Objective Ultranarrow bandpass optical filters are key components for signal processing in the fields of microwave photonics, dense wavelength-division multiplexing, coherent communication, and optical fiber sensing. The ideal ultranarrow bandpass optical filter has a rectangular frequency response composed of an ultraflat 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 the adjacent bands. Fiber Bragg gratings (FBG) that achieve various frequency responses are commonly used in bandpass optical filters towing to their small size, anti-electromagnetic interference, low insertion loss, and compatibility with other optical fiber devices and systems. However, the ordinary uniform FBG bandwidth is relatively large, in a dozen GHz or hundred GHz. Although the phase-shifted fiber Bragg grating can achieve a bandwidth below 100 MHz, its Lorentz or quasi-Lorentz line frequency response limits its application in high-resolution signal processing. Recently, the multiphase-shifted FBG (MPSFBG) containing multiple phase shifts have been used to design the ultranarrow bandpass optical filters. The MPSFBG can obtain the narrow band flat-top filter response with small insertion loss and good rectangularity by optimizing the position of each π phase shift. In actual preparation, there are errors in the control of the phase-shift amount and phase-shift position, which result in the difference between the actual optical spectra and theoretically calculated optical spectra under ideal conditions. Therefore, the introduction of a high-precision phase shift is the key to the fabrication of the MPSFBG.
Methods This study presents a method for fabricating the MPSFBG by introducing a high-precision phase shift into the FBG using local temperature control. First, the influence of the phase-shift amount and position errors on the insertion loss, bandwidth, and shape factor of the MPSFBG filter is analyzed in the theoretical part of this study, and the phase-shift amount and position precision required by the MPSFBG to obtain the narrow band flat-top filter response with small insertion loss and good rectangularity are obtained. In the experimental part, the principle of introducing a phase shift into the FBG using the local temperature control scheme is analyzed, and the structure for local temperature control is designed. The phase-shift amount and position precision achieved using the designed local temperature control structure in uniform FBG are experimentally measured. Finally, a dual-phase-shifted FBG is fabricated using this method. In addition, its frequency response is measured.
Results and Discussions The method proposed in this study uses local temperature control to introduce high-precision phase shifts into the FBG to assist the MPSFBG in achieving a phase-shift amount and position precision of 0.0007π and 30 μm, respectively (Fig. 9 and Fig. 10). This precision meets the phase-shift precision required by the MPSFBG in the theoretical analysis to obtain the narrow band flat-top filter response with small insertion loss and good rectangularity. In addition, this phase-shift erasing is possible because the phase shifts introduced by it do not permanently change the structure of the FBG, providing an efficient and economical method for fabricating the MPSFBG. The frequency response of the dual-phase-shifted FBG filter fabricated using this method is consistent with the theoretical fitting result under ideal conditions, and the insertion loss of about 0.5 dB, 3 dB bandwidth of about 366 MHz, 20 dB bandwidth of about 972 MHz, and shape factor of about 0.38 are realized (Fig. 11).
Conclusions In this study, a method for fabricating an MPSFBG filter using high-precision phase-shift control technology based on local temperature control is presented. First, in the theoretical part of this study, the effect of the amount and position errors of the phase shifts on the insertion loss, bandwidth, and shape factor of the MPSFBG filter is analyzed. The amount precision (0.0029π) and position precision (368 μm) requirements of the phase shifts are obtained for the MPSFBG achieving the narrow band flat-top filter response with low insertion loss and good rectangularity. The experimental part confirmed the feasibility of using local temperature control to introduce the high-precision phase shifts (phase-shift amount precision is 0.0007π, phase-shift position precision is 30 μm). Finally, the MPSFBG is fabricated using this method, and its optical spectrum is tested and theoretically simulated. The results show that the frequency response of the MPSFBG filter fabricated using this method is close to the ideal filter characteristics from the theoretical simulation, and insertion loss of about 0.5 dB, 3 dB bandwidth of about 366 MHz, 20 dB bandwidth of about 972 MHz, and the shape factor of about 0.38 are realized. The method proposed in this study for fabricating the MPSFBG is accurate, simple, and economical. Moreover, the phase shifts generated using this method do not result in permanent changes to the structure of the FBG; thus, it is erasable, which can be used to fabricate the new tunable fiber filters.
