David Robson kicks off a new series of articles on the different types of laser with a focus on solid-state rod lasers
Solid-state lasers and their applications are more ubiquitous than you might imagine; if it weren’t for micromachining using modelocked solid-state lasers, mobile phones would still be the size of bricks. There are three kinds of solid-state lasers: rod (which we will be dealing with in this feature), thin-disc, and fibre. Diode lasers also use a gain medium in their solid state, but they are considered a different type of laser by most engineers and scientists.
Rod lasers are the simplest lasers you could find, and are usually applied to rough-and-ready applications. They can be compact, and have a high power output. They consist of a cylindrical emitter contained in a solid lattice, such as crystal or glass, and are pumped with energy from either an electrical, or a light source. The way they are pumped is very important; they can be pumped transversely (through the sides), or longitudinally (end to end) by a diode, which produces a rounder beam profile useful for more delicate applications. By aligning the pumping zone with the cavity mode, it is possible to use the whole length of the crystal optimally, giving a high pumping density and high efficiency.
This kind of pumping does, however, have its difficulties. The harder you pump, the hotter the crystal gets, which can change the refractive properties of the medium. This effect is called thermal lensing, and does not give such a good beam profile necessary for many applications, although changing the doping of the material can reduce this effect. The engineering surrounding the laser design is also very important to remove heat and prevent this.
There are three main types of rod lasers: continuous wave, q-switched and modelocked, each with their own unique applications. Continuous wave lasers are now dealing with many applications typically associated with gas lasers. For example, where previously Argon lasers were used to pump Titanium-Sapphire lasers, Coherent now uses its solid-state Verdi green laser for the same purpose.
Laser Quantum specialises in producing green, 532nm, continuous-wave lasers with an M2 parameter of 1, meaning they have perfect diffraction. Rod lasers ideally suit these specifications, as the mode can be constructed very carefully by the cavity design. It may seem unlikely, but such lasers may be used in artificial insemination in the near future. Laser tweezers are a sophisticated way to manipulate microscopic components by focusing laser light onto an aqueous solution containing components that are between 1 and 10µm in size. Dr Lawrie Gloster, the marketing supervisor at Laser Quantum, says: ‘A very good M2 is required to give a good focus of the laser beam. At the research level groups are doing clever things with ceramic spheres – but at the moment its main employment is in applications where human contact would contaminate the sample, such as biopsies, or insemination.’
They are also used in Raman spectroscopy, and for the illumination of samples in DNA sequencing. ‘Since the human genome was studied there is now a move to commercialise DNA sequencing so people can have their whole genome sequenced,’ says Gloster. ‘Due to the nature of human biology, it is very important to do this quickly, with high flow rates and DNA sampling.’ It has been anticipated that this will be needed in the medical industry, such as for the screening of genetic diseases.
The benefits of these continuous wave rod lasers are not limited to biology; they are also used in holography and shearography. The applications of holography are as diverse as data storage, advertising, and the 3D modelling of dentures.
Q-switched lasers give high-energy pulses of light, and are especially useful in micromachining. The way they work can be explained by the analogy of a man with his foot on a hosepipe, who then takes it off, in rapid succession, allowing a greater burst of energy in a short time, typically tens of nanoseconds. The high-intensity pulses are commonly used to drill holes that connect the multiple layers of circuit boards, with higher yield and efficiency than the mechanical means used in the past. ‘Mechanical drills cannot achieve the required hole diameter, so we use lasers to drill the micro-vias,’ says Terry Hannon, director of the DPSS business group at Coherent. ‘The advantages are that you don’t need to replace a blade, or sharpen it. It produces a steady output with no vibration, with no wobble. Mechanical blades may cause chipping and cracking in the silicon as the thickness is reduced.’
For this kind of materials processing, 355nm, which is in the UV range, is the ideal wavelength, at a rate of 100kHz. The average power for this kind of laser is now 20-30W, but when Coherent started producing them in 1998, it was just 1.5W. ‘This improvement was driven by applications,’ says Hannon. ‘We make reliability and performance our focus, so we understand the technology at a deep level.’
Q-switched rod lasers are similarly used in hard disk texturing, to create bumps in the aluminium hard drive that can reduce friction. In the past diamonds were used for this purpose, often unsuccessfully. The lasers are also used to write tiny code markings, such as serial numbers, on the glass in PDAs and TV screens for improved traceability necessary for quality standards. More unusual applications include the detection of biohazards, such as airborne anthrax, and in flow cytometry systems that are used to analyse a patient’s blood and detect the HIV virus.
Modelocked lasers work in a similar way to q-switched lasers, but they have a much higher repetition rate, with periods so small (fewer than 10ps) that they can almost be considered continuous wave. These lasers are also used in silicon processing, this time to write directly onto the circuit board. ‘The high accuracy of modelocked lasers has made the shrinkage of phones and the increase in technology possible, and allows manufacturers to include video, photography and MP3 playing capabilities,’ says Hannon.
Arnd Krueger of Newport Spectra-Physics has an example of how modelocked lasers are also used to select the sex of calves. The bulls’ sperm are treated with a dye that binds differently to X and Y chromosomes, giving different fluorescence responses when activated by a UV laser. Charged plates would then deflect the sperm according to what chromosome it carried. The lasers are used similarly in confocal microscopy, where the samples are stained with different dyes, then excited, and then scanned to take a 2D picture. Krueger says: ‘You would use different wavelengths for different dyes, and a mixture of rod and diode lasers would be necessary for this.’
The Spitfire Pro XP from Newport Spectra-Physics, ideally suited to precision material processing
John O’Connor, marketing manager at Photonic Solutions, says: ‘A neat application is stereolithography, or rapid prototyping for small, high-cost ergonomics packages such as MP3 players or mobile phones. With a rapidly changing production line, you need to make the prototypes very quickly, which would not be possible using mechanical means. You need a very high beam quality and stability to make sure the design is correctly reproduced.’ A 355nm UV beam is scanned across resin, curing it according to the CAD design. According to Krueger, BMW uses this method to make prototypes of cars in a matter of hours.
Beam quality and stability are obviously important for all these applications. It would be easy to assume that thin disc and fibre lasers, which suffer less from thermal lensing, would now be the lasers of choice for many applications. However, none of the engineers consulted in the course of this feature seem to agree with this. It appears that rod lasers are already perfectly suited to what they’re used for. Dr Lawrie Gloster believes that development is currently focused on improving what we already have. ‘Innovation is coming in the engineering surrounding the lasers, to make them smaller, more efficient, and more powerful, while still keeping the costs low.’ It seems we’ll still be making use of the many varied applications of solid-state lasers for a while yet.