Single-Frequency Operation of a Widely Tunable SOA-Based Fiber Ring Laser

In a SOA-based fiber ring laser, tunable over the range 1450 nm to 1640 nm, a "wide" intra-cavity filter leads, counter-intuitively, to singlemode oscillation, whereas a "narrow" filter results in multimode oscillation. However, multimode oscillation is always observed in the red part of the gain curve, above about 1615 nm.