GaAs-based lasers technology
semiconductor diode laser. Whichever route is chosen will only lead to commercial success if it does not compromise power, power conversion efficiency and lifetime. Any changes must also maintain a sufficiently small far field emission angle: This is essential for realising low-cost, high-yield fibre coupling. Specifically, what is needed is a divergence angle of less than 50° for 95 percent of the power content of the laser.
Maintaining device efficiency while decreasing spectral width is not easy. State-of-the-art high-power, broad-area diode lasers emitting in the 900-1000 nm range can deliver CW output powers of 10 W at conversion efficiencies of over 65 percent, but fall to efficiencies of around 50 percent when internal gratings are added.
Our team at the Ferdinand Braun Institut (FBH), which is located in Berlin, Germany, has overcome this loss in performance with the addition of a grating that enables the fabrication of high-power lasers with 60 power conversion efficiency. Several technologies can be used to make high-power, broad-area diode lasers with internal gratings, and we adopt a two-stage epitaxy approach with the growth process halted part way through so that a grating can be patterned uniformly over the wafer using holographic techniques to deliver distributed feedback (DFB). The remaining portion of the vertical structure is then grown over the patterned surface.
realising this high-performance from our DFB broad-area lasers is the result of extensive development. Efforts have focused on edge-emitting single-emitter diode lasers with a 90 µm stripe width operating at 970 nm, grown using MOCVD on GaAs substrates (see Figure 2).
Accommodating aluminum
High efficiencies are only possible with high-performance laser architectures that accommodate a grating layer while complying with the constraints associated with making and designing this class of device. AlGaAs layers are often used in part of the vertical structure of high-performance lasers. However, integrated overgrown gratings must be patterned outside of the growth reactor, leading to oxidation of the aluminum, which generates defects.
We overcame this problem by turning to aluminum-free grating regions. These structured layers must be overgrown, which leads to further defect generation, partly because aluminum concentration varies in the layers grown over a patterned surface. Reducing the aluminum concentration helps, and we have found that Al0.15
Ga0.85
layers can minimise the defect density and still prevent carriers leaking from the active region.
Grating-based lasers that are suitable for fibre coupling must also have a small vertical far-field angle. This is realised by employing a relatively thick waveguide that helps to direct 95 percent of the light into a vertical angle below 45°. This 2.1 µm-thick waveguide also provides
As
Fig 2: (top) Schematic representation of a DFB, broad- area diode laser produced at the FBH. The inset shows an example TEM cross-section of the grating region. (Above) All devices are mounted junction- down on copper-tungsten submounts for assessment and use in external optical systems. The tips of a pair of tweezers are shown as a size reference. Credit: FBH, FBH/
schurian.com
design flexibility in the later placement of the grating layer, so that the grating strength can be varied as needed. With this approach we have fabricated reference 90 µm stripe single lasers without a grating emitting at 975 nm that deliver a peak power conversion of 65 percent at 10 W output power. This performance, which is suitable for grating integration, was realised despite design limitations and reported at Photonics West 2010.
Overgrown grating layers must combine low loss with high material quality. This was not the case for the earliest AlGaAs-based overgrown gratings in GaAs diode lasers produced in the early 1990s, which introduced optical losses of over 20cm-1
. The aluminum-free overgrown
grating regions in long-term use at the FBH typically have losses of 1cm-1
, and we have made further improvements in their performance to yield lasers with high-power conversion efficiencies. Gratings can compromise laser performance by increasing operating voltages by 0.2V or more, which impedes realising really high efficiencies. These issues were addressed by minimising the material
January / February 2011
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