By Silke Pflueger
Invented two short years after the ruby laser in 1962, diode lasers are now taken for granted in many areas of our lives. You use them each time you pick up the phone, use a DVD or Blu-ray Disc, print or are at the grocery store checkout. All in all diode lasers make up a market of more than $3 billion, 50 percent of the total laser market.
Of course all these are very low power applications, with power levels in the milliwatt range, not quite usable for cutting or welding. Getting to higher power levels was driven mostly out of the need for better pumps for solid state lasers starting in the 1980s. Lawrence Livermore National Laboratory demonstrated a 1.45 kW stack in 1990, with high power laser bars cooled by silicon microchannel coolers. The late 1990s saw the first companies producing high power diode lasers for direct use in industrial applications. Limited by their brightness, these diode lasers were mostly used for plastic welding and heat treatment.
The main drivers to go to direct diodes are higher efficiency, fewer components, a choice of wavelength, and more compact and reliable laser systems. Several companies are building direct diode lasers based on diode bars and reshaping optics for applications such as heat treating, cladding and brazing. To access the cutting and welding market, the beam quality has to be increased, by accessing the full brightness available from the diode emitters.
To get to higher brightness, we have based DirectPhotonics’ lasers on single emitter diodes. Each of those typically delivers 10-15 W, and requires a low drive current of up to 15 A. The low current can easily be switched with less than 15 µs rise time, and due to the low current, cost effective power supplies are available. Also, single emitter chips are established components, which are available from various suppliers at many different wavelengths, with exceptional reliabilities.
The challenge is to combine enough of those single diodes to generate multi-kW systems and to preserve the brightness of the diodes. We have licensed a technology developed by the German Fraunhofer Institute for Laser Technology and its US subsidiary that allows doing so efficiently.
Several steps are being used to overlap the light out of the individual diodes: First, they are optically stacked, and then many slightly different wavelength lasers are overlapped with the help of gratings and thin-film filters. Both steps are key to this technology. The optical stacking is automatically performed in a pick-and-place machine, allowing for cost-effective and reliable manufacturing. The dense wavelength combining allows adding power to the laser beam without losing the ability to focus the laser to a tight spot necessary for metal processing.
Our DirectProcess lasers, tailored for material processing, have power levels of 500 W, 1 kW and 2 kW ex-fiber, and are scalable to higher power levels. The beam parameter product for all these power levels is 7.5 mm*mrad, which makes them almost 3 times as bright as the diode lasers built with legacy technology, and enables them to compete directly with fiber and disk lasers in many applications.
New ultra-high brightness diodes, enabled by advances in semiconductor and packaging technology, are well on their way to become the new standard lasers in the 1 µm wavelength range. With a good enough brightness to tackle most common metal manufacturing jobs, it will ultimately be their efficiency that will turn them into the leading lasers in the market. While immediate energy savings may not be large, the higher efficiency causes an entire slew of advantages: Diode lasers require fewer and simpler power supplies, smaller chillers, a less complicated optical design, all leading to reduced cost, both for the operation and the original investment and a much smaller footprint.
Dr. Silke Pflueger is the General Manager for DirectPhotonics Inc.