UV inscribed fiber Bragg gratings (FBGs) are ideal optical devices in telecom,lasers and sensing systems. Phase mask scanning technique offers high quality FBG fabrication with high reproducibility and ease of design. At present, most of the FBGs reported using a phase mask scanning method are based on the 1st order Bragg diffraction of the fiber grating structure. Comparing with the 1st order Bragg resonances, the higher-order Bragg resonances has the shorter wavelength and narrower bandwidth, which has its special application value. So far, most of research of the high order Bragg resonances are about the FBG and the applications in temperature and strain sensing. However, there is few reports about the 2nd order Bragg resonance of tilted fiber gratings.

In general, the grating fringes induced by UV inscription through phase mask have a quasi-ideal sinusoidal refractive index distribution along the fiber axis. However, when the refractive index changes to saturation, which means the fiber has a large UV-exposure dose, the refractive index distribution tends to be square. According to Fourier series analysis, high-order frequency components will be generated in the frequency domain, and a series of high-order Bragg resonances peak will appear in the spectrum. For tilted fiber gratings (TFGs), the tilted fringes destroy the sinusoidal refractive index distribution along the fiber axis to a greater extent, and the higher-order Bragg resonance is easier to obtain.

The research group led by Yan Zhijun, Associate Professor of the School of Optics and Electronic Information, Huazhong University of Science and Technology, systematically analyzed the generation principle of 2nd order Bragg resonance of tilted fiber gratings fabricated, and experimentally observed the 2nd-OBR of TFG with various tilt angles from 0° to 45° fabricated by using UV inscription via a phase mask. The research results are published in Chinese Optics Letters, Volume 20, Issue 1, 2022 (Qingguo Song, et al, Theoretical and experimental analysis of second order Bragg resonance of 45° TFG and its fiber laser application.).

When the tilt angle of the grating is ~45º, the Bragg resonance exhibits very strong polarization dependence. Finally, we have employed the 2nd-OBR of 45° TFG that shows very strong polarization dependence to achieve an all fiber polarized laser system, in which the laser output shows high DOP (>99.9%), high OSNR (>50 dB). By applying mechanically stretching on the grating, a wavelength tunable laser output has been achieved. The output laser shows ~0.2 dB amplitude variation within one-hour continuous monitoring of the laser. The 2nd -OBR in a TFG would be a simple and effective way to produce single polarization fiber distributed feedback laser (DFB), as shown in Figure 1.

Fig.1 Schematic of the demonstrated fiber laser using the 2nd -OBR generated by a 45° TFG.

Professor Lin Zhang in Aston University evaluated the work: "This work has presented theoretically explained and experimentally characterized formation and feature of second-order Bragg resonance of TFG, and firstly revealed that the TFG with a tilted angle of 45 ° has a strong polarization dependent 2nd order Bragg resonance, which is cleverly applied to the polarization laser system. The experimental results show that the 2nd order Bragg resonance of 45 ° TFG has high application value in fiber lasers." In the next work, the research group will explore the application of 45 ° TFG 2nd order Bragg resonance in single polarization, ultra-narrow optical fiber lasers.