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State-of-the-art diode-pumped lasers in the low to medium power range now provide greater reliability than a light bulb in addition to providing turnkey simplicity, superior beam quality, and rugged, compact construction. In this article we examine how advances in laser design deliver these benefits, and briefly discuss industrial applications that rely on these lasers.

End-Pumping
A diode-pumped laser consists of a crystal or neodymium-doped material (e.g. Nd:YAG, Nd:YVO₄) that is optically pumped by a laser diode or laser diode array. One potential obstacle to using diode arrays is that the light is emitted from a series of facets arranged in a line or two-dimensional matrix (laser stack). One solution to this problem is to use a configuration where the extended output of the diode array is coupled to a fiber optic array that is then reshaped as a cylindrical bundle. This allows the pump light to be coupled into the end facet of a rod of laser crystal using simple optics. A pump light that is well matched spatially to the TEM₀₀ volume of the laser crystal ensures good beam quality. Just as important, this configuration allows a module to be located in the power-supply rather than the laser head. This makes module replacement trivial, eliminating the need to realign the laser head or any of the downstream optics.

Remote placement of the pump lasers has also enabled manufacturers to perfect the concept of the permanently sealed laser head. To completely implement this approach, it is also necessary to use long-lived optical components, permanently mount and assemble the components in a monolithic structure, design this structure to be thermally stable, and build the laser in a cleanroom atmosphere.

 

One way to achieve higher output powers with diode-pumped lasers
is to use a Periodic Resonator, as in Inazuma™ from Spectra-Physics.

In the latest lasers, the mechanical assembly also uses a novel approach to deliver cavity stability. The most rugged lasers produce an output consisting of multiple-longitudinal modes. This minimizes power fluctuations, is relatively insensitive to minor changes in cavity length and allows for stable frequency doubling. A very successful design is to arrange the optics in a Z configuration on a monolithic base formed by an aluminum I-beam. Because of the excellent torsional stability, any thermal expansion merely changes the cavity length. The mode quality and beam pointing direction are completely unaffected. Consequently, there is no need for special cooling measures such as heavy heat sinks. In addition, the optics are rigidly mounted using precision mounts. The use of bonding agents, such as epoxies, is completely avoided since these could outgas and affect laser performance over time. The end result is a compact laser head that can be operated for many months, or even years, without ever removing the cover.

Product Diversity
For end-pumped lasers containing a single Nd:YVO₄ (or Nd:YAG) rod, typical performance levels reach several watts of average power. However, one of the advantages of diode-pumped lasers is their design flexibility. These lasers can be designed to provide continuous wave (CW) output for applications such as inspection, where high peak power is not required. But a more common configuration is to incorporate a Q-switch in the laser cavity so that the output consists of short (< 15 nsec) pulses. With pulse repetition rates adjustable from 1 - 200 kHz, this results in peak powers as high as 30 kilowatts, enabling materials processing. In the past year, passively mode-locked industrial models have also reached the market. Here the output is characterized by very short pulse duration (<100 femtoseconds) and very high repetition rates (10's of MHz). These lasers are useful for precision materials processing, where the ultra short pulses virtually eliminate peripheral thermal damage.

This technology is also flexible in terms of scaling up the average output power. One way to do this is to use a so-called Periodic Resonator, where two laser rods operate in series in a single laser head. Each rod is end-pumped at both ends using the output of a pump module. As with lower power lasers, the output can be in a CW or Q-switched format.

It is not economically practical to further extrapolate this process by ganging multiple lasers in an industrial laser. A practical way to reach higher output powers is to pump the laser rod with the output of a high power, two-dimensional diode array. Once again, the problem becomes coupling this pump light into the end of the laser rod in a way that allows a sealed cavity head and simple field replacement of the pump module. One approach to this problem is to couple a factory-aligned laser diode stack to an optical funnel which delivers the pump energy into the end of the Nd:YAG rod. The pump light then enters the cavity through a window that eliminates the need to unseal the cavity. Moreover, factory alignment provides simple physical registration for field replacement with no need to optimize alignment, and no shift in the laser output beam. An example of this type of laser is the Tornado from Spectra-Physics, which delivers over 50 watts of output in either CW or Q-switched formats.

Flat panel displays require the production of repetitive and accurately registered
patterns of T shaped electrodes, which are produced by laser ablation of a thin
layer of transparent oxide. Image courtesy of Exitech.

Industrial Applications
Thanks to a range of output powers and flexible formats, diode-pumped infrared lasers have found diverse applications in a number of industries. They are used for marking metal parts, drilling small holes in medical devices, inspecting products, fabricating circuit boards, and direct-to-plate printing, to name just a few applications. To see the utility of these lasers, it is useful to look at a couple of very different applications - welding plastics and patterning electrodes in flat panel displays.

According to Ric Allot, R&D group leader for Displays at Exitech, "There is a fast growing need to pattern the transparent electrodes for flat panel displays and solar panels, an application for which diode-pumped infrared lasers are particularly well-suited." According to Allot, Exitech uses a novel 'bow tie' writing method for this application that combines galvanometer scanning with repetitive use of a photomask.

The transparent electrodes are formed of a thin layer of tin oxide, or indium tin oxide, with a typical layer thickness of 100 nm. In flat panel displays, it is necessary to produce a large number of densely spaced electrodes, which each have a characteristic T shape. These are produced by depositing a uniform oxide layer and then using laser light to remove the material around the T shape. The Exitech system uses a single T shaped photomask in conjunction with a Q-switched laser. This mask pattern is projected onto the work surface via scanning galvanometers. The part to be worked is moved using a continuous motion translation stage. Allot explains, "This combination of galvanometer scanning and part translation delivers the fast throughput demanded by the economics of this application." He also points out that, "It is essential to use diode-pumped lasers in this application. The typical customer uses many workstations around the clock. Traditional, lamp-pumped lasers would not be economically viable because of the down-time, expense and hassle associated with replacing lamps in so many lasers."

Diode-pumped lasers are slowly starting to find applications in plastics welding as a fumeless, precision alternative to bonding, where the bonding agent (glue) often produces toxic fumes during curing. Kevin Hartke, sales and marketing manager at the Mound Laser & Photonics Center, explains that, "A common technique is transmission welding. Here, we typically look to join two different plastics — one transparent at the laser wavelength and one that absorbs at that wavelength. The laser beam travels through the transparent plastic and is absorbed at the interface by the 'dark' plastic, causing local melting of the thermoplastic. Typical materials might be clear polycarbonate and carbon black-filled polycarbonate." Hartke noted that new, alternative materials such as Clearweld™ could also be applied to the interface of two transparent plastics with the same result.

The joint is held under physical pressure, to avoid expansion during the process time, which varies from milliseconds to 1-2 seconds. Overall weld speeds as high as hundreds of meters/second can even be achieved under optimum conditions. In addition, the use of beam scanning enables very complex or extended welds to be created. Just as important laser welding produces little peripheral damage, in comparison to ultrasonic welding. This is a major reason for the use of laser welding to seal delicate electronics in small enclosures, the most famous example to date being the Keyless-Go card used for remote entry with many Mercedes automobiles.

In conclusion, diode-pumped technology has taken industrial lasers to a level of reliability and operational simplicity that was unthinkable a few years ago. As a result, these lasers now support a broader base of applications than any other single type of laser.

This article was contributed by Dafydd Thomas and Michael Watt. Both authors are product managers in the OEM Business Unit of Spectra-Physics (Mountain View, CA). Dafydd Thomas can be contacted at dthomas@splasers.com. For more information on Spectra-Physics, visit the company's web site at www.splasers.com.