827 nm Bragg grating sensor in multimode microstructured polymer optical fibre

Abstract

We report on the fabrication and characterisation of a Bragg grating in multimode microstructured polymer optical fibre with a Bragg wavelength of 827nm. This is the smallest Bragg wavelength reported to date for a polymer optical fibre grating and the relatively low loss of the fibre at this wavelength considerably enhances the utility of the device compared to gratings at longer wavelengths. IntroductionSilica fibre Bragg grating (FBG) sensors have become well established as an important sensing technology [1]. However, despite the fact that it is a decade since the first FBG was reported in a polymer optical fibre (POF) [2], applications of the polymer devices away for the optical bench have only just started to be reported [3]. Even though polymer FBGs posses some attractive features (e.g. a lower Young's modulus than silica and a higher failure strain[4]), a considerable barrier to their take-up has been the availability of gratings only at longer wavelengths, mainly around 1550nm [2], [5], though there have been reports of gratings at 980 nm [6]. The problem with these devices is the high attenuation of POF at longer wavelengths, typically around 100dB/m at 1550nm and 10 dB/m at 980nm [7].FBGs at these wavelengths in POF have only become useful with the development of glued connections to silica fibre down-leads that allow POF sensors just a few centimetres in length to be deployed [3]. However, the presence of the relatively fragile and bulky connection so close to the FBG is a significant limitation on its deployment. Consequently, we have been working towards the inscription of FBGs in the 800nm region where fibre loss is much lower (2dB/m) [7] and broadband semiconductor sources are readily available.Work from the early 1970s on the photosensitivity of bulk poly(methyl methacrylate) (PMMA) reported a recording resolution of better than 200nm sufficient for the creation of Bragg reflectors for a red dye laser [8] and this suggested that the inscription of FBGs in POF at 820nm should not be problematic. Indeed, in our laboratory, we quickly succeeded in recording gratings with a period of 279 nm in 2mm thick slabs of bulk PMMA using a phase mask illuminated by a HeCd laser. However, numerous attempts to use the same laser to record similar gratings in samples of POF (both doped stepindex fibre and pure PMMA microstructured fibre) were completely unsuccessful. These attempts were made using two methods, the conventional phase mask approach [9] and an interferometric technique based on Lloyd's mirror [1].In this paper we report on our eventual success in producing gratings at 827nm and provide full details of the inscription arrangement that permits exposure times of around 2 hours. ExperimentalMultimode microstructured polymer optical fibre (MMmPOF) was used in this work. The fibre was obtained from Kiriama Pty. Ltd. of Sydney, Australia, and is made purely of PMMA with an outer diameter of 150µm along with a core diameter of 50µm. The photonic crystal fibre (PCF) design consists of three rings of holes ...

Fabrication and characterization of polycarbonate microstructured polymer optical fibers for high-temperature-resistant fiber Bragg grating strain sensors

“…The Bragg wavelength exhibited a linear response over the strain range 0-3%, with a sensitivity of 0.701 ± 0.003pm/µε calculated by linear regression. This sensitivity is very close to the value of 0.71 pm/µε reported for PMMA 3-ring mPOFBGs at 850nm [49] and 827nm [50], and similar to that of 0.64 pm/µε measured for a TOPAS 2-ring mPOFBG at 870 nm [13]. Moreover, the fiber did not show any hysteresis in the unloading phase, since the forward and reverse curves were perfectly overlapped, as shown in Fig.…”

Fast and stable gratings inscription in POFs made of different materials with pulsed 248 nm KrF laser

“…An appropriate wavelength regime is around 650 nm, where PMMA has low material loss, but the more common wavelength for POFBGs is around 850 nm, which we will also focus on here [26,27], since the polymer presents high losses at the 1550 nm region [27]. Conventionally, the inscription of POFBGs is made with the phase mask technique using continuous wave (CW) 325 nm HeCd UV lasers [25].…”

High-T_g TOPAS microstructured polymer optical fiber for fiber Bragg grating strain sensing at 110 degrees

“…It was first about 5 years later that FBGs at around 850 nm, where the mPOFs have lower loss, were reported [12,13]. Traditionally, POFs, mPOFs, and polymer FBGs have been made of PMMA [1,3,6,[10][11][12][13][14][15][16][17][18][19][20][21], which makes the response of the FBGs dependent on both temperature and humidity [17][18][19][20][21]. This fact, together with the issue of high loss and low operating temperature, has been hindering the application of polymer FBGs in key strain sensing and biosensing areas [8].…”