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Linewidth narrowing and stabilization of semiconductor laser light generation is of great research interest governed by the huge demand of compact cost-effective narrow-band laser sources for many potential applications. In 2012 we have demonstrated a simple kHz-linewidth laser just splicing a standard distributed feedback (DFB) laser diode and a few passive telecommunication components. The principle of operation employs the mechanism of self-injection locking that significantly improves DFB laser performance. While a typical linewidth of free-running DFB semiconductor lasers ranges from a few to tens MHz, self-injection locking of a DFB laser through an external fiber ring cavity causes a drastic reduction of its laser linewidth down to a few kHz. The advantage of the proposed configuration is that the same external fiber ring cavity could be used for self-injection locking of a DFB laser and as Brillouin scattering media to generate Stokes shifted optical wave. However, a continuous laser operation at two frequencies has not been reported yet preventing it from many prosperous photonic applications. Here, we introduce a simple dual-frequency laser configuration. In our approach, the implementation of self-injection locking into the Brillouin ring fiber laser helps to maintain coupling between the DFB laser and an external high-Q fiber cavity enabling dual-frequency laser operation. Specifically, the same ring fiber cavity is used to generate narrow-band light at the pump frequency (through self-injection locking mechanism) and narrow-band laser light at Stokes frequency (through stimulated Brillouin scattering). The system is supplied by a low-bandwidth active optoelectronic feedback circuit controlled by a low-cost USB-DAQ card that helps the laser to maintain the desired operation mode. The fiber configuration reduces the natural Lorentzian linewidth of light emitted by the laser at pump and Stokes frequencies down to 270 Hz and 220 Hz, respectively, and features a stable 300-Hz-width RF spectrum recorded with the beating of two laser outputs. We have explored key features of the laser performance, revealing its stability and applicability to RF harmonic generation of high spectral purity as an additional benefit of the proposed technique.
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