An optical scheme for markedly reducing the linewidth of a semiconductor laser diode has been reported by Japanese scientists at Tohoku University and the NTT Device Technology Laboratories. Laser sources with a narrow linewidth are required for coherent optical communication schemes, such as those that use quadrature amplitude modulation. The technique relies on coherent optical negative feedback from a Fabry-Pérot etalon that is coupled to a 1,550 nm distributed-feedback laser diode (DFB-LD) via a Selfoc microlens. As the optical frequency of the DFB-LD increases or decreases with respect to its operation point, the reflectivity of the etalon changes, providing optical negative feedback that 'pulls' the laser back towards a central single frequency of operation. Konosuke Aoyama and co-workers report reducing the linewidth of the laser diode from 6.4 MHz to 6.5 kHz as well as a 30 dB reduction in the power spectral density of frequency noise for frequencies below 100 MHz.
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Graydon, O. Linewidth reduction. Nature Photon 9, 7 (2015). https://doi.org/10.1038/nphoton.2014.318
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DOI: https://doi.org/10.1038/nphoton.2014.318