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Lamp-pumped solid-state lasers offer bright future

21 Jun 2007

Neil Ball and Matthias Stein argue that rapid improvements in performance and lifetime will ensure that lamp-pumped solid-state lasers continue to outsell diode-pumped systems.

The humble continuous-wave (CW) lamp or flashlamp provides the lifeblood for the majority of industrial solid-state lasers, but it is rarely afforded the importance it deserves. While the spotlight of attention has fallen on diode-pumped lasers for some years – with market interest fuelled primarily by their lifetime and stability – lamp-pumped lasers have been quietly outselling diode-pumped systems year after year (see figure 1). Such sustained success can be explained at least in part by continuing innovation by lamp manufacturers, which in turn has delivered significant performance improvements.

For a start, improvements in lamp technology have helped to increase the power output from industrial lamp-pumped lasers, with the result that flashlamp-pumped lasers are able to produce much more powerful bursts of energy than diode-pumped systems. While the average power of an industrial lamp-pumped laser was 20 W in the 1970s, and 500 W in the early 1990s, a modern laser lamp can easily achieve a peak power of 100 kW and average power levels of 1 kW. Such high-power outputs are crucial for material-processing applications such as high-power drilling, cutting and welding, where the process speed depends on the energy input.

Lamp-pumped CW lasers also continue to thrive in areas where precision and robustness are important. One example is diamond cutting. Lamp-pumped lasers operate reliably in this application every day under harsh conditions in India and Africa. Due to the high price of the raw diamonds, the lamp output must be highly stable to avoid any unnecessary waste.

Important improvements have also been made in the lifetime and stability of lamp-pumped lasers, so much so that it's worth taking a second look at the cost of ownership of these systems. A useful example is provided by a CW lamp designed for automotive applications. About 10 years ago, a lamp that lasted for 250 h would have been considered as offering good lifetime. Improving the cathode material has doubled the lifetime to 500 h, while more recent advances (discussed in more detail below) have further extended the lamp lifetime to 2000 h under the same operating parameters.

Similar progress has been recorded for flashlamps. For example, Heraeus' NextGen flashlamp series, introduced in 2006, has boosted the typical lifetime from 0.5 million to 3–4 million shots under harsh long-pulse, high-transfer conditions. At high frequencies and lower powers, a lifetime of over 40 million shots has been achieved. This means less downtime for changing the lamp, and it also improves the cost of ownership of the overall laser system.

Improvements in cathode design
To understand the impact of cathode design on lamp performance, let's first look at the typical failure mechanism of a laser lamp. During operation, the cathode can reach temperatures of more than 2000 °C, while the temperature gradient can reach 10,000 K/mm. Such extreme temperature changes lead to sputtering of cathode material onto the wall of the lamp, and eventually the tip of the cathode can crack or break up.

This sputtered material blackens the internal wall of the lamp, which in turn reduces the light output from the lamp and consequently the laser output. If the lamp is not replaced, blackening increases further and at some point the internal wall absorbs so much energy that the quartz material that forms the body of the lamp fails completely.

Heraeus addressed this problem in the 1980s with a cathode design that incorporates a heat choke. This so-called HiCharge design allows the electron-emitting surface temperature to be controlled, and also reduces temperature variations. Combined with a new processing technique for the cathode material, this work doubled the lifetime of many lamp types.

Optimizing lamp manufacture
It is conventional wisdom among engineers that it's not only the design of the product that impacts on performance, but also the way that it's made. For a laser lamp, this means reducing tolerances and optimizing the quality of critical lamp elements such as gas fill, the seal and the cathode. Heraeus Noblelight is unique in the laser-lamp industry in that it achieves improvements in these areas through automation.

For years, laser lamps had to be made by hand because of the complexity of the processes involved. Skilled lamp makers could achieve very good quality, but we have made further progress by automating and optimizing key steps in the production process.

First of all, lamp-to-lamp tolerances become smaller when key manufacturing steps are automated. For the laser user, this means less set-up time after changing a lamp and more repeatable laser performance from lamp to lamp.

Secondly, the process on our automated line makes it possible to produce a stronger seal, which is needed to ensure the hermetic structure of the lamp. The seal must be able to withstand temperatures of 250 °C long term and 600 °C short term, as well as large mechanical stresses. A weak seal could break under these harsh conditions, which results in leaking and failure of the lamp.

Tighter tolerances on the shrinkdown also improve the critical thermal management of the electrodes, which again has a positive impact on lifetime. And last but not least, a new pumping process increases the purity of the gas used to fill the lamp, which again extends the lifetime of the lamp.

All of these improvements have delivered substantial increases in lamp lifetime. Also, they demonstrate the capabilities of lamp technology to deliver performance improvements that will keep lamp-pumped lasers competitive for many years to come.

Besides process-related benefits, a lamp-pumped system also offers real economical advantages. For a start, lamps are more temperature tolerant than diodes, which allows the use of simpler cooling mechanisms. The replacement costs for lamps are also lower than for diode bars.

For a lamp-pumped laser, one leading laser-system manufacturer estimates that the service and lamp costs in every 2000 h maintenance interval is around 3% of the laser purchase price. In contrast, the maintenance cycle for a diode-pumped laser may be more than 10,000 h, but replacing the diodes can cost up to 80% of the laser purchase price.

Progress undoubtedly remains a part of daily life in the lamp-pumped laser industry, and lamp companies that continue to invest in R&D and modern manufacturing will continue to drive this progress.

• This article originally appeared in the June 2007 issue of Optics & Laser Europe magazine.

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