Narrow-linewidth semiconductor laser with highly-linear frequency modulation response for coherent sensing

Abstract

A narrow-linewidth semiconductor laser chip with highly linear frequency modulation response is presented and validated in two coherent sensing test experiments. This distributed feedback laser monolithic chip has an intrinsic linewidth of less than 10 kHz and an output power over 60 mW. When its injection current is modulated by a triangular function, the laser optical frequency can be modulated by more than 7 GHz at rates up to 100 kHz. The laser frequency modulation response is extremely flat up to 100 MHz, which allows correcting the residual sweep nonlinearities by a proper pre-distortion of the modulation signal. In a first test experiment, the laser was used into a monostatic FMCW lidar system. A point cloud was acquired with a field of view of 20°(H) x 10°(V) and an angular resolution of 0.05° along both axes. The acquisition was performed without averaging using a 7 mm diameter output beam of 100 mW. A high-quality point cloud including several objects of varying reflectivity was measured. In a second test experiment, the laser was used into an OFDR system for a distributed acoustic sensing (DAS) experiment. A short portion of a 50 m long SMF-28 fiber was exposed to a 2 kHz acoustic signal. Processed data clearly shows a strong 2 kHz tone at the location of the acoustic perturbation. In both test experiments, the laser was successfully linearized using modulation signal pre-distortion based on interferograms obtained with a Mach-Zehnder interferometer.