaner surfaces when cutting."

Time-Bandwidth Products, which is a spin-off company from Keller's group, has serious plans for commercializing these mode-locked ytterbium systems. Ytterbium really is leaving its mark everywhere - and the key Yb:YAG wavelengths of 1030 and 515 nm should soon become as recognizable as those neodymium numbers.