Chip makers chase fiber-laser boom
Wind the clock back a few years and efficiency was the key metric for high-power laser diode manufacturers. It was a prominent feature in the technical sessions at the Photonics West trade show, and companies involved in DARPA s SHEDS (super-high efficiency diode sources) program used the event to detail their progress towards an 80% efficiency target.
Fast-forward to this year s Photonics West and efficiency is no longer the key talking point. That s not because the goal of the SHEDS program has been met. Instead, it is a reflection of a shift in prioritization of laser diode characteristics. Top of the list is a need to serve today s key applications.
One of the largest applications to emerge is for fiber lasers. This has grown tremendously in recent times, and this year Stephen Anderson of Laser Focus World estimated that the market for fiber lasers would be worth more than $300 million in 2009. The role of the diode source, which can be a single emitter or a bar, is to pump a doped optical fiber featuring Bragg gratings at either end. These act like mirrors to produce a coherent, powerful beam with excellent beam quality, which is able to cut metal sheets and serve defense applications.
Like any source, a fiber laser benefits from a high-efficiency pump, because it reduces overall running costs and cooling requirements. However, the more important issue is the amount of optical power that can be coupled into the fiber. This is governed by the output power of the diode and its beam quality, which in turn depends on the geometry employed in the chip itself, such as the stripe width and the thickness of various epitaxial layers.
At this year s show, Alfalight unveiled a promising approach for improving beam quality. The US company s new distributed feedback (DFB) laser has an innovative design that emits light through a large rectangular window in the top of the chip, rather than the more conventional edge emission. The new technology features a curved grating in the p-type region, which diffracts light out of the chip, locks the wavelength and maintains an emission bandwidth of less than 0.2 mm.
The company cannot claim that it invented this technology – its roots can be found in the research conducted at the Hughes Research Labs (now HRL) in the 1990s. However, Alfalight s four-year development program has taken the performance of these devices to new levels.
Speaking at the Alfalight stand, vice-president of marketing and sales, Ron Bechtold, explained that the device has several benefits over the traditional edge-emitting design. It is immune to facet damage because the optical power density in the structure is reduced by four orders of magnitude. In addition, the output can be coupled into a fiber laser with relatively simple optics. And, as with VCSELs, these devices can be tested while still on-wafer.
Bechtold revealed that Alfalight s surface-emitting DBRs include a 6 W, 975 nm laser with a power conversion efficiency peaking at more than 50%, as well as a 2 W version with a diffraction-limited beam. An array of 20 of these devices produced more than 200 W, and another with 30 elements can deliver more than 300 W.
Paul Crump from the Ferdinand Braun Institute for High-Frequency Technology in Berlin, Germany, has addressed the issue of increasing the output power from a single emitter source. According to him, commercial 980 nm broad-area laser diodes used for pumping erbium- or ytterbium-doped fiber lasers produce reliable outputs in the 8–10 W range. His team, however, has now fabricated reliable chips that can produce twice this power.
Crump describes the devices as "super-large optical cavity designs", which have robust AuSn-based packages. They are grown by MOCVD and contain a 2.4 µm thick waveguide core with an asymmetrically positioned InGaAs/GaAsP double quantum well that minimizes excess resistance and optical absorption from the p-side of the mirror. Reliability tests have been performed on 4 mm long lasers with various stripe widths. They determined that devices with a 96 µm stripe, operating at 20 °C, could produce a continuous-wave output of 20 W for more than 4000 hours.
A claim for a completely different concept for coupling laser emission into a fiber laser was made in a talk given by Uwe Strauβ from Germany s Osram Opto Semiconductors. He argued that single emitters cannot produce enough output for some fiber lasers, but typical laser diode stacks suffer from cross-talk between the various elements because of the high fill factor used. The ideal solution, according to him, is to combine many single emitters in bars that have far smaller fill factors.
To test this idea Osram has built 808 nm, 880 nm and longer-wavelength laser bars with five emitters and fill factors of less than 20%. These are designed to couple into a fiber laser with a 200 µm core and a numerical aperture of 0.22. Emission from these bars was compared with that produced by five isolated, single emitters. "They are identical and there is no need to use a single emitter," claimed Strauβ, who went on to say that the reliability of the bars is similar to that of the single emitters.
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