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Fiber Bragg Gratings Sensor Inscribed in Highly Birefringent Fiber

In an article published in the journal Materials, researchers proposed cure stage detection with Fiber Bragg Gratings (FBGs) inscribed in single mode and highly-birefringent side-hole fiber compared to the reference gel point detected with an in-mold DC sensor.

Study: Gel Point Determination in Resin Transfer Molding Process with Fiber Bragg Grating Inscribed in Side-Hole Elliptical Core Optical Fiber. Image Credit: Connect world/

Embedded sensors such as fiber optic sensors (FOS) have recently drawn much interest in the scientific community. Because of their tiny size, resilience to electromagnetic fields, and multiplexing capabilities, FOSs primarily utilize structural health monitoring (SHM) of composite components. However, since FOS are integrated into the composite production process, there is a fantastic chance to utilize them as manufacturing process sensors.

Fiber Bragg Grating (FGB)

Fiber Bragg Grating (FGB) is consistently ranked as one of the most popular FOS. A simple FBG is a fiber segment just a few millimeters long with an in-written periodic refractive index modulation.

A wavelength of light reflected by the FBG depends on the separation between these modulations, also known as grating periods, and the effective refractive index of the fiber's core (Bragg wavelength). The wavelength of light reflected from the FBG may be adjusted to evaluate changes in the grating's period, which are brought on by axial strain applied to the optical fiber.

Gel Point Detection Using FBG Sensor

In thermosetting composites, FBG sensors identify gel spots and calculate the cumulative residual strain.

The search for minute variations in strain trends captured by FBGs serves as the foundation for gel point detection using FBG sensors.

According to research, gelation solidifies the resin enough to transmit stresses to the sensor, allowing strain-building detection using sensors. After the complete curing process, these trend changes are tiny, often on the scale of 100 με.

The measurement of deformations that occur in the composite plane is often the primary way of detection with FBGs, even though the residual in-plane strain is sometimes considerably lower than in the out-of-plane direction.

A Bragg grating positioned perpendicular to the composite plane is one approach to quantify out-of-plane strain. However, this approach only works with thick laminates and cannot be used with closed-mold techniques. However, it may be used to measure out-of-plane strain directly.

Comparing Single-Mode Fibers Embedded FBG Sensors with HB Optical Fibers Embedded FBGs

This study compares FBG sensors embedded in single-mode fibers with FBGs embedded in highly birefringent optical fibers for monitoring the resin transfer molding process.

Gel point detection using a reference technique, curing process monitoring using standard FBG, and curing process monitoring using FBG inscribed in high birefringent side-hole optical fiber were all carried out.


In a closed steel mold with a cavity that had the shape of a flat, square plate, the Resin Transfer Molding (RTM) process was performed. First, a homogenous and continuous mold temperature was attained by connecting both mold halves in parallel to a single heating/cooling unit. After achieving the processing temperature, an additional 10 minutes of waiting time ensured that the whole mold attained a steady temperature level. Next, the cavity was filled with six layers of bonded carbon fiber non-crimp fabric (NCF).

A flushing step was applied to eliminate air bubbles after the Tartler injection unit was employed. The outlets were shut after the cavity was cleared of all air bubbles. Between the fifth and sixth layers, optical fiber with FBGs was positioned to provide maximal bending sensitivity during mechanical testing.

Investigation Techniques

HBM SI405 was used for the investigations, and Bragg wavelength was determined with a 1 pm resolution. For the first 30 minutes, optical spectra were taken every 10 s, and peak identification was carried out using the cross-correlation approach to boost resilience in the case of HB sensors.

A 1 Hz automated peak tracking was also used and the in-mold temperature sensors helped calculate the temperature adjustment. Commercial fiber optic sensors with polyimide coatings and side-hole, highly birefringent fibers were employed as FBG sensors, and SEM was employed to check both fibers' degree of roughness.

Significant Findings of the Study

This study observed the RTM process using FBG sensors embedded in conventional single-mode and highly birefringent side-hole optical fiber. The in-mold DC sensor was used as a standard technique for gel point estimation. The monitoring results using optical sensors were contrasted with the calculated gel point. In-mold sensors were employed for the temperature compensation of FBG strain data.

The study concludes that the gelation point during the RTM process may be estimated using regular FBG sensors. But since chemical residual strain builds up gradually in the composite's plane, it is difficult to pinpoint the exact limits of the gelation point. Even little temperature fluctuations significantly impact the measurement; it seems that integrated temperature sensors are necessary for appropriate compensation when using normal FBG in the RTM process.

With HB FBG sensors, it is possible to monitor the accumulation of transverse residual strain build-up, which is more evident than the trend seen with normal FBG sensors. In addition, low-temperature sensitivity exists for peak separation. For this reason, a separate integrated sensor for temperature adjustment is not needed for transversal strain measurement with FBG inscribed in the side-hole elliptical core fiber.


Karol Wachtarczyk, Marcel Bender, Ewald Fauster, Ralf Schledjewski, Paweł Gasior and Jerzy Kaleta (2022) Gel Point Determination in Resin Transfer Molding Process with Fiber Bragg Grating Inscribed in Side-Hole Elliptical Core Optical Fiber. Materials.

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Taha Khan

Written by

Taha Khan

Taha graduated from HITEC University Taxila with a Bachelors in Mechanical Engineering. During his studies, he worked on several research projects related to Mechanics of Materials, Machine Design, Heat and Mass Transfer, and Robotics. After graduating, Taha worked as a Research Executive for 2 years at an IT company (Immentia). He has also worked as a freelance content creator at Lancerhop. In the meantime, Taha did his NEBOSH IGC certification and expanded his career opportunities.  


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