Feedback phenomena in lasers induce modulation signals that can be used for ranging and velocimetry. The modulation depth of a backscatter-modulated semiconductor laser diode is considerably enhanced by operating the diode near the lasing threshold. In the present work an external cavity is used to maintain a spectral linewidth of less than 1 MHz in a diode running at 2.5% of its rated power of 30 mW, resulting in a modulation depth enhancement by a factor of 30 over the high-power limit. Coherent velocimetry has been performed for targets at a distance of 50 m, or 20 times farther than the previously published operational range for backscatter-modulated laser diodes. Light power of less than 10 pW at the collection aperture produces useful modulation signals.
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