Beating signal power level improvement in ring lasers based on coupled ring resonators

Intensive industrial and academic research is oriented towards the design and fabrication of fiber ring lasers for application in different sensing systems. Using long fiber drum in the cavity has advantages concerning system parameters such as linewidth and frequency stability, which are vital parameters for ring laser sensors. However, the long drum increases the number of oscillating modes and decreases the power level per mode. Hence, the power level of the beating signal, in the RF domain, between the modes is deteriorated affecting the signal-to-noise ratio of the sensors. In this work, a fiber ring laser with a semiconductor optical amplifier (SOA) gain medium is presented with relatively high beating signal power level and small linewidth based on the insertion of a fiber ring resonator into the main ring. The effect of the inserted fiber ring resonator on the beating linewidth and signal power level is studied for different SOA pumping currents. It is found that the dispersive effect of the ring resonator harms the linewidth of the beating signal, if the SOA pumping current is much larger than the lasing threshold current. The presented system is implemented experimentally and optimized leading to about 34 dB improvement in the beating signal power level without sacrificing the linewidth.