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Diversity is strength for high-power laser diodes

14 Jan 2009

Innovation is the name of the game as laser diode makers strive to open up new markets for their products. Bookham explains that the trend towards higher powers must be matched by higher brightness and lower cost.

As the global recession becomes a reality, technology companies of all shapes and sizes are under pressure to broaden their product portfolio to meet the needs of diverse markets. The photonics industry is fortunate that a myriad of applications exist for optical technologies, but the challenge for manufacturers is to deliver the performance required for each application while also lowering costs and maintaining margins.

For laser systems manufacturers, the overriding goal is to reduce the dollar-per-watt ($/W) ratio. This trend towards higher powers and lower costs has been driven largely by the fibre laser market, since laser diodes are an essential, and costly, part of a fibre laser. With sales of fibre lasers growing in the double-digit range year on year, significant investment has been made in the development of cheaper and more powerful laser diodes.

There's no doubt that further reductions in cost could propel the fibre laser market to even greater heights. The aim for fibre laser manufacturers is to compete in the multi-kilowatt power range, which is currently dominated by CO2 lasers. Such high power levels are needed for heavy-duty applications such as metal cutting, and CO2 lasers have been valued for their good performance at the lowest possible cost.

But fibre lasers have the potential to take a slice of this substantial and lucrative market if manufacturers can deliver the right technology at the right price point. Driving down the $/W ratio, and reducing the size and complexity of fibre laser systems are key targets for manufacturers.

What's interesting is that it's the diode manufacturers – both in-house at vertically integrated laser systems players and the merchant diode makers – that are making the real strides in this direction. These diode makers are not only keen to reap the rewards of the growing fibre laser market, but also to gain traction in other markets and so diversify their customer base.

One of the main market sectors now being targeted is materials processing, which includes marking, rapid tooling, rapid prototyping, hard soldering and welding. Other key areas of interest are laser cleaning, medical and dentistry procedures – including vascular surgery, pigment colouration treatments and tooth repair – and computer-to-plate pre-press printing.

Within that broad range of applications, laser diode manufacturers are particularly excited about the prospects for direct-diode systems, which propel the laser diode from its conventional pump function to the primary source of light emission. One of the largest potential markets for these systems is laser cladding – used widely in the automotive industry to add extra material layers onto existing machine tools for purposes of repair or adaptation. This fast, efficient process enables expensive capital equipment to be used repeatedly without replacement, and to be repurposed to achieve different performance criteria – which leads to significant cost savings for car makers.

Diode lasers are also gaining traction in other materials applications. Welding and hard soldering, for example, have so far been the domain of traditional laser sources, such as CO2 or Nd:YAG lasers. Now, however, diode lasers are starting to be chosen for their lower cost and smaller footprints, combined with improved wall-plug efficiencies and high process speeds.

For some welding jobs on small materials that demand precision, accuracy and a "seamless" finish, diode lasers are also finding a niche. Conventional Q-switched laser sources have become the most popular choice for these applications because of their high peak pulse powers, but the high peak pulse output can, in precision welding, cause "beading" of the seam where weld spots have overlapped. When the goal is an imperceptible seam, a diode laser with a small spot size and high power in continuous-wave operation is ideal.

Meanwhile, in computer-to-plate pre-press printing – which already represents a high-volume market for laser diodes – manufacturers often quite specifically use 830 nm single-emitter pumps, although we are now seeing some interest in 10 W (and higher) single emitters at 9xx nm.

Each of these applications may require very different laser systems with different specifications, and the task for the systems manufacturer is to meet those many and varied needs. That task is, of course, borne at least in part by the manufacturers of the laser diodes that underpin, and enable, the systems and applications.

Despite the diversity, just about all laser systems designed for the applications described here are heading in a very clear direction: smaller, simpler packages, but with higher powers and higher brightness.

Increasing power is eminently achievable and significant advances in output power have been demonstrated in the last 12 months. And yet, on its own, the extra power is not enough to meet customers' needs; it must be delivered in a way that ensures an incremental scaling of brightness, while also remaining cost effective.

Brightness is defined as the power per emitting surface and solid angle. This can essentially be thought of as the concentration of power in one spot: if there is high divergence of the beam, the brightness, irrespective of the power, is reduced.

That has been a problem for direct-diode systems, since the light emission from laser diodes is both asymmetric and highly divergent. As a result, beam shaping optics is essential in many applications to provide the required brightness. It is the quality of these optics – as well as the raw performance capability of the diodes – that is determining which laser diode manufacturers are succeeding in the direct-diode market.

An output beam with low divergence is also essential to couple the emission into an optical fibre without excessive loss. The fibres being used by systems manufacturers are decreasing in size all of the time: 100 µm fibre is now commonplace for single emitter and bar-based solutions, which makes beam shaping even more critical.

For the power from the laser diode to be used efficiently, the output beam must be symmetrical and have low divergence. The two types of asymmetry that generally arise in diode beams are astigmatism and an elliptical beam profile. Meanwhile, each axis of the beam demonstrates a different divergence angle: the typical fast-axis divergence is around 40° full-width half-maximum (FWHM), while the slow-axis divergence is around 6–8° – although both figures can be significantly higher.

The result is a large, often elongated, beam spot that features "dead spaces" of low brightness caused by the configuration of the emitter. This is referred to as beam quality, often measured as beam parameter product (BPP) – the product of the beam's divergence angle and its radius at its narrowest point (the beam waist).

The methods to combat this involve reshaping using lenses and collimating using mirrors. The long-standing technique is to use a cylindrical lens to couple each emitter into individual optical fibres arranged in a bundle, but the resulting BPP is lower than desired by today's applications. More common methods involve the use of two reflectors or step mirrors. The latter of these two is often thought to provide the strongest results, yet new and innovative solutions are being developed by R&D teams all the time.

As well as beam shaping, diode manufacturers are achieving efficient coupling by optically stacking the emitters using imaging and beam transformation technologies. In this case, a fill factor as low as 18–20% is ideal – in stark contrast to non-imaging approaches, where a high fill factor is ideal. These hugely diverse requirements serve to demonstrate the range of products that diode players are bringing to market in order to target different applications.

Further to beam shaping and imaging, diode manufacturers are getting creative with bar and pump configurations, and with cooling designs, as they strive to get more power and brightness out of smaller products. Increased power into half bars is one example; reducing bar width while increasing the fill factor is another. For many products of this nature – where the final performance is dependent upon the configuration, the cooling designs and the micro-optics – it is the performance of the single-emitter laser diodes that is of the utmost importance.

Single-emitter pump modules are now achieving output powers of more than 10 W from a 105 µm fibre, while further increases will be made as chip technologies improve and design processes are refined. At the same time, diode manufacturers are combining more than one chip into the same package. In a recent example demonstrated by Bookham, 20 W was generated from a pump module package that combines three single-emitter chips. The packaging also allows a number of different types of chip to be combined, allowing the product to be offered at wavelengths ranging from 780 to 980 nm.

It is clear that the direct-diode market is now benefiting from many years of sustained innovation in laser diodes. We are now reaching a point where the beam quality, power, brightness and cost have reached a level where existing markets are showing increased reliance on the technology, and new markets are looking away from conventional laser systems.

Diode lasers are also highly suited to the economic climate. Above all else, diode lasers offer cost-effective alternatives to existing technologies, and may even enable cost savings on existing equipment through innovative cladding applications. For high-power laser diode manufacturers, this is a crucial time for developing more flexible platforms that can bring high-performance products into a wide range of applications, and to develop the broad customer base that is so important for long-term commercial success.

• This article originally appeared in the January 2009 issue of Optics & Laser Europe magazine.

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