Objective A tunable fiber filter (TFF) takes an optical fiber as the medium to realize wavelength-selective reflection or transmission of optical signals. TFFs play an important role in optical fiber sensing and communication owing to their inherent merits of anti-electromagnetic disturbance, compact size, and low fabrication cost. Compared with traditional interferometer TFFs, such as Mach-Zehnder interferometer, fiber Bragg gratings, Fabry-Perot interferometers, and microstructure interferometers, an optical fiber microcavity has a high quality factor, high energy density, and whispering-gallery mode (WGM) resonance spectrum with an ultra-narrow band. Moreover, research on WGM microcavities based on new materials is of great significance for realizing an all-optical controllable TFF with high flexibility and tunable filtering. An all-optical TFF with a simple structure can eliminate the need for applying additional mechanical devices or heating devices to realize dynamic tuning. In this paper, an all-optical TFF based on phosphate glass microspheres (PGMS) is proposed to facilitate a systematic study of optic-thermal tunability. With the advantages of all-optical control, compact structure, high stability, and ultra-narrow bandwidth, all-optical TFFs based on PGMS could be widely used in fiber sensing elements or mode selection of fiber lasers, providing good application prospects in the field of optical fiber communication.

Methods The components of a PGMS microcavity coupling system include the phosphate glass optical fiber (PGOF), PGMS and single mode tapered fiber. The PGOF was prepared by preform-drawing method. After high temperature melting, stirring, pouring, annealing and cooling to room temperature slowly, the optical fiber preforms of core and cladding were prepared, respectively. Then, the PGOF was fabricated by wire drawing with the preforms and then fused and stretched by CO₂ laser heating with a certain power. At the same time, the diameter changes of the PGOF were observed by high resolution microscope, and the microsphere was formed based on surface tension effect. The single mode tapered fiber with low loss and good size was tapered by controlling the cone drawing speed and hydrogen flow rate strictly. The sizes of PGMS and the single mode tapered fiber were characterized by optical microscope, including the diameter and waist width. According to the above preparations of three optical components, the WGM resonance spectrum with high Q value and low insertion loss was obtained by efficiently coupling PGMS with the single mode tapered fiber.

Results and Discussions The optic-thermal tunability of PGMS was studied by scanning WGM resonance spectrum under transient process. In the measurement of low power optic-thermal tuning, with the increase of optical pump power, the WGM resonance wavelength drifts to a shorter wavelength (blue shift). The maximum drift is about 18.25pm and the optic-thermal tunable sensitivity is about 49.46pm/mW with a linearity more than 0.99. In the test of high power optic-thermal tuning performance, the WGM resonance wavelength of PGMS drifts to the shorter wavelength (blue shift) with the increase of optical pump power. The maximum drift is about 179.58pm and the optic-thermal tunable sensitivity is around 72.727pm/mW with a linearity more than 0.99. According to the experimental results, the WGM resonance spectrum drifts and optic-thermal tuning sensitivity of PGMS increases with the increase of optical pump power. Furthermore, in order to verify the effectiveness of the experimental results, the all-optical tuning characteristics of the TFF based on silica microsphere were studied with the same method. The experimental results show that the WGM resonance wavelength of silica microsphere shifts to the longer wavelength (red shift) with the increase of optical pump power. The results of silica microsphere and PGMS are different, which depend on the characteristics of the material itself. The maximum drift is about 0.28pm and the optic-thermal tunable sensitivity is only about 0.086pm/mW with a linearity less than 0.7. By comparison, it can be seen that the all-optical TFF of PGMS based on strong optic-thermal effect combines with high Q value, high energy density and narrow linewidth characteristics, and achieves an optic-thermal tunable sensitivity up to 72.727pm/mW.

Conclusions An all-optical TFF based on PGMS was proposed and demonstrated. Fabricate the microsphere with high Q value was frabricated using the high power CO₂ laser by melting and heating the fiber. The microsphere was coupled efficiently by a single mode tapered fiber and the WGM resonance was excited by a tunable laser source. With a varied optical pump power, the PGMS has higher optical sensitivities than the silica microsphere. As for the PGMS, an increased pump power results in WGM resonance wavelength shifting to a shorter wavelength (blue shift), and the optic-thermal tunable sensitivity is about 72.727pm/mW with a high linearity of >0.99. However, in the same condition, the WGM resonance wavelength of the silica microsphere shifts to a longer wavelength (red shift), and the optic-thermal tunable sensitivity is only about 0.086pm/mW with a much lower linearity. The proposed all-optical TTF based on PGMS has the advantages of all-optical control, compact structure, high stability, ultra-narrow bandwidth and highly tunable efficiency, which will be used widely in the field of power system, optical fiber sensing and optical fiber laser.