INDUSTRY LASERS
high-power, laser cavity. In this form of laser system, gain is generated in the fibre by active ions, such as ytterbium or erbium, that are doped into the fibre core. The light that is coupled into this fibre from the GaAs diodes pump the active ions, generating light at a longer wavelength, which is guided within the fibre waveguide. The longer-wavelength light resonates between the two gratings, while being amplified with every pass through the cavity. Making one of the gratings partially transparent allows some light to pass through and impinge upon any material that needs to be processed.
Figure 1. Left side: CW power versus drive-current at 25 °C for the latest laser diode and its predecessor. Both chips have 4.1 mm cavity length. Right side: near-field images of the intensity at the front facet at 12 W, 25°C. The new chip has better uniformity and lower peaks, which should lead to better facet reliability.
Constructing a fibre laser in this manner allows the production of high-power fibre lasers with a single resonator that can deliver an output of over 2 kW. That may not sound like much, since it is only equivalent to the power drawn by a hand-held hair dryer. But the key difference is that the output from the fibre laser can be focused to a spot the size of a few diameters of human hair – several hundred microns.
At such a high power density, the laser light can rapidly cut through mild and stainless steels, aluminium, brass, and copper, as well as weld, mark, or braze a wide variety of materials. It is possible to cut mild steel up to 25 mm-thick with a 2 kW fibre laser, while a cousin with double that power can cut 1 mm-thick aluminium at 75 m per minute.
The most costly component within the fibre laser is the set of fibre-coupled pumps, which can account for more than half of the bill of materials. This makes the macro fibre laser market a considerably significant one for manufacturers of high-power GaAs-based lasers, which can pump at wavelengths of 910 nm to 980 nm to enable lasing further in the infrared – doping a fibre with ytterbium, for example, leads to lasing around 1060-1080 nm.
Figure 2. Reliability analysis employed the maximum likelihood method to fit a Weibull model with acceleration factors based on diode junction temperature and optical power. Activation energy, Ea temperature, Tj
, was fixed to 0.45 eV as in prior models, and laser diode junction , is calculated from thermal resistance and dissipated power.
The extracted shape parameter β is 1.7, and the power acceleration factor n is 6.5. The resulting scale parameter at 12 W, 25 °C is η = 570,000 hours, and cumulative failures from the model at 20,000 hours is 0.3 percent. Three early failures fell outside of the model and were removed. These can be treated as infants and eliminated with a longer burn-in.
To succeed in this market, laser diode manufacturers have to focus on supplying adequate pump brightness – that is, the power divided by the optical aperture in physical and angular space – at the lowest dollars-per-watt. There are also other criteria to consider: the laser must be reliable, because how long it lasts governs the lifetime of the system; and the packaging of the laser and the coupling into the fibre must preserve the hard-fought-for brightness from the chip, while ensuring that the product is competitively priced.
Serving the market At JDSU of Milpitas, CA, we are meeting all of these requirements with our latest generation
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www.compoundsemiconductor.net Issue VI 2014 Copyright Compound Semiconductor
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