It’s been more than 50 years since Theodore Maiman demonstrated the first laser. His device used synthetic ruby as the gain medium emitting a deep red laser line at 694nm. Solid-state laser technology was born and since then numerous active laser materials have been developed for different wavelengths. Ruby, a sapphire, is still used today in areas like tattoo removal and holography.
The fact that Maiman’s laser was based on synthetically grown ruby is noteworthy – man-made crystals are a stable source of material and are uniform in quality. Ruby is typically grown by the Czochralski method, a common crystal growth process used for many materials, including Nd:YAG, as well as silicon for the semiconductor industry.
‘The Czochralski process is the most common method used to grow Nd:YAG,’ states Scott Griffin, director of sales and marketing at US-based crystal grower Northrop Grumman Synoptics. The company has around 100 Czochralski growth stations, as well as hydrothermal stations for growing KTP.
Czochralski is a high temperature growth method – Nd:YAG is grown at around 2,000°C – whereby the rare Earth metals are melted in an iridium crucible. A seed crystal is then dipped in the melt, rotated until the boule reaches the required diameter, and then pulled out until it reaches its given length. Nd:YAG boules produced at Synoptics are between 76mm and 100mm in diameter and around 250-350mm in length. The process takes around eight weeks.
Synoptics produces a standard grade and a trademarked OPTO-lase grade of Nd:YAG. The difference between the two is the wavefront distortion of the material, with the standard grade providing around 1/2 fringe or 1/4λ per inch of crystal, whereas the OPTO-lase grade is significantly better at around 1/8 fringe or 1/16λ per inch. The wavefront distortion will ultimately affect the beam quality.
According to Andrew Turner, sales manager at laser crystal distributor Roditi International, manufacturing a high quality boule is firstly about choosing high quality, pure raw materials, and secondly the way in which the growth process is controlled. ‘What you are making is a very large single crystal of material and to achieve that without any problems within the boule itself takes a great deal of control.’
Different materials will have different growth periods – the sapphire-based materials and vanadate tend to grow quicker, while YAG materials take longer. Nd:YAG, in particular, is a very slow growing crystal. It needs to be grown slowly, because the neodymium ion is a lot larger than the yttrium ion, for which it is a substitute. ‘The material has to be grown slowly enough to keep the concentration of neodymium next to the crystal at the average concentration in the melt,’ explains Paul Collins, president of Laser Materials Corporation (LMC).
Constitutional supercooling occurs if the material is grown too fast, which results in an unwanted build-up of neodymium in the region next to the crystal.
LMC, based in the US, manufactures Nd:YAG crystals, as well as ruby and CTH:YAG (chromium, thulium, holmium-doped YAG). Its products are also distributed in Europe by Roditi International.
‘The upshot of this slow growth is that a very stable environment has to be maintained over a long time period,’ continues Collins. ‘There also has to be no fluctuations in the growth, because of constitutional supercooling. The key thing in growing high-quality Nd:YAG is having a very stable environment. This involves ensuring the cooling water, the air temperature and the heat source remains constant over that growth period.’
Because of the requirement for slow, stable growth, Nd:YAG crystals 15-20 years ago were much smaller, half the size of today’s crystals, according to Collins, which now measure 100mm in diameter or larger.
‘The increase in size has economic implications, but it also improves the quality of the crystal,’ says Collins.
The way in which Nd:YAG grows means there is a core region that’s unsuitable for use. That core affects the surrounding material in a small crystal, whereas the region between the core and the useable material is optically significantly better in larger crystals, Collins says. This makes a big difference in the quality of the beam.
Hydrothermal growth
Hydrothermal growth, whereby crystals are fabricated in an autoclave under high temperature and pressure, is the preferred method for growing materials, such as the nonlinear optic, KTP.
The hydrothermal process is a solution-based crystal growth method that’s modelled on the way crystals grow naturally – all gems except for diamond grow in this way in the Earth’s crust. The process begins with an aqueous solution exhibiting a thermal gradient.
Crystal salts are dissolved in the hot zone and move to the cooler zone by thermal convection. As the salts are transported, they shift from being saturated to super-saturated and are deposited on the seed crystal.
The flux method, in which all the salts are placed in an open crucible and heated up, is a third crystal growth process, which can also be used to produce KTP. ‘Both methods have their advantages,’ says Griffin of Northrop Grumman Synoptics. ‘The flux process allows much larger crystals to be grown, whereas hydrothermal is a much cleaner process as the growth is more linear.’
Dr Henry Giesber, principle scientist at US crystal grower Advanced Photonic Crystals, explains that it’s generally accepted that the hydrothermal method produces higher quality material than the flux process, although it is marginally slower.
Advanced Photonic Crystals (APC) runs hydrothermal growth stations producing a variety of crystals from nonlinear optics to laser active crystals. The company can produce a large KTP nonlinear boule yielding up to 120 pieces (3 x 3 x 5mm) for second harmonic generation in around eight weeks.
The hydrothermal method is also suitable for growing crystals of very refractive oxides, which are difficult to fabricate by other means. Materials like scandium oxide or lutetium oxide, which would both be good high-power laser host materials because they have high thermal conductivity, can be hydrothermally grown at high purities. Due to the nature of the molten oxides, the iridium crucible in which they are heated with the flux method is attacked, resulting in a crystal impregnated with iridium. This reduces the material’s thermal conductivity and its laser host properties. The hydrothermal process, on the other hand, allows the material to be produced at 600°C in a solution at high purities.
‘APC provides crystals for military and medical applications, but we’re really interested in advancing commercial applications,’ says Dr Jack Egan, founder and CEO of Advanced Photonic Crystals. ‘The solid-state laser industry, which has been around for 50 years, has reached a a plateau, because everyone wants Nd:YAG – that’s what people know and that’s what they want. What we’re trying to do is develop new materials for industrial lasers, which will be far more efficient and give industrial laser users higher power lasers. I view the hydrothermal crystal growth method as a way to enable this.’
Size matters
According to Griffin, the main advance in the growth processes has been centered on the size of the boule. ‘There are advantages to this, both in terms of productivity – in that the larger the boule the more rods can be extracted – and in terms of cost,’ he says. ‘In addition, the larger the boule the higher the quality of the crystal, as any wavefront distortions are spread over a greater area – Synoptics can offer the OPTO-lase high-grade material with low wavefront distortion due to the increases made in boule diameter.’
Wavefront distortion provides an indication of the optical quality of the material – if there is any strain in the crystal the light will have greater divergence, because of inhomogeneities in the index. Modern YAG has a very low wavefront distortion.
Producing high-quality laser crystals is reliant on the level of control in the growth process, as well as the sites in the boule from which the rod is selected. ‘There are going to be areas of differing quality throughout a boule,’ states Griffin. ‘Crystals are not perfect and it’s a question of selecting the material within a boule that meets the specification.’
Turner of Roditi says that customers are looking for consistency in laser crystals: ‘If a customer buys an Nd:YAG crystal at a particular size and concentration and then orders another batch at the same specification a year later, they want the crystals to be the same. Consistency of material is very important and that comes back to the control of the growth process.
‘The crystal is the gain medium; it is the medium that gives the lasing action, so it’s absolutely critical,’ he adds.
According to Turner, laser manufacturers generally have their own bespoke laser cavity designs and require bespoke laser rods. ‘You would think that after 50 years of the laser there would be standard designs and sizes of crystals, but this isn’t the case. As manufacturers push the boundaries of their cavity designs to increase efficiency, it becomes increasingly important that the crystals supplied for that cavity are consistent.’
There are also variations in the concentration of dopants in the crystal depending on the laser’s intended use. The concentration of neodymium in an Nd:YAG crystal, for instance, will vary depending on the final application.
High concentrations of neodymium cause the lifetime of the lasing medium to drop, which reduces the lasing efficiency. With short pulses, a short lifetime matters less. A CW laser crystal, or a long pulsed laser uses less neodymium to avoid that and maintain a high lasing efficiency. ‘Neodymium concentration can be very important in achieving the desired output power,’ says Collins of Laser Materials Corporation.
Crystal growers are still developing their processes in terms of growing larger boules with better yields, lower costs and higher quality. There will also be variation in the crystals supplied to different market segments. These advances are set to continue, as well as the development of processes for manufacturing more novel laser crystals.
