In this paper, we study experimentally and analytically the stimulated Brillouin-induced self-heterodyne technique for linewidth measurement of lasers having narrow spectral widths. The output of a laser under test (LUT) is modulated by a RF signal to generate the sidebands. The carrier and the sidebands co-propagate in a single-mode fiber (SMF) along with the stimulated Brillouin scattering (SBS) signal generated by a counter-propagating pump. The SBS process amplifies one of the sidebands, which beats with the LUT carrier wave. The beat signal is detected using a photodiode and analyzed using an electrical spectrum analyzer to compute the laser linewidth. We analyze the SBS-induced self-heterodyne (SISH) scheme mathematically to obtain expression of the power spectral density of the photocurrent. Experimental results show 20 dB and 3 dB linewidths of 15 kHz and 5.5 kHz, respectively, which are in excellent agreement with the estimated theoretical values, thereby validating the analysis. We also perform the conventional delayed self-heterodyne measurement and obtain 20 dB linewidth of 15 kHz, which compares well with the SISH method. However, the SISH technique reduces jitter by 10 dB. We also study the dependence on linewidth measurement of the SISH scheme on fiber length. For 2 km SMF length, the 20 dB linewidth is 23 kHz, which is higher compared to the measurement obtained by using 25 km SMF.
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