Quality to drive industrial revolution
While the telecom boom was driving 980 nm pump laser innovation at an astonishing rate, a largely distinct set of companies was servicing industrial customers with lower-brightness diode lasers. Telecom pumps enabled fiber-optic system bandwidth to progress rapidly, while their industrial diode cousins reliability and cost often disappointed, ultimately slowing the transition from lamp-pumped to diode-pumped solid-state lasers (SSLs). The telecom bust left a handful of high-performance suppliers eager to diversify their sizeable wafer fabs and offshore diode laser assembly and packaging facilities. The resulting influx of quality and manufacturing excellence into the industrial diode marketplace has begun to spark an industrial laser revolution.
Diodes are the single most efficient method of converting electricity into useful optical power. The most mature semiconductor diode laser material is AlGaInAs, which offers the highest demonstrated optical power density and electrical-to-optical power conversion efficiency, while covering the 800-1000 nm band that is critical for neodymium- and ytterbium-based SSL pumping. If reliable AlGaInAs diode lasers are finally brought to market at industrial laser price points, the effect will be dramatic.
Industrial diode laser reliability is bounded by problems of heat removal and power-cycled operation. To remove heat from multi-emitter diode arrays (bars), purified water is pumped through copper-microchannel coolers, which erode and oxidize to the extent that plumbing and water purity are as much of a talking point in the industrial laser community as semiconductor physics. Bars are commonly bonded to heatsinks with soft, indium-based solder, which has proven unreliable for industrial on-off cycles. In contrast, AuSn-soldered single-emitter and arrayed diode lasers introduced by telecom houses have set new benchmarks in sustainable power and mean time before failure (MTBF). Telecom-grade hard AuSn-soldered laser diodes show few to no ill-effects under power-cycling. Single-emitter versions save by using simpler, more reliable cooling systems.
If thermal limitations are sidelined, sustainable AlGaInAs laser power is limited by facet oxidation which liberates metallic arsenic. Eventually the facet is so absorptive that runaway heating drives failure via catastrophic optical mirror damage (COMD). Telecom suppliers developed various facet passivations to eliminate COMD more than a decade ago, but were unmotivated during the go-go 1990s to offer their technology to industrial customers. Filling the void were cheaper, unpassivated AlGaInAs devices, which delay COMD onset by de-rating optical power density, and "aluminum-free" InGaAsP diode lasers aiming to circumvent the facet corrosion mechanism altogether.
Aluminum-free devices generated considerable excitement and telecom-driven venture capital investment, but are still virtual no-shows in the telecom pump marketplace. InGaAsP enjoys more success in low-brightness industrial applications, at least matching the benchmark 1 W, 100 μm emitter for around 20 kh performance of AlGaInAs 1 cm × 1 mm 50-emitter bars. Bookham has shaken up the industrial bar market of late, introducing hard-soldered AlGaInAs bars based on its telecom diode technology with life-test data suggesting that reliable 100 W operation is possible. Much higher-brightness AlGaInAs single-emitter diodes that are available from both Bookham and JDSU sustain 8 W, 100 μm power at 940 nm with an MTBF of at least 300 kh.
While telecom migrants have greatly raised the bar for reliable power, they remain challenged to deliver their product at prices attractive to industrial users. But cost reduction track records suggest that the dollar/watt metric will soon be disrupted. Telecom revenue bases bolster companies like JDSU and Bookham, providing wafer fab economy of scale and access to offshore assembly unavailable to the smaller industrial diode shops. As impressive as the latter s early offerings are, they perform well below commercialized telecom pump facet power densities, leaving room to improve performance while reducing cost. If the industrial marketplace s unquenchable appetite for lower dollar/watt snowballs into rapid-fire diode laser innovation alongside volume-driven cost reduction, the solid-state industrial laser market could see fireworks.
To see where a second wave of innovation might lead, let s extrapolate commercially available fiber-coupled 100 μm aperture single-emitter diodes, now available at $35/W in volume, at the rate of progress sustained by telecom 980 nm pumps over the last decade (see figure). Assuming operating power increases at 17.5% annually, with manufacturing efficiencies enabling dollar/watt prices to fall by 26%, the year 2016 may see a reliable 35 W fiber-coupled 100 μm aperture single-emitter diode laser source selling for $60, or
