technology GaAs-based lasers
Internal gratings create powerful, spectrally pure lasers with high efficiencies
The combination of reliable output powers of 7W, peak power conversion efficiencies in excess of 60 percent and spectral widths below 1nm can be realized by incorporating integrating distributed feedback gratings into broad-area lasers, says Paul Crump from the Ferdinand Braun Institut, Germany.
H
igh-power diode lasers are increasingly important sources for direct use in many
industrial applications, such as cutting and welding. In direct application systems, the output from many diode lasers is optically combined into a single high brightness source, typically coupled into an optical fibre and directly delivered to the work surface – as illustrated in figure 1.
Today direct diode technology is a promising alternative to solid-state and fibre-laser systems for many industrial applications. Its performance is limited predominantly by the semiconductor lasers themselves, which operate with over 60 percent power conversion efficiency.
Further order-of-magnitude class improvements in the fibre-coupled power-density are possible via spectral multiplexing – a technique that combines multiple diode laser modules using spectrally selective optics. However, the practical maximum number of useable, combinable wavelengths is limited by the performance of the diode lasers. These typically have 95 percent of the power
content (∆λ(95%)) spread over a spectral width of 5 nm, and exhibit a wavelength shift with temperature of 0.4 nm/K in 960-980 nm range. However, if smaller values for ∆λ(95%) and better temperature stability could be realised, this would pave the way to introducing more wavelengths into direct diode systems, thereby increasing their brightness. Diode lasers are also important commercially as pump sources for solid state and fibre lasers. These systems will also benefit from a smaller value of ∆λ(95%) and a reduced wavelength shift with temperature: specific absorption lines could be targeted for higher performance (97x-nm in Yb:YAG for high absorption, or 88x-nm in Nd:YAG for higher efficiency). Additionally, efficient high-power pump lasers with narrow stable line widths could also meet the needs of new technologies, such as diode-pumped alkali vapour lasers.
There are many techniques that can be used to narrow and stabilise the emission spectrum of high power diode lasers. Adding external optical elements is one option, and it is also possible to introduce internal gratings within the
Fig 1: Fiber coupled diode lasers (left) are the enabling technology for many of today’s state of the art, high-efficiency industrial laser systems (right). High-efficiency diode lasers with narrow, stable spectral lines are needed for further improvements in the brightness of such systems. Credit: FBH/Immerz, TRUMPF
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www.compoundsemiconductor.net January / February 2011
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