The Nd-YAG laser
The Nd:YAG laser is a solid-state laser, usually in the shape of a rod, operating at 1.06 mm.4 The active species are neodymium ions present in small concentrations in the YAG crystal. Both continuous wave and pulsed laser outputs can be obtained at an overall efficiency in the 3 to 5 % range. This laser is used in industry because of its efficiency, output power and reliability compared to other solid-state lasers. The crystal is grown using the Czochralski crystal growing technique5 which involves slowly raising a seed Nd:YAG crystal from the molten crystal constituents to extract an Nd:YAG boule. A single boule typically yields several laser rods. The concentration of Nd ions in the boule is carefully controlled and is no greater than about 1.1 %. Increasing the Nd doping further in order to increase the laser power produces unacceptable strain in the crystal and leads to a dramatic reduction in laser power.
Laser rods are typically 6 mm in diameter and 100 mm in length with the largest commercial size rods being 10mm in diameter and 200 mm in length. Because of the small size of the crystal Nd:YAG lasers tend to be much more compact than are CO2 lasers. Illustrated in Fig. 5.5 are the main components of a single-rod Nd:YAG laser.
Laser action is achieved by exciting the crystal optically by lamps placed in close proximity to it. The lamps have an emission spectrum, which overlaps the absorption bands of the Nd:YAG crystal at 700nm and 800nm. In order to couple the maximum amount of lamplight into the rod and to extract the maximum laser power from it, the rod and the lamp are enclosed in specially designed and manufactured cavities. The two most common pump cavity configurations are elliptical and close coupled. In the case of elliptical crosssections, the rod and the lamp are placed along the two foci, and in the case of close-coupled cavities, the rod and the lamp are placed close together at the axis. The inside surface of the cavity is normally coated with gold in order to maximise the coupling of lamplight into the rod. Some laser manufacturers also produce ceramic cavities which allow more uniform
5.5 Schematic of an Nd:YAG laser (courtesy Rofin-Sinar).
pumping of the rod but at the expense of lower efficiency (approximately 5% lower) compared with that of the gold-coated cavities.
For continuous operation, krypton arc lamps are most widely used while for pulsed operation high-pressure xenon and krypton flashlamps are used. Lamp lifetime dominates the service requirement of modern Nd:YAG lasers. For arc lamps, the lifetime ranges between 400 and 1000h while for pulsed lasers it is about 20 to 30million pulses depending on operating conditions.
Only a fraction of the emitted spectrum is absorbed by the laser crystal and the rest of the emitted light is dissipated as heat in the cavity and it must be removed for efficient laser operation. This is usually achieved by flowing deionised water around the rod and lamp in a closed loop cooling system. The loop is coupled to a heat exchanger for efficient heat removal.
To increase the laser power above 500 to 650W, typically obtained from a single rod, requires an increase in the laser volume. However, increasing the rod volume has fundamental limitations. Heat generated within the rod causes large thermal gradients which lead to variations in the refractive index, lowering beam quality, as well as large mechanical stresses, which can cause rod fracture. To obtain higher laser powers involves the use of multiple laser rods. The rods are arranged in series and located either entirely within the resonator or with some being placed outside the resonator to act as amplifiers. These configurations are discussed and described in more detail by Rofin-Sinar.6 There are now on the market several systems all giving in excess of 2kW of laser power with the highest power commercial device producing 5kW from eight cavities.7
While lamps have been an integral part of the Nd:YAG laser technology to date and will remain so for the foreseeable future because of their relatively low cost, another technology is now emerging for high power laser applications both as a pumping source for Nd:YAG lasers and as a laser source in its own right.8,9 This technology is the high power laser diode and its main advantage lies in having a very narrow spectral output compared with that of the lamp which is matched to the absorption bands of the Nd:YAG laser thus increasing considerably the efficiency of the laser system. Diode-pumped Nd:YAG lasers have much better beam quality because of lower induced thermal stresses, are more compact, require smaller chillers and have much longer lifetimes in comparison with those of the lamps. A schematic of a diode - pumped Nd:YAG laser arrangement is illustrated in Fig. 5.6 for a single rod system. Rofin-Sinar is now offering commercially a 4.4kW diode-pumped Nd:YAG laser with guaranteed 15000h diode operation.
As well as pumping, Nd:YAG rods diode lasers are now being offered as standalone units for high power surfacing and welding applications. The advantages of diode lasers include high efficiency (up to 50%), which leads to lower operating costs and small size and so makes integration into existing production systems relatively simple. Units with output powers in the 3 kW to 5 kW range are now available commercially. The main disadvantage of laser diodes is their cost, which is about US$150-200 per watt of diode power, and the lack of field lifetime data. However, regardless of these
5.6 Schematic of a single rod diode-pumped Nd:YAG lasers (courtesy Rofin-Sinar).
factors, the future for high power diode lasers in manufacturing is bright and their price will decrease as the production volume increases.