Ultra-narrow-band optical filters are key components for signal processing in the fields of microwave photonics, coherent communication and dense wavelength-division multiplexing. The ideal ultra-narrow-band optical filter has a rectangular frequency response composed of an ultra-flat passband and a very steep edge. The flat passband has high signal fidelity and can prevent the signal from being distorted, whereas the steep edge can suppress the crosstalk between adjacent bands. Fiber Bragg gratings (FBGs) that achieve various frequency responses are commonly used in bandpass optical filters owing to their small size, low insertion loss and compatibility with other optical fiber devices and systems. However, for the application of ultra-narrow-band optical filters, the spectral and performance parameters of FBG are extremely demanded to obtain the rectangular spectral response with flat passband and high side-mode suppression ratio (SMSR). In the process of FBG fabrication, the stability of the writing optical path, the uniformity of the ultraviolet (UV) spot and the accuracy of apodization process are highly required whether the UV irradiation method of the phase mask or the point-by-point scanning method is adopted. The writing error will lead to the reduction of the spectral performance of the grating, which directly affects the quality of the signal. Therefore, the influence of UV irradiation inhomogeneity on the spectral properties of FBG is analyzed in this paper.

Solid-state laser point-by-point writing method is used for the fabrication of uniform narrow?band optical FBG. The uniformity of UV spot and the accuracy and stability of the optical path will affect the optical fiber grating refractive index modulation distribution, thus affecting the performance of FBG spectrum. By theoretical simulation, the influence of different UV irradiation inhomogeneity on the spectrum of FBG is analyzed. The apodization function is modified based on the analysis results to compensate for the effect of irradiation inhomogeneity, so as to achieve the improvements of the grating spectral performance. Using the proposed method, FBG is prepared after correcting the apodization function, which can compensate the writing errors caused by UV spot and optical path adjustment, and the SMSR of FBG is increased to 30 dB, greatly improving the performance of optical filter.

When the UV irradiation inhomogeneity function F(z) is introduced, it will not only affect the photo-induced refractive index background, but also affect the modulation depth of the photo-induced refractive index. Through the transmission matrix method, the influence of F(z) function distribution on the spectral performance of the FBG under different conditions is theoretically analyzed by MATLAB simulation software according to Eq.(2). As can be seen from Fig.1, when the length of the grating remains unchanged and the reflectance of the FBG remains the same (about－20 dB), the error of the refractive index modulation depth nˉ₁ increases with the increase of the inhomogeneity, that is, the increase of the linear slope k of the F(z) function. The 3 dB bandwidth and 25 dB bandwidth of the FBG also increase, the side band steepness N of the grating decreases, and the rectangularity of the reflection of the grating becomes worse. In experiments, by specifically adjusting the height of the fiber fixture on both ends (Fig.4), UV irradiation error is introduced. Thus the FBG reflection spectra and the change trend of grating parameters are obtained for FBGs written at different fiber tilt angles. The experimental results show that with the increase of the fiber tilt angle, the 3 dB bandwidth and 25 dB bandwidth gradually increase, and the side band steepness decreases, that is, the rectangularity of the grating reflection spectrum deteriorates. This is not conducive to the writing of gratings with ultra-narrow bandwidth and high rectangularity requirements which are important performances for optical filter applications. In further experiments, the UV spot irradiation inhomogeneity function F(z) is calculated according to the energy meter measurement results. By modifying the apodization function based on F(z), the 3 dB bandwidth of the obtained FBG is 96.22 pm, the 25 dB bandwidth is 150.92 pm, the SMSR is greater than 30 dB, and the side band steepness N is increased to 0.8044 dB/pm. As can be seen from the comparison in Fig.7, the SMSR and rectangularity of the grating spectrum written after the correction of spot irradiation inhomogeneity function are much improved, which also verifies the theoretical and experimental results.

According to the requirements of ultra-narrow-band FBG used in optical filters, the inhomogeneity of refractive index modulation caused by UV spot or optical path adjustment in grating fabrication by point-by-point scanning method is analyzed theoretically and experimentally. We propose to modify the apodization function based on the UV spot irradiation inhomogeneity function F(z) to compensate for the effect of irradiation inhomogeneity. The FBG fabricated by the above method has a high rectangularity and the SMSR is greater than 30 dB. To further obtain an ultra-narrow-band FBG, only the length of the grating region needs to be increased. The method of fabricating ultra-narrow-band FBG is simple, which can solve the difficulty of high-precision adjustment of the writing optical path and reduce the influence of non-uniform spot distribution of the laser itself, and greatly improve the application ability of ultra-narrow-band FBG for optical filters.