Surface-Emitting Laser Array

Manoj Kanskar and François Brunet, Alfalight Inc.

High-power, multimode laser diodes are workhorses for many industrial applications. They are used as tools for cutting, welding, sintering and soldering various materials and as pump sources for fiber, disk and solid-state lasers. Many of these applications require fiber-coupled output or a uniformly focused beam.

The task of fiber-coupling and beam-formatting edge emitters is not simple because of large and asymmetric beam divergence. Expensive micro-optic arrays, interleavers and beam-transformation optics are needed to squeeze the power into a small fiber, and this drives the dollar-per-watt amount to more than an order of magnitude higher than the cost of manufacturing the laser diode.

Fiber-coupled or formatted beams are still too expensive or cost-prohibitive for many applications. Brighter diodes with a useful beam format that requires inexpensive and low-cost manufacturing techniques can further push down the dollar-per-watt figure. Alfalight Inc. in Madison, Wis., has developed an architecture that achieves just this – a two-dimensional array of curved-grating surface-emitting distributed feedback (SE-DFB) lasers.

Why brightness matters

Spatial brightness is defined as power generated per given area and solid angle. Therefore, using brighter sources, more power can be focused on smaller areas or coupled into smaller fibers. An ideal way to generate high power would be to combine single-mode diode lasers. Unfortunately, such sources produce, at most, 1 W of useful power and would require an unmanageable number of diodes for power-hungry industrial applications. Instead, multimode 100-μm-wide, broad-stripe laser diodes typically are used.

A single such device produces about 10 W of power out of a 100-μm, 0.15-NA fiber with a relatively simple coupling scheme, but further power scaling remains a challenge.

Combining more chips helps generate higher power but at the penalty of higher cost and complexity. An alternative method uses fiber coupling of bars and stacks. In this case, diffraction-limited collimators, expensive microlens arrays, interleavers and precision beam-formatting optics must be used in high-tolerance, complex configurations. These sophisticated elements between the source and the fiber are what drive the cost of high-power fiber-coupled systems.

A lower-cost solution

Arrays of SE-DFB lasers have several key attributes, lifting many limitations and cost drivers of current high-power laser diode systems. Those attributes are enabled by one crucial feature: a curved second-order grating etched on the p-side cladding of the laser chip (See sidebar).

Producing SE-DFB chips is more efficient than making standard edge-emitting lasers because key steps are performed early in the manufacturing process. Because fabrication of the laser output window and probe testing of individual lasers are performed directly on the wafer, known good dies are selected before the laser chips are even cleaved, avoiding potential yield losses at expensive downstream packaging steps.

The SE-DFB array architecture provides a number of additional cost-saving advantages over the current technology. Whereas edge emitters must be placed on the knife edge of expensive, diamond-turned heat sinks, SE-DFB lasers are simply picked and placed on a low-cost, flat heat sink with an order of magnitude looser tolerance. Lasers in an SE-DFB array are wired in series, substantially reducing power loss in cables and power supplies. Furthermore, low-current power supplies are cheaper, reducing the overall system cost.

Another key advantage of the SE-DFB architecture is that the electrical connection is isolated from the coolant. This avoids galvanic corrosion that plagues microchannel-cooled bar stacks. Even at the kilowatt level, SE-DFB arrays are cooled with standard house water.

Customized beam with simple optics

The simple way that SE-DFB lasers are laid out on a heat sink makes it straightforward to customize the geometry of an array for a given application. For example, certain applications require an asymmetrical, thin rectangular beam. This is the case for laser-based surface treatment, hardening and cladding operations, where the beam is used as a broad optical brush to sweep large surfaces quickly. An SE-DFB laser array can be arranged into a few columns, each containing a number of laser chips. Because the beam is readily collimated straight out of the chip in one direction, no collimating optics are needed for individual chips.

In the orthogonal direction, the beam diverges slowly with a full angle of about 8°. Each column therefore can be collimated with a single standard cylindrical lens. This architecture yields important cost savings with respect to the collimation of edge-emitting bars, which requires expensive, diffraction-limited fast-axis collimation microlenses.

Figure 1. Shown is a 200-W surface-emitting distributed feedback (SE-DFB) laser array optimized for fiber coupling. Laser chips are arranged on a flat heat sink in a 4-6-6-4 configuration.

Fiber-coupled SE-DFB arrays

Figure 1 shows a 200-W quasicircular SE-DFB array that is coupled into a fiber cable with high efficiency without using beam transformation optics. In this configuration, four cylindrical lenses are used to collimate four columns in a 4-6-6-4 arrangement, and a single aspherical lens focuses the beam into a 200-μm, 0.22-NA fiber. The most stringent mechanical tolerance in this module with respect to fiber coupling is about 3 μm, two orders of magnitude looser than the 50-nm precision required for performing the same operation with a stack of laser bars. More rugged fiber-coupled SE-DFB products can be envisioned, handling shocks, vibrations and thermal gradients better than the current technology.

Alfalight’s 200-W fiber-coupled module has been designed for pumping ytterbium-doped fiber lasers (Figure 2). The important challenge of wavelength yield for 976-nm modules is waived with SE-DFB technology because the grating precisely determines the output wavelength and guarantees a wavelength yield of virtually 100 percent across the wafer.

Figure 2. Alfalight builds fiber-coupled 200-W SE-DFB array lasers. The 976-nm beam is coupled into a 200-μm, 0.22-NA fiber connector.

Besides delivering a narrow spectrum centered on the ytterbium absorption peak, the technology comes with other practical advantages; for example, the wavelength shift over temperature is only 0.07 nm/°C – five times slower than standard laser diodes. The pump absorption in the doped fiber consequently has only a weak dependence on the temperature of the cooling water. On a system perspective, this means that several pump modules can be cooled with a unique chiller with no temperature tuning required to optimize absorption.

Wavelength-locked high-power laser diodes bring benefits to industrial applications other than pumping solid-state media; for example, laser soldering of thermoplastics is generally realized by overlapping a transparent with a strongly absorbent polymer. The laser beam transmits through the top layer to melt the bottom, absorptive material, joining both pieces upon cooling. Materials must be carefully chosen to meet the respective requirements of high transmission and high absorption at the laser wavelength. SE-DFB arrays could provide low-cost solutions for this application, with an operating wavelength tuned on absorption and transmission bands of specific polymers.

Figure 3. The achievable output power of an SE-DFB laser array scales quickly as the constraint on the beam quality is relaxed.

Power scaling

Power scaling of SE-DFB arrays is possible using a larger number of emitters per array, as plotted in Figure 3. An additional level of power scaling is achievable by interleaving two arrays, as illustrated in Figure 4. Alfalight is developing a kilowatt SE-DFB array based on this latter approach and has demonstrated polarization combining of SE-DFB arrays with efficiency better than 98 percent, thanks to a polarization extinction ratio better than 1:1000 for single SE-DFB lasers. Moreover, the wavelength-locked, narrow spectral bandwidth of each array makes power scaling beyond 1 kW achievable through wavelength beam combining over a relatively narrow band.

Figure 4. Using an interleaver, two arrays can be combined to create a 1-kW SE-DFB laser module.

Multikilowatt SE-DFB arrays would be a good fit for pumping high-power thin-disk lasers. A thin-disk laser can be pumped with a single beam, reflected several times by the pump cavity and absorbed by the thin solid-state gain medium over multiple passes. The benefits of simpler pump architecture and an emission wavelength locked on the absorption band of the disk would generate important cost savings, especially when scaled at the kilowatt level.

Limitations taken for granted on brightness, architecture and yield of high-power laser diode manufacturing are being lifted as the very first generations of SE-DFB lasers are being integrated into prototypes. Whether used as direct laser sources or for pumping fiber and solid-state lasers, SE-DFB laser arrays will quickly become a game changer for industrial applications.

Meet the authors

Manoj Kanskar is vice president of research and development at Alfalight Inc. in Madison, Wis.; e-mail: mkanskar@alfalight.com. François Brunet is product manager at Alfalight; e-mail: fbrunet@alfalight.com.

What Is a Surface-Emitting Distributed Feedback Laser?

(A) A surface-emitting distributed feedback (SE-DFB) laser is based on a semiconductor quantum well and waveguide structure similar to more common edge-emitter laser diodes. Instead of being emitted through the edge of the chip, however, the output beam shines through a window on the top of the laser. Optical feedback from the edges of the chip is suppressed by an absorber region.

(B) The optical cavity-generating laser effect is formed by a curved second-order diffraction grating etched at the bottom of the laser.

(C) The first-order diffraction is steering the beam orthogonally from the waveguide, projecting it through the output window. The grating curvature shapes the wavefront to enhance brightness and collimate the beam in one axis.

(D) In the orthogonal direction, the beam slowly diverges with a full angle of 8°. A simple cylindrical lens collimates the beam.