Fast Bragg Grating Inscription in PMMA Polymer Optical Fibres: Impact of Thermal Pre-Treatment of Preforms

Abstract

In this work, fibre Bragg gratings (FBGs) were inscribed in two different undoped poly- (methyl methacrylate) (PMMA) polymer optical fibres (POFs) using different types of UV lasers and their inscription times, temperature and strain sensitivities are investigated. The POF Bragg gratings (POFBGs) were inscribed using two UV lasers: a continuous UV HeCd @325 nm laser and a pulsed UV KrF @248 nm laser. Two PMMA POFs are used in which the primary and secondary preforms (during the two-step drawing process) have a different thermal treatment. The PMMA POFs drawn in which the primary or secondary preform is not specifically pre-treated need longer inscription time than the fibres drawn where both preforms have been pre-annealed at 80 °C for 2 weeks. Using both UV lasers, for the latter fibre much less inscription time is needed compared to another homemade POF. The properties of a POF fabricated with both preforms thermally well annealed are different from those in which just one preform step process is thermally treated, with the first POFs being much less sensitive to thermal treatment. The influence of annealing on the strain and temperature sensitivities of the fibres prior to FBG inscription is also discussed, where it is observed that the fibre produced from a two-step drawing process with well-defined pre-annealing of both preforms did not produce any significant difference in sensitivity. The results indicate the impact of preform thermal pre-treatment before the PMMA POFs drawing, which can be an essential characteristic in the view of developing POF sensors technology.

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

“…Nevertheless, the time taken for a POFBG inscription can be reduced by several orders of magnitude with the application of a 248 nm laser with low fluence and repetition rate through the phase mask technique [35]. It was also demonstrated that the inscription time can be lowered by performing a thermal treatment on the fiber preforms [33].…”

“…Such treatment provides a reduction of the internal stress in the fiber created during fabrication process. For this reason, it reduces the POFBGs sensors hysteresis, and can also decrease the inscription time and increase the grating stability [33]. The annealing can be made before the grating inscription (preannealing) or after the inscription (post-annealing).…”

“…In addition, they are unsuitable for in-vivo applications and the dopants employed generally lead to higher transmission losses in the final sensor [33]. Since most polymers are intrinsically photosensitive, the doping is not necessary for gratings inscription [34] and the only issue of undoped POFs is the longer inscription time.…”

Dynamic mechanical characterization with respect to temperature, humidity, frequency and strain in mPOFs made of different materials

“…In addition, it can enhance the sensor strain sensitivity [39]. If the annealing is made on the fiber preform, it can increase the grating stability and reduce its inscription time [41], whereas, if it is made after the grating inscription, it can result in large blue-shift of the Bragg wavelength, especially when it is made under high humidity conditions [7]. Aiming at these advantages, the annealing was made in all POF samples employed.…”

Dynamic mechanical analysis on fused polymer optical fibers: towards sensor applications

“…Moreover, their strain performance can be enhanced through annealing and etching treatments [28]. The annealing can be made in the fiber or in the preform [29] and is composed of keeping the fiber in a temperature below the fiber material glass transition temperature (T g ) for some hours [30,31]. In order to increase the strain performance of the POFs, the annealing is made on both non-fused and fused POFs, which were kept in a temperature of 90°C for 24 h. The annealing temperature was chosen considering that the employed POFs present a structure similar to CYTOP fibers, which presents a T g of about 120°C.…”