1.3 - Fibre-Bragg-Gratings in Highly Birefringent Optical Fibres for Advanced FBG Sensing Applications

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings OPTO 2011
O1 - Components for Sensing and Detection
F. Jülich, A. Koch - Technische Universität München, München (Germany), J. Roths - Munich University of Applied Sciences, München (Germany)
27 - 32


With spreading of the Fibre-Bragg-Grating (FBG) based sensor technology, there is also a growing demand on high accurate sensors. Standard FBG Sensors are typically inscribed into single mode optical fibres, which have some disadvantages compared to other types of optical fibres.

It is known that polarisation effects play an important role when using single mode fibres. Therefore Standard FBG Sensors show a dependence of their Bragg wavelengths λB on the state of polarization of the light waves that interact with the FBG. With this type of FBG we measured variations of λB in the range of ~5pm to ~20pm. When these sensors are used for temperature sensing, the typical temperature sensitivity of FBGs of ~10pm/°C turns these variations to uncertainties of 0.5°C – 2.0°C in temperature sensing. The dependence of λB on the state of polarisation of the light wave is due to a certain amount of birefringence present in the optical fibre at the location of the FBG. The origin of this birefringence is from both, the residual birefringence of the pristine fibre and the birefringence that was induced during the inscription process.

Since it is difficult to produce FBGs with ultra-low birefringence in a reproducible way, our intention is to employ FBGs that were inscribed into highly birefringent or polarisation maintaining optical fibres. A FBG under high birefringence is characterised by the spectral separation of the Bragg reflection peak into two separated peaks. While standard single mode fibres are not capable of maintaining the input state of polarization (SOP) for more than a few meters, highly birefringent fibres maintain the SOP and therefore, interrogation of FBGs with a controlled SOP is possible in HiBi-fibres.

As a first step we investigated the inscription process of HiBi-FBGs. We inscribed FBGs in different types of birefringent optical fibres (Panda and Bow-Tie types) employing the so called phase mask technique. During the inscription process, we measured the Bragg wavelengths λB for both the slow and fast optical axes. With these data we were able to determine the birefringence of the pristine fibres and the inscription-induced birefringence as a function of the UV-exposure dose. These investigations are important to establish a reproducible fabrication process for HiBi-FBGs that can be used for highly accurate fibre based measurements.