An automatically power-stabilized (with power fluctuation < 0.155 dB), narrow-linewidth (0.0171 nm), wavelength-tunable (10.69 nm) erbium-doped fiber laser has been proposed by cascading two fiber Mach-Zehnder interferometers (MZI) without using any temperature controlling device. One of the MZIs (here called the 1st MZI) is composed of two 3 dB couplers to form interference patterns while the other MZI (here termed the 2nd MZI) is constructed with a tapered seven-core fiber (SCF) and based on the principle of supermode interference. For the two MZIs, the free spectral range (FSR), the passband bandwidth and the extinction ratio (ER) at 1560 nm are 0.37 nm, 0.19 nm, 16.6 dB and 13.93 nm, 7.93 nm, 10.1 dB, respectively. Due to the major difference between the two FSR values, the 1st MZI and the 2nd MZI respectively play a role in controlling the laser linewidth and suppressing the homogeneous broadening effect to reach to a satisfactory level of power stability. The 2nd MZI is also used to fine tune the laser wavelength by applying strain to the tapered SCF (TSCF) over the spectral range of 1570.22 - 1559.33 nm, with an incremental step of 0.37 nm being used. The side-mode suppression ratio (SMSR) of the tunable fiber laser can be up to 45 dB. By appropriately adjusting the polarization controller, dual wavelength lasing can also be achieved. For single wavelength lasing, the 3 dB laser linewidth is 0.0171 nm. The power fluctuation, without a temperature controlling device being used and operating at room temperature, is found to be less than 0.155 dB over 1 hour while the central wavelength drift is less than 0.19 nm.

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.

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.

Intensity interrogation-based surface plasmon resonance imaging (ISPRi) sensing has a simple schematic design and is the most widely used surface plasmon resonance technology at present. In this study, we report the successful development of a novel high-sensitivity ISPRi biosensor and its application for apoptosis detection in cancer cells. By optimizing the excitation wavelength and excitation angle, we achieved a refractive index resolution (RIR) of 5.20 × 10⁻⁶ RIU. Importantly, the biosensor has been tested and validated for high-throughput and label-free detection of activated caspase-3 with its specific inhibitor Z-DEVD-FMK in apoptotic cells. Therefore, this study describes a novel molecular imaging system to monitor apoptosis in cancers for disease diagnosis and/or evaluation of therapeutic efficacy of anti-cancer drugs.

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.

Rapid and accurate determination of target proteins in cells provide essential diagnostic information for early detection of diseases, evaluation of drug responses, and the study of pathophysiological mechanisms. Traditional Western blotting method has been used for the determination, but it is complex, time-consuming, and semi-quantitative. Here, a tapered seven-core fiber (TSCF) biosensor was designed and fabricated. By immobilizing protein kinase B (PKB), also known as AKT, antibody onto TSCF surface, the microfiber biosensor can be used for quantitatively detecting the AKT level in solution concentrations as low as 0.26 ng/mL. To test the reliability of the TSCF sensing method in a medical application, the TSCF biosensor was used to study the relationship between chrysin’s anticancer effect and the concentration of AKT in a human colorectal cancer cell line (LoVo cells). The results reveal that the inhibitory effect of chrysin on LoVo cells is positively correlated with the dose, agreeing well with the equivalent results using the traditional Western blotting method.