Internal Gratings Create Powerful, Spectrally Pure Lasers With High Efficiencies
High-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.
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
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 (Δλ(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 Δλ(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 Δλ(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 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).
Fig 2: (top) Schematic representation of a DFB, broadarea diode laser produced at the FBH. The inset shows an example TEM cross-section of the grating region. (Above) All devices are mounted junctiondown 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
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 aluminium concentration helps, and we have found that Al0.15Ga0.85As 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 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 perturbation due to the grating, in order to cut defect generation. A very thin aluminum-free grating layer of 10 nm-thick InGaP was used in the new design, located a long distance (0.77 μm) from the active region of the device to ensure a low grating coupling strength (κ ~ 0.5 cm1). Introducing this layer and fine-tuning etch and overgrowth conditions eliminated both excess optical loss and operating voltage – current can now flow without restriction through the p-side grating layer. The deployment of a low strength grating has an additional benefit: Increased slope efficiency. That’s because lower-strength gratings provide less feedback, increasing the proportion of light leaving the laser. These factors combine to enable the fabrication of more powerful lasers.
Matching gains and gratings
To optimise laser efficiency, it is also essential to select the best relative location of the material gain and the grating wavelength. The active region generates optical gain, and the grating provides reflection – together they combine to produce lasing. However, the grating wavelength and the lasing wavelength vary very differently with temperature. For every °C change, the grating shifts by 0.08 nm and the gain moves by five times as much.
These differences hamper laser performance. In 7-10 W lasers driven in continuous wave operation internal current heating raises device temperature by 30°C, preventing gain and grating wavelengths from being in sync over the whole operating range. When the gain is offset from the grating wavelength, light is less strongly amplified, causing the device to operate less efficiently.
In our design, gain and wavelength are selected so that they meet at the operating point of 7W. The downside of this approach is that the laser has a large threshold current, reducing overall power conversion. But if we had designed our device so that the gain and grating matched at the threshold current and the peak efficiency of the device was higher, this would compromise the output power – gain would drop so much at high currents that the device would over-heat. Thanks to all this detailed development work, we have produced diode lasers with an integrated overgrown grating that deliver a peak power conversion efficiency of 62 percent, as reported at Photonics West 2011. This falls by just 4 percent at the operating power of 10W. Spectral width with 95 percent power content at 10 W is 0.7 nm. These efficiencies are slightly lower than those of the grating-free reference device. The overriding reason for this slight reduction in performance is the offset between the grating and the gain wavelength. Reliability tests reveal that these lasers operate failure-free for over 4000 hours (to date) at an output power of 7W at 25°C (see Figure 3).
Fig 3: High-power diode lasers with integrated overgrown gratings reach high powers with a peak power efficiency of over 60 percent (left). The spectral width, δλ, at 10 W continuous output power is substantially reduced in comparison to a reference device without internal grating (right). Credit: FBH
Although these newly developed, grating-stabilised devices have substantially increased laser efficiency, further gains are necessary. Specifically, the efficiency should be maintained at more than 60 percent at higher output powers. There are two possible pathways to success: improving the performance of the baseline design; or reducing the influence of detuning, for example through improved heatsinking. On top of this, the process must be scaled for high volume manufacture - for larger volumes, 4-inch production will be necessary. However, the breakthrough efficiency achieved in these gratingstabilised, high-power lasers will enable a wide range of new and improved industrial laser systems.
This work was funded within the German Federal Ministry of Education and Research (BMBF) development program, INLAS. For more details see: “Reliable operation of 976 nm high power DFB broad area lasers with over 60 percent power conversion efficiency" Paul Crump et al. Paper 7953-50, Photonics West 2011
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