Mode instability phenomenon acts as a common feature in single-frequency fiber ring lasers. Pump power and cavity length are two important control parameters affecting the output SLM stability. In this work, mode instability of an EDFRL has been experimentally investigated by utilizing two effective methods. On the whole, a mode stability map of the EDFRL scaled by pump coefficient is measured and discussed completely through the interferometer scheme, which helps to evaluate the mode stability dependent on pump power in a form of global visualization. Besides, real-time and detailed detection of various mode instability behaviors, including occasional mode hopping, periodic mode hopping and intermittent MLM oscillation, is carried out using the optical heterodyne scheme. The dynamics of mode instability can also be visualized by on-line time-frequency diagrams. This work will contribute to the analysis, understanding and suppression of mode hopping in fiber ring lasers.
Chaotic fiber ring lasers (CFRLs) can be regarded as a type of complex multi-longitudinal-mode (MLM) lasers in optical frequency domain. However, most experimental investigations on laser chaos generation are only restricted to measurements of total intensity dynamics, with frequency-domain longitudinal mode information neglected. In this work, we experimentally study the longitudinal mode dynamics of a CFRL with pump modulation by utilizing a heterodyne detection scheme, in which a beat signal between the chaotic laser and a reference laser is generated. High-resolution instantaneous emission spectra reflecting the fine longitudinal-mode structure of the CFRL in overall are measured through heterodyne detection. Besides, longitudinal mode frequency and intensity dynamics of the CFRL are monitored simultaneously via time-frequency analysis and discussed in detail. Experimental results show that the CFRL exhibits dense and irregular MLM oscillation all the time when operating at intensity chaos state. Meanwhile, each oscillating longitudinal mode in the CFRL is broadened in spectral line due to pump modulation, and can perform chaotic or random-like behaviors in mode intensity. This work will play a significant role in the further analysis, understanding and application of chaotic fiber ring lasers.
We have demonstrated that a polarization-insensitive anisotropic dynamic grating can be used to detect accident
mode hopping in long-cavity-length fiber ring lasers. The polarization-insensitive property of the grating is realized by a
Faraday rotator mirror and validated experimentally. The mode hopping properties of a particular single-longitudinalmode
erbium-doped fiber ring laser is examined by an anisotropic grating with 3-m long erbium doped fiber. When
mode hopping occurs, the grating responds a negative intensity peak. Repeated measurement shows that the continuing
cavity length wandering during the laser operation is the origin of the accident mode hopping when there is no
modulation applied to the cavity length.
It is known that the incorporation of a self-induced grating in a saturable absorber can be used to drive fiber ring lasers to
single-mode operation with ultra-narrow linewidth output. However, ultra-narrow linewidth erbium-doped fiber ring
lasers (EDFRLs) usually suffer inevitably environmental perturbations for the relatively long resonant cavity, which may
deteriorate the single-mode stability and practical applications in optical coherence detection. In this paper, the singlemode
stability of the EDFRL under low-frequency resonant cavity disturbances has been investigated experimentally,
and the induced mode hops have been measured to demonstrate that the occurrence of mode hopping can be triggered
regularly by stretching the cavity fiber. The phenomenon of induced mode hopping indicates that the self-induced
grating can only dynamically adjust itself to small disturbances in the cavity parameters within a finite range.
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