Optical Phase Lock Loop (OPLL) with tunable frequency offset for distributed optical sensing applications

Abstract

OPLL based on 1550 nm narrow linewidth Planar External Cavity Lasers (PLANEX) with PM output offer several advantages over OPLL based on the conventional DFB or fiber lasers such as bandwidth requirements, high coherent efficiency, absence of the phase reversal and long term locking stability over ambient temperature changes. Such requirements are critical in the field of microwave photonics, LIDAR, coherent optical communications and optical metrology. We report a development of OPLL, optimized for distributed BOTDA/BOTDR Brillouin sensing applications. Conventional approach for distributed fiber optic Brillouin scattering (BOTDR) use a heterodyne architecture for detection of Brillouin scattering signals. With such approach bandwidth (BW) of the optical detector play one of the most critical roles in accuracy of the BOTDR detection. Such coherent detection require a 12 GHz bandwidth of microwave detector which bring excessive noise and high cost of the implementation. OPLL with LO having frequency offset of the order of Brillouin frequency, i.e. 9-12 GHz allow to use low RF BW detection of BOTDR signal. Such detection allow much higher sensitivity, lower noise contribution and offer considerable cost saving for BOTDR distributed sensing and monitoring Beat frequency stability of OPLL was on the order of few kHz and linewidth of the locked lasers was less then 20 kHz. Coherent efficiency of OPLL was better the 85%. Inherent wavelength stability of ECL (order of magnitude better then any of DFB lasers) allows continuous operation of OPLL without losing locking accuracy. OPLL stability was demonstrated over 48 hours of continuous operation.