Investigating key factors for optimizing FBG inscribed by femtosecond laser

Abstract

As the key device of optical fiber sensor, the manufacturing methods of FBG have attracted much attention. Point by point inscribing method using femtosecond laser has some great advantages of modulating the central wavelength, grating pitch and length according to the needs of sensing information. However, the relationship between the spectrum and the characteristic structure of FBG has not been completely clear. Among them, the key factors related to process implementation and their influencing laws are a very important part. Facing the current dilemma, the key factors for optimizing FBG inscribed by femtosecond laser are investigated in this paper, such as power, fiber type, polarization effect, and grating length. The direct relationship between different influencing factors is explored through experimental analysis and a method to optimize the polarization effect is proposed by increasing rotational degrees of freedom. At the optimal power with the grating length of 5 mm, cascaded FBGs on ordinary single-mode fiber with a reflection peak height of more than 20 dB and 3 dB bandwidth of less than 0.3 has been manufactured. This research is an improvement of the point by point method for obtaining FBG with optimized performance and spectra, which provides a good support for the development of optical fiber sensor with FBG as sensitive device.