Waveguides Force Diode Laser into Single Mode
Semiconductor lasers oscillating in a single, fundamental lateral mode produce a better-quality beam than multimode lasers, but single-mode oscillation is difficult to obtain. One approach has been to fabricate a waveguide into the semiconductor device and to keep the waveguide small enough that higher-order modes cannot oscillate. Although this approach has been successful, the tiny waveguide limits the power available from the laser because of its smaller gain volume and because the high power densities can damage the facets.
Figure 1. Far-field intensity patterns of lasers with different waveguides show the patterns at pump currents from 100 to 1200 mA in 100-mA intervals. A conventional straight-waveguide laser shows beam steering as pump power increases, indicating higher-order mode oscillation (a). A waveguide with two circular sections between straight sections limits oscillation to a single lateral mode, but a secondary beam is present even at the lowest currents (b). The optimal design with sinusoidal curves and no straight section shows Gaussian, single-mode oscillation and an absence of any secondary beam until the highest pump currents (c).
Recently, a collaboration between scientists at the University of Illinois at Urbana-Champaign and at Nuvonyx Inc. of Bridgeton, Mo., has shown that a larger waveguide can be forced to oscillate in a fundamental lateral mode if it is bent appropriately. Higher-order modes experience greater bend losses than the fundamental mode, so placing a bend in the waveguide ensures that only the fundamental mode reaches threshold.
The far-field intensity pattern of a conventional, straight waveguide in a semiconductor laser is shown in Figure 1a. As the current increases above 300 to 400 mA (corresponding to an output power of 200 to 250 mW), a directional shift in the peak intensity appears, indicating that a high-order mode has begun to oscillate.
Figure 2. The researchers investigated two shapes of the waveguides. In one, two sections of circular arc were fabricated between straight-waveguide sections (a). The other featured sinusoidal curves with no straight sections (b).
To suppress this high-order mode, the scientists fabricated a similar laser with a curved waveguide instead of a straight one. The waveguide consisted of two circular arcs of opposite curvature in between straight sections of waveguide (Figure 2a). Although this geometry prevented higher-order modes from oscillating, there were several drawbacks (Figure 1b). Because nearly all the bend loss occurs at the point where the bend is sharpest, that light is emitted from the facet and creates a secondary beam beside the main beam. Moreover, because the mode passing from the curved section of waveguide into the straight section is asymmetric, it tends to shift position as the pump current is increased.
A different design of the curved waveguide compensated for these problems. In this design, the curves were sinusoidal rather than circular, distributing the bend loss more evenly along the length of the curved waveguide (Figure 2b). Also, the straight section of waveguide was eliminated, ending the mode-mismatch problem. The far-field intensity pattern produced from this laser has no high-order lateral modes, and only at the highest currents does the light lost at the waveguide bend form a secondary beam (Figure 1c).
Comparing index-guided lasers with and without the bend, the researchers found that they couldn't obtain more than approximately 225 mW of pure single-mode power from the straight-waveguide laser, but they typically observed 0.5 W or more of single-mode power from the bent-waveguide lasers. Although additional loss due to the bend did raise the laser threshold by 5 to 10 percent, the bend loss was very low above threshold so that the power available at high pump currents was essentially the same for straight and bent waveguides.
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