Semiconductor lasers JULY 2007

Transcription

1 Semiconductor lasers JULY 2007

2 MARKET ANALYSIS TECHNOLOGY FOCUS Acquisitions galore in the diode sector The growing importance of high-power diode technology for materials-processing applications has fuelled a series of acquisitions in the area. Oliver Graydon Nature Photonics, Chiyoda Building, 2 37 Ichigayatamachi, Shinjuku-ku, Tokyo , Japan o.graydon@natureasia.com It s been an interesting few months for the laser-diode sector, with two wellknown names in the laser business Rofin-Sinar and Coherent investing heavily in the technology through a series of acquisitions. First up was the news in March that Rofin-Sinar was acquiring 80% of the stock of m2k-laser, a spin-off, founded in 2001, from the Fraunhofer Institute of Applied Science in Freiburg, Germany. M2k-laser has developed proprietary high-power tapered laser-diode and amplifier technology, which enables the creation of diodes that emit several watts of diffraction-limited output power in the infrared. Examples include 6-W GaAs tapered diodes that emit at a wavelength of around 1 μm and 2-W GaSb diodes that emit at around 2 μm. Applications for the firm s products include pumping solid-state lasers, materials processing and medical procedures. The remaining 20% of the stock of m2k-laser is held by the Fraunhofer Gesellschaft and the founding members of the company. The acquisition will help bolster Rofin-Sinar s diode-component business and give it valuable access to high-power technology for pumping its own solid-state lasers and directdiode-based materials-processing applications. We see this acquisition as an opportunity to strengthen our technology base and our market position in the component sector, said Günther Braun, Rofin-Sinar s CEO. Then just three weeks later in April, the news broke that Rofin-Sinar was purchasing Corelase, a Finnish developer of fibre-coupled laser diodes and diode-pumped fibre lasers for materials processing. Founded in 2003, Corelase has been busy developing a suite of solutions for a wide range of applications, ranging from welding and soldering to cutting and drilling. Today its product range spans from W Günther Braun, the CEO of Rofi n-sinar Technologies: We see this acquisition [of m2k-laser] as an opportunity to strengthen our technology base and our market position in the component sector. fibre-coupled laser diodes, in both raw form and integrated into a system, through to a 20-W-average-power megahertz-repetition-rate mode-locked fibre laser that emits picosecond pulses. However, Rofin-Sinar is not alone in investing in laser-diode technology. In April, Coherent announced that it had acquired Nuvonyx, a developer of high-power diode arrays that emit up to 50 kw of power allegedly the most powerful commercially available (see Technology Focus, Business News, page 390). The diodes are powerful enough for direct application in materials processing, without the need to pump another laser. The move signals Coherent s attempt to muscle in on the high-power materials-processing market a sector that is set to grow and is traditionally dominated by laser firms such as Rofin-Sinar, Trumpf and GSI Lumonics. Interestingly, the two deals described above are not the first acquisitions of ROFIN-SINAR diode technology that Rofin-Sinar or Coherent has made. Both firms made acquisitions in the area about ten years ago, with Rofin-Sinar purchasing a major stake in the German firm Dilas, which is one of the leading high-power diode makers today, and Coherent purchasing Tutcore of Finland, which specializes in high-power GaAs lasers. So the obvious question is why all these most recent acquisitions in diode technology? According to Michael Lebby, president of the Optoelectronics Industry Development Association (OIDA) in the USA, it s a case of gap filling, with firms completing product portfolios in order to help them compete and win market share. In general, these players and others, Trumpf for example, probably now feel that they need to have all the tools in the [laser] toolbox going forward, and those tools are fibre, solid-state, CO 2 and diode technologies, Lebby told Nature Photonics. In general as [diode] materials and packaging technologies improve and powers increase, as they have done in the past year or two, we can see a trend of the low-power end of the [materials-processing] market turning increasingly to diode-based solutions. Although heavy-duty cutting and welding of thick metal will probably still remain the territory of CO 2 and Nd:YAG lasers for some time, laser diodes are increasingly being used for less-demanding tasks, such as surface treatment, micromachining and processing of thinner samples. It seems then that big players in the laser materials-processing sector who initially built very strong business around non-diode laser technologies, such as gas lasers and flashlamppumped solid-state lasers, are now heavily investing in diodes and other complementary technologies. For example, Rofin-Sinar and Trumpf, two specialists in CO 2 and thin-disc solid-state lasers, are now investing in diode and fibre-laser technology through both in-house R&D and acquisitions. nature photonics VOL 1 JULY

3 INDUSTRY PERSPECTIVE TECHNOLOGY FOCUS LASER DIODES Pump up the power There is keen interest in extending the use of laser diodes beyond traditional low-power devices. Now the techniques needed to increase the achievable output power have become available. Joerg Neukum DILAS Diodenlaser GmbH, Galileo-Galilei-Strasse 10, Mainz, Germany Laser diodes are a staple of many gadgets around us, including CD and DVD players and computer drives, laser pointers and the lasers that are used for transmitting telecommunications signals. What they offer is low electricalpower consumption, a long lifetime, high stablility when operated in continuouswave mode and the ability to be directly modulated by electrical current. Despite the benefits they bring, these lasers typically only produce optical power in the milliwatt range. By boosting their optical power a thousand or a million times, laser diodes could become useful in many applications. But this in turn calls for a new set of manufacturing and cooling techniques, as well as qualification and testing technologies. Laser diodes are tiny devices containing an active semiconductor medium formed from a p n junction and are powered by injected electrical current. The semiconductor wafer is produced using conventional manufacturing methods molecular-beam epitaxy or metal organic chemical-vapour deposition. However, special emphasis has to be placed on achieving a high electro optical efficiency (typically about 50%) and designing a device that can resist the much larger currents and increased optical fields associated with high-power operation. Moreover, if laser diodes are to satisfy a variety of applications ranging from materials processing to pumping of solid-state lasers, medicine, printing and military work, they must produce light with a broad range of wavelengths. Once semiconductor wafers have been fabricated, laser diodes are arranged into arrays in which the wafers are cleaved into pieces typically ten millimetres wide and one to several millimetres long (Figs 1 and 2). For effective high-power operation, an efficient cooling method is crucial for removing the large amounts of heat that ~100 μm are generated. The semiconductor array is therefore soldered onto a heat sink, which consists either of a bulk copper block in contact with a cooling device, or of a copper block with an internal structure for water cooling. Today the output power of an individual laser-diode array is more than 50 W continuous-wave, depending on the cooling method used and the mode of operation. To reach the regime of hundreds or thousands of watts, laser-diode arrays have to be combined together into stacks so that the power provided by individual arrays can be added together. A stack of just 20 laser-diode arrays, for example, can deliver a power of more than 1 kw, all from a package the size of a matchbox. STACKING ~1 mm ~90 20 ~10 mm The basic idea behind stacking is to bring the light beams from several laser-diode bars Non-lasing zone between emitters very close together for further beam-shaping and imaging. In practice stacking can be achieved in two ways: physically or optically. The physical approach involves simply stacking individual heat sinks (attached to diode arrays) on top of each other. The optical approach involves distributing the heat sinks in space and combining the laser beams from each laser diode through optical means. Both methods of stacking yield a laser beam with the same resultant power. Physical stacking has the advantage that the laser-diode arrays occupy less space, but the placement of heat sinks on top of one another calls for complex cooling structures. Optical stacking, on the other hand, takes up more space but cooling can be achieved with relatively simple, bulk copper blocks. MULTIPLEXING Laser emitter Light divergence pattern of a single laser emitter Figure 1 Schematic of a laser-diode array. Despite the benefi ts of laser diodes, they can be disadvantageous when used in edge-emitting confi gurations. In particular, the light divergence out of the chip is high and the outgoing angle depends strongly on the direction of propagation. Micro-optical elements are therefore used to capture and collimate the light in one or two directions. In addition to stacking, can be used to combine several intense laser-diode nature photonics VOL 1 JULY

4 INDUSTRY PERSPECTIVE TECHNOLOGY FOCUS Conduction-cooled laser-diode array Staircase manifold Laser chip Deflection mirrors on prisms Cross-section of optically stacked combined beam Figure 2 Optical stacking of laser-diode beams using a step manifold. beams into a single beam, thus boosting the output power further. As can be seen in Fig. 3, using spatial the laser beam from a stack made up of, say, 20 single heat sinks can be combined with two or three other such beams to double or triple the overall power of the beam, all within a given beam aperture. After, the beam is still polarized, so that additional polarization with another beam of orthogonal polarization can further enhance the power by another factor of two. Beam sources based on the above techniques offer a single wavelength and can be used for applications such as the pumping of solid-state lasers and fibre lasers, or other wavelength-sensitive applications in the printing and medical fields. For processes based on heat treatment, which are fairly insensitive to the wavelength of the laser light used, it is possible to combine several intense beams of different wavelengths into a high-power beam. For reasons of simplicity, the wavelengths that can be multiplexed with dichroic filters need to be about 30 to 40 nm apart. So far, wavelength has already been accomplished using laser-diode wavelengths at 808, 880, 940 and 980 nm, allowing the power to be scaled up by another factor of four. As Fig. 3 illustrates, in an optically stacked system the paths of individual laser-diode beams may vary, and as a result often well-collimated beams are used. If semiconductor laser bars are arranged on a step manifold structure (Fig. 2), optically stacked, collimated beams are produced, which can then be further polarization or wavelength multiplexed for extra power increases. Alternatively, the beams can be coupled into optical fibres for ease of use for the end customer. In this way, using three 40-W semiconductor laser bars, conduction cooling and simple optical stacking of the three bars, laser-diode modules that provide 100 W of power out of a 400-μm-core fibre can be produced, with a numerical aperture of Such 100-W modules can be cooled using only forced air or peltier (active heat element) cooling. Further boosts the power to several hundred watts. Higher-power laserdiode modules are normally equipped with sensors for temperature and optical power control, fibre interlocks and other accessories. Modules coupled to 200-μm-core fibres are useful for fibre-laser pumping but not necessarily for direct material processing. Turn-key laser-diode systems are cooled laser-diode modules integrated with an optical fibre and imaging equipment, a control unit and power supply, all contained within a 19-inch rack-mountable chassis. Their advantage is that they require virtually no maintenance. Cooling is again achieved using either forced air or peltier heat pumps, and such systems now offer up to 100 W of optical power. For higher powers (200, 400 and 600 W), the amount of waste heat generated is large enough that the most efficient way to remove the heat is through the use of industrial water chillers. All of these systems have a computerized control interface for further integration and synchronization into production equipment. ENTERING KILOWATT TERRITORY Not all applications call for the highest power levels. The soldering of electronic components, plastic welding or microhardening processes can all be accomplished using laser diodes with powers ranging from several tens of watts up to several hundred watts. But with kilowatt powers additional applications open up, particularly in the area of materials processing, for example brazing and surface treatments, including laser-assisted machining, hardening, annealing and cladding. The most effective way of generating kilowatt and multikilowatt laser diodes involves stacking several hundred semiconductor laser bars together. But with such a density of semiconductor lasers, even the most electro optically efficient chip material requires significant heat dissipation, and the size of the cooling equipment becomes a non-trivial issue. In addition, the size of the power supplies injecting electrical current into such modules also grows. Today, laser heads offering up to 3.6 kw are available Stack 2 as standard products with direct beam and spot sizes of mm 2. At present, the higher-power laser-diode systems are mostly used for thermaltreatment applications, where the beam can be made rectangular to match the process requirements as needed. DILAS Diodenlaser GmbH has delivered customized products to clients in which beam homogenization optics are used to produce up to 11 kw in an homogenized area of mm 2 (B. Köhler et al. Proc. SPIE 6456, 22; 2007). The dimension of the laser head itself is 1, mm 2 and has a weight of up to 100 kg. With these sorts of lasers, the chillers and power supply occupy entire wall-sized racks. OUTLOOK Stack 1 Spatial λ 2 λ 3 Spatial Stack 3 λ 2 λ 3 Improvements in the electro optic efficiency and beam parameters of laser diodes have paved the way for compact, high-power beam sources. Laser beams with simple air-cooled systems and kilowatt-class lasers with more elaborate cooling systems are now a reality. With further improvements in beam quality, high-power laser diodes will soon displace lamp-pumped Nd:YAG lasers and become a common sight in a range of applications. Waveplate Polarization Stack 4 Wavelength dichroic filters Figure 3 Multiplexing technology used to boost the output power of a laser-diode system. 386 nature photonics VOL 1 JULY

5 PRODUCT HIGHLIGHTS TECHNOLOGY FOCUS JDS UNIPHASE Fibre-coupled laser diodes with unrivalled performance JDS Uniphase, a major supplier of optoelectronic components to the telecoms sector and other industries, is now offering a new series of high-power fibre-coupled laser diodes for pumping fibre lasers, as well as other applications. The 6397-L3 series of laser diodes are single emitters that deliver 8.0 W of output power from a fibre with a core diameter of 105 μm into a 0.2 numerical aperture at wavelengths of 915, 940 and 975 nm. The company also claims that by using multiple 6397-L3 laser diodes and a multimode fibre coupler or fibre bundle in a distributed architecture, hundreds of watts of highly reliable pump power can be easily achieved. Apart from pumping up fibre lasers, the product also suits use for materials processing, graphic arts, medical and dental applications, remote power generation and pyrotechnic ignition. Rack system offers kilowatt output Lumics, a Berlin-based manufacturer of laser-diode modules, has released a 1-kW output power pump laser diode. The LU0940C1000 is an industry-grade system designed for laser pumping and materials-processing applications. The device is composed of multiple singleemitter laser diodes that are hermetically sealed, fibre-coupled and packaged into a 19-inch rack that features the necessary water cooling and drive electronics. The laser output is available at wavelengths of 915, 940, 960 or 975 nm. The system can run on an input power of 110 V or 220 V, a.c., and comes with a diode mean time-to-failure of 400,000 hours. Lumics claims that the device offers very high reliability, owing to its proprietary laser-facet passivation technology. The firm has also introduced a fibre-coupled laser module that offers 8 W of output power at a wavelength of 915 nm. The device measures mm and has an electrical-to-optical power efficiency of more than 60% and a rated lifetime of 100,000 hours. Diode exploits passive cooling Jenoptik Diode Lab, a subsidiary of the German semiconductor laser specialist Jenoptik Laser Diode, has released a passively cooled laser diode. The device, which is packaged in an industrystandard conduction-cooled mount measuring mm, emits 80 W in continuous-wave mode or 200 W in quasi-continuous-wave mode, at a wavelength of 808 nm. Continuouswave powers of up to 100 W at 915 nm and 976 nm are also available. Typical operating characteristics are a drive current of 112 A and a voltage of 1.8 V. Fast axis collimation of the laser output is also available to ensure that 90% of the output power falls within an angle of 0.5. Applications include pumping of solid-state lasers, materials processing and medical tasks. Coaxial lasers target telecoms Laser Diode Incorporated (LDI), part of the Tyco Electronics Division, has announced the addition of a high-power uncooled coaxial packaged laser to its SCW series of laser products. Devices emitting at wavelengths of 1,310, 1,550 or 1,625 nm, with output powers of up to 200 mw, are on offer. The fully hermetic 3-pin design suits applications in optical time-domain reflectometry, spectroscopy and photon counting. The coaxial laser is RoHS (the Restriction of Hazardous Substances Directive) compliant and fibre coupled with 9/125 μm fibre. Laser Diode Incorporated says that the cooled 14-pin digital-image-processing, Butterfly package options, as well as custom screening and qualification, are available on request. Laser-diode platform promises high power and efficiency Photodigm, based in Texas, USA, has introduced a high-power, singlefrequency semiconductor laser diode that it says rivals the power levels only previously attainable by bulk conventional lasers. Through the use of their newly developed distributed-bragg-reflector ridge-waveguide process, the photonics technology specialist claims that it has created the world s first semiconductor laser diode that combines the features of high-power beam quality, low astigmatism and single-frequency operation. Based on a quantum-well epitaxial design, the PH9/10xxSF edgeemitting laser is capable of delivering more than 500 mw of output power with an efficiency of up to 50%, while maintaining single-frequency operation with a full-width at half-maximum bandwidth of less than 10 MHz. Evaluation units are now available in wavelengths ranging from 920 to 1,100 nm. The company predicts that this device will provide lower-cost and higher-efficiency solutions to digital imaging, defence and free-space communications applications. Laser-diode system for material processing Dilas, a world leader in the design and manufacture of high-power semiconductor lasers based in Mainz, Germany, has unveiled a new series of laser-diode systems designed for materials processing. The Line Focus Diode Laser Systems LF 40 are direct-delivered water-cooled laser diodes integrated with a microscale controller and chiller. They are available at wavelengths of 808 nm and 940 nm, with an optical output power of 400 W in continuous-wave mode. Potential applications include welding of plastics, and annealing and re-crystallization of surfaces. The lasers feature a line-focus beam profile about 100 mm in length and an efficiency of more than 50%. They offer increased process reliability, lower operating cost and higher efficiency over conventional processes. Their compact and robust design also allows easy integration into any manufacturing machinery or robotics, making them attractive for industrial applications. DILAS nature photonics VOL 1 JULY