Feedback Stabilizes High-Power Diode Lasers
High-power, broad-area semiconductor lasers exhibit spatial and temporal instabilities due to fundamental nonlinear interactions between the intense optical field and the semiconductor material. These interactions result in a broad spectral emission and a double-lobed far-field intensity profile that impede the usefulness of the devices for spectroscopy, materials processing, medicine and other applications. A group at Darmstadt University of Technology in Germany has produced streak-camera images of these instabilities and has demonstrated the effects of damping them with spatially filtered feedback.
Figure 1. The spatial and temporal instabilities apparent in these streak-camera photographs of the output of an InGaAsP laser diode are the result of nonlinear interactions between the optical fields and the semiconductor material.
The scientists chose to study a 1-W InGaAsP device with a 100-µm lateral stripe, operated in a pulsed mode to avoid thermal effects. They focused the 807-nm output onto the slit of a streak camera to produce the images shown in Figure 1. The vertical axis in these photos represents time, and the horizontal axis represents the displacement across the laser's aperture.
The spatial and temporal instabilities are visible in these streak traces. The left trace shows the first four nanoseconds of laser operation. There are a few cycles of relaxation oscillation at the beginning, as energy flows back and forth between the population inversion and the circulating power in the laser. Even as these oscillations are damping out, however, the beam is breaking up into spatial filaments. In the middle trace, the filamentation displays an oscillating behavior, with an intensity peak that bounces back and forth across the aperture. At 9 ns, for example, the peak hits the left side of the aperture and bounces back. In the magnified trace on the right, further temporal spiking behavior is apparent. The 28-ps period of these spikes corresponds to the round-trip transit time in the laser resonator.
To control the laser instabilities, the researchers employed a spatially filtered feedback scheme. A collimating lens and a 50 percent mirror selectively reflected the radiation in the zero-order lateral mode back to the laser. The radiation transmitted through the mirror and focused onto the slit of the streak camera produced the traces in Figure 2. The initial relaxation oscillation is visible in the left trace, but unlike the case without feedback, the subsequent emission from the laser was spatially and temporally homogeneous. Even after the laser had run for 8 to 12 ns (right trace), the filamentation and temporal pulsations were strongly reduced.
Figure 2. Feeding spatially filtered radiation back to the laser eliminates the instabilities.
The scientists believe that the instabilities in the first set of traces are linked to laser oscillation in multiple lateral and longitudinal resonator modes. By providing feedback to one lateral mode, they are forcing oscillation only of that single mode. Future studies will examine the behavior of longitudinal modes under conditions of spatially filtered feedback.
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