A Quantitative Analysis of Tunable Long Period Grating Technology and Its Application

Open Access
Lee, Jonathan Edward
Graduate Program:
Electrical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
September 24, 2007
Committee Members:
  • Shizhuo Yin, Committee Chair
  • Karl Martin Reichard, Committee Chair
  • Qiming Zhang, Committee Member
  • C Russell Philbrick, Committee Member
  • Qi Li, Committee Member
  • long period grating
  • high index overlay
  • ITO
  • tunable filter
In this thesis, a design for an electro-optically tunable, fiber optic filter based on a long period fiber grating (LPFG) is introduced and is the motivation for the quantitative analysis that follows. The basic fundamentals of coupled mode theory are presented and used to distinguish LPFGs from fiber Bragg gratings (FBGs). A three-layer model is then used to demonstrate the feasibility of the tunable filter design. The model results show that an LPFG with a thin cladding (~35-40 µm) will have an enhanced sensitivity to changes in the ambient index and will have a single resonant band in its output spectrum. The results of the three-layer model are experimentally verified, but unexpected LPFG tuning behavior is observed when a high index indium tin oxide (ITO) overlay is coated on the fiber. The ITO overlay, which had been omitted in the three-layer model, is shown to induce cladding mode transitions in the fiber as the ambient index is increased. These transitions significantly affect the tuning performance of an LPFG, and a new four-layer model, which includes the overlay, is developed to more accurately predict an ITO coated LPFG’s tuning behavior. The four-layer model is used to confirm and quantify the effects of mode transitions. The measured data from a number of ITO coated LPFGs is then compared to the model data to see how well the model predicts the tuning behavior of real LPFGs fabricated in our lab. By making a few adjustments to the model constants to account for variations in the fabrication processes, the model is made to fit to the various data sets. The tuning performance of one LPFG fabricated in our lab nearly matched the performance of the ideal-case LPFG predicted by the model. However, the errors that are consistently introduced by our equipment during fabrication prevented us from replicating the sample and proved to be the limiting factor in our research effort. Although we were unable to produce a working, electro-optically tunable filter prototype, this research effort was not pursued in vain. The four-layer model presented in this thesis is a comprehensive analytical tool that can be used to predict both the tuning range and peak depth of a tunable LPFG that has been coated with a high index overlay. Using the model as a guide, we fabricated an ITO coated LPFG whose resonant peak tuned in excess of 200 nm when the ambient refractive index was increased by 0.01. To the best knowledge of the author, this is the highest sensitivity reported for an LPFG to date. In addition to the tuning performance, the resonant peak remains within 1 dB of its maximum depth for nearly 150 nm of the tuning range. Furthermore, we examine several future research topics that could benefit from the analysis in this thesis.