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For Scientific Research & Industry Modernisation.
Fiber Bragg gratings (FBGs) manufactured by femtosecond laser technology possess significant advantages over conventional FBGs fabricated by ultraviolet lasers. This is due to the large refractive index difference introduced by a femtosecond laser, which can be up to 10-3 (Δn ∽ 10-3). Femtosecond laser manufactured FBGs offer two major advantages:
Therefore, the future trend in the field of FBG manufacturing is to achieve high-quality FBGs manufactured using femtosecond lasers. During the manufacturing process of FBGs with femtosecond lasers, the refractive index difference introduced by the laser directly determines the quality of the produced FBGs. When the refractive index difference produced by the laser processing is well controlled, each point’s size and refractive index distribution can be kept consistent for achieving high-performance FBGs. Each point’s refractive index difference is mainly determined by the interaction of light and material, which can be expressed as:
Δn = f(Pt)
where Δn represents the refractive index difference introduced by a laser, f is an arbitrary function determined by the material being processed, P is the laser power, and t denotes the exposure time. Thus, the refractive index difference is determined by both the material’s response and the laser’s parameters. The f varies depending on the materials.
Therefore, ensuring the consistency of the laser power and exposure time is the determining factor for achieving a consistent refractive index difference (Δn).
High power stability and precise power control are essential to guarantee the high-quality manufacturing of fibre Bragg grating (FBG). PowerFlex is a fully automated laser power control technology that enables precise control of laser power at a step of 1µW.
Fig 1. Comparison of FBG grid points for stable and unstable laser power processing. The blue line represents the fiber core and the dot represents the grid point of laser processing.
During the manufacturing process, the controlled exposure time can be very precise, at the order of 0.001ms, so the exposure time can be maintained very consistently. In addition, controlling laser power stability is essential to ensure consistent refractive index differences. In this regard, Innofocus uses a highly stable femtosecond laser with a real-time pulse power monitoring system. The monitoring system guarantees the stability of laser pulses during long-time continuous operation (>100 hours), and the fluctuation of a single pulse can be controlled within <1%. It enables continuous stable FBG manufacturing and ensures the consistency of the refractive index of each point. It lays a solid foundation for realizing high-performance FBG manufacturing. A schematic of the comparison is shown in Figure 1. Suppose the laser power is not stable. In that case, the distribution of refractive index within the fiber during manufacturing will be uneven, as shown in the case of Figure A, causing an overall lower quality of FBG. Innofocus’s high-stability laser can output stable power values for a long time to ensure a uniform refractive index distribution.
In addition to high power stability, precise power control to adjust the points’ size, and refractive index difference Δn are extremely important in optimizing FBG parameters and manufacturing different types of FBGs. To meet this requirement, Innofocus has developed the PowerFlex, a precise power control module. PowerFlex is a fully automated laser power control technology that enables precise control of laser power at a step of 1µW.
Our intelligent femtosecond laser power control technology can also meet the needs of introducing different refractive index distributions. As shown in Figure 2, the points in an FBG manufactured by a high laser power are larger in size with a higher refractive index change (Figure A), whereas points manufactured at a low power are smaller in size (Figure B). By changing the system’s parameters, various flexible needs can be met. The fully automated power system effectively avoids the variation in quality caused by laser power fluctuation. It allows for long-time, large-scale, repeated FBG manufacturing, resulting in high-quality mass production of FBG.
Fig 2. Schematic figure of FBG points processed with different laser processing powers.
