Using an optimized high-index ITO overlay on a single resonant band LPG to enhance the tunable range while maintaining the resonant peak depth

In this paper, we report laboratory test results of an LPG that can maintain a constant resonant peak depth over an enhanced tuning range when it is coated with an ITO electrode that has optimized thickness and refractive index. Without the ITO layer, LPG tuning ranges as large as 50 nm have been achieved when the ambient index is increased from 1.00 (air) to ~1.444 (index of the silica cladding), but the peak depth cannot be maintained. When a properly designed, high-index ITO overlay is coated onto the silica cladding, mode transition effects coincide with the LPG's intrinsic sensitivity to changes in the ambient index, resulting in a stable peak depth over an enhanced tuning range. The authors have experimentally demonstrated an LPG coated with ITO that can be tuned in excess of 150 nm with an ambient refractive index change of less than 0.01. To the best of the authors' knowledge, this is the highest sensitivity reported for an LPG to date. In addition to the tuning performance, the resonant peak remains within 1 dB of its maximum depth for at least 100 nm of the tuning range, which allows the tunable LPG to be used in real applications.