Laser linewidth characterization via self-homodyne measurement under nearly-coherent conditions

In this work we propose a novel and efficient characterization scheme for a narrow linewidth laser using a nearly-coherent delayed self-homodyne (NC-DSH) technique. The modulated signal of an analog coherent optics (ACO) transceiver, configured in optical loop-back, and the local oscillator (LO) are mixed after a very short optical path difference (OPD), corresponding to an interferometer operating in its nearly-coherent regime. The phase noise is extracted from a digital signal processing algorithm of carrier phase estimation (CPE), while data is transmitted. The interferometric pattern's E-field power spectral density (PSD) enables the extraction of the OPD and the linewidth of the transceiver's laser source in high accuracy. The proposed technique is demonstrated using a commercial integrated coherent transmitter and receiver optical sub-assembly (IC-TROSA).