Overwrite fabrication and tuning of long period gratings.

James Barrington,Ralph P Tatam,Matthew Partridge,Stephen W James

doi:10.1364/oe.24.022345

Abstract

The central wavelengths of the resonance bands are critical aspect of the performance of long period gratings (LPGs) as sensors, particularly for devices designed to operate near the phase matching turning point (PMTP), where the sensitivity to measurements can vary rapidly. Generally, LPGs are characterized by their period, but the amplitude of the amplitude of the index modulation is also an important factor in determining the wavelengths of the resonance bands. Variations in fabrication between LPG sensors can increase or decrease the sensitivity of the LPG to strain, temperature or surrounding refractive index. Here, the technique of overwritten UV laser fabrication is demonstrated. It is shown that, on repeated overwriting, the resonance bands of an LPG exhibit significant wavelength shift, which can be monitored and which can be used to tune the resonance bands to the desired wavelengths. This technique is applied to periods in the range 100 to 200 µm, showing the cycle-to-cycle evolution of the resonance bands near the PMTPs of a number of cladding modes. The use of online monitoring is shown to reduce the resonance band sensor-to-sensor central wavelength variation from 10 nm to 3 nm.

Highlights

The use of optical fiber long period gratings (LPGs) as a sensing platform has stimulated considerable interest over the last two decades
Where λx represents the wavelength at which light is coupled to the LP0x cladding mode, ncore is the effective refractive index of the mode propagating in the core of the fiber, nclad(x) is the effective index of the LP0x cladding mode and Λ is the period of the LPG
The influence of the amplitude of the refractive index modulation on the resonance bands suggest the ability to tune the properties of the LPG by repetition of the fabrication process, allowing the fabrication to be terminated once the required spectral characteristics have been attained

Summary

Introduction

The use of optical fiber long period gratings (LPGs) as a sensing platform has stimulated considerable interest over the last two decades. The difference in the photosensitivity of the exposed and unexposed regions of the fiber means that the amplitude of the index modulation would increase even when the fiber is UV irradiated uniformly [9], which, as will be discussed, will influence the resonance wavelengths and the strength of the grating and extinction of the resonance bands. In [12], MacDougall et al discussed the evolution of the spectrum of an LPG during fabrication, considering the influence of the amplitude of the index modulation They derived a modified expression for the phase matching condition that takes into account the influence of the increase in the refractive index of the core of the fiber; λx = (ncore + δ ncore − nclad (x) − δ nclad )Λ (2). This is investigated in detail by repeatedly using the point-by-point technique to create an LPG and tune its properties, but similar effects will be observed when using an amplitude mask

LPG fabrication system

Findings

Conclusions