6.1         Influence of humidity and temperature on polyimide-coated fiber Bragg gratings

6.1.3        Experiment

A bare fiber Bragg grating (FBG1) and a polyimide coated FBG (FBG2) have been exposed to temperatures and relative humidity in a climate chamber (Fig. 6-1).

Fig. 6-1 Tests fiber Bragg gratings (left) and polyimide coated FBG geometry (right)

The experimental setup is shown in Fig. 6-2. The first grating around 1535 nm without coating has been fabricated in a SMF 28 fiber using ArF excimer laser and phase mask. The second FBG around 1550 nm is a commercial FBG that was recoated with a polyimide by the manufacturer. The re-coated fiber diameter was measured under a microscope and the coating thickness was determined to 57.5 mm. The gratings were spliced together and integrated into an FBG measurement setup. The reflectivity of both gratings is measured using a tunable laser and a photo detector with A/D converter.

Fig. 6-2 Measurement set-up

A calibrated "Rotronic" electrical temperature (PT100) and relative humidity sensor (capacitive) are placed in the climate chamber. The sensor has a response time of less than one minute. A computer controls the environmental conditions in the climate chamber, controls the tunable laser, and performs the read out of the optical signal. From the reflection spectra of both gratings their Bragg wavelengths were obtained for different (RH, T) conditions.

The climate chamber maintains a constant temperature during RH variations. For six different temperatures from 23 °C up to 50 °C the relative humidity was changed from 10 RH% to 90 RH% in steps of 20 RH%. Unfortunately, the "Rotronic" sensor limited the maximal temperature. For every (RH, T) combination a measurement time of 120 minutes was taken to allow for a saturation of water within the polyimide. Every two minutes the changes of environmental conditions (RH, T) in the climate chamber were obtained from the "Rotronic" sensor and a full reflection spectrum of each FBG was taken. All data were stored on a computer for data processing.

The polyimide coating of the FBG has been removed after the experiment and the temperature sensitivity of both FBG's has been measured in a separate measurement setup. It consists of a temperature controlled water recipient with a mercury thermometer and the FBG reflection measurement set-up (Fig. 6-3).

Sensitivities of 6.78×10^-6 and 6.31×10^-6 K^-1 were obtained for the SMF 28 and the commercial fiber, respectively. These values are in good agreement with published results [2,3]. The temperature obtained from the reference grating agreed well with the value obtained from the "Rotronic" sensor.

Fig. 6-3 Temperature sensitivity of FBG measurement and set-up (insert)