Abstract

Recently, multilevel-modulation formats such as quadrature phase shift keying and 16-ary quadrature amplitude modulation are being introduced into optical communication systems. Since such signal forms are susceptible to laser phase instability, it is important to identify the linewidth of lasers used in such systems. As a measurement technology, the self-heterodyne method is widely deployed [1]. However, the method needs a large fibre delay to detect phase fluctuations of the laser, which is necessary for measuring narrower laser linewidth. Furthermore, rather complicated polarization alignment is required because the polarization states of laser output fluctuate over fibre transmission. The measurement scheme in [2], based on two-wave interference, can omit fibre delay and polarization alignment by using different two lasers; however, it cannot identify linewidths of the two lasers individually since we can obtain only the sum of two linewidths. To resolve these problems inherent in the conventional measurement technologies, we propose a novel linewidth-measurement scheme based on three-wave interference in conjunction with digital coherent reception.

© 2017 IEEE