45° tilted fiber gratings (45° TFGs) are an important class of polarization-dependent-loss-based polarizers. Unlike other fiber polarizers that require physical modifications to the fiber such as tapering, polishing, and etching the fiber, 45° TFGs can be fabricated noncontactly inside the fiber core with ultraviolet (UV) light exposure, preserving the mechanical strength of the fiber itself. This makes the 45° TFG-based polarizers ideal for applications that prioritize reliability and repeatability, such as polarization-mode-locking fiber lasers and polarization mode filtering in fiber-optic sensing. 45° TFGs utilize the Brewster angle effect, where the s-component of the light propagating in the gratings is resonantly radiated out, and in contrast, the p-component can propagate losslessly in theory. Thus, the contrast between these two polarization components, i.e., polarization extinction ratio (PER), is a fundamental parameter for evaluating the performance of the 45° TFGs. Unlike traditional fiber Bragg gratings, in which their key parameter, reflectivity, grows exponentially with the grating’s index modulation, the PER of 45° TFGs only grows with the square of their index modulation. Hence, a strong index modulation is often required to have a satisfactory PER for many applications. We propose a highly repeatable method to enhance the index modulation of the 45° TFGs and their PER by multi-pass UV light scan.

Conventional methods for fabricating fiber Bragg gratings (FBGs) include the two-beam interference method, point-by-point writing method, and scanning phase mask method. Specifically, the scanning phase mask is an important technique for fabricating low insertion loss 45° TFGs. It utilizes a tilted phase mask to spatially modulate UV light, creating the desired grating pattern. The UV light is then scanned along the length of the fiber to fabricate a 45° TFG. Due to the limitations of the grating writing system’s stability, traditional writing methods only employ single-pass scanning and do not control the polarization state of the incident UV light. As a result, the full utilization of fiber photosensitivity is not achieved, hindering the fabrication of high index modulation 45° TFGs. To address this, we propose an improved scanning phase mask method, allowing for multiple-pass scan and relaxing the stringent stability requirement of the fabrication system in practice during the entire scanning process required for the high PER 45° TFGs. Our innovative method takes advantage of the UV light polarization control and most importantly real-time feedback of the phase mask position using a high-precision piezoelectric stage integrated into our grating writing system. Using the real-time PER data during the grating fabrication process, a close-loop control is realized for the axial position of the high-precision piezoelectric stage, where the phase mask is mounted. The control parameters are optimized to ensure that the position of the phase mask for the writing segment of the fiber remains unchanged during a multi-pass scan.

Our theoretical analysis shows that the polarization control of the UV light enhances its interference fringe contrast after diffracting off the phase mask from about 91% to full 100%, resulting in higher index modulation of our 45° TFG. It is also found that the axial alignment error between successive grating writing passes should be controlled preferably within 10 nm. Experimental results show that with the optimized UV light polarization state and active feedback of the position of the phase mask, 45° TFGs with a center wavelength of 830 nm can be fabricated on hydrogen-loaded 40 μm ultra-thin polarization-maintaining fibers. These gratings only 30 mm in total length, scanned four passes during the writing process, all exhibit a very promising PER exceeding 35 dB, an insertion loss below 2 dB, and a 3 dB wavelength bandwidth exceeding 60 nm. They also demonstrate high annealing stability (only 3% variation) and low standard deviation of PER among multiple samples (0.2 dB), indicating excellent repeatability of our fabrication process and system. These fabricated 45° TFGs are well suited for applications such as fiber-optic gyroscopes and other fiber sensing systems. By reducing the system’s dependence on environmental stability, this adaptive multi-pass grating writing method enables efficient and large-scale production of stable 45° TFGs.

We first theoretically analyze the influence of the polarization state of incident UV light and the position error of the phase mask position during a multi-pass scan on the PER of 45° TFGs. Furthermore, we develop an improved scanning phase-mask fiber grating writing system, incorporating polarization control functionality for the UV light and a high-precision piezoelectric stage to accurately control the position of the phase mask. Real-time PER data obtained during the grating writing process is utilized in our developed closed-loop control algorithm to dynamically adjust the position of the phase mask. This innovative approach enables the development of a multi-pass scan system capable of significantly enhancing the PER of 45° TFGs and most importantly achieving repeatable fabrication of high-performance gratings. The stability and adaptability of the writing system are demonstrated, effectively mitigating environmental influences. Our findings provide a promising solution for the potential mass production of high-performance 45° TFGs, with broad application prospects in fiber-optic gyroscopes and other fiber-optic systems.