Behaviors of Photosensitive Mechanisms in a Low-Loss Fiber at Different Energy Densities

The photosensitive behaviors in laser grating formation is an important guideline for the preparation of drawing tower gratings (DTGs) in different optical fibers. We conducted static single-pulse laser accumulation experiments on a low-loss fiber to investigate its photosensitive behavior at different energy densities. By analyzing the relationships between grating reflectivity and the number of laser pulses, we found that the contributions of the structural adjustment (which is the combination of stress model and density model) and color-centered model to the grating reflectivity can be described by negative exponential functions and linear functions, respectively. At the first laser pulse, When the laser energy density is not high, the adjustment of the structure is dominated by the stress model, which counteracts the contribution of the color-centered model to the refractive index.; when the energy density is higher, stress relaxation occurs, density modulation is prominent, the grating reflective described by the two models are additive. Under the first laser pulse, the refractive index modulation due to the structural adjustment always dominates, and its external display, stress modulation or density modulation, is determined by the energy density. This provides theoretical guidance for online single pulse laser writing of high refractive index gratings.