The fiber-optic cascaded Fabry-Perot (FP) cavity structure is a significant interferometric sensor configuration, offering superior spectral resolution compared to Mach-Zehnder type sensors. In response to the comb-spectrum characteristics of fiber-optic cascaded FP sensors, we propose a novel optical vernier processing method. This method calculates the peak positions of individual resonance peaks without the need to track the drift of envelope modulation curve peaks, as required by traditional optical vernier scale methods. Utilizing this approach, we have significantly enhanced the sensitivity of cascaded FP cavity sensors. The strain and temperature sensing tests achieved sensitivities of 4.76 pm/μϵ and 271.61 pm/°C, respectively, representing an increase of over 20 times compared to a single-stage cascaded FP cavity. More importantly, the Figure of Merit (FOM) for the cascaded FP cavity sensors can be improved by more than two orders of magnitude under the same sensitivity amplification factor, compared to traditional optical vernier technologies based on envelope curve recognition. This also effectively avoids potential errors that may arise during the fitting process of the envelope modulation curve. These advantages endow the cascaded FP cavity sensors with significant potential for application in the field of high-performance fiber-optic sensing.

In this work, the design and numerical study of a MMI-based LiNbO₃ beam splitter were presented, with a combination of ion implantation and femtosecond laser ablation as a fabricating strategy. The designed splitters show advantages such as good fabrication tolerance, low insertion loss, compactness, and simple fabrication processes. We also demonstrated that the self-imaging principle could be applied to design low-loss MMI beam splitters for multi-mode input with mode number m = 1 and n>1 by carefully choosing the geometry parameters. The presented fabrication strategy provides an alternative choice for the fabrication of power splitters also with other MMI-based devices in various optical monocrystal chips.

The optical attractive force in tapered single-mode fibers (SMFs) is usually uniformly distributed around the tapered section and has been found to be important for trapping and manipulating targeted atoms and nanoparticles. In contrast, a peculiar phenomenon of the evanescent field splitting along the azimuth axis can be experimentally observed by tapering a weakly-coupled MCF into a strongly-coupled MCF to generate supermode interference. Moreover, the supermode interference produces a hexagonally distributed evanescent field and its six vertices give rise to the multiline optical attractive force. For such spectral resonances, the optimum extinction ratio for the transmission dips is given by 47.4 dB, this being determined using an index liquid to cover the tapered MCF. The resonant dips move to a greater extent at longer wavelengths, with the optimum tuning efficiency of 392 nm/RIU for index sensing. The split evanescent fields respectively attract the excited upconversion nanoparticles in the liquid to be linearly aligned and running down the tapered region over the fiber surface, emitting green light with 60° symmetry. The charged nanoparticles were periodically self-organized, with a period of around 1.53 µm. The parallel lines, with 60° rotational symmetry, can be useful for (1) indicating the exact locations of the side-cores or orientations of the tapered MCF; (2) as precision alignment keys for micro-optical manipulation; and (3) enhancing the upconversion light, or for use in lasers, coupling back to the MCF. The split evanescent fields can be promising for developing new evanescent field-based active and passive fiber components with nano-structures.

Here we report the fabrication of a magneto-optic waveguide based on TGG crystal via 15 MeV C³⁺ ion irradiation. The ion irradiation process leads to the optical anisotropy in the as-irradiated TGG waveguide, which hinders the magneto-optical rotation in the waveguide. To remove the irradiation-induced optical anisotropy, we annealed the as-irradiated TGG waveguide under different conditions. After annealing at 400 °C for one hour, the magneto-optical rotation of 14° per centimeter is observed in the waveguide at the wavelength of 632.8 nm, under the magnetic field of 0.24 T, which is comparable to that observed in the TGG crystal under the same magnetic field. This work paves the way for applications of TGG waveguides as integrated optical rotators and isolators.

The dependence of the output pulse characteristics of a Mamyshev fiber oscillator on cavity parameters is investigated in detail. We analyze the change in pulse spectrum bandwidth, pulse duration, dechirped pulse duration and chirp with the change in fiber group velocity dispersion, fiber nonlinearity, gain, and filters by putting forward a numerical model. In particular, as one of the most important components, the effect of filters bandwidth and the central wavelength interval between them is discussed. The passive fibers are classified into two kinds according to their locations in the cavity, which are the one before the gain fiber and the one after the gain fiber. Numerical simulation results show that a wide spectrum can be obtained by increasing the nonlinearity of the second passive fiber, while the change in nonlinearity of the first passive fiber has a weak effect on spectrum broadening. A wide spectrum could also be obtained by increasing the nonlinearity or the small-signal gain coefficient of the gain fiber. A Yb-doped Mamyshev fiber oscillator is demonstrated. The results show the increase in pump power, which agrees reasonably well with the numerical simulation results.